Last updated: 2026-09-05 · 5 min read
Corner Radius in Injection Molded Parts — How Small a Radius Causes Stress Concentration, Cracking and Mold Wear, and What Is the Minimum You Should Specify?
Every internal corner in an injection molded part is a stress concentration point. A sharp corner (zero radius) has a stress concentration factor of 3.0× — actual stress is 3× nominal. A 0.5mm radius drops it to 1.5×; 1.0mm drops it to 1.2×. The external radius should be at least 0.5× the wall thickness. Without adequate radii, parts crack under impact and mold steel erodes at the sharp edge.
Key Facts
- Stress concentration factor (Kt) depends on the ratio of radius (r) to wall thickness (t). At r/t = 0 (sharp): Kt ≈ 3.0. At r/t = 0.25: Kt ≈ 1.5. At r/t = 0.5: Kt ≈ 1.2. For a 2mm wall: a 0.5mm radius (r/t = 0.25) cuts peak stress in half. For glass-filled resins, glass fibers align perpendicular to the corner, amplifying the stress concentration by another 20–30%.
- The external corner radius must be at least 0.5× the wall thickness to avoid sink marks and voids. At a sharp external corner, the outer surface contracts faster than the inner — creating a void or sink. A 0.5× wall radius equalizes cooling. For a 3mm wall: minimum external radius is 1.5mm. If the drawing shows a sharp external corner, the mold maker adds the radius anyway — and the part dimension differs from the drawing.
- Mold steel at a sharp internal corner wears faster — EDM or milling leaves a micro-notch at the sharp edge. Under cyclic thermal loading, the micro-notch grows into a crack after 10,000–50,000 shots. A 0.5mm radius eliminates the micro-notch and extends mold life by 3–5×. For hardened steels (H13 at 48–52 HRC), the benefit is smaller but the radius still helps part quality.
How Does Corner Radius Affect Stress Concentration Across Resin Families?
| Resin | Kt (r/t = 0) | Kt (r/t = 0.25) | Min Recommended Radius (2mm wall) | Notch-Sensitive? |
|---|---|---|---|---|
| PC | 3.0 | 1.5 | 0.5mm | High (cracks without radius) |
| ABS | 2.8 | 1.4 | 0.5mm | Moderate |
| POM | 2.5 | 1.3 | 0.4mm | Low (inherently tough) |
| PA66 GF30 | 3.5 | 1.8 | 0.8mm | High (glass fibers amplify) |
| PBT GF30 | 3.3 | 1.7 | 0.7mm | High |
| PP | 2.2 | 1.2 | 0.3mm | Low (high elongation) |
PC and glass-filled resins are the most notch-sensitive — they crack at sharp corners even under static load (no impact). POM and PP are the least sensitive because they have high elongation and yield before cracking. For PC enclosures that must survive a 1m drop test: every internal corner needs at least a 0.5mm radius — without it, the drop test fails at the corner regardless of the wall thickness.
Stress concentration factor (Kt) — the ratio of peak stress at a geometric discontinuity to the nominal stress in the part. At a sharp internal corner (r = 0): Kt ≈ 3.0, meaning actual stress is 3× the calculated value. At r/t = 0.25: Kt ≈ 1.5. At r/t = 0.5: Kt ≈ 1.2. The snap-fit design guide shows how Kt affects cantilever root fatigue life.
What Three Rules Should You Follow When Specifying Corner Radii?
- Default internal radius: 0.5× wall thickness — never zero. For a 2mm wall: 1.0mm internal radius. For a 3mm wall: 1.5mm. This ratio keeps Kt below 1.3 for most resins and eliminates the mold steel crack risk. If the design has tight spacing and a 1.0mm radius does not fit, reduce to 0.25× wall thickness (0.5mm for 2mm wall) — this is the absolute minimum for all resins except PP and PE. For glass-filled resins, never go below 0.5× wall thickness. The screw boss design guide shows how fillets at boss bases follow the same radius rules.
- External radius = internal radius + wall thickness — this keeps the wall uniform at the corner. If the internal radius is 1.0mm and the wall is 2mm, the external radius should be 3.0mm. This maintains a constant material thickness around the corner — no thick section that creates a sink mark, no thin section that weakens the part. If the external radius is smaller (e.g., 1.5mm instead of 3.0mm), the outer surface is compressed and the inner surface is stretched — the wall is thinner at the outside of the corner and thicker at the inside, creating differential shrinkage and warpage.
- Blend radii into adjacent surfaces with a fillet — do not create a tangent discontinuity. Where the corner radius meets the flat surface, the transition must be smooth (G1 continuous — same tangent direction). A tangent discontinuity (where the curve meets the flat at a different angle) creates a new stress concentration at the blend point — defeating the purpose of the radius. In the mold, a smooth blend is achieved by CNC interpolation or by polishing the transition by hand. On the drawing, note “blend smooth, no tangent discontinuity” at critical corners.
Heat checking (in mold steel) — a network of fine cracks that forms on mold steel at sharp internal corners after 10,000–50,000 shots. The steel at a sharp corner has less mass, runs hotter than surrounding areas, and undergoes cyclic thermal loading each cycle. The micro-cracks grow into a visible network — initially cosmetic on the part (raised lines), then structural. Adding a radius increases steel mass at the corner, equalizes temperature and eliminates heat checking. The injection mold life guide covers how heat checking affects mold longevity.
Corner radius: internal ≥ 0.5× wall (never zero). External = internal + wall. Sharp corner: Kt = 3.0. r/t = 0.25: Kt = 1.5. r/t = 0.5: Kt = 1.2. PC and GF resins: notch-sensitive, need ≥ 0.5× wall. Mold steel at sharp corners cracks after 10k–50k shots.
- Internal radius ≥ 0.5× wall thickness — never zero; Kt = 3.0 at sharp corner.
- External radius = internal + wall — maintains uniform wall, avoids sink and warpage.
- Blend smooth, no tangent discontinuity — G1 continuous transition or new stress concentration forms.
- PC and GF resins: minimum 0.5× wall radius — notch-sensitive, crack without radius even under static load.
- Drawing note: “All internal corners R0.5 min” — covers 90% of corners in 2–3mm wall parts.
Frequently Asked Questions
Can I get away with a sharp internal corner if the part is small and the load is low?
For very small parts (<10mm) with low static loads and non-impact use, a 0.2mm radius (practically sharp) can work in PP or PE — these resins have enough elongation to absorb the stress concentration. For ABS, PC or any glass-filled resin: no. Even under low static load, PC develops micro-cracks at sharp corners within weeks from environmental stress cracking (contact with solvents, oils or even humidity). The 0.5mm radius is not optional for PC — it is the minimum for any corner that sees service.
If the design truly cannot accommodate a radius (e.g., a mating interface that must be sharp for sealing), add a 0.3mm radius and specify a local reinforcement — a rib or gusset that transfers the load away from the corner. This is a compromise, not a solution.
How does corner radius affect the molding process itself?
A sharp internal corner creates a hot spot in the mold steel — the steel at the corner has less mass to absorb heat, so it runs hotter than the surrounding steel. Over thousands of cycles, this thermal cycling causes the steel to crack (heat checking). The crack in the mold steel shows up as a raised line on the molded part — initially cosmetic, then structural. Adding a radius increases the steel mass at the corner, equalizes the temperature and eliminates heat checking.
A sharp corner also affects ejection — the part shrinks onto the sharp corner edge, creating a high local ejection force. The ejector pin at that location shows accelerated wear, and the part may drag or score at the corner during ejection. A radius reduces the shrinkage grip and distributes the ejection force over a larger area.
What radius should I call out on the drawing if I am not sure?
Use a default note on the drawing: “All internal corners R0.5 min unless otherwise specified.” This covers 90% of corners in parts with 2–3mm wall thickness. For thicker walls (4–6mm), increase the default to R1.0. For thin-wall parts (1–1.5mm), R0.3 is acceptable. The note should also say: “All external corners to match internal radius plus wall thickness.” This puts the responsibility on the mold designer to maintain uniform wall at corners — and it prevents the mold maker from guessing what radius to add.
Key Takeaways
- Internal radius ≥ 0.5× wall. Never zero — Kt = 3× at sharp corner.
- External radius = internal + wall. Keeps wall uniform, avoids sink.
- PC and GF resins: notch-sensitive, need ≥ 0.5× wall radius.
- Sharp corners crack mold steel after 10k–50k shots (heat checking).
Send the part drawing with the resin and load conditions — the DFM review flags every sharp corner and specifies the minimum radius for each, with the calculated stress concentration factor.
Sources: BASF — Design Guidelines for Corner Radii in Engineering Resin Parts · DuPont — Stress Concentration and Notch Sensitivity in Molded Parts · Plastics Technology — Corner Radius Design Rules and Mold Steel Heat Checking · Society of Plastics Engineers (SPE) — Stress Analysis of Molded Corner Geometries · MatWeb — Notch Sensitivity Data by Resin Family
— Arthur, Tooling Engineer at aCatMould