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For certain types of part geometry, the gas-assisted
injection molding process is a common alternative to conventional
injection molding. However, the level of processing complexity is
increased versus conventional injection molding due to the injection
of the gas within the polymer melt. Understanding and predicting
the behavior and flow of the gas within the polymer melt is critical
to achieving success in designing both parts and molds for the gas-assist
process. Through years of research and industry alliances, Moldflow
has developed an understanding of not only the gas-assisted injection
molding process, but also how to accurately predict the flow behavior
of both the polymer melt and the injected gas. Furthermore, in Moldflow
Plastics Insight™ 3.0 (MPI™ 3.0), the capabilities of MPI/Gas have
been significantly extended (MPI/Gas is the MPI module for simulating
the gas-assist process). MPI/Gas now interfaces with both MPI/Warp
and MPI/Fiber to predict both the shrinkage and warpage of gas-assisted
injection molded parts, as well as to predict the effects of fibers
during the filling of the part and the effect of the fibers on post-molding
shrinkage and part warpage. MPI/Gas recently introduced capabilities
that benefit those who need to optimize part and mold designs, as
well as the gas-assisted injection molding process itself.
The gas-assisted injection molding process has found
a niche among certain types of applications such as television cabinets,
automotive interior door handles, appliance parts, toys, and many
others. It is common knowledge that by using the gas-assist process,
it is possible to produce parts that are rigid, devoid of sink marks,
and have a lower tendency to warp. The gas-assist process also has
been proven to reduce material consumption, machine clamp tonnage
and cycle time. The gas-assist process is frequently used to produce
complex parts that consist of a combination of thick and thin sections.
Because the gas-assist process involves the dynamic
interaction of two rheologically dissimilar materials flowing within
typically complex mold cavities, the optimization of the part design,
mold design, and process conditions is, at best, extremely difficult.
Furthermore, even years of experience with conventional injection
molding are not sufficient to deal with the gas-assist process,
especially in designing the gas-channel network and optimizing the
processing window.
MPI/Gas is a powerful software tool that provides
the know-how to better understand the complexities of the gas-assisted
injection molding process. The knowledge gained from using MPI/Gas
typically equates to improved part and mold design and the reduction
or elimination of problems during the production of gas-assist parts.
The software assists part and mold designers in:
- Evaluating material selection for gas-assisted
injection molding part design.
- Optimizing the plastic part design to obtain
the desired gas penetration.
- Evaluating mold design options and their
impact on gas penetration.
- Determining the appropriate processing
conditions for producing acceptable quality parts, including
the volume of plastic to inject and the timing of the gas
injection.
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New
MPI/Gas Analysis Capabilities
New with the release of MPI 3.0, the MPI/Gas
analysis module supports the simulation of shrinkage and warpage
of a gas-assist part through an interface to MPI/Warp. As
a result, it is possible to estimate the residual stresses
in the gas-assist part and consider the effects on part warpage.
All things being equal, a gas-assist molded part typically
will be more structurally rigid versus the same part made
by the conventional injection molding process. Part designers
can now validate this in the early stages of part design,
where the cost of change is minimal and the impact of that
change is greatest.
On the left-hand side is an example of an
actual part produced by conventional injection molding, which
was found to have an unacceptable amount of post-molding warpage.
The same part was molded using the gas-assist process in an
attempt to reduce warpage to within acceptable tolerances.
The design was slightly modified to add flow leaders, which
served as the gas channels and which were the primary source
of gas coring.
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The images that follow indicate the out-of-plane
displacement using conventional injection molding and gas-assisted
injection molding. It can be seen that the image on the left, produced
by conventional injection molding, has significant warpage, while
the image on the right, the gas-assist part, is relatively flat.
The bottom image displays the predicted penetration of the gas channels,
seen as an X across the surface of the part.
In MPI 3.0, MPI/Gas users can also benefit from
a new interface to MPI/Fiber, which allows for the accurate simulation
of the gas-assist molding process with fiber-filled plastics. Fibers
such as glass and carbon are often added to plastics to improve
the structural performance of the final molded parts. These parts
are often larger in nature and lend themselves to the gas-assisted
injection molding process to reduce wall thickness, part weight,
warpage, and cycle time. The complexities of injecting fiber-filled
plastics with gas can now be simulated in MPI 3.0 to help ensure
that the molded parts will be of acceptable quality.
In addition, MPI/Gas flow results can be used as
an input to an MPI/Cool analysis, allowing users to study the impact
of a cooling-line layout on gas penetration and cycle time. The
cooling-line layout can be modified to improve the overall cooling
of the part, as well as to promote gas penetration.
Conclusion
Because of the inherent complexities of the gas-assisted
injection molding process, it is necessary for companies to understand
the behavior and flow of the gas within the polymer melt in order
to optimize part design, mold design, and the gas-assisted injection
molding process itself. The MPI/Gas module of the Moldflow Plastics
Insight product line provides the tools required to accurately predict
the behavior of both the gas and polymer within the gas-assist process
so that users can optimize their part and mold designs with confidence.
Furthermore, in MPI 3.0, the capabilities of MF/Gas have been significantly
extended. MPI/Gas now interfaces with MPI/Warp to predict both the
shrinkage and warpage of gas-assisted injection molded parts, and
with MPI/Fiber to predict the effects of fibers on the filling of
the part and the effect of the fiber orientation on post-molding
shrinkage and part warpage.
For more information about Moldflow Plastics Insight
products, visit www.moldflow.com
or contact your local Moldflow representative.
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