Vacuum-sealed casting process under pressure

CHINA FOUNDRYAug.2006Vacuum- sealed casting process underpressure*Ll Chen- -xi1, GUO Tai-ming, WU Chun-jing, WANG Hong*(1. School of Materials Science and Engineering, Shenyang University of Technology, Shenyang, 110023, P. R. China; 2.Department of Mechanical Engineering, University of Akron, Akron 44304, USA; 3. School of Materias Science and Engineering,University of Science and Technology Beijing, Beiing 100083, P. R. China; 4. School of Foreign Languages, Shenyang Universityof Technology, Shenyang, 10023, P. R. China)Abstract: A new casting method, the vacuum-sealed mold casting under pressure, has beendeveloped, and thin wall iron castings with high precision and smooth surface have been pro-duced successtully with this casting method. The experimental results show that the liquid ironhas a very excellent fling ability because a high negative pressure is formed in the mold cavityduring flling process. The vacuum- sealed mold under pressure has very high compressivestrength greater than 650 kPa, which is 3- 4 times as high as that of the molds produced byhigh-pressure molding process or vacuum-sealed molding process.Key words: casting process; vacuum-sealed mold; pressure castingCLC number : TG249.9Document Code: AArticle ID ; 1672- -6421(2006)03 -0190- -05T t is ofgreat importance to investigate the castingV-process. After the mold is made, it is placed into a vessel,processes for high precision thin wall ferrous metal partsas shown in Fig. 1. By removing the air through exhaustingpipe, the vacuum level in the mold can be maintained at aconsumption, saving raw materials and decreasing machinefixed value. The liquid metal is poured into the ladle insideweight. To achieve these objectives, many casting methodsthe vessel. Then the vessel is sealed; and the air pressure inhave been developed. V acuum- -sealed molding process, thethe vessel is increased to the designated value by pumping airV-process for short, is widely used to make iron castings withthrough the channel. After that, the liquid metal is poured intorelatively thin wall, high precision and smooth surface (1-4the mold cavity by turming the rocker arm. During the processHowever, the process cannot be used to pour castings withof flling and solidification, the air inside the mold isvery small wall thickness, because the liquid metal flling in acontinuously sucked out through the pipes and the mold ismold cavity relies on only the static pressure. head in thekept in a vacuum state. Hereafter, liquid metal flls andV-process. Moreover, the process cannot produce castingssolifies under the high pressure.that require very high dimension accuracy due to therestricted compressive strength of the mold.In order to improve the flling ability of the liquid metal andincrease the compressive strength of mold, the authors havedeveloped a new casting method named as thvacuum- -sealed mold casting under pressure, or P- V process.Although this method is based on V-process, it is differentbecause in the P-V process the liquid metal flls andsolidifies in a vacuum- -sealed mold under high pressure. Byusing the new method, castings with thin walls, smoothsurface and accurate dimensions have been producedsuccessfully.The mold used for P-V process is similar to that used for中850中国煤化工，*L Chen-xir; 3. Channel for thermalMale, bom in 1956, Prof., Ph.D. Being engaged in the re-HHC N M H G6. Exhausting air vave; 7.search on casting alloy and casting processPressure merer; 8.vessel cover; y. Hocker arm; 10. Pumping airE-mail: lichx@sut.edu.cnchannel; 11. MoldRecelved date: 2006- -02-16; Accepted date: 2006 05 -10Fig. 1 Sketch of the equipment used for P-V processVol.3 No.3.In P-V process, a small section of the plastic flm (area 1-2 for P-V process.cm2) on the top of the mold cavity is cut, with being replacedThe photographs of the actual castings are shown in Fig. 3.by the vent screen (teel mesh). The vent screen is located at The casting on the left in Fig. 3 is the casting made with P-Vthe highest point of the mold cavity, as shown in Fig. 2. Theprocess, and the casting on the right is the casting made withpurpose of the plastic film on the mold is to form pressure V-process. The castings in Fig. 3(a) and (b) are the same, butdifference between the inside of the mold and the mold cavity.these two photographs were taken from the different directionsGenerally speaking, the mold can be formned and kept from to show the shapes and sizes of the castings in 3D.collapsing when the pesure diference is more than 50kPa29.There was obvious difference in appearance between theIn P- _V process, the function of the vent screen connecting the castings shown in Fig. 3. The casting made with P- -V processmold cavity to the mold is to promote the liquid metal flowing was complete without defects such as cold shut and misrun.into the mold cavity by pulling gas or air from the mold cavityThe casting surface was. smooth, and the thickness variationthrough dry sand in the mold. When there is such a vent was smaller. However, the casting made with V. -process wasscreen, the pressure difference decreases during pouring; butincomplete with rough surface and bigger thickness variation.it is sill higher than 150 kPa, far greater than 50 kPa. The photographs show that the P-V process is suitable forTherefore, the vent screen doesn't destroy the function of thethinner castings.plastic flm on the cope mold.The experimental results showed that in P-V process liquidmetal has excellent flling ability in the mold cavity, and thecastings have good surface quality and dimension precision.Therefore P _V process can be used to produce thin wall ironcastings with high precision. In microstructure there was noobvious difference between the two castings, both consisting>4of pearlite and graphite.(a)1.Pipes for sucking air; 2. Pressure dfference sensor;3. Vent screen; 4. Plastic fimFig. 2 Skelch of the spot of the vent screen and themeasuring method of pressure dfference.Experimental materials and equipmentThe experiments were conducted with normal dry silica sand.The mold density was 1.57 g/cm3 after the flask was flld with(b)sand and well vibrated. The composition (wt.%) of the ironcastings made with P-V process was 3.2%C, 2.0%Si, 0.9%Mn, 0.12% P and 0.10% S with balance of Fe. .The compression strength of mold samples was measuredwith a SQY Strength-Testing Device. The pressure differencebetween the inside and outside of a mold was measured with aFig.3 Castings cast by P-V process (a) and V-process (6)Model MPX-270 Pressure Difference Sensor. The experimentequipment/setup used in the study on the P-V process isshown in Fig. 1.2.2 Flling ability of liquid metal2 Experimental results and analysiselThe measuring method is shown as in Fig.2, in which the2.1 Pouring experimentschange of pressure difference was revealed by a pressuredifference sensor. The variation of pressure difference be-For comparison, gray iron plates were cast respectively usingtween中国煤化工mold cavity was mea-V-process and P-V process. The dimension of the castingmade with P- -V process was 240 mm x 240 mm x 1.7 mm,suredYHC N M H Ghe xpermental resulsand that made with V-process was 240 mmx 240mmx 2.4mm. are shown'n ng. 4. ine vananon 01 the pressure diferenceThe absolute pressure in the molds was 0.04 MPa at thewas very small for V-process. While in P-V process, the airbeginning, and the pouring temperature was 1 300C for bothpressure in mold cavity increased slightly at the beginning ofprocesses. The absolute pressure in the vessel was 0.30 MPathe pouring process, and then the pressure decreased rapidly|1911CHINA FOUNDRYAug. 2006resulting in the quick increase in pressure difference. Thehighest-pressure diference was 60 kPa during the fllingously increase the flling ability of liquid metal.process. As shown in Fig. 2, the exhausting opening (ventIn practical casting production some common approachesscreen) connected the mold cavity to the mold ofP-V process,are applied to improve the fling ability of liquid metal,which promoted the air or gas in the mold cavity to exhaustincluding increasing the static pressure head of the liquidthrough the opening during the flling process. Consequently,metal, to increase the mold's temperature and to increase thethe air pressure decreased in the cavity to the level lower thanflling pressure. The above analysis showed that decreasingits environmental pressure.the pressure in the mold cavity is also an ffctive way toincrease the flling ability.10-2.3 Compressive strength of the mold samplesFor comparison the compressive strength of the mold samples-10of both V-process and P-V process were tested. The samples+Cwere made of no- -binder dry sand in the shape of eylinder,-30which were 50 mm long and 50 mm in diameter.The compressive strength of the V-process mold samples-5C~ 50was measured under atmospheric pressure (0.1 MPa). Duringthe measuring process, the absolute pressures in the samples-70545were 0.02, 0.04, 0.06 and 0.08 MPa respectively. Hence theTime. spressure diferences between the inside and the outside of the●V- Proces;■P-V processmold samples were 0.08， 0.06， 0.04 and 0.02 MPaFig. 4 Pressure diterence changes with time in therespectively.process of f侧igFor P-V process the compressive strength was measured inthe vessel. The absolute pressure in the samples was 0.02Based on Bermoulli equation同, the efet of the pressure dif-MPa and the absolute pressures in the vessel were 0.2, 0.3ference on the fling ability could be analyzed as fllows: as-and 0.4 MPa respectively. Thus the pressure differencessuning that the air pressure diference between the inside andbetween the inside and the outside of the mold samples werethe outside of the mold cavity is zero during the flling process0.18, 0.28 and 0.38 MPa respectively. The testing method offor V-process, the sprue height is z, the gravity acceleration isthe compressive strength of the mold samples was such thatfirstly the strengh-testing device was placed in the vessel,g, and the all pressure loss is Eh,in the pouring system, thenthen the prepared sample was mounted on the device whilethe flow velocity of the liquid metal at the gate of V-process isits vacuum level was maintained unchanged, and then thevessel was sealed, lastly when the pressure in the vessel was(1u=[2g(z- Zh)]主increased to the designed value, the compressive strength ofFor P-V process, if the average pressure diference betweenthe mold samples was measured. The loading process and thetested results were observed through the observing window onthe inside and outside of the mold cavity is OP, the density ofthe vessel's cover.the liquid metal is γ and other parameters are the same asThe measurement results on the compressive strength arementioned above, then the flow velocity of the liquid metal atshown in Fig. 5. The compressive stress given in the figure isthe gate isthe ratio of total load along the axial direction to the area ofthe original cross- section of a mold sample. The compressiveu=2gr-Zh,+AP )(2)strain is the ratio percentage of the length variation to theoriginal length of a sample. The values marked next to thecurves in the figure are the pressure diferences between theConsequently, Equation 3 is gained by equations () and (2)inside and the outside of the mold samples. If the values areu=(u'++ 2gAP广(3)0.08, 0.06, 0.04 and 0.02 MPa, the curves represent therelation of stress with strain for V-process and if the values ofAs shown in equation (3), the pressure diference SP is thethe∩12_ ^28 and 0.38 MPa, thesource to promote the liquid metal to flow more quickly. Forcurv中国煤化工on for P-V pocse.TYHC N M H Gd compesive strengthexample, ifu =0.6 m/s, y=7 500 kg/m' and AP is 25 kPa (av-of the V-process Is a lttle hugher than that of high-pressureerage value), the flow velocity calculated by the equation (3)molding process and the ltter is at about 150 kPa间In thisis un=0.98 m/s, or 4-4 x 100% =63%. As shown by thepaper, the compressive stress corresponding to 5%U1compressive strain is defined as the compressive strength. AtVol.3 No.3the 5% compressive strain, the compressive stress for thepressure diference on the burming-losing velocity of the flmV-process was about 150 -230 kPa (Fig. 5). It can be seen inwas specifially investigated. .Fig. 5 that the greater the pressure diference between theWhen the pressure differences were 0.05 MPa and 0.25inside and the outside the mold, the higher the moldMPa, the flm's buming-losing velocity was measured. For thecompressive strength. The compressive strength for P-VV-process, the absolute pressure in the mold was 0.05 MPa,process was higher than 650 kPa,3- -4 times as high as that ofthe atmosphere pressure was 0.1 MPa, s0 the pressurethe molds produced by high- pressure molding process or bydifference between the inside and the outside mold was 0.05V-process.MPa. For the P-V process, the absolute pressure in the moldwas 0.05 MPa, the absolute pressure in the vessel was 0.310000.38MPa,0.18MPaMPa, thus the pressure dfference was 0.25 MPa.P 0. 28MPaThe bumning- _losing velocities of flm were measured when a800coating layer was and was not applied on the plastic flm. The里e000.08MPacoating was composed of water, clay and graphite powder.Afer the coating was sprayed on the platic flm, it was dried黑400in the air., 0.04MPaFigure 6 shows the experimental method. When the2000.02MPatemperature of the electric stove was heated to 900C(1 652F ), the mold was mounted over an electric stove forbaking. The surface temperature of the electric stove was5253Strain,Gmaintained at 900 C(1 652F ) in the baking process. Thedistance from the plastic flm of the mold to the top surface ofFig. 5 The relationship between stress and strainthe electric stove was 2.0 cm. During baking the film bumtunder various pressure diferenceslosing gradually and the pressure difference declinedThe above experimental results suggested that the moldgradually. The velocity of the pressure difference decreasingreflected that of burming-losing of the film. The changes of thecompressive strength can be greatly raised by increasing thepressure dfference were measured with a pressure differencepressure difference between the inside and the outside thesensor as shown in Fig. 6.mold. This conclusion has a high practical significance.The mold compressive strength in P-V process results fromthe pressure diference between the inside and the outside the_mold. The larger the pressure diference, the larger the frictionamong sand grains, and the more difficult the movement ofsand grains against one another, leading to higher moldcompressive strength. High compressive strength is beneficialin producing castings with high dimension accuracy.Although approaches such as increasing binder content,baking green mold and using resin bonded sand can allimprove the mold compressive strength, they will also increase1. Plastic film; 2. Pressure diference sensor;the production cost grealy. Similar to the V-process, the3. Mold; 4. Electric stove; 5. Thermal coupleP-V process only requires that, before pouring liquid metal,Fig. 6 Testing method ilstraton of pressure dterencethe pressure in the vessel should be increased to a designedvalue, in which the cost increase is limited.Figure 7 shows the measured results. There was not coatinglayer on the plastic film for the V-process. When the mold2.4 Fim's burming- _losing velocityas baked more than 90 seconds, the pressure differenceUnder high temperatures the plastic film on the surface of thebegan decreasing. In the P-V process, when there was nomold cavity softens and melts, then the film evaporates andcoating on the plastic film, the pressure diference started todifuses into the mold sand under the effect of pressuredecrease greatly after the mold was baked for 50 seconds.difference, and in the process the mold loses its airproofBut when there was a coating layer on the plastic film for P-Vcapability gradually (-4. Such process is named as the process, the pressure diference began to decrease after theburning- -losing process of the plastic flm. Many factors affectmol中国煤化工he experimental resultsthe velocity of buning-losing of plastic film, such as the typeshowEeity of buming-losing ofand thickness of plastic film, casting size, pressure differencethe|YHC N M H Groces, so that the moldthe inside and the outside of the mold, thehas better airproof property. The burning _losing velocity wastemperature of liquid metal and whether there is a coating higher for P-V process when there was no coating on thelayer on the plastic flm. In the present study the efct of theplastic film. But when there was a coating layer on the plastic1931CHINA FOUNDRY. Aug. 20063 Conclusions(1) A new casting method, the vacuum- -sealed mold casing士process under pressure, has been developed.200(2) Thin wall iron castings with high precision and smooth星150surface have been made successfully with this new process.(3) The high pressure difference formed between the insideg 10and the outside of the mold cavity is the promoting source forthe liquid metal to have excellent flling ability.(4) The compressive strengh of the mold is higher than 6505010140kPa for P-V process, which is 3- 4 times as high as that of theTime, smolds produced by high-pressure molding process or byvacuum- -sealed molding process. The high compressive■V-Process without a coating layer;◆P-V Process withoutstrength is beneficial to improve the dimension precision ofa coating layer;●P-V Process with a coating layercastings.Fig. 7 Pressure diference changes with time during baking(5) The buming- _losing velocity of plastic film on the moldflm, the velocity of buming-loss reduced greatly and thesurface is higher for P-V process; however, when a coatingmold had good sealing capability.layer is sprayed on the film, the velocity of burning-losingIn the V-process the burning -losing velocity of the film was reduces grealy and the mold has good airproof property.slower because smaller pressure difference was formedbetween inside and outside the mold. In the P-V process,Referenceswhen there was no coating on the film, the greater pressuredffererce was formed, promoig the dfusios of the [1] schnedton P. Production of castins by he vacwum selemolding process using binderless sand. Foundry Trade Journal,evaporated flm into the mold sand. Therefore the process of1974,136: 723-733.film burning- -losing was quicker. When there was a coating[2] Noble Z P. Report on the second annual European V-processlayer on the film in P-V process, the velocity of filmtechnical meeting. Foundry Trade Journal, 1978, 144: 741-750 .burming-losing decreased obviously because the coating layer[3] Jain C K. Simulation behavior of sand under vacuum in V-prodecreased the pressure diference and lowered the filmcess. APS Transactions, 1986, 94: 657 662.temperature.[4] Ryojir 0. Accuracy of iron castings production by the V-pro-Figure 8 shows some samples used for measuring the fllingcess. The Brish Foundryman, 1995, 68: 70-78.ability ,the pressure in the mold cavity, solidification rate and [5] Kuisheng L. Casting Process and Principle. Beijing : Mechanicaltemperature distribution. Now the research tasks in laboratoryIndustry Press, 1998. (in Chinese)have been basically completed and the investigation on how to [6] Bishop D. Mechanical propertes of Vprocess molded steelproduce castings with the P-V process will be conducted incastings. APS Transactions, 1983, 91: 441446.7] Enderle R J. Introduction of vacuum molding at Buckeye Steel.the near future.AFS Transactions, 1979, 87: 59 64.[8] Murton A E. An evaluation of the vacuum mold process. AFSTransactions, 1978, 86:43-49.The research is supported by The Science Research Project ofthe Education Department of Liaoning Province, P. R. China, No.05L294Fig. 8 Some samples cast by P- Vprocess中国煤化工MYHCNMHG