Industry News

Variable temperature control, so that injection molding technology can be solved

2018-07-30
The automotive and consumer goods industries are increasingly demanding the quality of injectables, so new requirements are placed on the injection molding process, such as seamless surfaces, shorter injection cycles and high molding accuracy – these three challenges require injection molding. Developing a special cooling technology, variable mold temperature control is an efficient solution.
    Variable temperature control tool
    Variable mold temperature control means that the mold temperature is not constant, but is controlled according to a defined temperature profile during an injection cycle. At the time of injection, the temperature is almost as high as the temperature of the injected material; after injection, the mold needs to be rapidly cooled down to quickly solidify the material. Therefore, with this technology, it is possible to produce plastic parts without any streamlined, deformed, high-gloss surface.
    There are many techniques for variable temperature control, such as the use of two cooling circuits (steam/water or oil/oil), liquid cooling combined with induction heating, infrared heating combined with liquid cooling, or electrothermal heating and liquid with heating elements. The combination of cooling, as well as the dynamic mold temperature precision control technology of CPH high performance ceramics or two cycles of temperature control.
    The implementation of dynamic temperature control of injection molding requires the opening of a temperature bath close to the contour of the mold for rapid heating and cooling. Considering the different wall thicknesses everywhere, heat is conducted in different ways in different areas of the workpiece. The more the cooling passages pass through the mold or the closer to the mold cavity, the more accurate the adjustment of the cooling process. The premise is that each cooling channel has its own temperature control. For smaller tools, the channel is located below 1.5 mm of the cavity. The large mold is built in two halves and requires a long temperature control channel along the contour of the tool.
    Closed cavity temperature control
    Variable mold temperature control conflicts with short-cycle time requirements to some extent, but this conflict can be solved by new technologies such as “segmentation tool temperature control”. Using these techniques, the cooling channels will be very close to the mold cavity, and a three-dimensional channel structure can be implemented in the mold to achieve the desired cooling channel configuration.
    Temperature control can be achieved by measuring the in-mold temperature near the mold cavity or measuring the temperature of the refluxing coolant. A thermocouple or resistance thermometer is usually used, such as using a PT100 as a sensor to measure the current temperature and pass this information to a controller, and then adjust the flow of coolant in each channel. The means of regulation is to inject steam or hot water to heat and cool with water. This type of temperature control greatly improves surface quality and significantly reduces cycle time.
    Valve technology innovation
    New developments in proportional solenoid valves have enabled multi-circuit parallel operating temperature regulation circuits to be implemented. The advantage of these valves is their core-free friction-free bearings and the stick-slip effect is avoided by special springs. This is reflected in excellent records, including the most important reaction sensitivities (0.1% of the final value), minimal back error and excellent regulation. The new solenoid valve has a measuring range of 1:100, which allows it to adapt to even very fine temperature changes, such as very subtle changes caused by valve corrective action.
    The decisive test values used to measure the workpiece cooling process are return temperature and effluent flow, which helps to adjust the amount of coolant in an absolutely reliable and accurate manner. The most advanced sensors detect changes between 100 and 300 milliseconds, and the control valve adjusts the flow in 0.3 to 1 second. Impeller sensors are particularly suitable because of their small frame size and excellent response time.
    Actuator engineering
    The choice of valve depends on the flow and the degree of contamination of the coolant. For high flow rates and contaminated liquids, direct acting valves are recommended. Pneumatically operated On/Off or regulating valves are appropriate if the temperature is as high as 180 °C. Depending on the amount of heat lost, if the valve opening time is to be controlled, on/off temperature control or pulse cooling is used during the injection molding cycle cooling phase.
    For low flow rates and clean cooling fluids, such as central water treatment equipment, and the water temperature is below 90 degrees Celsius, the servo-assisted solenoid valve is very good.
    For more or less cooling depending on the nature of the workpiece, the use of multi-channel temperature control, the use of pneumatic control valves, direct acting or servo-assisted solenoid valves provides a clear advantage. Their crack opening can be quickly adjusted between 0-100%. Such dynamic regulating valve technology makes it possible to pre-set the temperature profile according to the workpiece. Quickly achieve temperature regulation protection and enhance valve durability. With pneumatic control valves, the nominal diameter is not limited by fluid pressure. Usually it ranges from 4 to 20 mm or even higher. To adjust the specific crack opening, a complementary electronic control is required. This proportional control valve is realized by pulse adjustment (PWM). Relative to a normal process control valve, this is a positioner control valve assisted by the combined valve position control. It adjusts the process valve piston to a specific crack opening.
     Decentralized mold temperature control
     Typically, an integrated process system includes a flow sensor, proportional valve, and flow controller. The controller plays a decisive role in the system because it must be able to handle different sensor signals, such as temperature, pressure or flow, and must be able to communicate with pneumatic and electrical control systems.
     Its most important features are: temperature regulation; pressure control; flow control; control On/Off solenoid valve, proportional valve, process control valve and electric control valve; input normal sensor signal (standard signal 4 to 20 mA, frequency, PT100) ; communicate with the central control unit for 4-20 mA at the set point, process value feedback; easily set by selected application (flow, temperature or pressure control); memorize data for most valves and sensors. The eCONTROL can also be made into a panel that is perforated according to the 1/16 DIN standard and can be integrated into existing control cabinets. The decentralized mold temperature system simplifies machine control and allows them to focus on their right tasks.
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