Processing on extruders

Frequently Asked Questions

About PLEXIGLAS® Molding Compounds

Does PLEXIGLAS® molding compound require drying prior to processing?
PLEXIGLAS® molding compound is dry when packaged and can be processed without predrying even after several weeks' storage, thanks to the polyethylene lining. We nevertheless advise our customers to use suitable drying equipment. 





At which temperatures can PLEXIGLAS® molding compounds be predried?

Predrying should generally be performed at the highest possible temperatures without causing the pellets to cake. Temperatures of 10 to 15°C below the Vicat softening temperature should therefore be observed. You will find the Vicat softening temperature in our product information leaflets on the respective material or in the CAMPUS database. We recommend the use of desiccant-type driers. 





Which screw speeds are specified for processing PLEXIGLAS® molding compound?

Plasticization should be performed as gently as possible (at low speed). The usual peripheral speed for processing PLEXIGLAS® is 0.3 m/s. 





How can streaks be avoided in the direction of extrusion?

This fault points to various possible causes: 





1. Moisture

Remedy:

Predry material or examine drying process (see under "Predrying")




2. Inadequate venting

Remedy:

- Examine vacuum (at least 100 mbar)

- Increase temperature before the vent zone

- Reduce screw speed (recommendation: do not exceed peripheral screw speed of 0.3 m/s)




3. Output too high

Remedy:

Reduce screw speed




4. Melt temperature too high (material decomposition)

Remedy:

Reduce processing temperatures or screw speed




5. Die lips and / or calibrating unit damaged

Remedy:

Grind and polish affected parts




6. Material deposits on die lips

Remedy:

Remove with copper or wooden rod, clean with cloth and polishing paste




7. Previous processing of plastic with higher molecular weight

Remedy:

Clean unit, especially screw




How can bubbles be avoided in the extrudate?

This fault points to various possible causes: 





1. Moisture

Remedy:

Predry material




2. Inadequate venting

Remedy:

- Examine vacuum (at least 100 mbar) - Increase temperature before vent zone

- Lower screw speed




3. Output too high

Remedy:

Decrease screw speed (recommendation: do not exceed peripheral screw speed of 0.3 m/s)




4. Melt temperature too high (material decomposition)

Remedy:

Reduce processing temperatures or screw speed




5. Contamination with foreign material

Remedy:

Pay attention to cleanliness




6. Die temperature too high

Remedy:

Reduce die temperature




How can bubbles be avoided in the bead?

This fault points to various possible causes: 





1. Bead too thick

Remedy:

Check bead, keep as small as possible




2. Inadequate venting

Remedy:

- Check vacuum (at least 100 mbar)

- Increase temperature before vent zone

- Reduce screw speed




3. Melt temperature too high (material decomposition)

Remedy:

Reduce processing temperatures and / or screw speed




4. Moisture

Remedy:

Predry material




How can clouding of colored extrudates be avoided?

This fault points to various causes: 1. Residence time in extruder too long Remedy: Avoid flow restiction, particularly in the vent zone 2. Inadequate distribution of colorant Remedy: Revise formulation, install static mixer 3. Contamination (e.g. by other plastics) Remedy: Clean unit, possibly clean drier and conveyors, avoid flow restriction 





How can dull surfaces be prevented on extrudates?

This fault points to various possible causes: 





1. Die temperature too low

Remedy:

Increase die temperature




2. Temperature of calender rolls too low

Remedy:

Increase roll temperature




3. Melt temperature too low

Remedy:

Increase melt temperature




How can rough surfaces be avoided on extrudates?

This fault points to various possible causes: 





1. Die temperature too low

Remedy:

Increase die temperature




2. Melt temperature too low

Remedy:

Increase melt temperature




3. Poor contact with calender rolls

Remedy:

Increase roll temperature




4. Nip wrongly adjusted

Remedy:

Adjust nip




5. Calender rolls uneven

Remedy:

Polish calender rolls




How can streaks or lines be avoided at right angles to extrusion direction?

Possible cause:  


- bead too thick (rotates)

Remedies:

Reduce bead (reduce screw speed).




How can porous, eroded surfaces be avoided on extrudates?

This fault points to various possible causes: 





1. Poor contact with calender rolls

Remedy:

- Clean calender rolls

- Increase roll temperature

- Increase contact pressure of calender rolls




2. Inadequate degassing

Remedy:

- Check vacuum (at least 100 mbar)

- Increase temperature before vent zone

- Reduce screw speed




3. Moisture

Remedy:

Predry material




How can chatter marks (waves at right angles to extrusion direction) be avoided?

This fault points to various possible causes: 





1. Contact with last calender roll too close

Remedy:

- Decrease roll temperature

- Change temperature of individual rolls

- Increase contact pressure of calender rolls

- Increase take-off speed




2. Different roll speeds (peripheral speed)

Remedy:

Examine synchronism of calender and take-off drives




How can pimples or specks be avoided in extrudates?

This fault points to various possible causes: 





1. Rolls contaminated (regular return of surface defects according to roll circumference)

Remedy:

Clean rolls




2. Impurities in melt stream

Remedy:

- Check melt stream for impurities

- Increase contact pressure of calender rolls

- Increase roll temperature

- Clean extruder, remove screw




3. Impurities from air (dust, fines)

Remedy:

Reduce distance between die/calender stack




How can a yellowiing of extrudates be avoided?

This fault points to various possible causes: 





1. Melt temperature too high

Remedy:

Decrease melt temperature




2. Residence time of melt in barrel too long (material decomposition)

Remedy:

Increase extrusion speed




3. Avoid backflow of condensate in degassing systems

Remedy:

Modify design




How can shrinkage be reduced?

For the further fabrication (thermoforming) of extruded sheets, sheet orientation and thereby shrinkage often have to be kept as low as possible. Shrinkage in machine direction always poses a problem, since it is higher. Shrinkage is mainly influenced by the speed of the take-off roll in relation to the speed of the last roll of the calender stack. The peripheral speed of the take-off roll should be 2-3 % lower than that of the last roll in the calender stack. 


The roll temperatures also play an important part. The last roll of the calender stack should be hot enough for the sheet to just about adhere to it. If the roll is too hot, the melt sticks too strongly during detachment. The force required for detachment provokes shrinkage. If the roll is too cold, the sheet becomes wavy upon detachment.

The ideal roll temperatures depends on the roll diameter, the sheet thickness and the extrusion speed.

See sketch for recommended roll temperatures (typical values):

The following measurement can be performed to check shrinkage:  

A sheet strip is placed on a tray sprinkled with talc for about 90min at 160°C. The change in dimension is subsequently measured. A shrinkage of c. 3% in a 3mm thick sheet can be termed good. Thinner sheets show higher shrinkage, and thick sheets less.




How can fluctuations in feed be avoided?

Possible cause:  





Machine settings are not ideal

Remedial measures:

The pellets normally show very good feed behavior. There is no need for special measures to support feed, e.g. by cooling the screw. Temperature control of the feed bush is advantageous, but cooling with water is usually sufficient. The melt throughput of an extruder can be strongly influenced by changing the temperature in the feed zone. Normally, the feed is more regular with a cold feed zone (zone 1 c. 180°C, zone 2 c. 200°C) than with a hot feed zone (zone 1 c. 230°C, zone 2 c. 240°C).




Feed behavior when processing regrind, or with added regrind:

Owing to the irregular particle structure and the increased content in fines, the feed behavior of regrind is often somewhat uneven. It is recommended to add only a certain percentage of regrind (up to 25%). It is quite possible to employ 100% regrind, but this should be done on extruders with grooved feed bushes.

The following procedure is recommended in the event of fluctuations in feed:

- Reduce temperature in zones 1 and 2

- Cool feed bush

- Switch on screw cooling (possibly by rinsing with water)

In rare cases, it is more successful to increase the temperature in the feed zone (depending on machine).




Another possible cause:

- Fluctuations in feed owing to foreign material in PMMA. Even the smallest quantities of PE (e.g. from the protective masking on extruded sheets that gets into the melt stream with the regrind) may cause extreme fluctuations in feed.

Remedy:

Examine cleanliness of pellets or regrind




How can thickness fluctuations be avoided at right angles to machine direction?

This fault points to various possible causes: 





1. Bead not ideal

Remedy:

Check bead, keep as small as possible (adjust by means of restrictor bar)




2. Temperature differences at the die

Remedy:

Check die temperature for fluctuations, adjust to uniform temperature




3. Rolls not optimally aligned

Remedy:

Align calender rolls




How can thickness fluctuations be avoided in machine direction?

This fault points to various possible causes: 





1. Insufficient roll pressure

Remedy:

Increase contact pressure of calender rolls




2. Pronounced fluctuations in melt temperature

Remedy:

Check barrel temperatures for fluctuation




3. Take-off is irregular

Remedy:

Check take-off speed




4. Fluctuations in feed

Remedy:

See section "How can fluctuations in feed be avoided?"




5. Inadequate back pressure

Remedy:

Install restrictor bar or throttle valve




How can the sheet edges be prevented from curving upward or downward?

This fault points to various possible causes: 





1. Temperature of polishing rolls too low

Remedy:

Increase roll temperature




2. Extrusion speed too low

Remedy:

Increase extrusion speed




3. One-sided cooling due to unfavorable temperature conditiones

Remedy:

Check temperature control




Which driers are suitable for drying PLEXIGLAS® molding compounds?




Should the supplied molding compound show too high a moisture content for processing after prolonged storage (bubbles or streaking), predrying is recommended. If the recommended drying temperatures and minimum drying times are observed, simple ovens with or without airflow are sufficient, but shorter drying times are obtained with so-called desiccant-type driers. Vacuum cabinets are often preferred in climates with high atmospheric humidity. Small portable units with high performance are available in addition to stationary units.

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Disclaimer

This information and all further technical advice is based on our present knowledge and experience. However, it implies no liability or other legal responsibility on our part, including with regard to existing third party intellectual property rights, especially patent rights. In particular, no warranty, whether express or implied, or guarantee of product properties in the legal sense is intended or implied. We reserve the right to make any changes according to technological progress or further developments. The customer is not released from the obligation to conduct careful inspection and testing of incoming goods. Performance of the product described herein should be verified by testing, which should be carried out only by qualified experts in the sole responsibility of a customer. Reference to trade names used by other companies is neither a recommendation, nor does it imply that similar products could not be used.
The Business Unit Performance Polymers of Evonik is a worldwide manufacturer of PMMA molding compounds sold under the PLEXIGLAS® trademark on the European, Asian, African and Australian Continent and under the trademark ACRYLITE® in the Americas.