Manufacturing Tolerances – Molding

Unlike rigid machined materials, thermoset molded elastomers do not lend themselves well to the same level of tolerancing. Being thermally molded, elastomers are subject to many variables. Temperature, cure time, mold tolerance, mold registration, compound variation, and shrinkage are just some of the factors all molders encounter.

The Association for Rubber Products Manufacturers (ARPM) has updated tolerance tables with ranges to provide communication between user and provider across a wide range of industries, from precision aerospace electronic components to open-tolerance products for consumer goods. These tolerance designations relate to the variability inherent in processing molded rubber parts and are referred to as ARPM “A2” Precision and ARPM “A3” Commercial dimensional tolerance designations.

There are obviously costs involved as the rubber molder prepares to meet customer requirements at the ARPM “A2” level. This includes preparations for tooling, extra features, cavity finishes, and cavity flow provisions. In processing, very close temperature control and timing of molding cycles may add to the cost of the part.

The type of rubber material, and particularly its durometer hardness, will determine if the part will experience substantial size change during its cooldown.

• Very soft rubber (15 to 30 durometer) will fall in a 3 to 4% shrink category.
• Firm to almost rigid compounds (65 to 85 durometer) will shrink 1-1/2 to 2%, allowing for tighter tolerances.

Shrinkage occurs on all rubber components and is a volume effect. Although built into the mold, it will vary depending on the part complexity within the same cavity. It occurs when the part is removed from the heated mold and allowed to cool. The engineering challenge is to cut a steel cavity that will reliably provide acceptable toleranced rubber parts. Given today’s close tolerance thin cross-section designs, your need for precision molded parts has never been more apparent.

“A2” Precision Drawing Designation
Dimensional Tolerance Table for Molded Rubber Products

When applying tolerances, the following rules should be kept in mind:

• Fixed dimension tolerances apply individually to each fixed dimension by its own size.
• Closure dimension tolerances are determined by the largest closure dimension and this single tolerance is used for all other closure dimensions.
• Fixed and closure dimensions for a given table do not necessarily go together, and can be split between tables.
• Capability studies can be run with a cavity segment to aid efforts to assign tolerances for a given material. Please contact us for assistance.

“A3” Commercial Drawing Designation
Dimensional Tolerance Table for Molded Rubber Products

Tolerances as shown in the ARPM: MO-1 2015 Rubber Handbook