Meaning
Thermal or environmental treatments applied to molded polymer parts after ejection from the tool stabilize material dimensions and optimize mechanical properties. Using post moulding conditioning is especially common for hygroscopic materials that require controlled moisture absorption before they can achieve their design toughness. This procedure prevents the part from undergoing unpredictable dimensional changes when exposed to ambient humidity in service.
Moisture Equilibration
Polyamide components are stiff and brittle immediately after molding due to their dry-as-molded state. Exposing these parts to a heated water bath or a high-humidity chamber accelerates the absorption of moisture, which acts as a plasticizer to increase impact strength. This controlled post moulding conditioning ensures that the parts reach their equilibrium moisture content and dimensionally stabilize before being subjected to mechanical assembly forces.
Without this step, parts might fail during snapping or press-fitting operations right after molding, even if they would eventually become tough in the field.
Stress Relief
Annealing is another form of post-molding treatment that involves heating the parts to a temperature below their melting point and then cooling them slowly. This thermal cycle relieves the frozen-in tensile and compressive stresses that develop during the rapid cooling of the injection molding process. Relieving these stresses prevents the parts from warping or cracking when they are exposed to elevated temperatures or chemical solvents in their end-use environment.
Quality Control
Implementing these finishing treatments requires careful monitoring of time, temperature, and humidity levels to ensure consistency across batches. Moulders test sample parts using moisture balances and dimensional gauges to verify that the target properties have been successfully achieved. Incorrect treatment can lead to over-saturation or thermal degradation, both of which ruin the structural integrity of the components.
