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

back