Why Do Corners Need Radii in Molded Plastic Parts?

Last updated: 2026-09-01 · Reading time: about 4 min

A sharp inside corner in a molded part is three problems stacked in one line of geometry: a stress concentrator that seeds cracks, a hot spot that sinks and warps, and an ejection hazard that scars.
The fix is the radius — sized as a share of the wall (the inside radius commonly half the wall thickness or more) and paired with the mating outside radius so the wall stays uniform around the bend — and it is one of the least expensive strength upgrades in part design. This page collects the working rules.

Key facts

<>

  • Plastics are notch-sensitive: a sharp inside corner multiplies local stress under load and impact, and fatigue or drops find it first — the crack history of molded parts is largely a history of corners.
  • The corner is also a thermal event: material piles up in an un-radiused corner, cooling slows, and the sink and warp that follow are drawn, not accidental.
  • Inside and outside radii are a pair: the outside radius equals the inside radius plus the wall, so the section keeps its thickness through the turn — radius them independently and you have drawn a thin spot or a thick one.

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

    Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

    What does a sharp corner actually do to a part?

    Structurally, it concentrates stress: load flowing around a zero-radius turn has nowhere to spread, and the local stress climbs to a multiple of the nominal — the multiplier grows as the radius shrinks, which is why molded parts crack at corners long before their bulk is in danger.
    Thermally, it distorts the process: an inside corner cut sharp leaves the mating outside corner thick with extra material, that accumulation cools last, and the shrinkage differential draws sink against the corner and bows the flanks — a warpage signature drawn by the drawing itself. In the mold, sharp corners scrape: edges catch on parting lines and ejection paths, wearing both the part finish and the tool.
    None of these costs anything to prevent at the desk — a radius is the least expensive line on the drawing and the most costly one to omit.

    How big should the radius be?

    The working rule pairs the two radii to the wall: inside radius at least half the wall thickness — more where impact, fatigue or chemical exposure ride on the part — and the mating outside radius set to inside radius plus wall, so the section bends at constant thickness.
    Follow the pair rule and the corner cools like the walls around it; ignore it and every radius becomes its own wall-thickness conversation. Where the corner hosts a rib or boss junction, the radius joins the junction’s mass budget — generous enough for flow and stress, restrained enough that the root does not become the thickest point of the part.
    Draft interacts gently: radiused corners traveling along the pull direction keep their draft continuous, while sharp corners localize wear at the exact line the tool rubs hardest. Tolerances finish the job — a functional radius is dimensioned like any other feature, not left to whatever the cutter leaves.

    Which drawing checks catch the classic corner errors?

  • Inside corners radiused as a wall share — stated on the drawing, not implied; the value grows with impact or fatigue duty.
  • Outside radius equals inside plus wall — the pair checked together; a mismatched pair is a hidden thickness change.
  • No un-radiused junction at ribs or bosses — the corner where structure meets wall follows the junction’s mass rules.
  • Corners on the parting line reviewed — radii that cross the split are either broken into the tool deliberately or dressed in finishing; surprises here read as flash and burrs.
  • Cosmetic faces keep their radii — a radius sanded off for appearance still carries the stress; the fix is a designed radius, not an erased one.
  • Radius tolerances named — functional corners dimensioned like any other feature, inside the standard ranges the process can hold.

    • Frequently asked questions

    Doesn’t the radius cost interior space?

    A little, and the trade is almost always favorable: the volume a radius claims is volume that was manufacturing risk, not usable space.
    Where a mating part truly needs the square corner — a seal seat, a register face — the answer is a designed local feature with its own radius breaks, not a global sharp corner. Spend the radius where loads travel; keep the square corner only where function genuinely demands it, and know why.

    Can a sharp corner be fixed by the molding process?

    No — the stress concentrator and the mass accumulation are geometry. Packing profiles can shave a shallow sink margin, and annealing can relieve some residual stress, but the crack initiator and the thermal signature survive every process setting. The radius is a drawing decision; by the press, it is history.

    Why did our part crack at a corner the FEA called safe?

    Models answer the load case they were given; corners fail on the ones they were not — impact at low temperature, assembly wedging, chemical exposure at the highest-stressed fiber. A corner modeled at nominal radius and molded at whatever the cutter left is two different parts. Verify the steel against the drawing, the drawing against the load cases that include the bad days, and the corner’s history changes.

    Key takeaways

    1. A sharp inside corner concentrates stress, slows cooling and scars on ejection — three failures for one missing radius.
    2. Inside radius half the wall or more; outside radius equals inside plus wall, keeping the section uniform through the turn.
    3. Radii at junctions join the mass budget; radii on the parting line and cosmetic faces are designed, never erased.

    If your drawing carries corners — every drawing does — send it over with the load duty. You will get the radius check corner by corner, the pair rule verified, and the junctions reconciled with their mass budgets — from a named tooling engineer, before any steel is ordered.

    Sources
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    Plastics Industry Association — industry resources on part design practice
    DME — mold technologies and components, reference for corner and cooling tooling practice

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet