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Comparison of Ring Die Pellet Machine and Flat Die Pellet Machine

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Working principle of flat die pellet mill

The flat die pellet mill was the first pellet mill design at the turn of the 20th century based on flat dies. Generally, there are two types of flat die pellet mills in the market, the rotating die type and the rotating roller type. The former one has a stationary roller with the rotating die while the latter has a stationary die with a rotating roller. Adopting vertical principle, the raw materials drops down by the its own weight into the pelletizing chamber where they are compressed between the rollers and die to form pellets by going through the die holes.

flat die pellet mill
flat die pellet mill

Advantage and disadvantages of flat die pellet mills are as following:

Flat die pellet mills are small in size and light in weight because of the compacted structure design.

Flat die pellet mils are easier to be cleaned and maintained and cheaper because of the design of portable rollers and die.

Flat die pellet mils are more convenient for you to monitor the process of pelletizing by watching the pelletizing chamber which make the experiment of indeterminate materials for raw materials expansion easier.

The disadvantages of pellet mills are they are not suitable for large scale pellet production. And also the gap between the roller and the die is tricky which should be adjustable according to the species of the raw materials especially the hardness of the raw materials.

All in all, the flat die pellet mills are usually used to process materials with large adhesive force for both fuel pellets and feed pellets in small scale and medium scale pellet production. They are known for the features, such as, reliable operation, greater mobility, low noise, low energy consumption and relatively low productivity, comparing with ring die pellet mills.


Working principle of ring die pellet mill

The ring die pellet mills were designed at around the 1930’s based on the design of ring die. The basic pelleting principle of the ring die pellet mill is a simple operation where feed mass is distributed over the inner surface of a rotating, perforated die ahead of each roll, which compress the feed mass and compress it into the die holes to form pellets.

ring die pellet mill
ring die pellet mill

Advantage and disadvantages of ring die pellet mills are as following:

Ring die pellet mills generate less wear and tear because the inner and outer edges of the rollers traverse the same distance.

Ring die pellet mills are more energy efficient than that of the flat die pellet mills.

One problem of the ring die pellet mills is side slip. Side slip is one undisputable and unavoidable fact on ring die pellet mill. It refers to having feed mash squeezed out sidewise from the nip between the roller and dies, which can be minimized, depending on the die configuration. One way to do this is to counter bore or drill larger hole in the two outer rows of the die so as to reduce the friction caused in the holes.

In one word, the ring die pellet mills also can be used to produce fuel and feed pellets. However, compared with the plat die pellet mills, the ring die pellet mills are well known for their large capacity for pellet production. They are featured as large capacity, with nice pellet forming shape and high density and high energy consumption.


Model MZLH320 MZLH350 MZLH420 MZLH520 MZLH768 MZLH858
Anti-caking Feeder Power 1.5KW 1.5KW 1.5KW 2.2KW 3KW 3KW
Forced Feeder Power 0.55KW 0.55KW 0.55KW 0.75KW 1KW 1KW
Capacity 300-400kg/h 500-700kg/h 1.0-1.2 t/h 2.0-2.5 t/h 2.5-4 t/h 3-4.5 t/h
Pellet Machine Power 37kw 55kw 90kw 110/132kw 250/280kw 280kw
Final Pellet 6-12mm 6-12mm 6-12mm 6-12mm 6-12mm 6-12mm


The complete wood pellet production process can be separate as the following sections:

Raw materials collection: there are many different biomass raw materials can be used to produce wood pellets, such as crop stalks, rice husks, peanut shell, wood branches, sawdust, wood chips, etc. Before starting to produce wood pellet, there must be raw materials quantity for at least one week production in stock. And the warehouse for biomass raw materials must be rain proof and fire proof.

Raw materials crushing: during ring die pellet mill working process, the size of biomass raw materials must remain the diameter of 3-5mm. Except the sawdust, other biomass materials must be crushed and grinded before it can be fed into ring die pellet mills. Typically, wood crusher is used for the initial wood material crushing, and hammer mill is used for wood material final grinding.

Raw materials drying: ring die pellet mill has strict raw material moisture content limit, the moisture content of biomass material must be less than 18%, for the best quality wood pellet, the moisture content must be lower than 10%. If the moisture content is too high, the wood pellet may failed to form into uniformed solid fuels; If the moisture content is too low, the wood pellet may be cracked. So an industrial rotary dryer is one of the main equipment in wood pellet production line.

Raw materials conveying: manual feed method can not satisfy industrial wood pellet production. In order to save labor, pellet production lines always go with an screw conveyor that can convey the biomass material to ring die pellet mill automatically.

Pelletizing: ring die pellet mill is the core machine of the complete pellet line. When biomass materials have been fed into pellet mill chambers, pellet mill ring die and rollers extrude the materials to pass through the holes on pellet mill ring die, and outside the pellet mill ring die, there is the knife cut the discharged round biomass battens into small pellet.

Pellet cooling: the temperature of the just-made wood pellet can reach 80 or 90 degree, it is too hot to pack, and if we let the wood pellet cooling itself, it may regain moisture. So after pelletizing process, wood pellet will be conveyed into pellet coolers directly.

Pellet package: after cooling process, wood pellet will be conveyed to pellet package machine. There are normally two types of package quantity, the bulk package, and retail package. Bulk package will pack wood pellet into bags with 1ton quantity, and retail package will pack wood pellet into bags with 15kg quantity.


The proper maintenance of a pellet making machine will definitely enhance its serve life. Both the pellet machine die and rollers are wear parts, if there is no proper maintenance or use wrong maintenance procedures, the wear parts can be quickly wore, and only the spare parts fee can cost you a fortune. In the following parts, we will talk about some standard pellet machine maintenance procedures.

How to Maintain a Ring Die Pellet Making Machine
How to Maintain a Ring Die Pellet Making Machine

Before you start the wood pellet machine, you should check all the screws, see if there is anyone loosening up.

Before you start the pellet machine, check the distance between pellet machine rollers and die, normally the clearance is about 0.1mm – 0.3mm, you can adjust the clearance through the housing pin of the rollers.

Before using the pellet machine for the first time, you should use oil materials to grind the pellet machine rollers and die for at least one hour. Moreover, when you stop pelletizing process each day, you can use the oiled material to run the pellet machine for a while, therefore, the residual wood materials will be press out of the pellet machine die holes.

When the pellet machine runs for about six months, the pellet making machine should have an overall maintenance and add lubricant to the pellet machine regularly.

Despite of the above maintenance method, there are more complicated maintenance methods for large pellet making machines. For wood pellet plants, the maintenance of pellet machine is more complicate, as the productivity are different and pellet plant has more complex pellet equipment combinations. In that way, find a professional pellet plant supplier is very important. They will help you set your complete plant layout, pellet equipment purchase, complete pellet line construction, etc.

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