The first is the ring die type pellet machine. As the name implies, it uses a ring die to form the pellets. The ring die itself is a thick metal cylinder with holes drilled through the cylinder around the circumference. Inside the ring are the rollers that are used to push the material through the die. Raw materials in the form of wood waste or biomass are fed into the center of the ring, and the rollers force them through the holes of the ring die. The formed pellets then emerge from the outside of the die.  This type of pelleting machine is what you d see in a commercial pellet plant that produces a high volume of pellets. The ring die type mill is a high cost, heavy duty machine that s largely out of the price range of most people that make their own wood or biomass pellets.
Next is the flat plate die type pellet mill. This machine utilizes a flat plate to form the pellets. If you can imagine a meat grinder, the die used in it is very similar to that of the flat plate style pellet mill. The die is made from a thick round flat plate with many holes drilled through it. A set of rollers ride on top of the plate, and when the raw feedstock is fed into the machine, the die turns along with the rollers.
To get a better idea of how this arrangement works, imagine a wheel on the edge of record player. This combined die and roller action forces the raw material through the die causing formed pellets to exit on the underside of the die, where they fall into a chute and exit the machine. The flat plate type pellet mill is typically much less expensive than its ring die counterpart, and are mostly used by do it yourself pellet producers.
They re also relatively simple to work on, and can be powered by a number of different methods. This in turn lends to the appeal of the flat plate mill to the small scale producer. By utilizing a power source that you may already own, you can save a lot of money by buying only the mill portion.