IMM 12/99

How do you calculate Cpk of part dimensions in a multicavity tool (64, 96, and 128 cavities)?

The Cpk value for each cavity or part must be determined. Samples are taken in chronological order throughout a run and then grouped (25 subgroups of five parts each is recommended for determining capability). Using R bar divided by D2 to calculate sigma, the cavity with the lowest Cpk value is the governing cavity that determines the tool's true capability. Programmable optical measuring equipment (smartscopes) greatly assist in gathering and processing the data needed for high-cavitation tools.
-A. Chmura, Tessy Plastics Corp., Elbridge, NY, (315) 689-2077.