The key component of the injection device – the nozzle
The nozzle is the connecting component between the injection unit and the mold, located at the front end of the barrel. Its primary function is to smoothly and accurately inject the plasticized melt from the barrel into the mold cavity and prevent melt backflow during the injection and pressure-holding phases. As a key component of the injection unit, the nozzle’s structural design, dimensional accuracy, and material properties directly impact the melt’s flow characteristics, injection pressure loss, and the quality of the molded part. Therefore, nozzle selection and maintenance are crucial in the design and use of injection molding machines.
Nozzles come in a variety of structural styles, including straight-through nozzles, self-locking nozzles, and lever-type needle-valve nozzles. Different nozzle designs are suitable for different plastic materials and molding requirements. The straight-through nozzle is the simplest type of nozzle, with a straight, unobstructed flow path and low melt flow resistance. It is suitable for plastics with good flow properties, such as polyethylene and polypropylene, as well as for simple, thick parts. The advantages of a straight-through nozzle are its simple structure, ease of manufacture, and minimal pressure loss. However, its disadvantages are that it is prone to melt backflow during the pressure holding phase, affecting the dimensional accuracy of the part. Self-locking nozzles, equipped with an internal check ring or check valve, automatically close the flow path after injection to prevent melt backflow. They are suitable for plastics with poor flow properties, such as polycarbonate and polyoxymethylene, and for parts requiring high dimensional accuracy. The advantage of a self-locking nozzle is that it effectively prevents melt backflow, improving part dimensional accuracy. However, its disadvantages are its relatively complex structure, high melt flow resistance, and high manufacturing and maintenance costs. The lever needle valve nozzle controls the opening and closing of the needle valve through a lever mechanism to achieve precise control of the melt flow. It is suitable for the molding of precision plastic parts and multi-cavity molds. Its advantages are high control accuracy and the ability to effectively eliminate gate marks. Its disadvantages are complex structure, high cost, and the need to work in conjunction with the control system of the injection molding machine.
Nozzle dimensions, including nozzle diameter, nozzle length, and nozzle tip angle, have a significant impact on melt flow and injection quality. The nozzle aperture size should be determined based on the part’s volume, wall thickness, and plastic fluidity. An excessively large aperture will result in prolonged melt retention in the nozzle, easily leading to cold slugs and poor part quality. A too small aperture will increase melt flow resistance and injection pressure loss, potentially leading to filling difficulties. Generally, nozzle apertures range from 2 to 6 mm. Larger apertures are recommended for plastics with good fluidity and large parts, while smaller apertures are recommended for plastics with poor fluidity and small, precision parts. The nozzle length should be kept within a reasonable range, as this increases the melt’s flow distance and cooling time within the nozzle, leading to lower melt temperature and reduced fluidity. A too short nozzle tip angle can result in insufficient contact between the nozzle and the mold’s sprue bushing, potentially causing melt leakage. The nozzle tip angle typically matches the mold’s sprue bushing angle, with common angles being 120° and 90°. This ensures a tight fit between the nozzle and the sprue bushing, minimizing pressure loss and melt leakage.
When selecting nozzle materials, consideration should be given to properties such as wear resistance, corrosion resistance, and heat resistance. Since nozzles frequently come into contact with the high-temperature melt and mold during operation, subjecting them to significant pressure and friction, high-strength alloy steels such as 38CrMoAlA are typically used. These materials undergo heat treatment processes such as quenching and tempering, and nitriding, to enhance their surface hardness and wear resistance. The nozzle’s inner surface requires precision machining, with a roughness of less than Ra0.8μm, to ensure smooth melt flow and minimize flow resistance and pressure loss. Furthermore, the nozzle’s inner surface should possess a good polishing property to prevent melt stagnation and decomposition during flow, which could affect part quality.
Nozzles require regular maintenance and care during use to ensure stable performance and longevity. First, regularly clean the nozzle of any residual melt to prevent it from solidifying and blocking the flow path, affecting melt flow. Use specialized cleaning tools such as copper rods and wire brushes to avoid damaging the nozzle’s inner surface. Secondly, inspect the nozzle head for wear. If severe wear or cracks are observed, replace the nozzle promptly to prevent melt leakage. Additionally, check the clearances between the nozzle and barrel, and between the nozzle and mold sprue bushing. Excessive clearances can lead to melt leakage and pressure loss, while too small clearances increase wear and shorten service life. When installing and removing nozzles, observe proper handling and avoid excessive force, which can cause deformation or damage. For self-locking nozzles and lever-type needle valve nozzles, regularly check the internal check mechanism and needle valve to ensure smooth and reliable operation.
The precision of the fit between the nozzle and the mold sprue bushing is also a key factor influencing the injection process. Good concentricity and a close fit should be maintained between the two. Otherwise, pressure loss and leakage of the melt will occur during the injection process, affecting the filling and molding quality of the plastic part. When installing the mold, ensure that the center of the mold sprue bushing is aligned with the center of the nozzle, with a deviation of no more than 0.1mm. At the same time, sufficient clamping force should be applied between the nozzle head and the mold sprue bushing. The magnitude of this clamping force should be determined based on the injection pressure and nozzle diameter. Generally speaking, the clamping force should be greater than the axial force generated by the injection pressure on the nozzle head to prevent melt leakage. In actual production, the clamping force between the nozzle and the mold sprue bushing can be controlled by adjusting the movement distance and pressure of the injection seat to ensure that it is within the appropriate range.
