Why corner radii matter in thermoforming
Sharp corners are the most common design error in thermoforming. They cause two compounding problems:
Material thinning
The hot plastic sheet cannot conform to a sharp corner — it bridges across the corner, creating a thin, highly stressed zone. This zone is typically 30–50% thinner than the surrounding wall.
Stress concentration
Sharp corners act as stress risers. Under impact, thermal cycling, or sustained load, cracks initiate at sharp inside corners and propagate through the wall. This is the primary cause of in-service thermoformed part failure.
The fix is simple: always specify a corner radius. Even a small radius (1.5× sheet thickness) dramatically reduces both thinning and stress concentration.
Inside vs outside corner radius
Inside corners (concave) and outside corners (convex) have different minimum radius requirements because the material behaves differently at each:
| Corner Type | Material Behaviour | Minimum Radius | Recommended Radius | Risk if Too Small |
|---|---|---|---|---|
| Inside (concave) | Material stretches to fill corner | 1.5× sheet thickness | 3–5× sheet thickness | Thinning, cracking, stress failure |
| Outside (convex) | Material compresses slightly | 0.5× sheet thickness | 1–2× sheet thickness | Appearance defects, minor stress |
| 3D inside corner (vertex) | Stretches in two directions simultaneously | 2× sheet thickness | 4–6× sheet thickness | Severe thinning — highest risk zone |
Minimum corner radius by sheet thickness
| Sheet Thickness | Min Inside Radius | Rec. Inside Radius | Min Outside Radius | Min 3D Vertex Radius |
|---|---|---|---|---|
| 1.5mm | 2.25mm | 5–7mm | 0.75mm | 3mm |
| 2mm | 3mm | 6–10mm | 1mm | 4mm |
| 3mm | 4.5mm | 9–15mm | 1.5mm | 6mm |
| 4mm | 6mm | 12–20mm | 2mm | 8mm |
| 5mm | 7.5mm | 15–25mm | 2.5mm | 10mm |
| 6mm | 9mm | 18–30mm | 3mm | 12mm |
| 8mm | 12mm | 24–40mm | 4mm | 16mm |
| 10mm | 15mm | 30–50mm | 5mm | 20mm |
Common corner radius failures and fixes
✗ Cracking at inside corners in service
Cause: Inside corner radius too small — stress concentration exceeds material yield strength under cyclic loading
Fix: Increase inside corner radius to minimum 3× sheet thickness. For impact-critical parts, use 5× sheet thickness.
✗ White stress marks at corners after forming
Cause: Material over-stretched at corner — radius too small for the sheet thickness and draw ratio combination
Fix: Increase corner radius. Reduce forming temperature slightly to reduce over-stretching. Consider plug assist to redistribute material.
✗ Mould marks visible at corners on finished part
Cause: Mould corner radius is sharp but part corner radius is larger — the part bridges the mould corner and marks are visible
Fix: Match mould corner radius to the desired part corner radius. Polish mould corners to Ra < 0.4μm.
Frequently asked questions
What is the minimum corner radius for thermoforming?
The minimum inside corner radius for thermoforming is 1.5× the sheet thickness. For a 3mm sheet, the minimum inside corner radius is 4.5mm. Outside corner radii can be as small as 0.5× sheet thickness. Sharp inside corners (radius < 1× sheet thickness) cause stress concentration, localised thinning, and potential cracking during forming or in service.
Why do thermoformed parts need rounded corners?
Sharp corners in thermoforming cause two problems: material thinning and stress concentration. During forming, the plastic sheet cannot conform to a perfectly sharp corner — it bridges across the corner, creating a thin, highly stressed zone. In service, this thin zone is the first to crack under impact or thermal cycling. Rounded corners allow the material to flow smoothly and maintain more uniform thickness.
What is the difference between inside and outside corner radius in thermoforming?
Inside corner radius (concave corner) is the more critical of the two. The material must stretch to fill an inside corner, causing thinning. Minimum inside radius = 1.5× sheet thickness. Outside corner radius (convex corner) is less critical because the material compresses slightly rather than stretching. Minimum outside radius = 0.5× sheet thickness, though larger radii improve appearance and reduce stress.