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CHrNESE JOURNAL OF MECHANICAL ENGINEElUNG 、，ol-23，No．5，2010 。547’ DOI：10．3901／CJME．2010．05．547，available online at www．cjmenet．corn；www．cjmenet．com．cn Design and Verification of an Auxiliary System for High Vacuum Die Casting GE Xiaohon91'2一，HUANG Hongwul，LI Hui2，and LIU Yadanl 1 School ofPhysics and Mechanwal＆Electrical Engineering,Xiamen University,Xiamen 361005．China 2 Institute ofMaterial Processing and Die＆Mould Integrated Technology,Xiamen University ofTechnology, Xiamen 36102幺China Received January26，2010；revisedAugustl8，2010；acceptedAugust25，2010；published electronicallyAugust26，2010 Abstract：Vacuum die casting is the optimal method to produce high quality aluminum alloy components．At present，there arc still very few systematic studies on Vacllum die casting theory and equipment design．On the basis of the existing theories of the vacuum die casting pumping and venting systems．a(chǎn) simplified model is established in this research．The model has an aggregate unit consisted of ‘'vacuum pump+buffer tank”and a cylindrical container(including the shot sleeve，cavity and exhaust channel)．The theoretical analysis is carried out between the cavity pressure and the pumping time under different volume models．An auxiliary system for high vacuunl die casting is designed based on the above analysis．This system is composed of a vacuum control machine and a new vacuum stop valve．The machine has a human-computer control mode with“programmable logic controller(PLC)+touch SCl'eal'’and a real-time monitoring function of vacuunl degree for buffer tank and die cavity．The vacuum stop valve with the“compressed gas+ piston rod+labyrinth groove”structure can realize the function of whole-process vacuum venting．The henri,,system shows great advantages on vacuuming the cavity with a much faster speed by making tests oil all existing die casting mold and a 250 t die casting machine．A die cavity pressure less than 10 kPa can be reached within 0．8 s in the experiment and the porosity ofcastings can be greatly decreased．The systematic studies on vacuum die casting theory and equipment have a great guiding significance for high vacuuin die casting，and call also be applied to other high vacuum forming in related theoretical and practical research． Key words：vacuum die casting，pumping model，high vacuum,vacIllmll stop valve，auxiliary system 1 Introduction Higll vacuum die casting technology is a special die casting process during which a cavity pressure less than 10 kPa call be obtained．This technology can reduce or eliminate the porosity in die castings，with the air entrainment content being merely 1-3 mL per 1 00 g．The obtained castings can be used for further processing such as heat treatment and welding。etctl-21．The key technology of lligh vacuun'l die casting is to design a high—sensitive vacuum pumping system．The difficulty lies in establishing a modeling of pumping process．Some scholars have studied on the theoretical model or practical application in this aspect． BAR-M匝IlL et alt3-41．studied the air venting of die casting and the ph)rsical modeling of vacuunl pumping process．They believed there was a critical vent area during the die casting process．Whca the effective vent area of a vent valve was smaller than the critical area，the venting process was insufficient．hence a lot of gases were involved in the liquid metal．When the effective vent area was larger, the liquid metal was overflowed easily．NOURI—BORUJE一 ’Corresponding author．E—mail：xmlggxh@163．corn This project is supported by Fujian Provincial Natural Science Foundation of China(Grant No．2007J0 1 70)，and Xiamen Municipal Natural Science Foundation ofChina(Grant No．3502220093034) RDI．et alt51．built a transient model for the residual air mass in the die cavity during the injection process．Thev considered the influence of the molten metal viscosity． temperature，injection velocity and friction factor，and thus improved the numerical simulation calculation method in this field．HERNANDEZ．et al【…．improved the model of BAR．M【EIR et a1．a(chǎn)nd took into account such unsteady factors as friction and air flow variation during vacuum pumping．1nhis improved physical model was suitable for both conventional die casting and vacuum die casting． There were many other studies on the die casting venting and vacuum die casting pumping internationally，but most of them stayed at the theoretical level and were mostly numerical simulations for the venting process． HU．et al【卜Ⅵof Tsinghua University，China，carried out the theoretical calculation and experimental study of the decline of vacuum pressure in the cavity pumped directly by a vacuunl pump．They compared a theoretical curve with a measured curve of the pressure in the cavity．Only when the time of vacuum pumping exceeded 2．5 s．a(chǎn)n ideal cavity vacuum pressure less than l 0 kPa could be obtailled． 、)liA N．et alL。厶刈from Huazhong University of Science and Technology,China，independently developed a key technology related to high vacuum die casting based on minimum filling time(MFT)method．They designed an auxiliary system with a bufrer tank that the mechanical 萬方數(shù)據(jù) ·548· GE Xiaohong,et al：Design and Verification ofan Auxiliary System for High Vacuum Die Casting valve could be closed by the impact of liquid metal flow． This system could decrease the pressllre of cavity to 10 kPa within 1 s． Based on the current Iiterature，few studies provide theoretically sound analysis and practical sound design for a vacuum die casting system with a buffer tank．This paper is to carry out theoretical research and calculation analysis of the high vacuum pumping system model with a buffer tank，to study the relationship between pressure in die cavity and the pumping time，and to develop a high vacuum die casting auxiliary system which Call satisfy"the high vacuunl experiment requirements． 2 Theoretical Model of Pumping System with a Buffer Tank The vacuum pumping scheme with a buffer tank is shown in Fig．1【4】．A large buffer tank，as a vacuum source， is added to extract the gas out of the cavity between the vacuum pump and the mould．The gas in the buffer tank is drawn out unceasingly by the vacuum pump． Fig．1．Scheme ofvacuum pumpkng system with a buffer tank A simplified model is given in Fig．2 for the theoretical calculation of hi【曲vacuum venting based on the above scheme．必n is the Math number at duct entrance，Jjl‰t is the Mach number at duct exit，and re(t)is the number of moles of gas in die cavity． vacutlrtl entrance Fig．2．Theoretical calculation model for air venting In this model。the unfilled shot sleeve。the runner and the die cavity are combined and regarded as a cylindrical container fcalled the cylinder)【jJ．Gases are vented through the duct to the buffer tank．功e main resistance to gas flow is in the duct．The following assumptions about this model are made for the theoretical calculation： (1)AU gases are ideal gases． (2)1nhe venting period is very short，the gases temperatu- re is at 300 k and the heat transfer of gases is not considered． (3)111e effect of gases leakage on the venting process is neglected． (4)The volume of vacuum valve duct is sinail compared to the gas duct，thus can be negligible． According to the model established for the venting process in high vacuum die casting，the following formulas can be established to relate the gas pressure in the cavity to the pumping time．In performing the calculations，‰and 朋rout at the duct entrance and exit can be obtained by solving Eqs．(1)一(3)‘3一'10l as follows： 警=愕L一陪1‰， m 篇P=(·+盟2 M二廠老。(f) I ”J M。 ，+等M二 ·+竿帆2 t+l I—I (2) (3) Within the time interval At，the gases mass Am flowing from the cylinder through the vent to the outside can be obtained by Eq．(4)t5-6]： Am=pgAAt=PoM,．(-b]i[·+等M：產(chǎn)觸∽ The cavity pressures po(t)and p“力can be obtained by Eqs．(5)，(6)，respectively： (5) (6) where the dimensionless quantity 4fL／D is the resistance coefficient in the duct，usually taking 3 to 7．p“f)is the pressure at duct exit，po(t)is the pressure in die cavity,k is the specific heat ratio of air,Am is the number of moles of gas entering the duct in unit time．P is the air density,R is the molar gas constant．“is the gas velocity at the vent entrance，A=n(D／2)2 is the vent cross-sectional are41，K is the cylinder volume，瑪is the buffer tank volume，and K is the duct volume，V4=“+％． The gas duct has diameter of 20 mill and length of 3 m． The buffer tank is selected with l 50 L．The vacuum pump with the pumping speed of 8 L／s is to be used，and the bufrer tank is pumped to have an initial pressure of 0．8 kPa． r— r一 豎 堅守半 ∽一 ∽一 m一 陋～ II =、●，、J f f，L，L 風(fēng) 風(fēng) 萬方數(shù)據(jù) CHINESE JOURNAL OF MECHANICAL ENGⅢEERING ·549· Table presents the values for the calculation． Table．Values for the initial calculation conditions Initial condition Value Cylinder volume VI／L ‰dllct Di8n柵D／mm Length L／m Pressure at 0 spo(O)／kPa Pressure in final po(t)／kPa Pressure at 0 sp，(O)／kPa Specific heat ratio ofairK Resistance coefficient 4fLIrt o．5，1．0，1．5 20 4 101．3 lO 8 1．41 In the range of3—7．here taking the value of5 Vacuum tank volume圪／L 150 Duct volume n／L 9．4 Pumping speed of the vacuulll pump 。 S／(L·5-1) 6 Molar gas constant R 8．3 I 4 Ambient temperature r／K 300 Cylinder(shot sleeve．runner and die cavity)with the volumes of 0．5 L．1．0 L and 1．5 L are taken for the calculation．ne calculation results are as shown in Fig．3． Whell¨=1．5 L，it only takes 0．73 s to decrease the pressure in the die cavity to 9．885 kPa by pumping and the lowest pressure can reach 8．974 kPa．taking only 0．83 s． ￡ jd 瓷 善 躥 蘭 營 § ．竺 凸 Fig．3．Relation curves between the die cavity pressure and time The calculation equations for the above theoretical model are established without considering the unceasing evacuation of the vacuum tank by the vacuum pump．The time of pumping the buffer tank call be estimated by Eqs． (7)，(8)[1l】： 一咕·g營， v=巧+％． (7) (8) where t is the pumping time，昂is the nominal pumping speed,V is the volume of the target container,n is the volume of the buffer tank,坎is the volume of the pumping duct，pi is the initial pressure in the target container,P is the target pressure in the target container, and the dimensionless quantity K is the correction cocfficient， taking l herein】． 111e time to pump the buffer tank to 0．8 kPa in advance is calculated as follows： ，=2·3量，q喜÷·g—P歹l一．．?！ぁひ?，、-··．，，、150i+94。x lg—100—000≈63 s． (9)l口一≈R． ●V-9 8 000 、7 There is a relatively long time not necessarily to be of vacuum before the liquid metal filling the cavity．This time can be used to pre·evacuate the vacuum tank．． 3 Design and Realization of a High Vacuum Auxiliary System On the basis of the above theoretical model．a(chǎn) high vacuum die casting auxiliary system is designed with following core parts such as the control system．vao]unl valve and pneumatic system．The design and realization processes will be introduced briefly in the following sections． 3．1 Design and realization of the control system The control system adopts the industrial control mode of “PLC+touch screen”．It can perform real—time monitoring of the vacuum pump，various solenoid valves，buffer tank， vacuum valve and die cavity．The schematic diagram and photo of the control system are given in Fig．4．The operation interfaces are given in Fig．5． 3．2 Design of vacuum valve and pneumatic control To achieve the best vacuum die casting result，it must be guaranteed that high vacuum level in the die cavity can be maintained until the cavity is completely filled with liquid metal．The vacuum pumping charmel should be closed at the moment that the filling ends．Therefore。the vacuum valve working mode is the key to realize this process．The developed vacuum valve has different structure from the existing onestl2-19]．A mode of“compressed gas+piston rod J．1abyrinth groove’’is originally created and the inertial impact of the liquid metal is used to close the vent channel．whose response time is only 1．5 ms．Therefore the ‘'whole·process vacuum venting'’is realized in die casting． Finally,the automatic reset of the vacuum valve is achieved with the assistance of the compressed gas．The structure and photos of the vacuum valve are given in Fig．6．The vacuum valve can also be controlled by the compressed air． The circuit diagram of pneumatic control of vacuum valve isgiven inFig．7． 萬方數(shù)據(jù) ·550‘ GE Xiaohong，et al：Design and Verification ofan Auxiliary System for High Vacuum Die Casting (a)Control system diagram I，2-Programmable controller；3-Touch screen； 4，5-Solenoid valve；6-Pressure transmitier； 7-Pressure regulating valve (b)Control system object Fig．4．Schematic diagram and object ofthe control system Fig．5．Operation interfaces ofthe control system (a)Inlemal structure ofthe vacuLim valre module ∞Outside configuration andphoto ofthe vacuumvalve Fig．6．Structure and photos of the vacuum valve Fig．7．Circuit diagram ofthe pneumatic control ofthe vacuum valve 1，5-2／2 way pilot-operated solenoid valve； 2，6～3／2Way pilot—operated solenoid valve； 3-3／2way direct-operated solenoid valve； 4-2／2way direct．operated solenoid valve； 7，8一Pressure regulating valve； 9一Pressure transmitter；10一Filter 4 Verification and Analysis of the Vacuum Pumping with the High Vacuum Auxiliary System 4．1 Experimental equipment and method ADCl2 alloy is used as the test material and the vacuum 萬方數(shù)據(jù) CHINESE JoURNAL OF MECHANICAL ENGNEERING ·551· die casting mould is designed for an automobile part(a bar)， whose mass is 1．04 kg containing casting itself,the gating system and the vent．As shown in Fig．8(a)，the developed vacuum valve and mould are installed on the die casting machine，F(xiàn)ig．8(b)shows the bar produced by this system． Fig．8．Experiment ofvacuum die casting A ccrtain amount of ADC 1 2 iS melted to 953 K．refined for 1 5 rain by blowing argon．a(chǎn)nd stood still for 1 0 min for thermal insulation casting．The mould iS heated to a preheating temperature 523 K with all electric heating rod． The vacuum pressure in the buffer tank iS set to 1 0 kPa． The vacuum levels following time in the cavity are shown in Fig．9．One curve is obtained by theoretical calculation and the other by measuring in the experiment process．ne two curves have the salne trend in the pressure drop section． 芒 暮 l 璺 魯 套 § 者 Pumping time t／s Fig．9．Measurement and theoretical calculation resuk 4．2 Observation of porosity castings The fatal defect of aluminum alloy die castings iS the porosity．By sampling，the X-ray inspection of the bars in connecting part without heat treatment indicates that the porosity of castings produced by conventional die casting is 1 00％．And the gas holes are widely distributed，whereas barely any gas hole can be seen in the castings by high vacuum die casting．The typical distribution of the gas holes are as shown in Fig．1 0．The‘'whole-process vacuunl'’ of this system iS synchronized with the iniection．a(chǎn)chieving the whole-process venting，and satisfying the requirement of low porosity to the greatest extent． Fig．10．X—ray pictures ofcastings produced by conventional and vacuum die casting 5 Conclusions (1)The cavity venting process with a buffer tank in high vacuum die casting iS calculated and analyzed．On the basis of the specific parameters selected for the developed high Vacuum die casting auxiliary system．the theoretical curves of cavity venting with different volumes are plotted． ResulB show that，for aluminum castings less than 4 kg，the cavity pressure Can be decreased to l 0 kPa within 0．8 S theoretically using the vacuum pumping with a buffer tank of 1 50 L．To a certain degree．it reflects the vacuum pressure variation仃end in the cavity． (2)A high vacuum die casting auxiliary system is developed based on the above calculation and analysis． This system has a touch control interface with advantages of the simplicity and fast response．a(chǎn)nd a mechanical 萬方數(shù)據(jù) 。552。 一鯉Xiaohong,et al：Design and Verification ofan Auxiliary System for High Vacuum Die Casting vaL：Illlnl valve that call achieve vacuunl venting during the whole process with the response time of only 1．5 ms． (3)The higIl vacuunl die casting auxiliary system has been used for trial production of ADCl2 alloy bars．The porosity decreases significantly compared to the conventional die casting technique． (4)The systematic studies on vacuunl die casting theory and equipment have a great guiding significance for high vacutlnl die casting，and can also be applied to other hiigh vacuum forming such as vacuulTl injection． References 【l】ZHAO Yunyun，WAN Li，PAN Huan，et a1．Design and application of the vo,c'tlllll,!shut-off valve and vacullln system for high vacuulll die casting[C]／／The 12th National Special Casting＆Nonferrous Alloys Annual Conference,Fuzhou,China,July 26—3 1．2008： 451—454．(in Chinese、 【2】ZHAO Ytmyun,WAN Li，PAN Huan，et a1．Development and application of high vacuum die casting in aluminum alloy[J]． Special Casting＆Nonferrous Alloys，2008，28(11)：858--861．(in Chinese) 【3】BAR-MEIR G，ECKERT E R G，GOLDSTEIN R J．Air venting in pressure die casting[J]．■Fluids Eng．，1997，1 19(2)：473_476． 【4】BAR-MEIR G，ECKERT E R G，GOLDSTEIN R J．A model of vacumpumping[J]．爿m紀(jì)Z Manuf．Scf．Eng．，1996，118(2)：259- 265． 【5】NOURI-BORUJERDI A，GOLDAK J A．Modeling of air venting in pressure die casting process[J]．ASME』Manuf Sci．Eng．，2004， 12“3)：577-581． 【6】HERNANDEZ J’LOPEZ J'FAURA F．Influence of unsteady effects on air venting in pressure die casting[J]．ASME Trans．，2001， 1 23(4)：884-892． 【7】HU Bo，ⅪONG Shoumei，MASAYAKI M，et a1．Calculation aad experiment oll cavity vacuum p托ssurc in v∞Bom die casting process[J]．China Foundry，2007，56(3)：258-261． 【8】HU 13,o，ⅪoNG Shoumei，MASAYAKI M，et a1．Comparison experiments on evacuating capability of a high vacum valve and a zig-zag valve in vacBunl die casting process叨．China Foundry, 2006，55(4)：355-359． 【9】WAN Li，PAN Huan，LUO Jirong．Application and development trends ofhiigh vacuu／ll die casting process and die casting aluminum alloys with high strength and ductility[J]．Special Casting＆ Nonferrous Alloys，2007，27(12)：939__942．(in Chinese) 【i0]WANG L H．Mathematical modelling of air evacuation in die casting proc6絡(luò)$via CASTvac and other venting devices[J]． International Journal of Cast Metals Research，2007，20(4)： 19l—197． 【1l】DAO Daan．Vacuum des／gn manua／[M]．Beijing：National Defense 【12】 【13l 【14】 【15】 【16】 【17] 【l 8】 【19】 Industry Press．2004．(in Chinese) IWAMOTO Termyu．ZI Ninngyoku．Vacuum control equipment and VacBLIm die casting method of die casting machine：China-CN 1709613【P】．2005—12—21． KAToU Koumei．Method to eontroI the vacllunl level in vaGuunl die casting and vacuum control to accomplish this mahod：China, CN 115092lfPI．1994一_06—04． L1 Yufeng．Die casting die designed and made by Japan-die casting method+local pressure pin[J]．En舒ne Fuel System，2000，12(4)： 2肚24． WISER J．A valve device used in die casting venting：Chi眠CN 1095000[P]．1994一l l—16． HODLER F．Die casting 1095653[P]．1994-1 1-30． LI Q B．Vacuum valve 25 1 8623[P]．2002—10-30． module with rant valve：China,CN of die casting machine：Chin亂CN GU Y W，L1 J X．Vacuum valve sllaletttre of a die casting machine： China,CN 2301318[P1．1998-12-23． ZHAO Y Y，WAN L，PAN H．Vacuum stop valve used for high vac'uum die casting：China,CN 201061824[P]．2008-05-21． Biographical notes GE Xiaohong。bom in 1964．is currently a Phi)candidate in School of Physics and MechanicalInstitute of Material Processing and Die ECKERT E R G;GOLDSTEIN R J Air venting in pressure die casting[外文期刊] 1997(02) 2.ZHAO Yunyun;WAN Li;PAN Huan Development and application of high vacuum die casting in aluminum alloy[期刊論文]-Special Casting PAN Huan;LUO Jirong Application and development trends of high vacuum die casting process and die casting aluminum alloys with high strength and ductility[期刊論文]-Special Casting XIONG Shoumei;MASAYAKI M Comparison experiments on evacuating capability of a high vacuum valve and a zig-zag valve in vacuum die casting process[期刊論文]-China Foundry 2006(04) 5.HU Bo;XIONG Shoumei;MASAYAKI M Calculation and experiment on cavity vacuum pressure in vacuum die casting process[期刊論文]-China Foundry 2007(03) 6.HERNANDEZ J;LOPEZ J;FAURA F Influence of unsteady effects o