Raw Tooling Trial Witnessing Protocols for Multi Cavity Injection Moulds
Press side multi cavity trial witnessing mandates independent volumetric fill verification, gate seal determination, and 24 hour conditioned metrology
Individual Cavity Cpk
Meaning
Statistical stability measures the capacity of a single moulding location within a multi-cavity tool to produce parts within tolerance limits. Individual cavity cpk quantifies the performance of each specific aperture during an injection cycle by comparing the distance between the process average and the nearest specification limit to the spread of the process variation. This metric ignores the performance of other cavities to isolate variables such as gate restriction, cooling inconsistency, or uneven pressure distribution.
The calculation relies on the standard deviation of dimensions taken from a stable production run where external influences like material batch fluctuation are minimized.
Cavity Performance
Process control across a multicavity tool requires evaluation of each gate and runner geometry to identify mechanical bias. A discrepancy between cavity outputs demonstrates that thermal or hydraulic imbalances exist within the hot runner system or the cooling channels. If the index drops below the baseline requirement for a specific station, technicians adjust the valve gate timing or the local cooling flow to force the output back into alignment.
High variance between stations drives the consumption of regrind or the need for excessive sorting of finished goods.
Production Variance
Resin viscosity changes and fluctuations in barrel temperature affect all stations differently due to their varying distances from the manifold source. The individual cavity cpk isolates whether the tool geometry compensates for these changes or amplifies the shift in part dimensions. Consistency in wall thickness depends on the balance of these individual indices across the full tool layout.
A process with identical values for every location indicates a balanced flow and efficient pressure transfer from the nozzle to the final part boundary.
Economic Consequence
Scrap rates rise when one cavity within a tool fails to meet the minimum threshold for dimensional repeatability. Parts produced in a low index location possess higher risk of dimensional failure during secondary assembly or end use application. Tooling maintenance schedules rely on the degradation of these values to determine when a mould requires welding, re-cutting, or vent cleaning.
Constant monitoring of these values secures the dimensional integrity of the final moulded component.