Injection molding materials combination
In injection molding, for any faults error on any 3M (Material, Machinery/Molding variables, Mold) maybe responsible. If you have earlier used similar combination of material & got satisfactory results (Reference sample may be available), then compare MFI values of base resins, if the values are same, then grade may differ & you have to get characterization results ( DSc/TGA) to ensure same combination being used.
Further, if any molding conditions (Temperature, pressure /speed of Injection, mold cooling etc.) are changed which might have created “stressed” points at which part may be getting cracked. You have to analyze within your manufacturing which “M” has problem.
For satisfaction, you may test MFI, density of raw material (Resins) used for product molding to compare with old raw material specs. Chemistry wise, materials molecular weight may differ (MFI will change) or MWD (molecular weight distribution) may not same as old stock. If you are getting compounded PC+ABS blend from compounder, then percentage of individual resin components (PC & ABS) may vary or they might have used different grades of PC & ABS in preparation of blend.
As my knowledge every raw materials supplier used to provide test certificate. It may fulfill your requirement but you may check properties by yourself (in different lab) and match with TC, it may help short out the problem (If you are using same materials with previous supplier).
When checking the composition differences between the “good” and “poor” batches, look at the additive package (i.e. the internal or external mold release or flow promoters). These are lower molecular weight and tend to accumulate at the flow fronts and last place to fill. You may not notice them unless you run an FTIR of the surfaces and compare spectra closely. Look for relative quantity and type. Sometimes people can forget that not all metal stearates are compatible with PC and make a substitution without realizing they just created problems for their customers. Also the aliphatic chain length of the amide or ester lubes can create issues: slightly shorter chains can lower the boiling point and degradation point, again accumulating at flow fronts and causing cracking or lower strength. If your cracks appear shiny or glossy instead of dull and rough, my bet is on the additives, a mold release, or a contaminate like mold oil/grease not seen before.