RMA tolerance classes for rubber parts
Rubber drawings give tolerances as RMA classes instead of a plus or minus on every dimension. The classes come from the Rubber Manufacturers Association (RMA) handbook, now published by the Association for Rubber Products Manufacturers (ARPM); the values are unchanged.
We hold extrusions to RMA E1, E2 or E3. E2 is our typical class, and tighter tolerances are available on request. Every quote states the class it is based on.
The designations
- E1, E2, E3: cross-section dimensions of extruded parts. E1 is high precision, E2 precision, E3 commercial.
- L1, L2, L3: cut length of extruded parts. L1 is precision, L2 commercial, L3 non-critical.
- A1 to A4: dimensions of molded parts. A1 is high precision, A2 precision, A3 commercial, A4 basic.
- Sponge extrusions carry their own, wider classes for cross-section and cut length.
- A drawing calls them out together, for example "RMA E2, L2" for a cut extrusion.
Extruded cross-sections: E1, E2, E3
The tolerance grows with the size of the dimension. Representative values, plus or minus, in inches:
| Dimension | E1 | E2 | E3 |
|---|---|---|---|
| 0.06 to 0.10 in | 0.008 | 0.014 | 0.020 |
| 0.10 to 0.16 in | 0.010 | 0.016 | 0.027 |
| 0.25 to 0.39 in | 0.016 | 0.027 | 0.039 |
| 0.63 to 0.98 in | 0.027 | 0.039 | 0.063 |
| 1.57 to 2.48 in | 0.039 | 0.063 | 0.098 |
Solid extrusions are quoted to E2 unless the drawing calls for another class.
Cut lengths: L1, L2, L3
Cut-length tolerances apply to extrusions cut to a set length. Representative values, plus or minus, in inches:
| Cut length | L1 | L2 | L3 |
|---|---|---|---|
| Up to 1.6 in | 0.03 | 0.04 | 0.06 |
| 4.0 to 6.3 in | 0.05 | 0.08 | 0.13 |
| 16 to 25 in | 0.10 | 0.16 | 0.25 |
| 40 to 63 in | 0.16 | 0.25 | 0.50 |
| 100 to 160 in | 0.25 | 0.50 | 0.80 |
| Over 160 in | 0.16% of length | 0.32% of length | 0.50% of length |
Molded parts: A1 to A4
Molded tolerances separate two kinds of dimension. A fixed dimension lies within one half of the mold. A closure dimension crosses the parting line, so it varies with how the mold closes and carries a wider tolerance. Representative values, plus or minus, in inches:
| Class | Fixed, up to 0.40 in | Closure, up to 0.40 in | Fixed, 1.00 to 1.60 in | Closure, 1.00 to 1.60 in |
|---|---|---|---|---|
| A1 high precision | 0.004 | 0.005 | 0.008 | 0.010 |
| A2 precision | 0.006 | 0.008 | 0.013 | 0.016 |
| A3 commercial | 0.008 | 0.013 | 0.016 | 0.025 |
| A4 basic | 0.013 | 0.032 | 0.025 | 0.045 |
Each molding quote states the class it is based on.
Sponge extrusions
Closed-cell sponge expands as it cures, so its cross-section varies more than solid rubber of the same size. The handbook gives sponge its own classes for cross-section and cut length, wider than E1 to E3 and L1 to L3, and the tightest sponge class is not applied to very soft sponge. Call out the sponge class on the drawing, or ask for the values that apply to your size.
Die cut and flash cut parts
Flat gaskets cut from sheet are held to a plus or minus on the cut outline; ours are cut to plus or minus 0.02 in. The sheet thickness carries the tolerance of the sheet it is cut from.
What sets the tolerance a part can hold
- Hardness: soft compounds deform more in the die, in the mold and when measured.
- Shape: thin walls, hollows and unsupported legs move more than solid sections.
- Shrinkage: rubber shrinks as it cures, and each compound shrinks by a different amount.
- Measurement: a soft part has to be measured without compressing it.
Questions we hear
- What RMA class do you quote?
- Extrusions are quoted to RMA E2 unless the drawing calls for another class. We hold E1, E2 or E3, and every quote states the class.
- Can you hold tighter than E2?
- Yes, E1 is available on request. Send the drawing with the dimensions that matter, and the quote states the class for each.
- My drawing gives no tolerance class. What applies?
- We quote to E2 for cross-sections and state the cut-length class, so the tolerance is written down before the order.
- Are RMA and ARPM tolerances the same?
- Yes. ARPM took over the RMA handbook; the classes and values are the same, though some table numbers changed.
Request a quote
Send us a drawing, a sample part or a rough sketch. We will review it, recommend the right material and process, and give you a firm ship date with your quote.