Meaning
Anisotropic distribution of reinforcing fibres within a polymer melt determines the directional strength and shrinkage of injection moulded parts. The resulting glass fiber orientation varies across the thickness of the part, depending on the shear and elongational flow fields during the injection phase. This alignment is highly directional, with fibres pointing along the flow direction near the walls and perpendicular to it in the core.
The difference in orientation causes variation in mechanical properties and thermal expansion.
Flow Influence
High shear rates near the mold walls force the fibres to align with the direction of the flow. In the centre of the cavity, however, the glass fiber orientation is more random or transverse due to the lower shear rate. This skin-core structure is created during the filling phase as the polymer melt solidifies against the cold metal surfaces.
Faster injection speeds increase the thickness of the aligned skin layer. This variable alignment must be analyzed during part design to predict mechanical behavior, especially in areas with thin walls or complex geometries.
Mold Design
Gate location and runner geometry determine the flow path and the resulting fibre alignment in the cavity. Diverging flow patterns can cause fibres to align perpendicular to the stress direction, which reduces the strength of the part. In addition, weld lines where two flow fronts meet are particularly weak because the fibres align parallel to the joint instead of crossing it.
Moulders can use simulation software to optimize the gate position, reducing these weak spots. This optimization is crucial for structurally loaded parts.
Structural Consequence
Parts with highly aligned fibres exhibit high stiffness along the direction of alignment but are brittle in the transverse direction. This anisotropy can cause the part to warp during cooling as the shrinkage rate differs along the axes. Selecting the correct gate location minimizes this warpage and ensures that the part can withstand the service loads.
This design control yields flat, strong components.
