INDUSTRY TOLERANCES FOR CUSTOM MOLDED PARTS
If rubber is considered to behave more like a highly viscous, elastic liquid, rather than a classic solid, tolerance requirements for custom molded rubber products can be more easily understood. Because rubber is a thermoset material, it is affected by more factors than most other materials. Some of these factors are discussed below.
- Shrinkage
This is the difference in dimension between corresponding features on the molded part and the mold from which it was produced, when both are measured at room temperature. All rubber materials have different shrinkage rates, which may necessitate the construction of separate molds if the same molded part is to be produced from more than one material. For example, fluorocarbon rubber has a much higher shrinkage than nitrile, and must be run in ”compensated” tooling. Although every mold is designed to take shrinkage into account, minute variability in mold temperature, cure time, materials, pressure, cavity size etc. must be considered, and covered by dimension tolerances.
- Environmental Factors
Rubber has a relatively high coefficient of thermal expansion, when compared to other engineering materials. In order for the molder and end user to agree on dimensional measurements, it is important that the temperature at the time of measurement be agreed upon beforehand. A good rule of thumb is to allow the molded part to rest at room temperature (23° C) for 12 hours before any measurements are taken. Some rubber materials absorb moisture, which also needs to be considered in determining tolerances. Ideally, such products should always be measured in a temperature and humidity controlled environment. Additionally, a few rubber compounds will crystallize when stored at low temperatures. Parts molded from these materials should always be allowed to come to room temperature before any measurements are taken.
- Mold Design
Mold costs are in direct proportion to the precision required for the final molded rubber product. While a product which specifies “A2” tolerances (see tables below), may require only a simple, relatively inexpensive compression mold, tighter tolerances frequently require a much more expensive transfer or injection mold. Fewer cavities may also be required, increasing piece price. Mold register (the relative alignment of the two or more plates in a rubber mold), is also a consideration. Parts with very close tolerances will require precise machining to control register, and therefore be more expensive.
- Inserts
All inserts, typically metal, plastic, or fabric, have tolerances of their own, and these must be taken into consideration when determining what the tolerances of the final product will be. Also, in most cases, room temperature inserts are placed into a heated mold, so the mold design must accommodate this, as the coefficient of thermal expansion of the rubber and the insert are rarely similar.
The tables below were developed by the Rubber Manufacturers Association to guide those involved in the design of molded rubber parts when deciding what the dimensional tolerances should be. The term “Fixed” refers to those dimensions that are not affected by variation in flash thickness. In other words, they are unaffected by the opening and closing of the mold. “Closure” dimensions are just the opposite; they are affected by flash thickness.