Last updated: 2026-09-04 · 6 min read

Why Do Molded Parts Need Fillets and Corner Radii? — Stress, Flow and Steel

A sharp internal corner is the most expensive feature in a molded part — it fails as stress riser, flow trap and steel knife-edge simultaneously. A fillet replaces the zero-degree corner with a smooth arc, buying back strength, fill quality and tool life for a fraction of a millimeter of material.

Key Facts

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  • A sharp internal corner has a stress-concentration factor that can more than triple local stress under the same nominal load, so a part that looks adequate at the corner is in fact primed to fail there first.
  • Radii also govern melt behavior: plastic flows around curves, and a sharp corner forces an abrupt direction change that can freeze the flow front, trap gas, or leave a weak knit line at the point of highest stress.
  • On the tool side, an inside corner in the part becomes an outside edge on the core — thin steel that erodes, is hard to polish, and where the mold runs hottest; a generous radius gives the mold maker solid steel and a ball-nose cut in one pass.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

    What Does a Radius Buy You?

    Five concerns, each resolved by the same fillet:

    Concern Sharp Corner Filleted Corner
    Stress concentration High; corner cracks first Spread; higher fatigue life
    Melt fill Flow stalls; weld or short Smooth turn; uniform packing
    Sink and void Thick mass shrinks inward Gradual transition; less sink
    Mold steel Thin hot edge that wears Robust, polishable radius
    Machining cost EDM and hand polish hard Ball-nose cuts it cleanly

    The stress-concentration effect is the headline: the sharper the inside corner, the higher the local stress multiplier for the same applied load, and cracks initiate at stressed notches. A fillet that cuts the concentration factor buys disproportionate gains in fatigue and impact life for pennies of added material.

    The fill effect is just as real: a corner is where the flow front slows, cools and can stall; a radius keeps the section from thickening and lets the melt pack evenly. The last two rows move the argument into the tool: an inside corner in plastic is a matching outside edge on the steel core — thin, hot, hard to polish — whereas a generous radius gives solid steel and lets a ball-nose mill do in one pass what EDM and hand-lapping do slowly.

    How Big Should an Inside Radius Be?

    Four rules, in order:

    1. Size the inside radius off the wall thickness. The workable rule is an inside radius around a quarter to a half of the adjacent nominal wall — so on a 2 mm wall, an R0.5 to R1.0 inside fillet — because what matters for stress and flow is the ratio of the corner to the section feeding it. A radius tiny relative to the wall barely dents the stress concentration; one that is generous keeps the transition smooth. Where the part allows, push toward the larger end.
    2. Keep the outside radius consistent with the inside. A common mistake is a big inside fillet paired with a sharp outside one, which reintroduces both a flow problem and a cosmetic sink because it makes the corner locally thick. The outside radius should equal the inside plus the wall thickness, so the wall stays uniform through the bend. Uniform section through the fillet is the actual goal — the radius is the means.
    3. Use a smaller radius only where geometry forces it, and say so. Sometimes a mating part, a sharp cosmetic edge or an assembly datum genuinely needs a tight inside corner. When you specify a reduced radius, flag it on the drawing as a deliberate deviation so the mold maker is not left to infer whether you meant it. Radii are a default to deviate from knowingly, not a rule to break silently.
    4. Let the radius inform draft and the tool maker’s input. Inside radii and draft angle work together: a rounded corner is far easier to draft and eject than a sharp one. This is a place where a few minutes of the mold maker’s feedback beats a rule memorized in CAD — ask them to confirm the radii are cuttable, polishable and ejectable as drawn, because they will see the core edge you are about to create.

    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. The related guide covers adjacent topics.

    Add the radius once and collect strength, fill, surface and tool-life benefits together. It is the cheapest multi-tasking feature in injection molding.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Is a bigger radius always better?

    Up to the point where it starts thickening the corner, generally yes — a larger inside radius keeps lowering the stress concentration and improving melt flow. The exception is when a generous radius adds so much local material that the corner becomes a heavy section that sinks, voids or extends the cycle.

    The right move is to increase the inside radius and, if it thickens the section, hollow it out behind with a cored radius or a nearby rib so the corner stays strong without becoming a mass of plastic that shrinks. The honest rule is “bigger until the wall thickens, then keep the radius and manage the bulk.”

    Do outside corners need radii too?

    Yes, though for partly different reasons: an outside radius mainly protects against a sharp cosmetic edge that is unpleasant to handle and prone to chipping, keeps the wall uniform where it bends, and gives the mold a durable inside feature instead of a fragile thin wall. Where the inside fillet is mostly about stress and the tool’s core edge, the outside fillet is mostly about appearance, ergonomics and holding section constant.

    The two are linked: the outside radius equals the inside plus the wall. Round both, keep them consistent, and the corner stops being the part’s weak point.

    Why does the mold maker care about my part’s radii?

    Because your part geometry literally becomes the tool’s geometry, and radii decide how expensive and how durable that steel is. A sharp inside corner means a matching sharp edge on the core — thin steel that runs hottest, wears fastest, is hard to polish and is a repair liability — while a proper radius lets the mold maker cut the feature with a ball-nose end mill in one clean pass.

    That difference shows up directly in the quote’s machining hours and in how the mold holds up over its rated life. Catching a too-tight radius on screen costs nothing; discovering it in hardened steel means a slower, dearer mold and a corner more likely to fail in service.

    Takeaways

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  • A sharp internal corner fails three ways at once — stress riser, flow trap and steel killer — and a fillet fixes all three for a fraction of a millimeter of material.
  • Size the inside radius at a quarter to a half of the wall thickness, and keep the outside radius consistent so the wall stays uniform through the bend.
  • Ask the mold maker to review your fillets before you freeze the model — catching a too-tight radius on screen costs nothing; finding it in hardened steel costs everything.

    • Fillets are the cheapest multi-tasking feature in injection molding.
    Add a radius once and collect strength, fill, surface finish and tool life together — the kind of design decision that costs nothing to make and everything to skip.

    Sources

    1. Society of Plastics Engineers — Technical Papers
    2. Plastics Technology — Processing Tips
    3. HASCO — Mold Base Standards
    4. MMTech — Mold Making Technology
    5. MatWeb — Material Property Data

    Sources: Plastics Technology · Manufacturing.net · Society of Plastics Engineers (SPE) · ThomasNet