Plastic Chair Injection Molding Issue - White Marks

Hi all,
I was wondering if anyone of you has experienced similar issue with the one we currently face.
Alongside a chair manufacturer we work on a chair design (full plastic, PP + glass-fiber) and in the edge of the seat and back (front side as shown on the picture bellow), after the injection and the chair’s cooling-down process, we notice these white marks. 300 chairs were tested and about 50% showed this problem.
Please note, that the chair after is injected looks fine, but after some hours or even a week, we notice gradually appearing these marks. Also, the injection gate (just one) is located exactly behind these marks. Then, we need strong pressure to inject the plastic and to ensure that the material fills all the cavity.

What we have already tried is to :

a) Raise the mold’s temperature
b) Adjust the injection pressure, time and temperature (various combinations) but still no result
c) Kept the chairs a bit longer in the mold before ejecting them
d) Modified the mold in the injection gate area (even enlarged for better flow).

If any of you has any idea how to solve this issue, or at least suggest a new direction towards its solution, please drop a line. Thanks!

I had a friend who had an issue with a plastic part that was cracking weeks (sometimes months) after manufacture. After a lot of trial and error they discovered it was the release agent they put in the mold that was reacting with something else that was off gassing in the product (this was a CE product). I don’t think that would be the case here, but could be something to try. I suppose it is too late in the process to add a second gate?

Michael, thank you for your quick response and suggestion. Interesting point about the release agent, I will suggest to the engineers to consider it as well. As for the 2nd injection gate, it is a solutions that it is been discussed, so it is not a “no”. May I ask though, despite faster injection time and the material’s lower temperature, is there as well any other reason you suggest this solution?
Thanks again.

Do you have any better images?

Is there any indentation at the area of the marks or is it completely smooth?

Have you sliced through one to evaluate the layout of the fibers?

My two thoughts are either it’s a jet mark that’s appearing on certain parts due to the temperature of the tool (may require modification of the cooling design) or an instance where the glass fibers are clumping up due to some resistance inside the mold or the speed of injection and not getting enough material surrounding them (why cutting it into cross sections might be useful).

Has a mold flow analysis been done on the tool?

My guess is also that the fibers are accumulating there causing the plastic matrix to yield. Otherwise the radius may be too small or not curvature continuous causing local stresses.

Cyberdemon, ralphzoontjens thank you for the advice. Much appreciated your time and input.

The area is designed to be smooth and most of the case after the injection, it remains smooth. However, in some failed test injections there were some few chairs that had a small bump there.
The manufacturer has done the slicing but it’s very difficult to observe flow/direction of the glass-fiber as it is used the 1mm length one. From what engineers saw, they could not notice something strange. All seemed usual they said.
The mold flow analysis did show the stress is focused on the gate area. But the white mark didn’t happen on all the products. Some are just fine.
Here are some more pics of the injection gate area and some other failed chairs.

Thanks again!

The fact that it doesn’t happen on all of them probably indicates its a temperature fluctuation problem then. If they shot a sequence of these and left them in a row, I suspect that it’s happening either as the tool gets hotter or is too cold. Adjustment of the cooling lines could help.

Cyberdemon, interesting thinking. As far as I know, the manufacturer records the production sequence of each test injection so I will pass to them your remark to check if there is any correlation between time of injection and defect.