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Cavity pressure measurements and process monitoring formagnesium die casting of a thin-wall hand-phonecomponent to improve qualityK.K.S. Tong*, B.H. Hu, X.P. Niu, I. PinwillPrecision Metal Forming Group, Gintic Institute of Manufacturing Technology, Singapore, SingaporeAbstractThe die casting of thin-wall magnesium components free of voids and having complete filling, resulting in high strength, can only beachieved under optimum cavity pressure. The pressure peaks occurring in the cavity are an important criterion for the consistent quality ofthese parts. The results from the measurement and monitoring of the cavity pressure can achieve minimum scrap and ensure constant quality.# 2002 Elsevier Science B.V. All rights reserved.Keywords: Cavity pressure; Die casting; Thin-wall hand-phone component; Magnesium1. IntroductionThe cavity pressure during the casting of a magnesiumthin-wall hand-phone component is one of the criticalfactors in determining the quality of the part. The commontraditionally measured parameters in die casting are thetemperature, the plunger velocity and the machine hydrau-lic pressure. However, the cavity pressure, the obviouslydeciding parameter, was never really looked at in the past,mainly because of the lack of reliable pressure sensors towithstand the hostile environment of die casting. In thisinvestigation, indirect piezoelectric cavity pressure sen-sors with a miniature 4 mm diameter pin are utilisedbecause of the geometry and size of the component.The metal pressure at the runner and cavity were measuredduring casting. The measured metal pressure profiles candetermine earlyclose-offof thegate due to freezing, whichwill result in improper liquid metal flow into the cavity,lack of pressure transmission from the machine injectionsystem to the casting and reduction in gate freezing time1. The effect of process parameters on the gate freezingtime such as the hydraulic pressure and metal velocitywere studied.2. Process monitoring development and description2.1. Hand-phone die, pressure sensors andtemperature sensorsThe experiments were conducted using a thin-wall hand-phone component die; Fig. 1 shows the fixedand moving diehalves.In order to measure the metal pressure and temperaturevariations during the casting process, pressure and tempera-ture sensors were incorporated into the die. Due to the smallhand-phone size, direct pressure measurement was physi-cally limited because the minimum diameter of sensoravailable commercially was 15 mm. Therefore, an indirectpressure measurement was used with a 4 mm ejector pinplaced in the cavity with a quartz force sensor behind theejectorpin.Thesensorandthedetails ofthespecificationareshown in Fig. 2 and Table 1, respectively.The temperature sensor used was a normal K-typethermocouple of 2 mm diameter, inserted in the die10 mm away from the cavity surface. The temperaturesensor was to ensure that the die temperature was main-tained during the casting process. The indirect pressuresensors were placed at the runner area (before thegate) andin the cavity (after the gate), the objectives are to evaluatethe gate freezing behaviour and the gate freezing timeduring the casting process, which will effect the behaviourof the metal pressure profiles of the cavity and the runner2.Journal of Materials Processing Technology 127 (2002) 238241*Corresponding author.E-mail address: .sg (K.K.S. Tong).0924-0136/02/$ see front matter # 2002 Elsevier Science B.V. All rights reserved.PII: S0924-0136(02)00149-82.2. Die casting processThemagnesiumhand-phonecomponentwascastusingan80 t hot-chamber machine. Various casting conditions wereused to obtain an acceptable quality component. The con-ditionsusedforthediecastingofthemagnesiumthin-walledhand-phone component are given in Table 2. The settingsgivenwereasdisplayedinthemachine:theactualvaluescanonly be obtained from the monitoring curvesof the machine.3. Results and discussion3.1. Metal pressure during castingTwo cavity pressure sensors were placed into the die; onein the runner just before the gate and one after the gate at thecavity, for measurement of the pressure variations duringcasting. Fig. 3 shows typical pressure profiles measuredduring the casting. The pressure variation during the castingcan be characterised as three distinct stages, as indicated inthe figure. Stage 1 is associated with the melt flow throughthe sprue post and runner. The pressure initially is almostzerobecauseoftheverylowresistancetothemetalflow.Thepresence of a small pressure peak (approximately 50 bar,arrowed) indicates the melt reaching the gate. The slightincrease in pressure is due to the cross-sectional reductiontowards the gate. Upon further filling to the cavity, thepressure is established rapidly. As soon as the cavity isfilled, there is a relatively stable stage (stage 2) to maintainthe high pressure to be transmitted from the hydraulicsystem to the cavity. After this period, the pressure at thecavity decreases, whereas the pressure in the runnerincreases (stage 3), indicating that the gate has been frozen,resulting in a high resistance to the metal flow in the runner.Thismetal pressurebehaviourisverysimilartothatreportedby Komazaki 3.3.2. Metal pressure profiles against casting defects(part quality)Themetal pressure profilesinthe runner andcavitycan beused to determine various casting defects. These defects orinconsistencies are pre-freezing of gate before cavity fill,sprue and nozzle blockages, incomplete filling, insufficientmetal flow, severe turbulent flow due to the jerking effectsof plunger, flow lines, insufficient pressure build-up in therunner, separation of liquid metal from the main streamand incorrect settings of the machine 4. Examples ofthis phenomenon (casting defects) with the metal pressureFig. 1. Fixed and moving halves of the thin-wall hand-phone die.Fig. 2. The Schlaefer quartz force sensor used in the experiments.Table 1Specification of the force sensorTypeRange(kN)Sensitivity(pC/N)Linearity(%)Temperature(8C)Quartz force sensor0104.00?2400Table 2Die casting conditions for the 80 t hot-chamber machineStroke (mm)Plunger slowspeed, firststage (%)Plunger fastspeed, secondstage (%)Hydraulicpressure(%)7621020457590Fig. 3. Typical measured cavity and runner pressure profiles duringcasting.K.K.S. Tong et al./Journal of Materials Processing Technology 127 (2002) 238241239profiles are shown in Figs. 46. Fig. 7 shows an optimummetal pressure profile to obtain a part of reasonable quality.The pressure profiles can be used as a reference processsignaturetoverifyanyinconsistenciesintheprocesssettingsand part quality.3.3. Estimated gate freezing timeThe behaviour observed in stage 2 of the measured metalpressure profile (Fig. 3) is very interesting as it is closelyrelated to the gate freezing which is important for theFig. 4. Pressure profiles of insufficient metal flow into the cavity because of nozzle blockage resulting in low metal pressures.Fig. 5. Partial runner and gate blockage resulting in the runner pressure at the sensor location not building-up. The higher cavity pressure was probably theresult of the unblocked gating and transferring of the pressure from the hydraulic system. The result for the part will be incomplete part filling and pre-solidification.Fig. 6. Jerky flow of the metal resulting from inconsistency in the movement of the plunger. This will result in the liquid metal separating from the mainstream. The part will consist of flow lines and turbulence effects characterised by irregular flow patterns.240K.K.S. Tong et al./Journal of Materials Processing Technology 127 (2002) 238241pressure intensification stage. It is assumed that, particularlyfor the sensor located just after the gate, the pressure shouldcontinue to increase or to be maintained at a high pressure aslong as the pressure applied from the hydraulic system canstill be transmitted to the cavity 5. The starting of thepressuredropatthesensoraftergateindicatesthefreezingofthegateandthatthepressure fromthehydraulicsystemisnolonger being applied to the cavity through the gate, resultingin the pressure build-up at sensor in the runner (locatedbefore the gate). These significant findings lead to theestimation of the gate freezing time from the start of thecavity fill to the drop in pressure seen in sensor in the cavity.The gate freezing time, the metal pressures in the cavity andgate speed were obtained from the series of plots for eachcasting condition. Figs. 8 and 9 show the estimated gatefreezing time with respect to the metal pressure and the gatespeed, respectively. It is seen that the gate freezing timecould be prolonged by applying a higher pressure and ahigher gate speed. The results indicate that the higherpressure and gate velocity help to keep the gate open longerby offering the possibility to remove any partial gate block-age during cavity filling with a higher momentum of metalflow through the gate. The present finding is significant forprocess design, as free flowing of the in-gate facilitatesmaterial feeding during filling and solidification, which isimportant in producing high integrity component.4. Conclusions1. Capabilities for the monitoring of cavity pressure inthin-wall magnesium die casting have been successfullydeveloped. The measurement of the metal pressure willensure optimum and high integrity part quality.2. A relationship between the gate freezing and the processvariables has been identified.3. The process signature (finger printing) technique againstthe reference metal pressure for each casting c