Plastic Chair Injection Molding Issue - White Marks

S

steppenwolf

New Member
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!


 
Does it occur only after they're fully cooled?
 
It looks like something I have seen before, so when I used to live in Dubai, where temperatures rise up to 50 degree Celsius. And by chance we would leave a plastic chair outside for a couple of days. These white marks would show up.

Mind you, they are not there in the beginning when we bought the plastic chairs. They also don’t appear when we use it inside the home. They don’t even appear when we leave the chair only for one day outside. It ONLY appears after several days outside in the sun. So I am guessing they appear from the cyclic thermal loading somehow. If it’s the same case in your factory then try not to thermally load the chairs a lot. Keep them in constant temperature rooms.

Also if the structural load carrying capacity of the chair doesn’t change with the white lines. i.e. it still functions well and passes all the tests. Use a dye in the injection so that the lines are not visible and you can use the product or sell it in the market. I don’t think it’s a defect that’s very serious. Because the chairs from Dubai are still usable. We used them for years until they finally broke even with the lines in them.
 
I actually thought something similar but not the same - I've seen similar marks from being out in the sun too long, so I thought it might somehow be UV related, although your answer makes much more sense, as the UV damage would probably take much longer to occur.
 
Hi Steppenwolf

1) Residual granulate moisture is too high.

Check the dryer or the drying process, measure the temperature of the granulate, withstand the drying time according to the instructions for preparing the material for casting.

2) If these shiny defects then this silver schlieres.

The thermal stress on the melt is posed due to: too high melt temperature, too long exposure in the melt state or too high auger rotation speed, too small nozzle section and flow channel.

Check the temperature of the melt, select the appropriate screw diameter, reduce the number of revolutions of the screw, increase the cross sections of nozzles and flow channels.
 
You see, I knew you'd get an answer from a pro eventually :)
 
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