IC LEARNING SERIES
Rapid Prototyping
& Manufacturing
Technologies
The Hong Kong Polytechnic University
Industrial Centre
IC LEARNING SERIES
Rapid Prototyping &
Manufacturing
Technologies
Suitable for the following learning modules offered by the Industrial Centre:
TM0203 Use of Plastics in Our Society
TM0205 Plastics Technology Practice and Metal Surface Finishing
TM4001 Integrated Training I for ME DG Students
TM4009 Integrated Training for ISE DG Student
TM4011 Integrated Training for PIT HD Student
TM4012 Integrated Training II for PIT HD Student
Rapid Prototyping & Manufacturing Technologies
Rapid Prototyping &
M an u f act u rin g
Technologies
Objectives:




To understand the importance and applications of rapid prototyping and
manufacturing (RP&M) technologies in the product design and
development processes.
To understand the general function of prototype in the product design and
development process.
To appreciate the common types of rapid prototyping systems.
To understand the basic steps in RP process.
To learn the basic of Rapid Tooling process.
1.
Introduction

In recent years, opening up local markets for worldwide competition has led to a
fundamental change in new product development (NPD). In order to stay
competitive, manufacturers should be able to attain and sustain themselves as
"World Class Manufacturers". The manufacturers should be capable in delivering
products in fulfilling the total satisfaction of customers, products in higher quality,
short delivery time, at reasonable costs, environmental concern and fulfill all the
safety requirements.
In many fields, there is great uncertainty as to whether a new design will actually
do what is desired. New designs often have unexpected problems. A prototype is
often used as part of the product design process to allow engineers and designers
the ability to explore design alternatives, test theories and confirm performance
prior to starting production of a new product. Engineers use their experience to
tailor the prototype according to the specific unknowns still present in the
intended design.
Rapid Prototyping technology employ various engineering, computer control and
software techniques including laser, optical scanning, photosensitive polymers,
material extrusion and deposition, powder metallurgy, computer control, etc. to
directly produce a physical model layer by layer (Layer Manufacturing) in
accordance with the geometrical data derived from a 3D CAD model. RP can
deliver working prototype at the early design stage of the new product cycle. So
manufacturers can use the working prototypes in bridging a multi-disciplined
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team composed of the marketing, design, engineering and manufacturing people
to design right at the first instance in catering for the customers.
Rapid prototyping (RP) is rapid creation of a physical model. However, rapid
prototyping is slowly growing to include other areas. So, Rapid Prototyping,
Tooling and Manufacturing (RP&M) should be used to include the utilization of
the prototype as a master pattern for tooling and manufacturing.
1.1
Types of prototype for NPD
Before getting involves to the topic of RP&M, the fundamental concepts and
applications of prototypes should be obtained. Prototypes that will be required in
the product design and development process are commonly divided into the
following types.
•
•
•
•
•
1.1.1
3D sketches
Cosmetic prototypes
Engineering / Functional prototypes
Samples for safety approval
Marketing Samples
3D sketches
Traditionally product designer use foam, cardboard, clay or wood to make quick
models. The designers may generate a lot of simple model a day to visualize
different ideas. They also interchange portions of different models by cut and
paste, or reshape the model to explore different possibility. Since it is the early
stage of the project, usually the models are not presented to outside clients,
therefore colors and textures and surface finishing are not an issue.
Among various RP technologies, Multi-jet Modeling (MJM) and 3D-printing (3DP)
technologies is most suitable for 3D sketches not only because they are faster, but
also because they are less expensive and the models are easier to reshape.
1.1.2
Cosmetic prototypes
As the name implied, the prototypes are used to evaluate the appearance and
feeling of the design, and get early comments from potential clients. Apart from
solid and non-functional, cosmetic prototypes are the same as finished product in
aspect, such as shape, color, texture and hardness.
Most RP technologies can be used for cosmetic prototypes. However, those with
better post-processing properties will have a competitive edge because a lot of
polishing and painting will be applied on the model. In this perspective, SLA &
MJM are easier to polish because of its smooth surface. FDM is more difficult
because the part have to be cover with a layer of putty. SLS and LOM are among
the most difficult, due to the tough nylon material and fiber strands of paper.
'Concept modelers' is the choice where time is the most critical concern. These
include droplet deposition systems such as InVision, Objet and ZCorp's machine.
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1.1.3
Engineering / Functional prototypes
In engineering development, Engineers will evaluate the form, fit and function of
the parts. The prototype will use the same material of the final product if
technology permitted. If exact material is not available, the materials with similar
mechanical, thermal and electrical properties are used. The SLS and FDM allow the
use of production grade plastic material, although the mechanical properties are
not exactly the same.
1.1.4
Samples for safety approval
For those products that are regulated by safety standards of importing country, at
lease 5-10 product samples have to be submitted to the laboratory for each
application. If pilot run of the product is ready, the finish products will be
submitted. However, in most case the project is still in tooling stage, and samples
by PU duplication may be used if the laboratory accepts. 5 to 10 duplicates can be
produced in a day. PU duplicate cannot be used as substitution if the part under
consideration is subjected to high temperature, voltage and mechanical abuse. In
this Rapid Tooling techniques can be used. Manufacturers have a choice to quickly
build a mould and produce a small batch of sample parts in actual production
material.
1.1.5
Marketing Samples
At a later stage when all design details are finalized and the project is in tooling
stage, the marketing department will start the promotion campaign. A lot of
samples may be distributed to clients. In this case, up to a hundred products may
be needed. Normally the samples will be made by RTV mould and PU duplication
technique. In many cases, these models are also used to take the picture that
appears in the package box or user manual.
1.2
Differences between conventional machining and rapid prototyping
Conventional machining can produce prototypes using metal removal method
with almost any type of engineering materials however there are only a limited
type of materials for any particular type of RP&M systems. RP&M systems are
usually used for making one or a few sample of prototype, while the number of
prototype produced in conventional machining can be adjusted as required. The
basic working principle of RP technologies is totally different from conventional
machining processes which build up a solid by mean of addition approach like
construction of the building. Hence the RP also started from foundation then
gradually increase in height until it reached the top layer.
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Fig. 1.1 Differences between conventional machining and rapid prototyping
1.3
Why RP&M
The application of the RP&M in the NPD can resolves the Fuzzy Front End – the
messy getting started period of NPD process - would be relatively costly and
timely. However using RP&M enable manufacturers to schedule the right product
being developed in a timely manner is the most important winning strategy. The
combination of the RP&M and CAD technologies in blending with traditional
technologies and gradually formation of cross-functional team are the strategies
for survival or strategies for empowering organization, people and facilities.
1.4
Limitations of RP&M
High precision RP machines are still expensive at around US$ 100 thousands to 1
million and not easy to justify economically and many service bureaus in providing
the physical prototypes output services. However, as the RP technologies are
getting more mature while RP manufacturers are facing more head-to-head
competition, the price will go down significantly in the near future such as the
launch of V Flash priced at US$ 10 thousands. Most of the budget RP systems are
difficult to build parts with accuracy under +/-0.02mm and wall thickness under
0.5mm. This is sufficient for prototypes built for concept evaluation and functional
test.
However, prototypes that will be used as master pattern for downstream
processes always require a much higher and consistent accuracy. Although there
are quite a large variety of materials that can be used in most RP processes, the
physical properties of the RP parts are normally inferior to those samples that
made in proper materials and by the traditional tooling. The RP parts are not
comparable to traditional Computer Numerical Control machined (CNC) prototype
parts in the surface finishing, strength, elasticity, reflective index and other
material physical properties.
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1.5
The future market profile of RP&M systems
The future market profile of the rapid prototyping/manufacturing industry will
have two specific niches. The first one will be focused on Digital Direct
Manufacturing (DDM) of engineering parts for not only testing form fit, functional
evaluation, but also can be directly used in products. The second niche will be
looking at the market requirement in another point of view. It will focus on
concept modeling or 3-Dimensional printing for design verification, preliminary
marketing tool and manufacturability studies. In this case, the most important
considerations are speed and low cost. The accuracy and resolution requirement is
minimal so far it can provide the designer a reasonable representation of the
design.
2.
Common Types of Rapid Prototyping Technologies
While there are many ways in which one can classify the numerous RP systems in
the market, one of the better ways is to classify RP systems broadly by the initial
form of its material, i.e. the material that the prototype or part is built with. In this
manner, all RP systems can be easily categorized into (1) liquid-based (2) solidbased and (3) powder-based.
Liquid-based RP systems have the initial form of its material in liquid state.
Through a process commonly known as curing, the liquid is converted into the
solid state. The Stereolithography Apparatus (SLA) falls into this category.
Solid-based RP systems encompass all forms of material in the solid state. In this
context, the solid form can include the shape in the form of wire, roll, laminates,
pellets and powders. The Selective Laser Sintering (SLS), Three-Dimensional
Printing (3DP) Fused Deposition modeling (FDM) fall into this definition.
2.1
Stereolithography Apparatus (SLA) – Liquid based RP
Stereolithography Apparatus (SLA) technology fabricates three-dimensional, solid
objects resin using a computer-directed, ultraviolet (UV) laser beam to cure
successive layers of photo-sensitive polymer resin in a vat.
SLA part is fabricated by laser spot instead of continues scanning on the resin
which forms a thin solid layer on the surface of the liquid resin through the
photopolymerization. Figure 2.1 shows the schematic diagram of this spot
scanning mechanism.
Polymerization is the process of linking monomers and oligomers into larger
molecules (polymers). When photoinitiators in the SLA resin are energized by the
laser spot energy exposure (Ec value in mJ/cm2) will undergoes the
photopolymerization process. It creates a thin layer of solid resin with thickness
about 2-3 times thicker than the layer thickness (overcure). The total thickness of
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the layer is depending on the scanning speed of the laser and the Depth
penetration factor of the resin (Dp value in mm).
Resin Surface
Fig. 2.1 Schematic diagram of SLA laser processing mechanism
2.1.1
Processes Description
In SLA process, the software firstly interprets and pre-process the CAD data and
slices it into a series of thin horizontal layers and converted to machine specified
control data files based on the part, building and recoating parameters. The
machine control data is then downloaded into the equipment for part building. A
perforated stainless steel building platform attached to a vertical elevator is
moved to the start position which is just below the resin surface.
An X-Y electronic motor driver optical scanning mirrors directs the laser beam,
which cures the borders and cross sections of the built parts one layer at a time on
the surface of the resin. Photopolymers are converted into solid state instantly
after irradiation of laser beam
The elevator then lowers the newly built layer by a distance of one layer thickness
after a short period of time to allow the newly formed layer to increase the green
strength (pre-dip delay), the vacuum activated re-coater blade which is separated
from the surface of the resin by a predetermined blade gap then coats a new layer
of resin with thickness equal to the layer thickness on the newly formed solid layer,
and the process is repeated until the object is completed.
The elevator then rises out of the resin surface and the object is removed from the
vat for the post processing. Figure 2.2 shows a schematic diagram of SLA system.
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Fig. 2.2 Schematic diagram of stereolithography process
Mirrors
Laser – concentrative UV beam to transom liquid resin
into solid state.
Elevator – control the movement of platform upward
and upward in order to
Platform – a steel plate with plenty of holes as the
basement for part building
Resin vat – contain raw material to form SLA model
Sensor
Mirrors – control the path of movement of the laser
beam at X and Y axis
Sensor – locate the coordinate and instant power of
the laser beam and feedback to the control unit for
fine adjustment
Fig. 2.3 Basic components of SLA system
2.1.2
SLA Material – Liquid Resin
A variety of resin is available for SLA, each with its own advantage and weakness.
Typical SLA resins only react to a narrow bandwidth of UV ray, both Viper Si2 and
SLA-3500 employs a Diode-pumped solid-state (DPSS) lasers Nd:YVO4
(Neodymium Yttrium Vanadate) as a energy source of polymerization. Generally,
the resins need to be kept in an environment with tight temperature and humility
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control in ensuring the reaction conditions. For example, the chamber of SLA is
maintained at 28°C ± 1°C.
Resins used in SLA process are mixture of photo-initiator, linkers, oligomers and
monomer mixture in liquid state, photonic energy source will trigger the chain
reaction of polymerization. Two parameters penetration depth (Dp) and critical
exposure (Ec), which are vary from different resins, are as the primary input to the
SLA machine for controlling the power and time of laser emission. The following
illustrate the underlying principle of the two important parameters;
Penetration Depth (Dp) and Critical Exposure(Ec)
When a beam of light hit the resin surface, it will cure a region of resin in the
shape of a bullet. The intensity of the beam determined the extent of reaction and
the size of the bullet.
The threshold exposure, Ec, is the energy required for the photopolymer changes
from the liquid to the gel phase. In the process, Dp and Ec is primary characters of
the resin provided by manufacturer. The SLA machine determine PL and Vs on the
fry by the relation that the draw speed of the laser Vs proportional to (PL/EC)exp(Cd/Dp).
Cp
Critical depth, given by the function
Cd=Dpln(Emax/Ec)
Dp
The depth of penetration which the
power of laser decay to approx. Emax/3
Emax
The power of laser at resin surface
Ec
Critical Exposure, a primary resin
character, unit in mJ/cm2
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Physical properties of SLA material available in IC
Accura 25
Accura 55
Accura 60
Accura
Bluestone
Appearance
White
White
Clear
Opaque Blue
Penetration
Depth(Dp)*
4.2mils
5.2mils
6.3mils
4.1mils
10.5mJ/cm2
7.4mJ/cm2
7.6mJ/cm2
6.9mJ/cm2
Tensile Strength
38MPa
63-68MPa
58-68MPa
66-68MPa
Elongation at
Break(%)
13-20%
5-8%
5-13%
1.4-2.4%
@66PSI
58-63oC
55-58 oC
53-55 oC
65-66 oC
@264PSI
51-55 oC
51-53 oC
48-50 oC
65 oC
Critical
Exposure(Ec)*
267-284 oC
@66PSI with
120oC Thermal
Postcure
Hardness, Shore D
80
85
86
92
For more information about SLA material, please refer to 3D systems web site
(http://www.3dsystems.com/)
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2.1.3
Support Structure
Support usually need for liquid-based and solid-based RP systems in order to anchor
the part to the platform hence the part can be separated from the platform thus
preventing floating layers and make removal of the part became simple.
Also support is needed for overhanging and tilted surfaces hence minimize part curl
and stabilize the part while being built. Self support angle is a predetermined angle
for the particular SLA RP system such that tilted surfaces with this angle from the
horizon needs no support.
The supported surfaces are the overhanging down face of the part. Since supports
need to anchor the surface at the contact point. Hence the surfaces in contact with
the support are rough and coated with resin gel after the part build completed. So do
not place supports on surfaces where finish is important.
Part
Part
Projection
Stand
Fig. 2.4 SLA support structure, Left: curtain support. Right: fine-point support
Support can be classified according to the support structure and the type of surfaces
being supported. Support types include solid, box, web and fine point supports.
Besides supports classified according to the geometry of the down face include point,
line and surface supports.
SLA uses Fine-point support style as the default support style, however when long
and slender support is need then Curtain support style can be used to ensure the
strength of the support while the prototypes are build in the vat. Figure 2.4 shows
both SLA support styles.
2.1.4
Building Styles
The rapid prototyping systems usually bundled with software which can control the build
styles, generate supports, slicing of the STL files and creating control file for building
the prototype in the RP machine. There are Part build style, Support build style, Part
Recoat style and the Support Recoat style in the SLA software called 3DLightyear.
Part build style controls the laser when drawing the part. Different part build style
can produce part in different patterns which include the solid part – the exact build
style, the hollow part for investment casting – Quickcast build style. The fast build
style which shortens the build time of the part by filling the part in alternate layers.
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Support build styles control the laser when drawing the support. Curtain support or
Fine point support can be created by choosing different support build styles in SLA
building platform.
Part recoat styles controls the recoating process including the dipping depth and
speed of the build platform and the speed and the number of recoats of the recoat
blade.
Support recoat styles controls the recoating process while building the support.
2.2
Fused Deposition Modeling (FDM)
2.2.1
Processes description
Fused Deposition Modeling (FDM) was developed by Scott Crump, the founder of
Stratasys. It was commercialized by Stratasys in 1991. FDM process create component
by extruding material (normally a thermoplastic material) through a nozzle that
traverses in X and Y to create each two-dimensional layer.
Heaters which surround two separated nozzles keep the plastic at a temperature just
above its melting point so that it flows through the nozzle and forms the layer
according to the tool path. In each layer separate nozzles extrude and deposit
material that forms the parts and the support structure. The plastic hardens
immediately after flowing from the nozzle and bonds to the layer below. Once a layer
is built, the platform lowers, and the extrusion nozzle deposits another layer and the
process is repeated until the object is completed. Figure 2.5 show a schematic
diagram of FDM process.
Fig. 2.5 Schematic diagram of FDM processes
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Support structure can be removed manually or, when water soluble supports are
employed, they can be simply dissolved by put into particular chemical solution.
However, soluble support is only available for building ABS model, other high melting
point plastic material such as polycarbonate (PC) and polysulphone (PPSF) are not
applicable.
2.2.2
FDM material
The build material is usually supplied in filament form with 1.8mm diameter, but the
layer thickness and vertical dimensional accuracy is determined by the extrusion
nozzle diameter and the rate of feeding of the building material, which ranges from
0.018 to 0.008 inches. In the X-Y plane, 0.001 inch resolution is achievable.
Thermoplastic material is most often used which including Acrylonitrate Butadiene
Styrene (ABS), Polycarbonate (PC), polysulphone (PPSF), and investment casting wax is
designed for investment cast.
ABSi
Appearance
ABS-M30*
PC-ABS
Ivory
PC*
ULTEM908
5
White
PPSF*
Tan
Tensile Strength
(MPa)
38
36
34.8
52
72
55
Tensile Modulus
(MPa)
1993
2413
1827
2000
2220
2068
Flexural
Strength (MPa)
59
61
50
97
115
110
Flexural
Modulus (MPa)
1797
2317
1863
2137
2507
2206
139
123
53.39
106
59
95
96
326
127
153
189
Density g/cm )
2
1.09
1.04
110
1.2
1.34
1.28
Elongation at
Break (%)
>10
4
1.2
3
4
3
Support
Structure
Soluble
Soluble
Soluble
Bass
Bass
Bass
Notched Izod
Impact (J/m)
Heat deflection
(oC)
*Material Available in IC
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2.2.3
Support Structure
Supports in the form of solid type or box type are usually required for the Solid-based
RP system. Box type support is used in FDM. The support material used for FDM
depending on the type of model mater used for making the prototype and can be
classified into two main type – soluble support and break away support. The soluble
support uses a "water-soluble" material which turns into liquid in an ultrasonic
heating tank filled with hot (70 degree F) amino based water solution for 4 hours, the
support is then dissolved in the tank. However the soluble support is only suitable for
ABS grade model material as shown in the table above. Break away support is used
for model material which melts at a higher temperature than ABS. Break away support
need to be removed by hand tools upon the completion of the building of the
prototype.
The self support angle of FDM is 45 degree and 4 types of support styles are available
for the creation of support namely Basic support, Sparse support, Surround support
and Break away support. The support curves can be created after the appropriate
supports style is determined.
Basic supports will create supports under all part features that are exposed to air on
the underside. The top layer of each support column will have a solid fill and the
layers underneath will have a small air gap between toolpath passes. These passes are
called the support raster curves. This type support is particular suitable for model
with fine details in the down face however the build time is long and the more
support material is used compared with the Sparse support.
Sparse supports will create supports that use less material than basic supports. This is
accomplished by creating a new raster fill pattern that has a much larger air gap
between raster. Like basic supports, the top layer of a support column is built with a
solid pattern of raster. The next several layers have the basic raster pattern that has a
small air gap. Progressing downward, a new sparse support region is used below the
several layers of basic support raster fill. The sparse support regions use a much larger
air gap when building the tool path raster. In addition, the region is surrounded by a
single closed tool path-perimeter curve.
Surround supports are used to surround small features or parts that require few
actual supports, but cannot stand upright on the platform during the part build.
Surround supports will create the support tops just like basic supports but will also
surround every layer of the model with a minimum thickness of supports.
Break-away supports are similar to sparse supports but consist of boxes instead of a
continuous raster. There is no closed toolpath-perimeter curve around the breakaway supports. They are easier to remove than other support styles for some
materials but build slower than sparse supports. Break-away supports are not
recommended for use with WaterWorks.
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2.2.4
Toolpath
The software which bundled with FDM is Insight which transforms the STL file into a
CMB file. The CMB file contains information about the hardware setup, the tooth path
(Road) for building the part and the details which control the movement of the
building platform and the FDM head. Curves in Insight are group into different set
namely part set and support set. The set contain information about the fill up pattern
of the empty space inside the slice curve.
Fig. 2.6 Example of FDM Toolpath
The fill up pattern usually contain a perimeter and a raster fill as shown in figure 2.7.
Different fill pattern for support and model can be achieved by configuring the
parameters of the fill so 4 types of support styles is available to suit different
conditions.
Fig 2.7 Illustrations of Raster and Perimeter
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The fill up pattern of the part need to be investigated layer by layer to ensure there is
no overfill. Overfill occurs when too much material is squeezed into a geometry of
less volume. Underfill occurs when the software doesn't detect enough room in which
to squeeze the raster road. Overfill will hinder the parts surface finish or even cause
part to topple over. Hence overfill usually need to be fixed before the CMB file is
loaded to the machine for building. Figure 2.8 shows the overfill and underfill of
deposition.
Perimeter
Ovefill Raster
Underfill raster
Fig. 2.8 Descriptions of overfill and underfill
2.3
Selective Laser Sintering (SLS)
Selective Laser Sintering (SLS) was developed and patented by Ross House-holder in
1979, but it was commercialized by Carl Deckard at the University of Texas in Austin in
mid-1980s. DTM Corporation was first the company taking this technology into
commercial market and it was acquired by 3D Systems in 2001. The major competitor
of 3D systems; EOS, a Germany based company existed in the market in 1994, which
still enjoys a significant market share. The major difference of the machine among 3D
systems and EOS is that 3D’s SLS machine can be processed multi material including
plastic, metal and resin sand but EOS’s machine is targeted for single application, i.e.
EOS-S machine is solely for sand material processing.
2.3.1
Processes Description
The basic concept of SLS is similar to sterolothography, but the powdered polymer
and/or metal composite material is sintered or melted by a laser that selectively scans
the surface of a powder bed to create a two-dimensional solid shape. Three
dimensional object is then created by attaching together of the two-dimensional
layers.
As in all rapid prototyping processes, the parts are built upon a platform that adjusts
in height equal to the thickness of the layer being built and the commonly used layer
thickness is 0.15mm. After every layer of scanning, building platform lower one layer
thickness distant and a roller convey a new layer of material on top of the part surface
for the next scanning and the process is repeated until the object is formed. Figure
2.9 show a schematic of the selective laser sintering process.
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Fig. 2.9 Schematic of selective laser sintering process
(Source: CustomPartNet)
Since the sintering operation is performed by high power laser, the building platform
and the powder bed need to be pre-heated by infrared heaters and kept in a certain
temperature during processing to avoid part deformation. Nitrogen gas, as protective
gas, is launched into the working chamber throughout the sintering process to
prevent oxidization reaction. Unlike SLA, special support structures are not required
because the excess powder in each layer acts as self-support function while the part
being built except processing of metal powder.
Direct Metal Laser Sintering (DMLS) is the parented term used to describe the process
of metal powder sintering developed by EOS Company. The major competitive
advantage among DMLS and SLS provided by 3D systems is no further infiltration
process required in which SLS metal “green” part is required to be placed into a
furnace, temperature in excess 900 °C, for removal of polymer binder and infiltrating
with bronze.
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2.3.2
SLS Material (Polymer)
Physical Properties
Units
PA
PA-GF
Alumide
Tensile Modulus
N/mm²
1700+/- 150
3200 +/- 200
3800+/- 150
Tensile Strength
N/mm²
45 +/- 3
48 +/- 3
46 +/- 3
%
20 +/- 5
6 +/- 3
3.5 +/- 1
N/mm²
1240 +/- 130
2100 +/- 150
3000 +/- 150
Charpy – Impact strength
kJ/m²
53 +/- 3.8
35 +/- 6
29 +/- 2
Charpy – Notched Impact
Strength
kJ/m²
4.8 +/- 0.3
5.4 +/- 0.6
4.6 +/- 0.3
Izod – Impact Strength
kJ/m²
32.8 +/- 3.4
21.3 +/- 1.7
NA
Izod - Notched Impact
Strength
kJ/m²
4.4 +/- 0.4
4.2 +/- 0.3
NA
Ball Identation Hardness
kJ/m²
77.6 +/- 2
98
NA
Shore D-hardness
kJ/m²
75 +/- 2
80 +/- 2
76 +/- 2
Heat Deflexion t°
°C
86
110
130
Vicat Softening
Temperature B/50
°C
163
163
169
Vicat Softening
Temperature A/50
°C
181
179
NA
Elongation at Break
Flexural Modulus
Remark: PA is Polyamine (Nylon), GF is glass fiber reinforcement, Alumide is PA powder mix
with aluminum powder; the materials are used in EOS INT P system.
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2.4
Three Dimensional Printing (3DP)
The three Dimensional Printing (3DP) technology was invented at the
Massachusetts Institute of Technology and licensed to several corporations. 3DP
indeed is the innovation and further evolution of two-dimensional printing
technology. The process is similar to the Selective Laser Sintering (SLS) process,
but instead of using a laser to sinter the material, an ink-jet printing head
deposits a liquid adhesive that binds the material. The following mainly describes
3DP technology which is employed in Z-corp 3D printing.
2.4.1
Process description
The 3D printing process begins with the powder supply being raised by a piston
and a leveling roller distributing a thin layer of powder to the top of the build
chamber. A multi-channel ink-jet print head then deposits a liquid adhesive to
targeted regions of the powder bed. These regions of powder are bonded
together by the adhesive and form one layer of the part. The remaining free
standing powder supports the part during the build. The processes repeated and
solid model is competed and underneath inside the powder bed. Figure 2.10
show a schematic illustration of the 3DP process
Input Binder Material
Print Head
Feeding
piston
Fig. 2.10 Schematic of 3D printing technology
After the part is finished, the loose supporting powder can be brushed away and
the removed by vacuum cleaner. Since the “green part” is very fragile, infiltration
of glue/ epoxy is needed to be applied on the part surface to improve the
strength of part. It should be noticed that any loose supporting powder should
be removed completely before performing infiltration to avoid generation of
ugly surface.
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2.4.2
3DP Material
Material used in Z-corp 3DP include plaster, starch and ceramic powders, plaster
powder is commonly used for making concept design model, it is most suitable
for concept proofing as the build speed is very fast, typically 2-4 layers per
minute. Starch powder is particular used to produce master patterns for
investment cast since it can be vaporized in high temperature. Ceramic powder
coated with resin is specially designed for making sand casting mould, i.e. Z-cast
is a special mixture of sand particle and resin for casting purpose.
Although the build speed of 3DP is relatively fast compared with other RP
processes, post-process is time-consuming and laborious. Infiltration is usually
performed by manual dipping of glue or epoxy resin. Wax-dipping has being
developed and utilized to speed up the post-process but the part strength is
much weaker than glue infiltration. A new powder system zp140 has been just
developed in which the parts can be cured by dipping into water or spaying on
the part surface.
3.
Workflow of RP processes
There are basically three stages of building physical RP models based on the
CAD data, namely the pre-processing, building and the post processing.
Whatever the CAD model is generated by solid modeling or surface modeling
approaches, most of RP machine accept only STL file as digital data input in
which most commercial Engineering CAD system is capable to convert 3D CAD
model into STL format. The STL model is then slice into layers data set and
transfer to machine for building model. Completed RP model is then performed
corresponding post-processed operations such as cleaning, post-curing and
infiltration. Figure 3.1 shows the typical workflows of RP processes
Fig. 3.1 RP processes workflow
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3.1
Pre-processing
The first step in the RP&M process is virtually identical for all of the various
systems, and involves the generation of a three-dimensional computer-aided
design model of the object. A good, preferably solid CAD modeling or a total
enclosed surface – water tight – CAD model is a key component of success RP
processing. The CAD file is then translated into a triangulation tessellated STL
format, which is the standard of the RP&M field. Figure 3.2 shows a typical
example of STL model which is composed of triangles and each triangle is
described by a unit normal vector direction and three points representing the
vertices of the triangle.
Fig. 3.2 Example of STL model
3.1.1
Verification and fixing of STL file
When one create 3D surface model using common surface modeler, there is very
little concern on the orientation of the surface normal, as most of the tool path
generation algorithms can detect the material side correctly.
However, when one generate STL data using these surface models, many
converters just use the normal data straight from the NURBS surfaces – free form
surfaces, thus the STL files generated are not useable without repair.
Triangles in an STL file must all mate with other triangle at the vertex and must
be properly oriented to indicate which side of the triangle contains mass. Many
STL viewers like Magics RP from Materialise read a STL file to analysis and to
correct, the connectivity and gap in the three-dimensional triangle matrix. This
process is totally automatic and simple to operate. Figure 3.3 shows a “bad” and
“good” STL file.
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Hole exists in STL file
Hole is filled by
“Automatic Fixing”
Fig. 3.3 “Bad” STL file is fixed “Good” STL file
3.1.2
Part Orientation
Part orientation has a significant effect on the final part quality and prototyping
cost. The switching between individual layers takes a significant part of the
overall building time and hence must be properly optimized hence to reduce to
building time and the building cost. The part should be orientated with minimum
height in order to reduce the number of layers.
For processes that need supports structures, part orientation should also be
optimized such that it would require minimal support hence reduce to building
time and the building material. The ultimate strength of the part will be affected
by its orientation within the print box. The part will be strongest along the Y-Axis
and the X-Axis and less strong along the Z-Axis which is illustrated in figure 3.4
Fig. 3.4 Effect of orientation to part strength
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Furthermore, staircase effect will be appeared on the near flat curve surface.
Hence proper orientation can produce a smooth external curve surface of the
prototype. In addition the minimum wall thickness of the part can only be built in
some particular orientation.
3.1.3
Support Generation & Editing
The rapid prototyping systems usually bundled with software which allows the
automatic creation and editing of the supports. The software will initially
generate the supports for all overhang regions based on the default support
parameters; figure 3.5 shows the problem of missing support for overhang
geometry. After the creation, the support structure can be individually modify,
edit, delete or add based on the part geometry. The region by region editing or
customization for supports generation has strengthened the essential support
and also minimizing the building of unnecessary supports.
Desired part or model
geometry
Without supports, overhanging areas of
part may peel away and damage the
whole model
Fig. 3.5 Result of missing supports for overhanging areas
Projection
Down-facing Region
Stand
Up-facing Region
Fig. 3.6 SLA Fine-point support
Support can reinforce delicate part while building and in the post processing
stage. However over support for delicate part will increase the difficulties of
support removal and even destroy the fine details on the down face. Figure 3.7
shows an example of over support.
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Fig. 3.7 Over support of SLA part
3.1.4
Slicing (Layer thickness)
A STL model used for RP contains a collection of planar triangular surfaces. These
faces define an approximate boundary for the object. Horizontal layers of equal
thickness are produced while slicing to produce the outer boundary of the part –
slice curve. Then void and solid region of the slice curve can be identified and
proper fill pattern can be created for part filling. Typical layer thickness of
commonly RP system is ranged from 0.05mm to 0.15mm.
3.1.5
Part Building
The prototype can be built in the RP machine according to the toolpath and
control codes generated by the software of the RP system. By laser scanning,
disposition, sintering, etc and under limited working envelope with closed
control of the processing environment the machine will start the build at the
bottom layer. Subsequent layer is added after the completion of the previous
layer until the final layer was build. Hence, RP process also refers to layer
manufacturing.
3.1.6
Post processing
Once the last layer on the part has been built, the prototype need to have
undergo some post processing processes such as removal of support, cleaning,
depowder, drain excessive resin, post curing in SLA, infiltration of resin/wax, etc.
The post processing is aimed to clean and reinforce the green part.
Additional surface finish procedures such as sanding, sand blasting, painting or
even electroplating are normally employed for cosmetic prototypes.
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4.
Rapid Tool Production
Only when the production quantity is massive that the expensive tooling cost can
be justified. As a result, the way of how to produce tooling quicker and more
economically especially for small batch manufacturing becomes a big concern.
Furthermore, in the product design and development process, there is always in
need of some intermediate tooling to produce samples for marketing, functional
test, or production process planning and evaluation purposes. In this respect, RT
is the ideal mean to fit the needs.
Rapid Tooling (RT) is the result of combining Rapid Prototyping techniques with
conventional tooling practices to produce small quantity of plastic or metal
components from electronic CAD data directly or indirectly. Direct RT technology
such as Direct Metal Laser Sintering (DMLS) which fabricate production tooling
from CAD data whereas Room Temperature Vulcanization (RTV) silicone rubber
mould is the most commonly used indirect approach for plastic components
duplication.
4.1
Type of RT Processes
Low volume (from 10 to 100)
•
RTV Soft Tooling
Intermediate volume (from 100 to 1000)
•
•
4.2
Metal filled Epoxy Tooling
Direct Metal Laser Sintering
RTV Soft Tooling
RTV silicone rubber mould is one of the important kinds of soft tooling which is
an effective, high fidelity and inexpensive way to create multiple copies of a
master prototype part. Indeed, this technology has been used by the industry for
many years. The only thing new is that the master pattern is produced by the RP
technology. The other common kind of soft tooling material is tooling grade
Polyurethane.
This technology is not specially designed for a particular RP process. In fact,
master patterns produced from most RP processes are suitable to apply for this
technology.
RTV molds can faithfully duplicate details and textures present on the master
pattern. Apart from detail, geometry can also be fully duplicated from the master
part when prototypes are removed from the mould. Great care should be taken
to ensure that the pattern is in perfect condition. After RTV mould is completed,
it can be used to further produce limited quantity of prototypes with a wide
variety of material properties.
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Its application is mainly to produce plastics or metal prototypes in small batch by
vacuum assisted casting or gravity casting method. The casting materials
normally used are PU, polyester, epoxy, tin-lead alloy (200 °C), pewter (230 °C)
and zinc alloy (400 °C).
The batch size is from several pieces to hundreds. Multiple moulds, sometimes,
are required depend on the complexity of the parts. In fact, the ease of
producing multiple moulds is one of the advantages of this technology.
However, the process is tedious and required high skill workers attendance.
4.2.1
Process Description
One-piece mould approach
This process begins with a master pattern which normally output from RP system
directly. It requires to sand and polish the part surface well since the RTV mould
will reproduce any and all surface defects on the master pattern, and in turn will
transfer them onto the final model. Then, the pattern attached with gating
system is mounted in a mould box. Silicone rubber is mixed with specific amount
of hardener and poured into the mould box. Degassing in vacuum chamber is
preferred to avoid trapped air caused by mixing and pouring. After solidification,
the mould is then spitted into mould halves according to the predetermined
parting lines. Figure 4.1 shows the RTV mould making process of one piece
mould approach.
Fig. 4.1 Processes of making RTV mould (one-piece mould approach)
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Two-pieces mould approach
Firstly, master pattern is prepared and laid horizontally inside a mould box with
the parting line built up by hand with model clay. Prepared silicone rubber is
poured inside the mould box to form one half of the mould. After solidification,
clay is removed, the other half of the mould is produced by repeating the above
steps with the master pattern turned upside down. Figure 4.2 shows the different
parting line surface of one-piece mould and two-pieces mould.
Fig. 4.2 Left; one-piece mould Right; two-pieces mould
Two-pieces mould is typically required when the parting line is difficult to be
determined. Two-pieces mould approach requires less skill compared with onepiece mould. However, it requires double time to make the mould and the
dimension accuracy of part is poor than one piece mould.
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4.2.2
Type of RTV Silicone Rubber
RTV silicone rubber can be divided into two types; condensation and addition
types, the following table shows the differences among them.
Condensation Type
Features
Lower costs
Broader product range
Less sensitive to exact mix
ratio
Accelerator
catalysts
available to speed up cure
•
•
•
•
Mixing of grades possible to
achieve desired hardness
•
Addition Type
• Low shrinkage, below 0.1%
• Marginally higher tensile
strength
• Slightly tougher rubber
• Need for careful and accurate
mixing
• Good abrasion resistance
• Can be accelerated using heat
• Tolerant to the addition of
silicone fluid as a softener
Shrinkage
Slightly higher than the addition
type
Low
When
Temperature
Increase
Curing Time decreases
Curing Time decreases
o
(5 C----7hrs;
o
(50 C-----2hrs;
o
25 C-----5hrs;
o
100 C-----30min;
o
50 C-----4hrs(max temp))
o
150 C-----10min;)
When
Humidity
Increase
Curing Time decreases
Curing Time decreases
When
Quantity of
Curing Agent
Increase
Curing Time decreases but there
are limits
Curing speed will not be altered
but will adversely affect the
physical properties.
Curing
Impediments
No
Yes
Remarks
By product will come out.
Dimensions of mould will be
slightly affected. (about 0.5%)
Warm the rubber mold to the
curing temperature before pouring
resin into a thermally cured the
rubber mold to raise dimensional
accuracy.
*Quantity of curing agent must be
carried out as accurately as
possible.
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Mixing Ratio is a term used to state the amount of each material to be mixed in
a multi-component material. The mixing ratios for two-part RTV products are
described on the individual product data sheets and are given as a ratio by
weight of each material.
Pot life is the length of time that a catalyzed resin system retains a viscosity low
enough to be used in processing. This is also known as WORKING LIFE or
USABLE LIFE.
Curing condition is the condition which can provide the optimum properties for
a silicone rubber. This typically depends on time and temperature.
4.2.3
Considerations of making RTV mould
To duplicate a good product by soft tooling, planning is very important as the
part may be blocked inside the mould if the parting surface is not well defined.
All holes should be filled by tapes to separate individual mould half before
pouring of silicone rubber.
For master pattern with flat surface, the parting surface should be located on the
flat surface and use plastic type as a marking. For master pattern with free-form
geometry, parting line should be located on the position which gives lowest
effect to the appearance.
It is important to evaluate the pattern if there are any undercuts that would lock
the casting in the mold. Small undercuts can be ignored since the silicone rubber
is flexible material. To deal with some deep holes feature such as boss, metal
inserts can be used to replace the silicon material by attaching the inserts to the
part before pouring of moulding material.
4.3
PU Casting
Even RTV mould can be used for different material, PU is the most popular
material accompany with RTV mould for producing plastic components. There
are two different approaches for PU casting, Top-down approach (direct gate)
and Bottom-up approach (by gravity).
For Top-down approach, PU is poured into the mould directly through a gate
connected on top of the part. Firstly, weighted material and RTV mould are put
into a vacuum chamber for degassing. PU resins are mixed together inside a
vacuum chamber, it is then poured into the mould via a funnel and air is
reintroduced simultaneously to force the PU material into the mould. This
method is suitable for casting small components such as jewelry that has fine
detail. Figure 4.3 shows the processes of PU casing by this top-down approach.
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Fig. 4.3 Steps of PU casting
For Bottom-up approach, PU material is poured into the RTV mould and filled
into the moulding cavity by gravity. Vent channels are need to be added at the
top areas of part to allow air out. This method is most suitable for large casting
part in which the mould is unable to put inside a vacuum chamber. Figure 4.4
shows the different set up of top-down and bottom-up approaches
RTV Mould
Material input
Vent channels
Material input
Cavity
Fig. 4.4 Left; top-down approach Right; bottom-up approach
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4.4
Epoxy Tooling
Epoxy tools are used to manufacture parts or limited runs of production parts.
Expoxy tools are used for:
•
•
•
•
•
Plastics injection prototype Mould
Mould patterns for casting
Vacuum forming moulds
Sheet metal forming moulds
Reaction injection moulds
Mould that is made of plastics is built from casting some special grade epoxy
resins directly onto the RP master model. This mould making method does not
require high precision machine tools as with conventional metal mould
production. This technology of direct transversal from the master model allows
large reduction in mould production costs and time.
In the past, plastics materials are not suitable for injection mould due to the lack
of strength and the high shrinkage during curing. Many problems arise such as
damage during mould making and moulding process. However, a special grade
epoxy resin is developed for better strength and stiffness. Epoxy resin is a
thermoset plastic reinforced with composite materials that can be cast to shape
before cured. This special grade epoxy resin is aluminum powder filled for
strength, stiffness and thermal conductivity improvement.
The mould made by this process is only suitable for injection moulding of
plastics parts. Common plastics materials like ABS, POM, etc. can be produced
from this mould in small batch size up to 3,000 pieces.
4.4.1
Process Description
The process of producing epoxy tooling is somehow similar as making RTV twopiece mould but usually double duplication techniques is used. A RP master
pattern is sanded and polished and the parting line is formulated by clay. A thin
layer of mould release agent is applied on the surface of the master pattern, RTV
is then poured into the mould box. The RTV is used as the negative mould
master patterns for casting of metal epoxy in forming the mould cavity and
mould core. As the density and viscosity of epoxy resin is relativity high,
degassing is carried out by several times in order to extract out all of the trapped
gases. After pre-curing, the master pattern is removed and the mould halves are
put into oven step by step increasing the temperature to 280℃ for post thermal
curing.
The mould halves are then turned to CNC machine for producing the sprue,
runner, gate and ejecting system. The mould is completed and ready for plastic
injection.
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4.5
Direct Metal Laser Sintering (DMLS)
Direct Metal laser Sintering (DMLS) builds solid metal parts directly from
powdered metals. It always used to build simple rapid tooling because of short
lead times, eliminate the cavity machining required. For advanced, cooling
channels and inserts can also be built in the rapid tools. Rapid tools using harder,
tougher materials can be used to inject hundreds to thousands of plastic parts.
Fig. 4.5 Example of DMLS insert for injection moulding
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References
•
Wohlers, Terry T. (1999), Rapid prototyping & tooling, state of the
industry : 1998 worldwide progress report, Fort Collins, Colo. :
Wohlers Associates, Inc.
•
Friedrich B. Prinz (1997), JTEC/WTEC panel on rapid prototyping in
Europe and Japan : final report, Baltimore, Md.: Rapid Prototyping
Association of the Society of Manufacturing Engineers
•
Lamnont Wood (1993), Rapid Automated Systems: An Introduction,
New York: Industrial Press
•
Dearborn, Mich. (1994), Rapid prototyping systems : fast track to
product realization : a compilation of papers from Rapid Prototyping
and Manufacturing '93, Society of Manufacturing Engineers in
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•
Chua Chee Kai, Leong Kah Fai. (1997), Rapid prototyping : principles
& applictions in manufacturing, New York : John Wiley & Sons
•
Marshall Burns (1993), Automated fabrication : improving productivity
in manufacturing, Englewood Cliffs, N.J. : PTR Prenrice Hall
•
Jerome L. Johnson (1994), Principles of Computer Automated
Fabrication, Irvine, Calif. : Palatino Press, c1994
•
D. Kochan (1993), Solid Freeform Manufacturing - Advanced Rapid
Manufacturing, Amsterdam ; New York : Elsevier (Request)
•
Joseph J. Beaman (1997), Solid freeform fabrication : a new direction
in manufacturing : with research and applications in thermal laser
processing, Dordrecht ; Boston : Kluwer Academic Publishers(Request)
•
øyvind Bjørke (1996), Layer manufacturing : a tool for reduction of
product lead time, Trondheim, Norway : Tapir Publishers
•
Graham
Bennett
(1996),
Second
National
Conference
on
Developments in Rapid Prototyping and Tooling, Bury St. Edmunds :
Mechanical Engineering Publications
•
Preston G. Smith, Donald G. Reinertsen (1991), Developing products
in half the time, New York : Van Nostrand Reinhold
Bibliography
http://www.custompartnet.com/
Rapid Prototyping Journal, Bradford, West Yorkshire, England Birmingham, AL: MCB
University Press Ltd.
Prototyping Technology International, Surrey : UK & International Press
Time-compression technologies. Europe, Tattenhall [England] : Rapid News Pub
The rapid prototyping directory, San Diego, Calif. : CAD/CAM Pub
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