Meaning
The allowable limit of variation in the physical measurements of a manufactured component ensures proper fit and function in assembly. Defining a dimensional tolerance for an injection-moulded plastic part is necessary because of the inherent thermal contraction of polymers as they cool in the mould. This specification dictates how much the final part can deviate from the nominal computer-aided design model.
Shrinkage Variation
High shrinkage rates of semi-crystalline resins like polypropylene and nylon make holding tight limits difficult compared to amorphous resins like polycarbonate or polystyrene. While amorphous resins shrink uniformly by roughly half a percent, semi-crystalline materials can shrink by up to two percent, often varying along the direction of polymer flow. To control this variation, the moulder adjusts the packing pressure and the cooling rate during the injection cycle to keep the part dimensions within the specified envelope.
If these parameters drift, the parts may exceed the limits, leading to scrap or assembly line failure.
Process Control
Moulders must maintain stable press conditions across the entire production run to minimize dimensional drift. Variations in melt temperature or gate wear will change the cavity pressure, causing the parts to grow or shrink unevenly. Consistent monitor tracking of these process variables prevents part dimensions from drifting out of bounds.
Inspection Limit
Quality control teams verify compliance by measuring critical dimensions on a coordinate measuring machine. These measurements are taken after the parts have fully stabilized, typically forty-eight hours after moulding, to account for post-mould shrinkage. This step confirms the parts are fit for purpose.
