Engineering tools

Draft angle calculator — angle to offset, offset to angle

Draft is the small taper on every vertical wall that lets a moulding leave its tool without scuffing or sticking. Its geometry is simple — dimensional offset per side = draw depth × tan(draft angle) — but its consequences surprise people: one degree on a 50 mm-deep wall moves the dimension 0.87 mm per side, 1.75 mm across the part. This calculator converts in both directions: angle to offset, or the offset your tolerance allows back to the angle you can afford.

Why draft exists at all

As a moulding cools it shrinks onto cores and grips them; ejection then drags plastic across steel. Draft turns that drag into instant release — the first fraction of a millimetre of ejector travel separates the surfaces completely. Without it: scuff lines on visible faces, white stress marks at corners, bent ejector pins, and cycle times stretched by cooling the part enough to survive ejection. Draft is the cheapest cosmetic and cycle insurance in moulding.

The stack-up nobody budgets

Draft means a wall's top and bottom differ dimensionally — so decide early which end carries the tolerance and dimension the drawing there. Mating parts, snap fits and seals on drafted walls need this decision made explicitly; it is a standard item in our DFM review, and one of the commonest sources of "the sample doesn't match the CAD" surprises when it is skipped. Pair this tool with the shrinkage calculator when dimensioning, and the cycle-time estimator when the wall thickness is also in play.