Help: Acrylic Black Specks and Pitting

[iamplwirtz]

Was the dryer and hoses cleaned properly from the last job? I know you said you cleaned the screw and barrel, but how about the feed throat from the hopper into the press? Usually it's a square with circular opening. Under and around that circular opening there's creases. If you rub your finger inside of there, there's usually always some build up that will randomly get pulled into the new material.

Fairly difficult to see the defects - but why the 50 degree jump from rear to front zones? Does it freeze off if lowered at the tip / front?

[Joel JS]

Hi,

Yes, Dryer recleaned, hoses recleaned, loader removed and put in hopper via clean bucket, with no difference. Can't see any ej pin flash, dme lowered for stringing, but no difference seen in parts otherwise.

Thx!
Joel

[Jendalf]

Hi,

two things.

1) it looks like wrong mixing - the screw is not able to mix the colour into the natural homogenously. Than when injected the non disperged colour makes the dark marking.
2) to much shear during screwing put inside the tesion which is than relaxed by micro orientation or micro cracking - changing the colour (optical properties) for light coming thru.
2a) this can appear during cooling time
2b) after some time of cooling at area temperature

s:
1) check the mixing, the full screw capacity isn´t good so try to proove that on the larger machine.
try to setup the rounds / backpreassure to have the colour-homogenous film when purging out.


2) a) if its there after ejection - try to prolong the coling time to 1 minute or more (this depends on the wall thickness).
b) if the part is ok after ejection and the issue appears later - put the parts into the oven tempered about 210°F for 5 mintes, than check

opt. additional share can be there applied also by wrong hot runners chanels, sharp edges 90° curves.

Acrylates are very sensitive to any share.

J.

[Joel JS]

OK J

The material is pre-colored grey, thx for the feedback!

Joel

[rickbatey]

Has this ever ran good? The pitting sounds like gels or unmelted pellets. The nozzle tips looks incorrect for the sprue too. I would use the Eliminator from Sam Kern or properly sized full internal nozzle tip AND nozzle body! What is the extruded pressure when running? It should be around 800-1200psi or so. If higher then resin is being over heated. Is this resin UV or FR? That will cause it to be VERY shear sensitive and degrade. I think the water temp could be hotter. Are the parts hazy when ejected? That’s typical for this resin because the part isn’t fully setup but you can drop them into clean plastic totes with clean water.
Rick.

[Joel JS]

Thanks Rick, ... will investigate your ideas. Joel