Ring Die Pellet Presses, what are the Benefits compared to Flat Dies?

After you start to learn the process of how to make wood pellets, you start to think about purchasing equipment. When it comes to purchasing a pellet press you have two main options. Either a flat die or ring die pellet mill. There are a few variations over these designs. For instance, while with a flat die pellet mill the die is always mounted horizontally in the machine, this is not always the case with ring die presses. With ring die machines the die can be orientated in either a vertical or horizontal design. Flat die pellet presses dominate the small scale market. While ring die pellet presses dominate commercial pellet plant operations. But why is this the case, what are the benefits of ring die pellet presses?

Ring Die Pellet Mill
The basic principle of the ring die pellet press design is material is fed into the centre of the die and the rollers compress the material so pellets emerge from the outer surface.

Ring Die Pellet Presses Don’t Suffer from Roller Slip

One of the reasons that ring dies are a superior design is that they do not suffer from roller skip. Due to the speed differences between the point of contact between the rollers and die, roller skip is a significant issue for flat die pellet machines. Ring die pellet mills do not suffer from this issue. The reason being there is no difference in speed between the die and rollers in any position.

Flat Die Pellet Mill Roller Slip
Flat die pellet mill roller skip. There is a slower rotation speed at the blue dots than the red dots. Hence the rollers have to ‘skip’ during operation leading the various issues.

The benefits of eliminating roller skip are reduced power consumption, extended consumable parts life and improved pellet quality. There is only one feature that flat die pellet mill manufacturers claim is superior to a ring die. That is the material flow of low-density materials such as straw and other grasses. While this can potentially be an issue for some ring die pellet mills, it’s generally not a significant issue.

Ring Die Pellet Press
Ring die and rollers.

Are there any downsides to Ring Die Pellet Mills?

From personal experience, I will say that ring die pellet mills are a bit more awkward to deal with when it comes to blocked pellet mill die. With any pellet mill, if the die is not properly prepared then the chances of blockage are increased. With some blockages, it’s possible to run an oily material mix through the machine to address the issue. First, you fix the rollers against the surface of the die (no roller gap) to and then try an process the oily material. If you are not getting anything coming out the other end then it will require you to take the die out of the machine for manual cleaning.

Blocked Pellet Mill Die
It is often easier to deal with blocked flat die pellet mill compared to a ring die pellet mill.

With small flat die pellet mills its often easier to get them out more quickly. Partly because there are fewer parts to take apart, and because the die will be small and light. When it comes to large ring die pellet mill the die will weigh several hundred kilos, even up to half a ton! Once the die is removed, to get the material out will require a punch and hammer or a drill. This process can take a very long time and if you’re not careful you can easily damage the die.

Ring Die Machines provide a more consistent Quality of Wood Pellets

As described above ring die pellet mills do not suffer from roller slip. Therefore when the wood is processed through a ring die pellet mill the material is equally exposed to the same amount of heat and pressure. This means under the right conditions a ring die pellet mill is able to process wood pellets of more consistent quality. The ‘right conditions’ means having the raw material at the best moisture content and the best particle size.

Wood Pellets
Ring die pellet mills generally produce a better quality of wood pellet.

When it comes to pellet binders some people are dismissive, implying that the use of pellet binders means the machine is not designed to process that particular material. Now while this may be the case in some circumstances and it is important to use a die designed for specific raw material, pellet binders should not automatically be dismissed. For instance, even Traeger who make some of the best quality BBQ wood pellets add in small amounts of soybean oil into their process. Granted the oil is not actually to specifically ‘bind’ the wood pellets but to lubricate the process, they still feel the need for an additive. But as you should not in their video, they use large scale ring die pellet mills in their BBQ wood pellet plants.

Conclusions on the Benefits of Ring Die Pellet Mills

When it comes to pellet production there are really very few reasons not to choose a ring die pellet mill. However, you still have to choose a manufacturer who produces their equipment to a high quality and can provide good spare parts support, that’s essential. If you want to learn more about the wood pellet production process please start with my page on how to make wood pellets.

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