Selection of hot injection nozzles
The selection of injection molding hot nozzles is a key step in the design of hot runner systems, directly affecting the surface quality of the product, molding efficiency, and material adaptability. As the final channel for the melt to enter the mold cavity, the hot nozzle must be accurately selected based on factors such as the plastic properties, product structure, and the number of cavities to ensure that the melt enters the mold cavity at the appropriate temperature, pressure, and speed to avoid defects such as excessive gate marks, wire drawing, and burning. Different types of hot nozzles have significant differences in heating methods, sealing structures, and applicable scenarios. Choosing the wrong type may lead to unstable production or scrapped products. Therefore, it is necessary to systematically evaluate various parameters and select the optimal solution.
Hot nozzles can be categorized by heating method into three types: internal, external, and adiabatic. The choice should be based on the thermal stability of the plastic and the required molding temperature. Internally heated hot nozzles incorporate a heating rod embedded within the nozzle, enabling direct heat transfer and high temperature control accuracy (±1°C). These nozzles are suitable for temperature-sensitive plastics (such as PVC and PET), effectively preventing localized overheating and degradation. However, their complex structure, high cost, and maintenance difficulties are significant. Externally heated hot nozzles utilize an external heating coil, offering a large heating area and rapid temperature rise. These nozzles are suitable for high-temperature molding of plastics (such as PC and PA66), and the coils are easily replaceable, resulting in low maintenance costs. However, temperature uniformity is slightly lower (±2°C). Adiabatic hot nozzles lack independent heating mechanisms and rely on the melt’s own heat and insulation to maintain temperature. They are suitable for low-viscosity, high-flow plastics (such as PE and PP). Their simple structure and low cost make them suitable only for short molding cycles, as temperature drops can lead to flow difficulties.
Hot-dip nozzles, categorized by gate type, must be tailored to the surface quality and structural requirements of the product. Common types include needle-valve, open, and latent. Needle-valve hot-dip nozzles use a piston rod to control the gate opening and closing, enabling precise control of the hold time and gate closure. They are suitable for products requiring no gate marks (such as automotive exteriors and medical devices) and effectively prevent wire drawing and flow casting. However, their complex structure and high cost require a timing controller. Open hot-dip nozzles, with their gates directly connected to the mold cavity, offer a simpler structure and lower cost. They are suitable for products with less stringent requirements for gate marks (such as industrial parts and turnover boxes). However, they are prone to gate wire drawing and flash, making them unsuitable for low-viscosity plastics. Submerged hot-dip nozzles, with their gates recessed into the interior or side of the product, create a minimal and concealed gate mark. They are suitable for products requiring high aesthetics (such as appliance housings), but attention must be paid to the strength of the gate location to avoid compromising product performance.
The dimensional parameters of a hot-dip nozzle, primarily including nozzle length, outlet diameter, and heating power, must be determined based on the product weight and cavity size. The nozzle length should match the mold thickness, generally 5-10mm shorter than the mold plate thickness. Excessive length will result in significant heat loss, while too short may lead to unstable installation. For deep-cavity products, extended nozzles up to 100-150mm in length can be used, but this requires increased heating power. The outlet diameter should be calculated based on the product’s minimum wall thickness and injection volume. For small products (weighing less than 50g), a diameter of 0.8-1.5mm is recommended; for medium-sized products (50-200g), a diameter of 1.5-3mm is recommended; and for large products (>200g), a diameter of 3-5mm is recommended. Excessively small outlet diameters can easily lead to shear overheating, while excessively large ones may result in noticeable gate marks. The heating power needs to be determined according to the nozzle volume and the plastic molding temperature. Usually, 1.5-2W of power is required for every cubic centimeter of nozzle volume. For example, for a nozzle with a length of 50mm and a diameter of 15mm, the power is about 15-20W to ensure the heating speed and heat preservation effect.
When selecting materials for hot-dip nozzles, consideration must be given to wear resistance, corrosion resistance, and thermal conductivity. The material of core components directly impacts service life. The nozzle body must be made of high-strength, heat-resistant steel (such as H13, SKD61), with a hardness of HRC50-55 after quenching to resist melt erosion and wear. The inner surface must be polished to below Ra0.2μm to reduce the risk of melt retention and degradation. For processing reinforced plastics containing glass fiber (such as PA66+30% glass fiber), the nozzle outlet must be inlaid with a cemented carbide (such as tungsten steel) with a thickness of 3-5mm to improve wear resistance and prevent expansion of the outlet after long-term use. When processing corrosive plastics (such as POM, PVC), the inner surface of the nozzle must be chrome-plated or made of stainless steel (such as SUS316) to prevent surface damage and melt contamination caused by chemical corrosion.
The selection of a hot nozzle also needs to consider compatibility with the hot runner plate and mold to avoid assembly interference or performance mismatch. The connection method between the nozzle and the hot runner plate must be consistent. Common connections include threaded and flanged connections: threaded connections are suitable for small and medium-sized nozzles. They are easy to install but have limited pressure resistance (<100 bar); flange connections are suitable for large nozzles. They can withstand high pressure (>150 bar) but have a complex structure. The positioning method of the nozzle must be coordinated with the mold guide pin and sleeve system to ensure that the coaxiality error with the cavity does not exceed 0.05mm to avoid eccentricity or flash of the product caused by off-center loading. In addition, the rated parameters of the nozzle must be selected based on the maximum injection pressure and flow rate of the injection molding machine. The maximum operating pressure of the nozzle should be at least 20% higher than the actual injection pressure to ensure safety and reliability. By comprehensively evaluating the above factors, the selected hot nozzle can achieve efficient and stable production while ensuring product quality, reducing maintenance costs and the risk of failure.
