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In-depth analysis of ring die and flat die of pellet machine

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PELLET MILL DIE HOLES

Pellet mill die hole is the place where raw materials are formed into uniformed wood pellets. There are many different types of pellet mill die holes, such as the straight die holes, release stepped die holes, external conical die holes, etc.

Pellet Mill Die Hole Compression Ratios

Different wood pellet mill die holes have different compression ratios, in another word, different wood pellet die holes are suitable for different types of raw materials. Take different types of wood materials as an example, the compression ratio of poplar wood is 1:6, the compression ratio of pine wood is 1:7, the compression ration of hardwood is 1:5, the compression ratio of cornstalk is 1:8, etc. From these numbers we can easily find out that different types of raw materials has different compression ratios, and the harder the raw materials are, the smaller the compression ratio will be.

Therefore, when you are choosing complete pellet mills or pellet mill dies, the compression ratio is an important factor which partially determined the wood pellet quality you produced.

Pellet Mill Die Materials
Pellet Mill Die Materials

Pellet Mill Die Hole Diameters

The diameters of pellet mill die hole is related to the diameter of pellet mill die and the raw material types. Take Richi pellet mill as an example, Richi manufactures pellet mills with die hole diameters from 2.5mm to 12mm. But not all these sizes pellet mills are suitable for wood pellet production.

If you have read EU wood pellet standard, you may find out the top quality wood pellet normally with the diameter of 6mm, or 8mm. If the diameter of wood pellets is over 12mm, the form of wood pellet requires larger pressure, which means the abrasion of pellet mill die and rollers increases. Therefore, for both ring die pellet mill and flat die pellet mill, 6mm and 8mm pellet mill die holes are used to produce wood pellets.

For feed pellets production, it is another story. Different types of feed pellets also needs different diameters pellet mill die holes. For examples, the feed pellets used for fish and chicken are normally 2.5mm-4mm, which means we should use the pellet mill with 2.5mm-4mm die holes, to make feed pellets for pig, sheep, rabbit, we should choose the pellet mills with 4mm-6mm die holes, etc.

WHAT DETERMINES THE PERFORMANCE AND SERVE LIFE OF PELLET MILL DIE?

Abrasion resistance: the majority pellet mill die damages are caused by abrasion. During the wood pellet manufacturing process, there is constantly friction between the pellet mill die, rollers and raw materials. The friction causes the abrasion of the pellet mill die surface and enlarge the die holes. And the abrasion resistance of pellet mill die will change along with its surface hardness, microstructure and the chemical components.

Corrosion resistance: there might be pitting under high temperature or high pressure environment, which will cause the corrosion problem of pellet mill dies. Normally, pellet mill die with high carbon and chromium content has better corrosion resistance performance.

Toughness: during wood pellet manufacturing process, pellet mill die suffers strong impact, which may cause the damage of pellet mill dies. And the long time non-stop running will also cause the fatigue damage of pellet mill dies.

Pellet Mill Die Materials
Pellet Mill Die Materials

THE RELATIONSHIP BETWEEN THE FACTORS

There is no such a perfect pellet mill die which has the perfect abrasion resistance & corrosion resistance performances, and at the same time, the strongest toughness. These three factors can be sometimes conflict.

Abrasion Resistance & Toughness

If you enhance the hardness of pellet mill die, then the abrasion resistance of pellet mill die will increase, too. But it will also reduce the toughness of pellet mill die, and increase the pellet mill die fragility. Therefore, there is no such a pellet mill die has the perfect abrasion resistance performance and toughness, find the right balance between the two factors is the key point.

Abrasion Resistance & Corrosion Resistance & Toughness

These three factors are not totally conflict. If the pellet mill die metal structure and corrosion resistance performance are poor, it will surely reduce the abrasion resistance of pellet mill die, and the impact resistance performance of pellet mill die will be reduced, too, which makes it easy to crack.

Toughness & Porosity

It is easy to understand the relationship between pellet mill die toughness and porosity. If there are more die holes on the pellet mill die, of course the pellet mill die are easier to crack. So high porosity makes poor toughness, if we want to enhance the toughness of pellet mill die, then we must reduce the numbers of die holes on pellet mill dies.

PELLET MILL DIE MATERIALS

There are many different types of steel materials can be used to manufacture pellet mill dies, such as alloy steel, stainless steel, carburized stainless steel. Different steel has different features, you can choose the suitable die materials for your pellet mills.

pellet mill die materials
pellet mill die materials

Alloy Steel Pellet Mill Dies

Alloy steel pellet mill die manufacturing adopts carburizing technology, which enhances the hardness of the surface of pellet mill dies. The core part of alloy steel has low carbon content, which is softer but with a stronger capability of resisting impact, so the pellet mill die is not easy to crack. But there are also some disadvantages of alloy steel pellet mill die. The anti-corrosion capability of alloy steel pellet mill die is average. After the hardened layer of pellet mill die surface has been worn, the inner material will wear faster than the hardened surface. So when you choose the suitable alloy steel materials for pellet mill die, it’s good for you to pay attention to the nickel content, high nickel content can increase the toughness of pellet mill die.

Stainless Steel Pellet Mill Dies

The stainless steel materials to make pellet mill must go through quench hardening process or vacuum hardening process. These types of stainless steel materials has carbon content of 0.4% – 0.5%, and chromium content of 12% – 14%, which makes the pellet mill die has strong corrosion resistant ability. But the abrasion performance and toughness of stainless steel pellet mill die is weaker than alloy pellet mill dies. But the hardness of both the surface and core part of stainless steel pellet mill die are equal.

Carburizing Stainless Steel Pellet Mill Dies

Carburizing stainless steel pellet mill die also has the hardened surface just like the alloy steel pellet mill die, which makes carburizing stainless steel pellet mill dies have strong abrasion performance. The hardness of carburizing stainless steel pellet mill dies is decreasing progressively from the surface to the core part. The corrosion performance of carburizing stainless steel pellet mill die is stronger than alloy steel pellet mill die, but weaker than stainless steel pellet mill die. The hardness of the pellet mill die core part is stronger than alloy steel pellet mill die. Carburizing stainless steel pellet mill die has perfect abrasion performance, which makes it has long serve life and low cost.

Table Comparison of Three Pellet Mill Die Materials

Die Type Abrasion Resistance Corrosion Resistance Toughness
Alloy Steel Pellet Mill Die 70 30 90
Stainless Steel Pellet Mill Die 60 90 50
Carburizing Stainless Steel Pellet Mill Die 90 75 80

PELLET MILL DIE SHAPES

Taking about pellet mill die shapes, you can easily form a picture of the shapes of different pellet mill dies in your mind by only know the names of different types of wood pellet mills.

pellet mill die shapes
pellet mill die shapes

Small Pellet Mill Die Shape

Small pellet mill is used for small scale wood pellet production, normally under 800kg/h. This type of pellet mill is mainly used for farmers, which can help them turn the crop wastes into useful pellet fuels for their household or even commercial purposes. For small pellet mills, aka flat die pellet mill, the shape of its pellet mill die is a round flat steel plate with multiple die holes on it. The flat pellet mill die will be installed in the small pellet mill horizontally. With different manufacturing requirements, the flat die can be rotated or fixed, and the compression ratios of the pellet mill die holes are various according to different types of raw materials.

Large Pellet Mill Die Shape

Large pellet mill is used for large scale wood pellet production, normally over 1ton/h. Large pellet mill is the core equipment of complete wood pellet lines, which is aimed to produce large quantity commercial wood pellet fuel for end users and industrial plants. For large pellet mills, aka ring die pellet mill, the shape of large pellet mill die is a steel ring with multiple die holes on the surface of the ring. During pellet mill manufacturing process, pellet mill ring die can be installed in pellet mills both vertically and horizontally. For different pellet mill manufacturing purpose, you can choose the suitable types of ring die pellet mills. And the pellet mill ring dies can also be rotated or fixed during wood pellet manufacturing process, and the compression ratios of pellet mill die holes can be customized.


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