The main qualification that a material to be bonded, or a substrate, must meet is the ability to withstand the high pressure and high heat involved in the process. However, just because a material can be used in rubber bonding, does not mean that it will turn out well every time.
Rather, the results of bonding rubber to an alloy depend on the composition of the alloy. As for rubber, any type may be used, as long as, one, it can flow into the mold without generating any notable level of cross-linking, and, two, as long as the materials that make up the rubber will not bleed onto the surface of the uncured stock too quickly.
Usually, the rubber material is chosen based on the part being manufactured or the metal being used. Types of rubber that are commonly used in the bonding process include nitrile, neoprene, silicon and a number of other synthetic and natural rubber materials.
The process of rubber bonding involves three main components: the metal substrate, the rubber material, and the bonding agents. The first two have already been covered in brief above; bonding agents are water or solvent based solutions that are topped with a primer coat made up of phenolic-style resins and a topcoat of a variety of materials, including polymers. Most solutions are patented and available for standard purchase.
The number of coatings required depends on the kind of exposure the part will experience. For example, if the part will not likely be exposed to high humidity, pressure, or heat, it may bond successfully with only one layer of bonding adhesive. Similarly, the thickness of the bonding layer depends on the nature of the rubber material upon which it is coated.
The choice of bonding agent or adhesive material depends on the rubber being used, its elasticity, and the component design. However, in all cases, as the effects of manufacturing processes on the environment have become clearer, solvent-based adhesives have largely been replaced by water-based adhesives. This is because water-based adhesives are far more environmentally friendly than the alternative, but have been found to be just as durable and provide equally secure and reliable seals.
Once the substrate, rubber material, and bonding adhesive have been chosen, the bonding process follows a fairly universal procedure. To apply a bonding agent, both the rubber material and the metal substrate must undergo some treatment.
First, the rubber must be sprayed with a gray primer coat via a low pressure, high volume barrel spraying machine. Note that manufacturers must make sure that the area over which it is sprayed is slightly wider and the area occupied by the black topcoat to be applied.
Next, before a substrate can be coated with an adhesive, it must be properly cleaned; no oils, grime, or dust may be left over from the fabrication process. This may be accomplished using chemical cleaning, sandblasters, or degreasers.
The next step, rubber molding, is the most important part of the rubber bonding process. To carry it out, the metal component is first positioned within a mold. After this, it is usually injected with heated, almost molten, uncured rubber. At this stage, plated inserts may or may not be added for additional strength.
Either way, after it is set in the mold, the rubber is left to dry and cure. Once completely dry, the part is considered done. Sometimes, the mold dries in such a way that it is slightly larger than the substrate, so that it forms a thin coating of rubber around the metal. Other times, the mold and the substrate are quite dissimilar from one another because the rubber part design is so complex.
The applications of rubber bonded to metal parts are numerous and varied. In the aerospace industry and automotive industries, they help with the installation and function of foot pedals, bumpers, steering wheels, reinforced tires, engine mounts, and more.
In construction and manufacturing, their innumerable applications include conveyor belts, noise and vibration isolation, gaskets, bearings, rubber lined rollers, and more. They also serve the medical industry with silicone bonded parts, such as surgical instrument handles. In addition, they are found in components of electrical cables, plugs, and rubber lined pipes and tanks.