The applicability of various plastics to hot injection nozzles
As a core component of a hot runner system, the hot nozzle’s material, structure, and heating method must match the plastic’s characteristics. The differences in melt temperature, fluidity, corrosiveness, and viscosity between different plastics directly determine the applicable hot nozzle type. For example, high-temperature plastics (such as PEEK) require a hot nozzle with a temperature resistance of over 400°C, while corrosive plastics (such as PVC) require the nozzle’s inner wall to be chrome-plated for corrosion protection. Proper hot nozzle selection can reduce problems such as raw material degradation, clogging, and flash, ensuring stable production. Otherwise, it can lead to mold damage or a surge in plastic part defect rates. Understanding the hot nozzle’s applicable requirements for various plastics is a key step in hot runner mold design and production. Hot nozzle parameters must be specifically matched based on the plastic’s processing temperature range, mechanical properties, and chemical characteristics.
Polyolefin plastics (such as PE and PP) have relatively relaxed requirements for hot-dip nozzles. Due to their low melt temperature (160-220°C), good fluidity, and non-corrosive properties, most standard hot-dip nozzles are suitable. These plastics have low melt viscosity and are prone to drooling. Therefore, hot-dip nozzles should utilize a needle-valve design, which closes to prevent material leakage at the gate. For example, hot-dip nozzles for PP bottle cap molds typically feature a pneumatically actuated needle valve that closes immediately after the pressure hold period to prevent drooling and gate blockage. Hot-dip nozzles made of S136 stainless steel can meet these requirements. Heating utilizes an internal heating rod (power density 20-25W/cm²) with a temperature control accuracy of ±2°C. For thin-walled PE parts (thickness <1mm), a large-diameter hot-dip nozzle (8-12mm diameter) is required to reduce flow resistance while reducing heating power to prevent excessive degradation. For example, the nozzle temperature for PE film molds must be strictly controlled at 180-190°C. Deviations exceeding ±5°C can lead to crystallization in the film.
Engineering plastics (such as ABS, PC, and PA) require higher heat resistance and precision in hot-dose nozzles. This is due to their wide melting temperature range (220-320°C), and some materials (such as PA) are hygroscopic and prone to hydrolysis at high temperatures. For ABS plastic, hot-dose nozzles should be made of 316 stainless steel to prevent micro-corrosion at high temperatures. The heating power density should be increased to 25-30W/cm² to ensure that the melt does not cool within the nozzle. For example, the hot-dose nozzle temperature for ABS appliance housing molds must be maintained at a stable 230-250°C. Excessive temperature fluctuations can cause silver streaks to appear on the surface of the plastic part. PC plastic melts at temperatures as high as 280-320°C. Therefore, hot-dose nozzles must be made of a high-temperature resistant alloy (such as H13 steel) and equipped with an independent temperature control system to prevent degradation caused by localized overheating (PC degradation can produce black streaks). The nozzle flow path should also be polished to an Ra of less than 0.2μm to reduce flow resistance. PA plastics require a backflow prevention structure in the hot injection nozzle to prevent bubbles generated by moisture absorption from the melt, which can affect the quality of the plastic part. For example, the hot injection nozzle of a PA66 gear mold requires a check ring and a mold temperature of 120-140°C to reduce the risk of hydrolysis.
High-temperature specialty plastics (such as PEEK, PI, and LCP) place stringent demands on the heat resistance of hot-dose nozzles. Their melting temperatures generally exceed 350°C, and some even reach over 400°C. Conventional hot-dose nozzle materials will soften or oxidize at these temperatures. Hot-dose nozzles for PEEK plastics must be made of nickel-based high-temperature alloys (such as Inconel 718), capable of operating at temperatures up to 420°C. Heating is achieved through embedded heating elements to ensure uniform temperature (with a deviation of less than ±1°C). For example, hot-dose nozzles for PEEK medical component molds require rigorous temperature calibration to prevent localized overheating that could lead to material degradation and the production of toxic substances. Liquid crystal polymer (LCP) exhibits orientation in the molten state, so hot-dose nozzles must be designed with streamlined flow paths, avoiding right angles or abrupt constrictions. The nozzle diameter is determined based on the part thickness (typically 5-8mm), and low shear rates (injection speeds less than 30mm/s) are used to prevent part warping caused by excessive orientation. The hot injection nozzle of high-temperature special plastics also needs to be cleaned of the oxide layer regularly. It needs to be disassembled and inspected every 500 molds to remove the surface oxide scale and ensure heating efficiency.
Corrosive plastics (such as PVC, POM, and fluoroplastics) require extremely high corrosion resistance from hot-dip nozzles. These materials release acidic gases during processing (e.g., HCl produced by PVC decomposition) or are inherently highly polar (e.g., fluoroplastics), which can easily corrode metal surfaces. Hot-dip nozzles for PVC must be chrome-plated (chrome layer thickness > 0.01mm) or constructed of Hastelloy alloy to prevent nozzle leakage caused by HCl corrosion. Heating temperatures must be strictly controlled between 170-190°C. Temperatures exceeding 200°C can exacerbate decomposition. For example, hot-dip nozzles for PVC pipe fitting molds must be equipped with exhaust gas collection devices to promptly exhaust corrosive gases. POM plastics are prone to formaldehyde gas at high temperatures. Hot-dip nozzles must be made of stainless steel and equipped with venting grooves. The runner surface roughness should be below Ra 0.4μm to prevent melt retention and decomposition. The nozzle temperature should be controlled between 190-210°C, and the holding time should be limited to less than 5 seconds. Fluoroplastic (such as PTFE) has extremely high melt viscosity and strong adhesion to metal. The hot nozzle needs to be coated with Teflon to reduce melt adhesion. At the same time, the nozzle diameter needs to be 20% larger than the conventional one (such as 8-10mm) to reduce flow resistance. The heating temperature is determined according to the type (usually 300-380℃).
