Cosmetic Process Window

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

I'm not sure how to solve it, but it looks like a delay in the response of the machine. Is this an electric or hydraulic machine?

This is a hydraulic injection unit.

Hello Rook,

1. You are very correct to cap off the pressures. My limit is actually about 6000 plastic psi. Byond that you can over pack the part and/or damage something. And so one has to be careful.
2. Looking at the dip, is there a ramp down speed (from Inj to Pack, Hold)? You may need to adjust that. I also think Nikom above is on the right track - there could be some delayed response from the machine. See if you can adjust the Pack & Hold speeds. May help.

Let us know, we all want to learn.

Suhas

Thanks for replies!

I did some processing this afternoon and hope my findings can spark a good discussion!

It might help to explain the graphic. The red line indicates the injection velocity set point and the machines actual injection velocity. The blue line shows the melt pressure that the screw is generating during injection and after transfer it shows the pack pressure. The yellow line displays the cushion (screw position) after transfer.

I began molding with the same process as yesterday and only made one adjustment at a time.

The first thing I noticed was that the cushion has a little bump in the trend line. It seems to be aligned with the pack pressure dip. I was thinking a lot about it, and I thought maybe I’m not filling the cavity enough. When it switches to second stage, the pack pressure quickly packs in material with little resistance, so the pack pressure dips until the cavity is full to provide resistance and level off? I think of it like walking down a staircase and expecting one more step that’s not really there. We have that feeling of falling until we touch the ground.

So I increased the shot size from 105mm to 110mm and it actually improved the dip. It was nearly gone!

I thought more about increasing the shot size and decided to set it back to 105mm and increase just the injection velocity. Here I went from 400 mm/sec to 600 mm/sec. Making a velocity change would normally force me to take off pack and hold to measure fill only weight, but here I just wanted to see what happened. And to my surprise, the dip flattened out!

But I didn’t stop here! I went back to 400 mm/sec and 110mm shot size. The dip was nearly gone and I decided to increase the pack pressure from 10,000 to 12,000 psi. Here is where I began to scratch my head, because the dip came back.

I ran out of time to check velocity alone with increased pack pressure. Any ideas to help me sort out the correct solution?

Attachments
1 - Normal Process
2 - Shot Size Increase
3 - Velocity Increase normal shot size
4 - Normal velocity, increased shot size, increased pack pressure


Rook

The images you posted are fairly small and make it difficult to read the graphs, you may want to reupload them to somewhere like Imgur and link them here.

Is there any sort of velocity limit on the holding stage in the machine controller?
Is the pressure on the graphs from a cavity sensor or from the machines transducer on the back of the screw?