Injection Moulding Tolerances: What Matters and What Does Not
Most first-time founders arrive at their first tolerance conversation expecting a number. They want the tightest one available, because tighter sounds like better, and better sounds like safer for the money they have put in.
That is not how tolerance works, and treating it that way is one of the fastest ways to burn through a budget before a single part ships.
This article is about the pattern we see, not a checklist. Three observations, from parts that came across our bench, that separate the tolerances that protect a product from the ones that just protect a nervous founder’s feelings.
The Surface Nobody Touches
A founder once asked for a tighter tolerance on a housing’s outer shell than on the part that actually snapped into it.
The outer shell is cosmetic. Nobody presses it against another component. It sits there and looks like plastic.
The tolerance she skipped over was on the internal boss where two halves clicked together. That is the surface that decides whether the product feels solid in someone’s hand or rattles on day one.
Tight tolerance on a surface that never mates with anything else is money spent on a feeling, not a function.
Where Function Actually Lives
Tolerance matters where parts meet. A snap fit, a bearing seat, a thread, a living hinge.
These are the places where a fraction of a millimetre changes whether the product works or breaks on the first use. This is where we spend the tolerance budget, because this is where it earns its keep.
Everywhere else on the part, the tolerance can relax. Not because it is cheaper, though it usually is, but because relaxing it changes nothing a customer will ever notice.
The Cost Shows Up Later, Not Now
The founders who get burned are rarely burned by the mould quote itself. They are burned three months later, when a tight tolerance that was never load-bearing turns a routine reorder into a rework.
Every tolerance you set is a promise the factory has to keep, run after run. A tolerance set tighter than the part needs does not just cost more once. It costs more every single time the tool cuts steel again.
The parts that stayed on budget were the ones where tolerance was set against what the part had to do, not against what felt reassuring on a drawing.
Pro-Dev designs parts to be manufactured from day one, which means tolerances get set against function before a mould is ever cut. Our factory relationships, built over more than ten years, tell us where tight tolerance earns its cost and where it is just an expensive habit.
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