Meaning
Dimensional reduction along the direction of polymer flow occurs as the plastic cools and the oriented polymer chains relax. This contraction is known as longitudinal shrinkage, and it is usually higher in unfilled polymers than in fiber-reinforced grades. When fibers are present, they align with the flow and mechanically restrict movement along this axis.
Anisotropy Effect
Fibers align in the direction of the flow during injection, leading to unequal shrinkage in different directions. While longitudinal shrinkage is minimized by this alignment, transverse shrinkage remains much higher because there is no reinforcement across the flow. The difference causes internal stresses that can warp the finished component.
Processing Variables
Injection pressure and melt temperature have a major influence on determining the final part dimensions. Higher holding pressures squeeze more material into the cavity, which directly reduces longitudinal shrinkage by packing the polymer chains closer together. Conversely, higher melt temperatures increase the total thermal contraction that occurs during cooling.
Moulders must balance these parameters during the trial stage to meet tight tolerances.
Mould Correction
Tool steel dimensions must be adjusted using different shrinkage rates for the flow and cross-flow directions. Failing to account for low longitudinal shrinkage in fiber-filled resins results in parts that are longer than specified. Toolmakers use directional shrinkage data from material suppliers to size the cavity steel correctly.
