posted July 24, 2002 04:31 AM               

Our sheet extrusion team is experiencing high (12% - 15%) shrinkage rate on HIPS sheets.

Occassionally, we also encounter sheet warping with our Acrylic and HIPS sheets.

What could be the cause of these problems? (Too fast output, raw mat, MFI?)

Our machine is from Cin-Milacron of about 7 years old. We also use about 20% regrind material which sometimes we pelletize first.

Allen

posted July 24, 2002 05:04 AM               

Try setting the lip of your sheet die to the required gauge Plus 10%- then set the gaps on your chill roll stack to the desired gauge. Run the roll temps as hot as posible, you may need to consider adding an extra chill roller (or two) to your line. With HIPS, higher roll temps improve gloss, increase impact strenght and tend to lower orientation shrinkage.

You might want to look at independantly controlling the chill rolls and the rubber pull rolls to further reduce sheet orientation.

Check the temperature variation along each chill roll-there shouldn't be any more than
1C difference, while you should have at least
5C difference between the top and middle, and 15 between the middle and bottom (reverse if your sheet is running up stack.

posted July 25, 2002 10:38 PM               

I'm not sure if the HIPS material's MFI has anything to do with the high shrinkages. Yesterday, I learned shrinkage rates reached 22%! Sheet thickness is 1.8-2 mm and 550-570 mm width. Our standard extruder output for these sheets is 190 kgs/hour.

We also suspect that the high MFI of the HIPS material we are using was causing the intermittent problem. The material's lot MFI (just one average MFI per lot is indicated) is usually 5 - 6 according to the supplier's QC certificate . But the local supplier's lot size is 10 to 20 metric tons. And they don't have general guidelines on how to process their material for extrusion.

We had a sample tested from 2 independent labs and the MFI was 3.4 and 3.7 so the average lot MFI the supplier was providing us may not be representative due to wide variations. We've already asked the supplier to reduce the lot size.

Do you think slowing down the line speed or lowering the temp to compensate for the high MFI is a good idea? Can the high MFI be considered the cause of the shrinkages?

Thanks again for your help.

posted July 25, 2002 11:31 PM               

Just checked with our production and it seems we are already doing what what you have suggested (die and roller gaps).

Upper roll temp is 10C, middle 95C and lower 80C. But there might be 2C fluctuations in the upper and lower chill rolls.

The die, upper and lower lips are set at 200C.

The conveyor is synchronized with the chill roll speed.

If you increase the line speed, would the chill roll temperatures go down drastically?

posted July 26, 2002 12:58 AM               

Are the chill rollers on your stack independantly driven, or driven by a single drive unit via a chain? It might solve your problem if you can reconfigure the haul off rollers so that you can control the rate you
pull the sheet away from the final chill roller. What temp is the sheet when it leaves the final chill roll? This could be where your problem is occuring.

The die temps sound good, what melt temp are you getting. It would be worth trying a die set of ,gauge plus 5%, then theoretically you are only drawing the material down by 5%.

You need to check how the temp varies along each chill roll- this shouldn't be more than one degree C. Otherwise you are cooling one side of the sheet at a different rate than the other (instant curly sheet).

Have you got your material supplier to send a tech to help suss out the problem? maybe worth a call.

posted July 27, 2002 04:27 PM               

This may be a silly question, but how are you determing the shrinkage occuring in your sheet?

This link is the generally accepted method http://www.plasticsheet.org/Guide/HOTAIR.pdf

When I refered to cooling the sides of the sheet at different rates, I was talking about the edges of the sheet.

posted July 28, 2002 11:09 PM               

The die temps ranges from 199C to 201C at different zones. The polishing rolls may be fluctuating by more than 2C. The melp pump temp is 180C, though I still need to verify and I'm trying to get the answers to your other questions.

The HIPS material supplier is a local Manufacturer (Philippines). Unfortunately, they are not able to give in-depth help and they are not too familiar with running an extruder. They don't even have guidelines for processing their material - unlike big name suppliers.

The material is customer-supplied so we do not have much choice. And I do believe we can accomodate the material with proper machine configuration.

BTW, we use 20%-30% regrind (same HIPS from extruded and thermoformed trims).

posted July 29, 2002 04:56 AM               

Made a typo error in the polishing roll temps...

Upper roll = 102C
Middle roll = 95C
Bottom roll = 80C

Should the middle roll temp be higher than the top and bottom? We use an up stack.

I need to check if the rolls have consistent temp along the cylinder.

Should the temps at the die ends normally be higher by 5C than the center zone?

posted July 29, 2002 05:46 AM               

How many heater zones does your die have? for an easy flow material like HIPS, especially with 20-30%RG you shouldn't need to raise the end zone temps.

The chill roll temps should be reversed, the exit temp of the sheet from a top roll of 102C is too high- you could try setting the bottom roll at 37C, and the middle and top rolls at 65C. See what effect that has on the orientation, gloss etc propeties of your sheet. Then raise the bottom and middle roll temps.

The speed differential between the chill rolls and the rubber pull rolls should allow the sheet to be a little loose, if you have it really tight and the sheet is hot, then you could be inducing additional orientation into the sheet.