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What You Need to Know About Molded Rubber Tolerances

All materials and industrial manufacturing processes create some degree of variability from one product to the next, no matter how automated the processes are. Depending on the materials and the needs of the project, these tolerances can be relatively large or incredibly minute so the products meet their required quality standards. Metals can often hold tighter tolerances as they can be cut or formed, without too much deformation. Compared to metals, different types of rubber offer different porosities, strength levels, and degrees of malleability, each of which can affect the tolerance of the final product.

What Are Tolerance Schedules?

Manufacturers use tolerance schedules that denote acceptable size variations and the degree to which a product’s dimensions can differ from the original design; these numbers are given as plus or minus (±) values. However, to account for the elasticity and flexibility of rubber in different conditions, manufacturers and engineers need a standardized approach to deciding the molded rubber tolerances for a project.

You cannot use metal tolerance schedules when working with rubber. The tolerances used in metal or even plastic fabrication are just too tight to be applied to many rubbers and gasketing materials. A soft sponge rubber will have a much harder time staying within a tolerance as its shape will distort and change to to its soft and malleable nature. In fact, many elastomers are designed to expand and contract. This is essential for products like gaskets, as they need to squeeze into specific cavities and re-expand. Because many of these elastomers simply cannot meet metal tolerances due to their very nature, you could be wasting money and time trying to create a product to meet metal tolerances. Therefore, engineers need to consider common rubber tolerance schedules in their drawings.

Learn more about the differences between metal tolerance schedules and the Rubber Manufacturers Association’s (RMA) rubber tolerance schedules, as well as how to make sure your products are manufactured according to your preferred standards.

RMA Rubber Tolerance Schedules vs. Metal Tolerance Schedules

Metals and alloys have stricter tolerance schedules because of the rigid nature of these materials. However, thermoset molded elastomers like rubbers cannot be made according to the same tolerances because, unlike metals, elastomers do not stay the same shape when compressed or under other conditions. Creating elastomer products using metal tolerance schedules would result in wasted money and time.

RMA rubber tolerance schedules apply to elastomers like natural rubbers, ethylene propylene rubber, silicone elastomers, nitrile rubbers, and others. Each one reacts differently during the manufacturing processes based on molding temperatures, batch or compound variations, cure times, anticipated shrinkage, and even the textural features of the mold.

Below are crucial considerations that influence elastomer and rubber tolerances.

Mold Design

The design of a mold itself should account for the variances in the rubber material. A mold is designed to create a part by pressing two plates (or more, depending on the complexity of the mold) together with a cavity in the middle that forms the rubber material into a shape. There are a few dimensions to consider in a mold’s design: the fixed and closure dimensions. The actual size of the plates themselves needs to be factored in (fixed dimension tolerance) as does the gap created when the plates are pressed together to form the product (closure dimension tolerance).

Trim and Flash

The flash (extra material) that is created in the parting lines of the mold where the rubber prevents it from closing needs to also be measured and controlled. If a material is prone to shrinkage, manufacturers can add more material to fill the mold. It is important to not only account for the outside flash but also be aware of any internal flash that might develop; if there is too much material on a critical part of the product, this can affect the functionality.

Distortion/Shrinkage

The inherent characteristics of different elastomers will result in different types of distortion and shrinkage based on how sensitive they are to heat or their ability to spring back to their original shape after being flexed (tensile strength). Many rubbers expand when heated and shrink when cooled, but the degree of size change varies. If a rubber shrinks after cooling, this is also essential to know when filling a mold.

Environment

To control for variances, manufacturers can manage humidity and temperature conditions, especially for rubber substrates that are more likely to absorb moisture from the air and swell.

RMA Rubber Tolerance Schedules

The Rubber Manufacturers Association (RMA) establishes standardized rubber tolerance schedules so manufacturers and engineers can effectively communicate and produce goods that meet all necessary standards. These rubber tolerance standards denote four tiers of necessary precision, ranging from a very high level of control to relatively lax (but still precise) standards. The levels are:

• A1, High Precision: This is the most tightly controlled tier, and manufacturers must take all possible steps to reduce variance and distortion. It requires very precise molds, molds with fewer cavities, closely monitored mixing, and extensive inspections.
• A2, Precision: This is a step down from A1 but still requires tightly controlled manufacturing and machining. Inspections are mandatory but less in-depth.
• A3, Commercial: Commercial-grade rubber products are the most common, and they must generally fall within reasonable tolerances based on the industry and type of product.
• A4, Non-Critical: These products have the least amount of precision and inspection. For this level, cost is often a priority over tolerance control.

In addition to adhering to RMA standards, it’s also important to consider how the inherent characteristics of different materials affect final product tolerances. For example, soft rubbers with a 30 durometer hardness or lower can shrink by 3 to 4%. Harder rubbers between 65 and 85 durometers, on the other hand, only shrink up to 2% and thus can have tighter tolerances.

High-Quality Rubber From RubberMill

Rubber goods manufacturers, engineers, and product designers should adhere to and communicate through RMA rubber tolerance schedules to keep everyone on the same page throughout production. At RubberMill, our processes and products adhere to RMA standards, and we work with every client to engineer and produce rubber goods that fulfill their needs. We prioritize clear communication regarding molded and die-cut rubbers to guarantee project success.

Contact us today to learn more about molded rubber tolerances or to start your project.