Overall flat die size

Outer diameter

Outer diameter of flat die is based on flat die slot in pellet mill. Larger pellet mills designed for using large flat dies, have to be equipped with proper motors, because larger flat dies results in growing power consumption. Small flat dies (200 - 260mm outer diameter) works with 15kW pellet mills, but larger pellet producers uses flat dies 300, 335 or 400 which require pellet mills with larger motors. Industrial production of pellet usually starts with machines equipped with 400mm diameter flat dies and larger, up to 1250mm. Larger flat dies have higher productivity because of larger working area.

Inner diameter

For D-type flat dies, inner diameter of the die is equal to pellet mill main shaft diameter. Starting with inner diameter equal 40mm for 200mm dies, and growing with outer diameter. Additionally, rotating dies comes with shaft key grooves which provides rotational torque transfer.
R-type flat dies are rarely used smaller than 300mm diameter flat dies, because this type of flat die is more expensive and harder to maintain.

Die thickness

Another important parameter of flat die is its thickness. Pelleted material is compressed by pressing it through heated die holes. Pelleting pressure value is a result of holes length, therefore of the flat die thickness. Choosing the proper flat die thickness is important for pelleting process. If holes are too short, pelleting pressure will be too low causing final product to crumble. Too long holes will exceed pelleting pressure over proper value, and cause material to stuck in die. The other factor is overall hardness required to keep flat die resistant to cracking.

Flat die keyway

For proper operation of pellet mill, flat die must be mounted to pellet mill shaft by a key. D-type flat dies has a keyway on inner diameter in order to transfer rotational torque, while R-type flat dies have one or more keys placed in keyways on outer diameter to hold it still in flat die. For flat dies with outer diameter 200 and 230mm, 10mm key is used, for flat dies 260 and 300mm - 16mm key. Key size depends on diameter of pellet mill shaft.

Flat die holes

Flat die holes size

Flat die holes size should be chosen depending on desired pellet diameter and can be chosen regardless other dimensions or parameters. Each pellet mill can have a bundle of flat dies with different holes size, which allows to produce pellet with different diameters. Please note that the quality of final product depends on size of the pelleted material particles. There are some advices for holes size for popular materials. 4mm holes are commonly used for pelleting feed, while most proper holes size for pelleting sawdust would be 6mm or 8mm for large dies

Holes pattern geometry

There are two most popular types of holes pattern geometry. First one - hexagonal pattern is commonly used in small and medium dies with diameters up to 400mm. The second one - radial pattern is efficient only for large flat dies (450mm diameter and more).

Holes to working area coefficient

Holes to working area coefficient is a quotient of total holes area to the die working area. Increase in this coefficient results in lower power consumption, while lowers the quality of final product.

Compression factor

Compression factor is a quotient of flat die hole length to hole diameter. Depending on required pellet quality, compression factor value may vary between 4 and 7,5. This parameter should be chosen according to type of the pelleted material. Usually, higher compression factor value has a positive influence on pellet quality.

Countersinks and compression factor

Total thickness and effective thickness

Total thickness is one of main flat die parameters and it's value should be determined in product specification. Among small flat dies with diameter up to 350mm, there is possibility of using both of their surfaces as a working surfaces, which highly extends flat die operation time. To achieve this, countersinks must be present on both sides of flat die holes. In this case, countersinks works also as a hole relief. For large industrial dies with diameter 400mm and more using both side countersinks is impossible, because due to die thickness required to for proper die durability, we need to use long relief holes to avoid material from being stuck in holes.
Total thickness of double-side flat dies is a sum of effective thickness and relief length, which has the same size as countersink. For one-side flat dies total thickness contains effective thickness and custom hole relief length. In all other cases, total flat die thickness is equal to effective thickness.

COUNTERSINK

At the first stage of pelleting process in pellet mill raw material is compressed by pressing them through countersinks placed on inlets of holes. Countersink size type are selected to all raw materials independently. In our production, we use proper countersinks for pelleting coniferous sawdust (spruce, fir), soft sawdust (pine), hard sawdust (oak, beech), wood chips, bran, straw (cereal straw, rape straw), cereal, maize, sunflower, manure and other. Even small differences in composition of pelleted material, can have a large influence on quality of the final product. Thus choosing proper countersink should be based on the composition of raw material.
Countersink angle and length are two basic parameters describing the countersink. For larger angle and length, more material is compressed at the same time which results in higher efficiency and power consumption. Smaller countersinks can have a negative influence on quality of final product. Proper countersink selection is usually based on experience and preceded by many tests and analyses.

Holes relief

When flat die thickness based on required durability is larger than optimal working length, holes reliefs must be used. For small flat dies relief holes are usually redundant, and double-sides die can be used. Holes reliefs have usually shape of tapper, but straight or stepped holes reliefs can be used.