SIGMASOFT® Virtual Molding Solves Ejection Problems in Injection
Contact pressure and ejection-induced part warpage predicted
As the demand for complex parts increases, so does the request for more effective ejection
systems in injection molds. While the normal approach is to troubleshoot the system during
mold trials, an extremely cost-effective alternative is now available to simulate the ejection
process, to predict possible issues and to optimize the mold performance.
Figure 1 – Contact pressure in the part shows that the holes are gripping to their molding
The design of ejector pins in injection molds is a task that often lacks a rigorous mechanical
analysis. The geometrical configuration, as well as the contact area and the ejection
pressure are normally based on previous experiences and problems are only identified once
the mold is set up on the machine and first trials are started.
This approach, while common, is risky and expensive: it is not unusual to find situations
where the required ejection pressure perforates the part, or where the ejection pins are
ineffective; in these cases some “last minute” changes in the ejection pin configuration must
be done, delaying the production start. It is also common to find ejection configurations
where some pins are not active in the ejection process, as the part shrinks “away” from them,
and thus the manufacturing costs – and perhaps the mold complexity caused by them – is
One of the most problematic situations is, however, the configuration where the part “sticks”
to the wrong mold half, so that it is practically impossible to eject it. With a general demand
for a single plastic part to integrate functionalities of several previous parts, as well as the
increasing need for design differentiation, the complexity of the molded component increases, and in some cases previous knowledge related to effective ejection systems may
For an example part an ejection system had been designed as seen in Figure 2. In this
approach, the action of three large pins in the corners was supported by eight small pins on
the cylindrical reverse. In practice, the part was sticking to the wrong mold half, and the
company reached out to SIGMA Engineer Timo Gebauer to solve the problem.
“The first approach was to run the mold virtually, just as in reality, to observe what was
causing the ejection problem”, explains Gebauer. To accomplish this, SIGMASOFT® Virtual
Molding used the complete mold information: all mold components where integrated in the
analysis, including the ejection system. Several molding cycles were simulated sequentially,
and afterwards a filling, packing and ejection analysis of the molded part was completed.
“It is important to consider all mold elements in the simulation, and to simulate several
thermal cycles. Some factors as thermal expansion, and thermal induced molded stress are
decisive to evaluate the ejection behavior of the part”, states Gebauer. A viscoelastic
material model, available in SIGMASOFT® Virtual Molding, was used to simulate the residual
stresses present in the part as it shrinks against the mold cavity, producing contact pressure.
SIGMASOFT® Virtual Molding was able to reproduce the way the part deformed during
ejection, as well as the contact pressure present at each location in the part, as seen in
Figure 1. It was evident that the small holes were gripping the part to the wrong mold half.
“The part stayed in the movable half because the inserts used to shape the holes were
retaining it, as the part warped constraining these inserts. This effect was evident when the
contact pressure in the holes was analyzed”, explains Gebauer.
The proposed solution was to make the ejector pins act as inserts to mold the part. “If the
ejector pins build the cavity, the ejection system will work correctly”, declares Gebauer. “In
this case, a detailed analysis of the deformation behavior was enough to identify the root
cause of the problem and to find a suitable and low cost solution”.
According to Gebauer, most of the ejection systems operate far from their optimum. This
increases both costs and mold complexity, and in some cases it may compromise cycle time
or part quality. With SIGMASOFT® Virtual Molding, an inexpensive approach is available to
get insight in the mold performance and to predict the contact pressure produced at each
part location. “This understanding is impossible to get in real life: all you see is if the part
ejects or not, if it cracks or not. And if it does, you are not only wasting production time, you
may also need to start rethinking your ejection system, without really knowing where to start”,
Besides the reduction in iteration time in the real mold, ejection simulation in SIGMASOFT®
Virtual Molding delivers understanding about the root causes of ejection problems. “The team
can learn from these problems and avoid them in the future”. Also the mold performance is
increased, building the reputation of a mold making department or company to deliver
functional, reliable molds, even for complex parts. The productivity is increased, as instead of
spending machine and personnel resources in iterations the mold’s start up is shortened,
releasing resources for development and growth. “And last but not least, the design team
becomes confident to engage in new, more complex projects, as a powerful predictive tool is
there to back them up”, explains Gebauer.
Figure 2 – Example part with layout of ejector pins
For further information on this story, please contact:
SIGMA Plastic Services, Inc.
11 N. Martingale Road, Suite 425
Schaumburg, IL 60173
SIGMA Plastic Services, Inc.