The failure of pellet mill ring die to discharge materials is a common problem in the pelleting process, which is usually related to material properties, equipment status, operating parameters and other factors. The following are the main causes and corresponding solutions summarized in the industry in 2025, supplemented with modern solutions based on equipment technology development (such as intelligent monitoring, new ring die materials, etc.):
I. Abnormal Material Properties
1. Inappropriate Moisture Content of Materials
- Causes:
◦ Excessive moisture (>18%): Materials are prone to adhere to the inner wall of the ring die or in the die holes, causing blockage;
◦ Insufficient moisture (<10%): Poor fluidity and high friction of materials make it difficult to be squeezed into the die holes by the pressure roller.
- Solutions:
◦ Use online moisture monitors (such as near-infrared sensors) to detect material moisture in real time, and adjust it to 12%-15% through the steam volume of the conditioner or water adding device (different materials have different adaptation ranges, e.g., feed is usually 13%-15%, and biomass pellets are 10%-12%);
◦ For excessive moisture, extend the conditioning time through the conditioner and add hot air drying assistance; for insufficient moisture, properly introduce saturated steam (to avoid condensation) or add a small amount of water during the conditioning stage.
2. Improper Material Crushing Granularity
- Causes:
The crushed material particles are too large (exceeding 1/3 of the diameter of the ring die holes), or there are excessively long fibers and impurities (such as stones, metals), leading to blockage of the die holes.
- Solutions:
◦ Check the screen aperture of the crusher to ensure that the crushing granularity is ≤ 1/3 of the diameter of the ring die holes (e.g., φ4mm die holes correspond to crushing granularity ≤ 1.3mm);
◦ Install impurity removal equipment (such as magnetic separators, vibrating screens) to remove hard impurities; fibrous materials (such as straw) need secondary crushing or cutting treatment to avoid entanglement.
3. Unreasonable Material Formula
- Causes:
◦ Excessive oil/sticky components (e.g., oil >5%): Materials are prone to slip and cannot be effectively grabbed by the pressure roller;
◦ Excessive crude fiber/inert components: Lack of viscosity, making it difficult to form pellets and easy to block die holes.
- Solutions:
◦ Adjust the formula: Control the oil addition within 3%-5%; if exceeding, add in stages (spray after pelleting);
◦ Add binders (such as bentonite, starch) to improve material viscosity; for crude fiber materials, soften the fiber structure by conditioning and heating (80-90℃).
II. Failures of Ring Die and Pressure Roller System
1. Blockage of Ring Die Holes
- Causes:
◦ Residual materials harden in the die holes due to incomplete cleaning after shutdown (especially for high-temperature and high-humidity materials);
◦ After long-term use of the ring die, the inner wall of the die holes is unevenly worn or forms an “inverted cone” (more wear at the inlet end than at the outlet end), causing material stagnation.
- Solutions:
◦ Clear blockages: Manually dredge with a special pin (matching the diameter of the die hole), or run “die cleaning materials” (such as stearic acid + broken rice mixed at a ratio of 1:5) at low speed for 3-5 minutes to discharge blockages through lubrication and extrusion;
◦ Repair the ring die: Ream the worn ring die (only for uniformly worn die holes) or replace with a new wear-resistant ring die (such as the mainstream tungsten carbide coated ring die in 2025, with 50% improved wear resistance);
◦ Intelligent prevention: Monitor die hole blockages through ring die temperature sensors (such as thermocouples) (abnormal temperature rise at the blocked area) and alarm for shutdown in time.
2. Improper Gap Between Ring Die and Pressure Roller
- Causes:
◦ Excessive gap (>0.5mm): Materials cannot be squeezed by the pressure roller and ring die, only slipping between them;
◦ Insufficient gap (<0.1mm): Direct friction between the ring die and pressure roller generates high temperature, causing material adhesion and accelerating wear.
- Solutions:
◦ Measure the gap with a feeler gauge in the shutdown state and adjust it to 0.1-0.3mm (smaller value for new ring dies, larger value for old ones);
◦ Modern intelligent pellet mills can automatically adjust the gap through servo motors (real-time optimization based on material feedback) to reduce manual operation errors.
3. Abnormal Pressure Roller
- Causes:
◦ Wear on the pressure roller surface (such as grooves, depressions) makes it unable to effectively grab materials;
◦ Jamming of the pressure roller bearing causes the pressure roller to stop rotating or have abnormal speed.
- Solutions:
◦ Replace the pressure roller shell (using new high-chromium cast iron material in 2025, with 30% improved wear resistance) to ensure a smooth and undamaged surface;
◦ Check the lubrication of the pressure roller bearing (using long-acting grease, such as polyurea grease) and perform regular maintenance to avoid jamming due to lack of oil.
III. Problems in Equipment Transmission and Power System
1. Abnormal Rotating Speed of Ring Die
- Causes:
Failures of the motor or reducer (such as belt slipping, gear wear) result in the ring die rotating speed being lower than the set value (e.g., designed 300r/min, actual only 200r/min), leading to insufficient extrusion kinetic energy.
- Solutions:
◦ Check the transmission system: Tighten the belt, replace worn gears to ensure the rotating speed meets the standard;
◦ Adopt a frequency conversion motor + PLC control system (mainstream configuration nowadays) to monitor the rotating speed in real time and automatically compensate for power output.
2. Unstable Feed Rate
- Causes:
Feeder failures (such as worn spiral blades, silo bridging) cause the feed rate to fluctuate, resulting in insufficient material filling in the ring die and inability to form continuous extrusion.
- Solutions:
◦ Check the feeder: Repair/replace spiral blades, and install arch-breaking devices (such as pneumatic knockers) in the silo;
◦ Use a loss-in-weight feeder (high-precision configuration) to adjust the feed rate in real time through load cells, ensuring the material filling rate in the ring die is stable at 70%-80%.
IV. Improper Operation and Maintenance
1. Unrun-in New Ring Die
- Causes:
The inner wall of the new ring die holes is rough, and direct production is prone to material blockage, especially for high-hardness ring dies (such as alloy ring dies).
- Solutions:
The new ring die needs “running-in”: Run materials containing 5%-8% oil (such as corn flour + oil) at low speed for 10-15 minutes to polish the inner wall of the die holes until the discharge is smooth.
2. Incomplete Cleaning After Shutdown
- Causes:
Residual materials in the ring die are not emptied after shutdown, and long-term storage causes materials to harden and agglomerate, blocking the die holes during the next startup.
- Solutions:
Establish a standardized shutdown process: Stop feeding first, idle for 30 seconds until no materials are discharged, then purge the inside and outside of the ring die with compressed air, and apply anti-rust oil if necessary (for long-term shutdown).
Summary
Troubleshooting for ring die non-discharging should follow the logic of “materials → equipment → operation”. Priority should be given to locating abnormalities through intelligent monitoring functions of modern equipment (such as moisture, temperature, and rotating speed sensors), and then solving them targeted. Daily attention should be paid to equipment maintenance (such as regular inspection of ring dies/pressure rollers, lubrication of transmission systems) and control of material pretreatment, which can significantly reduce the probability of failures. Pellet mills in 2025 are generally equipped with fault diagnosis systems, which can automatically analyze data combined with AI algorithms to assist in quickly locating causes (such as judging blockage through die hole pressure sensors), improving solution efficiency.