A tool changes dimension as its temperature changes. Uniform expansion can be predictable, but temperature gradients can distort the forming surface, shift cavity spacing, alter sealing, and move critical dimensions during a run.
Large aluminum tools, thin shell molds, composite tooling, and poorly supported structures are especially sensitive to uneven heating and cooling. The first parts after startup may differ from parts made after the tool reaches thermal equilibrium.
Control the thermal state
Mold surface temperature should be mapped during startup and steady production. Coolant inlet and return conditions, branch flow, and mounting points should be checked. A single sensor in a low-load area cannot represent the entire tool.
Dimensions should be qualified at a defined stabilized tool condition. Cooling channels and supports may need balancing or redesign. Process offsets should not be used to conceal a tool that continues to change shape throughout the run.