Manufacturers in a wide variety of industries rely on molded rubber parts for high performance in their products. With rubber being only one of many materials used in developing a new product, OEM engineers and purchasing managers pose similar questions about how elastomers work and which rubber material is right for their application. Following are answers to eight questions that OEMs often ask.
Q. What is a Thermoset Elastomer?
A. An elastomer is a polymer with viscoelasticity, which simultaneously exhibits the properties of a very thick fluid and a flexible solid. Thermoset elastomers are a class of polymers that are cured by cross linking molecular chains during the molding process, which requires the right amount of pressure and temperature to set the part into its permanent shape. Unlike plastics, which can break under pressure or melt in intense heat, rubber and elastomers hold up well to dynamic operating conditions, temperature variations, and harsh environments.
Q. What are the most commonly used elastomers in industrial applications?
A. Rubber is the common name used for a variety of elastomers, but natural rubber is one of the least commonly used materials in industrial applications.
The 12 elastomers used most often for industrial applications are:
- Natural Rubber
- Butyl Rubber (IIR, Isobutene-isoprene)
- Chloroprene (CR, Neoprene®)
- Ethylene Propylene Diene (EPDM)
- Fluorocarbon (FMK, Viton®)
- Nitrile Butadiene (NBR)
- Saturated Nitrile (HNBR)
- Silicone Rubber (SI, Gum and Liquid)
- Styrene Butadiene (SBR)
- Urethane (PU, Polyurethane)
® Neoprene and Viton are registered trademarks of DuPont.
Q. Which elastomer is best for my new product? Although the molded rubber part is only one small piece of the product, I need to understand the chemical-resistance values of the different elastomers so I can decide.
A. The first step in determining the right material for the molded part is to discuss the end product’s operating environment with the rubber molder’s chemists and engineers. The answer is not as simple as picking one of the 12 commonly used elastomers. When selecting the right elastomer for the application, rubber molding chemists and engineers consider such factors as the function of the part, the end product’s operating conditions, and the material’s attributes, such as its ability to resist such environmental hazards as contact with oil and other fluids, abrasion, and temperature fluctuations, as well as the material’s cost.
Rubber molders develop custom compounds for specific applications. OEM engineers and purchasing managers may want to consult rubber material specifications to understand more about the complexities of elastomers.
Q. How is a rubber compound developed?
A. Rubber molders develop a custom formula of an elastomer and additives for each component they produce in keeping with the molding process that best suits the part and its operating environment. Some 80 organic elastomers and more than 350 raw materials are considered in the process of developing natural, synthetic and high-performance compounds.
The first step is to review and understand the manufacturer’s requirements for the part, including:
- Chemical resistance needs
- Operating temperature ranges
- Other important characteristics of the operating environment
- The end manufacturing and assembly process
- Expected annual production volume
- The OEM’s budget and material costs
After a thorough review with design for manufacturability in mind, a rubber chemist will provide advice on the best material for the part and its operating environment. In preparing a rubber compound for a specific application, the chemist will collaborate with the molder’s and manufacturer’s engineers to develop a recipe of the proper elastomer plus a mix of specialized additives to address such issues as resistance to UV, ozone or oil, bonding, and electrical conductivity.
Then the chemist will test one or more compounds in a lab to determine how it can be expected to perform. Unlike plastic materials, which computers can model to predict performance, rubber is difficult to model, yet it is essential for certain applications that require flexibility and durability.
Many OEMs take the extra step in new product development of having the molder prototype both the custom compound and the molded part for proof of concept, to ensure it will perform as needed.
Q. Can we find out the exact formulation of the materials used in our part?
A. Custom formulations are proprietary to the rubber molder, which draws upon its library of compounds and employs its own R&D to develop each formula. If independent verification of a compound is required, the molder can work with a third-party testing company to ensure that the materials meet your specifications and any applicable regulations.
Q. Cost is always an issue with us. How can we know if a lower cost material would work just as well as a higher cost elastomer for our new part?
A. Developing the right rubber compound for the application takes a number of factors into account, balancing cost with functionality. The more specific data you can provide to the rubber compounder on the part’s functions and operating conditions, as well as specific budget considerations, the better the result. You can also request prototypes of one or more rubber compounds and/or parts so you can test the material or component under actual operating conditions to see if a lower cost material will perform well.
Q. Can I use molded rubber for a part with tight tolerances?
A. Yes. Rubber can be used for parts with tight tolerances, depending on the component’s specifications and operating conditions. In general, the flexible properties of rubber mean that tolerances tend not to be as tight as metal or rigid plastics. However, rubber parts can be developed to meet a wide range of tolerances, including RMA A1 tolerance standards.
Different rubber molding techniques are used to produce parts that require tight tolerances and precision functions. Rubber molding engineers work closely with their OEM clients to recommend the best molding process for the part, which include:
- Compression molding
- Transfer molding
- Liquid injection molding
- Insert molding
Any of the first three molding processes above can be used to provide insert molding and over-molding of rubber bonded to metal, plastics and other substrates. The process selected depends on the characteristics and cost of the material to be molded, the configuration of the part, and the projected annual production volume.
Rubber molding works by adding precise amounts of heat and pressure to set the part. Once a rubber part is cured, the rubber has gone through a chemical change. The chemical crosslinking that takes place gives the rubber the ability to flex but still retain its original shape, which is one of the key advantages of thermoset elastomers for certain applications that must tolerate temperature variations and contact with fluids.
Q. For materials that require government approval, how do I know the supplier will meet regulatory standards so I don’t have to switch suppliers mid-stream?
A. Ask the rubber molding company upfront about their experience with meeting your industry standards as well as regulatory and testing requirements. They should have experience working with independent testing organizations and also adhere to a policy of never changing ingredients in a rubber formula once that material has been approved and qualified.