There are basically two types of mold temperature control machines on the market, one using water as the medium and the other using heat transfer oil as the medium. The difference between these two types of mold heaters is that the advantages of using water as a medium (referred to as a water heater or water mold heater) are (1) no pollution. (2) Low operating costs and almost no money to buy media. (3) Fast response to temperature rise. The advantages of using heat transfer oil as the medium (referred to as oil temperature machine or oil mold temperature machine for short) include: (1) It is safer to operate at high temperatures and low pressures. (2) It can achieve a larger temperature control range than the water mold temperature machine. The function of the mold temperature machine is to heat and increase the mold temperature or cool and reduce the mold temperature to reach a reasonable temperature and control the injection working temperature, ensuring stable quality of injection molded parts and optimizing processing time. In the injection molding industry, mold temperature plays a decisive role in the quality and injection time of injection molded parts. This article will explain the application of mold temperature machines in injection molding from the following seven aspects: thermal balance of injection molding; The purpose of controlling mold temperature and the impact of mold temperature on injection molded parts; Preparation conditions for effective control of mold temperature; Mold temperature machine; Heat conducting fluid; Advantages and economy of mold temperature machines.
1. Controlling the heat balance of injection molding machines and the heat conduction of molds is the key to producing injection molded parts. Inside the mold, the heat brought by plastics (such as thermoplastics) is transferred to the material and mold steel through thermal radiation, and to the thermally conductive fluid through convection. In addition, heat is transferred to the atmosphere and mold base through thermal radiation. The heat absorbed by the heat conducting fluid is taken away by the mold thermostat. The heat balance of the mold can be described as P=Pm Ps. Where P is the heat taken away by the mold heater; Pm is the heat introduced by the plastic; Ps is the heat emitted by the mold into the atmosphere.
2. The purpose of controlling mold temperature is thermoplasticity and the impact of mold temperature on injection molded parts. In injection molding processes, the main purpose of controlling mold temperature is to heat the mold to operating temperature, and to maintain a constant mold temperature at operating temperature. If the above two points are successful, the cycle time can be optimized to ensure stable and high-quality injection molded parts. Mold temperature can affect surface quality, fluidity, shrinkage, injection cycle, and deformation. Excessive or insufficient mold temperature can have different effects on different materials. For sexual plastics, higher mold temperatures typically improve surface quality and fluidity, but can extend cooling time and injection molding cycles. A lower mold temperature will reduce the shrinkage within the mold, but will increase the shrinkage of the molded part after demolding. For thermosetting plastics, higher mold temperatures typically reduce cycle time, and the time is determined by the time required for the part to cool. In addition, in the processing of plastics, higher mold temperatures will also reduce the plasticizing time and the number of cycles.
3. The temperature control system consists of three parts: a mold, a mold temperature machine, and a heat conducting fluid. To ensure that heat can be added to or removed from the mold, each part of the system must meet the following conditions: first, within the mold, the surface area of the cooling channel must be large enough, and the diameter of the flow channel must match the capacity of the pump (pump pressure). The temperature distribution in the mold cavity has a significant impact on the deformation and internal pressure of the part. Reasonable setting of cooling channels can reduce internal pressure, thereby improving the quality of injection molded parts. It can also shorten cycle time and reduce product costs. Secondly, the mold temperature machine must be able to keep the temperature of the heat conducting fluid constant within the range of 1 ℃ to 3 ℃, depending on the quality requirements of the injection molding part. The third is that heat conducting fluids must have good thermal conductivity, and most importantly, they must be able to introduce or export a large amount of heat in a short time. From a thermodynamic perspective, water is significantly better than oil.
4. Working principle: The mold temperature machine is composed of water tank, heating and cooling system, power transmission system, liquid level control system, temperature sensor, injection port, and other components. Typically, a pump in a power transmission system causes hot fluid to reach the mold from a water tank equipped with a built-in heater and cooler, and then return from the mold to the water tank; A temperature sensor measures the temperature of the hot fluid and transmits the data to a controller of the control section; The controller regulates the temperature of the hot fluid, thereby indirectly regulating the temperature of the mold. If the mold temperature machine is in production and the mold temperature exceeds the set value of the controller, the controller will open the solenoid valve and connect the water inlet pipe until the temperature of the hot liquid, that is, the mold temperature, returns to the set value. If the mold temperature is below the set value, the controller will turn on the heater.
5. The types of mold heaters are divided according to the heat transfer fluid (water or heat transfer oil) used. The maximum outlet temperature of a water transporting mold heater is usually 95 ℃. The oil transporting mold temperature machine is used for occasions where the working temperature is ≥ 150 ℃. Generally, a mold heater with an open water tank for heating is suitable for water or oil heaters, with a maximum outlet temperature of 90 ℃ to 150 ℃. The main characteristics of this mold heater are simple design and economical price. On the basis of this machine, a new type of high-temperature water heater has been developed, which can allow an outlet temperature of 160 ℃ or higher. Due to the fact that when the temperature is higher than 90 ℃, the thermal conductivity of water is much better than that of oil at the same temperature, this type of machine has outstanding high-temperature working capacity. In addition, there is also a forced flow mold temperature machine. For safety reasons, this mold temperature machine is designed to operate at a temperature above 150 ℃ and uses heat transfer oil. In order to prevent overheating of the oil in the mold heater, the machine uses a forced flow pumping system, and the heater consists of a certain number of stacked tubes, in which there are heating elements equipped with fins for guiding water. heater
6. Controlling the non-uniformity of temperature at various points in the mold is also related to the time points in the injection cycle. After injection, the temperature of the mold cavity rises to the maximum, when the hot melt hits the cold wall of the mold cavity, and when the part is removed, the temperature decreases to the minimum. The function of the mold temperature machine is to keep the temperature constant at θ 2 min and θ Between 2max, which means preventing temperature difference △ θ W fluctuates up and down during the production process or gap. The following control methods are applicable to controlling mold temperature: Controlling fluid temperature is the most commonly used method, and the control accuracy can meet the requirements of most situations. Using this control method, the temperature displayed on the controller is not consistent with the mold temperature; The temperature fluctuation of the mold is quite large because the thermal factors affecting the mold are not directly measured and compensated for. These factors include changes in the injection cycle, injection speed, melting temperature, and room temperature. The second is the direct control of mold temperature. This method is to install a temperature sensor inside the mold, which is only used when the mold temperature control accuracy requirements are relatively high. The main features of mold temperature control include: the temperature set by the controller is consistent with the mold temperature; The thermal factors affecting the mold can be directly measured and compensated. Generally, mold temperature stability is better than by controlling fluid temperature. In addition, mold temperature control has good repeatability in production process control. The third is joint control. Joint control is a synthesis of the above methods, which can simultaneously control the temperature of the fluid and mold. In joint control, the position of the temperature sensor in the mold is extremely important. When placing the temperature sensor, it is necessary to consider the shape, structure, and location of the cooling channel. In addition, temperature sensors should be placed where they play a decisive role in the quality of the injection molded part. There are many ways to connect one or more mold temperature machines to the injection molding machine controller. Digital interfaces such as RS 485 are best used for operability, reliability, and anti-interference considerations. Information can be transferred between the control unit and the injection molding machine through software. The mold temperature machine can also be automatically controlled.
7. The configuration and use of the mold temperature machine should be comprehensively determined based on the processed material, mold weight, required preheating time, and productivity kg/h. When using heat transfer oil, the operator must comply with the following safety regulations: Do not place the mold heater near a heat source furnace; The joints shall be made of tapered leakproof and temperature and pressure resistant hoses or hard pipes; Regularly check the temperature control circuit, mold temperature machine, connectors, and molds for leaks, and whether their functions are normal; Regularly replace the heat transfer oil; Synthetic oil should be selected with good thermal stability and low coking tendency. In the use of mold warmers, the selection of suitable heat transfer fluids is crucial. Using water as a heat transfer fluid is economical, clean, and easy to use. Once a temperature control circuit, such as a hose coupling, leaks, the water flowing out can be directly discharged into the sewer. However, using water as a 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 a decrease in the efficiency of heat exchange between the mold and the fluid. When using water as a heat transfer fluid, the following precautions should be considered: pre-treatment of the temperature control circuit with anti-corrosion agents; Use a filter before the water inlet; Regularly clean the water heater and mold with rust remover. When using heat transfer oil, there is no disadvantage of water. Oil has a high boiling point and can be used at temperatures above 300 ℃ or even higher, but the heat transfer coefficient of heat transfer oil is only 1/3 of that of water, so oil temperature machines are not as widely used in injection molding as water temperature machines.