November 2005 Design Tip

Minimize material to maximize options

Resin Additional Tolerance
Polycarbonate or ABS 0.002 in/in
Unfilled Nylon 66 0.003 in/in
Polypropylene 0.005 in/in
Example additional tolerances for some resins

The accuracy of the rapid injection moulding process is limited by our standard machining tolerances of 0.003" plus an additional tolerance for the moulding process based on the resin you've selected (see table).

Because these combined dimensional tolerances are highly dependent on the part design and resin selected, Protomold cannot guarantee that a specific tolerance will be met. Conventional injection moulders address this through the use of iterative "steel safe" mould-making techniques. But with the automated, fast-turnaround rapid injection moulding process, we use the geometry specified in your 3D CAD model, incorporate a published shrink factor for the resin you've chosen and output CNC toolpaths to make the mould components.

So if you have features on your part that require tighter tolerances than our standard process can accommodate, consider a planned mould modification aimed at improving the accuracy of those features?. essentially a "simulation" of the conventional "steel-safe" approach.

The way to do this is to design your plastic features in the "minimum material condition" - because it is much easier to remove aluminium than add it (removing aluminium on the mould adds plastic on the part). As an example, consider the situation where the diameter of a through-hole is critical to the functionality of a design. As illustrated in the figure, it is better to initially design the hole in the part (and therefore the aluminium core in the mould) too large and then adjust to fit after some sample parts can be checked.

Because this type of mould modification and a subsequent run of new sample parts can typically be done for under $1000, by planning an iteration you might have a very cost effective way to get the greatest possible accuracy out of your rapid injection moulded parts.

Free Sample Part and Key

In our October Design Tip we offered you a free : Rapid Injection Moulding Sample Part and Key. Well, we've already shipped about 1000 of them! If you'd like us to mail one to you just send an email to and we'll get right on it.