Core Tip: Thermal balance of the injection mold control The heat transfer of the injection molding machine and mold is the key to producing injection molded parts.
1. Thermal balance of injection mold control The heat transfer of injection molding machines and molds is the key to producing injection molded parts. Inside the mold, heat from plastics (such as thermoplastics) is transferred by heat radiation to the steel of the material and the mold, and is transmitted to the heat transfer fluid by convection. In addition, heat is transferred to the atmosphere and the mold frame by thermal radiation. The heat absorbed by the heat transfer fluid is taken away by the mold warmer. The heat balance of the mold can be described as: P=Pm-Ps. In the formula, P is the heat taken away by the mold temperature machine; Pm is the heat introduced by the plastic; Ps is the heat that the mold disperses to the atmosphere.
2. Pre-conditions for effective control of mold temperature The temperature control system consists of three parts: mold, mold temperature, and heat-conducting fluid. In order to ensure that the heat can be added to the mold or removed, all parts of the system must meet the following conditions: First, inside the mold, the surface area of the cooling channel must be large enough to match the pump's capacity (pump pressure). The temperature distribution in the cavity has a great influence on the part deformation and the internal pressure. Properly setting the cooling channels can reduce the internal pressure and improve the quality of the injection molded parts. It also reduces cycle times and reduces product costs. Followed by the mold temperature machine must be able to make the temperature of the heat transfer fluid constant within the range of 1 °C -3 °C, according to the quality requirements of the injection molded parts. The third is that the heat transfer fluid must have good thermal conductivity, and most importantly, it must be able to introduce or export a large amount of heat in a short time. From a thermodynamic point of view, water is significantly better than oil.
3, the working principle of the mold temperature from the water tank, heating and cooling systems, power transmission systems, liquid level control systems and temperature sensors, injection ports and other devices. Usually, the pump in the power transmission system causes the hot fluid to reach the mold from the water tank with the built-in heater and cooler, and then returns from the mold to the water tank; the temperature sensor measures the temperature of the hot fluid and transmits the data to the control part. Controller; The controller regulates the temperature of the hot fluid and indirectly adjusts the temperature of the mold. If the mold temperature machine is in production, and the temperature of the mold exceeds the set value of the controller, the controller will open the solenoid valve to connect the inlet pipe until the temperature of the hot fluid, ie, the temperature of the mold returns to the set value. If the mold temperature is lower than the set value, the controller turns on the heater.
4, the type of mold temperature machine is based on the use of heat transfer fluid (water or heat transfer oil) to divide. With water transport mold temperature machine usually the maximum outlet temperature of 95 °C. Use oil-type mold temperature machine for operating temperature ≥ 150 °C occasions. Under normal circumstances, the die temperature machine with open tank heating is suitable for water temperature or oil temperature machine. The maximum outlet temperature is 90°C to 150°C. The main features of this type of die temperature machine are simple design and economical price. Based on this type of machine, a high-temperature water temperature machine has been derived. Its allowable outlet temperature is 160°C or higher. Since the temperature is higher than 90°C, the thermal conductivity of water is lower than that of oil at the same temperature. A lot better, so this machine has outstanding high-temperature working ability. In addition to the second, there is a forced flow die temperature machine. For safety reasons, this die temperature design operating temperature is 150 °C or more, the use of heat transfer oil. In order to prevent overheating of the oil in the heater of the die warmer, the machine uses a forced-flow pumping system. The heater consists of a stack of tubes with finned heating elements for diversion.
5, control the uneven temperature within the mold, but also with the injection cycle of the time point. After the injection, the temperature of the cavity rises to the maximum, and the hot melt hits the cold wall of the cavity. When the parts are removed, the temperature drops to a minimum. The function of the mold temperature controller is to keep the temperature constant between θ2min and θ2max, that is to prevent the temperature difference Δθw from fluctuating above and below the production process or the gap. The following several control methods are suitable for controlling the temperature of the mold: controlling the fluid temperature is the most common method, and the control accuracy can meet most of the requirements. Using this control method, the temperature displayed in the controller does not agree with the mold temperature; the temperature fluctuation of the mold is quite large, and the thermal factors affecting the mold are not directly measured and compensated. These factors include changes in injection cycle, injection speed, melting temperature, and room temperature . The second is the direct control of the mold temperature. The method is to install a temperature sensor inside the mold, which will be used when the mold temperature control accuracy requirements are relatively high. The main features of the mold temperature control include: the temperature set by the controller is consistent with the mold temperature; the thermal factors that affect the mold can be directly measured and compensated. In general, mold temperature stability is better than by controlling fluid temperature. In addition, mold temperature control is more reproducible in production process control. The third is joint control. Joint control is a combination of the above methods, it can control the temperature of the fluid and mold at the same time. In joint control, the position of the temperature sensor in the mold is extremely important. When placing the temperature sensor, the shape, structure, and position of the cooling channel must be considered. In addition, the temperature sensor should be placed in a place that is decisive for the quality of the injection molded part. There are many ways to connect one or more mold temperature controllers to the injection molding machine controller. It is best to use a digital interface such as RS485 in terms of operability, reliability, and anti-jamming. Information can be transmitted via software between the control unit and the injection molding machine. The mold temperature controller can also be controlled automatically.
6, the configuration and use of mold temperature machine The configuration of the mold temperature machine should be based on the processing of materials, the weight of the mold, the required warm-up time and productivity kg / h to comprehensive judgment. When using heat transfer oil, the operator must comply with such safety regulations: Do not place the mold warmer close to the heat source stove; Taper leakproof and hose with temperature and pressure resistance or hard pipe; Check regularly Temperature control circuit mold temperature machine, joints and molds have no leakage, function is normal; regular replacement of heat transfer oil; artificial synthetic oil should be selected, good thermal stability, coking tendency is small. In the use of mold temperature machine, choose the appropriate heat transfer fluid is critical. The use of water as the heat transfer fluid is economical, clean, and easy to use. Once the temperature control loop leaks, such as the hose coupling, the water flowing out can be directly discharged into the sewer. However, the use of water as the heat-conducting fluid also has disadvantages: the boiling point of water is low; depending on the composition of the water, corrosion and scaling may occur, resulting in increased pressure loss and decreased heat exchange efficiency between the mold and the fluid. When using water as a heat-conducting fluid, the following preventive measures should be taken into consideration: pre-treating the temperature control circuit with an anticorrosive agent; using a filter before the water inlet; periodically cleaning the water temperature machine and the mold with a rust-removing agent. There is no water disadvantage when using heat transfer oils. The high boiling point of oil, they can be used for temperatures higher than 300 °C or even higher, but the heat transfer coefficient of heat transfer oil is only 1/3 of the water, so the oil temperature machine has no water temperature in the injection molding machine widely [3] .
Two-color injection molding
The so-called two-color injection molding refers to a molding method in which two different color plastics are injected into the same mold. It can make plastic parts appear in two different colors, and can make plastic parts show a regular pattern or irregular moire patterns, in order to improve the practicality and aesthetics of plastic parts.
(1) Double color injection molding
Figure 1 shows the principle of double-mix injection molding. It has two barrels. The structure and use of each barrel is the same as the ordinary injection molding barrel. Each barrel has its own channel that communicates with the nozzle, and an on-off valve 2, 4 is provided in the nozzle path. During molding, after the melt is plasticized in the barrel, the sequence of the melt entering the nozzle and the ratio of the discharged material are controlled by the opening and closing valves 2 and 4 and injected into the mold cavity by the nozzle. Different plastic products with different color mixing effects can be obtained.
