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
<>
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:
- 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.
- 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.
- 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.
- 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.
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
<>
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
- Society of Plastics Engineers — Technical Papers
- Plastics Technology — Processing Tips
- HASCO — Mold Base Standards
- MMTech — Mold Making Technology
- MatWeb — Material Property Data
Sources: Plastics Technology · Manufacturing.net · Society of Plastics Engineers (SPE) · ThomasNet