Meaning
Statistical capability metrics measure the short-term performance of a specific piece of manufacturing equipment under tightly controlled, stable conditions. Processing specialists apply the cmk index to isolate the mechanical repeatability of an injection moulding machine by running a brief, uninterrupted series of shots. This index evaluates whether the equipment itself is capable of producing parts within tolerance when variables such as ambient temperature, operator changes, and resin batch switches are held constant.
Equipment Evaluation
Calculations of machine precision rely on gathering consecutive samples from a single production run, typically consisting of twenty-five to fifty continuous shots. Machine performance improves when the calculated cmk index exceeds a threshold of one point six seven, which indicates that the machine possesses sufficient margin to absorb slight natural fluctuations during an extended run. Shot weight and peak cavity pressure represent the common metrics monitored to generate the data for this analysis, ensuring that the mechanical components like the screw and check ring are sealing consistently without introducing volumetric drift.
Moulding Application
Precision moulding of engineering thermoplastics with narrow dimensional limits requires a machine that exhibits exceptional consistency. When a moulder holds a cushion size to less than a millimetre of deviation across consecutive shots, the cmk index provides mathematical proof of this stability. A low index value suggests that the hydraulic valves or electrical drives are failing to maintain uniform pressure, leading to part defects such as short shots or flash.
Production Boundary
Machine capability represents a temporary condition that does not guarantee long-term process stability. The cmk index remains distinct from long-term process capability metrics because it excludes the inevitable variation introduced by raw material lot switches and tool wear over months of operation. Maintaining a high machine index is necessary but insufficient on its own to prevent part reject rates from climbing during multi-week production campaigns.
