When is extrusion molding beneficial in the production of rubber components?

Extrusion is a long-established process in the rubber industry. It has significant advantages over other methods such as injection molding. Extrusion molding is a very efficient method of large volume production, where the total length of the products can be expressed as a multiple of 500 meters. However, it has limitations too, such as the fact that the rubber does not vulcanize during extrusion, so an additional step is needed in the production process for curing.

It is still regularly used in rubber manufacturing today, for products including tyres, seals, hoses, O-rings and gaskets. In this article, we present:

• the typical extrusion processes,
• how does extrusion molding differ from injection molding,
• the limitations of extrusion,
• for which products extrusion molding is recommended, and
• the extrusion services SIC offers.

Examples of extrusion molding processes

In the rubber industry we can distinguish between simple or complex extrusion processes, such as:

• Shear head: In this case, the extruder head is cylindrical, heated, and has a heated core rotating inside at an adjustable speed. The mixture is sheared as it passes between the cylinder and the rotating core. This heats the material, so the vulcanization starts in the shear head and is completed in a continuous vulcanization chamber after leaving the extruder.
• Roller head: extruder variant suitable for the production of sheet products, as the head ends in a two-roller calander.
• Roller die: A solution in which the sheet product is formed by a roller and a drawbar positioned parallel to it. It is used to produce small widths of sheet or strip.
• Crosshead: This extruder can be used to coat some other material with raw rubber. In the extruder head, the die is placed on the screw at an angle of 45, 60 or 90° and the material is pressed onto the thread drawn through it. The thickness of the layer is determined by the size of the die.

During extrusion, the rubber is not vulcanized, so this must be done separately. Several different processes are used for this, such as microwave, salt bath, heat tunnel vulcanization and autoclave. It is recommended to choose the most appropriate curing method for the specific material or product.

What you need to know about rubber extrusion molding

How does extrusion molding compare to injection molding?

The key difference is that extrusion molding only shapes, while equipment used nowadays for injection molding vulcanize as well. In addition, the injection method allows more complex spatial shapes to be created. In return, the tooling cost of extrusion molding is typically an order of magnitude lower than that of injection molding.

Extrusion is a good solution when the most typical measure of the product is length and the total volume can be expressed in hundreds of meters or kilometers. Indeed, the main advantage of extrusion is that it is a cost and time efficient process for products that can be expressed in such lengths.

Limitations of extrusion molding

Extrusion is only applicable in a limited hardness range, Shore A 60 – 90. With softer materials, the product leaving the machine may swell and deform, becoming unusable.

Another limitation is that the technology cannot be used to produce small quantities. This is limited by the mixing of components and the pressure required to extrude, as insufficient quantities of raw materials hinder or make impossible the homogenization process and system pressure build-up.

As well as that, color choice is also limited. In practice, almost all extruder machines are used for only one color. The reason for this is that there is always raw material residue in the extruder. This can only be removed after the disassembly of the machine.

For which products do we recommend extrusion molding?

Extrusion is a basic step in tyre manufacturing. The technology is also used to produce products such as seals, glass pane edge protectors and trims, electrical wiring, gaskets, molded rubber tubes, hoses and elbows. In the case of hoses and tubes, it is possible to extrude the outer and inner layers at the same time with the same equipment, that also places a reinforcing layer between them. This results in a complex, very high quality product, but of course requires special equipment, which means a more costly process than simple extrusion.

Extruded edge protectors

Extrusion, a service of SIC

In our factory in Pásztó, we usually extrude from one material, in one color. On the other hand we undertake extrusion projects from as little as 500 meters of product. Beyond that, for all ends to which this is the best means, we offer the method as part of the engineering and product design.