posted June 13, 2003 01:33 AM            

We are experiencing gauge variation across the sheet. I have tried choker bar adjustments but to no avail. The weird thing I found though, was when I took out the mesh screens and breaker plate and continued running the gauge became more uniform. The product in question is a mix of milled resin, foaming agent (azo) and compounded fire retardent pellets. When we run straight powder product we don't seem to have the same problem.

Regards

posted June 17, 2003 11:47 PM               

From your posting I would say you are restricting melt flow with screenpack overkill.

Throw in a 20-40 and watch your PSI go down.

One extra 80 screen can increase flow temp as much as 80 deg.

Depending on your set material, take the damn screens right out!
Use the breaker as a poor-man's mixer and nothing more.

If you run regrind, it will run hotter with more PSI, and you may want to use a screen for degra-d.
If thats the case, use a 40.

posted June 18, 2003 04:01 AM               

What barrel settings, screw speed are you using, and what melt temp and pressure are you getting.

posted June 18, 2003 09:18 PM            

Steve, we run the following settings

B1 - 110
B2 - 110
B3 - 105
B4 - 105
B5 - 100
B6 - 100
Screw RPM - this changes depending upon gauge variation. We have run at 30rpm and got stable gauge, other times we don't. Therefore we are forced to run slower 25- 26rpm. It is very inconsistant. Obviously management want us to run at as high a screw speed as possible to improve output.
Melt Pressure - 2000PSI
Melt Temp - 127 (to reduce prefoaming)
Screen - 20/60/20 (I have tried a 40/20 with little improvement, should I go to a 20/20) What is the philosophy behind screen pack combinations. Is it purely based on trial and error?
Breaker Plate - Standard straight through holes. I have read that the holes on the downstream should be chamferred to reduce dead spots. Would this help?
Amps - 90 to 100 how important is this in terms of mixing?

posted June 20, 2003 04:53 PM               

A twenty mesh screen is only included to support a finer mesh screen. The "open" area of a 40 mesh is about 38% while the "open" area of a 200 mesh is 34%- But the 200 mesh will trap more crap from the regrind and do it quicker, than the 40. A more restrictive pack raises the back pressure and can be a way of improving mixing (as a temp fix). If this worked then you would get a better designed screw and look at valving an adaptor.

Can you measure the pressure drop across your screens ?

Tell management that they can either have 100% crap at 30 RPM or 97% sellable at 26 RPM and let them make the call. What diameter screw have you got?

Screw motor amps depend on the viscosity of the material, I don't think you can infer the degree of mixing from them. And I know you can't use them as a poor mans pressure gauge.

If you had a melt temp T/C that traversed the melt, and the variation in readings was 1/2C across the melt, that would be great. A 1C variation would be acceptable and a 2C would be bad, very bad.

posted June 24, 2003 03:23 PM               

A delta of 200PSI over your screen pack is low. With a 40-60-40, and HIPS, we get a delta of 400PSI (fresh screen pack), when the delta climbs to 650PSI, we change screens.

Increasing the pressure measured at the end of the screw (head) infers that more back pressure is being generated.This is accomplished by valving an adaptor, or restricting the melt path by a plate. This back pressure pushes material over the flights and generally improves mixing.

I like your use of a reverse barrel temp profile, have you tried slightly higher temps in the feed zone?