Is it stress or is it contamination?

I have a cooling fan (not a squirrel cage fan) that we have been running for a few years now. The customer has discovered 2 failures during testing. The blades of the fan tore free of the hub in both cases. One was after 250 hrs. of testing and the other after about 1,000 hrs. The base material is a Co-poly PP.

The break area is unusual looking in that it looks like a fracture and there is an appearance of stress whitening through the entire wall thickness where the blade meets the hub. I also sectioned part of the blade near the end of the blade and it shows an inner white layer that is sandwiched between an opaque material. I'm assuming that this opaque material is the base PP, but I am not sure what is sandwiched between. Could a PP material show stress whitening in the core of the wall thickness and not the skin? I would think that if this part of the blade was white due to stress then the white would be through the whole thickness of the part not just the core.

One root cause I am investigating is whether the press was fully purged of the purge compound before running parts. The purge compound we use does have a white foamy appearance. I will be conducting an experiment to see if I can reproduce that. Another root cause I am investigating is material properties or cross-contamination of the base resin.


My main question is if one was to see stress whitening in a part would it be through the entire wall thickness?

Contamination tends to be throughout a part. We have seen some odd issues with what appears to be lamination/ contamination. What we found was an additive building up on the mold surface and causing the part finish. We just pulled out the Scotch Brite pad and some Striper from Slide products to fix the issue.
Now I wonder if you have some molded in stress that is causing this failure? What is the cycle time, water temps, barrel and machine settings? If the part is fairly thick in this area, we could be seeing the separation of the slower cooling internal area from the set outer layers. Do you add any color to the resin? Could be from the color carrier. Whats the gate freeze time? Could be from voids in the thick areas. Where is this thing gated? Could be from cold slugs or knit lines
rick.