posted March 27, 2005 07:04 PM               

Vent flow problems can be broken down into two problem source;

Bad vent geometry

Presssure flow

Since the vent works with PS, probably the geometry is good.

Pressure flow can occur when the head backpressure is too high, or the volume of material in the screw at the vent is too great.

Potential surces of your problem are:

Screens blinded by unmelt causing high backpressure.

Foaming at the vent causing material to be pushed out the vent.

I would first try to deal with the unmelt problem. Barrel zones should be set to 520-540F. Take of the die head and see what the melt quality looks like. Slow the screw down until you get a good quality melt and no venting.

Put the head back on and see if you are able to run. If you are unable to run you may need a new screw design. I would be happy to quote you a new design should it come to that.

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Best Regards,

Tom Cunningham

Extrusion Technical Services