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Pellet mill engineer's tips on pellet mill dies

By Jack Huang    Feb 28, 2014
How to choose the quality pellet mills? The pellet die is the one of the key elements that needs to be thought of. The characteristics of the dies defines the quality of the pellet mills.

The die is the heart of the pellet mills in the pellet forming operation. Although both flat dies and ring dies may be used to produce biomass pellets, for large plants the ring die is generally more advantageous. This is due to two reasons. First, is that the roller and die wear evenly since both edges move the same distance during the production.

This causes the wear to occur more evenly, thus saving on the need for replacement parts. Second is that ring dies are more energy efficient for large scale production. Roller slip occurs more often in flat die mills, a process that increases friction between the die and roller, consuming more energy in the process.
 

dies for pellet mills

Dies for pellet mills
 

Many characteristics of the die can be varied to get the desired results on a particular formulation. Often one must review die characteristics with the pellet mill supplier to find a solution to a specific problem.

1. Diameter and effective thickness: The key characteristics of the die include the diameter of its holes, its effective thickness—which ultimately determines the length of the pellet—and the ratio of these two variables. Each raw material has its own optimal length to diameter ratio. Denser materials, like wood, generally require smaller length to diameter ratios than less dense materials such as straw. One cannot indiscriminately change formulation without changing pellet quality. Besides providing a means of discussing any particular feedstock and its relationship to die requirements, this concept gives the ability to scale up or down in pellet size and be sure of having essentially the same quality and production criteria.

2. Total thickness: Another die characteristic is its total thickness. In many instances the overall thickness of the die must be greater than the effective length because of stresses within the die from pelletization. The overall thickness of the die is required to withstand the structural stresses of the operation. The thicker the die, the stronger it is. Normal die thicknesses range from under 1 centimeter to nearly 13 centimeters.

3. Counterbore depth: This is the difference between the total thickness and effective length of the die. A die is counterbored by taking a larger drill and drilling in from the outside of the die, relieving the pressure of the die on the material. Counterbores can be supplied either with a tapered bottom or with a square bottom. In some special feed milling and industrial applications, there is excessive expansion of the material as the pellet leaves the hole. A tapered counterbore is effective in minimizing a material’s tendency to hang up in the counterbore and eventually form a pellet equal to the counterbore diameter. Certain materials may also require a tapered counterbore to gradually relieve the pressure of the material as it exits the hole. This can improve pellet quality for certain materials.

4. Inlet diameter: The majority of the dies produced have a tapered inlet to ease the flow of material into the hole. This taper also begins to compress the material as it enters the hole, thereby doing work on the material.

5. Inlet angle: This is normally a degrees on small-hole dies and just eases the feed into the hole. The die will eventually wear to its own angle after it has been in production, so the taper is normally supplied at just the start of the flow until the die begins to wear. In certain instances, where operator control is difficult, dies can be counterbored differently to minimize the potential for peening. Furthermore, the hole pattern of a die can be varied to improve productivity or increase abrasion resistant quantities. It also can be modified to add strength. The alloy of the die can be varied to produce maximum life. A variety of stainless steel dies are used in pelleting formulations carrying corrosive ingredients. Heat treating the die brings out specific properties and varies according to specific application, depending on whether abrasion resistance or toughness would be a major criterion.

These are tips from professional pellet mill engineer. With these knowledge in your head, it would be easy for you to find the quality and cost-effective pellet mill.