Meaning
Thermal energy generated internally through mechanical shear during the movement of a polymer melt represents a critical factor in tool design. This phenomenon, known as dynamic viscous heating, occurs when the friction between polymer chains under high flow velocity converts mechanical energy into thermal energy. It is most pronounced in thin-wall moulding and narrow gate regions where shear rates are highest.
This localized heating can raise the melt temperature beyond the set points of the barrel, altering the planned flow behavior.
Moulding Consequence
Temperature spikes within the runner system can lead to material degradation and cosmetic flaws. While moderate dynamic viscous heating reduces pressure requirements by lowering resin viscosity, excessive heat degrades the polymer chains. This degradation leads to weak points, burns, or splay on the surface of the moulded part.
Shear Optimization
Adjusting the injection speed allows the processor to control the amount of internal friction generated. By optimizing the velocity profile, the technician utilizes dynamic viscous heating to fill thin sections without overheating the material. This balance keeps the material within its safe thermal processing window.
Tooling Response
Mould designers use specific gate geometries to manage the heat generated during the injection cycle. Proper gate sizing prevents excessive dynamic viscous heating, protecting heat-sensitive polymers like polyvinyl chloride from thermal degradation. This engineering practice ensures consistent part weight and mechanical properties across long production campaigns.
