posted June 20, 2006 04:15 AM               

I am operating a new extruder with HIPS as a raw material and producing sheets for thermoforming.

When the sheets comes out of the die and onto the roller sheets, wavy lines are observed. Adjusting the roller temperature (increasing), improve slightly the wavy lines but not eliminate it and incresing roller temperature further will caused some bubbles to be seen on the undersurface of the sheet.
The bubbles are seen more at the edges of the sheet and when we thermoformed these sheets, the product cracked!

- Appreciate your inputs on eliminating the
wavy lines as well as the cause of the bubbles at the edges at the sheet.

2) I also have another problem where sometimes the melt from the die stick on the rollers. How could I improve this?

Thanks

posted June 20, 2006 01:19 PM               

If width direction, is the distance between waves constant from wave to wave?

Measure distance from wave to wave, d, cm.

Measure line speed, V, cm/sec.

f = V/d, 1/sec

Is this value equal to the rps of the extruder?

If so, then check the stability of the extruder speed.

If not, then check the thrust bearing on the extruder. A small axial motion will cause trouble.

Perhaps it is a material problem. Try changing the die lip temperature.

If the waves are in the MD, then dirt in the die or on the die lips is likely problem.

Steve Derzinski, Ph.D.
President, Extruder Tech, Inc. www.extrudertech.com
Education for Extrusion Professionals

posted June 20, 2006 02:37 PM               

Also, can you increase the pressure at the die? This can help dampen flow disturbances that occur upstream. I will refer you to some work on this that I did some time ago on flow transients. http://www.extrudertech.com/antec971.pdf

The effects discussed in this paper deal more with disturbances of higher frequency that the screw speed. High frequency disturbances can be attributed often to solids conveying problems. If this is the case, try changing the barrel temperatures a little (5 C) in the first stage.

Low frequency disturbances (slower than screw speed) can be the result of melting instability. If this is the case, try changing the barrel zone temperatures in the melt section of the extruder.

These kinds of disturbances would be of random frequency. You can determine the frequency from the simple calculation of the wave length of previous note.

Steve Derezinski, Ph.D.
President, Extruder Tech, Inc. www.extrudertech.com
Education for Extrusion Professionls.

posted June 20, 2006 06:41 PM               

Welcome to the single screw forum.

What thickness sheet are you producing? For hips you need to set the nips between your chill rollers at the target gauge and set the gap on your die lip 10% over the target gauge.

You use hauloff speed to "draw" the material down to the gauge. I'd guess that currently you have a "rolling bank" of melt present at the entry to your first nip point on your chill roll stack.

What temps are you setting on your die and chill rolls?

------------------
Steve Hodgson

posted June 21, 2006 10:23 AM               

The dynamic torque for different polymers is shown to be quit variable and depend on conditions. Perhaps, some adjustments of your speed and temps can help your problem.
http://www.extrudertech.com/antec2002.pdf

This one deals with two-stage extruders, but the big idea is to keep the metering secion full. Higher back pressure will do this.

posted June 22, 2006 03:15 AM               

Thanks for the pointers and advise. Here are more details of the procedures that we carried out during the trial.

Trial was done on 0.4mm thickness sheet. White color sheet was produced using white masterbatch (0.4% loading).

Some GPPS was added to ease the vacuum forming as well as some regrind.

Barrel Temperature used was 215 to 232 deg C.

We reduced the barrel temperature slightly to achieve a head pressure of 100 kgf/m2. Previously the head pressure was 74 kgf/m2.

Roller temperature was set at 30 C, 30 C, 20 C. When it was initially extruded out,wavy lines could be observed (wavy lines occuring in the width direction. We have yet to calculate the frequency but the pattern is very regular and visually we can see that it is occuring at the same frequency.

We managed to reduce the lines by adjusting the roller speed. At higher speed, there was no overlap (banking) at the nipping part of the calendar and this resulted in good cosmetics (less visible lines) but the thickness became uneven (+/- 0.04) across the sheet. When banking was allowed, thickness across sheet became even (+/- 0.01) but wavy lines observed.

Increasing the roller temperature to 60 C and 80 C, the lines got better and were slowly becoming straight and less obvious but strangely, bubbles started to appear at one side of the sheet at the underlayer.

Will increasing the die Lip help?

Is there a good source of information on where I can get information on extrusion Troubleshooting guide?
Also, if I would like to attend any training on solving extrusion problems, in Asia, is there any available training centres that I can go to?

Thanks again guys for your input.

posted June 22, 2006 07:56 AM               

Other simple things.
Roller(s) What is the roller configuration? Three roll stack? Is there a melt curtain or a just a small distance between roller nip and die lips (melt bead?) Check the roller concentricity and roundness, roller mounting. Check the die mount to see that the die is not moving. If the distance between the die lips and the contact point on the rollers is changing (even slightly,) waviness will result. This should all be easy, simple, and cheap. It should be eliminated as causes.

posted June 22, 2006 08:04 AM               

See this link for starters. http://www.4spe.org/elearning/digitizedcd.php

Many articles and materials are available on all many topics.

See Extrusion Division web site, www.spexdiv.com for sources of information.

posted June 22, 2006 05:24 PM               

Putting GP in with HIPS is probably the cause of your cracking problem.

For HIPS, your barrel temps are high, I'd typically run HIPS at 175C (Feed Zone)-190(Metering Zone) Adaptors/Screen Change 210C and Die Zones 215C.

If you have your Die Lip gap set at .44mm, nip gap set at .4mm and your sheet is varying in thickness along its lenght (MD-Machine Direction), then you need to check:
Is the screw speed constant
Is the hauloff speed contant
Are the chill rollers running true
Is the head pressure remaining constant- if it varies, then you have a surging problem.

In addition to Stephen's suggestions- contact your local Dynisco agent and obtain a copy of their "Extrusion Processors Handbook", also if you can, attend one of Allan Griff's extrusion seminars. You should also get a hold of DOW Chemicals HIPS Extrusion guide, this will also be of use to you (Downloadable from DOW's website)

------------------
Steve Hodgson

posted July 03, 2006 08:43 AM            

If lines go all the way across and far apart, look for sticking on last roll, or some drive problem (missing tooth on gear). With cut sheet, the shear stroke can cause this, too.

A 0.4-mm sheet is thin to run with a bank. Need to adjust gauge more precisely with bolts, then zone heat, to get uniform gauge. Changing anything (rpm, barrel temps) and you need to re-adjust die.
Beware squeezing sheet between rols to get even gauge. If it isn't coming out even across die, the squeezing is false security; it will relax back to original uneven dimensions in the heat of the former. It's the thickness variation of the product that counts, not the sheet.

Bubbles may be moisture if HIPS isn't dried and weather is humid.

Brittleness may be due to GPS if you put enough in, but a small amount (5-10%) should not affect much, unless not mixed well enough. Why do this -- HIPS is easy enough to form as is. Check Izod impact or resin, maybe you have a medium impact grade, Izod should be 1.0 or more on 1/4" (6mm) samples. Unfortunately most suppliers report Izods on 1/8" (3mm) injection-molded specimens, which makes them look tougher.

I agree with Steve: 30 C is cold on rolls, 70-90 is more reasonable, upper end will cause sticking especially if rolls not often flushed inside. If sticks on last roll, blow air with fan on back of prior roll.

Also, let melt temp find its own level, leave barrel heats low, except if you are surging (amps vary 5% or more) you may need to change rear barrel up or down (can't tell until you try).

posted July 03, 2006 08:44 AM            

If lines go all the way across and far apart, look for sticking on last roll, or some drive problem (missing tooth on gear). With cut sheet, the shear stroke can cause this, too.

A 0.4-mm sheet is thin to run with a bank. Need to adjust gauge more precisely with bolts, then zone heat, to get uniform gauge. Changing anything (rpm, barrel temps) and you need to re-adjust die.
Beware squeezing sheet between rols to get even gauge. If it isn't coming out even across die, the squeezing is false security; it will relax back to original uneven dimensions in the heat of the former. It's the thickness variation of the product that counts, not the sheet.

Bubbles may be moisture if HIPS isn't dried and weather is humid.

Brittleness may be due to GPS if you put enough in, but a small amount (5-10%) should not affect much, unless not mixed well enough. Why do this -- HIPS is easy enough to form as is. Check Izod impact or resin, maybe you have a medium impact grade, Izod should be 1.0 or more on 1/4" (6mm) samples. Unfortunately most suppliers report Izods on 1/8" (3mm) injection-molded specimens, which makes them look tougher.

I agree with Steve: 30 C is cold on rolls, 70-90 is more reasonable, upper end will cause sticking especially if rolls not often flushed inside. If sticks on last roll, blow air with fan on back of prior roll.

Also, let melt temp find its own level, leave barrel heats low, except if you are surging (amps vary 5% or more) you may need to change rear barrel up or down (can't tell until you try).