Molding PC in dark colors

Has anyone had positive experiences molding PC in dark colors? I’m having a terrible time with parts that necessarily have some thin sections that are cooling faster than the nominal sections leaving light “shadows” on various cosmetic areas of the parts. The parts looked good initially but texturing has brought out a great deal of imperfections.

I knew to expect more difficulty with darker parts but what I’ve got is far worse than what I expect or find acceptable.

If the part neds to have some translucency, that will be a problem. If it can be solid opaque, try adding more pigment. Or change material to PC/ABS.

Has your manufacturer done any exploration in tweaking the molding temp/cooling time?

Usually you can eliminate a good bit of that just with processing tweaks.

BTW, The parts are opaque and from a masterbatch so adding pigment is not an option right now. I don’t think that’s it in any case. Unfortunately pc/abs is not compatible with certain chemicals the parts will likely come into contact with. We’ve been cornered into a specific grade of pc.

The manufacturer is in China and there’s a layer of communication (engineering group also in china) that I have to go through. This makes communication challenging. I have requested information on any processing efforts that have been made to eliminate the defects but have not yet gotten a response.

The most frustrating part is that we inspect our first (untextured) shots very, very closely before signing off on texturing. These defects were invisible w/o texture.

almost impossible to help without seeing the part, but take a good look at your gate position as well. Make sure you’re not inviting a knit line where you don’t want it, make sure you’re not jetting into a cavity. Moving the gate closer to the problem area and controlling flow around there specifically may help control your surface. You’ll also get more reliable cooling conditions closer to the gate.

Usually the generic rule of thumb is to gate so flow is from thick section to thin section. If the thin section is between the gate and thick section, the melt flow front could cool faster and cause surface blemishes. If you can’t move the gate, you might try to add some internal ribs to facilitate the flow. And like spoonyd said jetting is a danger when gating into the thick section, but this can be remedied by modifications to the gate like adding a core pin to obstruct the flow.

yes, sort of.

PC-PBT blend, glass fill, black.

Parts had 1/2 thickness lap joint at parting line for assembly purpose. Same as you, samples were fine, textured tool produced a slightly different colour and gloss around the 1/2 thickness lap joint.

Solution was slightly heavier retexture, and some processing changes I don’t remember, but they were developed on the molding machine by material supplier engineer.

I’ve had some cosmetic issues on thin monitor front bezels shot in dark grey / black in the past. I know the manufacturer used gas assist injection that might have helped keep some blemishes down (it had several injection points and knit lines were an issue). They also painted the part since it wasn’t touched very often.