The basic knowledge about the thermal expansion of components in the molds for plastic injection molding is discussed below.

In the case of a mold for plastic injection molding, in order to maintain the appropriate cavity surface temperature, it is maintained at a temperature of 30 to 150°C. On the other hand, molten plastic flows into the sprue, runner, and cavity, which receive heat from the plastic at temperatures in the range of 180 to 300°C. Metals generally undergo thermal expansion when the temperature rises. Therefore, even the constituent parts of a mold for plastic injection molding undergo thermal expansion. Thermal expansion can disrupt the mating between the guide post and guide bush, or can cause bad movement of the slide core, or can enlarge the dimensions of the core pins.

The basic changes in the dimensions due to thermal expansion can be calculated using the following equation.

Where, λ is the expansion (in mm) in the dimensions expected to thermally expand, α is the linear thermal expansion coefficient (mm/mm) of the metal, ｌ_０　is the initial length (mm), t is the initial temperature (°C), and　ｔ_０　is the temperature after heating.

The linear thermal expansion coefficients for typical metallic material used in molds are given below.