PVC Extrusion/Calendering

Garbage in-garbage out.

If you put wet compound, with trapped air into your extruder, this is what you will get.

Extrusion does not keep the material under sufficient pressure while the extrudate is solidifying. So bubbles grow.

Dry the material, then remove excess air.

Thanks,

you're actually saying put in the dryer AND the vent.

All my experience is in injection molding.

Consider my opinion nothing more than a guess.

Talk to some extruders. Look for a thermoplastic processors forum and ask them.

We had 40 injection molding machines in one plant and most of them had dryers to maintain quality.. It appears to be mandatory in some conditions.

Putting the feed hopper on resilient mounts and vibrating it (small amplitude, high frequency) will knock air out of the mix, but moisture is a more likely cause for your problems. Note over vibration will create bridging problems.

interesting to say the least. Are there any papers on the subject showing the effect of vibration on the number of air inclusions in the sheet please?

I do not know of any papers on this subject, my suggestion is only based on experience if handling powder blends for other process feeds. Before spending heavily on permanent equipment you could improvise some tests with temporary kit to see what works.

A pneumatic vibrator suspended in the feed hopper from it's air feed line would simulate vibrating the hopper. Make sure you pipe the exhaust out of the hopper. If your product does not improve you have eliminated one possibility at a very low cost.

Make up a blend and dry it off line, (your ingredients suppliers or the extruder manufacturer may be able to make up test samples if you do not have the facilities in house), but improvising a solution is better because you can you can create different degrees of dryness to test. If drying out the mix solves the problem you will also have data to help specify the degree of dryness required then buying a permanent drier solution.

you blend must be dry this is typical of moisture also in hi humidity if the delivery system starts and stops can condensate it is best to dry product in a nitrogen atmosphere or best results