In the actual injection molding production process, different types of plastic raw materials have huge differences in melting point, fluidity, thermal stability, high-temperature corrosion resistance and filling modification characteristics, and blindly selecting the same type of hot runner system will easily lead to many production problems such as raw material decomposition, poor molding fluidity, serious equipment wear and unqualified product quality. Therefore, manufacturers need to select targeted hot runner structure type, internal runner design scheme and matched supporting accessories including thermocouples according to the physical and chemical properties of production raw materials, so as to realize the perfect matching between hot runner equipment and plastic raw materials.
For conventional general-purpose plastics such as PP, PE, ABS and ordinary PVC, such raw materials have low melting point, good fluidity, low requirement for heating temperature and strong thermal stability, and are not easy to decompose in a reasonable temperature range. In view of such raw materials, enterprises can choose cost-effective open hot runner systems or simple low-cost valve gate hot runner systems. In terms of matching temperature sensing accessories, ordinary standard K-type hot runner thermocouples with conventional temperature resistance can meet the use demands. The installation mode can adopt embedded fixed type with simple structure, which can stably complete daily temperature monitoring work, effectively control production cost on the premise of ensuring basic molding quality, and is very suitable for mass production of ordinary household daily necessities and low-demand plastic parts. It should be noted that soft PVC materials are easy to produce corrosive gas at high temperature, so it is necessary to select corrosion-resistant outer shell thermocouples to avoid rapid aging and damage of temperature measuring accessories.
For high-temperature engineering plastics such as PC, POM, PPS, PET and PA66, such raw materials have high melting point, narrow safe molding temperature range, poor high-temperature thermal stability, and are easy to produce decomposition discoloration, performance decline and harmful gas precipitation once exceeding the limit temperature. When matching hot runners, it is necessary to select high-temperature resistant integrated hot runner systems with uniform heating effect and partition precise temperature control function, and priority should be given to valve gate hot runners with stable flow control effect. The matched thermocouples must select high-precision high-temperature resistant models with temperature resistance above 450℃, which can accurately capture tiny temperature changes in real time, assist the temperature controller to strictly control the hot runner internal temperature within the safe molding interval of raw materials, avoid material thermal decomposition, and ensure the mechanical properties and surface quality of finished products. For modified engineering plastics added with glass fiber, talc powder and other fillers, it is also necessary to match wear-resistant hot runner nozzles and anti-scratch protective thermocouple accessories to reduce equipment wear.
For heat-sensitive special plastics and biodegradable environmental protection plastics, such raw materials have extremely poor high-temperature resistance, short safe heating residence time, and are easy to age and deteriorate after long-time heating. It is necessary to select low-shear streamlined hot runner systems with short runner stroke and fast discharging speed, minimize the residence time of raw materials in the hot runner runner, and adopt multi-point distributed layout of high-sensitivity fast response thermocouples to realize all-round temperature monitoring, avoid local dead angle overheating, and strictly control the overall heating temperature of hot runners.
In addition, for transparent plastic products with high surface light transmission requirements and flame-retardant modified plastics with special functional requirements, it is also necessary to optimize hot runner internal polishing treatment scheme and temperature regulation scheme, and select thermocouples with stable signal and no temperature drift. To sum up, material property is the core basis for selecting hot runner systems and supporting thermocouple specifications, only by selecting equipment in a targeted manner can we give full play to the performance advantages of hot runners, stabilize product molding quality and extend the overall service life of hot runner equipment and supporting accessories.
