DEVELOPMENT OF THE PROCESS PARAMETER MAP FOR
REFERENCE FREE PART ENCAPSULATION
BY
CEANI GUEVARA
S.B.,
MECHANICAL ENGINEERING, 2000
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
BARKER
JUNE2001
~iMS CHUSETTS INSTIrUTE
OFTECHNOLOGY
C 2001 Massachusetts Institute of Technology
JUL 16 2001
All rights reserved
LIBRARIES
-.-
Author:.-.-.
Certified by:
Accepted by:
Department of Mechanical Engineering
May 11, 2001
.................
.......................
S
a.
S
Sanjay E. Sarma
Associate Professor of Mechanical Engineering
Thesis Supervisor
Ain A. Sonin
Professor of Mechanical Engineering
Chairman of the Graduate Thesis Committee
Development of the Process Parameter Map for
Reference Free Part Encapsulation
by
Ceani Guevara
Submitted to the Department of Mechanical Engineering
on May 11, 2001 in Partial Fulfillment of the
Requirements for the Degree of Master of Science in
Mechanical Engineering
ABSTRACT
A paradigm shift needs to occur so that manufacturing processes are able to
simultaneously improve in quality, rate, and flexibility while reducing cost. Fixturing
methods currently available represent an important constraint on manufacturing systems.
The Reference Free Part Encapsulation (RFPE) concept, developed by Sarma and
Wright, seems to be the paradigm shift required to improve the capabilities of the
fixturing technology across the board.
While Lee has proved the viability of the RFPE process experimentally, much work
remains to be done before the process can complete the journey from idea to reality.
Therein lies the motivation for this study: to establish a framework to understand RFPE,
consolidate the existing knowledge, identify the critical factors that remain unexplored,
determine the impact these factors have on the process, and thus develop a process
parameter map of the RFPE concept.
The following process parameters were identified: melt superheat temperature, mold
preheat temperature, ejection temperature, cooling rate, packing pressure, aging, and
alloy composition. These parameters were measured via metrics such as surface finish,
porosity, dimensional accuracy, thermal drift, and rewelding strength. The RFPE Process
Parameter Map was completed based on the information obtained through
experimentation, analysis, and literature review. The melt superheat temperature, mold
preheat temperature, packing pressure, and cooling have the potential to impact the
precision of the system. These experiments have identified the process window within
which the RFPE concept is not sensitive to variations in these process parameters. The
ejection temperature, aging and alloy composition do not seem to have a significant
impact on the precision of the process. The process parameter map forms a solid
foundation from which future developments of the RFPE process can take place.
Thesis Supervisor: Sanjay E. Sarma
Title: Associate Professor of Mechanical Engineering
Table of Contents
CHAPTER 1: BACKGROUND AND MOTIVATION ............
1.1.
Role of Fixturing in Manufacturing.........................................................16
1.2.
Functional Requirements of Fixturing.....................................................16
1.3.
Current Fixturing Technology...................................................................19
1.3.1.
1.3.2.
Modular Fixturing Systems ...........................................................
Universal Fixturing Systems ........................................................
Conformable Clamps ....................................................
1.3.2.1.
19
20
20
Fluidized Bed................................................................
21
1.3.2.2.
1.3.3.
15
1.3.2.3. Phase Change Material ..................................................
Limitations of the Current Fixturing Technology ..........................
22
23
CHAPTER 2: REFERENCE FREE PART ENCAPSULATION....... 25
2.1.
The Ideal Fixturing System.......................................................................25
2.2.
The RFPE Concept ....................................................................................
26
2.3.
Capabilities of RFPE ...............................................................................
28
2.4.
Three Machining Strategies: 2D, 21/2 D, and 3D Milling .........................
29
2.4.1.
2.4.2.
2.4 .3.
2.5.
2D M illing ....................................................................................
2 2D Milling ..................................................................................
3D M illing ....................................................................................
29
30
31
Motivation...................................................................................................32
CHAPTER 3: FRAMEWORK TO ANALYZE AND DEVELOP
RFPE .................................................................................
33
33
3.1.
The Building Blocks of a System .............................................................
3.2.
Design and Process Parameters................................................................33
3.3.
A Structured Approach to Developing the RFPE Process.....................36
CHAPTER 4: SYSTEMATIC EVALUATION OF
PARAMETERS AND METRICS................
4.1.
37
M etrics of the RFPE Process ...................................................................
37
4.1.1.
4.1.2.
4.1.3.
4.1.4.
4.1.5.
Surface Finish................................................................................
Porosity...........................................................................................
Dimensional Accuracy ....................................................................
Thermal Drift..................................................................................
Rewelding Strength ........................................................................
37
38
39
39
40
Process Parameters: an Overview...........................................................
41
4.2.1.
Temperature....................................................................................
4.2.1.1. Melt Superheat...............................................................
4.2.1.2. Mold Preheat Temperature ............................................
4.2.1.3. Cooling Rate .................................................................
4.2.1.4. Ejection Temperature....................................................
Packing Pressure.............................................................................
Aging .............................................................................................
Alloy Composition ........................................................................
42
42
43
43
44
44
47
47
4.3.
The A Priori RFPE Process Parameter M ap ...........................................
48
4.4.
Next Steps ..................................................................................................
50
4.2.
4.2.2.
4.2.3.
4.2.4.
CHAPTER 5: EXPERIMENTAL PLAN.................................................
51
5.1.
Objectives of the Experiments..................................................................
51
5.2.
Experimental Apparatus...........................................................................
51
5.2.1.
5.2.2.
51
5.3.
The Mold .........................................................................................
Instrumentation................................................................................52
Experimental Procedure ............................................................................
5.3.1.
5.3.2.
5.3.3.
5.3.4.
5.3.5.
5.3.6.
53
The Effect of and the Interaction between Melt Superheat and
Mold Preheat Temperature .............................................................
54
The Effect of Cooling Rate ............................................................
55
The Effect of Ejection Temperature ...............................................
55
Aging Effects..................................................................................
55
The Interaction between Packing Pressure and Cooling Rate........56
The Effects of Alloy Composition .................................................
56
CHAPTER 6: THEORETICAL BACKGROUND FOR
EXPERIMENTAL ANALYSIS .....................................
6.1.
The Impact of Surface Finish...................................................................57
6.1.1.
6.1.2.
6.1.3.
6.2.
57
58
The Mechanism of Surface Deflection Under Load ......................
Quantification of the Possible Deflection of the Asperities .......... 59
The Variables for Roughness Measurements as it Relates to
. 60
D eflection ......................................................................................
Im pact of Porosity.....................................................................................
6.2.1.
6.2.2.
6.2.3.
61
61
Form ation of Porosity.....................................................................
Classification of porosity: Macroporosity and Microporosity .......... 62
63
Porosity and the RFPE Process ....................................................
6.3.
Temperature Drop from the Mold Cavity to the Thermocouple Well...63
6.4.
Variations in Cooling Rate.......................................................................65
CHAPTER 7: EXPERIMENTAL RESULTS ..................
67
7.1.
The Effect of Melt Superheat and Mold Preheat Temperature................67
7.2.
The Effect of Ejection Temperature .........................................................
7.3.
The Effect of Cooling Rate.......................................................................69
7.4.
Aging Effects..............................................................................................
70
7.5.
The Interaction between Packing Pressure and Cooling Rate ..............
71
7.6.
The Effects of Alloy Composition...........................................................
71
69
CHAPTER 8: THE RFPE PROCESS PARAMETER MAP........ 73
CHAPTER 9: FUTURE WORK...............................................................
77
9.1.
Study of Design Parameters....................................................................
77
9.2.
Design of a Field Trial Prototype .............................................................
77
9.3.
Development of a Rapid Prototyping Center .........................................
78
APPENDIX A: AN INTRODUCTION TO THE RFPE DESIGN
PARAM ETERS ...........................................................
A.1.
Parameters Affecting the Alloy Storage and Delivery...........................
83
A.1.1.
The Reservoir .................................................................................
A. 1.1.1. Reservoir Temperature..................................................
83
83
A.1.1.2. Proportion of New to Recycled Alloy ..........................
A. 1.1.3. Reservoir Construction .................................................
Alloy Delivery...............................................................................
A.l.2.1. Injection Pressure...........................................................86
84
84
85
A.1.2.2.
Shot Size ........................................................................
86
A.1.2.3.
A.1.2.4.
A.1.2.5.
A.l.2.6.
A.1.2.7.
Rate of Change in Pressure ...........................................
87
Pressure on Opposing Sides of the Injection Piston .......... 87
Inertia Effects..................................................................87
Transfer Tubing Design.................................................87
Partial Blockages of the Metal Stream...........................88
A.1.2.
A.2.
81
Parameters Affecting the Introduction and Behavior of Alloy Inside the
Mold 88
A.2.1.
Gate Velocity..................................................................................88
A.2.2.
Cooling Direction...........................................................................89
A.2.3.
Heating and Cooling of the Mold..................................................
89
A.2.4. Mold Surface Finish......................................................................
89
A.2.5.
A.2.6.
Mold Material Selection...............................................................
Mold Locking Pressure .................................................................
90
90
A.2.7.
Mold Vents ....................................................................................
90
A.2.8.
Compatibility of Gate and Machined Section Geometry ..............
91
REFERENCES......................................................................................
93
List of Figures
Figure 1: The lack of locating stability or deterministic workpiece location
can introduce variations in the position or the orientation of the part. Even a
small variation in orientation can cause a significant error in the precision of
th e w ork p iece .....................................................................................................................
17
Figure 2: Excessive clamping loads can deform a workpiece. This
deformation can affect both the dimensional accuracy and the structural
integrity of the w orkpiece .............................................................................................
18
Figure 3: The inability of a workpiece to transmit the localized fixturing
support force throughout the part causes a loss of dimensional accuracy. In
effect, the workpiece deflection under machining loads results in a lack of
to tal restraint. .....................................................................................................................
18
Figure 4: Conformable clamp fixturing ......................................................................
21
Figure 5: Fluidized bed fixturing ..................................................................................
22
Figure 6: Phase change fixturing ..................................................................................
23
Figure 7: The Reference Free Part Encapsulation (RFPE) Process .............................
27
Figure 8: Sample parts produced with RFPE................................................................28
Figure 9: 2D M illing ....................................................................................................
29
Figure 10: 2!/2D M illing ................................................................................................
30
Figure 11: 3D M illing ..................................................................................................
31
Figure 12: A plot of the relationship between each process parameter and
each metric for a system facilitates an understanding of the system
characteristics.....................................................................................................................34
Figure 13: Each process parameter typically impacts multiple metrics of the
same system. Often, there is no optimum setting for the process parameter.
Therefore, tradeoffs between the metrics are required. .................................................
34
Figure 14: Design parameters determine the range of each process parameter
that is feasible for each implementation of the concept................................................35
Figure 15: For the particular implementation modeled in this diagram, the
choice was made to have the design parameters favor Metric #1 slightly over
M etric # 2 ............................................................................................................................
36
Figure 16: A surface finish can be categorized by the amount that it deviates
from a perfectly flat surface. There are three ways in which this deviation
occurs: form error, waviness, and roughness...............................................................38
Figure 17: H ow therm al drift occurs.............................................................................
40
Figure 18: Creation of weldline test specimens...........................................................41
Figure 19: The peak-to-valley heights of all specimens and inserts used to
characterize surface finish.............................................................................................
45
Figure 20: RFPE process window for packing pressure versus mold surface
rou gh ness ...........................................................................................................................
46
Figure 21: The A Priori RFPE Process Parameter Map ...............................................
48
Figure 22: Interaction matrix for the RFPE process parameters..................................49
Figure 23: Mold used to produce experimental specimens ..........................................
52
Figure 24: LabVIEW control program screen shot...........................................................53
Figure 25: Experim ental setup ......................................................................................
54
Figure 26: Model of surfaces as flat, and in perfect contact........................................57
Figure 27: Asperities reduce the real to nominal surface area ratio .............................
57
Figure 28: Profilometer trace for a 0.4 pm surface, with no wavelength
filterin g ...............................................................................................................................
59
Figure 29: Temperature drop due to conductive resistance as a function of
heat tran sfer rate.................................................................................................................64
Figure 30: Cooling rate of mold in air and in ice water...............................................70
Figure 31: Comparison of the cooling rate of a pressurized and unpressurized
m o ld cav ity .........................................................................................................................
71
Figure 32: Completed RFPE Process Parameter Map .................................................
74
Figure 33: Completed interaction matrix for the RFPE process parameters ...............
75
Figure 34: Rapid Prototyping Center...........................................................................
78
Figure 35: The Rapid Prototyping Center will transform the manufacturing
process from a heavy manual intervention process into a black box............................79
Figure 36: Proposed process flow for the Rapid Prototyping Center ...........................
80
List of Tables
Table 1: Peak-to-valley depth for various surface roughness standards........................60
Table 2: Roughness, summit curvature, and deflection values of the surface
roughness standards ......................................................................................................
60
Table 3: Description of the surface condition as a function of the injection
temperature and mold preheat temperature..................................................................
67
Table 4: Measurement of specimen diameters.............................................................68
TkanL You
To mg familg
To mg friends
To mg mentors
I dedicate this thesis to the memory of
M 3 Grandfather, Opa, Ernst 5.
Who taught me
Keizer
Math, Persistence, UJnderstanding
and to
M Grandmother, Oma, Kuth Keizer
Who taught me the
power of Kindness and Listening.
Chapter 1: Background and Motivation
Manufactured goods are prevalent in our society. These goods decrease the time, reduce
the effort, or provide the tools required to accomplish a task. The span of these goods
ranges from the very mundane, such as scissors or a wrench, to the highly complex, such
as an automobile or a heart pump. We use manufactured products daily, yet most of us
rarely stop to question how they were produced. When we purchase them, we want them
to be cheap, yet we expect them to be durable and perform as advertised. These
requirements percolate through to drive the goal of manufacturing systems: to produce
high quality, low cost products quickly in flexible set-ups. Unfortunately, these goals
tend to be competitive in nature, with advances in one area often requiring concessions
from the other metrics. In this way, incremental advances to the existing manufacturing
systems can only have extremely limited effects. Thus, a paradigm shift is needed to
raise the bar simultaneously on all four criteria used to evaluate manufacturing systems.
The first question we may want to ask is where this paradigm shift needs to occur. Do
we need to change the entire manufacturing process as we know it, or is it possible to
identify a key aspect that will enable the overall improvement we are looking for? In any
system, some areas are always more capable than others. Whichever aspect lags behind
the others becomes a critical link, limiting the overall potential of the system. Chapter 1
of this thesis argues that current fixturing technology is this critical link of both the
design process and of most manufacturing systems. Once the motivation for developing
a novel fixturing system is established, Chapter 2 introduces the Reference Free Part
Encapsulation (RFPE) concept-a paradigm shift. Chapter 3 establishes a framework for
the foundation necessary to analyze and develop the RFPE process. With this structure in
place, Chapter 4 both identifies the parameters of this process that have the potential to
affect the outcome and defines RFPE's measurable objectives. This chapter also
summarizes the insights gained from a literature review and from previous research.
Next, Chapter 5 discusses the design of experiments conducted to fill in the gaps in
understanding the impact of and interaction between the various process parameters. The
theoretical background necessary to analyze the data from these experiments is presented
in Chapter 6. Chapter 7 presents the results of these experiments. Then, Chapter 8
presents the RFPE Process Parameter Map. The understanding provided by this map will
help unearth the full potential of the RFPE process. Chapter 9 suggests avenues for
further research, and speculates about RFPE's effect on the manufacturing process.
To realize the potential contribution of RFPE, it is important to understand the role of
fixturing in manufacturing. Section 1.1 begins by discussing the role of fixturing in
manufacturing and its repercussions on the design process. Next, Section 1.2 reviews the
functional requirements of fixturing. Finally, Section 1.3 presents the current fixturing
technology.
15
1.1.
Role of Fixturing in Manufacturing
Manufacturing is the process by which a product is brought from design to production,
from idea to reality. In an ideal scenario, the design will achieve the functional
requirements set by the designer in the simplest possible manner. However, in reality,
the designer must take into account various limitations. In particular, the designer must
consider the cost and physical limitations of the different manufacturing processes.
One important limitation in manufacturing is defined by the capabilities of the fixturing
systems available. Fixturing is the critical link between idea and reality in a unit
production system. Fixturing holds the workpiece in the desired orientation so that
various manufacturing processes can be brought to bear on the component. In many
ways, the quality of the component is determined by the limitations of the fixturing
system itself. Furthermore, demands that a particular design places on a fixturing system
translate directly into financial and temporal costs born by the final product. These
challenges can be illustrated through an examination of the impact of fixturing on
machining processes.
Machine tools, especially with the aid of CNC, are capable of creating a great variety of
geometries. Recent strides in precision machine design have resulted in continued
improvement in the precision, power, and flexibility of tools. However, limitations in
current fixturing technology greatly restrict the shapes that can actually be produced. In
fact, designers must often increase the complexity of the part and the production process
to compensate for deficiencies in the fixturing systems. This, in turn, leads to an increase
in the cost and a decrease in the production in order to produce the part at all. Thus, the
creativity of the designer is curbed by the capabilities of the fixturing systems available.
Advances in this field would clearly have a positive impact on the manufacturing world.
Therefore, a new fixturing system would be most welcome. The goals of this new
fixturing system are driven by the functional requirements of fixturing and the
characteristics and limitations of the current workholding systems.
1.2.
Functional Requirements of Fixturing
In order to compare a variety of fixturing methodologies, it is important to establish the
goals of a fixturing system. This permits an objective evaluation of each system. Any
fixturing device must achieve some basic characteristics for it to be called a fixture. For
example, a fixture must provide the machine with access to the workpiece. Additionally,
a fixture should not destroy a workpiece. If the "fixture" does not meet either of these
characteristics, the device is effectively useless. For the purpose of discussion within this
thesis, these basic characteristics are assumed to be met implicitly by any device called a
fixture. Additionally, it is assumed that in evaluating fixtures, the goals of manufacturing
will be considered. Therefore, properties such as the production rate and production cost
dictated by the fixture need to be considered. However, before we can even begin
discussing these properties, we need to discover how well different fixtures perform their
16
duty. To do this, there are some finer requirements that these fixtures should achieve to
enable them to perform their function properly. These functional requirements of
fixturing systems are fourfold: locating stability, deterministic workpiece location,
clamping stability, and total restraint.*
Locating stability implies that the workpiece must achieve a stable position as it is placed
into a fixturing device. Deterministic workpiece location signifies that the position of a
workpiece must be precisely known to ensure the accuracy of the machining operations.
Clamping stability establishes that the equilibrium and accuracy of the workpiece
position should not be disturbed while the clamping forces are applied. Total restraint
means that once the clamping forces have been applied to the workpiece, it should remain
immobile for the duration of the machining operation.
The quality of a machined part is limited by the fixturing system used during the
production process. Even with the use of the best machine tools available, machining
processes can achieve the desired level of quality only if the fixture satisfies its four
functional requirements. The fixture's limitations in achieving these functional
requirements lead to errors introduced into the workpiece. For example, the lack of
locating stability or deterministic workpiece location can introduce variations in the
position or the orientation of the part. Figure li shows that even a small variation in
orientation can cause a significant error in the precision of the workpiece.
.,-Material to
DESIRED CUT
-Actual material
ORIENTATION ERROR
PART AFTER MACHINING
Figure 1: The lack of locating stability or deterministic workpiece
location can introduce variations in the position or the orientation of
the part. Even a small variation in orientation can cause a significant
error in the precision of the workpiece.
The fixture's ability to locate the part accurately is not sufficient in and of itself. Any
precision gained through locational accuracy and stability is readily lost when clamping
loads are applied incorrectly. Figure 2 shows how excessive clamping loads applied to a
*
i
Chou, Y.-C., Srinivas, R.A., and Saraf, S., "Automatic Design of Machining Fixtures: Conceptual
Design," InternationalJournalofAdvanced Manufacturing Technology, Vol. 9, 1994, pp. 3-12.
Kapoor, S., "New FEM Modeling Results for Fixture/Part Contact," AMRI Machine Tool Workshop on
Agile Fixturing, The University of Michigan, Ann Arbor, MI, July 25, 1996, p. 4.
Ibid.
17
workpiece can result in deformation of the part. This deformation causes too much
material to be removed during the machining operations. Such an error can affect both
the dimensional accuracy and the structural integrity of the workpiece.
Material to
DESIRED CUT
Actual material
EXCESSIVE CLAMPING LOADS
PART AFTER MACHINING
LEAD TO PART DEFORMATION
Figure 2: Excessive clamping loads can deform a workpiece. This
deformation can affect both the dimensional accuracy and the
structural integrity of the workpiece.
Finally, one cannot neglect total restraint, the fourth and final functional requirement of
fixturing systems. Unfortunately, this requirement is often difficult to achieve. Ideally,
once the workpiece has been clamped to its supporting elements, the fixture prevents the
workpiece from shifting from its desired position, regardless of the machining operations
that are performed on it. In reality, the supporting elements and holding forces are
typically applied over localized regions of the workpiece. This localized contact results
from tradeoffs between the capabilities of the fixturing system, the part geometry, and the
access required by the machine tool. Unfortunately, this localized contact means that
total restraint relies on the workpiece being capable of transmitting that support or force
throughout the part. Complex geometries, especially ones with thin cross sections, tend
to be unable to do so effectively. Thus, the workpiece can deflect under machining loads,
resulting in a loss of dimensional accuracy. Figure 3 illustrates how this loss of
dimensional accuracy can occur.
End'i1
renweaed
Materiatto-1
be reimavae-
DESIRED CUT
INABILITY OF PART TO
TRANSMIT LOCALIZED FORCES
LEADS TO PART DEFORMATION
PART AFTER MACHINING
Figure 3: The inability of a workpiece to transmit the localized
fixturing support force throughout the part causes a loss of
dimensional accuracy. In effect, the workpiece deflection under
machining loads results in a lack of total restraint.
18
Thus, the inability of a fixture to meet all four functional requirements of a workholding
device satisfactorily can result in errors introduced through a number of avenues. These
errors, if not corrected, combine to severely degrade not only the quality of the machining
process, but the quality of the resultant part. If the errors render the part out of
specification, significant time and effort must be expended to bring the part back to a
usable condition. Unfortunately, the error often cannot be mitigated and the part has to
be discarded.
Given the crucial role that fixturing systems play in determining the quality of unit
manufacturing processes, one must closely monitor their ability to meet their functional
requirements. Section 1.3 reviews the current fixturing technologies. In particular, the
advantages and limitations of each process, along with their impact on manufacturing, are
examined.
1.3.
Current Fixturing Technology
Current fixturing technologies can be divided into two broad categories: modular fixtures
and universal fixtures. Each category, along with their capabilities and limitations, are
discussed in Sections 1.3.1 and 1.3.2.
1.3.1. Modular Fixturing Systems
Modular fixturing systems are the dominant class of workholding devices used in
manufacturing today. As their name suggests, modular fixtures use a number of
elements, such as v-blocks, vices, and toe clamps, in conjunction, to fixture a workpiece.
These elements have standardized interfaces that allow them to be assembled into a
variety of configurations. Thus, the use of basic elements allows modular fixtures to hold
a variety of regular shapes.
Ball and socket assemblies, along with other innovations in the modular elements, have
greatly improved the modular fixturing system's flexibility. Work on fixturing design
and analysis* has improved the siting of clamping points. However, despite these
improvements, these systems still suffer severe performance degradations when applied
to complex geometries. Generally, even increasing the fixture's geometric complexity
significantly can only marginally improve the clamping stability. This limitation is due
to the constant tradeoff between the increased restraint that can be achieved by a fixturing
assembly and the resultant reduction in access for the machine tool.
*
Asada, H. and By, A., "Kinematics of Workpart Fixturing," IEEE International Conference on Robotics
and Automation, 1985, pp. 337-345.
Chou, Y.-C., Srinivas, R.A., and Saraf, S., "Automatic Design of Machining Fixtures: Conceptual
Design," International Journal of Advanced Manufacturing Technology, Vol. 9, 1994, pp. 3-12.
19
In addition to the geometric limitations of modular fixtures, one must take into account
the cost of a particular fixture in terms of time, money, and resources. Even slight
increases in part complexity can result in great leaps in the complexity of the fixturing
assemblies. This is costly not only in terms of the time and resources required to design
the workholding device, but in terms of the time and resources required to build and set
up the fixturing assemblies for each set of machining operations.
For modular fixturing systems, the cost of these activities is very high for low volume
production. This class of workholding devices becomes more reasonable only in mass
production processes, where the cost can be amortized over large unit volumes and long
production runs. Thus, the capital investment and lead-time required to construct new
fixtures becomes a significant cost that must be considered in bringing each new product
to market. Innovations such as pallet-mounted fixtures allow the set up to be
accomplished outside the machine tool, thus improving the uptime of the system.
However, the use of the pallets merely reduces the setup time while increasing the
temporal and material costs for design and storage of the fixtures. However, while these
innovations have brought about improvements in throughput, modular fixturing systems
remain a visible obstacle between design and production that limits the creativity of the
designer.
1.3.2. Universal Fixturing Systems
In order to minimize the impact of fixturing on the design-to-production process,
universal fixturing concepts were developed as potential replacements for modular
fixtures. Universal fixturing systems use a single fixture to handle a multiplicity of
geometries. The ability of these concepts to handle various workpieces without the need
for redesign provides a number of advantages. First, one avoids the cost of designing and
building unique fixturing elements and setups. Second, the use of a single flexible fixture
removes the need for storing and maintaining unique fixtures. Third, the use of a
standard fixture, or set of fixtures, greatly simplifies the requirements for toolpath
generation since, with known fixtures, there is no longer a need to constantly generate
new patterns to cope with the interference presented by the fixturing assembly.
Currently, there are three common types of universal fixtures: conformable clamps,
fluidized bed, and phase change technology.
1.3.2.1.
Conformable Clamps
Conformable clamps are a category of universal fixturing systems that use sliding and
pivoting elements to establish contact with the workpiece. These flexible elements adapt
to fit a large range of geometries. Once the workpiece is in the desired location and
20
orientation, the movable elements are locked into place. Figure 4* displays a
cross-sectional view of a conformable clamp fixture holding a part.
Figure 4: Conformable clamp fixturing
An application of conformable clamps can be found in turbine blade manufacturing.
During set-up, the stock from which the turbine will be machined is placed into the
clamping assembly. The blade is then adjusted until it is in the required location and
orientation. As the workpiece is adjusted, the movable elements shift to conform to the
part. Once the desired position has been reached, the clamping assembly is actuated to
lock the elements in place. The entire workpiece-clamping assembly can be transferred
from machine to machine so that the same reference frame is maintained throughout the
machining process. This greatly reduces the possibility of an error, or a stack of errors,
being introduced which would affect the locational accuracy of the part.
Due to the large number of moving parts, however, the conformable clamp concept is
vulnerable to jamming by the debris that enters the clamping assembly during the course
of machining. The introduction of this debris may impact the fixture's ability to
deterministically locate subsequent parts. Like modular fixturing systems, the
conformable clamps rely on the workpiece to transfer locally applied support and forces
to the whole structure in order to maintain clamping stability. While this is ideal for rigid
workpieces such as turbine blades, it is less than ideal for parts with thin cross-sections.
Furthermore, the fixturing elements can only provide limited damping for the workpiece.
This, along with the dependence on workpiece rigidity, limits the degree to which total
restraint can be achieved during machining. Finally, conformable clamps are generally
unable to provide adequate access to compact workpieces.
1.3.2.2.
Fluidized Bed
The second universal fixturing concept utilizes the functionality of a fluidized bed. In
preparation for workpiece insertion, a gas, typically air, is pumped into the bottom of the
bed of particles at a controlled rate. In this state, the air-particle mixture behaves like a
*
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 14.
21
fluid. This makes it quite easy to insert the workpiece into the fluidized bed. Once the
part has been positioned properly, the gas is withdrawn from the fluidized bed to compact
it. Often, vacuum or magnetic forces are applied to aid in the compacting process. The
compacted bed, which now behaves like a solid, grips the workpiece securely, as
Figure 5* shows.
Figure 5: Fluidized bed fixturing
The fluidized bed fixturing system has many advantages. The particle bed is infinitely
adaptable and can accept extremely complex geometries. The simple action of the
mechanism means that the cycle time for the device is very short. Fortunately, the
simplicity of the action required for fluidized bed fixturing also contributes to the
reliability of the system.
However, like the conformable clamp system, the fluidized bed system requires external
assistance to locate the workpiece. In addition, given that the penetration of the machine
tool into the bed would destroy the integrity of the bed, care must be taken to ensure
adequate access for the machine tool. Furthermore, while the pressure exerted by the
fluidized bed against the contours of the part fully supports the immersed workpiece, the
fluidized bed may not be sufficiently strong to retain the workpiece against machining
forces. Finally, the packing of the bed cannot reliably achieve the optimal packing
density. As a result, voids left in the bed may allow the workpiece to shift, adversely
affecting locational stability.
1.3.2.3.
Phase Change Material
In the third type of universal fixturing system, the phase change concept, a molten
material, such as low temperature plastic or metallic alloy, is poured into a mold. The
molten material flows around the contours of the workpiece and is allowed to solidify.
Upon solidification, the workpiece is firmly embedded within the phase change material,
as Figure 6 shows.
*
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 14.
* Ibid.
22
Figure 6: Phase change fixturing
The phase change material provides full support for the workpiece. In addition, it has
excellent damping properties, thus helping reduce errors introduced through vibrations
induced by the cutting process. Furthermore, if the phase change material is poured into
a regularly shaped die, the encapsulated workpiece can be handled and fixtured easily.
The regular geometry facilitates the handling process. Thus, the phase change concept
satisfies the locational stability, clamping stability, and total restraint requirements of
fixturing.
However, like the other two universal fixturing processes, a die is required to align the
part accurately. In order to assure deterministic location, care must be taken in the mold
and in the fixture design. In addition, the phase change process is highly
hardware-intensive. Finally, the heat transfer requirements of the process introduce an
added layer of complexity to phase change fixturing systems.
1.3.3. Limitations of the Current Fixturing Technology
Modular fixturing systems are capable of achieving the four functional requirements of
workholding devices when used to fixture workpieces with regular geometries.
However, as the geometries become more complex, modular fixtures become less and
less able to meet these requirements. This limitation is especially evident with regard to
clamping stability and total restraint. The precision that can be achieved with modular
fixturing systems is not aided by the need to switch to a new setup for each machining
operation. With each fixture change, the locational reference is lost and an opportunity
exists for errors to be introduced into the process.
Of the three universal fixturing concepts, the conformable clamping concept is the most
closely linked in functionality to the modular fixturing systems. It is not surprising that,
while they possess a number of advantages over the modular systems, these concepts
share many problems. The ability of the conformable clamps to secure irregular
geometries provides an improvement in locational and clamping stability over modular
fixturing. With proper metrological support during insertion, deterministic workpiece
location can also be achieved. Unfortunately, like modular systems, conformable clamp
systems cannot be adjusted during the machining process without the loss of locational
23
reference. In addition, conformable clamping is unable to secure compact shapes while
providing sufficient access for the machine tool. Finally, due to the limitations of locally
applied forces and supports, the use for this fixturing concept is limited to rigid
geometries for which it can comply with the total restraint requirement.
The fluidized bed concept can accommodate a wider variety of geometries than either
modular fixturing or conformable clamps can due to the ability of the particles to
conform to the workpiece. The simplicity of operations and the quick cycle time is also
quite desirable. However, the instability of the packing process, along with the low
retaining forces exerted by the particle bed, severely degrade the concept's ability to
achieve locational stability, deterministic work piece location, and total restraint. These
drawbacks limit the utility of the fluidized bed concept.
The phase change system shows the most promise of the three universal fixturing
concepts. The phase change material is capable of capturing a range of geometries that is
on the order of that captured by the fluidized bed concepts. In addition, the phase change
material can provide much higher holding forces than those exerted by the particles of a
fluidized bed. This increase in holding forces, damping ability, and support of the
workpieces is a function of the material property rather than a function of the geometrical
structure of the mechanism. This ensures that the fixturing quality remains stable
regardless of the complexity of the workpiece. While this has not been the general
practice, the phase change material also allows access by the machine. This can take
place over much of the enclosed surface without degrading the fixture's workholding
capabilities. The downside of the process is that it is hardware-intensive and the cycle
time is dictated by the physics of the heat transfer process involved.
From this examination of the advantages and disadvantages of the modular and universal
fixturing systems, it is clear that there are a variety of problems inherent in the fixturing
systems available today. In particular, it should be noted that none of the currently
available systems satisfies all four functional requirements of a fixture: locating stability,
deterministic workpiece location, clamping stability, and total restraint. In particular,
deterministic workpiece location has typically been sacrificed to allow for the other three
requirements and for machine access to the workpiece. This requires that a significant
portion of the manufacturing time be spent establishing reference planes each time the
part is fixtured. This process is not only time intensive, but also results in a loss of
accuracy since each of these references is established independently. Thus, there is a
need for a new fixturing system that is able to achieve all four functional requirements.
24
Chapter 2: Reference Free Part Encapsulation
From the discussion in Section 1.3, it is evident that modular fixturing systems, despite
improvements in quality and speed, place severe limitations on the design and production
process. In addition, the cost of maintaining, storing, designing, and building modular
fixtures plays a large role in determining product cost and viability. Universal fixtures,
on the other hand, provide a great deal of flexibility in terms of the geometry they can be
used for. These fixtures greatly simplify manufacturing planning and can be reused for
multiple products, but pay for it in terms of a higher degree of mechanical complexity. In
addition, the conformable clamps and the fluidized bed systems suffer from reduced
clamping reliability and total restraint in exchange for the flexibility they offer. The
phase change process is slow, hardware intensive, and requires precise secondary dies to
locate the workpiece. It is not immediately evident that any change to these existing
systems, modular or universal, would generate the meaningful, across-the-board
improvement in fixturing methods that is necessary to substantially improve the
manufacturing process. Clearly, a new approach is necessary to yield a significant
improvement over current fixturing methods.
2.1.
The Ideal Fixturing System
To enable this paradigm shift, it helps to develop an ideal fixturing system and determine
if it, or something close to it, is feasible. One could consider the imaginary scenario in
which a stock of metal could be suspended midair using non-contact forces, such as
magnetic forces. In this situation, all faces of the stock would be exposed for machining.
This means that any part could be machined from a workpiece in a single set-up, as long
as the machine has sufficient degrees of freedom. This imaginary scenario fulfills all
four functional requirements for fixturing systems. Thus, we can establish this as an ideal
fixturing system.
Initially, it seems impossible to bring this ideal system out of the imaginary world. In
reality, we must rely on physical contact and frictional forces to immobilize objects.
These means, by definition, require contact with the workpiece. Unfortunately, this
contact inhibits machine tool access to the workpiece in the regions where the forces are
applied. Thus, the need to access the entire workpiece requires moving the force
delivering mechanisms at least once during the machining process. However, all
locational information is lost when these force-delivering mechanisms are removed. It
does not seem possible to ensure free access to the workpiece while maintaining an
accurate locational reference. However, instead of discarding this ideal fixturing system
as not feasible in reality, one may question whether it is possible to conceptually freeze
the component in space a different way.
25
2.2.
The RFPE Concept
Sanjay Sarma* argues that this conceptual freezing of an object in space is possible.
RFPE locates, secures, and supports a workpiece by encapsulating it with another
material. This results in a cubical shape with the workpiece embedded inside the cube.
Thus, instead of suspending the workpiece in air, RFPE suspends it in a metal medium.
After the workpiece is initially encapsulated, it is cyclically machined and
reencapsulated. The reencapsulation step allows the position of the workpiece to be
stationary within a constant cubical structure. The material that the cube is formed of
may change, but the geometry in which the workpiece is suspended does not. In this
way, by keeping the workpiece stationary within the cube, one can always determine the
part's location and orientation. Thus, the workpiece is conceptually frozen in space.
This means that every time one places the part on the machine, one can choose the
orientation that one wants that particular time. However, it is not necessary to relocate
the part's references. Thus, by rotating the encapsulated workpiece, RFPE allows one to
access all sides of the workpiece without having to relocate the part. Once the machining
operations are completed, the encapsulation metal is melted away, leaving the part that
was desired. Figure 7t describes the RFPE process pictorially.
*
Sarma, S.E., A Methodologyfor Integrating CAD and CAM in Milling, Ph.D. Thesis, University of
California, Berkeley, 1995, p. 32.
Lee, E.C., Development of an EncapsulationProcessfor use in a Universal Automated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 21.
26
START:
RAW STOCK(EMBEDDED)
ENCAPSULATOR
ORIENT
MACHINE
REFILL
SIDE 1
SIDE2
SIDE N
FINISH:
Figure 7: The Reference Free Part Encapsulation (RFPE) Process
27
2.3.
Capabilities of RFPE
RFPE is clearly the paradigm shift that was needed to achieve the four functional
requirements of a fixture. Once the workpiece has been encapsulated, its position is
constant with respect to the surface of the encapsulated workpiece. Thus, locating
stability can be achieved. Furthermore, the regular geometry of the encapsulated
workpiece permits the use of simple, precise fixtures, thus enabling deterministic
workpiece location.
In addition, the RFPE process provides workholding capabilities superior to the current
fixturing systems. The use of a phase change material to secure the workpiece means that
no excessive force capable of deforming the workpiece can be applied during the
encapsulation process. Once the workpiece has been completely enclosed, all the
surfaces of the part are supported. This not only ensures clamping stability but also
allows the system to achieve total restraint. Thus, the encapsulated workpiece is totally
located, supported, and secured. Therefore, RFPE has the potential to produce high
quality components while minimizing the obstacles for both design and production.
Figure 8* shows some sample parts produced with RFPE.
Figure 8: Sample parts produced with RFPE
*
Lee, B.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 35.
28
2.4.
Three Machining Strategies: 2D, 2%D, and 3D Milling
RFPE suspends a workpiece in space and allows access to all of its faces. However,
having the encapsulated workpiece immobilize, support, and locate a part can result in a
high level of complexity. Sarma* recognized that not all tasks warrant a high level of
fixturing complexity. Therefore, in order to realize the RFPE concept with a level of
complexity commensurate with the difficulty of the task, he developed three distinct
machining strategies: 2D, 2 D, and 3D. The complexity permitted in the machining
operations increases as one progresses from the 2D to the 3D Milling Strategy. It should
be noted that, as one progresses from the 2D to the 3D milling strategy, the complexity
permitted in the machining operations increases. Unfortunately, tied to this increase in
flexibility is an increase in the complexity and cost of the encapsulation equipment.
2.4.1. 2D Milling
2D milling is the simplest machining strategy. As Figure 9 shows, this machining
strategy allows 2 parallel faces of the workpiece to be accessed, using the other four faces
to attach the workpiece to a fixturing device.
Figure 9: 2D Milling
This fixturing device provides the contact between the encapsulated workpiece and both
the machine and the encapsulation system. Additionally, if one leaves this fixturing
device attached to the encapsulated workpiece for the duration of the machining and
reencapsulation cycles, the fixturing device can take over the locational function from the
encapsulated workpiece.
Sarma, S.E., A Methodologyfor IntegratingCAD and CAM in Milling, Ph.D. Thesis, University of
California, Berkeley, 1995, p. 46.
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 48.
29
Since the fixturing device performs the locating function in 2D milling, the requirements
for the encapsulated workpiece can be relaxed. Given that the surface of the
encapsulated workpiece is not used to reference the location of the part, this machining
strategy requires neither precise surface finish nor exact dimensional repeatability. This
significantly reduces the complexity of the encapsulation machine.
2.4.2. 2%D Milling
Often, more than two faces of a workpiece need to be accessed to produce a part. In
these cases, 2D milling is clearly not applicable. However, only rarely does the
production of a part require all six faces of the encapsulated workpiece to be accessed. It
is in these cases that that 21/2D milling can be exploited. This machining strategy is not as
simple as 2D milling, but one can still reap significant benefits from not having to access
all six sides.
In 2/2D milling, a locating pallet covers one of the faces of the workpiece. As Figure 10*
shows, this leaves five faces of the encapsulated workpiece free to be machined.
Figure 10: 2%D Milling
The locating pallet serves as the link between the encapsulated workpiece and both the
machine tool and the encapsulation system. As in 2D milling, the fixture serves as the
locational device, while the encapsulating material provides the immobilization and
support required.
*
Lee, B.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 48.
30
2.4.3. 3D Milling
If the complexity of the part being produced requires all six sides of the workpiece to be
machined, the full power of RFPE needs to be unleashed. In this case, the 3D milling
strategy, the most flexible of the machining strategies, is the most appropriate.
In this strategy, the encapsulated workpiece can be placed on the machine with any face
up during any given portion of the machining cycle and can be turned as needed. Thus,
as Figure 11* shows, this strategy permits access to the workpiece from all directions.
(7E--
10
Figure 11: 3D Milling
The flexibility of the 3D milling strategy allows complex geometries, even those with
thin cross-sections, to be generated with great precision.
In this strategy, the encapsulation is responsible for ensuring compliance with all four
functional requirements of a fixture. This means that the encapsulation must not only
retain and support the workpiece against machining forces, but it must also maintain the
locational reference for the workpiece. Since the encapsulated workpiece may be placed
in any orientation, variations in the encapsulation may affect the precision of the RFPE.
This need for precision increases the rigor of the requirements for the encapsulation
process and equipment.
*
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 48.
31
2.5.
Motivation
Clearly, the RFPE concept can be the lever that is needed to significantly improve the
manufacturing process. It has the ability to satisfy-without compromise-all the
functional requirements of a fixture, and yet, in conjunction with the three machining
strategies, is flexible enough to work in a great variety of situations. The question then is
whether this concept is actually feasible in reality.
Given the difficulties in modeling the RFPE process analytically, initially much of the
work to prove the viability of the concept had to be done experimentally. Lee built an
encapsulation system at MIT, which has proved the viability of the process. However, as
expected, it also left significant room for improvement.
RFPE can only advance once basic knowledge has been gathered in all the critical areas.
Therein lies the motivation for this study: to establish a framework to understand RFPE,
consolidate the existing knowledge, identify the critical factors that remain unexplored,
determine the impact these factors have on the process, and thus develop the RFPE
Process Parameter Map. This map will guide future designs and developments of the
concept.
32
Chapter 3: Framework to Analyze and Develop RFPE
The goal of this study is to establish a foundation of knowledge on which the future work
on the RFPE process can be based. Without this foundation, systematic improvement of
the process cannot be achieved. The key to a solid foundation is a structured approach to
the discovery and layout of the basic building blocks. Section 3.1 begins with a
description of this approach.
3.1.
The Building Blocks of a System
For any engineering problem, the information can be divided into three parts: the system,
the inputs, and the outputs. For this study, the system in question is the RFPE concept.
At this level of abstraction, the concept encompasses all aspects of the process and
apparatus. The outputs are the values by which the system is judged. In particular, these
are the attributes, or metrics, that determine how well the RFPE process meets its goals,
including the functional requirements of fixturing and other aspects as esoteric as the
floor space that needs to be dedicated to the device. The inputs, in turn, consist of the
variables that can impact the quality of the process. These inputs can be further classified
into environmental variables and parameters. Environmental variables are those factors,
such as atmospheric pressure, that cannot easily be controlled. On the other hand,
parameters are set either during the design of the apparatus or during the operation of the
process.
3.2.
Design and Process Parameters
It is important to distinguish between design and process parameters. While both of these
affect the operation of the system and may affect the same metrics, they represent very
different things. Process parameters are indisputable properties of the concept, dictated
by the physical laws of nature. On the other hand, design parameters are the particular
characteristics selected for a particular implementation of the concept. Thus, any
implementation of a system has the exact same process parameters as another
implementation. However, two different implementations of a particular system are
likely to have different design parameters.
Process parameters denote the conditions that can be theoretically varied for a given
process. Depending on the sensitivity of the metrics to a particular parameter, the
variation can impact the quality of the product produced. Thus, it is important to identify
the process parameters for any system and investigate the relationship between each
process parameter and each metric for the system. Figure 12 shows how the plot of a
process parameter versus a metric can simplify the analysis of where the system should
be run to maximize that particular metric.
33
Metric
Process Parameter
Figure 12: A plot of the relationship between each process parameter
and each metric for a system facilitates an understanding of the
system characteristics
Theoretically, there are no limits to the extent to which a process parameter can be
varied. However, in reality, the range of each process parameter is bounded by physical
limitations. For example, if we consider two metrics that are affected by the same
process parameter, we could imagine facing the situation depicted in Figure 13.
Metric #1
Metric #2
Metric
Process Parameter
Figure 13: Each process parameter typically impacts multiple metrics
of the same system. Often, there is no optimum setting for the process
parameter. Therefore, tradeoffs between the metrics are required.
In this case, it would be impossible to select a setting for the process parameter without
knowing the details of the situation so that we can make an informed tradeoff. Ideally,
this means that in designing a system we would allow the user utmost flexibility to set the
process parameter to whatever setting is best for the application at hand. However, in
reality, we cannot feasibly build a machine that will allow all possible values of a given
process parameter. Thus, as Figure 14 shows, for any given apparatus, there is a window
within which the user can define the setting that is best for their particular application.
34
The boundaries of this window are defined by the choices made-implicitly or
explicitly-during the equipment design process.
Range permitted by
design parameters
Process Parameter Settings
Figure 14: Design parameters determine the range of each process
parameter that is feasible for each implementation of the concept.
Design parameters are the characteristics of the device that determine the extent to which
a piece of machinery can carry out its desired functionality. The particular design
parameters for a given system are selected based on the specific objectives of the
implementation being designed. These objectives guide the tradeoffs that are required by
the often-contradictory nature of the process parameters, as shown in Figure 13.
Additionally, factors such as the cost of the process and the apparatus, production rate,
and the quality specifications that the design has to meet also influence the choice of
design parameters. Ultimately, the design parameters determine the extent to which a
piece of machinery can carry out its desired functionality.
Ideally, design parameters are set such that they allow the end-user utmost flexibility in
choosing the process parameter setting. In the process modeled by Figure 15, it is not
possible to maximize both Metric #1 and Metric #2 simultaneously. Thus, the end user
needs to select the setting that is best for the case at hand. Unfortunately, physical and
cost limitations often do not allow for an apparatus that gives the user the ability to select
a setting in the entire range. Thus, a particular implementation of the concept becomes a
version of the concept optimized for a particular application.
35
Range permitted by
design parameters
selected.
Combination Metric
(a function of Metrics #1 & #2)
Metric #1
Vetric #2
Process Parameter
Figure 15: For the particular implementation modeled in this
diagram, the choice was made to have the design parameters favor
Metric #1 slightly over Metric #2.
This means that, in order to design an optimal system, it is important to first determine
the process parameters. With the process parameters defined, one can then investigate
the effect that they have on each other and on the metrics. Then, with this basic
understanding in place, one can choose the applications that one wants to satisfy with one
particular design and select the design parameters accordingly. In this way, systems
satisfying the requirements can be built at a reasonable cost within a reasonable amount
of time. For a different set of requirements, a slightly different implementation of the
same process can be built.
3.3.
A Structured Approach to Developing the RFPE Process
The goal of this study is to develop a roadmap that will guide the future development of
the RFPE concept. In order to do this, one must develop a means to systematically
evaluate the importance of various factors. This evaluation can be accomplished through
the identification of the critical metrics for the concept. By determining the process
parameters that can potentially influence these metrics, one captures the core variables
whose interrelationship will shape the quality of the product.
The next step in the journey lies in discovering the shape of these interrelationships. This
can be done through a combination of literature review of similar processes,
mathematical analysis, and experimentation. The factors that have negligible impact can
be discarded. Those that are crucial to the success of the concept will receive further
attention to determine the concept's sensitivity to variation in these areas.
36
Chapter 4: Systematic Evaluation of Parameters and Metrics
This chapter establishes the foundation for a systematic evaluation of the criteria that may
impact the RFPE concept. In particular, this chapter focuses on consolidating
knowledge, presenting parameters of known significance, and identifying parameters
whose effect remains to be determined. Section 4.1 presents the metrics for the RFPE
concept. Section 4.2 introduces the process parameters and summarizes both a literature
review and the current research on how the process parameters impact the metrics.
Section 4.3 consolidates this information and presents the RFPE Process Parameter Map.
Section 4.4 presents the next steps based on the current state of knowledge.
4.1.
Metrics of the RFPE Process
The metrics selected to evaluate the performance of the RFPE process are measures of
the quality of the encapsulated workpiece. These properties of the workpiece determine
how well the process fulfills the functional requirements of fixturing. The metrics are
divided into two categories: those that affect the precision of the process and those that
affect the integrity of the encapsulation.
The precision of the RFPE process is determined by the surface finish, porosity,
shrinkage, and thermal drift of the encapsulated workpiece. These factors, in turn
determine the minimal dimensional tolerance that one can achieve with this system.
Additionally, the integrity of the encapsulation is affected by the rewelding strength as
well as the presence of porosity. Sections 4.1.1 through 4.1.5 discuss these metrics.
4.1.1. Surface Finish
Surface finish refers to the relative roughness and flatness of the encapsulation surface.
Since the surface of the encapsulation forms the load-bearing surface through which the
encapsulated workpiece is fixtured, variations in surface finish can impact the
dimensional accuracy of the process.
The surface finish can be characterized by the amount that the surface deviates from
perfect flatness. As displayed in Figure 16,* there are three types of deviations: gross
deviation, surface waviness, and roughness. Gross deviation is called form error. Minor
deviations can be called surface waviness or roughness. The distinction between
waviness and roughness is a function of the frequency of the undulations. For the
purpose of this study, the two will be considered as one single category and referred to as
roughness.
Tencor P-10 Reference, P/N 412937 Rev. A, 1996, pp. 3-8.
37
Rougfroes
Formn Errr
Waviness
Figure 16: A surface finish can be categorized by the amount that it
deviates from a perfectly flat surface. There are three ways in which
this deviation occurs: form error, waviness, and roughness.
Form error can be measured with a coordinate measurement machine. Surface roughness
can be measured with a profilometer. Profilometers are capable of greater resolution than
the coordinate measurement machines and can provide a better characterization of the
micro-scale features of the surface.
While these methods provide a fairly accurate way to characterize the flatness of a
surface, this degree of precision is not always required. In many cases, the surface
deformities do not affect a process as long as the roughness does not exceed a certain
threshold value. In this instance, the use of a "go/no-go" gauge would be appropriate.
An example of such a roughness gauge would be a surface roughness standard, which
contains a variety of roughness samples that one can compare to the target surface.
4.1.2. Porosity
Porosity refers to the presence of voids in the encapsulation. These voids are formed
either by shrinkage or by gases trapped during the molding process. Section 6.2
describes the porosity generating mechanisms more in-depth. The presence of porosity
on the surface of the encapsulation forms defects that, in sufficient numbers, compromise
the load-bearing capability of the encapsulation. The added deflection resulting from the
compression of the defect-ridden surface impacts the accuracy of the process.
Furthermore, the presence of porosity inside the encapsulation can compromise the
workholding ability of the encapsulation against the embedded workpiece. Finally,
porosity in the encapsulation weakens the encapsulation since the voids reduce the
amount of material available to withstand tensile and compressive loads.
The presence of porosity, both internal and external, has the potential to impact the
quality of the RFPE process. The presence of voids in the encapsulation can be detected
by comparing the encapsulation density against the tin-bismuth density. The weight can
be obtained with a balance. The volume can be found by depositing the specimen into a
graduated cylinder filled with water. The displaced volume of water will be equivalent to
38
the volume of the encapsulation. With sufficiently fine resolution on the balance and on
the graduated cylinder, deviations from the tin-bismuth density can be determined quite
accurately.
Since it is also the location, not just the presence of porosity, that determines the impact
on the RFPE process, visual inspection and identification of the voids is an important
component of the measurement process. If necessary, instruments with the appropriate
resolution can measure the size of the voids. However, often the short sample length of a
profilometer limits the size of the voids that it can measure accurately. For internal
voids, it is necessary to section a specimen to determine their presence, frequency of
occurrence, and location.
4.1.3. Dimensional Accuracy
Dimensional accuracy refers to how closely the specimen geometry matches the expected
geometry. Ideally, to simplify the mold design process, this means that the specimen
geometry mirrors the geometry of the mold. Unfortunately, the expansion and
contraction of the mold and the specimen due to the changes in temperature makes this a
difficult goal to achieve. This variation is determined by the material's properties, in
particular the coefficient of thermal expansion and the geometry of the part in question.
If the dimensions of the part or the temperature change involved are sufficiently large, the
dimensional variation can impact the dimensional accuracy of the RFPE process. Thus, it
is important to determine the effect of each process parameter on this metric and methods
to mitigate unwanted variation.
Two-dimensional measurement of dimensional accuracy can be accomplished using
calipers and micrometers. Higher precision, non-contact instruments such as lasers can
also be used, although the precision advantages are often offset by errors introduced by
variation in the optical properties of the surface. For a three-dimensional measurement of
dimensional variation, a coordinate measurement machine is necessary.
4.1.4. Thermal Drift
One of the advantages of RFPE over conventional fixturing processes is that the
workholding is accomplished using the material properties of the metallic alloy. While
this is much more stable and offers better damping than the structural stiffness provided
by conventional fixtures, one must be aware of the errors that can be introduced by
thermal drift. Thermal drift describes the shifting of the encapsulated workpiece as the
encapsulation temperature reaches or exceeds the melting point. As Figure 17* shows,
this results in a softened zone within which the workpiece can shift. If this occurs, the
locational references for the part would be lost. Unfortunately, if thermal drift is present
*
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 23.
39
in the process, the error will only be spotted after the encapsulation material is removed
following the completion of the part. Therefore, it is important that no thermal drift takes
place.
I
-
-
New Encapsulatio.10
Workpiece
Sofiened Zone
Previous Encapsulation
Figure 17: How thermal drift occurs
4.1.5. Rewelding Strength
Rewelding refers to the process by which new alloy is introduced into the encapsulated
workpiece to restore it to its original shape. In order to ensure the integrity of the
encapsulation, a strong bond must exist between the new and the old alloy. Otherwise,
under thermal or machining-induced stresses, the encapsulation would disintegrate. This
would cause the locational references to be lost.
Like thermal drift, rewelding cannot be readily measured before the encapsulation fails.
As such, it is a factor that is better avoided in the RFPE process. The measurement of
rewelding strength is best carried out using tensile-strength specimens created according
to the flow chart in Figure 18.* The yield strength of the weldline can be determined with
the use of an Instron tensile test machine, as carried out by Valdivia.t
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 90.
t Lee, E., Valdivia, P., Fan, W., Sarma, S.E., "The Process Window for Reference Free Part
Encapsulation," To appear in ASME JournalofManufacturing Science & Engineering,p. 8.
*
40
Cavity
Gate
A ir Sprue
A
Molded specimen
SDecimen
broken in half
Parting line
Tensile test specimen mold
Final tensile test specimen
with desired weldline
0:-
Broken half placed back into mold
and re-injected
eldlile
Figure 18: Creation of weldline test specimens
4.2.
Process Parameters: an Overview
The process parameters of RFPE fall into four main categories: temperature, packing
pressure, aging, and alloy composition. The temperature parameter is separated into a
number of subcategories. These are melt superheat, mold preheat temperature, cooling
rate, and ejection temperature. Packing pressure is the pressure maintained during the
cooling process. Aging refers to the changes in a workpiece's dimensions as a function
of the time between a casting operation and the actual machining operation. The alloy
composition also affects the quality of the encapsulation.
Each of these categories will be examined for their potential impact on the RFPE process.
The information provided below is based on a literature review of related manufacturing
processes and on work previously conducted in the development of RFPE.
41
4.2.1. Temperature
There are four temperatures that affect the RFPE process: melt superheat, mold preheat
temperature, cooling rate, and ejection temperature.
4.2.1.1.
Melt Superheat
The melt superheat denotes the amount by which the molten material is heated above the
melting point of the alloy at the point of entry into the mold cavity. If the injection
temperature cannot be measured, the reservoir temperature can be monitored. The
reservoir temperature should be the desired injection temperature plus the temperature
difference necessary to account for the heat loss during alloy delivery to the mold. The
melt superheat has a direct impact on the quality of the casting. If the melt superheat is
not sufficiently high, cold shuts may occur and trapped air may not be able to escape
before the alloy solidifies. Furthermore, one must consider the effect of the melt
superheat on rewelding and thermal drift.*
During reencapsulation, contamination on the surface of the alloy prevents proper
diffusion and bonding between the existing and the new alloy. This results in poor
adhesion and greatly reduced structural strength. There are two ways to remove the
contaminants: chemical agents and superheating. Chemical agents, such as reagents,
fluxes, and solvents, will strip away the contaminants. However, the difficulties in
automation, as well as safety and environmental considerations, make the chemical
option impractical. Therefore, superheating is preferred.
By superheating the molten alloy, sufficient heat transfer takes place to displace the melt
front into the existing alloy. This allows the new alloy to penetrate the layer of
contaminants to achieve a good weld. Given that the contaminant layer has a thickness of
approximately 1 pm, a penetration of at least 1 mm must be achieved to ensure the
dispersion of the contaminant layer. However, caution must be taken as too much
superheat may result in thermal drift of the workpiece within the encapsulation. The melt
superheat temperature, along with other factors such as the mold preheat temperature,
determines the extent of the penetration. Lee has developed a model to analyze the
effects of melt superheat and mold preheat temperature on thermal drift and rewelding.
Valdivia experimentally verified the rewelding aspects of the model. They concluded
that good quality rewelding can be achieved through a combination of high melt
superheat and mold preheat temperatures. t The desired interpenetration between the
alloys can be achieved within twenty seconds. This rewelding time should be kept under
forty seconds to avoid thermal drift.
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p 22.
t Lee, E., Valdivia, P., Fan, W., Sarma, S.E., "The Process Window for Reference Free Part
Encapsulation," To appear in ASME JournalofManufacturingScience & Engineering,p. 12.
*
42
Depending on the design of the encapsulation system, the melt superheat may also play a
role in ensuring the dimensional accuracy of the encapsulation. If the encapsulation is
extracted from the mold based on a set cooling time, varying injection temperatures will
result in varied dimensions. Thus, to ensure dimensional stability, it is best if the melt
superheat stays as constant as possible. Further methods to maintain dimensional
stability will be addressed in the following sections.
Porosity is another metric that may be impacted by the melt superheat. If the injection
temperature is too close to the melting point of tin-bismuth, there may not be sufficient
time for the air to vent completely from the mold prior to alloy solidification. In this
case, the melt superheat will contribute toward the formation of porosity in the mold.
This porosity may be within or on the surface of the encapsulation.
Finally, high reservoir temperatures used to generate high melt superheat may result in
alloy segregation as well as the formation of oxides. Alloy segregation would lead to
variable structural strength and temperature response. On the other hand, formation of
oxides would result in contamination of the mold and encapsulation, degrading both the
surface finish and the integrity of the encapsulation.
4.2.1.2.
Mold Preheat Temperature
The temperature of the mold also plays an important role in determining the quality of the
encapsulation. During injection, a high mold preheat temperature is necessary to prevent
cold shuts and early solidification.* In addition to the impact on the surface finish, the
premature solidification can prevent trapped air from escaping, thus causing porosity.
Furthermore, a low mold preheat temperature will result in the rapid solidification of the
encapsulation. The surface of the encapsulation has the potential to warp due to stress
induced through thermal expansion and contraction. The final mold temperature is also
important as it determines the thermal expansion of the mold and the associated errors in
the encapsulated workpiece. Finally, the high temperature needs to be maintained during
packing so that rewelding occurs.t
4.2.1.3.
Cooling Rate
The cooling of the mold must proceed slowly enough that the material strength and
ductility are not compromised.t A slow cooling rate ensures that thermal factors
dominate and kinetic factors are damped out. This helps minimize the impact on both
*
I
Allsop, D.F., and Kennedy, D., PressureDiecasting; Part2, Pergamon Press, New York.1983, pp. 6-7.
Lee, E., Valdivia, P., Fan, W., Sarma, S.E., "The Process Window for Reference Free Part
Encapsulation," To appear in ASME JournalofManufacturingScience & Engineering,p. 12.
Murray, M.T., Griffiths, J.R., "The Design of Feed Systems for Thin-Walled Zinc High-Pressure Die
Castings," Metallurgicaland Materials Transactions,Vol. 27B, February 1996, p. 115.
43
surface finish and dimensional accuracy. Finally, too high a cooling rate may result in air
being trapped in the encapsulation, thus increasing the amount of porosity.
4.2.1.4.
Ejection Temperature
Many casting processes utilize cooling time to determine the ejection of the casting from
the mold. However, as Section 4.3.1.2 discusses, packing pressure and cooling rate have
complementary effects. These, along with the mold preheat temperature, the melt
superheat, and the cooling rate of the mold, impact the workpiece temperature at any
given point in time. Variations in the ejection temperature seem likely to have an
important impact on the precision of the RFPE process.
Ejection of the encapsulated workpiece at too high temperatures can result in warping of
the surface as shrinkage takes place. This may have an effect on both surface finish and
dimensional accuracy. Varying ejection temperatures will result in varying shrinkage
rates, thus introducing errors into the critical dimensions of the encapsulated workpiece.t
Therefore, the ability to reliably achieve consistency in casting dimensions can be better
accomplished by following a temperature-based ejection scheme than a time-controlled
one.
4.2.2. Packing Pressure
The packing pressure has the potential to impact the porosity and surface finish of the
encapsulation. Experiments have shown that, as long as packing pressures above 60 psig
are used, the reflected surface finish on the encapsulation will closely approximate that of
the mold. Figure 19 shows the difference between the mold surface finish and its
reflection on the corresponding specimen as a function of pressure. From the data
obtained in the experiments, a reflected surface roughness on the order of 64 to 125 micro
inches will have a negligible effect on the precision of the RFPE process. Thus, the mold
can be manufactured using standard machining processes.
* Allsop, D.F., and Kennedy, D., PressureDiecasting;Part2, Pergamon Press, New York.1983, p. 15.
t Ibid.
Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturing System, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 51.
44
60
- - - - Mold Ra =3.2
Roughness beyond
profilometer range
50
X Nom. Ra = 3.2
-..-.. Mold Ra= 1.6
Nom. Ra =1.6
- -Mold Ra =0.4
1040
5 30---
ModR=
:
10
.
om R2=0.
30
10
0
- -
- - -
- - - - -~-
40
20
-
-
- ------
- - -
60
80
Pressure (psi gauge)
Figure 19: The peak-to-valley heights of all specimens and inserts
used to characterize surface finish
The packing pressure also aids in the dimensional accuracy of the encapsulation. By
forcing the molten alloy to conform to the surface of the mold until solidification has
taken place, packing pressure minimizes the dimensional variation due to the expansion
or contraction of the alloy due to changes in temperature.
From the experimental data, Fan also developed a correlation between packing pressure
and the acceptable mold surface finish. Figure 20* shows the resultant process window.
*
Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturingSystem, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 57.
45
-
3.5
-.-0
32.5
:a
-
2-
0
W
Z,
1.5
Tolerable Roughnes s
(microns)
-
Process
Incapable
Process
Capable
10.5-
0
20
40
60
80
100
Molding Pressure (psi gauge)
Figure 20: RFPE process window for packing pressure versus mold
surface roughness
A higher packing pressure has the potential of compressing the air that remains inside the
mold. This results in smaller air bubbles. El Mahallawy has shown that there is no
macroporosity at pressures beyond 10,000 psig.* This should help ensure the structural
integrity of the encapsulation. This, in turn, would increase the strength and ductility of
the encapsulation. In RFPE, the use of packing pressure that is two orders of magnitude
lower than that spacified by El Mahallawy means that there is likely to be some porosity
present both on the surface and throughout the encapsulation. However, the level of
porosity should not be sufficient to affect the surface finish, the porosity of the surface, or
the structural integrity of the encapsulated workpiece.
High packing pressure also minimizes the air gap between the mold and the molten alloy.
This increases the heat transfer by eliminating the insulation layer. Since a high packing
pressure has a similar effect as increasing the cooling rate,t the former can be applied to
provide more flexibility for the latter parameter. However, one should note that
pressurized gas in the bubbles might form blisters upon reheating as the trapped air
expands.t This should not be of concern, as the reencapsulation will take place in a
pressurized environment.
*
El Mahallawy, N. A., Taha, M. A. and Zamzam, M. L., "On the Microstructure and Mechanical
Properties of Squeeze-Cast Al-7 wt% Si Alloy," JournalofMaterials ProcessingTechnology, 40, 1993.
p. 83.
Ibid, p. 83.
Allsop, D.F., and Kennedy, D., PressureDiecasting;Part2, Pergamon Press, New York.1983, pp. 6-7.
46
The most important effect of packing pressure is on the dimensional accuracy and
stability. The packing pressure helps offset shrinkage effects and maintain the
dimensional accuracy. Consistent packing pressure is required to ensure repeatable
results.
4.2.3. Aging
During casting processes, stress can be introduced into the material by effects such as
shrinkage, turbulence, and premature solidification. This results in a state of tension that
may dissipate slowly over a period of time following the casting process. The casting
will continue releasing the stress until equilibrium has been reached. Depending on the
material and the strength of the induced stresses, this may result in dimensional variations
that are large enough to affect the precision of the process.
Zinc castings have been noted to shrink between 0.075% to 0.12% over several months.*
It is unclear as to the effect that aging has on tin-bismuth. The effect that aging has on
the surface finish is equally unclear. This, along with the time period over which the
aging process takes place, form possible avenues for experimental verification.
4.2.4. Alloy Composition
As the RFPE process proceeds, new encapsulation alloy must be added to the reservoir to
replace the consumed alloy. As new and recycled alloy are mixed in the reservoir, the
introduction of contaminants is unavoidable. The contaminants can be impurities
contained within the ingots, chemicals used to treat the outside of the ingots to prevent
oxidation, particles and fluids introduced during machining, or oxides generated during
the heating process.
These impurities contaminate the mold. This, along with any contaminants that may
aggregate on the surface of the workpiece, may contribute to a poor surface finish. The
contamination of machined surfaces, along with the contaminants present in the alloy,
may lead to rewelding problems. The presence of contaminants on the mold and alloy
surface may lead to air being trapped on the surface, thus causing porosity. Finally,
sufficient oxide-induced defects may affect the dimensional accuracy of the
encapsulation. Repeated heating of the alloy may result in segregation of the elements
and variations in mechanical and thermal properties of the encapsulation material.
Careful selection and usage will be necessary to minimize the impact of alloy
composition on the RFPE process.
*
Murray, M.T., Griffiths, J.R., "The Design of Feed Systems for Thin-Walled Zinc High-Pressure Die
Castings," Metallurgicaland Materials Transactions,Vol. 27B, February, 1996, p. 115.
47
4.3.
The A Priori RFPE Process Parameter Map
From the information provided in Sections 4.1 and 4.2, a process parameter map was
created showing the relationship between the RFPE process parameters and metrics.
Figure 21 presents the RFPE Process Parameter Map. The process parameter effects are
divided into three categories. The shaded blocks indicate that the process parameter and
the metric are materially independent. The check-marked blocks have been investigated.
The remainder represents the relationships that have a potential to impact the RFPE
process and, thus, are worthy of further exploration.
Metrics
Dimensional
Thermal
Rewelding
Accuracy
drift
strength
Melt superheat
[Lee 2001]
[Lee 2001]
Mold preheat temperature
[/
[/
[Lee 2001]
[Lee 2001]
Cooling rate
[Lee 2001]
Process Parameters
.
Surface
Po__sity
finish
Ejection temperature
Packing pressure
[Fan 2000]
[Fan 2000]
[Fan 2000]
Aging
Alloy Composition
Relationship
to be
determined
/
[Source]
Map
completed
Materially
Independent
Figure 21: The A Priori RFPE Process Parameter Map
In addition to the relationship between the metrics and the process parameters, one must
also consider the relationship between the various process parameters. These
relationships are laid out in the interaction matrix shown in Figure 22. The question mark
indicates those parameters that have a potentially positive interaction.
48
r
q
Process
Parameters
Melt
superheat
Mold
preheat
temperature
Cooling rate
Ejection
temperature
Packing
pressure
Aging
Alloy
composition
Melt
superheat
Mold
preheat
temperature
Cooling rate
Ejection
temperature
Packing
pressure
Aging
Alloy
composition
Materially
independent
Relationship
to be
determined
Redundant
Spaces
Figure 22: Interaction matrix for the RFPE process parameters
4.4.
Next Steps
The research into the status of the RFPE process highlighted the areas that have been
mapped. Additionally, the literary research pinpointed many areas that are materially
independent and have no impact on the process. However, there still remains much
unexplored ground in the process parameter map that needs to be understood. The study
of these parameters will help identify the impact of and relationships between the process
parameters. This is the first step in further developing the RFPE process.
50
Chapter 5: Experimental Plan
Chapter 4 identifies a number of process parameters that have the potential to impact the
quality of the RFPE process. Experimentation was necessary to determine the actual
effects. Section 5.1 reviews the objectives of the experiments. Next, Section 5.2
provides an overview of the experimental apparatus designed for these experiments.
Section 5.3 provides, in detail, the experimental procedure.
5.1.
Objectives of the Experiments
The objective of the experiments carried out in this study was to explore the effects of
process parameters that could potentially impact the quality of the RFPE process. In
particular, the experiments explored the effects of melt superheat, mold preheat
temperature, and cooling rate on the surface finish, porosity, and dimensional accuracy of
the encapsulation. The effect of ejection temperature and aging on surface finish and
dimensional accuracy was also examined. By evaluating the results of the experiments,
information regarding the effects of alloy composition was collected.
5.2.
Experimental Apparatus
5.2.1. The Mold
A mold was made to produce tin-bismuth specimens for the various experiments.
Figure 23 displays the mold with the top removed. In order to increase the number of
experiments that could be carried out within a given period of time, two molds were used
in the experiments. The molds are labeled Mold A and Mold B respectively.
51
Figure 23: Mold used to produce experimental specimens
The mold used to produce the specimens is a four-part mold. It has a parting line along
the length of the cylindrical cavity to facilitate removal of the specimens. The air used to
supply pressure to the molds is introduced through the quick-release fittings attached to
the top plate. To facilitate removal of specimens, clamps were used to assemble the
mold.
Ideally, a thermocouple would be inserted into the mold cavity to monitor the specimen
temperature directly during specimen production. Unfortunately, the difficulties involved
in sealing the mold cavity and in extracting the specimen made this option impractical.
Thus, as an alternative, the mold thermocouple was inserted into a shaft on the side of the
mold that put the tip of the thermocouple 0.1 inches away from the wall of the mold
cavity.
5.2.2. Instrumentation
The packing pressure, the reservoir temperature, and the mold cavity temperature
required control and monitoring. A pressure regulator controlled the compressed air feed
from the building compressed air system. The pressure to the mold was switched on and
off via a SMC NVFS3300-3FZ automatic valve with a subplate. The temperature of the
mold and the tin-bismuth reservoir were measured using a J-Type thermocouple.
A LabVIEW program was written for three purposes: data monitoring, data collection,
and pressure actuation. The program tracked the temperature inputs from the various
52
thermocouples. Temperature alarms built into the system can be set to aid in the
operation of the apparatus. The temperature data was stored every second in a designated
text file, along with binary pressure data, and counter information. The program also
provided the interface displayed in Figure 24 to actuate the pressure.
Figure 24: LabVIEW control program screen shot*
A Gar Electroforming M- 15 Surface Roughness Standard Set was used for the threshold
surface roughness measurements. A micrometer accurate up to ± 0.0001 inches was used
for dimensional measurements.
5.3.
Experimental Procedure
Experiments were designed to test the effect of the process parameters on each other and
on the metrics of RFPE. For each experiment, the mold and the molten alloy were heated
in the setup displayed in Figure 25. The alloy was melted and brought to the desired
*
The LabVIEW program can gather data for 3 molds, however the cycle time of the experiments limited
the use to the two molds mentioned in Section 5.2.1.
53
temperature in a stainless steel pot on an electric heating element. The mold was heated
from the bottom surface using another electric heating element to a temperature slightly
above the desired set point. Then, the mold was removed from the heating element.
Figure 25: Experimental setup
Once the mold reached the desired set point, the molten alloy was poured in, the top plate
secured, and the mold pressurized. Eutectic tin-bismuth was used. The mold
temperature was monitored until it reached the desired ejection temperature. At that
point, the pressure was relieved, the mold disassembled, and the specimen removed.
The diameter of the specimen, which is the critical parameter for these experiments, was
measured with the micrometer upon extraction from the mold. The specimen then
underwent visual inspection for surface porosity and other defects. Surface finish
measurements were also made using the surface roughness standard. Finally, the
temperature information was extracted from the LabVIEW program for analysis.
5.3.1. The Effect of and the Interaction between Melt Superheat and Mold
Preheat Temperature
The effects of melt superheat and mold preheat temperature on surface finish, porosity,
and dimensional accuracy were determined using the same experiment. This experiment
also determined the relationship between the two parameters. The melt superheat
temperatures were selected to range from just above melting point to a high degree of
superheat. The temperature of the tin-bismuth should not exceed 450'F, since the
oxidation of the alloy increases significantly beyond this point. In particular, the melt
Widely available, this eutectic binary alloy is composed of 58% bismuth and 42% tin. The alloy has a
low melting point of 281*F. It has a reasonably high Young's Modulus of 7.5 GPa and an adequate
0
yield strength of 60 MPa. Its thermal conductivity is 18.4 W/m K, while its thermal expansion
6 0
coefficient is 15x10- / K [Lee 1999].
54
superheat was set to 293.5'F, 331 F, 368.5'F, and 406'F, while the mold preheat
temperature was varied between 200'F, 225'F, 250'F, and 275'F.
Aside from the two process parameters under consideration, all other variables were held
constant. The packing pressure was set to 70 psig. The specimen ejection temperature
was set to 200'F. Finally, natural convection was used to cool the mold.
5.3.2. The Effect of Cooling Rate
A second experiment was conducted to determine the effect of the cooling rate on surface
finish, porosity, and dimensional accuracy. There were two sets of test runs. The first set
was carried out utilizing natural convective cooling with ambient air. In the other set, the
mold was placed into a bath that contained an ice-water mixture. When the thermocouple
reached the ejection temperature, the mold was removed from the water and the specimen
extracted.
For this experiment, the melt superheat was set to 331 'F, while the ejection temperature
was set to 200'F. The mold preheat temperature was varied between 225 F, 250'F, and
275'F. The packing pressure was set to 70 psig.
5.3.3. The Effect of Ejection Temperature
A third experiment was conducted to determine the effect of ejection temperature on
surface finish and dimensional accuracy. The ejection temperature was tested at 150 0F,
200 0 F, and 250'F. For this trial, the melt superheat was set to 33 1F and the mold
preheat temperature was varied between 225'F, 250 0 F, and 275'F. The packing pressure
was set to 70 psig. The molds were cooled through natural convection in ambient air.
5.3.4. Aging Effects
The goal of this experiment was to determine whether aging has an impact on the surface
finish and the dimensional accuracy. To do this, the specimen diameter and surface
roughness of the parts created for the previous four experiments were measured both at
the time of removal and at discrete time intervals ranging from a few hours up to four
weeks after specimen production.
55
5.3.5. The Interaction between Packing Pressure and Cooling Rate
Literature research* suggests that the application of packing pressure can increase the
cooling rate. To determine to what extent this relationship can affect the RFPE process,
two sets of trials were conducted. In the first series, pressure was applied shortly after the
molten alloy was poured into the cavity. In the second series, the specimens were left to
cool at atmospheric pressure.
For this experiment, the melt superheat was set to 331 'F, the ejection temperature was set
to 200'F, and the mold preheat temperature was varied between 225 F, 250'F, and
275*F. The packing pressure was set to 70 psig.
5.3.6. The Effects of Alloy Composition
Finally, throughout all the experiments, observations were made regarding the formation
of oxides in the molten alloy and their effect on the process. Particular attention was paid
to the mold and the surface finish of the specimens. The porosity information of the
surface was also recorded.
El Mahallawy, N.A., Taha, M.A., Zamzam, M.L. "On the microstructure and Mechanical Properties of
Squeeze-Cast Al-7 wt% Si Alloy," Journalof MaterialsProcessing Technology, Vol. 40, 1994, p. 83.
56
Chapter 6: Theoretical Background for Experimental
Analysis
In order to analyze the data obtained from the experiments discussed in Chapter 5, some
theoretical background is necessary. This chapter begins with an analysis of the asperity
deflection mechanism. Then, the analysis of the experimental setup is presented. In
particular, the expected temperature drop from the tip of the mold shaft to the
thermocouple well is calculated. Additionally, the cooling rate of the mold in air and in
ice water needs to be determined.
6.1.
The Impact of Surface Finish
As Figure 26* shows, surfaces are often idealized as perfectly flat. As such, perfect
contact is assumed. However, in reality, the surface deviates from perfect flatness in a
number of ways.
Figure 26: Model of surfaces as flat, and in perfect contact
As Section 4.1.1 mentioned, gross deviations from flatness are called form error.
Smaller, higher frequency error is called waviness or roughness. These deviations result
in a real contact area that is a fraction of the nominal contact area, as Figure 27 shows.
Figure 27: Asperities reduce the real to nominal surface area ratio
*
Fan, W.,The Effect of Surface Roughness on the Precisionof the EncapsulatedFixturingSystem, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 27.
5Ibid,
p. 28.
57
As a result, the areas in contact are under much higher compressive loads than expected
for a given nominal area. Deflection of the surface under this load condition has the
potential of affecting the precision of the RFPE process. In order to understand this
potential impact, the mechanism of deflection under load must be understood.
6.1.1. The Mechanism of Surface Deflection Under Load
When a surface is placed under load, elastic and plastic deformation may occur. It is
important to determine how much of the deflection is plastic, as that will result in a
change in the encapsulation dimensions. It is also important to determine the maximum
surface deflection that is possible with a given surface.
The elastic deflection of asperities can be modeled by adapting the Hertzian theory of
elastic contact between solids of revolution.* Equation (1) shows the deflection sustained
by the asperity at the onset of yield.
81;- 2Y2 R
E*2
16
where the Y is the yield strength of the material, R is the radius of curvature, and E* is the
modulus of elasticity.
The radius of curvature of the asperity is difficult to measure, but can be approximated
from the profilometry data by the root mean square curvature, O . Equation (2) defines
the root mean square curvature.
i=N
K7
1/2
N j=1
where N is the number of samples and
Equation (3).
K
K is
the summit curvature as defined by
zz+1 -2z, +zu 1
2 +(3)
h2
where z is the height from the graphical centerline determined by the profilometer and h
is the sampling interval defined by Equation (4).
h =-
L
N
(4)
Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturing System, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 3 6 .
58
With the onset of yield, plastic deformation of the asperity material begins.
Theoretically, the material continues deforming until perfect contact has been achieved.
However, there has been documented research that shows that asperities typically persist
even under high compressive loads.* According to Childs,t the asperity interactions can
be modeled using plasticity mechanics. The attainable real to nominal contact area ratio
is expected to be between 0.56 and 0.65, with the lower theoretical limit being 0.5. Thus,
the maximum possible deflection of the asperities is half the peak-to-valley height of the
surface.
6.1.2. Quantification of the Possible Deflection of the Asperities
As Section 4.1.1 discusses, profilometers can be used to characterize a surface.
Figure 281 shows a typical profilometer trace of a surface.
20000
15000
U)
E 10000
0
5000
0
L-5000
-10000
dL
IL
r
111)
-200 IF
I HIVIV
ALJVV
3 Do
-15000
-20000
Data Number
Figure 28: Profilometer trace for a 0.4 ptm surface, with no
wavelength filtering
From the profilometer traces of a series of surface roughness standards, their
peak-to-valley depth can be determined. Table 1 presents this data.
Moore, A.J.W., "Deformation of Metals in Static and in Sliding Contact," Proceedingsof the Royal
Society (London), Vol. A195, 1948, pp. 231.
Uppal, A.H. & Probert, S.D., "Deformation of Single and Multiple Asperities on Metal Surfaces," Wear,
Vol. 20, 1972, pp. 381
Childs, T.H.C., "The Persistence of Asperities in Indentation Experiments," Wear, Volume 25, 1973,
p. 16.
Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturingSystem, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 40.
§ Ibid, p. 42.
59
Table 1: Peak-to-valley depth for various surface roughness standards
Ra (tm)
0.05
0.1
0.2
0.4
0.8
1.6
6 (PM)
0.0138
0.009855
0.008811
0.004428
0.004545
0.003258
Peak-to-valley Depth (pm)
0.397
0.753
1.528
3.167
4.239
9.680
The elastic deflection calculated using profilometry data for the same range of surface
roughness is shown in Table 2*.
Table 2: Roughness, summit curvature, and deflection values of the
surface roughness standards
Nominal
KLA-Tencor
Matlab calculations
Ra (pm)
Ra (pm)
RRMS (pm)
Ra (pm)
RRMS (Am)
K (M')
8y (pm)
0.05
0.1
0.2
0.4
0.8
1.6
0.0473
0.079
0.233
0.353
0.673
1.383
0.060
0.106
0.278
0.442
0.837
1.747
0.057
0.096
0.235
0.441
0.620
2.055
0.074
0.140
0.297
0.553
0.789
2.636
37150
51920
58040
115580
112540
157000
0.0138
0.009855
0.008811
0.004428
0.004545
0.003258
Clearly, the postulated elastic deflection is at least an order of magnitude less than the
half peak-to-valley height available. Thus, the majority of the deflection will be plastic,
with a permanent change in the dimension of the encapsulation.
6.1.3. The Variables for Roughness Measurements as it Relates to Deflection
With this understanding of the asperity deflection mechanism, one can develop a
methodology to evaluate the impact of surface roughness on the precision of the RFPE
process. The surface roughness is characterized by a number of roughness parameters.t
Equation (5) defines the first of these parameters, Ra, the average roughness. The
average roughness is a measure of the deviation from an artificially imposed centerline.
Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturingSystem, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 41.
* Tencor P-10 Reference, P/N 412937 Rev. A, 1996, p. 3-8.
60
Ra
=
fy I dx
(5)
0
where L represents the sampling length, while y is the height of the profile measured
relative to a graphical centerline computed by the profilometer. x is the incremental
distance traversed by the profilometer along the sampling length.
For most uniform, well-behaved surfaces, the Ra is a sufficient means to evaluate the
characteristics of the surface. Coupled with the data presented in Section 6.1.2, this
allows the evaluation of the impact that surface finish has on the RFPE process in
Section 7.1.
6.2.
Impact of Porosity
The encapsulation material is used to locate, secure, and support a workpiece during
machining operations. To fulfill its functions, the encapsulation material must remain
intact, except in the specific areas that need to be removed for the particular machining
operation under consideration. These excised portions should be planned out in advance
to prevent a loss of structural integrity of the encapsulated workpiece. Porosity disrupts
the microstructure of the encapsulation in an unpredictable fashion. This microstructure
directly affects the structural integrity and the surface topography of the encapsulation.
As such, it has the potential to impact the RFPE process.
6.2.1. Formation of Porosity
As mentioned in Section 4.2.2, porosity is composed of voids that remain when the
molten metal solidifies. Porosity has the potential to weaken the strength of the casting.
In particular, it can degrade the casting's fatigue resistance* and provide crack nucleation
sites.t The presence of surface porosity, through the generation of cold shuts, can impact
both the accuracy and the aesthetic quality of the casting.t
Anson, J.P., Gruzleski, J.E., "The Quantitative Discrimination between Shrinkage and Gas
Microporosity in Cast Aluminum Alloys using Spatial Data Analysis," Materials Characterization,Vol.
43, 1999, p. 319.
t Atwood, R.C., Sridhar, S., Lee, P.D., "Equations for Nucleation of Hydrogen Gas Pores during
Solidification of Aluminum Seven Weight Percent Silicon Alloy," Scripta Materialia,Vol. 41, No. 12,
1999, p. 1255.
I Boileau, James M., Zindel, Jacob W., Allison, John E., "The effect of solidification time on the
mechanical properties in a cast A356-T6 Aluminum Alloy," Society ofAutomotive Engineers Technical
Paper Series, no. 970019, 1997, p. 62.
*
61
These voids, or pores, are generated by three mechanisms. The first type of porosity is
generated by shrinkage. As the material in one location of the casting cools and
contracts, molten material is drawn from another part of the casting to compensate for the
reduction in volume. This mechanism, which usually takes place in long freezing range
alloys, is referred to as feeding. Feeding takes place through interdendritic channels. As
solidification continues, feeding becomes more and more difficult until the channels are
blocked off. To relieve the stress that builds up as shrinkage continues, small pores are
formed. These pores tend to form along dendritic channels and are characterized by their
long shapes.
The second mechanism of porosity generation is through the improper degassing of the
molten alloy. The most common of these is the presence of hydrogen in aluminum. The
change in solubility of hydrogen as the aluminum alloy transitions from liquid to solid
state causes gas pores to form.t
The third porosity generation mechanism is gas entrainment.t Gas present in the mold
during the injection process can become trapped. Common causes for pores generated by
gas entrainment are the presence of turbulence in the mold and inadequate vents. Both
types of gas pores are usually spherical in nature. Turbulence induced pores can be
distinguished due to their elongated profile.
6.2.2. Classification of porosity: Macroporosity and Microporosity
As discussed in Section 6.2.1, there are three types of porosity. While one can describe a
surface by the types of porosity present, often one is looking only for a characterization
of the surface regardless of the cause of the porosity. In this case, it is helpful to
determine whether there is microporosity or macroporosity, or both, present on the
surface. Microporosity refers to defects that are less than one millimeter in size. On the
other hand, macroporosity refers to any defects that are greater than one millimeter in
size. Macroporosity can include cold shuts and misruns.**
Anson, J.P., Gruzleski, J.E., "The Quantitative Discrimination between Shrinkage and Gas
Microporosity in Cast Aluminum Alloys using Spatial Data Analysis," Materials Characterization,
Vol. 43, 1999, p. 320.
Kuznetsov, A.V., Vafai, K., "Development and Investigation of Three-Phase Model of the Mushy Zone
for Analysis of Porosity Formation in Solidifying Castings," InternationalJournalofHeat Mass
Transfer, Vol. 38, No. 14, 1995, pp. 2557-2559.
t Saikawa, S., Nakai, K., Sugiura, Y., Kamio, A., "Effects of Hydrogen Gas Content on Generation of
Porosity in Al-Li Casting Alloys," Materials Transactions,JIM, Vol. 40, No. 1, 1999, p. 57.
I Bar-Meir, G., Eckert, E.R.G., Goldstein, R.J., "Pressure Die Casting: A Model of Vacuum Pumping,"
JournalofManufacturingScience and Engineering,Vol. 118, May, 1996, pp. 259.
§ Medeiros, S.C., "Failure Analysis of an Aluminum Casting," Advanced Materials & Processes,April,
1999, p. 4 2 .
** Ibid.
*
62
6.2.3. Porosity and the RFPE Process
Most of the research on porosity tends to focus on aluminum alloys, particularly on the
elimination of shrinkage and hydrogen gas pores. Others work on specific materials with
unique properties, geometries, and conditions. The models used for porosity prediction
are mostly derived from experimental correlations and are particular to the type of alloy
and operating condition. The material in these studies is of limited use because the
phenomena are extraordinarily specific. However, they do provide a sense of the
parameters that may impact porosity formation. In addition, design rules to significantly
reduce porosity, such as proper vent sizing and siting, were developed by Bar-Meir.*
One must also keep in mind that the function of the encapsulation differs greatly from
that of typical casting components. While most cast components are design to exceed the
service life of the systems of which they will be a part, the RFPE encapsulation is strictly
for temporary service. It is intended to function as the fixturing medium for the duration
of the machining operations on the workpiece. Then it will be melted down for
recycling. During this short life cycle, the encapsulation will only be exposed to a
limited number of load cycles. This means that fatigue failure is not of great concern in
this application. The only concern engendered by porosity is the presence of pores in
sufficient quantities that the strength and accuracy of the encapsulation is compromised.
This would prevent the encapsulation from carrying out its function to locate, hold, and
support the workpiece. The presence of internal pores that will affect the strength of the
encapsulation is largely a function of design parameters and is outside the scope of this
study. The experiments outlined in Chapter 5 examine the effects of various process
parameters on the amount on external porosity present on the encapsulation specimens.
6.3. Temperature Drop from the Mold Cavity to the Thermocouple
Well
To be able to interpret the data obtained during the experiments, it is important to
determine the temperature drop across the distance from the tip of the hole to the mold
cavity.
The thermal resistance across that distance is composed of two elements: conductive
resistance and contact resistance. The temperature drop due to conduction across that
distance was determined approximately using the equation for radial conductive heat
transfer.t Rearranging the equation to solve for the temperature drop yields Equation (6).
Bar-Meir, G., Eckert, E.R.G., Goldstein, R.J., "Pressure Die Casting: A Model of Vacuum Pumping,"
JournalofManufacturingScience and Engineering,Vol. 118, May, 1996.
t Mills, A.F., Basic Heat and Mass Transfer,Richard D. Irwin, Inc., Chicago, USA, 1995, p. 69.
63
AT
=
ln(r 2 / )Q
2,zkL
(6)
where rj and r2 are the inner and out radii respectively, Q is the heat transfer rate, k is the
thermal conductivity, and L is the height of the mold. Figure 29 shows this temperature
drop from the cavity of the mold to the tip of the thermocouple as a function of the rate of
heat transfer.
0
15-
o1
05-
E
0
0
0
800
400
600
200
Heat Transfer Rate (W)
1000
Figure 29: Temperature drop due to conductive resistance as a
function of heat transfer rate
Over the range of temperatures we are concerned with, the temperature drop as a function
of the heat transfer rate is effectively linear. In order to gain a sense of the maximum
pressure drop possible, an approximate Q was calculated using one-dimensional heat
transfer and the electrical analogy. The heat transfer from molten alloy at 406'F to
ambient air temperature at 60'F is approximately 42 W. Therefore, the maximum
temperature drop due to conductive resistance is 0.6'K, or 0.108'F. The measure of the
contact resistance is given in terms of an equivalent interfacial heat transfer coefficient,
hi. Equation (7) solves for the temperature drop given the equivalent heat transfer
coefficient.
AT=
h1A
Q represents
(7)
the heat transfer rate as determined above. A represents the tip area of the
thermocouple. AT represents the temperature drop across the interface. The contact
resistance for a stainless steel to aluminum interface is approximately 4000 W/m 2 K.*
Based on a heat transfer rate of 42 W and a tip area of 7.9x10- 6 M2 , this would only result
in a temperature drop of 0.133'K, or 0.24'F. When the contributions of the two thermal
resistances are added together, this results in a maximum temperature drop of 0.35*F.
This is insignificant when compared to the error margin of the thermocouple. Therefore,
*
Mills, A.F., Basic Heat and Mass Transfer, Richard D. Irwin, Inc., Chicago, USA, 1995, p. 69.
64
the thermocouple provides an acceptable means of determining the conditions within the
mold cavity.
6.4.
Variations in Cooling Rate
The experiment outlined in Section 5.3.2 explores the effect of different cooling rates on
the dimensional accuracy of the process. The cooling rates using different mediums are
calculated in the following analysis.
The simplest way to determine the cooling rate would be to use the lumped parameter
approximation* defined in Equation (8). This validity of this approximation can be
verified with the final result.
dT =
(T -T,)
pVc
dt
(8)
where h is the convective heat transfer coefficient, p is the density, V is the volume, c is
the heat capacity and T and Te represent the mold and environmental temperatures
respectively. The suitability of the lumped parameter approximation can be tested using
the Biot numbert defined in Equation (9).
Bi -
Rinternal _
Rexternal
hL
(9)
k
The laminar convective heat transfer coefficient can be calculated using the correlation
developed by Churchill and Chu shown in Equation (10).1
RaL =GrPr
Gr = ATgL
V2
9/16 -16/9
0
(10)
L=
NuL
0.68+0.67(RaLTY
h- NuLk
L
*
Mills, A.F., Basic Heat and Mass Transfer,Richard D. Irwin, Inc., Chicago, USA, 1995, p. 30.
SIbid, p. 32.
+ Ibid, pp.293-29 4 .
65
Where RaL, Gr, V NUL, and h are the Rayleigh number, Grashof number, Prandtl number
function, Nusselt number, and the convection heat transfer coefficient respectively. ,
A T, g, L, v, Pr,and k represent the volumetric coefficient of expansion, the temperature
change, gravitational acceleration, length of the convective surface, kinematic viscosity,
Prandtl number, and thermal conductivity respectively.
Using an initial mold temperature of 330'F, ambient air temperature of 70'F, ice water
temperature of 32'F, and a vertical distance of 4 inches, the heat transfer coefficients
were calculated. The heat transfer coefficient is 8.03 W/m 2 K for ambient air and
259 W/m 2 K for the ice-water bath.
The Biot numbers for the air and the ice water baths conditions are both much smaller
than 0.1. Therefore, the lumped parameter approximation is suitable for the current
calculations. From Equation (8), the predicted cooling rate for ambient air is 0.02750 K/s,
or 0.0500 F/s. The predicted cooling rate for an ice water bath is 0.886*K/s, or 1.60'F/s.
66
Chapter 7: Experimental Results
With the understanding of the experimental apparatus developed in Chapter 6, we are
now ready to consider the experimental results of the experiments described in Chapter 5.
7.1.
The Effect of Melt Superheat and Mold Preheat Temperature
The melt superheat and the mold preheat temperature have the potential to affect surface
finish, porosity, and dimensional accuracy. In particular, this effect was determined by
the difference between the melt superheat temperature and the mold preheat temperature
as well as the two temperature settings relative to the melting point of tin-bismuth. The
experiments were intended to determine the effect of these two process parameters on the
metrics.
Table 3 presents the visual observations of the surface. The surface finish was good
when the melt superheat was greater than or equal to 331 'F and the mold preheat
temperature was greater than or equal to 250'F. At these settings, only microporosity
was present on the surface. Below those temperature levels, the surface was marred by
both macroporosity and microporosity.
Table 3: Description of the surface condition as a function of the
injection temperature and mold preheat temperature
Mold Preheat
Temperature (*F)
275
250
225
200
Injection Temperature (*F)
293.5
331
368.5
406
Small pores distributed Smooth surface with tiny, almost
invisible pores.
across surface.
A few very small pores across surface.
Small pores with path
lines around edge.
No flow lines evident.
Flow lines and pores
Small pores sprinkled across surface.
present.
Small amount of flow lines.
Premature freezing of alloy. Pores and flow lines present.
However, aside from the extreme cases, these indications do not seem to impact the
surface finish beyond mere aesthetics. The specimen surface, as expected at this
pressure, is a near-mirror reflection of the mold surface. Regardless of the melt superheat
and mold preheat temperature, the surface roughness was always less than 1.6 pm. From
Table 1, this corresponds to a peak-to-valley height of approximately 9.68 pIm. The
maximum possible deflection of the surface is the half height, which is 4.84 pIm, or
0.00019 inches. This is more than an order of magnitude less than the desired tolerance
67
of 0.005 inches for the RFPE process. Therefore, one can conclude that the process is
robust against surface finish and surface porosity variations introduced by the melt
superheat and mold preheat temperatures.
Table 4 contains the specimen diameters measured upon extraction from the mold. The
average diameter is 1.0104 inches, with a standard deviation of 0.0004 inches. The
specimen diameters were found to be 1.0104 inches, with a standard deviation of
0.000378 inches, for Mold A and 1.0104 inches, with a standard deviation of
0.000417 inches, for Mold B. The variation overall and between the molds is within the
tolerance of the micrometer.
Table 4: Measurement of specimen diameters
Mold
Temperature
(*F)
275
250
225
200
Injection Terperature (IF)
293.5
1.011
1.011
1.010
1.010
331
1.0105
1.010
1.0010
1.0105
368.5
1.011
1.0010
1.0105
1.0105
406
1.0105
1.0010
1.0105
1.0105
Variations in the melt superheat and the mold preheat temperature did not seem to have a
significant difference on the dimensional accuracy of the specimens. The difference
between the measurements and the 1.008 inches diameter of the mold cavity is due to the
thermal expansion of the mold. Aluminum's coefficient of thermal expansion is
22x10-6/OK. The 211OF, or 1 17.2'K, difference between the ambient environmental
temperature and the melting point results in a thermal strain of 0.00258 for the aluminum.
At 281 OF, this would give the mold cavity an expanded diameter of 1.0106 inches. This
corresponds well to the measurements of the specimens. The coefficient of thermal
expansion seems to be the dominant effect affecting the dimensional accuracy of the
RFPE process.
For encapsulation mold design, this variation will have to be considered carefully in order
to achieve an encapsulated workpiece of the desired dimension. In addition, the mold
temperature at which the embedded workpiece is reinserted for reencapsulation will have
to be carefully controlled to minimize induced stresses due to thermal expansion.
Finally, the controlled preheating temperature will permit the mold to fit and seal
properly around the encapsulated workpiece.
68
7.2.
The Effect of Ejection Temperature
The temperature at which the specimens are ejected from the mold has the potential to
impact the surface finish and dimensional accuracy of the specimens. In order to
determine the sensitivity of these metrics, specimens were ejected from the mold at
150 0F, 200 0 F, and 250*F. When the specimen was produced at a high melt superheat
with an initial low mold temperature, there were some instances where the alloy had not
completely solidified when the specimen was extracted. There was insufficient time for
the entire specimen to cool below the tin-bismuth's melting point. Care should be taken
to ensure that this does not take place during the actual encapsulation. Despite this, no
additional variations in surface finish beyond those caused by melt superheat and mold
preheat temperature was noted. The difference in dimension was accounted for by the
thermal expansion of the mold. Therefore, one may conclude that both the surface finish
and dimensional accuracy of the RFPE process are independent of the ejection
temperature over the range tested.
7.3.
The Effect of Cooling Rate
One of the concerns with using a phase change process was the relatively long cycle time
compared to the other universal fixturing concepts. This is due to the requirement for
heat transfer into and out of the encapsulation. Therefore, it is desirable for the cooling
time to be reduced as much as possible in order to reduce the cycle time. However,
counterbalancing that requirement was the potential for a rapid cooling rate to negatively
impact the surface finish, porosity, and dimensional accuracy of the RFPE process.
Figure 30 shows the cooling rate of the specimen using both air and ice water as the
convective medium. The air and ice water medium yielded cooling rates of 2.407'F/s
and 0.0776 0 F/s. This is similar to the predicted cooling rates of 1.6 0 F/s and 0.05'F/s.
The difference can be explained by environmental factors that resulted in forced
convection, such as air conditioning and mixing of the ice bath as the mold was inserted.
There were no surface finish, porosity, and dimensional accuracy variations attributable
to the variation in the cooling rate. The results of the experiments show that, regardless
of whether the specimen was cooled in an ice water bath or in ambient air, the surface
finish, porosity and dimensional accuracy was not affected at a level detectable by visual
inspection and by the instruments used.
69
280
270
260
L-
-cAir Cooled
4) 250
i
240
E 230
-
Ice Water Bath
220
210
-IL
200
1
201
401
601
801
Time (s)
Figure 30: Cooling rate of mold in air and in ice water
7.4.
Aging Effects
As mentioned in Section 4.2.3, aging is caused by the presence of thermal and kinetic
stress. These stresses can be created by shrinkage, turbulence, and premature
solidification. The gradual relaxation of these stresses over a period of time as the
specimen tends towards equilibrium may result in changes in the specimen dimension
and surface finish.
The specimens from the previous three experiments were measured immediately
following ejection, several hours later and once weekly after that. The only variation that
was noted can be attributed to thermal expansion as discussed in Section 7.1. There were
no changes in dimension between the second, third and fourth weeks. The surface
inspections and comparisons between the specimens and the surface roughness standard
did not detect any variations. There were no detectable changes in porosity.
The lack of variation during to aging is due to two reasons. First, the experiments were
carried out by hand-pouring the alloy into the mold. The low rate of alloy introduction
into the mold minimizes the amount of turbulence present, thus neutralizing the kinetic
effects. Second, the slow rate of solidification, even with the ice water bath, provided
sufficient time for the thermal effects to dampen out. From this, one may conclude that,
as long as the process operates in moderate injection and cooling regimes, the metrics of
surface finish and dimensional accuracy are not sensitive to aging effects over the range
and duration of operations of the RFPE process.
70
7.5.
The Interaction between Packing Pressure and Cooling Rate
An experiment was conducted to determine the effect of pressure on cooling rate.
Figure 31 shows the cooling rate of the specimen at atmospheric pressure and at 60 psig.
The cooling rate with pressure was 0.089 0F/s compared to 0.049 0 F/s when the mold was
at atmospheric pressure. The 83% increase in cooling rate demonstrates the positive
correlation between packing pressure and cooling rate described in Section 4.2.2.
270
o-%
260
Atmospheric
250
L
Pressure
240
E 230
0,
M-
220
60 psig
210
200
M- C
LO,
04
0)
(O
MO
T_
(0j
Mv
MC
MC
NlIO- M
0M
N
T01
Cv,
0
L(
0)
N0)
0
a,
(%.
T_
CO,
Time (s)
Figure 31: Comparison of the cooling rate of a pressurized and
unpressurized mold cavity
7.6.
The Effects of Alloy Composition
Alloy composition plays an important role in the RFPE process. This is especially true
given the dependence of the process on the alloy material properties. Lee, Valdivia, and
Fan* have addressed a number of issues regarding alloy composition: rewelding, thermal
drift, and the use of eutectic tin-bismuth.
It was noted during the experiments that high temperature, rapid changes of temperature,
and agitation all result in the rapid generation of oxides. These oxides are less dense than
the tin-bismuth alloy and float to the top of the reservoir. While the presence of oxides
*
Lee, E., Valdivia, P., Fan, W., Sarma, S.E., "The Process Window for Reference Free Part
Encapsulation," To appear in ASME JournalofManufacturingScience & Engineering.
71
does not seem to affect dimensional accuracy, it has shown the potential of affecting the
RFPE process in terms of surface finish and porosity. The oxides tend to aggregate on
the surface of the specimen. This causes blemishes on the specimen surface. In addition,
the oxides contaminate the surface of the mold. This makes it easier for air bubbles to be
trapped on the specimen-mold interface.
The simplest solution to the oxide issue is to avoid introducing it into the alloy stream.
This can be best accomplished by locating the alloy intake at a small distance above the
bottom of the reservoir. This would allow the intake to both avoid the layer of oxide
lying on top and any debris that may have settled on the bottom of the reservoir.
Secondly, agitation of the alloy should be kept to a minimum to avoid both the generation
of oxides and the mixing of the oxides into the alloy stream. The amount of oxide
present in the system can be avoided by using filtering systems to continuously skim off
the oxide on the surface of the reservoir. Finally, temperature control, both in the
avoidance of high temperatures and of high rates of temperature change, would be
required to minimize oxide generation.
72
Chapter 8: The RFPE Process Parameter Map
Current fixturing technology is the critical link in manufacturing. Fixturing is the choke
point that impedes the flow of ideas from design to production. The limitations of current
fixturing systems severely restrict the component geometries that can be produced at a
reasonable cost. It forces the designer to take roundabout routes, such as dividing the
part into smaller, simpler components or embedding locational or workholding
attachments into the design, in order to produce the component. The universal fixturing
technologies provide a much greater flexibility. However, this flexibility comes at the
cost of robustness. Conformable clamp systems have poor damping capabilities and are
mechanically unreliable. Fluidized bed technologies are flexible and allow relatively fast
turnovers, but are limited to low workholding forces and often sacrifice accuracy when
the bed settles. Phase change systems have excellent workholding properties, but the
relatively slow cycle time and hardware-intensive systems used to establish the locational
reference form major detractors.
Sarma* developed a paradigm shift for fixturing systems: the Reference Free Part
Encapsulation Concept. Since the viability of this concept has been proven
experimentally, it was important to fully understand the RFPE process to allow further
development. In order to do this, a literature review of similar processes was conducted.
Additionally, research to date was examined to see what knowledge we have already
gained. With this foundation, the RFPE process parameters and metrics were defined and
an initial process parameter map was constructed. As Figure 21 shows, the effects on
packing pressure on all the parameterst had been examined in addition to the causes of
thermal drift and re-welding strength.t However, the effects of melt superheat, mold
preheat temperature, ejection temperature, cooling rate, aging, and alloy composition on
surface finish, porosity, and dimensional accuracy had yet to be examined. This study
explored these areas through a literature review, theoretical analysis, and
experimentation. This investigation resulted in a completed RFPE Process Parameter
Map, presented in Figure 32. This study also explored the relationships between the
following process parameters: melt superheat and mold preheat temperature, melt
superheat and ejection temperature, melt superheat and alloy composition, mold preheat
temperature and cooling rate, mold preheat temperature and ejection temperature, cooling
rate and ejection temperature, cooling rate and packing pressure, and cooling rate and
aging. The Final RFPE Interaction Matrix is presented in Figure 32.
Sarma, S.E., A Methodologyfor IntegratingCAD and CAM in Milling, Ph.D. Thesis, University of
California, Berkeley, 1995, pp. 28-57.
* Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturing System, S.M.
Thesis, Massachusetts Institute of Technology, 2000, p. 51.
Lee, E., Valdivia, P., Fan, W., Sarma, S.E., "The Process Window for Reference Free Part
Encapsulation," To appear in ASME JournalofManufacturingScience & Engineering.
4
73
-Metrics
*I
Process Parameters
Surface
finish
Porosity
Dimensional
Accuracv
Thermal
drift
Melt superheat
Mold preheat temperature
Cooling rate
Ejection temperature
Packing pressure
Aging
Alloy Composition
[This thesis]
[Source]
[This thesis]
Materially
Independent
Map
completed
Figure 32: Completed RFPE Process Parameter Map
74
Rewelding
strength
_____________
Process
Parameters
Melt
superheat
Mold
preheat
temperature
q
Cooling rate
p
Ejection
temperature
p
Packing
pressure
p
Aging
Alloy
composition
Melt
superheat
preheat
temperature
Cooling rate
Ejection
temperature
Packing
pressure
Aging
Alloy
composition
Materially
independent
Relationship
Characterized
Redundant
Spaces
Figure 33: Completed interaction matrix for the RFPE process parameters
Chapter 9: Future Work
The completion of the Process Parameter Map is the critical first step in further
developing the RFPE process. From this point, there are a number of avenues of research
that can be pursued. As discussed in Section 3.2, once the process parameters are
established, the design parameters can be coherently selected for any particular
implementation of the system. This is the next step in bringing RFPE to life. In order to
do this, the design parameters of the process must be defined. Once these design
parameters have been defined, any interdependencies between the process and design
parameters must be explored. This will allow us to expand the RFPE Interaction Matrix.
With this foundation in place, a field prototype can be developed. With the lessons
learned from this field prototype, a coherent rapid prototyping system can be built.
9.1.
Study of Design Parameters
Since the process parameters have been identified, the mapping of the design parameters
can begin. This will identify the factors that have a major impact on the quality, cost, and
cycle time of the RFPE apparatus.
As Appendix A discusses, there are many design parameters that must be considered
when designing the apparatus, including orifice size, injection pressure, shot size, cooling
direction, mold surface finish, mold locking pressure, siting of vents, and siting of
injection ports. These parameters denote the limits of the process that can be achieved
with a given apparatus built to certain specifications. Interrelationships between design
parameters also help determine additional design variables. An example of this is the
relationship between orifice size, vent siting, and injection pressure that determines the
injection velocity, the cavity fill time, and the turbulence during the molding process.
Finally, one must study the interaction between process and design parameters.
9.2.
Design of a Field Trial Prototype
Once the RFPE Design Parameter Map has been developed, the apparatus can be
designed according to the optimal settings and ranges. The design should include not
only the encapsulation machine, but also the associated encapsulated workpiece fixture.
In addition, procedures will have to be developed for the application, machining, and
removal of the tin-bismuth.
Field trials can then be carried out to validate the function of this design. From the
understanding gained during these trials, the design and procedures can be fine-tuned.
The objective of these trials will be to demonstrate the optimal capabilities of the RFPE
process. This test may include some or all of the machining strategies.
77
9.3.
Development of a Rapid Prototyping Center
Once the RFPE System is fully developed, we envision using it as the core component of
a Rapid Prototyping Center. Figure 34* shows an RFPE System and three milling
machines in a model of one of these Centers.
Figure 34: Rapid Prototyping Center
This Rapid Prototyping Center will further transform the manufacturing process. Unique
designs will no longer require individual attention. Instead, the manufacturing process
will be a black box. As Figure 35t displays, the inputs to this process will be a unique
G-code and raw stock while the outputs will be the unique products.
*
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 17.
Ibid, p. 378.
78
E~1\
Customer A
Customer B
>24
Unique G code
Figure 35: The Rapid Prototyping Center will transform the
manufacturing process from a heavy manual intervention process into
a black box
The power of this Center is that the planning and set up time will be virtually eliminated.
In fact, manufacturing will move from requiring intensive human decision making to one
where the majority of the energy can be dedicated toward design work. This will
drastically reduce the cost of manufacturing. In fact, in this Rapid Prototyping Center, a
central computer controller can control the entire process, as Figure 36* shows.
*
Lee, E.C., Development of an EncapsulationProcessfor use in a Universal Automated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 38.
79
Encapsulation Machine
Queue 2
Queue 1
noCentral Computer
Controller
MachineTool
Queue 4
Queue 3
Figure 36: Proposed process flow for the Rapid Prototyping Center
If we take this a step further, we can take advantage of other technology under
development and have the central computer controller have a single input: the CAD
drawing of each part that is desired. This controller uses tool path generation* to generate
the G-code. This G-code is parsed into machining passes. These machining passes are
then automatically controlled, with reencapsulation occurring in between consecutive
machining operations. Thus, the designer only needs to worry about putting his or her
ideas down on paper. The rest will be taken care of by the Rapid Prototyping Center.
Thus, Rapid Prototyping Centers will have the ability to "revolutionize the way engineers
and designers think about product development and manufacturing. Just as the concepts
of mass production-assembly lines, interchangeable parts--have revolutionize [d]
manufacturing, so shall rapid prototyping through machine tools. The ability to create
one part as quickly, efficiently, and as economically as creating a thousand of those parts,
that is the next frontier in manufacturing and product development."f
This may all sound like a dream-but dreams give us a goal. Lee took the first step in
proving Sarma's idea. The development of the Process Parameter Map completed the
next step in the process. With the efforts of others in pursuing the next stages of
development, Rapid Prototyping Centers can become a reality.
Institute
Balasubramaniam, M., Automatic 5 Axis NC Toolpath Generation,Ph.D. thesis, Massachusetts
of Technology, 2001.
Fixturing
t Lee, B.C., Development of an EncapsulationProcessfor use in a UniversalAutomated
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, p. 18.
*
80
Appendix A:
An Introduction to the
RFPE Design Parameters
In the course of conducting the literature review for the RFPE process parameters, some
information was gathered on the design parameters. In an attempt to avoid duplicating
effort in the next stage of RFPE process development, this Appendix presents these initial
findings.
The design parameters for the RFPE System can be divided into two categories. The first
category affects the storage and delivery of the molten alloy. The second category affects
the introduction and behavior of alloy inside the mold.
A.1.
Parameters Affecting the Alloy Storage and Delivery
In the RFPE process, the material properties of the phase change material determine the
workholding, damping and support characteristics of the fixturing system. While this
ensures that the fixturing quality remains stable regardless of the part geometry,
variations in the alloy composition will impact the reliability of the system. Therefore, it
is imperative that all avenues for potential degradation of alloy quality be identified and
neutralized. This section will examine a number of such avenues.
A.1.1.
The Reservoir
The reservoir forms an inseparable part of any encapsulation machine. It is the location
where the alloy is melted down and stored in preparation for injection into the mold. The
objective of reservoir design is to create a system that can provide alloy in the desired
state with a minimum of contaminants. There are three design parameters to be
considered: reservoir temperature, proportion of new to recycled alloy, and reservoir
construction.
A.1.1.1.
Reservoir Temperature
The reservoir temperature is the temperature of the tin-bismuth awaiting injection. The
desired reservoir temperature is determined by the melt superheat process parameter and
by the physical layout of the machine. The experiments discussed in Section 7.1
established the desired melt superheat. The physical layout of the apparatus determines
the heat loss that takes place between the reservoir and the injection point into the mold.
The reservoir temperature is, in essence, the desired injection temperature plus the
temperature difference necessary to account for the heat loss during alloy delivery.
The correct setting of this design parameter has an important impact on the metrics used
to measure the quality of the encapsulation. If the reservoir temperature is set too low,
there will not be sufficient melt superheat. This will result in poor surface finish and
porosity as outlined in Section 7.1. Variation in the reservoir temperature will lead to
83
variation in the thermal expansion of the encapsulation, thus resulting in varying
dimensional accuracy.* If the reservoir temperature is too high, the resultant excessive
superheat has the potential of causing thermal drift of the encapsulated workpiece.
Finally, the reservoir temperature will likely have an impact on the microstructure of the
alloy.' This, in turn, will impact the strength of the encapsulation.
The reservoir temperature also has an effect on the process parameters. The relationship
between the reservoir temperature and the melt superheat has already been outlined
above. In addition, the reservoir temperature, by setting one end of the temperature
gradient, plays a part in determining the cooling rate. Finally, the reservoir temperature
will have a large effect on the alloy composition. In particular, high reservoir
temperatures, as well as sharp temperature fluctuations, tend to encourage oxide
formation. This was born out by experimental observation noted in Section 7.6.
A.1.1.2.
Proportion of New to Recycled Alloy
Once machining operations are complete, the encapsulation is melted off the embedded
workpiece and, along with any scraps generated, returned to the reservoir. The alloy is
then recycled for future operations. The use of recycled alloy provides an avenue for the
introduction of contaminants such as cutting fluid, metal scraps, and oxides. Methods for
dealing with the contaminants are covered in Section A. 1.1.3. The inevitable loss of
alloy during the machining process means that new alloy will have to be introduced. The
tradeoff lies between introducing the minimum amount of new alloy necessary to
replenish losses to optimize the economics of the process and providing sufficient pure
alloy to ensure alloy quality in the process.
Inability to ensure alloy quality may result in poor surface finish and porosity as
contaminants are introduced into the mold. In sufficient quantities, these contaminants
have the potential to affect the strength of the rewelding between old and new portions of
the encapsulation. Finally, the introduction of metal chips and other contaminants into
the encapsulation machine may negatively impact the reliability of the system.
A.1.1.3.
Reservoir Construction
In addition to the above design considerations, the construction of the reservoir will
heavily influence the quality of the alloy available for encapsulation. This influence is
transmitted through three design considerations: construction materials, heater selection
and siting, and contaminant containment.
* Allsop, D.F., and Kennedy, D., PressureDiecasting;Part2,
Pergamon Press, New York. 1983, p. 15.
t Murray, M.T., Griffiths, J.R., "The Design of Feed Systems for Thin-Walled Zing High-Pressure Die
Castings," Metallurgicaland Materials Transactions,Vol. 27B, February, 1996, p. 115.
84
The material used to construct the reservoir must meet three basic requirements. It must
be suitable for long duration, high temperature service. The reservoir surface must be
durable against impacts and not contribute contaminants to the melt. Finally, the
reservoir material must be chemically inert when placed into contact with the molten
alloy at operating conditions.
The heaters must be sited to minimize the heat transfer resistance between the heater and
the molten alloy. The location of the heaters also plays an important part in ensuring the
even distribution of temperature. This will avoid the creation of hot spots that would
result in oxide generation. Finally, heaters and associated control systems must be
selected so that the system is responsive within the operational range and is capable of
maintaining the desired temperature with minimal fluctuations.
The final consideration of reservoir construction is contaminant containment. This refers
to the goal of ensuring alloy quality by preventing the introduction of contaminants into
the encapsulation machine. Since the oxides tend to float on the surface of the melt and
debris such as metal chips tend to sink to the bottom, one solution would be to locate the
alloy intake at a set distance above the bottom of the reservoir. This will not only avoid
the oxide floating on the surface, but will also provide a sump to allow metal chips to
settle. To further improve the alloy quality, filtration systems can be installed. Studies of
filtration systems used with other casting materials and processes will provide a useful
first step in this endeavor*.
A.1.2.
Alloy Delivery
Once the alloy has been heated to the correct temperature, it is ready to be injected into
the mold. The alloy is pumped from the reservoir to the mold using a piston cylinder
arrangement. The piston can be actuated pneumatically or hydraulically. While
hydraulic cylinders allow higher reaction times and pressures, these qualities are not
necessary for the RFPE process. The functionality of the pneumatic cylinders was quite
sufficient for this application. In addition, the pneumatics cylinders have an advantage in
terms of cost and ease of use.t A number of design parameters affect the efficacy of this
process, including injection pressure, shot size, rate of change of pressure, opposing
pressures on each side of the piston, inertia, transfer tubing design, and partial blockages
of the metal stream.
*
1
Neff, D.V., "The Filtering and Degassing of Aluminum Die Casting Alloy" Die CastingEngineer, Vol.
18 2 4
30, No. 5, September-October, 1986, pp. - .
Lee, E.C., Development of an EncapsulationProcessfor use in a UniversalAutomated Fixturing
System, S.M. Thesis, Massachusetts Institute of Technology, 1999, pp. 50-51.
85
A.1.2.1. Injection Pressure
The injection pressure refers to the pressure of the molten encapsulation machine as it
enters the mold cavity. This design parameter can impact the surface finish, porosity,
rewelding strength, and in extreme cases, the dimensional accuracy of the encapsulated
workpiece.
A high injection pressure leads to a high injection velocity, which results in a lower fill
time and increased turbulence. While the lower fill time may be desirable to improve the
cycle time of the encapsulation system, the increased turbulence causes the molten alloy
to mix with the air in the cavity. If the encapsulation solidifies before the alloy can settle,
air bubbles are trapped in the encapsulated workpiece. This trapped air causes porosity.*
If air bubbles are trapped on the surface, the surface finish of the part can be affected. In
extreme cases, the dimensional accuracy of the encapsulated workpiece can be sacrificed
as the voids on the surface compromise the load bearing ability of the encapsulation.
The injection pressure should not affect the location of the encapsulated piece unless the
internal porosity level is sufficiently high to degrade the mechanical properties of the
encapsulation. This may lead to the failure of the encapsulation during machining. There
is no known effect of injection pressure on rewelding strength. The injection pressure is
affected by a number of other parameters detailed below.
A.1.2.2.
Shot Size
The shot size is the amount of alloy that can be ejected by the piston-cylinder assembly.
A shot size must be larger than the mold volume in order to generate good quality
castings. Otherwise, incomplete filling of the mold would occur. The unstable injection
pressure, along with the increased presence of air in the system, would increase the
chance for porosity. This would compromise both the surface finish and, potentially, the
strength of the encapsulation.
When the injection piston plays an additional role as the packing pressure applicator, an
adequate shot size becomes even more important. An incomplete filling of the mold, or
short shot, will not only result in the defects detailed above, but it will also eliminate the
hydraulic lock the piston needs to apply the packing pressure. The lack of pressure on
the casting would result in an even larger percentage of porosity and lower dimensional
accuracy due to the lack of pressure compensation for shrinkage.
Finally, the shot size has no known effect on the thermal drift and rewelding strength.
*
Bar-Meir, G., Eckert, E.R.G., Goldstein, R.J., "Pressure die casting: A Model of Vacuum Pumping,"
JournalofManufacturing Science and Engineering, Vol. 118, May, 1996, p. 256.
86
A.1.2.3.
Rate of Change in Pressure
The rate of change of pressure can negatively affect the precision of the RFPE process.
The packing pressure effects on surface finish and porosity were explored by Fan.*
Variations in the injection pressure will result in varying degrees of air entrapment. This
air entrapment can impact surface finish, porosity, dimensional accuracy and rewelding
strength. If the rate of change of pressure cannot be applied in a consistent manner, the
reliability of the process will be negatively impacted.
A.1.2.4. Pressure on Opposing Sides of the Injection Piston
Another parameter that may cause variation in the injection pressure is the pressure
present on opposing sides of the injection piston. This pressure can vary depending on
the design of the piston cylinder, the environmental pressure, and the design of the
apparatus. The presence of pressure on the other side of the injection piston will reduce
the actual available injection pressure and potentially result in the negative effects
outlined in Section A. 1.2.1.
A.1.2.5. Inertia Effects
While inertia effects have played a part in determining the quality of die casting
processes,t it is not clear how inertia effect will impact the RFPE process. Further
research, both in the form of literature review and in experimentation, is necessary.
A.1.2.6. Transfer Tubing Design
Transfer tubing refers to the tubing that connects the reservoir to the mold. The tubing
should be as short as possible to minimize heat loss to the environment. In addition, high
and low loops should be avoided as much as possible to avoid trapping air or debris in the
line. The air and the debris, if injected along with the alloy, have the potential to affect
the surface finish, porosity, and dimensional accuracy of the encapsulation.
*
I
Fan, W., The Effect of Surface Roughness on the Precision of the EncapsulatedFixturingSystem, S.M.
Thesis, Massachusetts Institute of Technology, 2000.
Upton, B., PressureDiecasting;Part 1, Pergamon Press, New York, 1982, p. 42.
87
A.1.2.7. Partial Blockages of the Metal Stream
Partial freezing of the alloy in the metal stream or blockages in the pathway can affect the
surface finish and porosity of the encapsulated workpiece. Any blockages in the alloy
pathway can increase turbulence, increasing the amount of trapped air bubbles. These air
bubbles result in porosity as the encapsulated workpiece solidifies. The partial freezing
of the alloy in the metal stream introduces random variation in the encapsulation structure
with resultant variations on mechanical properties. This is a particularly important issue
to consider since it affects the reliability of the encapsulation strength. One approach
towards mitigating this issue is to minimize the flow path of the alloy from the reservoir
to the mold. Alternatively, heaters can be used on or near the transfer tubing to ensure
that the alloy remains above the melting point during the entire operation.
A.2. Parameters Affecting the Introduction and Behavior of Alloy
Inside the Mold
Once the alloy has been delivered to the mold, one must be aware of the kinetic and
thermal effects that will ultimately have an effect on the quality of the encapsulation.
The following sections will examine various design parameters that will control these
effects. These parameters include gate velocity, cooling direction, heat and cooling of the
mold, mold surface finish, mold material selection, mold locking pressure, mold vents
and compatibility of gate and machined section geometry.
A.2.1.
Gate Velocity
The gate velocity is the rate at which the alloy enters the mold. The velocity, which
determines the cavity fill time, is a function of the orifice size, injection pressure, and
frictional effects on the alloy. This parameter has the potential to impact the surface
finish, porosity and dimensional accuracy of the encapsulation. A high gate velocity will
result in a great deal of turbulence. On the other hand, too slow a gate velocity means
that the alloy will cool before the mold is entirely filled, causing cold shuts. Allsop
recommends a two stage filling strategy.t The first stage is at a lower velocity to reduce
turbulence and permit air to vent from the mold. The second stage is at a higher velocity
to complete filling the mold before solidification takes place. In this fashion, the
turbulence, and thus the amount of porosity generated, will be greatly reduced.
* Upton, B., PressureDiecasting;Part1, Pergamon Press, New York,
1982, p. 42.
t Allsop, D.F., and Kennedy, D., PressureDiecasting;Part2, Pergamon Press, New York. 1983, p. 6-7.
88
A.2.2.
Cooling Direction
The application of directional cooling to the mold may help ensure the quality of the
encapsulation by improving the surface finish and dimensional accuracy and lowering the
porosity. A mold that is cooled from the bottom up will provide more time for air
bubbles to escape, thus reducing porosity. In a mold where the gate is located at the top,
the shrinkage effect will also be minimized, as packing pressure will be applied until the
moment the gate freezes. Experiments should be carried out to determine the effect that
directional cooling has on the microstructure of the encapsulation. This may have an
impact on the strength of the encapsulation.
A.2.3.
Heating and Cooling of the Mold
The temperature of the mold, along with the temperature of the melt, controls the
temperature gradient and, thus, the heat transfer rate from the encapsulation to the
environment. The implementation of heating and cooling channels on the mold body
would allow the cooling rate to be varied in a programmed fashion. Although this would
increase the complexity of the apparatus, such a heat transfer control system may be
necessary to achieve the cooling requirements laid down by Lee* to maximize rewelding
strength while minimizing the risk for thermal drift. Directional cooling may also be
implemented once such a system is in place. Finally, this type of system will allow
cooling to be controlled so that the kinetic effects have an opportunity to dampen out and
allow thermal effects to dominate in the solidification process.t
A.2.4.
Mold Surface Finish
The mold surface finish is very important to the RFPE process. The process parameter
windows have been selected to replicate the mold surface finish. As long as a fine mold
surface finish is selected, coupled with a packing pressure above 60 psi, the surface finish
of the encapsulated workpiece will closely follow that of the mold. The mold surface
finish should be selected considering the tradeoff between a fine mold finish and its cost
and durability. Since a surface that wears out quickly can have a detrimental effect on
dimensional accuracy, it would be wise to select the roughest, least delicate surface
possible that would suffice. From the results in Section 7.1, mold surface finish with a
surface roughness of 1.6 pm is capable of generating an encapsulation surface with an
Lee, E., Valdivia, P., Fan, W., Sarma, S.E., "The Process Window for Reference Free Part
Encapsulation," To appear in ASME JournalofManufacturingScience & Engineering.
T Murray, M.T., Griffiths, J.R., "The Design of Feed Systems for Thin-Walled Zing High-Pressure Die
Castings," Metallurgicaland Materials Transactions,Vol. 27B, February, 1996, p. 115.
89
error that is more than an order of magnitude smaller than the tolerance of the RFPE
process. This is a surface finish that is achievable using normal machining techniques
and repressents a good compromise between fineness and durability.
A.2.5.
Mold Material Selection
The mold surface finish may also be affected by the mold material. In the experiments
conducted to examine the process parameters, aluminum molds were used. Aluminum
was sufficiently durable for the low number of trials carried out on the experimental
molds. However, some scarring patterns developed toward the end of the test. This may
be due to wear of the aluminum during the molding process, chemical interactions
between the aluminum and the tin-bismuth, and desposition of oxides or other
contaminants onto the mold surface. For improved wear resistance, the actual molds
used in the encapsulation machines will likely be fabricated out of harder and tougher
materials, such as steels. Combined with a surface finish selected with the criteria
outlined above, a durable mold that manufactures parts with consistent quality can be
fabricated at a reasonable cost.
A.2.6.
Mold Locking Pressure
Mold locking pressure is another avenue by which the dimensional accuracy of the
encapsulation may be impacted. Care must be taken during the design process to ensure
that sufficient pressure is applied toward locking the mold to overcome the injection
pressure. Otherwise, the injection pressure may force the mold open along the parting
lines, thus introducing dimensional errors into the encapsulation.
A.2.7.
Mold Vents
The use of mold vents may be necessary in order to evacuate air from the mold. The
successful location and sizing of the vents will significantly reduce the amount of air
trapped in the mold. Correspondingly, this will reduce the amount of porosity due to air
entrapment and improve the surface finish of the encapsulated workpiece. Bar-Meir has
developed a methodology for vacuum venting.* This methodology also includes a model
for optimizing the vent area, which may be a helpful first step should the need for such
vents appear.
*
Bar-Meir, G., Eckert, E.R.G., Goldstein, R.J., "Pressure die casting: A Model of Vacuum Pumping,"
JournalofManufacturing Science and Engineering,Vol. 118, May, 1996, pp. 256-265.
90
A.2.8.
Compatibility of Gate and Machined Section Geometry
Finally, the geometry of the machined section that needs to be refilled needs to be
considered. Depending on the mold design, some limitations may have to be placed on
the cavity geometries that can be refilled. This must be considered in conjunction with
the positioning of the encapsulated workpiece within the mold during reencapsulation.
Otherwise, if the cavity or the gate is poorly located, air may be trapped, causing
porosity. In addition, if the path is too long or complex, cold shuts may occur as the
metal freezes before filling the cavity completely.
91
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