Meaning
Boss root fillets are reinforcement radii positioned at the intersection of a vertical rib and a horizontal base plate in an injection moulded part. The boss root fillets reduce stress concentration by redistributing loads across a wider surface area during ejection and service. Designers define these fillets as a specific curvature value applied to the junction to prevent the formation of hairline cracks in the plastic wall.
Tooling Geometry
High stress typically occurs at the point where a thin rib attaches to a thicker wall section. These fillets allow resin to flow into the base of the rib without creating air traps or incomplete fill patterns. Sharp corners act as nucleation points for structural failure because the polymer chains cannot orient effectively around an acute angle.
A radius equal to at least one quarter of the adjacent wall thickness ensures that the molten material packs properly into the cavity during the hold phase of the cycle.
Resin Behaviour
Semi-crystalline polymers show sensitivity to sharp transitions during cooling. The fillet geometry moderates the rate of heat transfer through the intersection and prevents differential shrinkage that causes voids inside the rib. Small radii promote brittle failure in high modulus resins under impact loading.
Larger fillets increase the total mass at the junction and may require additional cooling time to prevent sink marks on the aesthetic surface of the part.
Production Economics
Mould manufacturers generate fillets by cutting a specific radius into the steel insert with an end mill or an electrical discharge machining electrode. Higher fillet precision minimizes the force required for part extraction and reduces the frequency of mechanical breakage on the mould surface. Excessive fillet size increases material usage per cycle and potentially lengthens the total manufacturing time.
Proper fillet execution prevents part rejection and lowers the life cycle cost of the tool by mitigating wear at the rib base.
