In the production process of large underwater pelletizers, precise control of the feed rate is crucial for ensuring consistent pellet size. Fluctuations in the feed rate directly affect the residence time, pressure distribution, and shear rate of the molten plastic in the extrusion die, leading to deviations in pellet length, diameter, or shape. Therefore, multi-dimensional optimization is necessary, encompassing equipment design, process parameters, raw material characteristics, and process monitoring, to achieve precise feed rate control.
At the equipment design level, a rational structure for the feeding system is fundamental. Underwater pelletizers typically employ forced feeding devices, using a screw or plunger to push the plastic raw material into the extruder barrel. The screw's geometric parameters (such as screw channel depth, pitch, and compression ratio) must be customized based on the raw material's flowability and melting characteristics. For example, high-viscosity raw materials require shallow screw channels and small pitch screws to enhance shearing and conveying capabilities; while low-viscosity raw materials require deep screw channels and large pitch screws to avoid excessive shearing and degradation. Furthermore, a sealing structure must be installed at the connection between the feeding device and the extruder barrel to prevent raw material leakage or air ingress, ensuring stable feed pressure.
Optimizing process parameters is crucial for precise control of the feed rate. Screw speed directly affects the material delivery rate and must be matched with parameters such as extruder back pressure and die temperature. If the screw speed is too high, the material's residence time in the barrel is too short, potentially leading to incomplete melting and uneven particle size. If the speed is too low, material accumulation in the feeding section can cause feed fluctuations. Therefore, the optimal speed range needs to be determined experimentally and continuously adjusted using variable frequency speed control technology. Simultaneously, the die temperature needs to be set in segments according to the material's melting point and viscosity curves to ensure uniform melt flow during extrusion and avoid inconsistent die exit speeds due to temperature differences.
The influence of material characteristics on feed rate cannot be ignored. Different plastics have significantly different molecular weight distributions, melt indices, and additive contents, requiring targeted adjustments to the feeding strategy. For example, materials containing fillers or reinforcing fibers have poor flowability, requiring a reduced feed rate and increased screw shear force to promote uniform dispersion; while high-flowability materials require a faster feed rate and optimized cooling system to prevent particle adhesion. Furthermore, the pretreatment of raw materials (such as drying and mixing) must be strictly controlled to prevent moisture or impurities from causing feed blockage or melt degradation.
Process monitoring and feedback mechanisms are the last line of defense to ensure the stability of the feed rate. Underwater granulators need to be equipped with high-precision sensors to monitor parameters such as the rotational speed, current, and pressure of the feeding device, as well as the temperature and pressure of the extruder barrel in real time. Data is collected and analyzed through an industrial control system (such as PLC or DCS) to establish a dynamic correlation model between the feed rate and process parameters. When feed fluctuations are detected, the system can automatically adjust the screw speed, die temperature, or cooling water flow rate to form a closed-loop control. For example, if the feed pressure suddenly increases, it may be due to raw material blockage; the system can reduce the screw speed and increase the die temperature to promote melt flow. If the feed rate is too slow, the screw speed can be increased and cooling conditions optimized to prevent particle deformation.
Die design and maintenance have a direct impact on particle size consistency. The number, diameter, and arrangement of the die holes need to be customized according to production requirements. Excessive orifice count may lead to insufficient die pressure and uneven discharge speed; insufficient orifice count may cause melt fracture due to excessively high local pressure. The die surface must undergo ultra-precision polishing to prevent melt adhesion or particle tailing caused by roughness. Furthermore, regular cleaning of the die channels to prevent abnormal discharge due to material residue or impurities is also crucial for ensuring particle size consistency.
Optimizing the cooling system can indirectly improve the control of the feed rate. The cooling water flow rate, temperature, and level of the underwater pelletizer must be strictly controlled. Insufficient cooling may cause particle adhesion or deformation, while excessive cooling may cause stress concentration or brittle fracture within the particles. By adjusting the water pump speed, water valve opening, and liquid level, it is ensured that the particles solidify rapidly after leaving the die, while avoiding dimensional deviations due to uneven cooling. In addition, the cooling water must be filtered and circulated to prevent impurities from scratching the particle surface or clogging the die.
By systematically optimizing equipment design, process parameters, raw material characteristics, process monitoring, die maintenance, and cooling systems, the feed rate control accuracy of large underwater pelletizers can be significantly improved, thereby ensuring the consistency of pellet size. This process requires combining simulation analysis, experimental verification, and long-term production data accumulation to form a standardized operating procedure that adapts to different raw materials and production needs, ultimately achieving efficient, stable, and high-quality pelleting production.
