Process

Developing the right compound (material) is a crucial step in the design process. In their original state, most elastomers have limited commercial value. Therefore, a compound "recipe" must be developed to enhance or add to an elastomer's inherent physical and chemical characteristics, and to ensure efficient molding or extruding. It is important that a material is further developed to meet any necessary industrial specifications, such as ASTM, BS, SAE, UL, FDA, NSF International and U.S.Military Standards.

In the extrusion process, the Rubber Compound, which includes the polymer (EPDM), fillers and additives such as pigments, curing agents, antioxidants and processing aids, is fed into an extruder. The extruder consists of a rotating screw inside a close-fitting heated barrel. The purpose of the extruder is to soften and pressurize the rubber as it is continuously transported to a die head containing a specific part profile at the extruder exit. Therefore, extruded products have a consistent cross-section along the entire length of the product. Typical extruded rubber products include continuous sheets, tubing, and specific part-profiles for seals and gaskets, and weather stripping.

The die is a metal head that is attached to the end of the extruder. Each die consists of an individually machined part-profile opening specific to every seal or gasket. Pressure, built up by heat and the rotating screw, forces the rubber through the die opening—forming it into one continuous profile that had been previously machined into the die.

When the rubber shape (extrudate) exits the die, the material swells so that the part cross-section is larger than the die cross-section. This phenomenon is called "die swell," and depending on the rubber formulation and the extrusion conditions, the dimensions of the extrudate may be increased by several percent to several hundred percent beyond those of the die. Furthermore, except for circular cross-sections (i.e., circular cords), the rubber does not swell uniformly in all directions.

For example, in order to extrude a square cross-section, the die must be cut so that the sides of the square are curved inward.

Upon exiting the die, the rubber extrudate must be cured or “vulcanized” to maintain its shape and acquire the physical properties necessary for the application. This is accomplished by a chemical reaction (the cross-linking of molecules) from peroxides or sulfur curing agents. Vulcanization is initiated when the extrudate temperature is raised by passing it through a curing media, such as a molten salt bath, microwave and/or a hot-air process.

Part dimensions change somewhat during the curing process, so this must also be accounted for in the die design. Curing is the final step in the rubber extrusion process, although the extrudate frequently undergoes a value-added operation, such as slitting, splicing, cutting to specified lengths or application of pressuresensitive adhesives.

Note: Thermoplastic elastomers by nature do not require vulcanization in order to achieve part shape. Instead, the material is hardened simply by cooling in a cold-water bath.

Polymer products may also be shaped into their final form through a molding process. In this process, the final shape of the end product, such as a ball, is determined by the shape and contour of a mold cavity. Molding is used when the final product needs to be conjoined and transitional between planes, as opposed to the continuous, non-transitional profile of an extruded product. There are three basic types of molding processes used to shape polymer products: compression, transfer and injection. Other forms of molding, such as bladder molding, are variations of the three basic types.

Compression Molding

Compression molding takes place as two plates of mold are forced together. A slightly oversized rubber preform (raw compound) is shaped to approximately fit into the mold cavity. When the plates are squeezed together, the preform is compressed and forced to conform to the mold cavity. A molding press is used to provide the necessary force to close the mold. Excess material, or flash, that squeezes out is pushed into grooves cut into the top plate and later trimmed off. Once the curing or vulcanizing process is complete, the mold is opened and the molded product can be removed).

Transfer Molding

Transfer molding can be considered a form of compression molding in that pressure forces the rubber to conform to the mold cavity. The main difference is that in transfer molding, the rubber preform is heated and forced by a plunger through a funnel-shaped opening, or sprue, into the mold cavity. (see Fig. 2)

Injection Molding

Injection molding is a different, more complex process than both compression and transfer molding. The most significant difference is that the mold is not forced together, rather the mold halves begin the process clamped together. Once the mold is clamped, preheated rubber is forced into the sprue of a hot mold. (See Fig. 3)