Molded Rubber Tolerance Classes A1 to A4

The four classes

• A1, high precision. Keep it for the dimensions the fit truly depends on. Holding it takes a precisely built mold with few cavities, tight control of the process and close inspection, so it is the most costly class to hold. Agree with the molder how the part will be measured before production starts, since a small measuring error can use up much of the band. Some compounds cannot hold it, post-cured ones in particular.
• A2, precision. One step looser. The mold still has to be machined to precision and maintained, and the parts are still inspected carefully, but measuring them is more straightforward than at A1.
• A3, commercial. The everyday class, and the one most molded parts are made to. A molder works to it when the drawing names no class.
• A4, basic. For parts where the dimensions need some control but cost matters more than precision.

Choose the loosest class the fit allows, and call out the few dimensions that need a closer class on their own instead of tightening the whole part.

Fixed and closure dimensions

ARPM sorts the dimensions of a molded part into two kinds. A fixed dimension is formed within one part of the mold, so the flash at the parting line does not change it. A closure dimension crosses the parting line, so it moves with the thickness of the flash, and one tolerance, set by the largest closure dimension on the part, covers all of them. Two molders can leave different flash at the parting line and both still meet the drawing, so mark where flash is not allowed, such as on a sealing face.

What tightens or loosens a tolerance

• Shrinkage. Rubber shrinks as it cools out of the mold, by an amount that changes with the compound, the cure, any insert, the shape and any post-cure. The mold is cut oversize to allow for it, but some variation always remains.
• The mold and its cavities. No two cavities are quite identical, so a mold with many cavities adds that spread to the part. A tighter class needs a more precisely built mold, which costs more.
• Register. How well the plates of the mold line up affects every dimension that crosses from one plate to another.
• Trimming. Removing flash can take material from the part itself, so a thin lip at the parting line may end up sized by the trimming rather than the mold.
• Inserts. A metal or plastic insert brings its own tolerances and has to locate in a hot mold; see rubber-to-metal bonded parts.
• Measuring. Rubber is soft, it changes size with temperature, and some compounds take up moisture, so parts are measured after they have settled at stated conditions, with a method that does not squeeze them.

How to state tolerances on a drawing

Put the ARPM class in a drawing note, such as "ARPM A3" for commercial tolerances, and call out any dimension that needs a closer class on its own. ARPM's designation system also covers the surface finish, with its F classes, and how far flash may extend at the parting line, with a T designation that carries the allowed extension; the dimensional class, the finish and the flash can be written together as one designation. Features measured against another feature, such as concentricity, flatness or the location of an insert, have no standard class, so state them as geometric tolerances and agree them with the molder. For the tightest tolerances, agree the measuring method before the first parts are made.

Other rubber processes have their own scales

The A classes are for molded parts. ARPM's handbook gives extruded profiles, lathe-cut parts and cellular or sponge parts their own designations, so a drawing for one of those names its own scale rather than an A class.

Tolerances on a Conquest Seal molded quote

State the class and the critical dimensions on the drawing you send. The quote is written to your drawing on either route, the domestic molders Conquest Seal distributes for or our overseas manufacturing program, and the RFQ checklist for custom molded parts lists what else a molded quote needs. The material side of the drawing is covered in the D2000 callout guide. The part types with their own pages are molded gaskets and seals, grommets and plugs and boots.

Sources

1. The ARPM Rubber Handbook (ARPM MO-1), Eighth Edition 2020, Association for Rubber Products Manufacturers: chapter 1, Molded Rubber Products (drawing designations, factors affecting tolerances, fixed and closure dimensions, the A1 to A4 classes, measurement, relative dimensions) and the chapters on extruded, lathe-cut and cellular products, read 2026-10-10. Designations and meanings only, no values reproduced