Rounded pellets usually are hot-cut, cylindrical pellets are usually strand-cut so most likely different extruders were used. This (change in extruders) may effect compounding of the materials and thus properties, but your supplier should have done QC testing to know product is the same. Again, this may be where pellet-to-pellet differences could show up in FTIR analysis. Also, most suppliers don’t mold sample test specimens that have knitlines that many production parts contain, and they wouldn’t be able to catch differences in batches if you are having knitline issues. Poorly compounded materials (not enough mixing) might show up in drop dart impact test comparisons, as I have seen this in some materials I have evaluated in the past.
A couple of years ago I analyzed an FR-PC/ABS painted bezel for an electronic enclosure molded in China and found that their injection molder had sworn they used the same material, but the batch that had poor performance had a bunch of silicone oil in it. This caused failing the heat performance test and very poor paint adhesion.
If the pellet shape look difference for sure the material was polymerized in other reactor as well as extruder and can affect the recopy of course the molecular chain, but also no forget the process can affect the moister in the material, cooling water in the tool, and the temperature profile I think before to say is the material double check the process.
There could be two major interrelated issues involved viz, material and its impact on injection molding process.
As per my knowledge, PC+ABS always has filling related issues; as the flow properties are not very good. The process is usually pressure limited with higher residual in-mold stresses. This can lead to premature failure of the part.
The answer should somewhat lie in the MFR which differs due to change in MWD and MW. Test the good batch and the newer one and compare the results. A relatively simple and cheaper MFI test is recommended. Then other characterization test may help in pinpointing the exact cause.
