Cosmetic Process Window

[The Rookie]

Hey all!

I’m working on a new valve gated hot runner mold and have reached the Cosmetic Process Window study to determine appropriate pack pressures.

Transfer pressure is around 17,500 psi

I started from no pack pressure and increased by 1000 until cosmetically acceptable parts were made. This turned out to be 8,000 psi. I then started to find the upper limit. I got up to around 15,000 psi and the parts still looked good. The goal is to go until parts stick in the mold, flash or warpage. But that makes me a little nervous to keep increasing pack pressure higher than that, so I capped it there for now.

The procedure itself went great but I noticed something from the injection/packing phase graph that I was hoping to get help on.

At 8,000 psi pack pressure the melt pressure readout is almost flat after transfer. When I increase the pack pressure, there is a dip in the melt pressure after transfer before it then gets to the set-point. I’ve attached some graphs at 8000, 11000 and 15000 psi pack.

Can anyone explain what is actually going on here? My colleagues are telling me that I need to profile the pack pressures to get rid of the dip in melt pressure. I trialed a few different profiles and it didn’t seem to make an impact with what I tested. It might be the correct procedure, but I’m hoping to get some feedback on what is actually happening in the mold and the proper solution.

Thanks!!

Rook