Finishing of rubber compounds

The finishing of rubber compounds involves fulfilling specific customer requirements that cannot be met during the actual compounding process by means of a subsequent processing step. A number of different finishing processes are used.

Custom conversion

As part of the conversion process, the finished rubber compound can be made into a wide variety of shapes and geometries. For example, the customer can receive the materials in the form of sheets or blanks, strips and profiles, extruded round cords, calendered sheets or wigwags. They can be supplied as calendered films, free-flowing pellets, continuous strips or sheets, continuously wound or cut to length.


Calendering is a means of rolling flowable rubber compounds under pressure and at moderate temperatures. It is used to produce films, sheets, strips and rubber-coated fabrics. The following methods can be used:

  • Sheet calendering: A homogeneous rubber compound is extruded without a backing material.
  • Doubling: A prefabricated web is coated with another web by calendering to obtain a greater thickness.
  • Frictioning: A soft rubber compound is "rubbed into" a reinforcing material, e.g. a fabric.
  • Coating or skimming: A sheet is laid on one or both sides of a reinforcing material.
  • Sheeting: Using a soft pressure roll, a thin fabric is pressed onto the calendered rubber web.

Calendering is especially useful when the specifications for the thickness of a product are within a very narrow range. For example, we can produce rubber films that are over a metre wide but no more than 4 millimetres thick. The maximum width of our calendered products is 1200 millimetres, with a thickness of 0.5 millimetres.


Using extruders we can produce seamless round cords, profiles or webs of any length and with a uniform cross-section. In this process the rubber compound is pressed through a die following homogenisation. The shape of the product is determined by the shape of the die.


Cutting rubber to the correct size is a particular challenge and requires special cutting machines. While rubber compounds are viscoelastic, vulcanised parts present a different problem because of their elasticity. In both cases, the firmer the material, the better and easier it is to cut.

We will gladly cut your material to size for blanks, for example square or round cross-sections in (almost) any size.


For companies requiring materials in conveyable or easily dissolved form, we can supply free-flowing pellets. The starting material is reduced to the specified particle size using a shredder and a cutting or pelletising unit.

Pelletisers chop up the compounds until the pieces are small enough to fall through a screen into a collecting device. Oversize particles continue to be picked up and shredded by the rotor blade. This variant differs from the strip pelletiser in that there are no fixed dimensions for the pellets, only a maximum size.

In both cases we add a release agent to prevent the pellets from clumping.


Tougher product quality demands require clean compounds that are free from impurities. To filter out any remaining particles from the rubber compound and obtain precision pellets and blanks, the material is screened or strained after production. The art here lies in balancing compound quality and screen size, and RADO can draw on years of experience in this.

Gear extruders supersede hot feed extruders

Which system is used for purifying rubber compounds depends on local conditions and on the requirements for the individual production process. In recent years, there has been a general move towards gear extruders for straining rubber compounds. There are three main design principles for the feed mechanism: screws, internal feed rolls or external feed rolls. In all systems the mix can be fed cold or hot.

Although screw-type extruders are by some way the most elaborate of the three feed mechanisms, because of the space required and the energy they consume, they are by far the most widely used in the rubber industry because of their rugged and flexible delivery mechanism. Feed roll systems are primarily used in mixing lines, where they are much easier to integrate because of their manageable size. Gear extruders with internal feed rolls are even more compact than those with external rolls. Irrespective of that, the operating costs in both cases are much lower than those for screw-type extruders because of the lower energy consumption.

Operating principle of gear extruders

Gear extruders operate in the same way as gear pumps. In the feed section there are two upstream feed gears, which draw in the rubber compounds and then press them through fine screens. The plant throughput depends on the screen size and the maximum thermal load of the compound.

A hot-feed extruder can be integrated directly into a kneader line, between the roll mill and the batch-off unit. Alternatively, a cold-feed gear extruder can be used locally, upstream of the process extruder of a production line. This concept is particularly useful for highly accelerated mixes.

Rolf Müller
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