International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Investigation of process parameters for an
Injection molding component for warpage and
Shrinkage
Mohammad Aashiq M1, Arun A.P1, Parthiban M2
1
2
PGD IN TOOL & DIE DESIGN ENGINEERING-PSG IAS
ASST.PROFESSOR DEPARTMENT OF MECHANICAL ENGINEERING-PSG CT
Coimbatore – 641004, India
Abstract— The purpose of the research is to explore the
influence of different mold temperatures on the warpage &
shrinkage of the injection molded component’s. The simulation
software MOLDEX 3D was used for this study, the simulations
were done by varying different mold temperatures and their
corresponding warpage & shrinkage were collected. It was found
that the different mold wall temperature causes the asymmetrical
polymer flow in the cross-section due to which the asymmetrical
structure in the parts cross-section occurs and this was observed
using the flow analysis software. So it is required to assure
homogeneous mold wall temperature across the entire cavity
during the production of injection molded parts. This research
finally concludes that warpage and shrinkage decreases for
increased values of mold temperature.
Keywords: Injection molding, Warpage, Shrinkage, Flow
Analysis.
I. INTRODUCTION
Injection molding is a process where by a solid
thermoplastic material is heated until it reaches a state of
fluidity, is then transferred under pressure(injected) into a
closed hollow space (mold cavity),and then cooled in mold till
it reaches a solid state, conforming the shape of mold cavity.
Assuring the proper mold temperature for a specified polymer
is a very important issue as well as keeping the temperature
constant and equal across the cavity surface. Differences in
mold temperature can lead to the problems with manufactured
parts like warpage [1-3]. The reason of this are stresses in the
parts.Since polymer with higher temperature exhibits more
intensive shrinkage than in lower temperature, the temperature
differences created during part cooling in the Mold lead to
part distortion. The deformation of a simple injection molded
part due to less and high temperatures are shown in Fig. 1.
Part becomes concave from the Mold “hot” side and convex
from the Mold “cold” side. The main cause of warpage is the
difference in internal stresses produced in the material due to
differential shrinkage.It needs to be addressed early on during
the molding process as the defects are hardly rectifiable later.
ISSN: 2231-5381
Fig. 1.Warpage of injection molded part due to different mold temperature.
The non-uniform mold temperature in the mold plate can be
also problematic in case of multi cavity Molds. If the parts are
formed in different temperature they can differ in structure
and properties
after manufacturing
process
[2].
The differences in properties are significant for semi
crystalline polymers that crystallize during solidification
process. The problem of different properties among the parts
from one mold is especially important in case of molding
small parts in molds with many cavities. Moreover, the
different temperature can also affect polymer flow in runners
of multi cavitymold and this can lead to non-simultaneous
filling
of
the
mold
cavities
[2].
Computer simulation programs are very good tools for
prediction of quality problems with injection molded parts [2].
It is possible to predict polymer flow inside the cavity as well
as other physical properties distribution of melt across the
entire cavity like pressure, shear stress, shear rate, temperature
etc. Weldlines and air traps are also identified. Minimum
warpage optimization can be done by CAE analysis by
processing parameters optimization [2].
Gate location
analysis also can be done to achieve this. The warpage is also
dependent on presence of filler in the plastic as well as on the
kind of filler. Particularly it can be different across the part
depending of the filler orientation [3]. The warpage is
dependent also on processing conditions. Computer
simulations are helpful with warpage optimization.
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
In case of thin-shell parts the melt temperature and holding
pressure are important parameters. The holding pressure is
especially important with minimizing the shrinkage of
semicrystalline polymers, like POM.The shrinkage can be also
the reason of warpage. To minimize warpage it is important to
assure the uniformity of the temperature across the part. It will
also result in preventing sink marks and different shrinkage in
the parts [2]. The cooling of the parts in injection molding
plays an important role. It is done by cooling channels in the
mold. These channels can be designed in many ways in the
same Mold but uniform mold temperature should be assured.
When there is only one and long cooling channel the coolant
heats up in the mold and is not efficient. One of good
solutions is to use special manifolds and design many coolant
inlets and also many outlets in the mold [3]. Sometimes
special injection molding techniques are used to avoid quality
problems with parts. For example to avoid sink marks in parts
with thick walls polymer with foaming agent is used. It makes
a possible manufacturing thick-walled part which is very
difficult in conventional injection molding because thick layer
of polymer is cooled very slowly and exhibits very high
shrinkage. This leads to sink marks occurring and to the
warpage problems. Minimizing warpage is also possible when
using compression injection molding, which is especially
recommended for thin-walled parts and parts with application
for optics. The reason of minimal warpage after this
manufacturing process are very low internal stresses in
injection Molded parts [1]. The reason of warpage is usually
unequal Mold temperature or differences in part thickness. In
addition, it is amplified by intensive shrinkage of fluid core
inside the part. If the fluid core is placed not symmetrically in
the middle of the part cross-section, one half of the part
contracts more because of intensive shrinkage . If melt
polymer flows inside a Mold channel (runner or gate) the flow
front is formed in a fountain flow [1]. The flow front is
symmetrical to axis of symmetry, placed in the half-distance
between Mold walls, if only the conditions of flow are the
same near both Mold walls – equal temperature, surface
roughness etc.
Fig. 2. The scheme of melt behaviour for less temperature
Fig.2. shows how a flow front of plastic material inside the
cavity plate will be when the mold temperature is less(for
example :30°C).The flow of plastic material will be different
for higher mold temperature.The flow (for example :60°C) is
shown in Fig.3.
ISSN: 2231-5381
The flow is symmetry since the mold temperature is high.
Fig. 3. The scheme of melt behaviour for higher temperature
The differences in Mold temperature can be caused by
different cooling.The location of cooling channels is often
determined by shape of molded part – cavity in the Mold. The
unequal cooling and flow asymmetry causes further problems
with molded parts – first of all warpage. The Molded part is
warped in the way like shown in Fig. 1. Due to Temperature
difference part is contracted. Therefore on this part wall the
surface is concaved. The theory presented in some
publications explains the reasons of the warpage. It is required
to do more research in this field to study about warpage. . This
could provide some information helpful in more detailed
description of warpage phenomenon.
II.
PROCEDURE
A. Materials
In this study, Acrylonitrile butadiene styrene (ABS) was
used as a polymer material. The grade of the ABS was LG
ABS AF-303 and its Melt Flow Index Value is MFI 50
g/10min at 230ᵒC.
B. Injection Molding
The basic function of a mold is two fold. Firstly it should
impart shape to the part and secondly it should cool the
molded part. Double cavity injection Mold is designed for
performing the tests. The specimens (bars: 55.5x20x2.95 mm)
was used for the investigation. The layout of component is
shown in Fig. 5. . The sketch of one of molding plates in the
Mold showing the layout of cavities and runners is presented
in Fig. 4. The mold base was designed in order to
accommodate cooling lines that are running around the
component.the mold base dimensions are(244x194mm). The
important aspect in injection Molding is cooling the part. In
the case of experimental Mold used here the cooling system is
very simple – one circuit in each Molding plate. The details of
cooling system in the Mold are sketched in Fig. 6. The entire
Mold consists of several plates (mounting plates, ejector
plates etc.) but cooling circuit is made only in two plates
(Molding plates) that form the parts. One of these plates is flat
the plate is with sprue bushing mounted. In the second one the
cavities are milled together with the runners and edge gates, as
it is shown in Fig4.
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Fig. 6.Scheme of mold cooling
Fig. 4. Sketch of the injection Mold used for the Analysis
 Injection temperature: 230°C
 Mold temperature: 30 - 60°C.
 Injection velocity: 30 cm/s
 Injection time: .37 s
 Holding pressure: 60 MPa
 Holding time: 5 s
 Cooling time: 17 s
For the Mold temperature values that were kept different
for each run in Flow Analysis (30°C, 40°C, 50°C, and 60 °C)
to cause the stress in the parts and, as the result, deformation
of the parts(to analyze warpage). The temperature values were
changed to compare the warpage to highly deformed parts and
to observe the asymmetry of flow front in the Mold.
III. RESULTS AND DISCUSSION
. Fig. 5. Sketch of the injection mold component
C. Processing Parameters
The polymer used for investigation was LG ABS AF303.This material is used to manufacture parts like caps,
blocks and automotive components. Even though ABS
plastics are used largely for mechanical purposes, they also
have electrical properties that are fairly constant over a wide
range of applications. The temperature values were changed to
compare the warpage to highly deformed parts and to observe
the asymmetry of flow front in the Mold. The Mold
temperature values were kept different for each run in Flow
Analysis (30°C,40°C,50°C,60 °C) to analyse the warpage.All
other parameters were kept constant for all the analysis.
ISSN: 2231-5381
The difference in value of warpage & shrinkage for different
mold temperature values were observed from the simulated
results. The parts deformation were showed in the following
(Fig.7.a,7.b,7.c,7.d.).
A. Observation of parts deformation
The parts after processing with different Mold temperatures at
30°C, 40°C, 50°C, and 60 °C are deformed. The variation in
warpage is noted along X-displacement, Y-displacement and
Total displacement. In practice the displacement along X and
Y can only be calculated using the Co-ordinate Measuring
Machine.So the Total displacement values are noted.The
shrinkage variations are also noted down.
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Fig. 7b.Warpage Displacement of Component at 40·C
Fig. 7a.Warpage Displacement of Component at 30·C
Fig. 7c.Warpage Displacement of Component at 50·C
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Fig. 7d.Warpage Displacement of Component at 60·C
The deformation measured with the results obtained from
Flow Analysis run clearly shows that warpage is caused due to Mold
temperature differences. The optimum Mold temperature helps to
avoid the warpage in the plastic component. Here the optimum mold
temperature is considered as 60°C.
TABLE I
EFFECTS OF PROCESSING PARAMETERS
Mold
Cooling time
Shrinkage (%)
Warpage (mm)
Temperature(°C)
(sec)
30
13
5.726
0.201
40
15
4.828
0.232
50
16
3.04
0.283
60
16
3.177
0.092
Fig.9. Measured deformation [mm] in the parts length
IV. CONCLUSION



If the Mold temperature is different, this leads to
asymmetry of melt flow. This, in turn, causes the
asymmetrical structure development in the part crosssection.
This study concludes that the warpage and shrinkage
value decreases for higher mold temperature
In further research the effect of variation of mold
temperature on the warpage and shrinkage obtained
using the simulated results will be verified by
experimental work.
REFERENCES
[1]
[2]
[3]
[4]
Fig. 8. Measured shrinkage [%] in total volume
[5]
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J.P. Beaumont, R. Nagel, R. Sherman, 2002, Successful
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J.P Beaumont., 2004 Runner and Gating Design Handbook.
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inatti.Smorawiski,1989 InjectionMolding technology,
WNT,
Warsaw, ,460 (in Polish).
T.A. Osswald, L-S.Turng, P.J. Gramann, 2001 Injection
Molding Handbook, Hanser Publishers, Munich.
Practical Guide to Injection Molding, 2004 Edited by Vanessa
Goodship, Rapra Technology Limited and ARBURG Limited,,
193- 94.
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