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Flat die or ring die?

There are two types of flat die pellet mills in the market, the rotating die type and the rotating roller type. The former one has a stationary roller with the rotating die, while the latter has a stationary die with a rotating roller.
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The flat die pellet mill was the first pellet mill design at the turn of the 20th century. Generally, there are two types of flat die pellet mills in the market, the rotating die type and the rotating roller type. The former one has a stationary roller with the rotating die, while the latter has a stationary die with a rotating roller.
Adopting vertical principle, the raw materials drop down by its own weight into the pelletizing chamber where they are compressed between the rollers and die to form pellets by going through the die holes.

 

flat-die

 

 Advantages of flat die pellet mills

 

  • They’re small in size and light in weight because of the compacted structure design.
  • They’re easier to be cleaned and maintained and cheaper because of the design of portable rollers and die.
  • They’re more convenient to monitor the process of pelletizing by watching the pelletizing chamber, which makes the experiment of indeterminate materials for raw materials expansion easier.
  • They are known for reliable operation, greater mobility, low noise, low energy consumption.
  • They are far cheaper than that of the ring die pellet mills.
  • The pressure of flat die pellet mills is adjustable.
  • The diameter of rollers in the flat pellet mill is not restricted by the diameter of the pellet die so that the roller pressure can be increased by enlarging the inside bearing space, adopting a big bearing which also expands the working life.

 

Disadvantages of flat die pellet mills

 

  • They’re not suitable for large-scale pellet production.
  • The gap between the roller and the die is tricky which should be adjustable according to the species of the raw materials, especially the hardness of the raw materials.
  • They provide relatively low productivity, comparing with ring die pellet mills.

 

The ring die pellet mills were designed at around the 1930s based on the design of ring die. The basic pelleting principle is a simple operation where feed mass is distributed over the inner surface of a rotating, perforated die ahead of each roll, which compresses the feed mass into the die holes to form pellets.

 

ring-die

 

 

Two or three rolls’ models are available on the market, although the two-rolls solution is more effective because

  • the rolls are much larger
  • the nip angle is smaller
  • the machine run with a much thicker pad of material
  • the more open area to allow the feed material to enter the die cavity pellet mill with two large rolls produce up to 15% more capacity
     
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Advantages of ring die pellet mills
 

  • They generate less wear and tear because the inner and outer edges of the rollers traverse the same distance
  • They guarantee a large capacity for pellet production.
  • They provide nice pellet forming shape and high density.


Disadvantages of flat die pellet mills
 

  • High energy consumption.
  • Side slip: it refers to having feed mash squeezed out sidewise from the nip between the roller and dies, which can be minimized, depending on the die configuration. One way to do this is to counterbore or drill a larger hole in the two outer rows of the die so as to reduce the friction caused in the holes.
  • Higher price, compared to that of flat die pellet mills.
  • The pressure of the pinch roller is limited because the diameter of the rollers is limited by the diameter of the pellet die.

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