Casting is the earliest metal hot forming process that humans have mastered. It has a history of about 6,000 years and is the basic process of modern machinery manufacturing industry. The casting process is the process of melting the metal into a liquid that meets certain requirements and pouring it into the mold. After cooling and solidification and cleaning, a casting (part or blank) with a predetermined shape, size and performance is obtained. The cost of the blanks produced by casting is low, and it is more economical for parts with complex shapes, especially with complex inner cavities; at the same time, it has wider adaptability and better overall mechanical properties.
Low-pressure casting is a method of making liquid metal fill the cavity under pressure to form a casting. Because of the low pressure used, it is called low-pressure casting. The process is as follows: In the sealed crucible (or electric furnace), dry compressed air is introduced, and the metal liquid rises along the riser tube under the action of the gas pressure, and smoothly enters the cavity (metal type) through the gate. And maintain the gas pressure on the liquid surface in the crucible (or electric furnace), after a period of holding pressure, until the casting is completely solidified. Then, the gas pressure on the liquid surface is released, and the unsolidified metal liquid crucible (electric furnace) in the riser is opened by the cylinder and pushed out of the casting. The unique advantages of low-pressure casting are shown in the following aspects: the liquid metal filling is relatively stable; the casting has good formability, which is beneficial to the formation of castings with clear contours and smooth surfaces, which is more beneficial to the formation of large-scale thin-walled castings; the castings are densely organized, High mechanical properties. Low-pressure casting is a low-pressure strong and low-speed filling casting method. Compressed air is used as the filling power. Liquid metal filling requires stable; good pressure retention, good casting formability, clear contours, and smooth casting surface; requires mold Good cooling, dense casting structure and high mechanical properties.
2. Low-pressure casting automation system
2.1 System structure
In order to meet the requirements of low-pressure casting, the system of industrial control computer and PLC in low-pressure casting constitutes an organic system of industrial control computer and plc, as shown in Figure 1. This system uses a hierarchical control method. The requirements of liquid level pressure control and mold temperature (cooling) pressure control casting are completed by the industrial computer.
2.2 System principle The computer control system principle of the pressure / temperature control system of the low-pressure casting machine is shown in Figure 2.
(1) Pressure control: The aerodynamic principle of the pressure control system of the low-pressure casting machine is shown in Fig. 3. The industrial control machine and the liquid level pressure i / o template controlled by it control the digital combination valve island to realize low-pressure casting pressure control. The industrial computer system uses a sensitive pressure sensor and a software pid controller to simultaneously display the set pressure curve and the actual control pressure curve of the casting process on the computer screen, and perform a superimposed comparison. The real-time temperature curve of four temperature measurement points is displayed in four colors on the computer screen. In addition, a graph is used to indicate the liquid level of the aluminum liquid in the heat preservation furnace to remind the operator to pay attention: when the metal liquid in the furnace is less than the minimum, the system automatically stops pressurization.
y1 ～ y9: regulating valve (normally closed);
y11: intake main valve (normally closed);
y13: Slow pressure relief of the furnace (normally open);
y14: all the pressure relief of the furnace (normally open);
yk1: pressure switch to monitor the pressure in the furnace at "0" mbar. Adjusted to about 50mbar when leaving the factory;
yk2: pressure switch, monitor the maximum pressure in the furnace, adjusted to about 950mbar when shipped from the factory;
b1: Pressure sensor, 0 ～ 1600mbar, 4 ～ 20ma. Detect furnace pressure;
b2: Pressure sensor, 0 ～ 7000mbar, 4 ～ 20ma. Check the air pressure of the intake pipe.
(2) Cooling control: The industrial computer and the cooling control i / o template controlled by it control 12 cooling channels (5-way air-cooled, 7-way air-cooled + water-cooled), and several channels can be arbitrarily selected to work. Each cooling channel is provided with three working modes: "on", "off", and "auto" and two control modes: time control and temperature control. The man-machine interface has a cooling screen, which can be switched on and off at will.
2.3 Hardware configuration
The configuration of the low-voltage control system is as follows:
(1) panel870 industrial computer: CPU above piii700, 40g hard disk, 128m memory, 12 "LCD screen, touch keyboard, floppy drive and DVD drive, USB interface and rj45 interface;
(2) Board: a set of 813ba / d board, a set of 1751di / o board, a set of 7216 relay board (including odc5 or idc5b solid state relay), a set of 785 relay board;
(3) S7-300 series PLC: CPU module: one set of CPU314, one flasheprom memory card (64k), one ps307 power supply 5a, three sets of sm321 input modules (32 points 24vdc), four sets of sm322 output modules (16 points 24vdc) , 3 40-pin terminals, 4 20-pin terminals, and 1 480mm rail.
3. Project evaluation
(1) The control system realizes effective control of the process parameters including pressure, temperature, time, and position in the machine operation, hydraulic mechanical action, casting process, holding furnace heating, and mold cooling process. The system has the function of saving data and can access, adjust and manage the casting parameters.
(2) The control accuracy is high, the pressure deviation value in the filling and holding phase is ≤3mbar, the pressure deviation value in the rising phase is ≤5mbar, and the pressure deviation value in the boosting phase is ≤10mbar.
(3) With automatic pressure compensation capability, the pressure in the holding furnace can be accurately and repeatedly reproduced according to the set curve, without being affected by the holding furnace leakage, gas pressure fluctuations in the gas supply pipeline and metal liquid level changes (except for serious holding furnace leakage ).
(4) Up to eight steps of boosting curve and one step of holding pressure curve can be freely set according to the process requirements. For furnace types with furnace volume ≤800l, the maximum boosting speed can reach 100mbar / s.
(5) With a friendly man-machine interface, various process parameters can be input conveniently.
(6) With data storage and recall functions. The casting process parameters of each wheel type can be input and confirmed and automatically saved, and can be directly called from the system in the future.
(7) The low-pressure casting control system has the function of fault self-checking, which is convenient for maintenance.
4 Conclusion
The rapid development of China's industrial control automation technology, especially the application of IPC and PLC is more mature. After nearly three years of efforts, we have successfully applied IPC and PLC at the same time in the low-pressure casting machine of wheel hub.
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