How to Optimize the Cycle Time of Your Injection Molded Part

Written by Sean Mertes, Account Development Engineer

In my years of tech service, I have spent many hours at a press trying to optimize the cycle time of a part, and discovered that not enough process technicians and engineers understand the demolding temperature of their plastic parts. We can all agree that the injection molding process starts with filling the part. We don’t want to fill too quickly or too slowly. We need to make sure we are not pressure limited, then fill to the correct switch over position, and finally transfer to pack/hold. During pack/hold, we need to optimize the pressure, understand gate freeze time, and fill the part, so that there are no sinks or voids. The last part of the process cycle for injection molding is cooling the polymer for ejection, but this step of the cycle is typically glossed over. What most people seem to neglect is that the cooling of the part should typically be the longest part of the overall cycle.

See Figure #1. Cooling in the mold beings during Fill/Pack/Hold and until the part is ejected.

Figure #1. Sequential timers on machine.

Many process technicians and engineers know and understand how to optimize the
filling/packing/holding portion of the cycle. However, when asked about the cooling, I never
get a great answer.

I have always been intrigued with the demolding temps of different polymers. Many years
ago, I created a spreadsheet and started to catalog how “hot” different molding shops were
kicking out the parts. It became difficult to try and classify each polymer, filler, filler
percentage, and the demolding temperature that was being utilized.

I recall one customer that was molding Nylon 6 for a fitting. I went in to optimize the cycle
and watch demolding temps, and I remember now that if we kicked out the polymer above
200 F (93 C), we had issues with the part retaining its shape. It was not until I was in a class
by independent materials and processing consultant Mike Sepe years later that the light
finally came on. Mike was showing a DMA curve (Dynamic Mechanic Analysis), and
everything started to make sense.

See Figure #2.

I realized that molding technicians should read the graph from right to left and the design
engineers read the graph from left to right. I am currently learning more about this in
Beaumont’s American Injection Molding Institute (AIM) Plastics Technology & Engineering
program (PTE). We are deep diving into DMA curves and what they represent and how to
utilize them when designing parts as well as understanding cooling demolding temps.

For example, there was a molding trial where we were trying to replace

FR PC/ABS with our Tristar FR PC.

I was not able to attend the trial; however, when I asked how our FR PC ran, I was told that it was a drop-in replacement. I asked if they were able to cycle faster using the PC versus the previous PC/ABS resin and was told that they keep the cooling time and mold

I explained that the PC could be demolded at a much hotter temp than the current PC/ABS
and we should be able to reduce the cooling time by a few seconds.
I illustrated this by showing them Figure #3 below.

Figure #3.

We submitted more material and decided to rerun the trial, but while monitoring the
demolding temperature of the parts. We were not only able to reduce the cooling time on
the existing process using FR PC/ABS, but we also saved even more cooling time when we
switched to our FR PC.

By using Scientific Molding procedures and understand the fill time, pack/hold time and
using these DMA curves along with monitoring the demolding temperatures of the parts,
we can minimize the cooling time and achieve the fastest cycle time.

Looking for more insight into demolding techniques?

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