How Does the Roller-die Gap of Feed Pellet Machine Affect Pellet Durability Index
● The influence of roller-die gap on the PDI
The increased pressure generated in the die hole of pellet mill increases the physical properties of feed pellets. Elevated pressure seriously depends on the coefficient of friction between conditioned feed compound and die wall, plastic properties of conditioned feed material and its conditioning time. In the gap between the die wall and roller, a layer of feed material with a specific thickness is forming. This layer of feed material and its thickness directly influences formation and compaction of the material inside the pellet mill die-hole by roller-die frictional force. By increasing the roller-die gap, several layers of feed mash are created and the coefficient of friction is increasing.
When increasing the roller-die gap to 2 mm as well as reducing feeder rate by 50%, PDI can be significantly increased. This is probably caused by prolonged kneading effect time. Obviously, by decreasing the feeder rate, the coefficient of friction helps forming higher PDI values, but energy consumption and production capacity reaches commercially non satisfactory level and reduced production capacity of feed pellet machine. In addition to the coefficient of friction influence on PDI, larger roller-die distance forms better pressing angle. This produces higher pressure on the material that enters the die hole, and the feed material in the hole as well. All this leads to better-compacted particles into feed pellets and elevates physical properties of animal feed.
When roller-die gap is set to be too big, it often results irritating choke-ups. Opposite, if rollers are adjusted hard against the die, some damages of the die-hole inlets and die blockages are possible.
● What is reasonable roller-die gap of feed pellet mill to improve PDI?
The main purpose of pelleting is to produce a palatable, high quality product with minimum production cost. Manipulation of feed pellets during storage, transporting, and transfer to feeding lots seriously reduce number of feed pellets that finally reach feed pans. Therefore the best solution for this problem is to increase Pellet Durability Index of feed pellets by using different settings in manufacturing process and different raw ingredients. The reasonable pellet mill roller-die gap is a good method to improve the PDI, as we have shown on the above.
If you want to achieve the best balance of power consumption and pellet quality, you are advised to maintain a gap of 1 mm. If you set a gap above 1 mm you will have issues with power consumption and potentially stall the motor. The greater the distance the more resistance, therefore above 1 mm is only recommended for certain animal feed pellets and other density materials. Last but not least, you’d better not to have metal to metal contact, that is to say do not adjust the roller-die gap to zero. You are advised to start the die and then slowly adjust the rolls to the die, as soon as you hear the contact, just lift back slightly and then lock the roller in position.
The Influence of Knife Condition of Feed Pellet Mill on the Feed Pellet Quality
In the feed pelleting process, the conditioned feed mash flows by the gravity into the die chamber where rollers press the softened feed mash through the holes in a circular die. At the same time the knives located outside the circular, rotating die cut off the shaped, dense pellet at the proper length. Different sized animals eat different sized feed, therefore cutting, thus uniformity of feed pellets, is essential procedure for feed producers. Therefore, the knives play an important role in cutting feed pellets. However, worn knives usually have randomized cutting area, feed pellets are more randomly stressed, therefore new abrasions would be formed, hence more dust in the end product obtained. All that leads to reduced physical quality of feed pellets and non-satisfaction from customers.
Research based on possibility that the knives condition of the feed pellet mill influence physical properties of feed pellets has been accomplished. The research showed that the knife condition（worn or sharp）can affect the physical quality of feed pellets.
Research has been based on two types of ingredient composition(table 1). Hypothetically, superior and poor for physical properties of feed pellets.
Table 1—Experimental ingredient composition
Wheat based (%);Maize based (%)
Feed mash conditioning was performed by the conditioning chamber of feed pellet mill with 75℃ preset temperature in both, test and its replica for both ingredient compositions. Samples taken after the conditioner have been analyzed for moisture. Moisture showed to be similar in both tests and diets. Finished feed samples were taken immediately from the feed pellet mill to avoid any breakage due to the transport to packaging line. Cooling was performed for 6 hours at the ambient temperature of 26℃ and relative humidity of 50% to avoid any forced moisture removal, thus more likely larger crack formation.
Two different knife conditions (figure 1&2) have been used as well as the new technique of pushing pellets thus breaking them where the sheer stress is most likely to be lowest, have been performed by pusher(figure 3).
Feed pellets, compressed and formed in the die of feed pellet mill at the die outlet are experiencing sheer stress formed by knife. Wheat based pellets did not show to be influenced by different knife condition as sharp neither worn. When knife is worn, theoretically impact of revolved pellets is let by chance, thus additional abrasions on pellets are likely to be formed.
Cutting technique by pushing the maize based feed pellets by angled and widen flat surface(figure 3) illustrated increasing the physical properties of maize based pellets compared to cutting the same pellets with the sharp knives.
Results from this research outcomes that feed pellets made of raw materials which are expected as good for physical properties (in this case, wheat based) can be cut with the sharp and maintained knives, but if knives are worn, additional maintenance investment is not necessary. Whereas pellets manufactured from raw materials which are assumed as bad for physical quality of feed pellets(in this case, maize based) should be pushed. Hence pellet is forced to break where its natural weakest crack exist with obvious increase of PDI and reduction of additional cracks formed by knife sheer stress.