Understanding The Demolding Temperatures of Your Parts

How to Optimize Cycle Times for Your Injection Molded Parts

Over my years in tech service, I've spent many hours at a press dialing in the cycle time of a molded part. One surprising thing I've discovered is that not enough process technicians and engineers pay attention to demolding temperatures.

The injection molding process starts with filling a part; you don’t want to fill too quickly or too slowly, so you constantly need to check that you're not pressure-limited. After filling the mold to the correct switchover position, you'll move on to the pack and hold steps.

During pack and hold, you have to apply the correct pressure and gate freeze time. (This makes sure no sinks or voids form.) The last step of the cycle for injection molding is cooling the polymer to prepare it for ejection — which is the step that's typically done wrong.

What most of my colleagues ignore is that the cooling of the part should typically be the longest part of the overall cycle if you want to get the most out of your material properties.

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

Figure #1. Sequential timers, as seen on the machine.

I've always been interested in polymer demolding temperatures. Long ago, I created a spreadsheet and started to catalog how hot parts were when our client molding shops kicked them out.

I remember one customer that was molding Nylon 6 for a fitting. I went in to help optimize their cycle and record demolding data; I found that if we kicked out the polymer above 200 F (93 C), we'd have issues with parts retaining their shape.

It wasn't until I took an independent materials and processing class with consultant Mike Sepe years down the road, though, that the culprit of our clients' issues came into focus. Mike was showing the following DMA (Dynamic Mechanic Analysis) curve:

Figure #2.

I realized that molding technicians should read the graph from right to left and design engineers should read it left to right.

Beaumont’s American Injection Molding Institute (AIM) Plastics Technology & Engineering program (PTE), a course I'm currently enrolled in, talks more about this divide. By studying DMA curves and what they represent, I can teach our clients to use them to their advantage (instead of working against themselves).

Just recently, I learned about a molding trial where a client replaced FR PC/ABS with our Tristar® FR PC.

Although I couldn't attend the trial in-person, when I asked how the Tristar® FR PC ran, the client claimed it was a drop-in replacement. I asked if they were able to cycle faster using the new resin, but they told me they'd kept the cooling time and mold steel temperatures constant.

I explained that the PC could be demolded at a much hotter temp than the current PC/ABS and that we should be able to reduce the cooling time by a few seconds. I used this graph to prove my point.

Figure #3.

When we reran the trial with my recommendations applied, we saved even more time than I'd anticipated.

Thanks to scientific molding procedures, including proper DMA curve analysis and closely monitoring demolding temperatures, we can minimize cooling times and achieve the faster cycles made possible by innovative materials.

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