Principle and development of ring die granulator

1.Machine base  2.Magnet   3.Feed chute  4.Pelletizing chamber  5.Door cover  6.Main transmission box  7.Speed control motor  8.Reducer   9.Hopper

10.Feed flap  11.Feeder  12.Conditioner  13.Conditioner reducer  14.Conditioner motor  15.End cover  16.Coupling  17.Main motor  18.Travel switch

19.Lubrication system

Figure 1: Structure of the Ring Die Pellet Machine

The press roller and ring die are crucial components that determine the pellet quality and working efficiency of the ring die pellet machine. The ring die demands high standards; factors like diameter, shape, aperture ratio, and materials must be carefully considered during its manufacturing. The speed of the ring die must also be strictly controlled, as it significantly affects product quality. Optimal ring die speed can effectively improve production efficiency, product quality, and reduce wear.

Given that the diameter of the press roller is much smaller than the inner diameter of the ring die, the press roller is more prone to damage and has a shorter lifespan. Therefore, high carbon alloy steel is typically used to manufacture the press rollers. As the cost of the ring die is much higher than that of the press roller, protecting the ring die by sacrificing the press roller is often a necessary measure to extend the ring die’s lifespan.

3. Working Principle of the Ring Die Pellet Machine

During operation, the ring die rotates clockwise, with conditioned raw materials entering the pelletizing chamber. As the material enters the working area, the press roller experiences friction and begins to rotate clockwise. The rotation of the die and roller propels the raw material forward at high speed. As the quantity of raw material increases, the mutual extrusion pressure also increases. When the extrusion pressure reaches a certain level, it overcomes the frictional force, forcing the material into the ring die holes. The continuous rotation of the die and roller ensures a steady feed of material into the holes, where it is extruded and cut to form the desired pellets.

The operation process can be divided into three zones: feeding zone, compression zone, and extrusion forming zone.

• Feeding Zone: When the wedge angle between the ring die and the press roller is larger than the maximum friction angle of the material layer, the material does not advance to the next zone. However, as the ring die continues to rotate, the material accumulates between the ring die and the press roller, following the movement of the material layer in the compression zone and gradually entering the rotation area formed by the die and roller. Due to the centrifugal force generated by the ring die’s rotation, the material adheres to the inner side of the ring die.

• Compression Zone: When the wedge angle between the ring die and the press roller is smaller than the maximum friction angle of the material layer, the material layer moves towards the extrusion zone along with the ring die. Simultaneously, the cross-sectional area of the compression zone decreases, compressing the material layer and increasing its density. However, the internal pressure is not yet sufficient to overcome the frictional resistance, resulting in further compression without moving into the ring die holes. As the extrusion pressure continues to increase, the material undergoes irreversible deformation.

• Extrusion Forming Zone: The gap between the ring die and the press roller narrows, and the extrusion pressure increases continuously. The powdered material is compressed to form cohesive pellets, which are then extruded through the ring die holes. The extruded pellets experience a slight rebound, making their diameter slightly larger than the ring die holes.

4.Current Development Status of Ring Die Pellet Machines

Since the 1980s, China has focused on researching biomass solid fuel molding technology. In recent years, with rapid economic development, domestic machinery levels have significantly improved. Coupled with the global emphasis on environmental protection, the development of ring die pellet machines has naturally progressed. As economic development continues, the demand for high-quality pellet products has increased, presenting an excellent opportunity for innovation and improvement in ring die pellet machines. Optimizing the key working components and structure of these machines, along with researching and exploring subsequent improvements, can enhance ring die pellet technology and improve product yield rates. This not only boosts the international competitiveness of Chinese pellet machine products but also advances China’s industrial technology, holding great theoretical and practical significance.

In terms of ring die pellet technology, some countries currently possess relatively complete technical systems. Developed countries have established a comprehensive technical system for ring die pellet equipment, providing various high-quality production equipment, practical production plans, and solutions for encountered problems. Leading companies in the ring die pellet field primarily come from developed regions abroad, where research in this area started earlier and has yielded substantial results, forming a complete system. With national and corporate emphasis on ring die pellet technology, significant optimizations in manufacturing processes, energy consumption, and structural design have been achieved. These improvements have notably enhanced energy efficiency, operational efficiency, and product quality. The technology is reliable and characterized by efficiency, energy-saving, and environmental protection, making it highly competitive. As the modern era progresses, these devices are evolving towards greater intelligence, eco-friendliness, and larger scales.

Researchers like Meng Haibo and Tian Yishui from the Ministry of Agriculture’s Planning and Design Institute have undertaken the design and exploration of biomass solid fuel molding equipment and production lines and participated in developing standards for biomass solid fuel molding. Gao Wei from Shenyang Agricultural University conducted digital design and experimental research on biomass pellet fuel pellet machines, summarizing the effects of factors such as moisture content on pellet formation rate, density, and impact resistance. Wu Yunyun from Shandong University used finite element analysis to study the fatigue life of ring dies in biomass fuel pellet machines. Jiang Qinghai from Nanjing University of Science and Technology analyzed the wear mechanisms of ring dies in biomass pellet machines, identifying abrasive wear, fatigue wear, and polishing wear as the primary failure causes. Domestic universities and research institutes are increasingly focusing on biomass molding equipment, generally divided into three research directions: finite element analysis of key components to optimize their structure and parameters, studying the wear or fatigue mechanisms of key components to address severe wear issues of press rollers and ring dies, and analyzing the application or marketization of biomass fuel or equipment. These studies provide theoretical and data support for the design and optimization of domestic biomass pellet machines. However, many research results are based on data or optimization outcomes under specific conditions, often idealized and simplified, limiting their practical guidance. China’s development of ring die pellet machines is still in its early stages, with ongoing issues such as poor performance, high power consumption, and severe wear. Nevertheless, the Ministry of Agriculture has completed independent research on ring die pellet machines and successfully established practical production lines.

5. Prospects for the Promotion of Ring Die Pellet Machines

Processing straw into biomass pellets is crucial, as biomass pellets contain fewer harmful substances, burn more completely, and perform comparably to traditional fuels, with less environmental pollution. This aligns with the concept of green industry. Therefore, the comprehensive promotion of biomass pellets produced by ring die pellet machines across various fields is essential. Currently, only a small portion of straw is processed into pellets, with most raw materials for pellet production being bamboo, wood, and other potentially polluting materials. Pellet machines play a vital role in recycling waste materials, and replacing coal with pellets would greatly benefit the environment. The operational efficiency, product quality, and energy consumption of pellet machines are commendable, and their products are environmentally friendly. In the market, pellet fuel has almost entirely replaced coal as a cleaner, more efficient fuel in Europe and America. However, in China, pellet fuel is only used in a few fields such as hot air stoves, boilers, and biomass gasifiers, not yet widespread in daily household use. With China’s increasing focus on environmental protection and further development of ring die pellet machines, replacing traditional fuels with pellet fuel is imminent, making the development and promotion of ring die pellet machines indispensable.

6. Domestic and International Development Trends of Ring Die Pellet Machines

Modern ring die pellet machines not only use active rollers but also the more common active ring dies. The ring die rotates driven by the main shaft, with 2-3 press rollers inside. During operation, the press rollers press the powder into the ring die holes, extruding it into small cylindrical shapes, which are then cut into pellets. This process does not generate additional frictional forces, resulting in lower energy consumption, higher production efficiency, and better product quality. Ring die pellet machines hold a significant position in the feed and agricultural industries, both domestically and internationally. To further improve production efficiency, optimize product quality, and extend service life while meeting actual production needs, countries will support the development of ring die pellet machines through various policies, leading to the creation of more efficient, user-friendly, and automated machines.

Although China’s development of ring die pellet machines has been rapid in recent years, there is still a technological gap compared to countries with advanced technology and specialized talent. China lacks high-power equipment with larger ring die diameters, and the lifespan of key components like ring dies lags behind other countries. Moreover, there is a shortage of skilled personnel in agricultural machinery. Therefore, China could consider introducing advanced foreign technology and equipment, adopting a critical approach to complete the development of domestic ring die pellet machines, aiming to catch up and potentially surpass international standards.

7. Main Research Directions and Prospects Domestically and Internationally

The ring die pellet machine initially took shape in the 1930s. Over several decades of development, it has undergone continuous optimization and innovation. Today, both domestic and international research on pellet compression materials has established a solid foundation, with numerous experiments conducted and future development prospects discussed based on experimental data. However, research experience on the ring die, the critical component for pellet formation, remains relatively scarce. Most papers assume boundary conditions and conduct numerical simulations for stress and strain analysis of the ring die. The optimization design of the equipment is often limited to simple simulation comparisons and analyses.

In the future, failure research on ring die pellet machines should be undertaken to explore structural compensation during the wear failure process of the ring die. This would provide a basis for designing new ring die structures and guide further innovations in ring die technology.