optimization of plastic parts obtained by injection molding · optimization of plastic parts...
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1
Optimization of plastic parts obtained by injection
molding
Rodrigo Ferreira
Departamento de Engenharia mecânica, Instituo Superior Técnico, Avenida Rovisco Pais, 1049-001, Portugal
Abstract
Due to their versatility, malleability and resistance to high temperatures, thermoplastics are being increasingly used
in all industries, including the automotive industry.
In the present work, the injection molding process to fabricate intercooler plastic tanks is studied, considering three
different materials with 50% and 30% glass fiber reinforcement: PA66+PA6 GF50, PAXT GF50 and PEEK GF30.
The aim of this study is not only to understand the impact and influence of these different materials in the injection
molding process, but also how each variable of the process influences the quality of the final parts produced.
Two different analysis are addressed: Finite element method using Autodesk Moldflow Insight with real processing
characteristics (for the three materials in study), and Design of Experiments analysis.
The capability of the finite element analysis as well as the Design of Experiment results were proved by a comparison
of the results with real parts produced under known conditions and parameters.
Keywords: Injection molding process, Thermoplastics, Intercooler tanks, Polyamides, PA6, PA66, PEEK, Finite element
method, Warpage, Glass fiber, Moldflow, Design of experiments.
1. Introduction
It was in 1886 that Karl Benz designed and built the first
practical car powered by an internal combustion engine,
to which was attributed the patent number 37435 [1].
Since then, the evolution of the cars has been increasing
on a global scale, not only in in a level of engines and
their efficiency, but also in aerodynamic and aesthetic
level. Being a highly competitive market, there is a
growing need to rely on a production of resistant and
low cost components, such as polymeric materials with
or without fiber reinforcement.
The same happens with the heat exchangers. The use of
plastic heat exchangers is something that has been used
for some time. Part of the initial interest in the
development of polymeric materials in heat exchangers,
was driven by its ability to be used in liquid and gas
environments and to its corrosion resistance [2]. Allied
to this, the use of polymers allows a reduction in weight
and manufacturing costs, giving them a substantial
advantage over those fabricated from metal alloys [2].
Thanks to its versatility, flexibility and resistance to
chemicals and high temperatures, the so-called
engineering thermoplastics have an important role in
the automotive industries. Within these, polyamides are
the most commonly used thermoplastic in this industry
[3].
Principles of Intercoolers
An intercooler is a heat exchanger that cools the air
before it admitted to the engine.
Their use is essential, since nowadays the laws relating
to fuel consumption and CO2 emissions are increasingly
stringent and restrictive. One of the solutions to comply
with the legislation, is to reduce the cylinder capacity of
the engines and associate turbochargers that allow an
increase in power. With the use of turbochargers, it is
essential to use a heat exchanger that reduces the
temperature of the compressed air prior to its admission
to the engine.
The thermodynamic objective of using turbochargers
followed by a cooling of the air through an intercooler,
is to obtain an increase in the mass of air that goes into
the cylinders of the engine for combustion [4]. This
compression of the air inside the turbo, is accompanied
by an increase in temperature [4]. Hot air is less dense
and contains less oxygen molecules per unit volume,
meaning that the use of a heat exchanger after the
compression in the turbo, increases the mass of air
2
admitted to engine cylinders, resulting in more torque
produced and less fuel consumption [5]. In addition, the
lower the inlet air temperature, lower the engine heat
load, improving the engine performance and preventing
premature wear [5].
Fig. 1 - Intercooler structure (adapted from [6])
Another solution that follows the reduction of emissions
of pollutant gases to atmosphere, including NOx, is the
use of exhaust gas recirculation systems. This
recirculation decreases the combustion temperature
within the cylinder reducing the formation of NOx.
Currently there are different architectures for such
systems, in which the gases can be recycled with or
without cooling and with low or high pressures [7], [8].
Experimental studies carried out by C. Cuevas et al. [9]
determined that an intercooler in an application of
exhaust low pressure loop recirculation system, can
reach efficiencies up to 0.97, for the parameters
studied.
Fig. 2 - Low pressure loop exhaust gas recirculation system
(adapted from[6])
Since the intercooler in service is subjected to high
temperatures and pressures, it is necessary to ensure
that the mechanical properties of the material support
the temperature gradients without compromise the
proper functioning of the component. So, it is
mandatory to know the typical operating conditions of
a car intercooler. According to [10]:
Inlet charge air temperature: 120 – 220 °C
Desired temperature of the outlet air: 50 – 90 °C
Pressure: 1.3 – 4.0 bar
Mass flow rate: 0.04 – 0.5 kg/s
Cooling rate: 10m/s
Desired pressure drop: should not exceed 600
Pa
Chemical resistance to all solvents used in the
automotive industry, including salt
For the use in high pressure exhaust systems:
resistant to acids.
Resistant to biological attacks and fatigue
cracks.
Thermoplastics with fiber glass reinforcement
The mechanical properties of thermoplastics are
sensitive when compared to metal materials. They are
strongly influenced by the deformation rate,
temperature and humidity resulting in a decrease of
yield stress with the increase of temperature and
decreasing of the strain rate [11].
However, reinforced thermoplastics, have advantages
over the traditionally used, which are the increase of the
fracture toughness, the damage tolerance and
durability.
Although yield strength and Young modulus increase
with the % of fibers, the new technical challenges arising
from high levels of temperature and pressure processing
demands make it very complex and may sometimes
impair the surface appearance of the final component.
Fig. 3 – Stress-strain curves for different % of glass fiber
reinforcement, temperature influence in the relative
modulus and stress. Adapted from [12].
Materials in study
3
The three materials in study are:
PA66+PA6 GF50 – This material consists of
polyamide 6 (PA6) and polyamide (PA66)
reinforced with 50% glass fiber and stabilized
term. Both polyamides exhibit good mechanical
strength, high impact resistance, and good
damping and wear resistance.
PAXT GF 50- This polyamide has good
mechanical properties even at high
temperatures. It is resistant to heat even for long
term exposures being capable to maintain
dimensional stability.
PEEK GF 30 – is a semi-crystalline thermoplastic
polymer reinforced with 30% glass fiber, in
which PEEK is an abbreviation for Polyether
Ether Ketone. PEEK is a polymer highly resistant
to thermal degradation and has excellent
mechanical and chemical resistance, which
remain unchanged at high temperatures.
Fig. 4 - Hierarchical performance of thermoplastics versus
temperature.
From the analysis of the Fig. 5 we can conclude that the
PEEK maximum service temperature is much higher than
the polyamides (nylons).
Fig. 5 - Ashby map - strength versus maximum service
temperature.
2. Numerical Experiment
Finite elements model
The material-properties of the three materials in study
are given in Table 1 (as obtained from Moldflow data).
Table 1 - Material-properties of the three materials in study
In Table 2, recommended process temperatures for each
of the materials are presented.
Table 2 - Recommended process temperatures
Material PA66+PA6 PEEK PAXT
Melting Temp. 260°C 343°C 300°C
Glass
transition
Temp.
N.D. 143°C 125°C
Mold Temp. 50°C 176°C 125°C
80°C 205°C 140°C
Melt Temp. 280°C 370°C 305°C
305°C 400°C 320°C
Extracting
Temp. 204°C 285°C 267°C
In order to try to replicate the injection process in the
most reliable way possible, cooling channels and
injection sprues were modeled in Moldflow as they are
in the real mold of the parts (2 cavities mold). However,
as the recommended process temperatures is different
for each material, the cooling fluid has also to be
different depending on the material in study.
Table 3 - Cooling fluids
Material PA66+PA6 PEEK PAXT
Cooling Fluid Water Oil Oil
Temperature
of the cooling
fluid
70°C 170°C 120°C
Despite the fact that the mold produces in each cycle of
manufacturing the two tanks simultaneously, the first
Moldflow study to compare the influence of each
material in the quality of the parts produced and in the
Properties PA66+PA6 PEEK PAXT
Young Modulus [GPa] 14.86 11.8 14.6
Rupture stress [MPa] 230/145 180 250
Strain at rupture [%] 2.4/4.6 2.7 2
Density [Kg/m3] 1580 1510 1580
4
process parameters, was made considering only the inlet
tank of the intercooler.
Several loops for numerical experiments are completed
varying the injection parameters until all necessary data,
including clamping-force, warpage values, weld lines,
pressure vs. time plots ant flow vs time plots are
obtained. For comparison purposes between materials,
the cooling time and compression time were kept
unchanged (25s and 10s respectively). Also for
comparative purposes the analysis were made without
limitations on the injection machine, in order to avoid
influences/limitations on the injection molding process.
2.1 Simulation and analysis
The simulation procedure is based on a few steps.
Models are first imported to Moldflow environment, and
after the generation and correction of the mesh, the
boundary conditions such as material properties (one
for each material in study), cooling channels, hot sprues
and process parameters are set and designed. Properties
and meshes must also be assigned to both cooling
channels and hot sprues.
Until the final mesh of the parts is meets the required
targets, several loops between Moldflow and CAD
software’s may occur. Sometimes, depending on the
parts complexity, targets aren’t even achievable with the
use of a dual domain mesh. In those cases a 3D mesh
must be used.
In this particular study, for the mesh to have the desired
requirements, all the small fillets (between 1 and 2mm)
as well as the thread of the spigots, had to be taken from
the original geometry. Final geometries of the tanks as
well as the mesh statistics are shown of Fig. 6 and.
Fig. 6 - Final geometries of a) inlet tank b) outlet tank
Fig. 7 - Mesh statistics for outlet and inlet tanks
Once the mesh and all the boundary conditions are set,
firstly it is performed for each material a so called
molding window analysis. This analysis calculates
preliminary parameters appropriate to each material
case study. The results of this analysis are then
subsequently used as a reference in a first
cooling+fill+pack+warp analysis.
Table 4 - Molding window recommended values
Material PA66+PA6 PEEK PAXT
Mold
Temp. 80°C 182.44°C 133.33°C
Melt Temp. 302.73°C 380.71°C 315°C
Injection
time 0.8304 s 2.0056 s 1.5766 s
Afterwards, another study is carried out, this time using
a variable profile that relates the % of shot volume with
the % of flow rate. As in the previous case, a
cooling+fill+pack+warp analysis is held.
Finally a last analysis is performed where the flow and
packing pressure are kept constant.
For all the studies listed above, results such as pressure,
flow, warp, welding lines and clamping force are
analyzed and compared for all materials to see how they
influence the quality of the parts produced.
For each of the cases highlighted above, packing
pressure is set to be a fixed percentage of the maximum
injection pressure during the packing process.
5
According to simulation results for all the materials, for
higher values of packing pressure, lower are the values
of the maximum deflections.
With the objective of analyzing the effect of the packing
pressure in the part deflections and in cavity residual
stresses, a study is made where the percentage value of
the packing pressure was varied between 80% and 150%
with steps of 10%. Results are shown in Fig. 10 and Fig.
11.
Fig. 10 - Deflections vs %Pressure
Fig. 11 - In cavity residual stresses vs %Pressure
2.2 Current processing conditions
To validate the simulation results presented previously,
another Moldflow analysis was carried. This time the
model was prepared with both tanks (2 cavities mold)
and a valve gate was set to assure that the filling of the
tanks is balanced. Processing conditions used in
Moldflow are the same that are currently used for the
production of the parts. This way it is possible to portray
faithfully the injection processing conditions and obtain
Fig. 8 - Pressure vs Time and Flow vs Time for a) Molding Window values b) Variable profile c) Constant flow and packing pressure
0
20
40
60
80
100
0 0,5 1 1,5 2
Pre
ss
ão
[M
Pa
]
Tempo [s]
PA66+PA6 GF50 - MW
PEEK GF30 - MW
PAXT GF50 - MW
0
20
40
60
80
100
120
0 20 40 60 80 100 120
Pre
ss
ão
[M
Pa
]
% Volume [%]
PA66+PA6 GF50
PEEK GF30
PAXT GF50
0
20
40
60
80
100
120
0 0,5 1 1,5 2
Pre
ss
ão
[M
Pa
]
Tempo [s]
PA66+PA6 GF50
PEEK GF50
PAXT GF50
50
100
150
200
250
300
0 0,5 1 1,5 2
Cau
dal
[cm
^3/s
]
Tempo [s]
PA66+PA6 GF50 -MW
PEEK GF30 - MW
PAXT GF50 - MW
0
100
200
300
400
500
600
0 20 40 60 80 100 120
Cau
dal
[cm
^3/s
]
% Volume [%]
PA66+PA6 GF50
PEEK GF30
PAXT GF50
0
200
400
600
800
1000
1200
1400
1600
1800
0 0,5 1 1,5 2
Cau
dal
[cm
^3/
s]
Tempo [s]
PA66+PA6 GF50
PEEK GF50
PAXT GF50
a) b) c)
Fig. 9 - Clamping force for a) Molding Window values b) Variable Profile c) Constant flow and packing pressure
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40
Fo
rça d
e F
ec
ho
[to
n]
Tempo [s]
PEEK GF30
PAXT GF50
PA66+PA6 GF50
0
20
40
60
80
100
120
140
0 10 20 30 40
Fo
rça d
e F
ech
o [
ton
]
Tempo [s]
PEEK GF30 - MW
PAXT GF50 - MW
PA66+PA6 GF50 - MW
0
20
40
60
80
100
120
140
160
0 10 20 30 40
PEEK GF50
PAXT GF50
PA66+PA6 GF50
6
results as close as possible to reality. It is important to
mention that the material used for this validation is the
PA66+PA6 GF50.
The simulation procedure is based on the following
steps.
After importing the injection machine specifications
(listed on Table 5) to Moldflow, the control variables of
the injection process were set accordingly to the real
parameters presently used for producing the intercooler
tanks.
Table 5 - Injection machine specifications
Maximum clamping
force [ton]
500
Intensification ratio
[MPa]
8.1
Maximum injection
pressure [MPa]
173
Maximum hydraulic
pressure [MPa]
21.4
Spindle diameter [mm] 80
Maximum flow [cm3/s] 460
Maximum injection
stroke [mm]
600
Filling and packing control
The filling control is performed based on the spindle
velocity profile versus its position, while the packing
control is accomplished by the pressure versus time. The
V/P changeover occurs when the spindle reaches the
position x=10mm. Control values for both Filling and
packing phases are listed on Table 6 and Table 7.
Table 6 - Filling control
Filling control
Velocity [mm/s] 55 41.2 27.4
Position [mm] 110 55 0
Velocity [%] 60 45 30
Flow [cm3/s] 276 207 138
Fig. 12 - Spindle velocity VS Spindle Position
Table 7 - Packing control
Packing control
Packing
pressure [MPa] 42.1 70.3
Position [mm] 10 -
Hydraulic
packing
pressure [MPa]
5.2 8.7
Pressure [%] 24.3 40.65
Since the geometry and volume of the tanks is different,
a carburetor with a valve gate system was used. By using
this system, it is necessary to set the opening and closing
timings, so the injection is balanced, preventing one of
the cavities to start the packing phase while the other
one is still in the filling stage. Knowing the volume of
each tank, it was calculated the time needed to begin
the filling of the second tank. The calculated volume
difference between the tanks was approximately 3.85%,
which means that the valve gate can be opened 0.185s
after the filling phase has started, considering the
current processing conditions.
In Fig. 13 and Fig. 14the filling pressures and flow rate
versus time, resulting from the conditions used, are
illustrated.
Fig. 13 - Pressure VS Time for the current processing
conditions
Fig. 14 - Flow VS Time for the current processing conditions
7
Pressure in the beginning of the filling stage, exhibits a
substantially higher value than over the remaining filling
phase. This is a result of the valve gate being closed in
the beginning of the process. At 0,185s, the pressure
stays approx. constant until the V/P change over. It is
also noticeable on Fig. 14, that 3D Mesh shows a lower
flow rate when compared to the dual domain mesh. The
reason for this discrepancy is the low refinement of the
dual domain mesh in the injection point of the parts.
Also in this zone there isn’t correspondence between the
triangle elements of the dual domain mesh, resulting in
a low quality result of the simulation. This is one of the
cases where the complexity of the part justifies the
usage of a 3D mesh.
Comparison between Moldflow and real results
Concerning the Moldflow warpage results, the outlet
tank has slightly higher deflections than the inlet tank.
The areas where the maximum deflections are identified
in the outlet tank, are in the tank foot, which can make
the assembly of the header plate and also the crimping
to be compromised.
For the inlet tank, the area where the greatest
deflections where identified, does not represent such a
major concern for the intercooler assembly. In this case
they were identified on the spigot, which is the
engagement zone between the intercooler and the hose
coming from the turbocharger. Since the deflections are
considered small, the flexibility of the hose assures a
good connection to the spigot which does not
compromise the proper function of the intercooler.
Fig. 15 and Fig. 16 shows the Moldflow warpage results
for both tanks.
Fig. 15 - Inlet tank Moldflow warpage results
Fig. 16 - Outlet tank Moldflow warpage results
In the real parts, the warpage was only analyzed in the
tanks foot, since this is one of the most critical zones for
the intercooler assembly.
The CAD model of the parts served as reference for the
deflection results measured.
Fig. 17 - CAD model of the outlet tank foot
Measurement of the deflections was made using a
digital caliper, and the results are illustrated in Fig. 18
Fig. 19.
Fig. 18 - Inlet tank measured deflections
Fig. 19 - Outlet tank measured deflections
Contrary to the numerical analysis, the warpage
measured on the parts has a convex curvature. This
results suggest that the mold used to inject these parts
has suffered corrections in its geometry in order to
minimize warpage. Indeed, despite the reverse in the
direction of the curvatures, the maximum values of
warpage have decreased in modulus relatively to the
ones obtained in the numerical analysis, being more
evident in the outlet tank where the deflections went
from 1.48mm (0.8357+0.6407) to 0.45mm.
Besides the deflection, the weld lines and the mass of
the components were also compared.
8
Table 8 - Comparison of the weight of the parts between reality
and numerical analysis
Real parts
Dual
Domain
Mesh
3D Mesh
Weight [g] 813.8 626.9082 765.95
Difference - 186.8918 47.85
As expected, the total weight obtained using the Dual
Domain mesh is considerably lower than the actual
parts, and also to the obtained for the 3D mesh. This
goes in accordance to what was explained previously. In
the areas where the elements of the Dual Domain mesh
do not match, the thickness of the component is
influenced and therefore the calculated weight of the
parts is compromised.
Fig. 20 - Weld lines comparison - Real parts VS Moldflow
analysis
The large percentage of glass fiber reinforcement (50%),
make it difficult to identify the existing welding lines by
visual inspection of the parts. However, in several cases,
the weld lines identified in Moldflow matched the ones
visualized in the parts. Some of them are displaced only
few mm from one case to another. Such small
differences may be due to the
corrections/simplifications made on the geometry of the
parts, thereby improving the quality of the mesh used
and slightly change the injected material flow displacing
the weld lines slightly.
3. Design of Experiments (DOE) analysis
The design of experiments is a structured and organized
statistical method to determine the relationship of
several factors in a particular case, and how they
influence the outcome of the process in question. It is
commonly used to assess the sensitivity of the results to
variables, such as processing parameters, thickness
change, or even to understand interactions between
variables and their influence on the results.
In Moldflow, the DOE uses user-defined input variables,
such as mold temperature, melt temperatures, flow, etc.
and performs a series of analysis based on statistical
models. The weight assigned to each variable, as well as
the quality criteria are also user-defined factors.
Simulation results are then displayed according to the
type of analysis made, and can be2D or 3D graphics with
information regarding the study conducted as well as a
rank of the variables influence[13], [14].
In this research a variable influences then responses was
performed with 4 sub variables.
The variables selected for the optimization analysis were:
Ram speed
V/P switch-over by Ram position
Duration of the packing pressure
Packing Pressure
Parameters and their levels are shown in Table 9.
Table 9 - Parameters and their levels used in the DOE analysis
Parameters -1 0 1
Ram speed
[mm/s]
21.92 to
43.92
27.4 to
54.9
32.88 to
65.88
V/P switch
over ram
position
8 10 12
Packing
Pressure
duration
[s]
6.4 8 9.6
Packing
Pressure
[MPa]
33.6 to
56.24 42 to 70.3
50.4 to
84.36
The quality criteria used to define the quality of the parts
are shown in Fig. 21.
Fig. 21 - Quality criteria menu
Two distinct results are obtained for this analysis:
9
Influence of each variable for each of the
defined quality criteria
The impact of these variables on the quality of
the final part.
Concerning the DOE warpage results, it follows that the
packing pressure and the V/P switch-over by ram
position are the variables with greater impact, with a
weight of approx. 40% each (fig.x).
Fig. 22 - Impact of the variables in the warpage results
The calculated optimized parameter values for the
warpage were:
Fig. 23 - Optimized DOE calculated parameters values for the
warpage
There is an improvement in the deflection results for
both tanks, compared to the results shown previously
considering the current parameters for the injection of
the parts. Also, as shown in fig. x, the maximum
deflection zone changes in the case of the outlet tank,
going from the tank foot to the spigot, which is a less
critical zone.
Fig. 24 - Optimized warpage results with the optimized
calculated parameter values
4. Conclusions
The study on the optimization of the intercooler tanks
obtained by injection molding, allowed not only to verify
the influence of each of the parameters of the process,
but also the influence of each one of them in the quality
of the molded parts.
Regarding the comparison between the three materials
studied, there are several points to retain. Considering
the mechanical behavior of the materials, it was found
out that the PAXT and PA66+PA6 GF50 characteristics
are very similar and slightly superior to those of PEEK
GF30, in particular with regard to the elasticity modulus
and rupture stress. This differences may be due to the
first two materials have a glass fiber reinforcement of
50% while PEEK as only 30%. However. The last is the
material that allows the maximum service temperature,
that is, material whose mechanical and chemical
properties provides the best characteristics even at high
temperatures.
Concerning the processing conditions, PEEK is the most
demanding, requiring mold temperatures substantially
higher.
Analyzing the results presented along this research, it is
found that the PEEK is the material that provides the best
warpage results. Although these results may be related
to the fact that higher pressures are used for filling and
packing this material, it is impracticable to obtain the
same values of deflections for the other materials
studied.
It has been proven that the packing pressure directly
influences the deflection results of the molded parts. The
higher the packing pressure, the lower the deflections
will be. However the mold cavity residual stresses also
increase with this increase of the packing pressure. It is
therefore important to find a compromise between the
packing pressure, the deflections of the part and the
residual stresses.
The validation of the Moldflow results was verified with
a comparison between the parts injected at J.Deus
Company and the numerical simulation. By comparison
of the weight, deflections and position of the welding
lines, it is concluded that Moldflow gives accurate
approximations especially if the system is well modeled.
This analysis made clear that the real mold of the parts
has already suffered compensations to reduce the
warpage.
The trend of the results obtained with the DOE tool, in
all cases analyzed, matched the results obtained in
simulations made later in which the parameters
obtained from this optimization were used. Being able
to show the influence of each variable in the process, as
well as presenting the optimized parameters for each
10
quality criteria, make this tool a fundamental help in the
variable optimization process.
5. References
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http://www.biography.com/people/karl-benz-
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