An increase in field failures of these parts was eventually traced to a change in the chemistry of one specific colorant. One of the ingredients in this colorant was also acting as a nucleating agent, which then caused a subtle variation in the degree of crystallinity in the molded parts and – more importantly – a subtle variation in the mold shrinkage. The six sigma affect was that in certain mold cavities the screw holes were a touch too small. So, for parts molded in this color, under long-term load, and when exposed to the end-use environment, they had a higher incidence of failure.
It took months of investigation to isolate this problem, and then several more to come up with a viable solution – a slight increase in the hole diameter on all screw bosses in all cavities. This solution was sublte, inexpensive, and 100% effective. A classic example of the unintended consequences of a change in the supply chain.