Meaning
Mould dimension calculation determines the physical boundaries of a tool cavity to compensate for polymer shrinkage during cooling. Machining dimensions are established through cavity steel sizing by adding expected shrinkage percentages to the target part dimensions. Tooling datasheets provide base shrinkage rates, although actual shrinkage varies with gate location and injection pressure.
Dimensional Allowance
Shrinkage behavior of semi-crystalline polymers like polypropylene differs along and across the direction of flow. Correct calculation of cavity steel sizing must account for these directional differences to prevent out-of-tolerance parts. Production risks increase when a single isotropic shrinkage value is assumed for complex geometries.
Machining Strategy
Toolmakers often build cavities slightly smaller than calculated to allow for subsequent metal removal. Adjusting cavity steel sizing after initial trials is cheaper than rebuilding an oversized cavity. Solid steel can be milled away easily, while adding material requires expensive welding or insert replacement.
Selecting the correct tolerances at the design stage minimizes the need for these post-moulding corrective operations.
Production Impact
Incorrect calculations lead to parts that fail assembly due to poor fit. Proper cavity steel sizing prevents these assembly line bottlenecks and reduces scrap rates. Stable tool dimensions ensure that part quality remains within specification across millions of cycles.
