ADVANCED COOLING SIMULATION TECHNOLGIES FOR THE INJECTION
MOLDING PROCESS
Lu Chen, Clinton Kietzmann*, David Astbury*, Liang Shao
Autodesk (China), 399 Pudian Road, Shanghai, China 200122
*Autodesk Australia Pty Ltd, Moldflow R&D Center, 259 -261 Colchester Road, Kilsyth Vic 3137
Abstract
Cooling circuit design in plastic injection molding has
become the focus for improving part quality and reducing
cycle times. In order to achieve both high quality parts
and low cycle times; technologies such as, rapid heat
cycling, and conformal cooling are commonly used. This
paper gives an extensive introduction to a 3D finite
element method based solver which has been developed to
simulate these molding technologies. Rapid heating and
cooling cycling heats the mold surface prior to mold
filling using steam, hot water or heater cartridges and
cools the mold during the cooling phase with chilled water
resulting in a very good surface finish without
compromising the cycle time. With conformal cooling, the
cooling channels follow the precise geometry of the part
in the mold. This aids in creating a uniform temperature
distribution across the surface of the part by targeting hot
spots. Rapid heating and cooling technology combined
with conformal cooling is also supported by this solver.
Typical cases for these technologies are presented in the
paper and discussed.
Introduction
Plastic injection molding is one of the most popular
manufacturing processes for mass production. In order to
obtain cost effective high quality parts consistently, many
molders turn to simulation technology for part and mold
design. Simulation technology aims to provide the analyst
with accurate mold cooling or heating, filling and part
warp results to guide their design decisions.
Traditional injection molding design aims to maintain
the mold at a constant temperature for the entire injection
molding cycle. In order to achieve this, coolant is pumped
through the mold cooling channels with constant inlet
temperatures.
Traditionally these cooling lines are comprised of a
network of holes drilled through the solid mold that are
plugged at various positions forcing the coolant to follow
a certain path. If more cooling is required closer to the
part in a certain region of the mold the traditional
approach is to insert a baffle or a bubbler in this region
[1].
Conformal cooling channels are made to any shape
that follows the precise geometry of the part in the mold.
This aids in creating a uniform temperature distribution in
the part by targeting hot spots on the part surface with
arbitrary shaped cooling channels in the mold. Ultimately
these result in better quality parts, shortened cycle times,
reduced waste and cost reductions. Traditional cooling
lines are used to supply and remove coolant from this
insert containing the conformal cooling channel.
These conformal cooling channels replace the
conventional cooling lines in the mold and no other
change in the injection molding cycle is required.
Conformal cooling channels are proving to be very
popular when used in conjunction with the Rapid Heating
and Cooling injection molding process.
Rapid Heating and Cooling varies the inlet
temperature of the coolant during the injection molding
cycle. The aim is to have a mold cavity surface
temperature close to melt temperature during the mold
filling stage and to reduce the surface temperature to
ejection temperature during the packing and cooling stage
of the injection molding cycle.
The high mold cavity surface temperature during
filling results in high gloss surface finishes of plastic
parts, avoiding the need for any other post molding
surface enhancements such as polishing and painting. A
hotter mold during filling also results in lower injection
pressures, allowing the molding of parts with reduced wall
thicknesses and the selection of gating locations which
reduce mold cost. The hotter mold also reduces the impact
of weld lines which improves the strength and aesthetics
of the part.
The benefits of rapid heating and cooling processes
are particularly favorable when molding clear components
such as lenses and flat screen television housings among
other components.
The use of conformal cooling channels can minimize
the distance from the cooling channels to the part cavity,
so allowing the mold in contact with the part to more
quickly heat up and cool down during each cycle.
However in order to utilize the full potential of
conformal cooling it is very important that the fluid flow
dynamics in the conformal cooling channel be understood.
Large hollow voids in the mold working as conformal
cooling channels may not be ideal for optimum heat
removal during cooling or heating for Rapid Heating and
Cooling. The fluid will always circulate along the path of
least resistance through the conformal channel. Hence if
the conformal channel has large variations in cross
sectional area it is highly likely that the flow will stagnate
in some pockets with very little fluid circulating through
those regions. If the fluid is not circulating in a certain
area of the mold poor heat transfer will result in this
region of the mold. This will lead to uneven cooling or
heating of the mold resulting in poor part quality. The
most common heating methods considered for simulation
are saturated steam, electric cartridge and pressurized hot
water heating.
Saturated steam is the most effective medium for
heating the mold due to the release of latent heat energy
when it condenses to water. The heat transfer capacity of
saturated steam is much higher than that of pressurized
hot water or superheated steam. Saturated steam provides
most heat via latent heat release when condensing from a
vapor to a liquid. Only saturated steam is considered in
the work presented here.
Even though saturated steam is the most effective
medium for heating the mold, some molders prefer for
safety reasons to use hot compressed or pressurized water
to heat the mold. Simulation of heating by pressurized
water is included in the work presented here.
After the part has filled the molding cavity at high
cavity surface temperature, the rapid heating and cooling
process aims to cool the part down to ejection temperature
by using conventional or conformal cooling methods.
Usually chilled water is circulated through the channels
for a period of time, reducing the cavity surface
temperature to a temperature suitable for ejection.
On certain parts such as flat screen television
housings or automotive interior components where only
one side of the part requires a high quality surface finish,
only the fixed half of the mold is exposed to the rapid
heating and cooling process. The moving half with the
side of the part where surface finish is not important can
rely on conventional cooling. However for lenses and
other optical applications rapid heating and cooling is
required in both halves of the mold.
Almost all the rapid heating and cooling systems
discussed use a closed loop control system where
thermocouples are placed in the mold at strategic
positions near the cavity to control the commencing and
termination of various stages of the cycle.
Rapid heating and cooling cycle
The rapid heating and cooling cycle differs somewhat
to a conventional injection molding cycle. With the closed
loop nature of the cycle, it is possible that no two injection
molding cycles are ever identical. The heating and cooling
stages could turn on and off at inconsistent times during
successive cycles. Our simulation of the process supports
closed loop control as well as timer controlled processes
where each cycle will be identical.
The rapid heating and cooling cycle usually begins
during the mold open stage just after the previous cycle’s
part is ejected. The mold is now in the mold open stage.
Saturated steam that is usually controlled by the pressure
of the boiler is allowed to fill the rapid heating and
cooling channel. Once the channel is filled with steam the
channel exit is blocked off and the saturated steam
pressure is maintained in the channel for a specified time
or until a thermocouple in the mold has reached a required
temperature. During this period the steam condenses,
releasing its latent heat of condensation to heat the mold.
Once the mold has been heated for a specified time or has
reached the required temperature, the mold closes and
mold filling by injection begins.
Usually once the part has filled and packing is about
to commence the heating stage is stopped by cutting off
the steam supply to the channel. The channel is then
purged with compressed air for a short time to expel the
remaining steam and condensate from the channel. After
this primary air purge the cooling stage of the rapid
heating and cooling process commences where chilled
coolant is cycled through the rapid heating and cooling
channel. The reason for the primary air purge is that
“explosive vaporization” may occur if coolant and steam
come into contact. This may result in pitting on the
surfaces of the channel reducing their heat transfer
effectiveness.
The cooling stage ends after a specified time or once
a thermocouple has reached a required temperature. The
part is then ready for ejection. Once the cooling stage has
completed a secondary air purge occurs where the channel
is cleared of coolant, allowing a clear passage for the
steam to enter for the next cycle. These cycles repeat
themselves for each part that is manufactured.
If heater cartridges are used in the mold the heater
cartridges are turned on and off at various times during
the cycle. The cooling channels are separate entities and
work in conjunction with the heater cartridges.
When using conformal cooling channels in the mold
it is very important that the flow regime is understood
before the mold manufacturing process begins.
In order to have a thorough understanding of the fluid
dynamics in the conformal cooling channel, the
manufacturing process needs to be used in conjunction
with simulation. Simulation allows the user to extract the
full potential of their investment in their manufacturing
technology or process providing them with the
consequences of the part and mold design decisions
before mold manufacturing begins.
Computational Fluid Dynamics (CFD) is a robust
technology that uses numerical methods and algorithms to
analyze fluid flows by solving the Navier-Stokes
equations. Commercial CFD packages have been in
existence for many years and are considered to be a robust
technology. The benefits of CFD technology can be
leveraged to design optimum conformal cooling channels
for plastic injection molds. In order to get the full benefit
of CFD for conformal cooling the CFD solver needs to be
customized to integrate it with plastic injection molding
simulation technology requirements.
Simulation technology
The transient mold cooling module that has been
developed to support 3-D tetrahedral meshes of the mold
is used for simulating conformal cooling and rapid heating
and cooling. [2].
In order to simulate the flow dynamics in the
conformal cooling channels an existing CFD solver was
used to provide the flow and temperature solutions in 3-D
conformal cooling channels [2]. Conformal cooling
channels can also be represented by several (1-D)
channels following the contour of the part closely. These
representations do not require the CFD solver for
simulation and a (1-D) solution is preferable in such
channels due to the complexity of the CFD solutions.
A rapid heating and cooling boundary condition can
be specified on a conformal or conventional cooling
channel, in order to simulate rapid heating and cooling.
The purpose of this paper is to present the conformal
and rapid heating and cooling capabilities of the transient
mold cooling solver. Conformal cooling and rapid heating
and cooling analyses are performed on both arbitrary
shaped 3-D cooling channels and conventional cooling
channels in the mold.
Conformal cooling
The incorporation and application of the CFD solver
in the transient mold cooling module is discussed. The
capabilities of the conformal cooling solver are
demonstrated on a fully 3-D conformal cooling channel.
Only CAD bodies are considered for conformal
cooling geometries to be analyzed with the CFD solution.
If a 3-D CFD solution is desired on 1-D geometry then the
geometry needs to be modeled in 3-D in CAD.
The CFD solver used for simulating the fluid flow in
the conformal cooling channel is a finite element based
solver requiring tetrahedral element meshes in the solution
domain [2]. The tetrahedral element mesh requires special
treatment relating to the geometry of the channel, hence a
standard tetrahedral element mesh cannot be used.
The CFD software package’s mesher places
enhancement layers close to the boundary wall of the
channel and allows the user the option of specifying the
number of enhancement layers. Enhancement layers
should not be placed on the inlet and outlet surfaces of the
conformal cooling channel. In order to guarantee this, the
user must specify the inlet and outlet faces on the CAD
body prior to meshing. The mesher then meshes the body
appropriately and the enhancement layers are clearly
visible on these inlet and exit regions. The CFD solution
is sensitive to the mesh density. Solution accuracy is
proportional to mesh density; however the analysis time
also increases with a finer mesh. Various other mesh
options are also supported allowing more complex flow
phenomena such as recirculation zones to be captured if
needed.
The CFD solver has been fully integrated into the
previously developed transient cooling module Error!
Reference source not found. and the combined operation
is fully automated. The CFD pressure and flow velocity
solution is a steady state solution, as is used for the 1-D
coolant solution. The CFD temperature solution in the
conformal cooling channels is a transient solution and is
solved in conjunction with the mold and part temperature
solutions. The cooling channel wall temperature is
constantly updated by the mold temperature solution
which is used as the boundary condition for the transient
CFD temperature analysis of the coolant. The transient
CFD temperature solution provides the transient mold
temperature solution with a transient convection boundary
condition. Once the transient coolant, mold and part
temperature solutions have converged for a particular time
step the computation is advanced to the next time step
until
the
entire
cycle
time
is
completed.
The velocity plot of the fluid in the conformal cooling
channel needs to be viewed with a cutting plane due to
there being a zero velocity boundary condition on the
channel wall. The velocity vectors can be shown as darts
with the magnitude shown by the color. The velocity can
also be shown as a streamline plot. A heat flux plot also
shows the heat transfer between the fluid in the conformal
cooling channel and the mold. The pressure of the fluid in
the conformal channel can also be shown.
When viewing the velocity plot in conjunction with
the temperature plot the correlation between the
temperature on the surface of the conformal cooling
channel and the flow inside the channel is evident. The
velocity plot is used to identify areas of flow stagnation.
The temperature and heat flux plot can therefore be used
to guide design changes to the mold.
The solver uses a zero outlet pressure boundary
condition in the solution. Hence the calculated inlet
pressure is equivalent to the pressure drop along the
cooling circuit. The inlet temperature is also specified as a
boundary condition and the outlet temperature is
calculated by the CFD solver.
Rapid Heating and Cooling
A rapid heating and cooling inlet can be specified on
both 1-D channels or on the inlet face of the conformal
cooling channel. This inlet boundary condition gives
access to specify the rapid heating and cooling cycle
process information. The solver supports both steam and
hot water heating and cooling fluids.
When considering rapid heating, the condensation of
steam in the cooling channels is modeled taking the latent
heat into account. To calculate the latent heat of
condensation during simulation, saturated steam tables are
used. [8] Appropriate, well known, film-wise
condensation formulations were applied to obtain the heat
transfer coefficient for each time step during the steam
heating phase. [9] As the steam condenses the changing
quality of the steam, which is the ratio of vapor mass to
total mass, is also considered.
In order to model the cooling stage in the rapid
heating and cooling solver the boundary conditions
specified on the rapid heating and cooling node are used.
The process settings dialog was modified for rapid
heating and cooling. A new “mold close time before
injection” option has been added. This time represents the
time duration after the mold has closed and before the
commencement of the next injection cycle. In some rapid
heating and cooling cases the mold is heated up whilst
awaiting injection in this mold close position. When
temperature control is selected the mold may remain in
this state whilst awaiting a trigger signal from a
thermocouple somewhere in the mold before injection will
commence.
On each rapid heating and cooling boundary
condition node the user can select “Time controlled” or
“Temperature controlled” boundary conditions for the
steam heating and cooling of the rapid heating and cooling
channel. The “Time controlled” option allows the user to
set the stages as fixed times during the cycle and the
solution should converge to identical cycles.
The “Temperature controlled” option allows the user
to set a threshold temperature on any node in the mold
that will stop the heating or cooling when the node
reaches this temperature. When using this option the mold
may require a long time to reach the specified temperature
in the cycle and this may alter the specified cycle time.
The user is given the choice of “Extend mold open time”,
“Extend mold close time before injection” or “Do not
delay injection”. If the mold is kept open during rapid
heating by using the “Extend mold open time” option,
heat will be lost through the parting plane via air
convection to the surroundings. If the mold is kept closed
by using the “Extend mold close time before injection”
option, heat conduction would take place through the
closed parting plane to the opposing mold half during the
rapid heating stage. Depending upon the thermocouple
settings this heat transfer could be significant. Hence, the
user has the option of setting the mold position during this
delay. Each mold component can be specified as
belonging to the fixed or moving mold half.
If the user chooses to use heater cartridges for the
rapid heating of the mold, three new heater control
options are available: The options are “Time”,
“Thermocouple” and “Time with Target Temperature”.
The “Time” option allows the user to turn on the
heater cartridge at a specified time late in the cycle and to
turn it off early in the next cycle, preferably just after
filling. The “Thermocouple” option allows the user to
specify a thermocouple node with two temperatures
representing the switch on and switch off temperatures
with accompanying delay times. When the node reaches
these temperatures the heater will behave accordingly.
“Time with Target Temperature” allows the user to
specify times in the cycle when the heater cartridge will
turn on or off together with a node on which the
temperature target is specified which will trigger the
heater to turn off. This can lead to cycle time extension
occurring as a result of the thermocouple not reaching the
required temperature within the specified cycle time. The
same options for extending the cycle time are available to
the user, namely “Extend mold open time”, “Extend mold
close time before injection” or “Do not delay injection”.
These options have the same meaning as for steam
heating.
Similar results are provided from the conformal
cooling rapid heating and cooling solver as are provided
from the 1-D channel rapid heating and cooling solver.
Results and Discussion
By referring to [10] it can be seen that the conformal
cooling channel results match those obtained from the 1-D
solution very closely.
The real benefits of the 3-D CFD solution become
evident when arbitrary shaped conformal cooling channels
need to be analyzed where the 1-D solution cannot be
used for these geometries. Figure 1 shows the conformal
cooling channels which may be used in a mold used for
manufacturing satellite dishes. The conformal cooling
channels follow the contour of the part as closely as
possible with solid mold sections obstructing the flow
where either the feed system or ejector pins are placed.
These obstructions have an effect on the flow pattern,
which in turn has an effect on the coolant temperature
distribution in the cooling channel. The temperature
distribution then influences the heat transfer in the mold.
Figure 3: Coolant temperatures in the lower channel.
Figure 4: Heat flux at mold-coolant interface.
Figure 1: Satellite dish model with conformal channels.
Figure 2: Streamline plot of velocity in lower channel.
Figure 2 shows the streamline plot of the velocity
distribution in the lower conformal cooling channel.
Streamlines close together signify a higher flow rate
though these regions, which means that the velocity is
also generally higher. This is confirmed by the color of
the streamlines. It can be seen that at the entrance of the
conformal cooling channel, on the left of the plot, a large
recirculation zone exists, while behind all the obstructions
the velocities are very low and streamlines are absent. By
looking at Figure 3, which shows the coolant temperature
in the lower conformal cooling channel, it can be seen that
where the velocity is high the coolant temperature is low
and vice versa. The coolant temperature is approximately
30C hotter behind the obstruction than at the inlet of the
conformal cooling channel. Figure 4, showing the plot of
the heat flux between the conformal cooling channel and
the mold, shows that where the coolant stagnates there is
very little heat transfer between the coolant and the mold.
In the regions where the velocity is high there is much
better heat transfer.
Figures 2, 3 and 4 show that proper simulation of the
fluid flow and heat transfer in the conformal cooling
channel is vital in designing an appropriate conformal
cooling system for the mold. Simulation can help the
mold designer refine the design to create optimum cooling
conditions. Without simulation a designer may arrive at a
very complex and expensive mold design that will have
very poor heat dissipation.
To demonstrate the rapid heating and cooling, the
component presented in Figure 5 is molded using rapid
heating and cooling technology. The fixed side of the
mold, the side with the feed system, is heated using the
rapid heating and cooling channel shown close to the
surface of the part. The moving half of the mold uses 4
conventional cooling channels using water set at 30 C
flowing with a Reynolds number of 10000.
Figure 6: Sensor node on mold in contact with the part.
Figure 5: Rapid Heating and Cooling part and mold.
The rapid heating and cooling boundary conditions
are specified on the rapid heating and cooling boundary
condition node as shown in Figure 5. This case uses the
“Time controlled” option for rapid heating and cooling.
As the mold opens for part ejection the rapid heating and
cooling begins with a 2 second air purge to remove all the
excess coolant in the channels. This air purge time
corresponds to a 2 second mold open time set in the
process settings. After the air purge, the channel is heated
with saturated steam at 10 bar pressure for 15 seconds.
The 15 seconds corresponds to the mold-close time before
injection in the process settings. After the steam heating
has finished, the filling process occurs during which the
cavity is filled with polymer and the cavity surface is at its
hottest. The start of filling also corresponds to the
beginning of the primary air purge where the cooling
channels are cleared of excess steam and condensate from
the steam heating phase. After the primary air purge has
completed the rapid heating and cooling channel is cooled
down with chilled water set at 25 C for 13 seconds
flowing with a Reynolds number of 10000.
Figure 7 shows the temperature change of that node
during the injection molding cycle. It can be seen from
Figure 7 that at the time of injection the surface
temperature of the mold is at 135.4 C and is still heating
up even though the rapid heating and cooling channel is
being purged with air. This phenomena is called “thermal
inertia” when the effects of the steam heating are still
being transmitted through the mold whilst the steam
heating has ended. The surface reaches a maximum
temperature of 143.6 C after 2 seconds which
corresponds to the start of the cooling phase in the rapid
heating and cooling channel. During the cooling phase
chilled water is circulated through the cooling channel and
it can be seen that the surface temperature decreases to
111.6 C after 18.8 seconds which corresponds to the end
of the mold open time, which is also the end of the
secondary air purge. This temperature is as expected.
After 18.0 seconds the mold surface starts to heat up again
very quickly. This corresponds to the steam heating phase
of the next cycle and is as expected. At the end of the
cycle the temperature has reached 135.4 C again which
corresponds to the temperature at the start of the cycle.
To see the effect of the rapid heating and cooling
cycling on the surface of the mold in contact with the part,
a sensor node N1930 (see Figure 6) is chosen to monitor
the temperature change during the cycle.
Figure 7: Temperature plot of sensor node in the mold.
Figure 8 shows the temperature change of the fluid
(steam, air or coolant) in the rapid heating and cooling
channel. For this model very low pressure air was used to
purge the channel. By looking at the temperature trace for
the inlet element B89, it can be seen that air enters the
channel at 25 C. It can also be seen that the air heats up
as it cycles through the channel by looking at the mid
channel element B64 and the exit element B76. After 2
seconds the air purge is completed and the cooling cycle
begins. It can be seen that the temperature rise of the
coolant across the cycle is significant. At the end of the
coolant cycling the inlet temperature of the coolant is 25
C and the exit temperature is 38.2 C. This 13.2 degree
temperature variation is a large rise indicating that a large
amount of heat is removed from the mold during this
phase. After 15 seconds the cooling phase is stopped and
the primary air purge begins. It can be seen that the air
heats up during this stage. After 17 seconds the steam
heating phase begins. For this model the saturated steam
is set to a pressure of 10bar in the boiler which, from
steam tables, corresponds to a saturation temperature of
179.9 C. The temperature of the steam remains at this
temperature as it condenses, heating the mold with the
latent heat of condensation.
Figure 8: Time dependent temperature plot of fluid in
rapid heating and cooling channel.
Figure 9 is a plot showing the surface temperature of
the rapid heating and cooling channel that is in contact
with the steam, air or coolant. It can be seen that at the
end of the steam heating cycle the temperatures are
around 150 C which are the hottest during the cycle.
During the air purge the temperature reduces on the
channels. However, after 2 seconds the coolant starts
flowing in the circuit and the surface temperature
decreases significantly very quickly. The inlet element
B89 cools to 80.78 C and the exit element B76 cools to
88.22 C during the cooling stage. The mid channel
element B64’s temperatures are midway between the inlet
and exit elements temperature’s which is as expected.
After 15 seconds the primary air purge begins and the
temperature starts to rise again, as air does not remove
heat as well as the coolant. Once the primary air purge is
complete, the steam heating begins and the surface
temperature of channel heats up quickly to 150 C. This is
as expected.
Figure 9: Time dependent temperature plot of the rapid
heating and cooling channel surface.
Figure 10 shows the mold cavity temperature in
contact with the part at the time of the flow front arrival.
This result is a Flow solver result which takes advantage
of the transient mold temperature solution. Figure 10
shows both the top of the part temperature exposed to the
rapid heating and cooling channel as well as the part
temperature on the other side that is exposed to
conventional cooling. By looking at these temperatures it
can be seen that the top part temperature is much higher
than the bottom temperature. This would be desirable if
the user would prefer a better surface finish on the top of
the part. However, the temperature distribution is not very
uniform across the top surface which may necessitate a
mold re-design so that the rapid heating and cooling
channel can be a consistent distance from the part.
Subsequent warp analysis to predict the final part shape as
a consequence of these uneven cavity surface
temperatures is also possible. Figure 10 is the most useful
plot for a part and mold designer as the ultimate goal of
rapid heating and cooling is to obtain a mold surface
temperature as hot as possible during the filling stage.
Figure 10 provides the user with this information.
Figure 10: Flow front temperature of the top and bottom
of the mold cavity.
Conclusions
The conformal cooling functionality added to the
transient cool simulation is a worthwhile enhancement to
the existing simulation technology when simulating
conformal cooling channels that follow the contour of the
part closely. Previous work has shown that the results
from the conformal cooling solver match those from the
1-D solution very well.
Rapid Heating and cooling functionality can also be
used with conformal cooling and the numerical results
obtained from these simulations agree with observations
made from actual processes.
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innovation.com.
manufacturing,
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Key Words: Injection molding, Autodesk, transient cool,
simulation,
conformal
cooling,
CFD