Moulding tolerances: what to expect

Why moulded parts do not hold machining tolerances

A machined part is cut to size. A moulded part is cut oversize in the tool and then shrinks to size as it cools, and the amount of shrink depends on the material, the wall, the gate position, the cooling and the direction of flow. The tool is cut with a shrink allowance, but that allowance is a single number applied to a part that shrinks differently in different places.

Two effects make this worse. The first is anisotropy: a glass filled material shrinks less along the fibre direction than across it, so a dimension depends on how the flow ran through that region. The second is warpage: uneven cooling and uneven section leave residual stress that relaxes after ejection and bows the part. A flat cover that measures well on the bench can be 0.5 mm out of flat a day later.

Hygroscopic materials add a third effect. Unfilled PA66 absorbs moisture from the air and grows, so a dimension measured dry is not the dimension in service. The measurement condition has to be part of the drawing, not an afterthought at inspection.

The standards and what they cover

ISO 20457 sets out tolerances and acceptance conditions for plastics moulded parts. It defines tolerance grades by nominal size and material class, so a general tolerance note can refer to a grade instead of listing every dimension. DIN 16901 covered similar ground before it was withdrawn in favour of the ISO standard, and many older drawings still reference it.

Neither standard tells you what your part can hold. They provide a vocabulary and a default. A general tolerance grade appropriate for an unfilled commodity part is usually too wide for a precision connector body and unnecessarily tight for a large cosmetic cover.

For features that are machined after moulding, a separate note is needed, because those features follow machining practice rather than moulding shrink. Mixing the two in one general note is a common source of argument at first article.

Typical bands to plan around

The table gives indicative general tolerance bands for unfilled and lightly filled engineering grades. They are a planning aid: the actual capability depends on the tool, the material and the feature. Use them to sanity check a drawing before it goes out, not to replace the moulder's capability data.

What actually moves a moulded dimension

Material is the first driver. Semi crystalline grades such as POM and PA shrink more than amorphous grades such as PC and ABS, and filled grades shrink less but more unevenly. A change of grade, even within the same polymer family, can move a dimension outside a tolerance that was set for the original.

Tooling is the second. The shrink allowance is a guess until the first parts are measured, and the tool is then adjusted by removing steel. Features that need added steel are expensive to correct, which is why a tool is usually cut slightly small in the direction that allows correction.

Process is the third. Pack pressure, melt temperature, tool temperature and cooling time all change shrink. A moulder can move a dimension within a band by adjusting the process, but doing so for one dimension usually moves another.

Geometry is the fourth. Long thin features warp, large flat faces bow, and features far from the gate shrink more than features near it. A tolerance applied uniformly across a part ignores all of this.

Datums and GD&T on a moulded part

A moulded part needs datums that exist in the mould. The parting line, a moulded boss face or a core face are natural candidates because they are formed by steel that is stable and easy to reproduce. A datum on a warped cosmetic surface is a datum on a moving target.

Position tolerance suits moulded holes and bosses better than plus or minus coordinates, because it controls the pattern as a group and allows the bonus tolerance that comes with a hole size at maximum material condition. Profile tolerance suits a sealing face or a curved cover, where the form matters more than the location of any single point.

Flatness on a large moulded face is often the hardest call. A cover that must seal needs a flatness or profile callout with a realistic value, and the value should come from the seal requirement, not from a default. A flatness of 0.1 mm on a 200 mm face is achievable on a well cooled tool with a stable material; the same call on a warping grade will fail at any cycle time.

How to write the tolerance note

Put a general tolerance in the title block and name the standard and grade. Then add tight tolerances only on the features that locate the part, seal it or carry a terminal. Mark those features on the drawing so the inspector knows which ones matter.

State the measurement condition. Name the time after moulding, typically 24 or 48 hours, the temperature and the moisture state for hygroscopic materials. A dimension measured on a hot part straight from the tool is not the dimension the customer receives.

State which features are machined after moulding, and to which tolerance. If a face is milled flat after ejection, its flatness follows the machining, and the moulding note should not be applied to it.

Finally, allow for the tool correction cycle. A first article is expected to show some dimensions out, and the tool is adjusted before the part is accepted. A drawing that demands full tolerance on the very first shot will either be ignored or will cost a lot of tool changes.

• General tolerance with the standard and grade named.
• Critical features identified, with tighter values only where needed.
• Measurement timing, temperature and moisture state stated.
• Machined features called out separately.
• A first article allowance for tool correction.

The cost of asking for too much

A tight tolerance on a moulded part costs money in the tool, in the process window and in the inspection. In the tool it means a stiffer construction, better cooling and more correction loops. In the process it means a narrower window, longer cycles and more scrap. In inspection it means more frequent checks and, often, a gauge rather than a caliper.

It also costs time. A part that is specified to a band the process cannot hold will fail first article, then pass after a tool change, then drift in production as the tool wears. The cost shows up as sorting, concessions and a supplier who prices the risk into the next quote.

The better approach is to spend the tolerance where it buys function. A terminal pocket position may genuinely need 0.1 mm. The overall length of the housing usually does not. Marking the difference is the single most useful thing a designer can do for a moulded part.

1. 01

   List the functional features

   Terminal pockets, seals, mounting and mating faces. These get the tight calls.

2. 02

   Set a general grade

   Pick a standard grade for everything else and put it in the title block.

3. 03

   State the conditions

   Time after moulding, temperature and moisture state for the measurement.

4. 04

   Review the first article

   Expect tool correction and judge the capability, not the first shot.