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Author Topic:   Vent pumps
Tom Byrne
Junior Member
posted 06-21-2001 12:04 AM     Click Here to See the Profile for Tom Byrne     Edit/Delete Message   Reply w/Quote
Why would there ever be required and wouldn't they encourage vent bleed?

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Tom C
Junior Member
posted 06-21-2001 08:37 AM     Click Here to See the Profile for Tom C   Click Here to Email Tom C     Edit/Delete Message   Reply w/Quote
Vacuum venting works great in some situations where the melt has a lot of air or other volatiles mixed in. This occurs frequently when compounding mineral powders or fibers into plastic. A lot of air is entrained in the mix, and a vacuum pump helps to densify the melt.

Conventional extrusion of resin or pre-compounded pellets should not require a vent pump, or even a vent if the resin was produced correctly, has been dried if required, and has a proper screw design.

A properly designed screw and vent with vent diverter should not have any problems working with a vacuum pump. Vent flow will occur under the same circumstances as without a vent pump. Usually the presence of a vacuum will make the melt foam more than without vacuum. If you design the screw so that all of the foaming occurs, and is devolatilized, well before the vent then you will have a nice dense melt passing by the vent.

Downsides of vacuum venting are:

1) You are concentrating sometimes not so nice chemicals and discharging them to the air or water.

2) Some of the volitile chemicals will condense in the vacuum system, causing problems.

3) Cleanout of fouled vacuum pipes can be difficult.

Tom @ CCS

[This message has been edited by Tom C (edited 06-21-2001).]

[This message has been edited by Tom C (edited 06-21-2001).]

[This message has been edited by Tom C (edited 06-21-2001).]

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2beeps
Junior Member
posted 07-01-2001 05:14 PM     Click Here to See the Profile for 2beeps   Click Here to Email 2beeps     Edit/Delete Message   Reply w/Quote
Hi Tom & Tom,

My experience with vacuum venting was primarily with both single and twin screw rigid PVC extrusion from powder blend compound. A good vent vacuum IS necessary to pull trace moisture, entrapped air, and any other volatiles out BEFORE the compound is fully fused,or melted----in order to prevent porosity in the finished product. Most 24:1 or 32:1 L/D single screws, and the conical and parallel twin screws have screws designed with a decompression zone at the vent, which allows a good 24-28" vacuum to be pulled with little or no vent flow, if the extruder running conditions (temperature profile, RPM,adapter orifice size) are balanced to the
fusion and melt viscosity characteristics of the formulation.

The same would hold true for flexible PVC extruded from powder blend, in my opinion.
I don't know of any other polymers that are normally extruded from powder. With pellets, of course, a vent is not necessary.

Regards,
Skip

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