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

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

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

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

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.

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

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

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

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

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.