The compression ratio of a pellet mill refers to the ratio between the diameter of the pellet die hole and the effective length of the die hole. The compression ratio varies depending on the raw materials being used. For example, the compression ratio for poplar is 1:6, pine wood is 1:7, hardwood is 1:5, wood shaving is 1:5.5, and corn straw is 1:8.
The degree of countersink, also known as the choke, is an important factor in determining the force at which the feed material is pushed into the pellet compression area. It affects the compression ratio and the overall pelletizing process.
When selecting pellet mill dies for different types of feed, it is important to consider the formulation of the feed. For example, broiler feed, which contains fewer fibers and more oil, requires a higher compression ratio of 1:12 or 1:13 to achieve better pelleting properties and less frictional load on the pellet die. On the other hand, using high compression dies for layer feed may lead to poor die life, die chocking, non-uniform pellet durability index (PDI), more power consumption, low output, and more wear and tear. In this case, a compression ratio of 1:9 or 1:10 is recommended.
For sinking aqua feed, better water stability is desired. Therefore, using a pellet mill die with a high L/D ratio (effective length divided by the hole diameter) is recommended for better PDI and good water stability.
The working width of a pellet mill die is measured between the two inside edges of the die grooves, while the L/D ratio is the effective length divided by the hole diameter. High L/D ratios provide high pellet die resistance as feed moves through the hole, while low L/D ratios provide low resistance.
The counter sink diameter of the pellet mill die refers to the diameter of the inlet where the feed enters the compression area. Different configurations of the inlet can be used to achieve extra compression if needed for specific materials.
The inlet depth, also known as the countersink depth, refers to the depth of the countersink. It is an important factor in the pelletizing process.
In general, when pressing pellets, the height (length) of a pellet should not greatly exceed the diameter. This is because the stresses in the compact are greater closer to the moving surface of the top plunger. If pellets become significantly longer than the diameter, cracking or other adverse effects may occur. However, this rule is not applicable in all scenarios and other factors should be considered.
If longer pellets need to be pressed, it is suggested to use lower forces, consider lubricating the die, and use a small o-ring between the base plate and sleeve to allow some compression of the lower pressing surface. This helps to distribute the stresses more evenly throughout the compact.
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