Blush or Graying at molded part edges

[stevemboise]

Good day all.

This is my first post to this forum so I'll try to be as specific and brief as possible.

I have been working in injection molding in China for 16 years and this is the first time I have experienced this issue.

We make a variety of part sizes (PC/ABS blend resin) and we are getting what might be called 'blush' on the textured appearance surface (MT-11020) and is quite undesirable.

The blush appears as a graying of the dark blue color along the sides of the parts adjacent to the terminating walls of the part, being the transition between core and cavity creating wall thickness.

There is no discoloration on the core half surfaces nor does the graying 'wrap' onto the edge of the part. Safe to say my issue seems to be strictly related to the texture and where the melt may be either cool enough and/or the injection pressure has decreased near the end of the shot.

Fill looks good and geometry as close to perfect as I need. There is no warp in the part(s).

Photos attached to save a bit of time.

Any ideas would be greatly appreciated. I'm thinking possibly the tool needs additional venting and perhaps higher tool temperature.

Thanks for any and all feedback.

[iamplwirtz]

It looks as if it could be sticking to the mold on that line... I'd try heating the mold more....

[stevemboise]

Thanks iamplwirtz.

The part pulls without issues, not sticking to either core or cavity halves of the tool and no scraping on the texture.

I appreciate the thought and reply though.

[iamplwirtz]

Has this always been an issue? Whats your mold temp?

[Pilot]

By my opinion it is a pressure issue. Wall thickness and cooling of the melt in the tool. Please try to check, if it appears also on the part without holding pressure. than you will see, from which stage it comes.

[rickbatey]

Looks like a wall thickness change reading through. I think you’ll have to adjust wall thickness change on underside to fix the issue.
BTW how much draft is on the part? Little draft could create a dragging incident with the grain as the tool opens.
Rick.

[Derrick]

Yes, I agree, it is probably due to the uneven thickness, in inner side, there is probably some cutout inside.

[djshauns]

I see this a lot myself being from a wall thickness area, someone has mentioned above about turning your hold psi off to see if it is there or not.
I've been able to fix these issues by dropping pack and hold time.

[Jendalf]

:X Someone! :)

So whats the result Steve? With PC/ABS it not so clear, can You also make some additional pictures (detail, B side, where are gates, crosscut of the part)
J

[Pilot]

:-) yes, someone...

[stevemboise]

Okay all...thank you for all of the suggestions and questions.

Both parts have consistent wall thickness at 4.0mm. There aren't any transitions on them from thin to thick or visa versa.

The graying does not appear until we 'pack' the mold and we've tried going just over the fill stage, decreasing the hold pressure and time with favorable results.

We still had some issues but while watching tool trials or T-1 shots, I had them reduce the injection speed and that made the biggest difference. We have a trade-off situation between slower injection time and knit line visibility but we've finally managed to get a good process that reduces the graying to a very narrow area around the parting line with knit lines that are barely discernable.

Mold temp is running between 50°C and 60°C using a water heater to maintain the temp.

Your tips and hints have helped a great deal. Thanks again...