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How Pellet Mill Affects Your Feed Quality

By Jack Huang    Oct 31, 2014

Pelleting has a long history since it was introduced into feed processing, which could be dated back to 1920. Today, feed pelletizing is widely used for its nutritional benefits and physical benefits such as, easy handling, ingredient segregation reduction, feed wastage reduction, and high bulk density. According to C.R. Strark’s study, the influence of the mentioned factors are feed ingredient formulation, particle size, conditioning and die specifications are the main factors that influence feed pellets quality, shown in percentage in the following picture.

Factors influencing feed pellets quality

According to his study, it is known that feed pellet quality is largely affected by formulation 25%, conditioning 15%, particle size 20%, cooling 5%, die specification and throughput 20%. Among them, factors concerning feed mill are condition, particle size, cooling, and die specification and throughput which take about 75 % all together. Therefore, how a feed pellet mill affect your feed quality will be discussed.

How feed pellets are made by pellet mill

How feed pellets are made through a pellet mill? It depends on the which pellet mill you are using, the flat die pellet mill or the ring die pellet mill. Generally, the pellet are made by the interaction of roller and die under the interaction of three main forces, which are roll force, radial force, and tangential force. Generally processing are like this: feed stocks are conveyed to the pelletizing chamber, where the rollers will press the mash into die holes with the pressure of rollers and the feed pellets are formed in the die holes and extruded and cut off by knife.

How feed pellets are made by a pellet mill

How pellet mills affect your feed pellets?

Generally, pellet mills are the core equipment is feed pellet production. The inherent specifications of a pellet mill will largely affect the pelletization of pellets. These specifications are die dimensions, die speed and roller-die gap, etc.

Effect of die dimension on feed pellet quality

Pellet die can affect the feed pellet quality on many ways by its characteristics such as, metallurgy which will affect friction and temperature build up during pelletization, die hole designs which will affect the formation of feed pellets, hole pattern and number which will affect feed pellets productivity and thickness of the die in relation to the die hole diameter known as L:D ratio or compression ratio, which is the most decisive factor that will affect pellet durability. In the following, compression ratio will be talked in length for your sake.

Before talking about L:D ratio,it’s necessary to know the die hole designs. Generally, there are several die hole designs are available, with the features of normal, deep , flat, well type, cylindrical, conical, stepped , shown in the following picture.

Pellet die hole designes and stepped pellet die hole
Pellet die hole designes and stepped pellet die hole

Among the above designs, the most common one are the first type which is used in flat die pellet mill and the seventh, the stepped one, which are the most developed design for flat die and ring die. Well, then we’d take the stepped die hole design to talk about how L:D ratio affect pellet quality.

Take the stepped pellet die design shown in the right picture for example. The LD ratio refers to the a ratio value of active length (L1)of the press channel in relation to the diameter(d1), which will be L1/d1 as the situation in the picture.

Here is how L:D ration influence pellet durability: at first phase, as the feed stock goes into die hole through d1, the friction will is increasing while at the same time L1:d1 ratio is decreasing. As soon as feed stock reaches to d2, the active length will is changed into L2 which is ensured that that L2:d2 equals to L1:d1. It means the same compression ratio is guaranteed through the pelletizing process and the durability of pellets is increased and the serving time of the die will be prolonged but the production rate will be decreased.

Pellet durability index and produciton rate

According to study of C.R.Stark, normally pellet durability increases with L:D ratio because the shear forces is increased which results from the increased friction between feed stock and die. However, when the compress ratio is too large, the possibility of blocking will be increased, which will increase the risk of damaging of pellet mill.

What’s more, the L:D ratio is varying with the different ingredient formation, which is a dynamic parameter for different feed pellets production. That’s to say, pellet dies with different specifications should be carefully selected based on the species of your feed formation. However, in actual feed pellet production, the requirement to L:D ratio is not so rigorous. Normally ingredient with similar formation can be pelletized by the same die, or the same pellet mill. That’s to say, one pellet mill can used to produce different feed pellets. And it is very usual that wood pellets are used to produce feed pellets because the L:D ratio requirement for feed pellet production can be met by a wood pellet mill.

Gemco pellet mill can be used to produce wood pellets and feed pellets
Gemco pellet mill can be used to produce wood pellets and feed pellets.

Effect of die speeds on pellet quality

Die speed of a pellet mill is the tangential velocity of the rollers during pelletizing. Die speeds varies with the production of pellets with different sizes. Normally, high die speeds around 10 m/s are utilized to produce pellets with small diameters ranging from 3mm to 6mm while lower die speeds are used to produce pellets with diameter from 6mm to 7mm. And also, lower die speeds around 4-5 m/s are usually used to process low density materials to produce high density pellets by expelling the air out of the feedstock.

Effect of roller-die gap on pellet quality

The gap between rollers and die directly influence the formation of the pellets, and the hardness and durability of pellets. The gap refers to the space between the annular matrix and the rollers, where interaction of roll force, radial force, and tangential force happens to force the feed stock to go through the pellet die to form pellets. Normally, pellet durability will be increased when the gap between the roller and die is narrowed down in a certain extent. What needs attention is the rollers and die can not contact with each other because the powerful contacting friction will damage both the rollers and the die.

The gap between the pellet rollers and the pellet die
The gap between the rollers and the die of a pellet mill

Pellet mill influences pellet hardness and durability during the procedure of pelletizing through its inherent characteristics, such as, die dimensions, die speed, and gap between the roller and die which are controllable and adjustable by the selection of quality pellet mills and adjustment in the processing.

At last, we’d like to share with you the video on how cattle feed pellets are made by a small pellet mill for your reference. If you have any question about feed pellet mills, please feel free to consult. Leave a message down below, free consultation will be provided.