Meccom Extruded Rubber Products
Rubber Extruding
Process
The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that this ratio can be very large while still producing quality parts.
Equipment
Movement of the extrusion with relation to the ram determines the machine configuration. If the die is held stationary and the ram moves towards it, then it is called “direct extrusion.” If the ram is held stationary and the die moves towards the ram, it is called “indirect extrusion.” When producing industrial rubber extrusions, equipment varies by three major characteristics:
- The position of the press, either vertical or horizontal.
- The type of drive, either hydraulic or mechanical.
- The type of load applied, either conventional (variable) or hydrostatic.
A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure, facilitates the flow. For specific sealing requirements, an EPDM rubber extrusion may be processed through specialized rollers inside a perforated drum.
Typical extrusion presses cost more than 100,000, whereas dies can cost up to 2000.
Direct extrusion
Direct extrusion, also known as forward extrusion, is the most common method for creating industrial rubber extrusions. It works by placing the billet in a heavy-walled container. The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated.
The major disadvantage of this process is that the force required is greater than that needed in the indirect process because of the frictional forces introduced by the billet traveling the container length. When forming a custom EPDM rubber extrusion, the greatest force is required at the beginning and slowly decreases as the billet is used. The end of the billet (the butt end) is not used because the material must flow radially to exit the die.
Indirect extrusion
In indirect extrusion, or backwards extrusion, the billet and container move together while the die is stationary. The die is held in place by a “stem” which must be longer than the container. Because the billet moves with the container, frictional forces are eliminated. This leads to several advantages:
- A 25 to 30% reduction of friction, which allows for extruding larger billets
- Less tendency for industrial rubber extrusions to crack as no heat is formed from friction
- The container liner lasts longer due to reduced wear
- The billet is used more uniformly, reducing defects and coarse-grained peripheral zones
The disadvantages involve surface impurities affecting the final piece. While this process creates long profiles, localized components like molded rubber suction cups require a different manufacturing approach entirely.
Hydrostatic extrusion
In this process, the billet is completely surrounded by a pressurized liquid, except where it contacts the die. This is highly effective when a dense EPDM rubber extrusion is required for high-pressure environments. The advantages include:
- No friction between the container and the billet reduces force requirements
- Increased ductility of the material under high pressure
- An even flow of material with no residue left on container walls
The disadvantages are that billets must be tapered to form a seal, and containing fluid under high pressure is difficult. While extrusions are ideal for lengths, small specialized parts like molded rubber grommets are better suited for compression or injection molding.
Drives
Most modern presses are hydraulically driven, utilizing either direct-drive oil or accumulator water drives. Direct-drive oil presses are reliable and can deliver over 35 MPa (5000 psi). Accumulator water drives are faster and used for materials requiring high temperatures. Understanding the advantages of using molded rubber parts is essential when deciding between extrusion drives and molding cycles for a specific project.
Die design
The design of an extrusion profile impacts its manufacturability. The maximum size for any of our industrial rubber extrusions is determined by the circumscribing circle. This diameter controls the size of the die required, which determines if the part fits in a given press.
The complexity of a custom EPDM rubber extrusion is quantified by the shape factor, which is the surface area generated per unit mass. This factor ultimately dictates both the cost of tooling and the speed of the production line.
