Meaning
Deforming a polymer or resin during processing requires a force that diminishes as the material temperature increases. This change in mechanical behavior is characterized by yield stress reduction, which describes the decrease in the stress at which a plastic material begins to deform plastically under load. In polymer processing, understanding this phenomenon is essential for optimizing thermoforming and demolding forces.
Thermal Impact
Elevated temperatures increase the molecular mobility of polymer chains, making them slide past one another more easily. This behavior leads to a significant yield stress reduction, which allows the material to be shaped with less energy and lower mechanical load on the equipment. If the temperature is too low, the material remains stiff, increasing the risk of part or tool damage.
Demolding Process
Pulling a warm molded part from the core pins requires careful control of the ejector force to avoid permanent deformation. Because of the yield stress reduction at demolding temperatures, the ejector pins can punch through the part if the cooling time is insufficient. The part must be cooled until it has regained enough strength to withstand these forces.
Structural Limit
Parts intended for high-temperature service must be made from resins that maintain their strength when hot. Selecting a polymer with a high glass transition temperature prevents premature failure caused by this strength drop under load.
