The questions of what is the ideal pellet mill roller and die gap may seem strange to some people. There are several websites and forums that state that with a pellet press you need to ‘clamp’ the rollers down against the die to ‘build up sufficient pressure to push the material through the die’. However, when you start to really learn about how to make wood pellets you will know that’s a false statement. I need to make it clear straight away. Clamping the rollers down against the die is terrible advice. It may seem necessary on some cheap Chinese pellet makers to force the material through the die. However, you should never ‘clamp’ the rollers to the die on any pellet mill.
By clamping the rollers against the die you will eventually damage the roller bearings. You will also dramatically accelerate die and roller wear and reduce pellet quality. If you go and visit a professional large scale wood pellet manufacturing plant they will tell you that the die and rollers should never touch, there should be no metal to metal contact.
So what is the Ideal Gap between the Pellet Mill Die and Rollers?
While the pellet mill die and rollers should never touch, that is not to say that the size of the gap is irrelevant. You want to maintain a gap of 1mm to achieve the best balance of power consumption and pellet quality. This will form a ‘carpet’ of material against the surface of the die. The carpet lay is the first stage of compression and conditioning before the material gets compressed through the die. If you set a gap above 1mm you will have issues with power consumption and potentially stall the motor. The greater the distance between the die and rollers the more resistance. Therefore a gap above 1mm is only recommended for certain animal feeds and other low-density materials. Also, you will only get a carpet to form if the moisture content of the material is correct.
The condition of many small pellet mill dies is also very poor. The dies are often not properly prepared and polished. Therefore to avoid a pellet mill die blockage users clamp the rollers against the die to force the material through.
Creating a Roller and Die Gap on Cheap Pellet Mills
The problem is, particularly on cheap pellet mill imports there is usually no method to set the distance of the die from the rollers. Therefore this usually means that you have to have metal to metal contact. Hence one of the reasons why it is very difficult to produce quality pellets on many small single phase pellet presses.
Top Tip: It is possible to set the roller gap on cheap Chinese pellet mills using the ‘double nut’ method. You place a piece of paper or cloth between the rollers and die. Then using two nuts tightened against each other on the bolt that fixes the rollers you can set the required 1mm roller and die gap.
Conclusions on the Pellet Mill Roller and Die Gap
While you can produce wood pellets by fixing the pellet mill rollers down against the die, sooner or later you will regret it. As the temperature builds the grease within the rollers will start of escape, especially if its not proper high temp grease. Then the rollers will cease up, then you will need a new set of bearings. Furthermore, for running the pellet mill with no roller and die gap you will get a much shorts parts life. The rollers and die will wear out very quickly from the metal to metal contact. There you operating cost per tonne of wood pellets will be much more than it should be.
However, to run a pellet mill with the 1mm roller and die gap you do need the material and pellet mill to be properly conditioned. If you properly prepare the pellet mill and the raw material you will get better quality wood pellets and lower production costs due to the pellet mill parts lasting much longer.