Radius tearing is a repeatable fracture at a corner or transition where strain is concentrated over a small area. It can occur on both positive and negative tools and is often intensified by a cold contact point or an aggressive plug.
There is no universal minimum radius for every polymer and sheet thickness. Required geometry depends on material ductility, surface finish, draw direction, final wall requirement, and whether the radius is on the plug, mold, or trimmed part.
Diagnostic clues
A tear that remains exactly on the tangent line indicates geometry or first contact. If it shifts with temperature, local sheet condition is contributing. Plug witness marks immediately before the tear suggest the plug is pushing or freezing the sheet at the wrong location.
The radius should be increased where design permits, and the forming sequence should spread strain over a wider area. Tool and plug edges must be free of burrs. When geometry cannot change, pre-stretch, plug contour, sheet grade, or starting gauge may need revision.