Stamp kneader

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The internal mixer punch kneader is a machine that is used in rubber processing. It is used to produce the rubber mixture from several individual components (natural, synthetic rubber, plasticizers, fillers, crosslinkers, accelerators, ...) in a batchwise mixing process. In the large-scale rubber industry, punch kneaders have largely replaced the rollers that were previously used as mixing units.

Construction principle

Construction principle of a stamp kneader

The stamp kneader consists of a closed mixing chamber (2) in which 2 counter-rotating rotors (4) knead the mix in the free mixing chamber volume (3). The punch kneader is usually filled via the shaft opening of the punch (1), which is raised for this purpose. When the filling is complete, the ram can be lowered and the chamber closed. The mixing chamber, rotors and the emptying saddle (6) are traversed by cooling channels (5) with which the punch kneader is heated to a process temperature that is optimized for processing the material to be mixed. Towards the end of the mixing process, the emptying flap (6) is opened and the mix is ​​emptied while the rotors are running. (Numbers in brackets refer to the graphic opposite)

Stamp kneaders are common in sizes from 100 ml to approx. 1000 l chamber volume. Variants without an emptying flap are also known, in which the mixing chamber can either be tilted or dismantled for emptying.

Working principle

Stamp kneader (laboratory system) with open mixing chamber

The mixing process takes place discontinuously. To do this, the components are placed in the kneader with the ram open. Then the ram moves vertically with high pressure on the mixture and closes the mixing chamber. Inside the kneader there are two counter-rotating rotors through which the punch presses the rubber mixture. The heat dissipation creates the temperatures necessary to produce the mixture. The mixing chamber and the rotors must be permanently cooled so that the temperature does not get too high. Cooling is very important so that the rubber mixture (with crosslinkers and additives) does not vulcanize (knot formation). At the end of the mixing process, the emptying saddle is opened and the compacted and homogenized mixture is discharged.

A completed mixing process is also referred to as a mixing stage. This process is repeated until the mixture has reached the desired degree of mixing.

Advantage of the punch kneader compared to mixing rollers

Compared to the open mixing rollers that were widely used in the rubber industry towards the beginning of the 20th century, the punch mixer has advantages in terms of efficiency, productivity, process control, occupational safety and occupational hygiene.

While fillers, oils and waxes used to be mixed into the rubber on open rollers, the use of a closed mixing chamber meant a significant reduction in the emission of dusts and volatile components. When working on mixers instead of rollers, the risk of accidents caused by the rotating hot rollers was eliminated.

While the production of a 30 kg mix piece on a roller usually took between 20 and 40 minutes (depending on the complexity of the mix recipe), today with punch kneaders mix weights of up to 400 kg can be produced in 2–4 mixing steps of 5 minutes each. This increase in productivity goes hand in hand with a reduction in staff compared to the mixing roller operation, which, depending on the degree of automation of the mixer line, can be 1 to 4 people per line. If you leave out the provision of raw materials and the removal of the products, even limited, fully automatic operation of a line can be realized.

Mixing rollers are usually operated by a roller operator with considerable manual and physical effort. At the same time, the roller operator is also responsible for the manual process control of the mixing process. In contrast, stamp kneaders can be operated automatically using an electronic process control. This makes the production of the rubber mixtures more reproducible, which is reflected in a more uniform and higher quality of the compounds produced and in a more precise process control with regard to the desired parameters (e.g. the mixture temperature). This enables manufacturing processes in punch kneaders, for example, that cannot easily be implemented on rollers, such as reactive mixing.

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