Water-Assisted Injection Molding System Based on Water Hydraulic Proportional Control Technique

CHINESE JOURNAL OF MECHANICAL ENGINEERING●418●Vol. 23, No. 4, 2010DOI: 10.3901/CIME 2010.04.418, available online at www cjmenet.com; www.cjmenet.cm.cnWater-Assisted Injection Molding System Based on Water HydraulicProportional Control TechniqueZHOU Hua' , ZHANG Zengmeng'"* , GAO Yuan' an' , and YANG Huayong'1 The State Key Lab ofFluid Power Transmission and Control, Zhejiang Universit, Hangzhou 310027, China2 Jiujiang Branch of 707 Research Institute, China Shipbuilding Industry Corporation, Jiujiang 332007, ChinaReceived July 4, 2009; revised April 30, 2010; accepted May 17, 2010; published ectronicall August 20, 2010Abstract: Water-assisted injection molding(WAIM), an innovative process to mold plastic parts with hollow sections, is characterizedwith intermittent, periodic process and large pressure and flow rate variation. Energy savings and injection pressure control can not beattained based on conventional valve control system. Moreover, the injection water can not be supplied directly by water bhydraulicproportional control system. Poor efficiency and control performance are presented by current trial systems, which pressurize injectionwater by compressed air. In this paper, a novel water hydraulic system is developed applying an accumulator for energy saving. And anew differential pressure control method is proposed by using pressure cylinder and water hydraulic proportional pressure relief valvefor back pressure control. Aiming at design of linear contollr for injection water pressure regulation, a linear load model isapproximately built through computational fluid dynamics(CFD) simulation on two-phase flow cavity flling process with variabletemperature and viscosity, and a linear model of pressure control system is built with the load model and linearization of water hydrauliccomponents. According to tbe simulation, model based feedback is brought forward to compensate the pressure decrease duringaccumulator discharge and eliminate the derivative element of the system. Meanwhile, the steady-state error can be reduced and thecapacity of resisting disturbance can be enhanced, by closed-loop control of load pressure with integral compensation. Through thedeveloped experimental system in the State Key Lab of Fluid Power Transmission and Control, Zhejiang University, China, the staticcharacteristic of the water hydraulic proportional relief valve was tested and output pressure control of the system in AcrylonitrileButadiene Styrene(ABS) parts molding experiments was also studied. The experiment results show that the dead band and hysteresis ofthe water hydraulic proportional pressure relief valve are large, but the control precision and linearity can be improved withfeed-forward compensation. With the experimental results of injection water pressure control, the applicability of this WAIM systemand the effect of its linear controller are verified. The novel proposed process of WAIM pressure control and study on characteristics ofcontrol system contribute to the application of waler hydraulic proportional control and WAIM technology.Key words: water-assisted injectio molding, water bydraulics, proportional pressure control, linear control, load characteristicof elongated parts, for example, automotive industry, white1 Introductiongoods, office equipment, medical products and consumergoods, etcl4!.Water-assisted injection molding(WAIM)， generallyHowever, the WAIM process is complicated becausesimilar to gas-assisted injection molding(GAIM), is thethere are many factors influencing molding patterm andnewest way to mold hollow or partly hollow plastic parts.quality. The load characteristic is coupling with pressureWater can be supplied by water hydraulic system andand flow. And it is necessary to control the pressure, flowinjected into the molten regions in mould. Then cavities arerate and temperature of injection water5-6]. In addition, thegenerated and polymer melt is displaced to mold a hollowtemperature and flling percent of polymer melt andcore. So the substitution of filtered water for pure nitrogentemperature of mould need to be regulated by injectionsed in GAIM brings many advantages, such as shortmachinel6-9. So the molding process and system arecooling cycle time, high quality of products and low cost.designed to make injection water supplier work well withinjection machineland if the injection waterEspecially, WAIM can produce larger void spaces andlonger hollow sections in parts than GAIM(I-3). All thepressu中国煤化Iddy and inerfacialadvantages lead the applications of WAIM to a broad fieldinstabiHCN MH GS reason to controlpressure mcicasuug sICp uy sIcp. The injection water●Corresponding author. E-mail: zzm. zju@gmail.comflow rate is large and the compensation flow rate replacingThis project is supported by National Basic Research Program of Chinamelt shrinkage is small. The energy saving and control(973 Program, Grant No. 2006CB705405). National Natural Scienceperformance are required in system design based on theFoundation of China (Grant No. 50775199). and Zhcjiang ProvincialScience and Technology Plan Program of China (Grant No. 2007C21057).CHINESE JOURNAL OF MECHANICAL ENGINEERING●419●intermittent periodic work cycle of WAIM. The presentexperimental and commercial WAIM systemsreDesign of the System and Pressure Controldeveloped from GAIM and water is pressurized bycompressed gas. The pressure and flow control ofThe WAIM process totally contains five steps marked byinjection water are low-stiffiness and slow-response. Alsonumbers in Fig. 1, i.e., polymer melt injection, waterlow eficicy is induced by the aditional air compresser inection, maintaining pessure, drainage and part eciono7.and exhaust valves. These problems can be solved whenthe The water hydraulic system is in ready mode beforeinjcetion water is supplied by water hydraulic systempolymer melt injection is finished. The delay time,directly.generally no more than 3s, is decided by polymerThe water hydraulic technique using filtered tap water orcharacteristic, pressure, temperature of melt, mould andseawater as working fluid offers advantages, such as low injection water{5-9. It is important that the molding qualitycost for medium, environmental friendliness, cleanness and is strongly infuenced by the set of delay time. The waternon-flammabiltyto. The typical application fields include will be injected into mould cavities at the end of time delay.water mist fire fighting, high pressure water cleaners, The injection water pushes the polymer melt forward andunderwater operation, water treatment, nuclear, mining, the hollow core is formed. The pressure and temperaturesteel, ship, food processing industry, el1-21., The fast are regulated by water hydraulic system at this step. Toresponse, high control sifness, constant viscosity and low avoid turbulence in the area of the water injector, thetemperature-induced drift are highly praised in water pressure profile is recommended to be adjusted startinghydraulic proportional control technique, for the reason of with low pressure and then increasing to high pressure. Sohigh bulk modulus and thermal capacity of water. Another the pessre should be controled in slope or segments. Thebenefit is high efficiency coming from low viscosity "[12-15]1、 process of WAIM is intermittent and the injection stageHowever, the nonlinearity in water hydraulic controltakes very short time, typically only one or two seconds,systems may be great due to the seal and lubrication inwhich is a moment in one work cycle. Large flow needs tocomponents. The static friction and leakage flow will causebe supplied at the same time. The pressure is maintainedlarge dead band and poor linearity in control valves,after water injection and the flow rate decreases from theeyinders or other atuators as dstrbace aidaila1 maximum toa very small lvel t0 spply the melt sinkgeThe steady-state accuracy of control valves can beduring the cooling time. Some imperfections, such asincreased by static compensation as well as the dynamicshrinkage hole, may be caused by the excessively lowperformance will not be afected. Simultaneously, thepressure. The water hydraulic system needn't supplyanti-disturbance performance may be improved throughinjection water with pressure and flow rate at the next steps,closed-loop control. Besides, the throttle valves,i.e., drainage, mould opening and product ejection. Toaccumulator, and wide variation of flow rate changeconclude, intermitence, large variation of injectionpressure and flow rate, and processing time control areparameters and equilibrium points during process of watermain working characteristics of WAIM.injection. Generally, hydraulic plants exhibit significantnonlinearities and parameters varying with time, whereas③the linear controller is classical, mature and easy to design.20The set of rules and methods is well developed in linears5-oQcontrol. The injection parameters are different for④variations of polymer materials, mold, process approaches.5①The simple control scheme design based on linear control0 2468101214161820theory contributes to the convenience of parameterTime 1/sadjustment and industrial process.Fig. 1. Load pessure profile of WAIMThe application of water hydraulic technique in WAIMshows a perfect combination of green transmission andThe circuit scheme of water hydraulic system is shownnovel injection molding technology with energy saving, in Fig. 2. An accumulator and a fixed small displacementhigh efficiency and low operation expense. So on the basis pump are used. The accumulator is charged during pressureof the analysis of the WAIM process and load characteristic，maintaining and other intermittent moments, such asa novel design of the water hydraulic system was designed drainage and part ejection. Large flow water injection isand modeled in MATLAB. The problems of linearization, supplie中国煤化工discharges. Theload characteristic and multi-point feedback need to be converhe pressure ofdiscussed in terms of tracking errors and stability. Then theaccumi:YHC N M H Ging valve or throttleexperiments of proportional pressure control on WAIM valve at the outlet. However, the increase of the loadwere conducted on compensated static control of water pressure is required to inject water into mold while thehydraulic proportional pressure relief valve.pressure of accumulator decreases for discharge. And theZHOU Hua, et al: Water-Assisted Iijection Molding System Based on Water Hydraulic●420●Proportional Control TechniqueWaterPressureMold Moldholding pressure is much higher than that at the beginninginjectionreleaseopencloseof the injection as shown in Fig. l. The original methodsMaintainingcontrolling pressure of accumulator are not suitable. So apressureIPart Inovel differential pressure control strategy is provided to0FPL! Drainagejectionregulate the water injection pressure by a proportionalpressure relief valve and a pressure cylinder. The cylinderTime IpelayPiston Charge ofis designed to boost the pressure and separ ate the injectionpushy i accumulatoriPe..warm water from the power pack. In discharge process of102030accumulator, the pressure before water injector pr can beMelit ijectionwritten asTime 1/sFig. 3. Pressure profile in a whole work cyclep4-P。4.1)ATable 1. Status of the system and control valvesWater hydraulic_ Control valvesdue to force balance characteristic of the cylinder in steadyStepsystemVA VIVDstate. Here Al, A2 and As are respectively the area of theMould closeCharge ofpiston near to port A, B and L, while Pa and Pg are thepressure at port A and B. Diameters of the piston are givenMelt ijectionaccumulatorto make A1= -2A2= =2A3 approximately according to thisTime delayPrepare todesign, then PL=2pA-Pg. Therefore, the pressure PL can be(Delay progam)inject watercontrolled through controlling the pressure Pg by aDischarge ofproportional pressure relief valve. So the water injectionWater ijectionpressure depending on PL can be regulated by following thePressure maintainingSmall flowset value or profile as shown in Fig. 3. Even the pressure Prcan be increased with large variation though the pressure ofPressure decreasingLift pressure atportBthe accumulator PA decreases.Piston pushDrainageback日VDMould open/LParts apartalve+四千因七Here the signals “+”and“_” indicate the states ofcontrolvalves respectivelyenergization andMoldde-energization. A signal will be sent out from the waterPressure eylinderBhydraulic system to the injection molding machine whenthe pressure of the accumulator is detected up to the setA回value and the polymer melt will be injected in after themould is closed. Another signal from the injection molding口machine will be sent back when the melt injection finishes.AccumulatorThe water hydraulic system will start time delay process atProportionalthe same time and the 3/2 directional control valve“VA”valvwill be switched. Water will be injected into polymer meltsafter delay time while the 2/2 directional valve “VL" isPumpswitched on. The accumulator discharges, and the piston ofthe pressure cylinder is pushed forward as well. Thepressure before water injector can be regulated by theproportional pressure relief valve in processes of injection,Fig. 2. Circuit of water hydraulic system for WAIMA, B,L- - Ports of pressure cylinder,pressure maintaining and decreasing. Here the lifing-VA - 3/2 directional control valve at Port A of pressure cylinder, pressure nrncess isthe. kev nart tn he studied in modelingVL- -2/2 drctional cotrol valve in front of water injector,nd中国煤化工rol. At the step ofVD- -Water drainage control valve,drainagCNMHGVD"openstodrini/p - Presse sensor,the Wauci Jul.' IIC valvc vn caul be switched and thepiston will be pushed backward. Then the accumulatorThe switches of on-off control valves in every process charges again as soon as the piston is pushed back to theare shown in Table 1.end. After drainage and dry of the inner surface of plasticCHINESE JOURNAL OF MECHANICAL ENGINEERING*421，parts, a signal will be sent from water hydraulic system top=2Ln(i)r(5)injection machine to open mould and ect the parts. Timeof WAIM processes can be set through touch screen andcotolled by PLC. The next is another work cycle fora where L and r are dimensions of the mould cavity. Thenew product.complex relationship between the load pressure and flowrate, influenced by temperature, polymer, pressure, and so3 Analysis of Load Characteristicon, can be estimated by computational fluid dynamics(CFD)simulation approximately. As shown in Fig. 4, there areThe water injection pressure depends on the flow three phases, i.e, water, polymer melt and air. A k-wresistance of the polymer melt and the part geometry. When turbulence model is used in the field of water and laminarinjected into the mold cavities, water cools the polymer in polymer melt. The flow front is solved by volume ofmelt and increases its viscosity. The leading edge of the fluid(VOF) model, and the pressre implicit with splitingwater forms a solid boundary or highly viscous membrane. of operators(PISO) coupling format of velocity andThe membrane forces the polymer melt forward, instead of pressure is employed in this unsteady computation. Thethe melt forcing the water to the sidell. The dynamic simulation results including volume fraction, temperatureinteraction of water and polymer melt with large diference and ratio of turbulent viscosity are shown in Fig. 5.in viscosity is involved in WAIM cavity flling process, and Temperature changes greatly at the interface between waterstrongly depends on the temperature due to viscosity- and polymer melt. The maximum of turbulent viscositytemperature charateristic of polymer material. So the ratio max(4/u)=1 380 and is in a crret range. The resultssimplification is necessary, and through simulation, the show that the thin residual wall gained in simulation iscoupling relation between load pressure and flow rate shall close to the experiment rsuts!7-?. The coupling betweenbe obtained.load pressure p and flow rate qL is shown in Fig. 6. TheOnly considering viscosity at first, Cross-Arrhenius approximate equation can be written asmodel is suitable to model the viscosity characteristic ofpolymer melt cavity fllig flow in molding injection[17-181._B.p=告9,The equation isn。(T,p)where AL is the cross section area of mould cavity and B isn(j,T,p)=;2)1+(nir/t)"the equivalent load visco-damping coefficient which can begiven by CFD simulation or WAIM experiments.whereTable 2. Cross model parameters of polymerPolymern%(T,p)= Bexp|(3)Model parameterPA66ABS0.239 0And n is the polymer melt index. The apparent viscosity ηShear stress i/MPa0.183 600.09841Viscosity cofficient B/(Pa.s)2.123x10*93.415x10*is affected by the melt temperature T, pressure p and shearTemperature factor Tg/K139199223ratej , and no is the zero shear rate viscosity. Eqs. (2) andPressure factor B/MPa'0.02000.0164(3) are under the condition that polymer melt temperature isVitrification temperature Tg/K325375100 K higher than virification temperature Ig This is- VscsiaIr nw/(MPa*5)35 190.0916.5applicable in GAIM. However, the temper ature of polymerInjected water Polymer metAirmelts may drop to T0n>250 rad/s, which is much higher than thewhere kta is the gain of the pressure transducer. In order toproportional relief valve. Therefore, the disturbance 0control the load pressure consistently with input, we shouldparameter variation and deviations imported b;make PB response following the decrease of thelinearization is negligible at low frequency .accumulator pressure PA. Therefore, the pressure ofBased on the model-based feedback of the accumulatoraccumulator pA is fed back to coree the input value ofthe pesure, the integral compensation is introduced toproportional pressure relief valve. The pressure P阳at inlet closed-中国煤化工，as shown inFig.8.of the relief valve is regulated according to Eq. (26). By the A serieMYHC ent gains of integralmodel-based feedback, the system is transformed to a typepart anC N M H Gshown Fig.90 system. The structure of pressure control is improved andThe magnitude of open loop is raised up with thethis simple controller is reliable, however, without any increase of the integral gain, so the ste ady-state error willother compensation and load pressure feedback, a large be reduced by closed-loop control. However, the systemZHOU Hua, et al: Water-Assisted Iiction Molding System Based on Water Hydraulic●424.Proportional Control Techniquebecomes unstable as the integral gain increases. The set main valve port. Due to the static and coulomb friction andvalue of k is limited by 0r2, the cut-off frequency of the the structure of force amplifying lever, the width of deadproportional relief valve. Further more, the actual value of zone and hysteresis of the valve is very largel2). The valvek is far less than 0r2, because of the poor control precisioncontrol board is “BLSI"-type digital PID contoller,and the nonlinearity, such as large dead band and hysteresis including three analogue inputs, PID parameters set,of the valve. The load viscosity varies more largely than open-loop and closed-oop mode for inlet pressure control.inertia and elasticity in WAIM for different polymers. Andn order to analyze the static characteristic of thethe load vicosity has rlatively large infuene on pesre proportional relief valve, the control board isset oncontol. Along with the increse of melt viscsit, the openloop mode. In this mode, the input signal can bemagnitude of load pressure A/B' decreases, and the gaindirectly magnified to Umx=+24 V, imax = 0.52 A toof the open-loop type 0 system with model-basedactuate the electromagnet.correction is reduced. So the crossover frequency of thesystem with integral compensation decreases. Also thedamping ratio of the system decreases as the visco-dampingcoficient of the load increases. We can draw a conclusion4-图-@-四+因φ@四个四由个四个that high viscosity of the polymer melt makes the injection困pressure fluctuate when a large input signal change occurs.So a small value of k should be set to ensure the enoughstability margin when high viscosity polymer is used for-四WAIM.团一PTable 3. Main parameters for simulation囚一JParameterValue_ Fig. 8. Closed-oop control structure of pressure control systemEquivalent tifnesss cofficient of accumulator k/(kN.m) 38Equivalent visco-damping cofficient of accumulator1 42250-B/kg.s")Cross section area of accumulator Aa/m20.015Bulk modulus of water风/GPaArea of piston near to portA A:/m0.012Area of piston near to port B A2/m20.006思-s0Area of piston near to portL As/m20.00Mass of piston m/kg31首-10No intgratorVisco-damping coffcient in pressure cylinder B/(kg.s5) 39Load mass m/kg90z2 -150--0 k=10MStatic friction f/N.. k=20Equivalent load visco damping coffcient B'/(kg*s-)60 000-200-0000 k=30Cross section area of mould cavity Au/m20.002Corner frequency of frst-order inertial loop a://rad.s5)321Damping ratio矣-250Cormer frequency of second- order oscillation loop10a/(rad.s')5 Experiments on Pressure Control System-270A fixed-displacement water hydraulic pump, with rated-360-pressure 16 MPa and small displacement 25 mL/r, is usedin the WAIM system. The injection water pressure can beboosted to 25 MPa by the pressure cylinder, and throughthe accumulator, the max flow rate is above 100 L/min. AFrequenecy @/(nd.stwater hydraulic proportional pressure relief valve, withFig. 9. Bode diagram of the system with integral compensationmax working pressure 31.5 MPa, is applied in the system.As shown in Fig. 10, this valve is three-stage with ball seat中国煤化工valve as the throttle control pilot stage, and the force of thejYHCN M H Gop and coesa-opproportional electromagnet is amplified by a lever. The mode ar oluwg 11.山uI upul-loop control mode,main valve is characterized with two-stage throttle and high the hysteresis occupies about 22% of the input range, andpressure inducting structure, which resists cavitations at the the linear range is only 7%, within which the ascending sectCHINESE JOURNAL OF MECHANICAL ENGINEERING●425●occupies only 1%. The reason comes from the structure of It is shown that the load pressure eror at the end of thepoppet valve, friction and the pilot stage. The linear control slope contol is 5.8%. Meanwhile, the maintaining pessurerange can be extended to about 56% and the hysteresis can is set as 19 MPa and the steady-state error is only 0.7%.be reduced to 17% under closed control, but severeWhen the directional control valve switches, the impactoscllatios are easily caused due to the large hysteresis of pressure occurs with the maximum overshoot close to aboutthe valve. As the key component of the whole WAIM 8 MPa, and the delay time for response of the on-off valvesystem, the water hydraulic proportional pressure reliefis about 0.27 s. The excessive time delay and pressurevalve can not meet the requirement of the system becauseimpact may cause problerms of quality of products. On oneof the high nonlinearity. Based on the least-square linearhand, the water hammer in water hydraulics is greater thanregression on the static characteristic of the valve, theoil hydraulics, for the reason of high sifness of waterhysteresis can be efectively reduced through inversehydraulic system and fast propagation of pressure in water.transformation and static feed-forward compensation. ForThis special property of water causes the high press urethe reason that the linear controllable range accounts forimpact and oscillation during process of valve switch. Onsmaller fraction of the increase of input signal than thethe other hand, the underdamping may be introduced bydecrease, the pulse modulation is added to one -sidesmall flow resistance at the inlet of the accumulator, and ithysteresis compensation. According to the staticalso can cause pressure oscillation and overshoot withcharacteristic with the compensation, the linear range isswitch of directional valve. From the experimental results,above 80% and the hysteresis is reduced to about 5% asit is realized that the water injection pressure can be raisedgradually in spite of the poor performance of the directionalshown in Fig. 12.valves and pressure control valve. Moreover, the peakvalue of the water injection pressure is controllable and theElectromagnetLever Ceramic ballholding pressure can be stabilized at a maximum value.i Throtle control pilot stageThese results show that the design approach meets therequirements of WAIM production.工wwqResistor12r0F8+MA7Pilot-stage spoolMain-stage spoolOulletInlet20406080 100Fig. 10. Structure sketch of the water hydraulic proportionalInput i/% .pressure relief valveFig. 12. Static characteristis of the water hydraulic14rproportional pressure relief valve with compensation1220 p5- o一Open-loop0t一一Closed-loopMceasured2Set value06(30100.1.05 2.02.5 3.0Fig. 11. Static characteristics of the water hydraulicTime 1/sproportional pressure relief valve under open-loop中国煤化Injection waterand closed-loop controlBSMHCNMHGThe load pressure curve of ABS molding experiment is6 Conclusionsshown in Fig.13. The delay time is setas tg= 0.5 s, andthe slope control time of water injection is setas 16g= 1.5 s.(1) Intermittence, periodic, large pressure and flow rateZHOU Hua, et al: Water-Assisted Injection Molding System Based on Water Hydraulic●426.Proportional Control Techniquechange are main work characteristics of WAIM. TheReinforced Plastics and Composites, 2007, 26(14): I 441-1 454.differential pressure control method with pressure cylinder9] LIUS I, CHEN Y s. The manufacturing of thermoplastic compositeparts by water-assisted injection-molding technology[J]. Compositesis provided to regulate the accumulator pressure throughPartA: Applied Science and Mangfacturing 2004, 35(2); 171-180back pressure control by water hydraulic proportional[10] BACKE W. 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An analyical comparison ofhigh melt viscosity leads to large oscillation and instabilit.hydraulic systems based on water and on oilCI/Proceedings of the(4) Based on the feed-forward compensation for the7th JFPS International Symposium on Fluid Power, TOYAMA,water hydraulic proportional pressure relief valve, theJapan, September 15-18, 2008: 679 684.experimental results show that the max steady-state error of [16] LIU SJ, LIN s P. Study of fingring' in water asisted injectionslope injection pressure and maintaining pressure aremolded composites[]. Composites Part A: Applied Science andManufacturing, 2005, 36: I 507-1 517.separately 5.8% and 0.7%. The developed linear controller17] u C T, SHIN J w, ISAYEV A L. Primary and secondary gameets the requirements of water injection pressure control.penetration during gas assisted inection molding, part Il: simulation(5) Pressure impact and overshoot occur on switch ofand experimen[]. 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Factors afecting the formation of fingering inregard to its entrance characteristics[]. Chinese Journal ofwater-assisted injection-molded thermoplastics[]. Advances iMechanical Engineering. 2007, 43(9): 28 -32. (in Chinese)Polymer Technology, 2006, 25(2);: 98 108[23] ZHANG Zengmeng, ZHOU Hua, GAO Yuan'an, et al. Simulation6] LIU s J, CHEN Y s. Warterasisted ijection molding ofand experiment on static characteristic of water hydraulicthermoplastic materials: efects of processing pararmeters[J].proportional pressure relief valve[J]. Journal of the China CoalPolymer Engineering and Science, 2003, 43(1): 1 806-1 817.Society, 2009, 3411): 1 569 -I 573. (in Chinese)7] PROTTE R, BANGERT H, COOPER C.et al. Waterassist ijectiomolding- a innovative process technology for produtity Biogra中国煤化工improvement: developments in peoessin, equipment andZHOUprofessor and a PhDmatialslC]/Proceedings of the 61st Annual Technical Conference candidaMYHCNMH GLab of Flud Powerof SPE. Nshille IN, USA, May 48, 2003.3 Bokiel: Transmission and Control, Zhejang ”University, China. He8] LIUS J, LIN C H. An experimental study of waeassisted ijectionSPE/ANTEC, 2003: 404 408.received his PhD degree from Huazhong University of Sciencemolding of plastic tubes witb dimensional transitions[J]. Journal ofand Technology, China, in 1998. His main research interestsinclude water hydraulic technology and electro-hydraulic controlCHINESE JOURNAL OF MECHANICAL ENGINEERING●427●Industry Corporation, China. He received his bachelor degreeengineering.Tel: +86-571-87951659; E-mail: Hzhou@zju edu.cnfrom Huazhong University: Science and Technology, China, in1990.ZHANG Zengmeng, bom in 1979, is curently a PhD candidate inthe State Key Lab of Fluid Power Transmission and Control, YANG Huayong, bom in 1961, is curenly a professor and a PhDZhejiang University, China. He received his bachelor degree fromcandidate superisor in the State Key Lab of Fluid PowerZhejiang University, China, in 2003. His rescarch interests includeapeChrins Hauite poro conrol d wtrasires ijectinTrasmission and Control, Zhejiang University, China. Hewater hydraulic proportional controlreceived his PhD degree from Bach University, England, imolding technology.His main research interests include mechachonics engineering,Tel: +86-571-87951271; E-mail: zm.zju@gmail.comfluid power tasmission and control, energy saving inGAO Yuan'an, bom in 1967, is curently the vic presideatof mechJujiang Branch of 707 Research Institute of China ShipbuildingE-mail: yby@zju.edu.cn中国煤化工MHCNMHG