Meaning
Dimensional reduction perpendicular to the direction of polymer flow occurs as the plastic cools and contracts. This cross-flow contraction is transverse shrinkage, and it is often higher than flow-direction shrinkage in fiber-reinforced materials. Since fibers do not align perpendicular to the flow, they cannot restrict contraction along this axis.
Anisotropic Behavior
Fiber-reinforced polymers exhibit different shrinkage rates along and across the flow direction due to fiber orientation. Transverse shrinkage is typically much higher than flow-direction shrinkage, which creates internal stress and causes part warpage. Tool designers must use multi-directional shrinkage factors when designing the mould cavity.
Gate Impact
Gate location and style dictate the polymer flow path and the resulting fiber orientation within the cavity. Since transverse shrinkage is highest across the flow direction, changing the gate location alters where the high shrinkage occurs. Moulders use this technique to shift warpage away from critical dimensions.
Cavity Dimensioning
Failing to account for the higher cross-flow contraction results in parts that are narrower than specified. Accurate cavity steel sizing requires separate calculations for the flow and transverse shrinkage directions based on material supplier data. The dual-axis approach ensures that the finished parts meet the assembly requirements.
Machining costs are reduced by eliminating the need for subsequent trial-and-error modifications.
