Molded Rubber Part Tolerances
Many factors involved in the manufacturing of molded rubber products affect tolerances. Some things to consider include shrinkage, mold design, trim and finish, inserts, distortion, and environmental storage conditions.
Classes determine the fixed and closure dimensional tolerances for products molded in solid rubber. The table below shows the 4 classes that are used by RMA.
| CLASS | NAME | DEFINITION |
|---|
| A1 |
High Precision
|
This is the tightest tolerance classification and indicates a high precision rubber product. Such moldings require precision molds, fewer cavities per mold, close mix controls, etc., which results in high cost. Optical comparators or other measuring devices may be required to minimize distortion of the rubber part by the measuring instrument. This part requires expensive control and inspection procedures. |
| A2 |
Precision |
These moldings involve much of the close controls required for the above class. Molds must be precision machined and kept in good repair. While measurement methods may be simpler than the above class, careful inspection will usually be required. |
| A3 |
Commercial |
This class indicates a general grade or commercial product. This is the most commonly used class. |
| A4 |
Non-critical |
Molds by this class apply to products were control is non-critical and secondary to cost. |
Based on the class selected, use the corresponding table below to apply tolerances as follows:
1. Fixed tolerances are related by size to each dimension but all closure tolerances are determined by the largest closure dimension.
2. Tolerances not shown should be determined in consultation with the manufacturer.
3. Take special care when applying standard tolerances to products having wide sectional variations.
SIZE RANGE
(INCHES) | PRECISION-A2 | COMMERCIAL-A3 | BASIC-A4 |
|---|
| FIXED | CLOSURE | FIXED | CLOSURE | FIXED | CLOSURE |
|---|
| 0 - 0.4 |
+/-0.006
|
+/-0.008 |
+/-0.008 |
+/-0.013 |
+/-0.013 |
+/-0.032 |
| 0.4 - 0.63 |
+/-0.008 |
+/-0.01 |
+/-0.01 |
+/-0.016 |
+/-0.016 |
+/-0.036 |
| 0.63 - 1 |
+/-0.01 |
+/-0.013 |
+/-0.013 |
+/-0.02 |
+/-0.02 |
+/-0.04 |
| 1 - 1.6 |
+/-0.013 |
+/-0.016 |
+/-0.016 |
+/-0.025 |
+/-0.025 |
+/-0.045 |
| 1.6 - 2.5 |
+/-0.016 |
+/-0.02 |
+/-0.02 |
+/-0.032 |
+/-0.032 |
+/-0.05 |
| 2.5 - 4 |
+/-0.02 |
+/-0.025 |
+/-0.025 |
+/-0.04 |
+/-0.04 |
+/-0.056 |
| 4 - 6.3 |
+/-0.025 |
+/-0.032 |
+/-0.032 |
+/-0.05 |
+/-0.05 |
+/-0.063 |
| 6.3+ multiply by |
0.004 |
0.005 |
0.005 |
0.008 |
0.008 |
0.01 |
SIZE RANGE
(MILLIMETERS) | PRECISION-A2 | COMMERCIAL-A3 | BASIC-A4 |
|---|
| FIXED | CLOSURE | FIXED | CLOSURE | FIXED | CLOSURE |
|---|
| 0 - 10 |
+/-0.16
|
+/-0.2 |
+/-0.2 |
+/-0.32 |
+/-0.32 |
+/-0.8 |
| 10 - 16 |
+/-0.2 |
+/-0.25 |
+/-0.25 |
+/-0.4 |
+/-0.4 |
+/-0.9 |
| 16 - 25 |
+/-0.25 |
+/-0.32 |
+/-0.32 |
+/-0.5 |
+/-0.5 |
+/-1 |
| 25 - 40 |
+/-0.32 |
+/-0.4 |
+/-0.4 |
+/-0.63 |
+/-0.63 |
+/-1.12 |
| 40 - 63 |
+/-0.4 |
+/-0.5 |
+/-0.5 |
+/-0.8 |
+/-0.8 |
+/-1.25 |
| 63 - 100 |
+/-0.5 |
+/-0.63 |
+/-0.63 |
+/-1 |
+/-1 |
+/-1.4 |
| 100 - 160 |
+/-0.63 |
+/-0.8 |
+/-0.8 |
+/-1.25 |
+/-1.25 |
+/-1.6 |
| 160+ multiply by |
0.004 |
0.005 |
0.005 |
0.008 |
0.008 |
0.01 |
NOTE: These tolerances follow RMA (Rubber Manufactures' Association) guidelines for molded rubber parts.
Fixed dimensions are those not affected by flash thickness from the parting lines of the mold.
Closure dimensions are those affected by flash thickness from the parting lines of the mold.
A2 tolerances indicate a precision product. Molds must be precision machined and kept in good repair. Typically requires careful inspection.
A3 tolerances indicate a “commercial” product and will normally be used for most products.
A4 tolerances apply to products where some dimensional control is required but is secondary to cost.
Flash
Excess rubber on a molded product resulting from cavity overflow at the parting lines where the mold sections are separated. Flash has two dimensions; extension and thickness. Flash removal can impact part cost significantly. Permitting the widest possible tolerance on flash will help contain costs.
Flash Designations (From Rubber Manufacturers' Association Handbook)
- Extension: Designated by a 'T' and the maximum extension in thousandths of an inch or millimeters. For example: T.032 indicates a 0.032 inch maximum extension, and T.80 mm indicates an 0.80 mm maximum extension.
- Thickness: May be specified following the extension designation if critical to the part's function. Closure tolerances from tables apply.
- Location: An arc marking the area and a leader to the trim symbol indicates the flash specifications at this location. If flash is unacceptable at the location, use T.000.
- Standards: The drawing may indicate maximum flash extension permissible, see the table below. Naturally, assigning a trim symbol to an area should be consistent with the function of the product and, for the sake of simplicity, only areas requiring a designation should be specified. A standard note can often be used without additional notation.
| DRAWING DESIGNATION | DISCRIPTION |
|---|
| T.000 |
(T.00 mm) No flash permitted on area designated. (Standard notation regarding other surfaces must accompany this notation.)
|
| T.003 |
(T.08 mm) This tolerance will normally require buffing, facing, grinding or a similar operation. |
| T.016 |
(T.40 mm) This tolerance will normally require precision die trimming, buffing or extremely accurate trimming. |
| T.032 |
(T.80 mm) This tolerance will normally necessitate die trimming, machine trimming tumbling or hand trimming. |
| T.063 |
(T1.60 mm) This would be the normal tear trim tolerance. |
| T.093 |
(T2.35 mm) This tolerance will normally require die trim, tear trim, or hand trim of some type. |
| T.8 |
(T.8) No flash limitation. |