Molded Pulp Edge Radius Design: Why Sharp Corners Fail and 0.8 mm Fillets Win

Molded pulp parts fail at edges — literally: cracks, tears, and fiber starvation all start at sharp corners. Unlike injection molding, where sharp corners are merely discouraged, in pulp molding they are structurally impossible to form well: fibers cannot pack into a zero-radius corner, and drying shrinkage concentrates stress there. A simple radius rule — never design below R0.8 mm internal, R1.5 mm external — eliminates a whole class of production defects.

The Scenario: The Corner That Cracked Every Shift

A part with a square internal corner cracked at the junction in drying, at a rate of 3-5% per shift. Adding a 1.5 mm fillet to the corner removed the crack entirely — zero failures across 20,000 parts. The fix was a CAD change and a mold polish; the part geometry was otherwise untouched.

Pain Points

• Zero-radius corners starve of fiber, producing thin, weak junctions that crack in drying and transit.
• External sharp edges tear during demolding when the part shrinks onto the mold.
• Inspection flags edge defects that are really design defects — geometry, not process, is the cause.
• Designers import injection-molding habits (0.2 mm radii) that cannot work in pulp.

The Solution: Radius Rules for Pulp Geometry

Three radius rules cover 95% of edge-failure cases:

1. Internal corners: minimum R0.8-1.5 mm A 1-1.5 mm internal fillet lets fibers pack and relieves drying stress. Testing across part families shows internal cracks fall by ~90% when the internal radius goes from 0 to 1.5 mm.

2. External edges: minimum R1.5-3 mm External radii protect the part through demolding and handling. A 2 mm external edge on a 2 mm wall survives demold tears that a sharp edge fails on 30-50% of parts.

3. Rib roots and bosses: R1-2 mm (same as rib fillet rule) Rib-to-wall junctions follow the same logic: a 1-2 mm root fillet eliminates the classic rib-root crack. This is the single most common mold-revision request in pulp programs.

4. Draft at 1-3° on all vertical walls Draft complements radii — it lets the part release without tearing and lets fibers pack at the base. No draft + sharp corner = guaranteed defect.

The Result: From 4% Cracks to Zero

A five-part product family revised to minimum R1 mm internal / R2 mm external radii eliminated edge cracks across the line: defect rate fell from 3-4% to below 0.2%, and the mold revision cost less than one week of scrap. The design guide now locks these minima, and new-part DFM reviews check radii before tooling.