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2 Design of the System and Pressure Control
The WAIM process totally contains five steps marked by numbers in Fig. 1, i.e., polymer melt injection, water injection, maintaining pressure, drainage and part ejection[7]. The water hydraulic system is in ready mode before polymer melt injection is finished. The delay time, generally no more than 3 s, is decided by polymer characteristic, pressure, temperature of melt, mould and injection water[5–9]. It is important that the molding quality is strongly influenced by the set of delay time. The water will be injected into mould cavities at the end of time delay. The injection water pushes the polymer melt forward and the hollow core is formed. The pressure and temperature are regulated by water hydraulic system at this step. To avoid turbulence in the area of the water injector, the pressure profile is recommended to be adjusted starting with low pressure and then increasing to high pressure. So the pressure should be controlled in slope or segments. The process of WAIM is intermittent and the injection stage takes very short time, typically only one or two seconds, which is a moment in one work cycle. Large flow needs to be supplied at the same time. The pressure is maintained after water injection and the flow rate decreases from the
maximum to a very small level to supply the melt shrinkage during the cooling time. Some imperfections, such as shrinkage hole, may be caused by the excessively low pressure. The water hydraulic system needn’t supply injection water with pressure and flow rate at the next steps, i.e., drainage, mould opening and product ejection. To conclude, intermittence, large variation of injection pressure and flow rate, and processing time control are main working characteristics of WAIM.
Fig. 1. Load pressure profile of WAIM
The circuit scheme of water hydraulic system is shown in Fig. 2. An accumulator and a fixed small displacement pump are used. The accumulator is charged during pressure maintaining and other intermittent moments, such as drainage and part ejection. Large flow water injection is supplied when the accumulator discharges. The conventional approach regulating the pressure of accumulator is to put a pressure reducing valve or throttle valve at the outlet. However, the increase of the load pressure is required to inject water into mold while the pressure of accumulator decreases for discharge. And the
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ZHOU Hua, et al: Water-Assisted Injection Molding System Based on Water Hydraulic
Proportional Control Technique
holding pressure is much higher than that at the beginning of the injection as shown in Fig. 1. The original methods controlling pressure of accumulator are not suitable. So a novel differential pressure control strategy is provided to regulate the water injection pressure by a proportional pressure relief valve and a pressure cylinder. The cylinder is designed to boost the pressure and separate the injection warm water from the power pack. In discharge process of accumulator, the pressure before water injector pL can be written as
Fig. 3. Pressure profile in a whole work cycle
p = pA A1 − pB A2 ,
A3
(1)
due to force balance characteristic of the cylinder in steady state. Here A1, A2 and A3 are respectively the area of the piston near to port A, B and L, while pA and pB are the pressure at port A and B. Diameters of the piston are given to make A1=2A2=2A3 approximately according to this design, then pL=2pA−pB. Therefore, the pressure pL can be controlled through controlling the pressure pB by a proportional pressure relief valve. So the water injection pressure depending on pL can be regulated by following the set value or profile as shown in Fig. 3. Even the pressure pL can be increased with large variation though the pressure of the accumulator pA decreases.