posted January 11, 2007 01:45 AM               

We are facing now another problem. there is indentation/pits on the sheet surface and it is occuring randomly all over the sheets.

When the sheet is thermoformed? the indentation remains there. Initial thought is that this is due to trapped gases/ moisture but the material was pre-dried for 3 hours and still the problem is there?

Material in used is HIPS + white masterbatch.
and sheet thickness is 0.7mm.

Whay can I do to remove this indentation marks?

Appreciate the feedback.

Thanks

Mazad

posted January 11, 2007 06:41 PM               

Are you running virgin HIPS or reclaim?

What extrusion parameters are you setting and getting (Sets=Barrel Zone Temps, Screen Pack, Die /Adaptor Temps, screw speed, Die Lip gap-Gets= Melt Temp)

Have you run some material without MBX in it, what is the carrier used in your MBX. I have had problems with MBX with a "universal" carrier (PE) and insist on GPPS as a carrier for MBX intended for HIPS use.

If you are running reclaim, dust etc could be the likely cause of the problem. Air entrapment from either running too fast, or a poor screw design is another posibility (what is the geometry of your screw- I assume it is not vented)

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Steve Hodgson

posted January 12, 2007 09:06 PM               

Try drying the material longer to see if there is any improvement. If so, then go to a good vented extrusion screw design.