Tip of the Day 161: How to Prove Material Change

Tip #160 triggered a surge of e-mails regarding material changes. Apparently this problem is more common that we realized. It causes aggravation for molders who are left with the burden of proof to show the material supplier that there is a documented change.

How can prove that the material has changed? In the previous tip we used process of elimination: The mold did not change nor did the process as measured from inside the cavity. The material was pronounced guilty on circumstantial evidence only.

Suggestion: During « Validation » or « PPAP » place a quantity of the original material in storage. You would, of course, save enough to allow a process to stabilize long enough to get valid parts for measurement. Perhaps more than once. Also, carefully document the in-cavity process and the cavity dimensions as built. Include with clamp style and tonnage.

Then, if you later discover a change in product that is not explained by changes in the mold or process^† you can re-run the validation with the original material. Run the exact same process and mold (i.e. cavity shape) with the original material. If the parts prove to be the same with the original material as they were during validation then you have the smoking gun: material. If the parts are different with the original material than they were during validation you need to look for mold (shape) changes or process changes.

^† Remember that the process is what the material sees in side the shape of the cavity (including mold deflection). For example, if the water temperature and flow are identical but someone put smaller water fittings on the mold you may have different cavity temperatures. To match the process you must match the in-cavity variables.