Heating of injection molds
Heating injection molds is a crucial process for ensuring smooth melt filling and high-quality finished products. This is particularly true for high-viscosity plastics, large, complex products, or when controlling melt flow properties is crucial. The design of the mold heating system must be tailored to the plastic’s characteristics, product structure, and production efficiency requirements. The appropriate heating method and power configuration ensure uniform and stable mold temperature, preventing product defects caused by localized temperature differences. A suitable heating scheme not only improves melt flow but also reduces internal stress and enhances product dimensional accuracy.
Common heating methods for injection molds include electric heating, hot oil heating, and steam heating. Electric heating is widely used due to its simple structure and precise temperature control. Electric heating elements primarily include heating rods, heating coils, and heating plates. Heating rods are suitable for deep cavities or localized heating, such as heating the interior of a core. Their diameter is typically 8-16 mm, and their power density is 10-20 W/cm². Heating coils are typically used to heat the barrel or mold edges and can be customized to suit the mold shape. Their power density is 5-15 W/cm². Heating plates are suitable for heating large areas, such as the mold surface. They are generally 10-20 mm thick and provide even power distribution. For example, a large automobile bumper mold utilizes a combination of heating rods and heating plates. Twenty 12 mm diameter heating rods (total power 8 kW) are inserted into the core, and two heating plates (total power 12 kW) are installed at the bottom of the mold. This allows for precise control of the mold temperature within a range of 80-120°C.
Hot oil heating is suitable for molds that require uniform heating over a large area. Heat is transferred to the mold through a hot oil circulation system, offering a wide temperature control range (50-300°C) and excellent thermal stability. The hot oil heating system consists of a heating oil tank, a circulation pump, piping, and a thermostat. The piping must be evenly distributed within the mold, with spacing of 50-100mm to ensure uniform heat transfer. For example, a polycarbonate (PC) mold uses hot oil heating with a 50L tank capacity, 15kW heating power, a 20L/min circulation pump flow rate, and a 10mm diameter piping. This serpentine layout keeps the temperature differential within ±2°C across all mold zones, effectively addressing the poor flowability of the PC melt. However, hot oil heating systems require high initial investment, and the piping is prone to leaks, resulting in relatively high maintenance costs.
Steam heating is suitable for high-temperature heating needs (100-180°C). It utilizes the latent heat of saturated steam to heat the mold, resulting in rapid temperature rise and high thermal efficiency. A steam heating system requires a boiler, steam valve, and condensate recovery device. Piping must be constructed of high-temperature-resistant stainless steel with a wall thickness of at least 2mm to prevent leakage of high-pressure steam. For example, a nylon (PA) product mold is heated using 0.6MPa saturated steam. The mold temperature quickly rises to 120°C, reducing the temperature rise time by 40% compared to electric heating and significantly improving production efficiency. However, steam heating has low temperature control accuracy, typically within ±5°C, and is not suitable for applications requiring precise temperature control. It is primarily used for simple products produced in large quantities.
The temperature control of the mold heating system needs to work in conjunction with the cooling system to form a closed-loop control. The accuracy of the temperature controller should reach ±1°C, and it should be equipped with multiple temperature measuring points to monitor the temperature of various areas of the mold in real time, such as the cavity surface, the inside of the core, near the gate, etc. For temperature-sensitive plastics, such as polyvinyl chloride (PVC), an overheating protection device must be installed in the heating system. When the temperature exceeds the set value by 10°C, the power will be automatically cut off to prevent the plastic from decomposing. In addition, the layout of the heating elements must avoid the cooling water channel and the ejection mechanism, and the distance between the two must be no less than 15mm to avoid heat interference. The layout of the heating elements can be optimized through simulation analysis software (such as Moldflow) to ensure a uniform temperature field. For example, a precision gear mold adjusted the position of the heating rod through simulation to reduce the temperature difference of the tooth surface from ±5°C to ±1°C, significantly improving the dimensional accuracy of the product.
In practice, heating parameters must be dynamically adjusted based on production needs. When weld marks appear on a product, the temperature near the gate can be raised by 10-15°C to enhance melt fusion. When cold spots appear on the product surface, the heating elements in that area must be checked for proper operation, and higher-powered heaters should be replaced if necessary. The heating system’s energy consumption must also be considered. Using zoned heating and intelligent temperature control technology to reduce heating power during non-production periods can save over 30% of energy. By continuously optimizing the heating solution, precise mold temperature control can be achieved, ensuring the production of high-quality products.
