Plastic Chair Injection Molding Issue - White Marks

Hi all and good to find you here.

I am not an ME, but an industrial designer, collaborating with a chair manufacturer for a full-plastic chair I have designed for them. Although they have quite some experience in production, yet, we are stuck with a problem we face with the latest design.

So, as shown in the pic bellow, the plastic (full plastic, PP + glass-fiber) chair, in the edge of the seat and back (front side as shown on the pic), 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. Thank you!


https://thumb.ibb.co/gpMBJ6/SY.jpg

Could it be residual stress resulting in whitening?

Hi magicniner and thank you for your response.

This is a cause the the factory is studying but until now, by adjusting temperature, flow, pressure etc., did not manage to solve. Definitely is a try/error process that they have done many times to solve other problems in the past, but this time it is not so obvious the cause of this particular issue.

I think it is gate problem,higher the mould temperature and lower gate injrction pressure,also adjust the speed here.