Meaning
Preferential alignment of polymer chains during the melt phase dictates the mechanical strength of the final part. In injection moulding and film extrusion, molecular orientation occurs when the shear forces of flow pull the coiled chains into a straightened, parallel arrangement. This alignment is most pronounced near the cold walls of the tool where the polymer freezes rapidly.
Factors such as the degree of orientation determine whether a part is tough and flexible or brittle and prone to cracking along the flow lines. The limit of this alignment is reached when the polymer chains are fully extended, a state rarely achieved in standard processing but approached in high tenacity fiber spinning.
Shear Influence
High injection speeds increase the shear rate and lead to a more intense molecular orientation in the skin of the part. This skin layer provides a protective barrier and enhances the surface finish, but it also creates internal stresses. If the melt is allowed to cool too slowly, the chains will recoil into a random state, a process known as relaxation.
Moulding cycles must be optimized to freeze the orientation in the desired state.
Mechanical Property
Physical characteristics vary depending on the direction of the measurement relative to the flow. Tensile strength is typically higher in the direction of the molecular orientation while impact resistance might be lower in the transverse direction. This anisotropy is a factor for structural parts that must withstand specific loading patterns.
Engineers use this effect to reinforce hinge areas in living hinge designs.
Optical Effect
Light passing through a part with extensive alignment will experience birefringence. Molecular orientation causes the refractive index to differ between the flow and transverse directions, which is visible under polarized light as coloured fringes. High levels of this optical distortion can be a defect in transparent lenses or covers.
Controlling the gate location and the cooling rate minimizes these unwanted visual artifacts.
