RMA Tolerance

The Rubber Manufacturers Association (RMA) has published the RMA handbook. The purpose of this handbook is to provide a method for standardizing design requirements for molded rubber products. The information included in the RMA handbook will give an overview of what to expect with dimension tolerances and will let customers know more about the manufacturing process.

Dimensional tolerances, or part tolerances, are acceptable variations in part size. These differences are caused by slight manufacturing variations and expressed as plus or minus (±) amounts in millimeters (mm) or inches (in). Precision parts such as aerospace components require tighter tolerances with less variation. Other types of components, such as consumer products, may allow for greater part-to-part differences.

The following will provide a common terminology for use in discussing dimensions of molded rubber products, and for distinguishing various tolerance groupings:

Fixed Dimensions: Dimensions not affected by flash thickness variation. (Mold Closure)

Closure Dimensions: Dimensions affected by flash thickness variation. (Mold Closure) 

In addition to the shrinkage, mold maker’s tolerance, trim and finish, a number of other factors affect closure dimensions. Among these are flow characteristics of the raw stock, weight, shape of preform and molding process. While closure dimensions are affected by flash thickness variation, they are not necessarily related to basic flash thickness. If a manufacturer plans to machine or die trim a product, the mold will have a built-in flash, which will be thicker than if hand deflashing or tumble trim were to be employed. Thus, products purchased from two sources could have different basic flash thickness at the parting line and yet meet drawing dimensions.

There is usually a logical place for the mold designer to locate the parting line for best dimensional control and part removal. If the product design limits this location, an alternate mold construction will be required, which may affect the tolerance control on the product, and may, in some cases, increase the cost of the mold.

Registration Dimensions: Dimensions are affected by the matching of the various plates of the mold that form the mold cavity.

Register is usually controlled by dowel pins and bushings or by self-registering cavities.

There are four RMA drawing designations for the dimensional tolerances of molded rubber products:

A1 – High Precision

A2 – Precision

A3 – Commercial

A4 – Basic

You’ll find these tolerances below so that you can add them to the part drawings that you send to us.

Drawing Designation: “A1” High Precision

Drawing Designation: “A2” Precision

Drawing Designation: “A3” Commercial

Drawing Designation: “A4” Basic