rubber seal design

Rubber Seal Design: Molding and Material Considerations

Rob PruynMolding Services, Value-Added Manufacturing

Do you need help designing a rubber seal for injection, compression, or transfer molding? Do you need to design quality sealing products for medical devices, or for transportation, process control, industrial, or consumer goods applications? Then remember to account for these molding and material considerations:

  • surface finish
  • flatness and parallelism
  • part tolerances
  • low temperature capability
  • coefficient of friction
  • compression set
  • stress relaxation
  • chemical compatibility and volume swell

Contact The Rubber Group to discuss your rubber seal design or keep reading to learn more about these key considerations.

Surface Finish

Surface finish is important because a rough surface can provide a path for potential leaks. In general, rubber seals with smoother surfaces last longer and mate with surfaces more effectively. For example, in a fluid power application, the surface finish of a rubber seal can influence how successfully a fluid is contained with a system.

With molded rubber products, surface finish is a function of four factors:

  • mold surface quality
  • type of rubber
  • amount of mold release (if any)
  • flash removal method

The Rubber Group uses drawing designations from the Association for Rubber Products Manufactures (ARPM) for surface finish. There are four different levels.

F1A smooth, polished, and uniform finish completely free of tool marks, dents, nicks, and scratches, as produced from a highly polished steel mold. In areas where F1 is specified, the mold will be polished to a surface finish of 10 micro-inches (250nm) or better.
F2A uniform finish as produced from a polished steel mold. In areas where F2 is specified, the mold will be polished to a surface finish of 32 micro-inches (800nm) or better but with very small tool marks not polished out.
F3Surfaces of the mold will conform to good machine shop practice and no micro-finish will be specified. This is “Commercial Finish”. 
F4Satin finish.
ARPM Drawing Designations

Unless surface finish is critical for the appearance or function of a rubber seal, F3 is probably appropriate since a tighter finish (F1 or F2) can incur higher mold costs.

Flatness and Parallelism

Like surface finish, flatness and parallelism are important because they help to prevent leakage. Flatness is a measure of deviation from a straight edge in one plane. Parallelism is the relationship of surfaces in different planes. Here are some examples of how these surface measurements are used:

  • If a rubber product has a metal insert, the mold’s surface may need to be flat within a specific measurement, such as 0.001”.
  • If a parallel plates are used during molding, they may need to be parallel to each other within 0.002”.

Part Tolerances

Part tolerances are allowable variations in the dimensions of molded rubber products. They are pivotal because they determine how well a rubber seal will fit into your application and how reliable the final product will be. By eliminating ambiguity about dimensions, part tolerances improve fit and function. The following factors affect them.

  • shrinkage amount
  • mold precision
  • trimming or deflashing
  • use and location of inserts
  • distortion during mold removal or packaging
  • environmental storage conditions

The Rubber Group uses ARPM part tolerances as a general guide. We can also customize tolerances for your sealing application.

There are four ARPM tolerance levels for molded rubber products:

  • A1 (high precision)
  • A2 (precision)
  • A3 (commercial)
  • A4 (basic)

As a rule, higher-precision rubber products cost more because they require greater mold quality, process controls and measurement frequency. Unless part tolerance is critical, a commercial tolerance (A3) may be suitable for your application.  

Low Temperature Capability

Low temperature capability is important because rubber becomes less flexible in the cold and can crack or break. Even if a rubber seal doesn’t fail, performance becomes less predictable at the limits of a material’s temperature range. By selecting an appropriate rubber material for your application’s service temperatures, you can reduce risk and promote long-lasting sealing.

The chart below shows the low-temperature flexibility of some common elastomers.

ElastomerLow Temperature Flexibility
Natural RubberGood
Common Elastomers for Low Temperatures

Note: Butyl, polyurethane, natural rubber, and SBR each have a minimum service temperature of approximately -65°F.

Coefficient of Friction

Coefficient of friction (COF) is important because a low COF may be needed for mating or sliding parts, or to assist in the release of sealing products from molds or cavities. COF, the amount of friction between two surfaces, ranges from 0 (no friction) to 1; however, it can also be greater than 1, such as with silicone. Because friction slows down a moving object, designers need to consider the movement of a rubber seal. If the seal needs to slide easily, an elastomer with a lower COF can help because less force is required.

Compression Set

To ensure long-lasting sealing and avoid leaks, molded rubber seals that are subject to compressive force need to resist compression set – the permanent deformation of a material after a compressive stress is released. For example, if a valve seal reaches compression set, the rubber won’t “bounce back” or return to its original thickness when the valve is opened. Because the rubber seal won’t compress when the valve is closed again, leaking will occur.

Stress Relaxation

Stress relaxation is important because, over time, the amount of force with which a seal pushes against the mating surfaces will decline. If your design doesn’t account for the effect of this load force over time, your rubber seal may become permanently deformed into a compressed shape and fail before the end of its service life. Sometimes, the cure system that is used with a rubber compound can improve its stress relaxation characteristics.

Chemical Compatibility and Volume Swell

Chemical compatibility and volume swell are critical if your rubber seal needs to resist specific types of chemicals in a specific environment. Different elastomers can resist different types of chemicals, but a rubber material can also behave differently when in contact with the same chemical under conditions of varying temperatures and times. Volume swell measures the deterioration of a rubber material in relation to the amount of liquid chemical exposure, such as with petroleum products.  

Get Help with Rubber Seal Design

The Rubber Group combines injection, compression, and transfer molding with design assistance and help with material selection. If you’re designing a rubber seal for a medical device or a transportation, process control, industrial, or consumer goods application, contact us.