Optimizing Thermoplastic Wall Transitions to Prevent Volumetric Sink Defects
Maintain wall transitions below a three to one taper ratio while balancing packing pressure to counter thermal volume loss before gate freeze.
Fillet Radius
Meaning
Internal curve transition represents the rounded corner connecting two intersecting surfaces in a plastic part design. A generous fillet radius reduces local stress concentration and improves the flow of molten polymer during the injection cycle. This dimension is typically specified as a fraction of the nominal wall thickness.
Flow Resistance
Molten plastic requires a smooth pathway to fill the mould cavity without experiencing excessive friction. Sharp corners restrict the advancing flow front, which creates high injection pressure requirements and increases the likelihood of shear degradation. A correct fillet radius lowers the pressure drop and allows the melt to pack the cavity evenly.
Load Distribution
Concentrated mechanical stresses often accumulate at sharp internal angles when the finished part is subjected to external forces. Incorporating a fillet radius distributes these forces over a larger area, which dramatically increases the impact strength of the component. Parts with sharp corners fail under much lower mechanical loads.
Warpage Control
Non-uniform cooling in thick corners leads to uneven shrinkage and internal stresses. When the fillet radius is too large, it creates a thick region that is prone to sink marks and prolonged cycle times. Keeping the radius within thirty to sixty percent of the wall thickness balances strength against cooling efficiency.
The designer must therefore maintain a uniform wall thickness by removing excess material from the opposite side of the transition to prevent void formation and structural distortion.