Help w Polycarbonate Black Specks

Hello,

I have run Polycarbonate clear in the past but this one job we always fight black specks. Looking for ideas to solve.

We have pulled and cleaned screw and barrel, and on the very start it comes with a lot of black specks. Then it gradually has lets black specks, but, never being completely gone.

Barrel holds 8 - 9 shots (too many) Barrel temps are 520 in rear to 630 in front. (Has to be hot to get to fill out, but even at 630 front, I don't see any yellowing.)

Have tried slow screw RPM with low back psi. Screw and Barrel are new / general purpose. We can only run in this machine because of the PSI reqd.

Thx for any help,
Joel

Hi Joel,
clean screw is a basic requirement. (but you know it:-) The screw must be also smooth without any damages or grows, where the material can stay. Low Screw RPM, low back pressure as much is possible. Also injection pressure only on minimum which is necessary. Please check also hoses for material conveying, filters on hopper loaders atc...
You cab also try to "shorten the screw" by lower temperature profile on rear zones.
Please check also injection nozzle mounting to avoid any dead spaces or sharp edges (different diam. of gate and inj. nozzle for ex.)
Generally the black specks in PC are coming from temperature stress (degradation)

Ok Thx for the tips,

Joel

We had similar issues. We resolved this by not leaving any material in the barrel over night, so in other words, purge out as much PC as possible and then even purge with PP. Its a pain in the ass but it works.

As Crabstix wrote. .. this is MUST. the screw must be purged out, PP for is a good for it. Somebody call it like "issue for Mondays" :-)

Have you had any luck with the black specks?

Sorry for the late response, we ended up moving the mold to another machine/facility where it works better..

but thanks for the feedback. Also now seeing and wondering what have you seen as best practices with shutting down clear PC, to restart the next day...

Purging it all out with PP OR leave Barrel full at reduced temps?

thx
Joel

PS Was this website down last week? ....wasn't loading for me.