The most effective machinery for making pellets is the pellet mill. Originally used to process animal feed, patents were registered for pellet mills in the United States as early as the 1950s. In these and in modern machines, a perforated plate of metal— the die—is fashioned either annularly (as a ring) or as a flat disc. Rollers are mounted adjacent to the die. Feedstock is placed between the die and the rollers, and through the rotation of either of the two components, material is carried by the textured surface of the rollers and forces material through the holes of the die to be condensed into pellets. Cutting blades are mounted near the outer surface of the die to cut the extruded material to a proper length. This process typically carries with it a host of auxiliary machinery and processes to generate the highest quality pellets.
There are two basic types of pellet mills. One is the flat die pellet mill and the other is a ring die pellet mill. The flat die pellet mill preceded the ring die mill and is more commonly used for small- to mid-scale pelletization. In contrast, ring die pellet mills are more commonly used in industrial scale pellet production. Variations also exist as to the arrangement of the die—vertical versus horizontal—and to the number of rollers.
Flat die pellet mills
The flat die pellet mill uses the force of gravity to pull the material downward to the rollers, which then create additional force as they rotate to push the materials through the holes of the die.
The material is then compressed between the roller and die surface through the die holes. Once the pellets emerge from the die a knife cuts the pellets off at a set length. A worm and wheel drive is used by some flat die pellet mills while others are driven via bevel gears.
In some flat die pellet mills the die is stationary and the rollers are driven. In other flat die pellet mills the die is driven and the rollers rotate as material passes between the roller and the die.
Advantages of flat die pellet mills: Flat die pellet mills are generally easier to clean than round die pellet mills. Quick access to the pellet mill chamber allows faster die and roller changes for more time in production. The compact design of the flat die pellet mill means that small, lightweight models are available, which are more suitable for small-scale production. Another key advantage of the flat die pellet mill is visibility. If a material is producing poor quality pellets or no pellets at all, viewing the material during the pellet process can give the best information as to problem and how to correct it. With many flat die pellet mills it is possible to see into the pellet mill chamber during the pelletization process, others have quick access doors to see into the chamber. Finally flat die pellet mills are regarded as more robust for pelleting problematic feedstock, such as agricultural materials. Therefore flat die pellet mills have a wider material tolerance than round die pellet mills.
Disadvantages of flat die pellet mills: With each rotation, the interior of the flat die travels a shorter distance than the exterior. This results in uneven wear of the roller and die, particularly at the exterior edge of the die. Some flat die pellet mills utilize tapered rollers to correct this problem.
The ring die pellet mill is comprised of a vertical ring die with rollers on the inside. Material is fed from a surge bin through a variable speed conditioner above the pellet mill; the conditioned material is then fed in through the machine’s door. A screw auger further feeds material into the center of the pelletization chamber. Inside the chamber the rollers are stationary while the die rotates. Once in the chamber, the material is spun by the die’s rotation for compression through the holes by the rollers.
Advantages of ring die pellet mills: First, ring die pellet mills do not suffer uneven roller and die wear as the inner and outer edge of the rollers cover the same distance. For this reason, ring die pellet mills are preferred for large scale production as the long-term costs from roller and die replacement are perceived to be lower.
Ring die pellet mills are also preferred for large-scale production for energy efficiency reasons. Roller slip in flat die pellet mills causes extra friction during the pelleting process, thereby using more energy during production. Yet this friction is not entirely disadvantageous, as friction results in more heat, which is needed to produce quality pellets. All ring die pellet mills come complete with a conditioner and variable speed feeding.
Disadvantages of ring die pellet mills: The first obvious disadvantage of the ring die pellet mill is its size and weight. This is usually not a problem for large-scale production, yet is often problematic for small-scale production. Also, changing rollers and dies in a ring die pellet mill is a far more labor intensive process. The dies in particular are large and heavy, in most cases requiring lifting equipment for removal or replacement. Also roller adjustment is more difficult, as a majority of ring die pellet mills require manual roller adjustment which can only be accessed by opening the pellet mill chamber. However this is not always the case, as some now come with optional remote roller adjustment at extra cost. Overall, ring die pellet mills are more costly than flat die mills, as the machines themselves and the dies and rollers are generally more expensive than flat die pellet mills. Finally, the user can’t view the pelleting process, as the die and roller are encased behind a solid door, thereby only allowing indirect methods to quickly obtain information for troubleshooting.