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Injection moulding tolerances — what to specify, what to pay for

A tolerance is not a wish; it is a price. Every tightening of a dimension on your drawing buys real things downstream — better steel, slower cycles, more measurement, sometimes fewer cavities — whether or not the function needed it. This guide explains why moulded dimensions vary, the three honest grades of tolerance, and how to spend tightness only where the part earns it.

Why a moulded dimension varies at all

The mould is machined to microns; the part is not, because the part shrinks on the way out of it. Shrinkage depends on the material (semi-crystalline materials shrink more, and less predictably, than amorphous ones; glass fibre shrinks differently along and across flow), on the wall (our wall-thickness guide is half a tolerance lesson in disguise — non-uniform walls shrink non-uniformly), and on the process (pack pressure, melt and mould temperature, cooling time — all move the final dimension). A tolerance, then, is a promise about how well all three are controlled, shot after shot. That control is what you are pricing.

The three honest grades

Harper’s handbook grades moulding tolerances by the care they demand, and the language is worth adopting on your drawings: ideal tolerance — achievable with minimum care, the band a healthy process holds without trying; commercial tolerance — possible with reasonable care, the everyday working grade of industrial parts; and precision tolerance — the tight band that demands the best steel, the most stable materials, closed control and continuous measurement. Each step between grades multiplies attention, and attention is cost. The mistake that pays for other people’s holidays: drawing precision-grade numbers onto commercial-grade features by habit.

What widens or narrows the achievable band

• Material choice — stable amorphous grades hold tighter bands than high-shrink semi-crystalline ones; filled grades change the picture again, direction-dependent.
• Part design — uniform walls shrink predictably; every DFM rule broken is a tolerance eaten.
• Cavity count — Rosato is blunt: fine tolerances usually cannot be held across many cavities, because each cavity is its own small world. Precision features push toward fewer cavities — which is a piece-price decision too (see how many cavities?).
• Process control — fine tolerances demand closely controlled conditions, consistent material lots and disciplined drying; this is where a supplier’s quality system stops being paperwork and becomes your dimension.

How to specify like a professional

Tolerance the function, not the drawing: pick the few dimensions that mate, seal, align or slide, and spend your tightness there — let everything else live at commercial grade. State the measurement conditions (a moulded part is measured after it has finished shrinking and at an agreed temperature — dimensions move for hours after ejection). Agree the method: the same feature measured by callipers, CMM and optical comparator gives three conversations. And ask your moulder one revealing question: which of these dimensions will you measure every lot, and which at first-article only? A supplier with a real answer has a real quality system.

Send the drawing with the tight ones marked

Tell us which dimensions truly matter and why — our DFM reply will say which grade each one needs, what holds it there, and what it does to the quote. Often the honest answer makes the part cheaper. Request a quote.