Being thermally molded, thermoset elastorners are subject to many variables which impact dimensional tolerances. Among these variables are temperature, cure time, mold tolerances and condition, post eure, number of cavities, and compound variation.

The Rubber Manufacturers Association (RMA) has developed tolerance tables with ranges to provide a basis for communication between the customer and the molder for a wide range of industries from precision aerospace to basic consumer. These tolerance designations relate to the variability inherent in processing rubber and silicone components. The designations are:

• "A1" High Precision
• "A2" Precision
• "A3" Commercial
• "A4" Basic

The type of rubber and its durometer or hardness will determine if the part will experience substantial size variation during cool down.

• Very soft rubbers (10 to 30 durometer) will shrink 3% to 4%
• Harder rubbers (50 to 90 durometer) will shrink 1-1½% to 2%

Mold cavity dimensions and the actual dimensions of the molded component will vary. The mold must be machined to compensate for the material shrinkage. Limtech engineers can apply past experience with similar parts and materials to design a mold which will achieve the required dimensional tolerance.

Definitions

• Fixed Dimension - Dimension determined by the mold steel. The dimension typically does not cross the parting line
• Closure Dimension - Dimension across the parting line; it is determined by the opening and closing of the mold. Typically, closure dimensions have more variation than fixed dimensions.

Measuring

• Limtech personnel measure elastomer parts using a computerized optical comparator
• Very flexible parts cannot be measured, but must be checked in application

Tables

STANDARD DIMENSIONAL TOLERANCE TABLE-MOLDED RUBBER PRODUCTS
DRAWING DESIGNATION "A1" HIGH PRECISION

STANDARD DIMENSIONAL TOLERANCE TABLE-MOLDED RUBBER PRODUCTS
DRAWING DESIGNATION "A2" PRECISION

STANDARD DIMENSIONAL TOLERANCE TABLE-MOLDED RUBBER PRODUCTS
DRAWING DESIGNATION "A3" COMMERCIAL

STANDARD DIMENSIONAL TOLERANCE TABLE-MOLDED RUBBER PRODUCTS
DRAWING DESIGNATION "A4" BASIC