The key component of the injection device – the barrel
The barrel is the core component of the injection molding machine, responsible for plasticizing, melting, and conveying the plastic. Its structural design, material selection, and processing precision directly impact the plasticizing quality and production efficiency of the injection molding machine. The barrel’s primary function is to heat and melt solid plastic particles into a uniform melt, and then, driven by the screw, convey the melt to the nozzle, preparing for the injection process. The barrel operates in an extremely harsh environment, needing to withstand high temperatures (typically 150-350°C), high pressures (up to 100-200 MPa), and intense friction and wear. Therefore, it must possess excellent resistance to high temperatures, high pressures, and wear. For example, when processing glass fiber reinforced plastics, the inner wall of the barrel is subject to severe wear from the glass fibers. Insufficient wear resistance can significantly shorten the barrel’s service life, affecting the plasticizing effect.
The barrel’s structural design significantly influences the plasticizing effect. It is typically divided into three sections: the feed section, the compression section, and the homogenizing section. These sections, along with the screw’s three-section structure, work together to complete the plasticizing process. The feed section primarily conveys plastic pellets from the hopper to the compression section. This section features a larger inner diameter and deeper screw grooves, facilitating pellet conveying and preheating. The compression section is where the plastic begins to melt. The screw’s compression and the barrel’s heating gradually melt the pellets and expel air. The homogenizing section further mixes the molten plastic, adjusting the melt’s temperature and viscosity for injection. This section features a smaller inner diameter and shallower screw grooves, facilitating melt homogenization and metering. The barrel’s length-to-diameter ratio (A/D ratio) is also a crucial parameter, generally ranging from 18 to 25. A larger A/D ratio increases the plastic’s residence time in the barrel and promotes more complete plasticization. However, a larger A/D ratio increases energy consumption and the risk of plastic degradation.
The material selection for the barrel should be determined based on the type and characteristics of the plastic being processed to ensure adequate wear and corrosion resistance. For processing common plastics such as PE, PP, and ABS, the barrel can be made of 38CrMoAlA alloy structural steel. After nitriding, the surface hardness can reach 900-1000HV, providing excellent wear resistance and fatigue resistance. For processing plastics containing fillers such as glass fiber and minerals, the barrel must be made of a more wear-resistant material, such as a bimetallic composite material with an inner wall of high-chromium cast iron or tungsten carbide alloy and an outer layer of carbon steel, which is formed by centrifugal casting. This material has a wear resistance 3-5 times that of ordinary nitrided steel, significantly extending the service life of the barrel. For processing corrosive plastics such as PVC and POM, the barrel must be made of stainless steel such as 316L to prevent erosion by corrosive media.
The barrel heating system is crucial for ensuring plastic melting. Electric heating coils or electromagnetic heating devices are typically used to heat the barrel. These coils are installed on the barrel’s outer wall and are divided into multiple heating zones (typically 3-5). Each zone can be independently temperature-controlled to achieve a temperature gradient along the barrel’s axis, meeting the plastic’s melting requirements at different stages. For example, the temperature in the feed section is kept low (close to the plastic’s glass transition temperature) to prevent premature melting and bridging. The temperatures in the compression and homogenization sections gradually increase to reach the plastic’s melting point. Electromagnetic heating devices generate heat in the barrel through electromagnetic induction. They offer advantages such as high heating efficiency, rapid temperature rise, and energy savings, and are gradually replacing traditional electric heating coils. Temperature sensors such as thermocouples are also installed on the barrel to monitor the barrel’s temperature in real time. The temperature control system precisely controls the temperature, achieving an accuracy of ±1°C, to prevent temperature fluctuations that could affect plasticization quality.
The clearance between the barrel and screw is a critical parameter that influences plasticization and melt leakage. Excessive clearance increases melt backflow between the barrel and screw, reducing plasticization efficiency and metering accuracy. Excessive clearance increases friction and wear between the screw and barrel, shortening the service life of both. The clearance size is determined based on the characteristics of the plastic and the diameters of the barrel and screw. It is generally between 0.1 and 0.3 mm. For smaller barrels and screws, the clearance can be reduced; for larger diameters, it can be increased. Regularly check the clearance during assembly and operation. If the clearance exceeds the specified value due to wear, replace the screw or barrel promptly to ensure plasticization quality. Furthermore, the front end of the barrel connects to the nozzle, and a good seal is required to prevent melt leakage. Typically, a tapered fit or O-ring seal is used to ensure stable pressure during the injection process.
