June 8, 2022 | Technical Note
Almost every appliance part gets bent, folded or drawn, which is why formability decides yield more often than weather resistance does. T-bend is the most direct read on that.
The sample is folded 180° around itself, then tape is pulled off the bend to see whether coating lifts. 0T is the tightest — the sheet folded flat against itself. 1T, 2T and 3T loosen from there, and a higher number means less flexible coating.
| Coating system | Typical T-bend | Suitable forming |
|---|---|---|
| PE (polyester) | ≤ 2T | Normal bending, folding, light stamping |
| SMP (silicone modified polyester) | ≤ 3T | General bending; assess before deep drawing |
| HDP (high durability polyester) | ≤ 3T | General bending |
| PVDF (fluorocarbon) | ≤ 1T | Best flexibility, highest cost |
These are typical, not guaranteed values. The same system in a different colour or film thickness can move a full grade — dark and metallic shades are usually more brittle.
1. Over-curing. Oven temperature too high or line speed too slow cross-links the film too densely and makes it brittle. Most common cause, and the hardest to spot by eye.
2. Film too thick. A 28 μm top coat looks more substantial than 20 μm, but the outer skin sees more tensile strain on a bend and cracks sooner.
3. Substrate too hard. A high-yield substrate springs back further and concentrates stress at the bend radius, tearing the coating off. For deep-drawn parts, do not simply pick the strongest grade available.
T-bend measures adhesion after deformation; cross-hatch measures it as supplied. They are not substitutes. Some sheet passes cross-hatch at grade 0 and still fails a fold — that is a flexibility problem. Others fold cleanly but lose corners on cross-hatch, which points to pre-treatment or curing.
Do not rely on a certificate alone. Send us the part drawing, or a section of the profile you form, and we will run the actual bend radius before fixing coating and film thickness. Lab T-bend numbers are no substitute for the one bend your die makes.
