Proceedings in Manufacturing Systems, Volume 11, Issue 2, 2016, 95‒100 ISSN 2067-9238

ANALYSIS OF TEMPERATURE INFLUENCE ON INJECTION MOLDING PROCESS

Karel RAZ1,*, Martin ZAHALKA2

1)
PhD, Lecturer, Eng., Regional Technological Institute, University of West Bohemia, Czech Republic
2)
Lecturer, Eng, Regional Technological Institute, University of West Bohemia, Czech Republic

Abstract: This article deals with possibilities of modern advanced simulation methods for determining
quality of mechanical systems. For temperature studies injection molding process was chosen. It is influ-
enced by many parameters such as temperatures and pressures. This article is focused on influencing by
temperature. The main aim is to determine optimal temperatures of injected plastic material, mold and
coolant. For each plastic material temperature range of mentioned parameters is given. Using modern
plastics flow simulation it is identified the exact influence of each parameter on final properties of prod-
uct. As main parameters for evaluation the followings are chosen: level of mold filling, number of weld
lines and total production times. As a reference state it is chosen the process with mean values of all in-
jection parameters. Simulations are verified by comparing with an experiment on real injection molding
machine.

Key words: injection molding, temperature, quality, molding simulation.

1. INTRODUCTION 1 cycle time. To do so, both CAD (NX 10 software) and

CAE (Moldex 3D software) tools are used to reproduce
Injection molding of plastics is worldwide one of the part and the process and then, after the simulations, to
most commonly used production methods. Nowadays, validate the results by experimental molding injection.
there are plastic parts used in almost all kinds of products An important thing is that all manufacturing parame-
and in all industrial fields. Quality is now with costs the ters of injection mold are highly interdependent. At this
main indicator, which will determine success on global point, a suitable method to conduct the simulations is
rapidly changing market. required to avoid unnecessary time spending and compu-
Our research is focused on improvement of techno- tational costs and to organize the results.
logical process of injection molding using new methods
of advanced simulations (CAE). In the past, adjustments 2. INFLUENCE OF TEMPERATURE
of all injection parameters were tested using real experi-
ments. It means that production was stopped and until Temperature can influence quality of final product.
setting up all parameters properly it was not producing All main parts of injection molding press have different
any parts. temperature during injection molding. The parameters
One of advanced methods for experiment is using vir- considered here are injection temperature, mold tempera-
tual simulation with finite element method. Using this, it ture and coolant temperature.
is possible to predict influence of all parameters on target
product. This article is focused only on influence of tem- 2.1. Injection temperature
perature. Changes in second main parameter, which is Injection (melting) temperature is the temperature of
pressure, will be under inspection in future work and plastic material in time of entering gate into cavity of
research. mold. All plastic materials have ranges for melt tempera-
The quality of the final product and the productivity ture. For general kinds of plastic materials the ranges of
of the injection molding process depend on several pa- injection temperatures are shown in Table 1.
Table 1
rameters. The manufacturing parameters, material char-
Melt temperatures of typical materials
acteristics and mold design have direct influence on the
product. Determination of the optimum parameters for Material Injection (Melting)
the process is mandatory for ensuring the product quality temperature [°C]
and to decrease production costs. HDPE 190‒240
Therefore, the main purpose of this work is to deter- LDPE 170‒270
mine the optimum manufacturing parameters for injec- PP 180‒280
tion molding of an existing part in order to decrease PA 6 240‒280
ABS 190‒270
* PS 170‒270
Corresponding author: Univerzitni 8, 306 14 Pilsen, Czech Re-
public, PMMA 200‒260
Tel. (+420) 377 638 751; PVC 190‒220
Fax: (+420) 377 631 112;
PC 270‒320
E-mail addresses: kraz@rti.zcu.cz (K. Raz).
96 K. Raz and M. Zahalka / Proceedings in Manufacturing Systems, Vol. 11, Iss. 2, 2016 / 95−100

Table 2
Mold temperatures of typical materials
Material Mold temperature [°C]
HDPE 30‒60
LDPE 20‒60
PP 20‒90
PA 6 40‒100
ABS 50‒80
PS 20‒80
PMMA 30‒80
PVC 20‒70
PC 85‒120

Table 3
Properties of Liten MB/ML 71 HDPE
according to producer
Property Value
Melt flow 8 g/10 min
rate
Fig. 1. 3D CAD model of plastic product and static half of
Density 963 kg/m3
mold.
Yield stress 25 MPa
Input factors were varied in range according to
2.2. Mold temperature Table 4. For parameters, such as pressures or time were
During production process, temperature of mold is considered as default values. The first simulation was
stabilized on almost constant value. This value is in di- done with all default parameters. Then, for the following
rect relation with coolant temperature and parameters simulation, temperatures were varied from minimum to
(Table 2). maximum with maintaining other parameters on default
values.
2.3. Coolant temperature By using this approach simulations are able to show
As coolant, in most cases it was used water supplied how each change of temperature is influencing the whole
from an external cooling system. Sometimes air, oil or process.
other materials are used. Problem is in material library in Moldex 3D, because
available testing material Liten MB/ML 71 HDPE is not
2.3. Properties experimental material worldwide used and is not available in material library. It
As a reference material was used High-Density Poly- is necessary for validating simulations with experiment
ethylene produced by Unipetrol with technical name having the same material or material with corresponding
Liten MB/ML 71 HDPE. It is a thermoplastic material properties.
with following properties. Most suitable library materials are shown in Table 5.
According previous table (Table 3) is visible that From these for simulations Lyondell Bassel Lupolen
temperatures for setting up molding machine are not 5031 L HDPE was chosen.
provided by producers. In this case is necessary to use
value from general range. For HDPE is injection temper- Table 4
ature from 190°C to 240°C. Generally there is a problem Input parameters for simulation
with ranges in injection molding, because all parameter Factor Units Minimal Default Maximal
are not pre-determined accurately [1, 2, 3]. value value value
Coolant
°C 35 40 45
3. SIMULATION OF PARAMETERS VARIATON temperature
Injection
Modern approach is in minimizing of delays in pro- temperature
°C 190 215 240
duction process. For this it is suitable the usage of virtual
Mold
simulation and testing [8]. temperature
°C 35 50 65

Injection
3.1. Simulation using Moldex 3D pressure
MPa - 85 -
Moldex 3D is one of commonly used software for in-
Packing
jection plastic simulation. It is based upon finite element MPa - 60 -
pressure
method and has comprehensive library of materials,
which is suitable for virtual testing. Packing
sec - 3 -
time
First step is creating a model in CAD software Sie-
mens NX 10. Testing model is visible on following pic- Cooling
sec - 10 -
time
ture. It is four pieces of shaft bearing´s halves. Visible
are also runners and sprue (Fig. 1). Filling time sec - 0.5 -
 K. Raz and M. Zahalka / Proceedings in Manufacturing Systems, Vol. 11, Iss. 2, 2016 / 95−100 97

Table 5
Available material models
Physical Unipetrol Chevron Lyondell Lyondell
property Liten Phillips Basell Basell
ML 71 Marlex Lupolen Lupolen
HDPE H516 6031 M 5031 L
HDPE HDPE HDPE
Density
0.963 0.961 0.963 0.952
[g/cm3]
Melt Flow
8.5 8.0 10.5 8.5
[g/10 min]
Yield stress
25 25.5 31 26
[MPa]
Poisson
0.40 0.38 0.40 0.40
ratio
Flexural
modulus 1.10 0.965 1.5 1
[GPa] 0 0.32 0.65
[sec]
Fig. 2. Front filling time with minimal
3.2. Injection temperature injection temperature.
The injection temperature was varied 25°C up and
down from the reference value of 215°C. The injection
temperature refers to the temperature of the material after
the plasticizing stage.
Influence of injection temperature on filling stage.
The temperature is influencing viscosity of the material.
From that knowledge is possible to understand why there
were short shots in the simulation with the minimum
injection temperature (Fig. 2). As is visible in figure, the
minimal injection temperature was unable to fill the
cavities and left there 6.65% of empty volume. That
characteristic also reflected in the melt front time that
was longer when the melt was cooler.
The number of air traps cannot be consider for the
minimum melt because of the short shot, that left a criti-
cal portion of the cavities without material.
The minimum melt temperature results caused also
increasing in the number of weld lines. Weld line is the
line formed by two different melt fronts joining together
with sharp angle during the filling stage. It usually de-
0 0.28 0.52
creases the strength of the final product and produces
[sec]
optical visible defects. Maximal injection temperature
(Fig. 3) also decreased the percentage of the part under Fig. 3. Front filling time with maximal injection tempera-
high shear stress (>1MPa) [5, 6, 7]. ture.
Influence of injection temperature on packing
stage. The short shot from the filling stage was almost
completely filled in the packing stage. Only 0.0025% of 3.3. Mold temperature
the volume remained unfilled. It is also possible to notice The mold temperature was varied from 35°C to 65°C
in the following images that there is no visible unfilled and compared to the reference simulation, which mold
volume, comparing to the filling phase, where the short temperature was 50°C. The mold temperature refers to
shot was visible. That is how the ideal packing phase the temperature of the cavity that is filled during the
should work. During the filling stage, the cavities should injection phase.
be filled up to 90% to 99% and then, during the packing Influence of mold temperature on filling stage. The
phase, also called second injection phase, the cavities melt front time increased with decreasing of the mold
should be completely filled. temperature due the effect of temperature on the material
The higher melt temperature leads to higher maxi- viscosity. There were no significant variations in the
mum average temperature during the cooling phase. The number of air traps and weld lines. There was a substan-
maximum cooling time needs to be increased by 13.5%. tial increase in the percentage of the part under high
The minimum melt temperature causes high values of shear stress (>1MPa) in the simulation with lower mold
volumetric shrinkage. Even being symmetrical, high temperature (Fig. 4). When the mold is cooler, the mate-
values of shrinkage may lead to warpage after the part is rial near the mold’s wall freezes and it causes increasing
ejected. of the shear rate.
98 K. Raz and M. Zahalka / Proceedings in Manufacturing Systems, Vol. 11, Iss. 2, 2016 / 95−100

0 0.31 0.62 0 15.6 31.2

[sec] [MPa]
Fig. 4. Filling Melt Front time Fig. 6. Packing shear stress with minimal
with minimal mold temperature. coolant temperature.

0 0.24 0.58 0 13.8 27.7

[sec] [MPa]
Fig. 5. Filling Melt Front time with maximal
Fig. 7. Packing shear stress with maximal
mold temperature.
coolant temperature.

Influence of mold temperature on packing stage. front with respect to time during the filling stage, has
During the packing analysis, no significant variation of decreased with the increase of the coolant temperature.
the parameters was noticed. The region under high shear The volumetric percentage of the part under high
stress after packing slightly decreased with the increase shear stress (<1MPa) during the filling stage is increased
of the mold temperature. The opposite also happened, significantly with decreasing of the coolant temperature.
with the decrease of mold temperature, the shear stress There were no short shots, in other words, the mold’s
increased (Fig. 5). The cooling time increased with the cavities were completely filled. There were air traps, but
rise of mold temperature. in the same number as in default simulation, so no influ-
ence was noticed. The number of weld lines from both
3.4. Coolant temperature
simulations with the coolant temperature variation is at
Coolant temperature is the temperature of the coolant
liquid that pass through the cooling channels inside the same level with default simulation [4].
mold. It can be seen in the following images. The mini- Influence of coolant temperature on packing stage.
mum coolant temperature applied at the simulation was There were no alterations in the size of short shots (it was
35°C (Fig. 6), the maximum was 45°C (Fig. 7) and the already zero in the filling phase), number of air traps and
reference value was 40°C [9, 10]. weld lines. The percentage of the part under high shear
Influence of coolant temperature on filling stage. stress (>1MPa) followed the filling results. It hase in-
The melt front time that shows the position of the melt creased with the decrease of the coolant temperature.
 K. Raz and M. Zahalka / Proceedings in Manufacturing Systems, Vol. 11, Iss. 2, 2016 / 95−100 99

4. EXPERIMENT 5. CONCLUSIONS
For validating of virtual simulation experimental in- The coolant temperature has a small/none influence
jection molding (Fig. 8) with default parameters men- in the process parameters and in the final product at the
tioned above was performed. The result (produced part) analyzed interval of temperature variation. The part per-
(Fig. 9,b) is than compared with virtual result from CAE centage under high shear stress (>1MPa) increased sub-
simulation (Fig. 9,a) [11, 12, 13]. stantially during the filling phase, but it came back to the
There are not significant differences in both parts. In expected values in the packing phase, similar to the ref-
real experiment there are no short shots and optical prop- erence value.
erties are at suitable level. According to warpage meas- There were no significant alterations in comparison
urement both products are comparable and virtual simu- with the reference simulation in the analyzed data.
lation gives us correct results. The injection temperature has important influence on
the process and on the final product. High injection tem-
peratures present lower viscosity, representing better
flow characteristics, as seen in the filling melt front time:
it was smaller for higher melt temperatures.
However, the injection temperature increase may lead
to lower density of the final part, as showed, which result
in worse impact resistance as well as higher energy con-
sumption in molding and longer cycle time. High melt
temperature also can lead to polymer degradation and
burn problems.
Low melt temperatures influences material flow, re-
sulting in short shots and weld lines. To work with lower
temperatures, higher pressure is necessary so the friction
heating, as a result of shear stress, helps the flow de-
creasing the viscosity of the melt.
The optimum temperature must be found to ensure
process and product quality. The combination of low
Fig. 8. Experimental injection molding device.
melt temperature with high mold temperature is suggest-
ed as an efficient alternative to parameters set up.
The mold temperature influences in the shear stress,
decreasing the shear stress values with the rise of mold
temperature. It also influences in the cooling time, hence
the cycle time. No further parameters variations were
noticed by results interpretation.
However, the mold temperature has a determinant
role in the microstructure of the final product. In amor-
phous polymers such as ABS and polycarbonate, higher
mold temperatures produce lower levels of molded-in
stress and consequently better impact resistance, stress-
crack resistance, and fatigue performance.
In semi-crystalline materials the mold temperature is
an important factor in determining the degree of
a crystallinity in the polymer. The degree of crystallinity
governs many performance parameters, including creep
resistance, fatigue resistance, wear resistance, and di-
mensional stability at elevated temperatures.
Crystals can only form at temperatures below the
melting point but above the glass-transition temperature
(Tg) of the polymer. As mentioned, the combination of
low melt temperature with high mold temperature is
suggested as an efficient alternative to parameters set up.
This study involved analyzing of the parameters in-
fluence on the final product using computer aided tools
and consisted of two parts:
• Variation of each one parameter at time and conduct
computer simulations to investigate the influence at
the part and other process conditions.
b • Applying the primary results to suggest an optimized
Fig. 9. Part: a ‒ CAD virtual products; b ‒ real final set of process parameters to implement in a real injec-
product from experiment. tion process. These two results (simulation and real
100 K. Raz and M. Zahalka / Proceedings in Manufacturing Systems, Vol. 11, Iss. 2, 2016 / 95−100

Table 6 REFERENCES
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