Quantifying Inter Cavity Thermal Drift Rates in High Cavitation Tool Steel Assemblies
Inter-cavity thermal drift in high-cavitation steel tooling shifts part dimensions; turbulent coolant flow and conductive inserts eliminate temperature spreads.
Linear Thermal Expansion
Meaning
Fractional changes in the length of a polymer or mold component per degree of temperature change determine the dimensional stability of a part during cooling and its fit within an assembly. This physical property is quantified by a coefficient that varies between the resin and the mold steel. Understanding linear thermal expansion is necessary for calculating the shrink rate and the final dimensions of the molded product.
Material Behavior
Polymers typically expand and contract at rates ten times higher than the metals used for tooling. Amorphous resins show a more predictable change than semi-crystalline materials, which undergo additional volume changes during the formation of crystals.
Mold Design
Tooling engineers must size the cavity larger than the desired part to account for the shrinkage that occurs as the plastic cools. If the mold is made of aluminum instead of steel, the different expansion rate of the tool itself must also be included in the calculation. Errors in these estimates lead to parts that are out of tolerance or assemblies that fail to snap together.
Tolerance Constraint
Environmental temperature changes can cause a part to change size even after it has left the factory. A part that fits perfectly at room temperature may bind or loosen when exposed to extreme heat or cold in the field. This behavior is a primary consideration when selecting materials for automotive or aerospace applications.