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Design Guide: The Extrusion Process

This guide is intended to give design engineers the basic facts needed to begin the process of designing effective rubber seals, gaskets and weather stripping. It is not intended to provide every piece of information available on the process or extrusion, the amount of information available is simply too great. However, it should serve as a starting point and give engineers the necessary information to better understand what they will need to consider during the design process.

Polymer products may be shaped into their final form by either an extrusion or a molding process. A three-dimensional object such as a ball is processed by molding in which the rubber is forced to conform to the surfaces of a mold cavity. Extruded products have a constant cross-section along the entire length of the product. Typical extruded rubber products include continuous sheet, tubing, seal and gasket profiles and weather stripping.

In the extrusion process, the rubber compound or formulation, which includes the polymer, fill 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, mix and pressurize the rubber as it is continuously conveyed to a shaping die at the extruder exit.

The die is a metal disk with a machined opening in the desired shape of the extruded part. The pressure built up by the rotating screw forces the rubber, which has been softened by the heating, through the die opening-shaping it into the profile cut in the die.

When the rubber shape (extrudate) exits from the die, the material swells so that the part cross-section is larger than the die cross-section. This phenomenon is called “die swell”; 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 (see below.) Furthermore, except for circular cross-sections (i.e., circular cords), the rubber does not swell uniformly in all directions.

After leaving 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 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 secondary processing such as slitting, splicing, cutting to specified lengths or application of pressure-sensitive adhesives.

Several variations of the basic extrusion process exist. In coextrusion, softened rubber streams from two (or more) extruders that are connected to a single die, permitting such features as two colors, dual durometers or sponge and solid combinations, without resorting to secondary operations.

Thermoplastic elastomers are specialty polymers that do not require vulcanization in order to achieve part shape. Instead, the material is hardened by cooling to room temperature in a cold water bath.

Example A: Sponge rubber may have an “open cell” structure in which each cell is connected to other cells, providing a continuous gas pathway throughout the part. The sponge may also have a “closed cell” structure (see figure1) in which each gas cell is completely surrounded by rubber, and thus isolated from all other cells.

Example B: Sponge rubber, also called “expanded” or “cellular” rubber, is extruded by including a gas-forming ingredient (blowing agent) in the rubber formulation. As the rubber extrudate passes through the successive heating zones in the cure system, it reaches a temperature at which the blowing agent decomposes and gas bubbles form, creating individual vulcanization chambers, or a salt bath, the extrudate is cooled to stabilize it dimensionally. The vulcanization process preserves the cell structure formed by the gas in the interior of the extruded part. However the surface of the extruded part is smooth, without exposed cells, and is referred to as a skin.