Heat conduction of injection molding machines and molds is the key to production of injection molded parts. Inside mold, heat brought by plastic (such as thermoplastic) is transferred to material and steel of mold through thermal radiation, and to heat transfer fluid through convection.
In addition, heat is transferred to atmosphere and formwork through thermal radiation. Heat absorbed by heat transfer fluid is taken away by mold temperature machine. Thermal balance of mold can be described as: P = Pm-Ps. Where P is the heat taken away by mold temperature machine; Pm is the heat introduced by plastic; Ps is the heat dissipated from mold to atmosphere.

Temperature control system consists of three parts: mold, mold temperature machine, and heat transfer fluid. In order to ensure that heat can be added to mold or removed, each part of system must meet following conditions: First, inside mold, surface area of cooling channel must be large enough, and diameter of runner must match capacity of pump (pump pressure).
Temperature distribution in cavity has a great influence on deformation of parts and internal pressure. Reasonable setting of cooling channels can reduce internal pressure, thereby improving quality of injection molded parts. It can also shorten cycle time and reduce product cost.
Secondly, mold temperature machine must be able to keep temperature of heat transfer fluid constant within range of 1 ℃ -3 ℃, depending on quality requirements of injection molded parts. Thirdly, heat transfer fluid must have good thermal conductivity, and most importantly, it must be able to import or export a large amount of heat in a short time.
From a thermodynamic point of view, water is significantly better than oil.

 

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 devices. Normally, pump in power transmission system causes hot fluid to reach mold from water tank equipped with built-in heater and cooler, and then return to water tank from mold; Temperature sensor measures temperature of hot fluid and transmits data to controller of control section; controller adjusts temperature of hot fluid, thereby indirectly adjusting temperature of mold.
If mold temperature machine is in production, temperature of mold exceeds set value of controller, controller will open solenoid valve to connect inlet pipe until temperature of hot fluid, that is, temperature of mold returns to set value. If mold temperature is lower than set value, controller will turn on heater.

Maximum outlet temperature is usually 95 ℃ when using a water transport mold temperature machine. Oil-transporting mold temperature machine is used for working temperature ≥150 ℃. Generally, mold temperature machine with open water tank heating is suitable for water temperature machine or oil temperature machine. Maximum outlet temperature is 90 ℃ to 150 ℃. Main features of this mold temperature machine are simple design and economic price.
On the basis of this machine, a high temperature water temperature machine is derived. Its allowable outlet temperature is 160 ℃ or higher. Since heat conductivity of water is much better than oil at the same temperature when temperature is higher than 90 ℃, this machine has outstanding high temperature working ability.
In addition, there is a forced flow mold temperature machine. For safety reasons, this mold temperature machine is designed to work at temperatures above 150 ° C and uses thermal oil. In order to prevent the oil in the heater of mold temperature machine from overheating, machine uses a forced flow pumping system, and heater is composed of a certain number of tube stacks, there are heating elements equipped with fins in tubes for diversion.

After injection, temperature of mold cavity rises to the highest. At this time, hot melt hits cold wall of mold cavity, and temperature drops to the minimum after parts are removed. Function of mold temperature machine is to keep temperature constant between θ2min and θ2max, which means to prevent temperature difference Δθw from fluctuating up and down during production process or gap.
Following control methods are suitable for controlling temperature of mold: controlling temperature of fluid is the most commonly used method, and control accuracy can meet requirements of most situations. Using this control method, temperature displayed on the controller is not consistent with mold temperature. Temperature fluctuation of mold is quite large. Thermal factors that affect mold are not directly measured and compensated. These factors include changes in injection cycle, injection speed, melting temperature, and room temperature. Second is direct control of mold temperature. This method is to install a temperature sensor inside mold, which is only used when mold temperature control accuracy is relatively high. Main features of mold temperature control include: temperature set by controller is consistent with mold temperature; thermal factors that affect mold can be directly measured and compensated. Generally, stability of mold temperature is better than by controlling fluid temperature. In addition, mold temperature control has better repeatability in production process control. Third is joint control. Joint control is synthesis of above methods, it can control temperature of fluid and mold at the same time. In joint control, position of temperature sensor in mold is extremely important. When placing temperature sensor, shape, structure, and position of cooling channel must be considered. In addition, temperature sensor should be placed where quality of injection molded parts is decisive. There are many ways to connect one or more mold temperature machines to injection molding machine controller. Considering operability, reliability and anti-interference, it is best to use a digital interface, such as RS485. Information can be transferred between control unit and injection molding machine via software. Mold temperature machine can also be automatically controlled.

 

Configuration of mold temperature machine should be comprehensively judged according to processed materials, weight of mold, required warm-up time and productivity kg / h. When using thermal oil, operator must comply with such safety regulations:
Do not put mold temperature machine near heat source stove; joints use tapered leak-proof and temperature- and pressure-resistant hoses or hard tubes; regularly check temperature control circuit mold temperature machine, whether joints and molds are leaking, whether function is normal; regularly replace thermal oil; artificial synthetic oil should be used, which has good thermal stability and little tendency to coking.
In the use of mold temperature machine, choosing right thermal fluid is extremely critical. Using water as a heat transfer fluid is economical, clean, and easy to use. Once temperature control circuit such as a hose coupling leaks, water that flows out can be directly discharged into sewer. However, use of water as a heat transfer fluid also has disadvantages: boiling point of water is low; depending on composition of water, it may corrode and scale, causing increased pressure loss, reduced heat exchange efficiency between mold and fluid, and so on. When using water as a heat transfer fluid, following precautions should be considered: pre-treating temperature control circuit with an anti-corrosion agent; using a filter in front of water inlet; regularly cleaning water temperature machine and mold with rust removing agent.
There is no disadvantage of water when using thermal oil. Boiling point of oil is high, they can be used at temperatures higher than 300 ℃ or higher, but heat transfer coefficient of heat transfer oil is only 1/3 of water, so oil temperature machine is not widely used than water injection machine.