Development of vacuum die-casting process

Overseas FoundryMay 2009Development of vacuum die-castingprocess*Masashi Uchida(Technical Development Center, Ube Machinery Corporation, Ltd, 1980 Aza Okinoyama, Oaza Kogushi, Ube, Yamaguchi 755-8633 Japan)Abstract: The vacuum die-casting process, started 25 years ago in Japan, has been widely applied. Thistechnology contributes very much to improvement of castings quality. The main factor causing the defects of diecastings is the trapped air in the mold cavity, while the key technology of vacuum die-casting process is to avoidthe trapped air effectively by evacuating the cavity before casting. At the same time, due to the shot speed andthe casting pressure reduced in half, the service life of the die is prolonged and the productivity is enhanced, aswell. Vacuum die-casting process is of great significance in improving the die castings quality and making up theshortcomings of super-high-speed shot casting.Key words: vacuum die-casting; die tooling; GF processCLC number: TG249.2Document code: AArticle ID: 1672-6421(2009)02-137-08Vr acuum die-casting process was introduced into massvacuum processes were proposed, designed and tested, butproduction in 1983, after gas free (GF) process wasnone was suitable for mass production. The reason is explainedinvented. Around the same time, OPTI-VAC process was using Fig.1.advocated in Europe, but it was not widely applied. On theThe process starts with evacuation when position signal Cother hand, GF process was introduced to all major die-castersis activated during low speed flling. The change of shot speedand automobile makers in Japan and 52,690 tons of castings to high speed mode is completed by position signal B; once(7.7% of total yearly die-casting production) was cast in 1992position signal A is activated, the shut off valve is closed nearwith this process. Soon after, GF process was introduced into shot end to avoid melt aluminum entering into vacuum line.European market and it became popular worldwide afterwards.The challenges for such process are the control of pouringIn these days, ultra-high-vacuum die-casting processweight of aluminum and also the motion time of shut off valve.has been developed in Europe and starts to be applied for If discrepancy or delay happens during the process (computerproducing large and thin parts, such as space-frames andtechnology was poor at that age and AC solenoid valve needspillars, which are usually welded to automobile chassis.8 ms error to be ON in the relay circuit), melt aluminum flowsThis report describes the mechanism of vacuum die-castinginto vacuum line and this would halt production. It will takeprocess, its development and applications.30 minutes to 1 hour to remove the intruded aluminum.GF process was invented in 1980 under the pressure of1 Circumstances of developing GFstrong demand for more reliable shut off valve. Figure 2 showsGF process schematically. The whole process is similar to thatprocess and its constitutionshown in Fig. 1, but the design for shut off valve is completelyMajor die-casting defects are blowhole, shrinkage, colddifferent. It closes passively using inertia force of metalshut, flow line and misrun. Except for shrinkage, all otherflow without being affected by porting weight error or shotdefects are associated with trapped air during feeding. Thisconditions. Such, the new vacuum process was establishedphenomenon had already been known and well-documented inshowing reliability and applicability for mass production.1980's, empiricism suggested to evacuate air in die cavity foreliminating these defects. According to these practices, many2 Know-how to use GF processeffectivelyPractical usage and features of GF process is summarized asMale, born in 1946, invited advisor of UBE Machineryfollows:Corporation, Ltd. He graduated from UBE National College(1) Vacuum time, should be within 03 s (maximum0.5 s). Ifof Technology, Japan in1967 and majored in Mechanicalpumping time (frorl中国煤化工to higl. to high speedEngineering. His research interests include die-casting,extrusion and semi solid casting technologies.command B) is mc:YHCNMHGofproducingE-mail: 11514u@ube-ind.co.jpblowhole defect increases.Received: 2008-06-15; Accepted: 2008-12-20Coating spray must be applied to die surface to prevent137.CHINA FOUNDRYVol.6 No.2Fig.1 Layout of early developed vacuum process中国煤化工MHCNMH GFig.2 Schematic drawing of GF process138Overseas FoundryMay 2009adhesion of aluminum before casting. Almost all water-basedenough evacuating ability as 20-27 kPa could be establishedsolvent is vaporized by the heat of die and the solute makeswithin 0.3 s.the coating film at the die surface. But very few amount ofCalculate airAssumeliquid may intrude into the gaps between die and cores, andvolume to beinspired airthose liquid remain liquid especially in the case of die toolingevacuatedvolumewith many slide cores. If die cavity is evacuated for more than0.5 s, remaining liquid can be sucked into the cavity surface几and vaporized rapidly when contacting the melt. The gas canCalculate| Calculate capacitybe easily entrapped into the casting and form blowholes. Sprayvent runnerof requiredliquid is diluted over 40 times with water and the vaporizedareaevacuationwater inflates 1 ,240 times in volume. Concerns about watervapor inflation must be considered even very small amount(4) Vent runner should be adopted at the final flling pointof spray liquid is employed. It has been demonstrated that determined by numerical metal flow analysis. One fifth of airvacuum within 0.3 s is appropriate to prevent above-mentioned still remains after evacuated to 20 kPa in the die cavity, so theproblem.remaining air should be pushed out by metal flow itself. Figure 3(2) Good vacum system and well designed die tooling needshows examples of defects expected from inappropriate designno sealing at the parting line of die or slide cores. An optimum of final flling point. Last remaining air could not be pushedcondition (without sealing) could be obtained by applyingout through air vent because it was already entrapped by melt.20-27 kPa vacuum.Modifying vent runner from side to bottom could remarkably(3) Die tooling for ventilation should be designed to have reduce blowholes.Fig.3 Partial shots showing wrong vent runner layoutFor design purpose, Fig. 4(a) shows an original die layout described previously. By moving the vent runner from side tofor a grill tray. This product is polished inside of tray portion bottom, Fig.4(c) shows the modified tooling and as a result,after deburring and then applied PTFE coating with burningthe production rejection rate has been reduced from 30% fortreatment. Blistering may occur at this treatment if blowholesthe design of Fig. 4(a) to 0.5%. The numerical simulation withexist at near surface of the casting. Figure 4(b) shows the firstCAE is shown in Fig. 5, where the final flling points can bedie layout for GF process with wrong vent runner design, aseasily identified without further casting trials.中国煤化工YHCN MHGFig.4 Modification of runner layout139.CHINA FOUNDRYVol.6 No.2Fig.5 Flow analysis result (ADSTEFAN)3 Characteristics of GF processFig.6 Mechanical type GF valveprocess to eliminate defects, such as blowhole, cold shut, balls to adjust locking force, it is always a challenge to adjustflow line and misrun. Like almost all other casting technique,two spring evenly. If this adjustment is not enough, shut offunexpected effect for GF process can also appear. That is,timing may be inaccurate and insufficient evacuation (shutgood parts can be obtained with lower shot speed and/or lower off faster with too small locking force by groove wear) ormetal pressure. For instance, shot speed can be reduced from 2.5aluminum intruding into the vacuum line and jam (shut offm/s to 1.5 m/s, or metal pressure can be reduced from 80 MPaslower with too much locking force) may occur. It is alsoto 50 MPa without sacrificing casting quality. This is becauserecommended that replacing valve set to a new one if there isexcess power was needed to cancel backpressure of remaininga sign of jam-like error detected by electric signal. Valve unitgas in the cavity without GF process. At casting site, reduction looks like a cassete unit and can be replaced easily within 10of shot speed and metal pressure prevents burrs remarkably,minutes. Some customers replace valve unit every 4 hoursand it also prolongs die life, eliminates deburring operationeven if there is clear evidence of jam.and increases up time of casting machine. In starting up newFurther investigation on the collision force applied toparts, good parts can be easily obtained with GF process sothe main valve indicates that jam might be caused by suchmodification process of die tooling can be reduced, and totallycollision force intermittently against supposition. Thisnew product can be introduced into mass production in a phenomena is so called“flying melt", as indicated in Fig.6.relatively short period of time. .If the vent runner is straight and short, jam can easily occur.But there are some disadvantages. One of them is thatEffective coutermeasures to avoid jam are to make vent runneraluminum melt can intrude into vacuum line when aluminumlonger and add more than two of 90 degrees turns, this isdreg stuck at shutting valve sheet during last shot. Theilustrated in Fig.7.procedure of trouble-shooting of this problem is described inFigure 7(a) shows the moment before GF valve opens. Asthe next section.air pressure is introduced into Room-A, the main valve ispushed downward and the piston touches O-ring. Under this4 Evolution of GF valvecondition, bring air pressure into Room-B continuously andFigure 6 shows a schematic of a mechanical type GF valve.exhaust air in Room-A, air pressure force in Room-B pushsthe piston downward and keep the main valve open shown inMain valve is locked by 'open lock ball' that holds valve inFig.7(b). Vacuum evacuation is done in this procedure.open position against shut force of‘shut spring' and vacuumShot velocity shall be changed to high speed mode at thesuction until melt front hits the valve. Open lock ball must bemoment that vacuum pressure reaches the set value. Figureadjusted to loose against shut off force just before metal front.7(c) shows the moment when main valve shuts off. If vacuumIf locking force is weak, main valve shuts with vacuum forceis high to increase the shot speed so that the front of the meltbefore melt comes to the valve portion. In this case, a largepushes the main valve hard and breaks the O-ring seal, theamount of air still remains in the cavity. On the other hand,air could leak through the gap. Then air pressure starts to actvalve does not shut at proper moment if locking force is strongon both upper and bottom sides of the piston, and upwardand aluminum may intrude into the vacuum line. Therfefore, .force dominates because the effective cross section areas areit is critical, and also difficult, to adjust locking force of‘opendifferent (Rod-A is thinner than Rod-B and bottom side arealock valve'. It has been suggested that the locking force isof the piston is I中国煤化Lrain valve goes upadjusted once a day with an exclusive jig.and shuts vacuuYHCNMHGEarly GF valve adopts mechanical lock system (shown inCharacteristics ot this air-driven type GF valve is easyFig.6), the locking force reduces with time as the groove into adjust locking force by simply controling air pressuremain valve wears. Though there are screws behind open lock140Overseas FoundryMay 2009(a) Just before valve open(b) During evacuation(c) Shut off valve(d) Shot endFig.7 Air drive type GF valveintroduced into Room-B. As a result, the locking force can beadjusted during continuous casting operation without usingspecial jig. Since there is no mechanical lock, wear is almostnegligible. Another feature of air-driven GF valve is its highsensitivity of starting shut off movement. In contrast to 0.6 mmvalve lifing distance for mechanical type valve, only 0.2 mmlifting is required for air-driven type GF valve.A recorded 92,000 shots run without a jam has beenreported for air-driven GF valve. Many user add their ownknow-how into die tooling for stable production. Details of dietooling is beyond the scope of this paper. As a matter of fact,Fig.8 Density of plate type castings at various shotsthe GF valve has promoted die design in Japan. In the meanime, UBE, the inventor of GF valve, has been continuouslyinvestigating and adopting new designs and technologies tominimize jam problem. For example, more sensitive reactionagainst melt collision in starting vacuum evacuation canbe achieved by changing operating sequence (i.e. reducingair pressure in Room B, open-holding power, and then addpressure in Room-A).5 Relationship between high-speedshot and vacuum die-castingFig.9 Gas volume involved in plate type castings atEffect of vacuum process was described in the previousvarious shot velocitysections. For a direct comparison, a same dimension ofgas content in the casting using both processes increases withADC12 plate (equivalent to AA383) was used (200 mmshot speed, GF vacuum casting dramatically reduces the gaswidth, 270 mm height and 4 mm thick), the gate thicknesslevel by almost one order of magnitude.was 2 mm, and a metal pressure 50 MPa was employed. TheIt has been found that blowhole defects seem to disappearadvantages of vacuum die-casting over high-speed shot castingwith increasing shot velocity over 4 m/s, especially on theare ilustrated in Figs. 8 and 9. Figure 8 shows the change inmachined surface. However, entrapped gas still remains, anddensity of castings as a function of shot velocity, produced by the small porosities became uniformly distributed and hardlynormal die-casting and GF ulra-high-vacuum process. It canseen with naked eyes. Small blowholes might cause leakage ifbe seen clearly from Fig. 8 that casting density using GF ultra-linked together anc中国煤化工8 is used. Thevacuum casting is consistently higher than those with normalcomment“ThereiFfor ultra-high-die-casting. It is also noted that the density does drop withspeed die-casting"YHCNMHGofdscussngincreasing the shot speed in both cases. Figure 9 shows gas defects on machined surfaces. It is believed that vacuumcontent in the casting as a function of shot speed. Though theprocess is necessary to get high quality castings combined with.CHINA FOUNDRYVol.6 No.2ultra -high speeds.ejector pinhole) using O-ring and piston ring around plungerData of vacuum process presented in Fig.8 and Fig.9 weretip. In comparison, 20-27 kPa vacum can be achieved in a .made with GF ultra-vacuum process, which will be describednormal GF process without any sealing.in the next section. Usual GF process can also get such a goodNormal GF process evacuates die cavity to about 20 kPadata if desirable conditions, especially die tooling, are met.Casting produced under such conditions involves gas content remnant gas must be pushed out through air venting slotof lower than 4 mL/100g Al, which is within the limit ofby metal flow (with adjusting shot conditions). Die toolingcastings for welding.having long flow length is not suitable for manufacturinglarge and thin castings because long metal flow reduces metal6 Ultra-high-vacuum die-castingtemperature, which in turn may cause defects like misrun orcold shut. By increasing the vacuum up to 5 kPa, remnant airprocessis reduced to one twentieth of atmospheric pressure, so flowUltra-high-vacuum die-casting process was first proposedroute can be minimized without considering exhausting airand designed in Europe around 1995 for producing large andfrom venting slot. Schematic diagram of ultra-vacuum systemthin automobile parts, such as pillars and space frames. Thesewith GF valve is shown in Fig.10. The system is almost sameparts need to be welded so gas containing must be loweras normal GF system except for adding seals between dies andthan 4 mL/100g AI. Ultra- vacuum as 5- 10 kPa is generallycores and piston ring around plunger tip.achieved by sealing between dies and cores (sometimes atWith the improved design for ultra vacuum, evacuation time :中国煤化工MHCNMH GFig.10 Ultra-high-vacuum die-casting system with GF valve142Overseas FoundryMay 2009during casting becomes more important. It is better switchingnewsystemhas1mhoseatpartAand5-7matpartB,andfrom slow speed to high speed within 0.5 S to prevent“flyingthe hose at part B is also doubled in thickness. Combined withmetal" from overhanging on the distributor and being inspiredlarge exhaust volume of GF valve, this unit pumps fast vacuumby vacuum flow into the cavity, and this has to be changedas 5 kPa within 0.32 s. An overall comparison betweenconventional GF process and ultra-vacum die-casting processOther featured changes for ultra-vacuum compared with the is tabulated in Table 1.normal GF process are: (A) doubled flow capacity of vacumswitch valve and vacuum tank capacity; and (B) the location7 Comparison of die tooling betweenof the vacuum switch valve. The switch valve is used to locateGF process and ultra-high-vacuumon tank side with the vacuum pump as one unit to be portabledie-casting processeasily, and is connected with GF valve by long hose. Refer toFig. 10, GF system has 5-7 m hose at part A and 0.5 m at part B.As described in section 6, die tooling for normal and ultra-So the system must evacuate two to three times air in the hosehigh-vacuum GF die-casting are different. Schematic drawingsthan cavity volume. It is not an issue for normal vacuum asshown in Fig.11 explain basic concept of both processes. Thelow as 20 kPa and it can be reached within 0.3 s. However, for keys for both processes are directional flling for normal GFultra-high-vacuum as high as 5 kPa, the vacuum switch valvevacuum die-casting and shortened flow length in ultra-high-has to be moved to the top of the ejector side platen. Such thevacuum, respectively.Table 1 Comparison of normal GF process and ultra-vacuum die-casting processUtra-high-vacuumTemNormal GF processUBE GF processOthersVacuum pressure20-27 kPa5-10 kPaTime to be evacuated0.3s0.32 s1.0-1.5sDie parting0-ringCore0-ingSeals---Ejector pinPiston ringPlunger tipSpecial vacuum sealingGas content in products2-10 mL100g Al2-4 mL/100gAIUsageGeneral die-castingPillar, sub-flame, node, bracket, upper-arm(a) GF normal vacuum(b) GF ultra-vacuumFig.11 Die tooling for normal and ultra-vacuum die-casting system with GF valveIt is now widely accepted that ultra-vacuum die-casting8 Summaryproduces best weldable products. But normal GF process can中国煤化工ind developedalso get such weldable quality with minimum blowholes underGF die-casting proproper conditions, as shown in Fig.9. Common practice inquickly into mass IYHC N M H Gocess, severalindustry is that apply ultra-high-vacuum process for thinnervacuum processes were proposed. In 1995, ultra-high-vacuumplates of less than 4 mm with flow length longer than 700 mm.process first appeared and several ultra-v acuum processes143.CHINA FOUNDRYVol.6 No.2were also developed in this decade. But application of thesetechnologies and make stable mass production at high shotultra-vacuum processes is limited in automobile body parts.velocity of 6-7m/s.All products are large and need huge sized die-castingRecent automobile parts, especially safety suspension partsmachine like 2,500-4,000 tons of die clamping force, so thesethat need high strength or rigidity, are converted from ductileprocesses did not become popular among car makers and largecast iron to heat- treated gravity aluminum casting, and then todiecasters. General die-cast parts do not need ultra-vacuum heat-treated aluminum die-casting. In this case, the cast wall isprocess. Normal vacuum process is good enough to eliminateas thick as 20- 30 mm. To get lighter and thinner suspensiondefects except for shrinkages.parts, the tendency of transition from thick aluminumIt is reported that the combination of ultra-high-vacuum andto lighter rib-structured thin aluminum casting becomesultra-high-speed shot process can produce high quality castingsnecessary. Such transition was already witnessed in Europehaving 8%- 15% elongation with lower metal pressure.more than 10 years ago.This indicates high quality castings may be produced withAs described above, die-casting machine and casting processthe casting machine now in use with further modifications.are now in progress to seek new technologies for the future.Technologies for ultra-vacuum process and multi-metal It is expected that vacuum casting process will play a majorpressure (including low pressure) casting have become mature.role in die-casting industry for both product quality and massIt will be challenging for the machine maker to combine thoseproduction efficiency.中国煤化工YHCNMH G144