Factors Affecting the Tolerances of Cut or Molded Rubber Parts

Many factors combine to affect the tolerances that can be achieved with cut or molded rubber parts. Due to their inherent physical properties and characteristics, rubber materials are not capable of supporting the same level of dimensional tolerances that can be achieved with some other materials such as steel or hard plastics.

Dimensional tolerances are an important factor to consider and specify when sourcing or buying cut or molded parts, including cut parts manufactured from solid sheet rubbers or cellular materials such as sponges and foams. Tolerance designations for cut or molded parts relate to the variability that normally occurs when producing parts from solid or cellular rubbers. In this blog, we examine some of the factors that can affect the tolerances of cut or molded rubber parts, and which in turn determine the final dimensions of a finished part.

Factors affecting the tolerances of cut or molded rubber parts: cut parts

Specifying tolerances for solid parts such as gaskets and seals, which are cut from sheet or strip rubber, can be tricky because these products are often designed to flex, expand, and contract when placed into use.

Numerous factors can cause variation in the dimensions of a cut part. Material age and cure time, storage and processing temperature, and the pressure imposed on the material as it is being cut can all affect the final dimensions. In addition, secondary operations, such as buffing and finishing, may affect the final dimensions as well as the reproducibility of those dimensions within a batch of parts.

At CRG, we use GFA (Gasket Fabricators Association) best practices in our production processes to assure that the tolerances agreed to with a customer are achieved when cutting solid rubber parts. When cutting parts from cellular rubber materials, such as open or closed cell sponges and foams, we follow the guidelines and tolerance tables published in the ARPM (Association for Rubber Products Manufacturers, Inc.) Rubber Handbook. In addition to these best practices for tolerancing, all production at CRG takes place under controlled conditions designed to assure quality, following the processes and procedures implemented under our ISO 9001:2015 quality management system.

Because of the variability inherent in both solid and cellular rubber materials, and the processes used to cut parts from these materials, it is unrealistic to expect the same precision of tolerances that are achievable when machining solid metals such as steel or aluminum. When specifying desired tolerances in a cut rubber part, design engineers and buyers should take care to refer to tolerance tables, such as those published by the ARPM, which are followed by reputable part manufacturers.

Factors affecting the tolerances of cut or molded rubber parts: molded parts

As with cut parts, there are numerous factors that can affect the tolerances of molded rubber parts. These include the following:

  • Material shrinkage. In molding rubber parts, there may be difference between the mold dimensions and the completed part. This difference can be caused by material shrinkage as the part cools and by shrinkage of the base compound used to produce the part. While the amount of shrinkage can be minimized, it cannot be eliminated entirely.

  • Mold design. The molds used to produce molded rubber parts can exhibit dimensional variation in their designs, such as variation in the dimensions of a mold’s cavities. These variations can result in dimensional differences of the completed part.

  • Trim and finish. As with cut parts, secondary operations, such as flash removal, can result in dimensional variation in the finished part.

  • Material distortion. Subjecting finished parts to stresses, such as those caused by moving or packing, can distort the part and result in dimensional differences.

  • Environmental and storage conditions. Rubber changes in dimensions as temperature and humidity changes. For molded parts that are used in critical applications that require precise dimensioning, the temperature and humidity under which a part is to be measured should be specified.

For customers seeking molded rubber parts, CRG follows the tolerancing guidelines and requirements contained in ISO 3302-1:2014 (Rubber – tolerances for products – Part 1: Dimensional tolerances). ISO 3302-1:2014 is an International Standard, published by the International Organization for Standardization (ISO), that specifies classes of dimensional tolerances and their values for molded, extruded, and calendared solid rubber products. The relevant test methods necessary for the establishment of compliance with ISO 3302-1:2014 are also specified in the standard. The tolerances specified in ISO 3302-1:2014 are primarily intended for use with vulcanized rubber but can also be suitable for products made from thermoplastic rubbers.

CRG supplies a variety of cut or molded rubber parts for a wide range of industrial and automotive applications. These include gaskets and seals, parts for isolation, damping and load bearing, connectors, bumpers, caps and grommets, and many more. We carry a wide range of performance materials which allows to produce cut or molded parts for almost any application.

To find out more about the cut or molded rubber parts which CRG can supply, please contact our sales team at crg@canadarubbergroup.com.