Shear Stress

Meaning

Mechanical flow forces describe the internal resistance of a polymer melt as it moves through a channel or die. High shear stress occurs when the velocity gradient across the flow path is steep, such as in narrow gates or thin walls. This force can cause molecular chains to break and lead to a reduction in the mechanical properties of the finished part.

It is measured in pascals and varies depending on the melt temperature and the flow rate of the resin.

Melt Viscosity

Fluids like molten plastic become thinner as the rate of deformation increases during processing. Under high shear stress, the polymer molecules align with the direction of flow and allow for easier filling of the cavity. Moulders must balance this thinning effect with the risk of thermal degradation from friction.

Molecular Degradation

Excessive forces within the melt can permanently damage the structure of the resin. When shear stress exceeds the threshold of the material, the polymer chains snap and create lower molecular weight fragments. This process results in parts with lower impact strength and poor surface finishes.

Flow Rate

Controlling the speed of the injection screw manages the amount of force applied to the resin. A slow injection speed reduces shear stress but might lead to premature freezing or weld line defects. Engineers use rheological data to calculate the optimal speed for each specific part geometry and material type.