Meaning
Physical property of the steel used in mold making that describes how the dimensions of the tool change in response to fluctuations in temperature. Knowing the tool steel thermal expansion is critical for calculating the final dimensions of a molded part, as the cavity itself grows larger when heated to operating temperature. This value is usually expressed as a linear coefficient in units of micrometers per meter per degree Celsius.
It defines the growth of the cavity from the ambient shop temperature to the target processing temperature.
Design Calculation
Mold designers must subtract the expansion of the steel from the shrinkage of the polymer to find the correct machining dimensions. Since tool steel thermal expansion is predictable, it can be compensated for during the computer-aided design process. Failure to account for this change results in parts that are consistently out of tolerance.
Precision components require the use of steels with low expansion coefficients to minimize these errors.
Mechanical Stress
Non-uniform temperatures across a mold plate cause internal stresses and potential warping of the tool. Different grades of steel exhibit different rates of tool steel thermal expansion, which can lead to fitment issues between the core and the cavity. Bimetallic effects occur if a mold uses different materials for its internal components.
Maintaining a uniform temperature through efficient cooling channel design mitigates these risks. This becomes particularly important in multi-cavity tools where even small variations in expansion can cause significant differences in part quality between the center and the edges of the mold base. The expansion of the manifold in a hot runner system must also be carefully coordinated with the rest of the tool.
Alignment Maintenance
Moving parts such as slides and lifters depend on precise clearances that are affected by heat. If the tool steel thermal expansion is not managed, these components may bind or seize during the production run. Proper lubrication and the use of wear plates are standard practices to handle the small movements caused by thermal growth.
Long-term stability of the mold depends on consistent thermal cycling within the design limits.
