Small black areas (spots) inside the material, mostly present in transparent resins.
• Causes:
machine
· down time too long
· barrel switched off over a long period of time
· poor purging of barrel
· dirty plasticizing unit
· inadequate nozzle
mould
· dead edges in gate/ runner system
material
· granule impurities
· degradation by other resins
· pick-up of degraded material from cylinder wall during cooling
• Actions:
1st
Purge with an appropriate material in general, or with a universal
purging compound like Kapronet. For PC, you can use ground acrylic
or regrind PC resin; for Ultem resin use regrind Ultem resin or
glass-filled PC resin, and so on.
Don't drop temperature settings while purging.
2nd
check for impurities: use uncontaminated material, do proper housekeeping
3rd
check and adjust melt temperature
4th
check for dead edges: nozzle, back flow valve gates/runners
