This example shows an injection molding actuation system. The model contains a set of cartridge valves that control pumps, motors, and cylinders to execute the steps of an injection molding process.
The system presented in the example is the actuation system of the molding machine injection unit described in full detail in the Vickers Industrial Hydraulics Manual (second edition, 1989), and shown in the schematic diagram below.
Figure 1. Working Members of the Molding Machine Injection Unit
The unit consists of a screw and barrel, hopper, extruder motor, and injection cylinder, which are all mounted on a carriage. The screw, extruder motor, and injection cylinder are arranged in a single unit that can slide along the carriage to eject the molten plastic from the barrel cylinder into the mold. The cycle considered in the example starts with the extruder rotation, during which molten plastic is pushed forward by the screw through the nonreturn (check) valve. The rotation is stopped when preset volume of molten plastic has been accumulated in the front chamber of the barrel. The injection maintains specified back-pressure while the chamber is filled and is pushed back by the molten plastic.
The next operation following the extruder run is called decompression. During this phase, the active chamber of the injection cylinder is decompressed by connecting it to the tank. The injector unit pauses after decompression for 2 seconds to allow the previously molded and hardened part to be removed from the mold. After the mold is closed, the injection cylinder goes forward and injects molten plastic into the mold.
The schematic diagram of the unit hydraulic system is shown in Figure 2.
Figure 2. Injection Unit Hydraulic System Schematic Diagram
The system is built of five cartridge valves that are arranged into the injection unit manifold:
The extruder cartridge valve controls rotation of the extruder motor. The motor starts rotating as electromagnet Y1 is energized.
The injection retract cartridge retracts the nozzle from the mold for a very short distance to allow the mold to be opened and the previously molded and hardened part to be removed. The cartridge poppet is opened as electromagnet Y2 is energized. When the electromagnet is deenergized, the poppet is closed and the control chamber of the tank cartridge valve is vented to allow free return flow from the extruder motor.
The tank cartridge valve connects extruder motor to the tank during the barrel fill
The injection forward cartridge valve pushes the nozzle forward to contact the mold and injects molten plastic into the mold. The cartridge is controlled by electromagnet Y3. At neutral position, one of the control chambers of the back-pressure cartridge is vented to allow this cartridge to work as a pressure relief valve and maintain pressure set by the supply pressure at port Z1 and openings of orifices in the back-pressure cartridge covers.
The back-pressure cartridge valve maintains low back-pressure in the injection cylinder chamber to assure required quality of the molten plastic. The cartridge poppet is opened when electromagnet Y3 is deenergized. Simultaneously, the poppet of the back-pressure cartridge is closed by pressurizing the previously vented control chamber.
There are five signals that control this system:
1. Extruder - electromagnet Y1 signal
2. Retract - electromagnet Y2 signal
3. Inject - electromagnet Y3 signal
4. Nozzle - simulates the changing gap between the nozzle and the mold as a 2-way valve. In a real machine, the nozzle remains in contact with the mold while the extruder runs and the front portion of the barrel is filled. This contact prevents molten plastic from the spill. Since no mold is considered in the model, the nozzle is kept closed with the 2-way valve controlled by the signal 'Nozzle'. The valve is opened as the nozzle is moved away from the mold to keep things close to the real behavior.
5. Backpr - controls pressure level at port S1 (Figure 2) which establishes the pressure maintained by the back-pressure cartridge.
The purpose of remaining blocks is explained on the model schematic diagram (Figure 1).
The plots below show the pressures in the injector chambers and the position of the barrel piston. The retract, decompression, and injection phases can be identified by the pressures and positions in the system.