Automotive quality standards have led to improvements in diecasting processes, specifically in the control of the mold temperature. Processes need to be exactly reproducible and retraceable, which essentially requires a reliable method for fine-tuning the cooling parameters for castings molds. Realizing such highly sophisticated cooling systems depends on state-of-the-art valve and sensor technology customized according for the application.
Modern automated pressure casting allows efficient production of a comprehensive series of lightweight and thin-walled castings with small tolerances, smooth surfaces, and precise shapes. Very sophisticated molds made from hot-forming steel even allow for more than 100,000 casts. Depending on the casting process (lowpressure or high-pressure diecasting), the pressure for injecting the liquid metal can vary between 0.7 and 200 MPa. Injecting into preheated steel molds ensures that even the finest structures are filled and the fabrication of very thin-walled and delicate products with a homogeneous surface is possible. Computer-controlled pressure casting equipment with automated mold temperature controls guarantee a reproducible and consistent quality of items produced in much higher quantities than is possible with standard equipment.
Controlled cool-down
Heat from the injected molten metal is conducted into the mold. It must be able to absorb and dissipate a considerable amount of heat to allow for the product to cool down and solidify in a controlled manner. Controlling the molding tools’ temperature using oil, water or an air/water mixture, provides the means for precise metallurgical control of the part. This tight temperature control enhances the reproducibility of the parts’ crystalline structure, as well as increasing process efficiency and reducing part defects due to the minimizing inner tensions and material porosity.
The precise regulation of the mold’s temperature has proven to be a crucial step for ensuring tension-free cooling of the workpieces. Cooling channels just beneath the surface of the mold’s formative structure provide the best path for good heat dissipation and relatively constant temperatures within the mold. The structure of those cooling channels and the maximum temperature of the mold are defined by the shape of the workpiece and by the cooling liquid in use. Low-inflamation synthetic oils enable coolant temperatures to rise as high as 350°C (662°F). They allow for an initial heating of the whole mold and for a constant warming of cooler areas. Water, or an air/water mixture for coolant temperatures up to 160°C (320°F), is far more efficient for heat dissipation and more economical than oil. Consequently, water or an air/water mixture is preferable to oil whenever it’s necessary to dissipate a high level of heat within a short period of time.
The number, design, and placement of the cooling channels within the diecasting mold determine efficient temperature regulation. Various small channels enable a more precise temperature regulation than a smaller number of larger conduits. Advanced diecasting normally features five to 30 (sometimes even up to 100) of these cooling channels in one mold.
The most important parameter to ensure a consistent cooling process of the workpiece is the outlet water temperatures or inlet air/water flow. “The feed coolant flow and temperature should be measured and controlled for every single cooling channel, making it necessary to have a valve and a sensor in each line to set the correct flow,” explains Joe Vandehey of Brkert Fluid Control Systems. “In order to improve the dynamics of temperature regulation, it is also permissible to measure the effluent temperature of the coolant and set the temperature with a cascadedflow control loop.”
State-of-the-art sensors detect flow changes within 100 to 300 milliseconds, and control valves immediately adjust the flow rate within 0.3 to 1 seconds. This control process significantly enhances temperature stability over that of a temperature-only feedback control loop. Basically, the choice of sensors and control valves depends on process dynamics and fluidic cycle time.
Advanced valves and sensors
Several independent cooling circuits arranged in parallel permit a more precise regulation of the cooling process, and thus improve workpiece quality. The latest development of proportional solenoid valves and controllers by Brkert makes it possible to implement multi-loop parallel-working temperature regulation circuits. The advantage of these valves lies in the frictionless bearing of the magnetic core and specially formed springs that avoid stick-slip effects.
Propelling this development is past performance history and outstanding response sensitivity (0.1% of the final value), minimal reversal errors, and excellent valve regulation repeatability. The turndown ratio for these new proportional solenoid valves is 1:100, making it possible to adjust to slight shifts in temperature or flow by very precise corrective moves of the valves. In combination with the new ultrasonic flow meter, which starts measuring at 0.06 l/min with a turndown ratio of 1:200, it’s possible to inject very precisely small amounts of water into an air stream for cooling the mold. Ambient conditions in diecasting plants are harsh: the air contains fine particles and operating temperatures are very high due to the liquid metals. Valves that regulate the influent fluid to the cooling channels of the mold therefore need to be environmentally resistant, both internally and externally. So, when determining actuator and associated sensor technology, polyphenylene sulfide (PPS), a thermoplastic polymer, is the material of choice.
Proper valve selection depends on the flow rate and the degree of coolant fluid contamination. “With high flow rates and dirty liquids, direct-acting valves are more recommended. In applications with temperatures up to 180°C (356°F), pneumatically operated on/off or regulating valves are the best technology,” says Vandehey. With low flow rates and very clean cooling fluids (i.e. if there is a central water-treatment unit involved) and with water temperatures below 90°C (194°F), servo-assisted or direct acting electromagnetic valves are perfect. Those valves are available in nominal diameters from 0.05 to 20 mm. Brkert’s latest on/off technology includes servo-assisted valves without these small orifices in the diaphragm. This makes the valve less sensitive to unforeseen contamination from the cooling fluids.
With multiple-channel temperature control that involves more or less cooling per channel depending on the physical dynamics of the workpiece, regulating valves — direct acting or servo assisted — offer a clear advantage. The opening of the valve can be quickly regulated to any value between 0 and 100%. This is how dynamic regulating valve technology permits the use of flow profiles that are pre-programmed and tailored to the workpiece. With pneumatically operated valves, nominal diameters are not limited by fluid pressure. They can range from 4 to 50 mm and the valves can be placed very closed to the molding die due to the high temperature stability.
Calibrated control
For managing the water or air/water flow, special controllers are necessary to set the water flow or to set the ratio between the airflow and water flow.
Brkert provides all the relevant actuators, sensors and controllers for decentralized control of temperature regulation for a broad range of diecasting molds. Decentralizing temperature control of the mold allows quick and easy retrofitting. It requires only simple optimization measures to bring existing facilities up-to-date.
The company offers a variety of valves, sensors and controllers for gas, liquid or a combination of both. The usual integrated flow system consists of a flow sensor (paddle wheel or ultrasonic), a control valve (electric or pneumatic) and a flow rate PI or cascaded temperature controller that can be combined with various valve types. The overall controller (for setting the flow rate and/or temperature) plays a decisive role within the system as it has to be able to process different sensor signals, such as temperature, pressure, and flow rate. The controller also must be diverse enough to control either a pneumatic or electric control loop. Brkert’s new universal controller eCONTROL 8611 encompasses all of these qualities. (eCONTROL is available also as a 1/16DIN panel version that can be integrated into an existing control cabinet.) Among its features are: temperature regulation, pressure control, and flow-rate control.
Also, the device controls on/off electromagnetic solenoid valves, proportional valves, process control valves and electromotive regulation valves. It recognizes normal sensor signals (standard 4-20mA, frequency, PT 100); and it communicates with the central control unit at 4-20 mA for setpoint and process value feedback. The developers indicate eCONTROL is easy to set up according to the selected application (flow, temperature or pressure control), and data for most Brkert valves and flow sensors are stored in the controller.