An extruded rubber product differs from a molded rubber product in that the rubber is manufactured in a continuous process. Raw rubber materials are forced through a die of the desired cross-section under pressure from an extruder created by a screw inside the extruder barrel. The extruded rubber product leaves the extruder in a soft pliable uncured state. The extruded product then needs to be cured / vulcanized to ensure that the rubber product will remain in the desired form to meet the desire applications requirements.
Uncured / Unvulcanized rubber compound is fed into the extruders feed section. The flutes of the revolving screw carry the rubber forward through the barrel to the die, building up pressure and temperature as it advances toward the die. The rubber is forced through the die by this pressure and swells in varying amounts depending on the type and hardness of the compound. Due to the many variables such as temperature, pressure, etc., the extrusion varies in size as it leaves the die, thus requiring plus or minus tolerances on the cross-section. During the vulcanization, the extrusion will swell or shrink in the cross section and length depending on the compound used. After vulcanization, a length of extrusion tends to be reduced in dimension more in the center of the length than the ends. The extruded product is vulcanized either in a heated pressure vessel (static vulcanization) or by the continuous vulcanization process. A brief Description of each follows:
Extrusion Curing Processes
Autoclave Curing / Valcanization Process
The extrusion is conveyed from the extrusion machine to a station where it is cut to varying lengths depending on the finished length and placed on a metal pan in a free state; that is, it is not contained in a cavity as in molding. The part is then vulcanized in a heated pressure vessel known as an autoclave. Generally, the autoclave is heated by steam which is allowed to fill the autoclave, building up the required temperature, which then vulcanizes the rubber into its usable form. This is known as open steam vulcanizing or open-cure. The pressure surrounding the extrusion during open steam curing minimizes porosity in the extrusion.
Microwave or Salt Bath Curing / Vulcanization Process
The extrudate is fed into the vulcanization process directly from the extruder permitting the extrusion to be vulcanized in a continuous length. Several media are employed in the continuous vulcanization of rubber, all of which shall be operated at elevated temperatures: air, molten salts, oils, and microwave. Microwave is a method whereby the extrudate is subjected to high frequency electromagnetic waves which raises the temperature of the extrusion to near curing state, uniformly throughout. The lack of pressure in most continuous vulcanization processes makes porosity in the extrusion difficult to control. For most rubber compounds the open cure process is most practical. Many variables are encountered in the extrusion process which make it necessary to require tolerances more liberal than molded parts. A design engineer should have a general knowledge of the extrusion process and its variables to enable him to design parts that can be extruded at reasonable cost.
The extrusion die is a precise tool which is made by cutting an opening through a blank of steel; the opening is shaped to form the rubber into the desired cross-section as it is forced through the die by the pressure from the revolving screw of the extruder. Most rubber compounds swell and increase in dimension coming through the die orifice. The die, by necessity, is made for a particular extruder and a particular compound.