The default standard high precision tolerance blocks found on most drawings for machined parts, like metals and alloys, are not feasible for rubber parts. Compound variation, shrinkage, cure time, cure temperature and batch variation all affect the tolerances variation on rubber parts.
The cost of the mold is determined by the varying degree of precision that is designed by the design engineer. Each cavity will have some degree of variation from one to another. Mold alignment is also a factor that must be used to calculate the proper tolerances. The mold alignment or registry is usually controlled by the pins and bushings, or in some cases by self-aligning individual cavities.
All rubber products exhibit some amount of shrinkage after molding as the part cools. The shrinkage of a part is affected by time, temperature, pressure, post cure and batch variation.
To assure a uniform method for use on drawings and in specifications, the tolerances have been standardized by the Association for Rubber Products Manufacturers (ARPM) for use in the molded rubber field.
Drawing Designation “A” is the tightest classification and indicates a high precision rubber product. Such products require expensive molds, fewer cavities, costly in-process controls and inspection procedures. The exact method of measurement should be agreed upon by the manufacturer and customer, as errors in measurement may be large regarding tolerance. Some materials, particularly those requiring post curing, do not lend themselves to Drawing Designation “A1” tolerances.
Drawing Designation “A2” tolerances indicate a precision product. Molds must be precision machined and kept in good repair. While measurement methods may be simpler than Drawing Designation “A1”, careful inspection will usually be required.
Drawing Designation “A3” tolerances indicate a “commercial” product and will normally be used for most products, and Drawing Designation “A4” tolerances apply to products where dimensional control is non-critical and secondary to cost.