There are several different pellet mill die types with regards to hole depth, hole pitch, and hole width.
Hole Depth: The depth of the hole, illustrated by the letter L, plays an important role in the pellet process. The deeper the pellet mill die hole, the long time the material is exposed to heat and pressure. With more heat and pressure the lignin melts better and a harder denser pellet is formed. However a long hole depth means more friction is the material passes through the pellet mill die, so the pellet mill has to have sufficient power to push material through the deeper pellet press die. This either means the pellet mill or pellet press has a low gearing or has a larger motor.
Hole Pitch: The hole pitch, illustrated by the letter I, also plays an important role in the pellet process. The pitch angle of the hole influences pellet compression and some pellet mill dies have specific angles for certain materials. There is also pitch angle as the pellet leaves the die, this is also variable on feed stock.
Hole Width: Different pellet mill dies have different sized holes, ranging from 1mm holes to 10mm and more. For fuel pellets the most common sizes are either 6mm or 8mm, 6mm pellets are used in stoves or 6mm or 8mm pellets are used in boilers. With a larger diameter hole there is less friction and resistance during the pellet process. This can lead to increased productivity of pellet presses; however it can also have a negative effect on pellet mill production quality.
