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Fig. 2. Circuit of water hydraulic system for WAIM
A, B, L —Ports of pressure cylinder,
VA —3/2 directional control valve at Port A of pressure cylinder, VL —2/2 directional control valve in front of water injector,
VD —Water drainage control valve,
i/p —Pressure sensor.
The switches of on-off control valves in every process are shown in Table 1.
Here the signals “+” and “–” indicate the states of control valves respectively energization and de-energization. A signal will be sent out from the water hydraulic system to the injection molding machine when the pressure of the accumulator is detected up to the set value and the polymer melt will be injected in after the mould is closed. Another signal from the injection molding machine will be sent back when the melt injection finishes. The water hydraulic system will start time delay process at the same time and the 3/2 directional control valve “VA” will be switched. Water will be injected into polymer melts after delay time while the 2/2 directional valve “VL” is switched on. The accumulator discharges, and the piston of the pressure cylinder is pushed forward as well. The pressure before water injector can be regulated by the proportional pressure relief valve in processes of injection, pressure maintaining and decreasing. Here the lifting- pressure process is the key part to be studied in modeling and simulation on the pressure control. At the step of drainage, the valve “VL” closes and “VD” opens to drain the water out. The valve “VA” can be switched and the piston will be pushed backward. Then the accumulator charges again as soon as the piston is pushed back to the end. After drainage and dry of the inner surface of plastic
CHINESE JOURNAL OF MECHANICAL ENGINEERING ·421·
parts, a signal will be sent from water hydraulic system to injection machine to open mould and eject the parts. Time of WAIM processes can be set through touch screen and controlled by PLC. The next is another work cycle for a new product.
3 Analysis of Load Characteristic
The water injection pressure depends on the flow resistance of the polymer melt and the part geometry. When injected into the mold cavities, water cools the polymer melt and increases its viscosity. The leading edge of the water forms a solid boundary or highly viscous membrane. The membrane forces the polymer melt forward, instead of the melt forcing the water to the side[16]. The dynamic interaction of water and polymer melt with large difference in viscosity is involved in WAIM cavity filling process, and strongly depends on the temperature due to viscosity- temperature characteristic of polymer material. So the simplification is necessary, and through simulation, the coupling relation between load pressure and flow rate shall be obtained.
Only considering viscosity at first, Cross-Arrhenius model is suitable to model the viscosity characteristic of
p = 2L(˙)˙ , (5)
r
where L and r are dimensions of the mould cavity. The complex relationship between the load pressure and flow rate, influenced by temperature, polymer, pressure, and so on, can be estimated by computational fluid dynamics(CFD) simulation approximately. As shown in Fig. 4, there are three phases, i.e., water, polymer melt and air. A k–ω turbulence model is used in the field of water and laminar in polymer melt. The flow front is solved by volume of fluid(VOF) model, and the pressure implicit with splitting of operators(PISO) coupling format of velocity and pressure is employed in this unsteady computation. The simulation results including volume fraction, temperature and ratio of turbulent viscosity are shown in Fig. 5. Temperature changes greatly at the interface between water and polymer melt. The maximum of turbulent viscosity