Heat Treatment of Plastic Injection Molding Products

Concept and function of plastic heat treatment

The heat treatment of plastic products refers to the method of holding molded products at a certain temperature for a period of time to eliminate internal stress. Heat treatment is the best method to eliminate orientation stress inside plastic products.

Applicable objects requiring heat treatment

For injection molded parts with high rigidity of polymer molecular chains and high glass transition temperature; For parts with thick walls and metal inserts; For components with a wide temperature range and high dimensional accuracy requirements; Parts with high internal stress and difficult to self dissipate, as well as parts that have undergone mechanical processing, must undergo heat treatment.

Mechanism of heat treatment stress relief

Heat treatment of the component can cause the polymer molecules to transition from an unbalanced conformation to an equilibrium conformation, allowing the forcibly frozen and unstable high elastic deformation to obtain energy and undergo thermal relaxation, thereby reducing or essentially eliminating internal stress. The commonly used heat treatment temperature is 10-20 ℃ higher than the operating temperature of the component or 5-10 ℃ lower than the thermal deformation temperature.

Heat treatment time setting

The heat treatment time depends on the type of plastic, thickness of the component, heat treatment temperature, and injection molding conditions. For general thickness parts, heat treatment for 1-2 hours is sufficient. As the thickness of the part increases, the heat treatment time should be appropriately extended. Raising the heat treatment temperature and extending the heat treatment time have similar effects, but the effect of temperature is more pronounced.

Heat treatment implementation methods

The heat treatment method is to place the workpiece in a liquid medium such as water, glycerin, mineral oil, ethylene glycol, and liquid paraffin, or heat it in an air circulation oven to a specified temperature and stay at that temperature for a certain period of time, then slowly cool it to room temperature. Experiments have shown that immediate heat treatment of the demolded parts has a more significant effect on reducing internal stress and improving the performance of the parts. In addition, increasing the mold temperature, extending the cooling time of the parts in the mold, and conducting insulation treatment after demolding all have similar heat treatment effects.

Limitations and precautions of heat treatment

Although heat treatment is one of the effective methods to reduce internal stress in parts, it usually only reduces the internal stress to the range allowed by the working conditions of the parts, and it is difficult to completely eliminate the internal stress. When subjected to prolonged heat treatment on PC components, the PC molecular chains may undergo ordered rearrangement or even crystallization, thereby reducing impact toughness and decreasing notch impact strength. Therefore, heat treatment should not be the only measure to reduce internal stress in the workpiece.