Why rubber tolerances are wide
Rubber shrinks as it cools after cure, by an amount that depends on polymer and filler (FKM about 2 to 3 percent, silicone 2 to 4 percent). Mould cavities are cut oversize, but small differences in compound, temperature and press cycle show up in the part. Rubber also deforms when measured.
ISO 3302-1 classes
| Class | Meaning | Typical use |
|---|---|---|
| M1 | Fine, precision | Seals, small parts, high-cost moulds |
| M2 | High precision | Technical parts, thin sections |
| M3 | Normal | General mechanical parts |
| M4 | Coarse | Large or low-cost parts |
For each class, the tolerance depends on the nominal dimension, with separate columns for fixed (closure-dependent) and non-fixed dimensions. Dimensions across the parting line are looser.
Typical numbers
For a 25 mm dimension: M1 about plus or minus 0.2 mm, M2 plus or minus 0.3, M3 plus or minus 0.5, M4 plus or minus 0.8. For a 100 mm dimension, M2 is about plus or minus 0.6 mm. These are indicative; use the standard tables.
Design rules for moulded parts
- Keep wall thickness uniform.
- Use generous radii.
- Avoid features that must hold tolerance on soft flexible regions.
- Put critical dimensions in fixed directions, not across the parting line.
- Specify flash limits and where the parting line and gate marks may be.
- Define the measurement method: optical, low-force, temperature.
Trimmed or ground parts
If tolerance must be tighter, cryogenic deflashing or grinding of cross sections can improve precision, at extra cost. Cut parts from sheet or tube have different tolerance rules.
Cross-reference
For seals, the standard may be ISO 3601 for O-rings and ISO 3302-1 for other moulded parts. Do not mix them. State the standard in the drawing note.
