Thermal Expansion Coefficient

Meaning

Physical material properties describe how much a substance increases in size as its temperature rises. The thermal expansion coefficient of a mould steel determines how much the cavity dimensions will change when it reaches its operating temperature. Polymers have much higher coefficients than metals, which is why parts shrink significantly after they are removed from the tool.

This value is expressed as the change in length per unit length for each degree of temperature change.

Dimensional Change

Calculating the expected growth of the tool ensures that the final part meets its tolerance requirements. A high thermal expansion coefficient in a resin means that the moulder must account for more shrinkage during the cooling phase. This calculation is a fundamental part of the initial tool design process.

Material Response

Different grades of steel and aluminum respond to heat in unique ways. When a mould uses multiple materials, the difference in their thermal expansion coefficient can cause components to bind or leak. Engineers select materials with compatible growth rates to maintain the integrity of the assembly.

Design Clearance

Leaving enough room for parts to move as they warm up prevents mechanical interference. The hot runner manifold relies on a known thermal expansion coefficient to seal against the nozzles without crushing them. Proper clearances are verified during the first heat cycle of the tool.

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