How can the die characteristics be determined?

This is where experience becomes an important factor. The selection of a die specification is often the result of a compromise between the different needs of the many formulas produced in a production plant and from the specific properties of the actual plant. The most common mistake is to assume that the die is the cause of all problems. Pellet quality can depend on many factors that have nothing to do with the die, the most common being poor steam-conditioning of the formulas. To overcome these problems, dies with a high compression index (i.e. large thickness) are used.
This is an analysis of the advantages and disadvantages.

THICK DIES

Advantages:

  • pellet quality is not dependent on operator experience
  • increased pellet durability.

Disadvantages:

  • lower capacity / productivity
  • frequent clogging
  • higher production costs
  • increased wear on dies and roller shells
  • lower conditioning temperature
  • lower moisture content of feed
  • higher temperature caused by friction
  • die track liable to damage.

THIN DIES

Advantages:

  • higher capacity
  • lower clogging risks
  • option of adding more moisture
  • option of conditioning the mash at a higher temperature
  • better suited to a wider range of formulations
  • cost savings in the reduction of die and roller shell wear
  • reduced stress on machinery
  • cost savings because of lower power consumption.

Disadvantages:

  • increased steam requirement
  • possible cooling problems due to higher conditioning temperatures
  • need for greater operator input and experience to ensure pellet quality
  • pellets have a mat appearance
  • lower pellet density /durability.

ROLLOVER

What is “rollover”?

Rollover is the condition of the die face when the hole inlets start to peen closed.

If the rolls come into contact with the working surface of the die and damage it, laminating its surface and deforming the entrance of the holes. This peening action has disastrous effects on both pellet quality and the die throughput.

When does “rollover” arise?

  1. when the rolls are too close to the die
  2. when the wear ring (or clamping ring) has excessive play
  3. when the main bearings of the die holder assembly have excessive play.

What problems can “rollover” cause?

  1. inadequate production with the motor at full-load
  2. excess mill vibration
  3. early breakdowns.