Posts: 11
From:pune,maharastra,INDIA.
Registered: Jan 2005

posted August 24, 2007 02:20 AM            

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I am working in a
company which produces pvc rigid,corrugated and foam
sheets.we use contra-rotating parallel twinscrew
extruder (Theysohn made).we are facing the problem of
yellow lines which we call burning-lines on the upper
surface of sheet after running the m/c for 10-12hrs of
foam sheet production.I have seen the plateouts in
upper-part of the adaptor and on screw at metering zone.I sugested to increase the heat in this region but my Boss doesn't accept my view,instead he says
that "adaptor temp. should be low to increase back
pressure & induce the gases in the foam cell.And the company which manufactures pvc sheet with lead stablizer faces the same problem"
we process the rigid pvc on back-pressur of
100-105 KN and foam sheet on 150-155KN. I have also
checked the adaptor temp. which shows minus 30 c of
its setting. I have also read in your forum that the
screw temp. should be equivalent to metering zone but
in your case we keep the screw temp.equivalent to
venting zone. so pl. clarify my doubt which is as
follows:
1.Is it neccessary to process foam sheet on low temp?
i.e with high back-pressure.?
2. If we increase the adaptor and screw temp.
equivalent to metering zone then what will happen at rear barrel temp.,how much heat we have to reduce?
our barrel parameter is as follow:
B1-135c,B2-145,B3-160c,B4-168c,B5-175c.
screw temp.=155c and adaptor=156c.
3.All company in the world which process pvc with
lead stablizer faces the problem of burning line,is it
true?If true,is there any solution?
I will be grateful to you if you clarify my doubt.

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Posts: 889
From:Brodheadsville, PA USA
Registered: Jun 2001

posted August 24, 2007 01:26 PM            

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Is the line in the center of the sheet where the screws meet?

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Best Regards,

Tom Cunningham

www.ExtrusionTechnicalServices.com

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Posts: 11
From:pune,maharastra,INDIA.
Registered: Jan 2005

posted August 25, 2007 01:43 AM            

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yes, the yellow line emerge from the centre of the sheet where screw meets.Earlier what happens the yellow line start from both the edge of the sheet and gradually increase upto two inche on both edge and then suddenly yellow line emerge from the centre of the sheet on upper surface. some time black burnt particle emerge from the die followed by yellow lines continuously.
This problem occur in foam sheet production after 10-12hrs of machine start and same problem observe in rigid and corrugated sheet production after 45-60hrs.

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Posts: 889
From:Brodheadsville, PA USA
Registered: Jun 2001

posted August 25, 2007 09:08 AM            

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The flow field shape from a figure 8 barrel to a profile can be very complex and can include dead spots and high shear points. Both can produce discoloration.

This can be a very difficult problem to solve. I suggest a flow analysis and redesign by a professional. Contact me if you would like additional information.

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Best Regards,

Tom Cunningham

www.ExtrusionTechnicalServices.com

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