Effect of cooling water flow rates on local temperatures and heat transfer of casting dies

Journal of Materials Processing Technology 148 (2004) 57–67

Effect of cooling water flow rates on local temperatures andheat transfer of casting dies

Henry Hua,∗, Fang Chena, Xiang Chena, Yeou-li Chub, Patrick Chengba Department of Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, Ont., Canada N9B 3P4

b Department of Research and Development, Ryobi Die Casting Inc., Shelbyville, IN 46176-9720, USA

Received 5 November 2002; received in revised form 20 January 2004; accepted 21 January 2004

Abstract

In high pressure die casting processes, proper temperature control of a die yields high casting production rates and superior qualitycomponents. Die temperature is usually influenced by various die design and process parameters such as size and location of internalwater cooling lines, flow rates of cooling lines, and pouring temperature of molten metal. Among them, the cooling water flow rate playsa major role in controlling thermal pattern of the die. In order to better understand the effect of water flow rates on die temperature, acomputer-based data acquisition and control system (DACS) has been developed and employed to monitor and record data of internal waterflow rates and local temperatures of an industrialized die insert installed in a die casting process simulator. The analysis of experimentalinformation collected by the DACS indicates that the water flow rates influence die temperatures especially at the early stage of die cooling.However, the die heat removal rate tends to decrease as cooling time extends.© 2004 Elsevier B.V. All rights reserved.

Keywords:Die casting; Heat transfer; Internal water cooling; Water flow; Data acquisition

1. Introduction

High pressure die casting is a manufacturing process forproducing accurately net-shape, smooth-surface metal com-ponents. It is accomplished by forcing molten metal into areusable metal mould, which is called the die. The processis often described as the shortest distance between raw ma-terial and finished products such as engine blocks and trans-mission cases. The die is made of two sections to permiteasy removal of the finished castings. These two sections aremounted in a machine. One section is stationary (fixed diehalf) while the other is movable (injector die half). To starta casting cycle, the two halves are clamped tightly togetherby the die casting machine. Molten alloy is injected into thedie cavity where it solidifies rapidly. Then, the die halvesare separated and the casting is ejected. The die serves twobasic functions: to retain the desired shape of the casting,and to remove the heat from the molten metal in a reason-able amount of time. During the cavity filling stage, moltenmetal is poured into the shot sleeve, and then injected intothe die cavity by the piston under an applied pressure. Thecavity filling time is extremely short, typically between 5

∗ Corresponding author.E-mail address:[email protected] (H. Hu).

and 150 ms depending on casting size. Pressure intensifica-tion (10 to over 30 MPa) occurs during the solidification ofthe metal[1]. The aim of the applied pressure is to reducethe amount of gas porosity and feed shrinkage porosity, aswell as to increase the dimensional accuracy of the part.Very often, the die is internally cooled to allow its temper-ature distribution and hence that of the casting, to be con-trolled properly. This can minimize defect formation insidecastings, increase the solidification rate of castings, and con-sequently yield a low scrap rate and a high production rate.

Conventionally, die temperature is monitored and con-trolled based on the experience of operators and trial-and-error. Dies containing internal cooling water lines arefrequently run with maximum flow rates through each line.Localized over-cooling which leads to the formation of de-fects such as poor fill can be manually adjusted by decreasingwater flow rates in a specific cooling line. However, recentstudies on the effect of die surface temperature distributionon casting quality have indicated that casting defects, such asporosity and hot tearing, result from hot spots with abnormalhigh temperatures present in the die[2]. With establishedcooling lines running already at maximum flow rates, it be-comes difficult to reduce the temperature of hot spots in dies.

In the past, a number of studies[3–8] have been carriedout in an effort to understand the influence of cooling water

0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.01.040

58 H. Hu et al. / Journal of Materials Processing Technology 148 (2004) 57–67

rates on die temperature distribution, and develop controlsystems. A number of temperature control units designedto help manage the temperature of a die are commerciallyavailable. Although, they can change the temperature of theentire cavity, most of them lack the capability of sensinglocal die temperatures, and are not able to correct local-ized problems which are currently present in the die cast-ing industry. The most recent control scheme developed byBishenden and Bhoia[8] is capable of varying water lineflow rates with an on/off solenoid valve being switched basedon the measurement of local die temperatures. However, theflow rates cannot be adjusted continuously in each coolingline by the on/off solenoid valve integrated in the system.In the present work, the effect of cooling water flow rateson local die temperatures has been investigated by employ-ing a PC-based data acquisition and control system (DACS)[9,10]. This has three channels for temperature signals andone channel for the water flow rate signal, and is capableof displaying this data, and of adjusting flow rates continu-ously up to the maximum value available. The present paperdiscusses informative results obtained with the DACS dur-ing the experiments which was carried out with a laboratorydie casting process simulator.

2. Experimental

2.1. A die casting process simulator

The purpose for building a die casting process simulator(DCPS) was to provide an experimental setup, in order toconduct representative experiments as it would have beenvery costly to conduct experiments on a real die castingmachine to generate sufficient information for the establish-ment of a reliable relation between cooling water flow ratesand heat removal rates.

Fig. 1. Schematic of a laboratory die casting process simulator.

The DCPS is schematically shown inFig. 1. Instead ofheating the die by directly pouring molten aluminum alloyonto it, a 3 kW furnace is employed in the DCPS to preheatthe die insert to a certain temperature. One die insert madeof H13 tool steel with an internal cooling waterline providedby Ryobi Die Casting Inc. (USA) was used in the DCPS. Thethree-dimensional coordinate system used for determinationof heat transfer occurring in the insert is depicted in Ref.[10]. The piece of the insert is mounted to a movable rackthat supports the insert. An insulating plate, made of fiberglass, was placed between the contact surface of the insertand the metal rack. The insert could be pushed into thefurnace until it is completely enclosed by the furnace. Todetermine the temperature distribution in the die insert, threethermocouples are placed in holes drilled from the rear ofthe insert toward the insert tip at depths of 0.07, 0.085,and 0.105 m.Fig. 2 shows the relative positions of threethermocouples.

2.2. A data acquisition and control system

2.2.1. HardwareThe DACS used for this study was developed on a

PC-based system with a plug-in data acquisition and con-trol card (type: CYDAS 1602DAP) due to its efficiency andeconomy.Fig. 3 shows schematically the entire DACS. Itconsists of a PC, an input and output interface, a water flowmeter, a pump and thermocouples. A RotoFlow flow sensormanufactured by Gems Sensors Inc., was used as a waterflow meter to measure the cooling water flow rate in dieinsert. The output of the meter is 0–10 V dc correspond-ing to the flow rate 2.08 l/min (0.55 GPM) to 18.9 l/min(5 GPM). This signal can be read directly by the data ac-quisition board and no signal amplifier is needed. A Wildenair-operated water pump was selected as an actuator; itcan be controlled by an electrical signal to increase water

H. Hu et al. / Journal of Materials Processing Technology 148 (2004) 57–67 59

Fig. 2. Relative locations of three thermocouples.

flow rate up to 9.46 l/min (2.5 GPM). Through changingthe pumping speed, which is denoted as stroke per minute(SPM), the flow rate is changed accordingly. OMEGA lownoise thermocouple probes, model number HUKMQSS,were chosen as temperature sensors for the system. Thistype of thermocouple has a ground strap connection thatprovides protection against electrical noise. The detaileddescription of the DACS can be found in Refs.[9,10].

2.2.2. SoftwareThe main functionality of the software program (written

in C) running on a PC was to acquire and monitor data,and to log them to the disk for later analysis. The soft-ware program acquired data by interfacing to the plug-indata acquisition board housed in the PC. The main pro-gram routine is divided into four sub-function modules:data-sampling module; data-display module; data-savingmodule; pump-control module. The data-sampling mod-ule is responsible for acquiring data by interfacing to thedata acquisition board at a sampling rate that is specifiedby software. The data-display module displays data on thescreen. The data-saving module saves data to the disk inan ASCII text file format. The pump-control module is re-sponsible for providing control functionality such as setting

Fig. 3. Schematic diagram of the DACS and a setup for experiment A.

the pump speed. The data-sampling module is triggeredonce every sampling period. The data-display, saving, andpump-control modules can be executed by user input. Refs.[9,10] discuss the software development in details.

2.3. Experimental procedures

Scientifically understanding the effect of cooling waterflow rates on local die temperatures during high pressure diecasting required the following questions to be answered:

1. How does the temperature of a die change when varyingcooling water flow rates are being applied to the internalcooling waterlines?

2. How does the heat flux change along the internal wa-terline when varying cooling water flow rates are beingapplied to the internal cooling waterline?

3. How do the overall heat removal rates change when dif-ferent cooling water flow rates are applied to the internalcooling waterline?

Since, the objective of each of the above questions isslightly different, two sets of experimental setups and pro-cedures were developed accordingly. Experiment A was de-signed to acquire necessary data for resolving questions 1


and 2. Experiment B was designed for question 3. The DACSdiscussed inSection 2.2was applied to all the experiments.

2.3.1. Design of experiment A—die temperatureThe objective of experiment A was to acquire tempera-

ture signals from three locations inside the die insert andthe cooling water flow rate signal simultaneously. Analysisof the experimental data generates an understanding of howthe internal die temperature and the internal local heat fluxchange with variations in the water flow rate.Fig. 3 showsthe schematic diagram of the setup for experiment A. The dieinsert was first heated to 550◦C. Cooling water with a fixedflow rate was then applied. Experimental data was recordedduring the cooling stage. Five different rates, 0, 3.79 l/min(1.0 GPM), 5.68 l/min (1.5 GPM), 7.57 l/min (2.0 GPM), and9.46 l/min (2.5 GPM), of cooling water were employed dur-ing experiment A. Local heat flux across locations 2 and 3can be estimated by using Fourier’s law:

q′′n = −k

dT

dn(1)

whereq′′n is the heat flux in then-direction (J/s m2), k the die

H13 thermal conductivity (J/s m K),T the die temperaturedistribution along then-direction (K), andn is the directionalong which temperature gradient exists.

2.3.2. Design of experiment B—heat removal rateThe objective of experiment B was to acquire the data nec-

essary to estimate the overall heat removal rates. Except thatboth inlet and outlet water temperatures were measured, thesetup for this experiment is identical to that for experimentA. The heat removal rate of the system can be estimated byusing the following equation:

qconv(t) = mcp(To(t) − Ti(t)) (2)

whereqconv(t) is the instantaneous convection heat removalrate (J/s),m the cooling water flow rate (kg/s),cp the spe-

Fig. 5. Insert temperatures at 3.79 l/min (1.0 GPM).

cific heat of water at constant pressure (J/kg K),To(t) theoutflow water temperature (K), andTi (t) is the inflow watertemperature (K).

Three quantities,To(t), Ti (t), and m, were measured inexperiments B to calculate heat removal rates. Hence, onlytwo thermocouples were used to measure temperatures ofthe inlet and outlet water.

3. Results and discussion

3.1. Experiment A

3.1.1. Effect of water flow rates on die insert temperaturesFig. 4 presents the temperature change at three locations

during the cooling time period when no water is flowing.T1, T2, andT3 represent the temperatures at locations 1–3,respectively. It is evident inFig. 7 that temperatures at allthe locations increase.T1 increases from 530 to 559◦C, T2from 535 to 545◦C, andT3 from 512 to 520◦C at 50 s afterthe furnace is turned off.Figs. 5–8reveal the variation of

Fig. 4. Insert temperatures with no cooling water.





temperatures versus cooling time when 3.79 l/min (1.0 GPM)5.68 l/min (1.5 GPM), 7.57 l/min (2.0 GPM), and 9.46 l/min(2.5 GPM) water flow rates are applied, respectively. Exam-ination of these figures manifests that the temperatures ofthe die insert at locations 1–3 decrease rapidly after coolingwater is applied. However, the decreasing rate slows downwith increasing cooling time. This may be due to the factthat, in the laboratory setup, the furnace temperature wasset at 800◦C to heat the die insert. During the die coolingstage, even though furnace is turned off, the temperatureof the furnace is still higher than that of the die insert.Therefore, heat continuously transfers from the furnace tothe die insert during the entire die cooling stage. The ex-perimental results plotted inFig. 4 show the occurrence ofthe continued heat transfer during the experiment.


It is worthwhile mentioning that the temperature dif-ference betweenT1 and T2 (see Figs. 4–7) is about15◦C when cooling starts. However, experimental data re-ported by Papai and Mobley[11] show the temperaturedrop in the die surface area is 100◦C after cavity fillingstage. The internal temperature distribution in the entiredie insert during simulated experiments is more uniformthan that in real die casting processes because of theextremely long preheating time in the experiments. De-creasing the temperature of the die insert becomes moredifficult in the laboratory simulated experiments than thatin real die casting processes due to the continuous heattransfer during the cooling stage and the more uniformtemperature distribution. Therefore, it is reasonable to as-sume that, in real die casting processes, the temperaturedrop of the die insert surface will be much larger thanthat of the simulated experiments during the die coolingstage.

Fig. 9 shows the temperatures of the die insert at threelocations when cooling water flow rates are 3.79 l/min(1.0 GPM), 5.68 l/min (1.5 GPM), 7.57 l/min (2.0 GPM),and 9.46 l/min (2.5 GPM) 50 s after the onset of cool-ing. When the water flow rate increases from 3.79 l/min(1.0 GPM) to 5.68 l/min (1.5 GPM), the temperature at lo-cation 1 drops 5◦C further. There is a 4◦C temperaturedrop when flow rate changes from 5.68 l/min (1.5 GPM)to 7.57 l/min (2.0 GPM) at location 1. There was al-most no temperature drop when increasing the flow ratefrom 7.57 l/min (2.0 GPM) to 9.46 l/min (2.5 GPM) atlocation 1. At location 2, the values of the tempera-ture drop were 4, 4, and 1◦C, for flow rates of 3.79,5.68, 7.57, and 9.46 l/min, respectively. At location 3,the values of temperature drop were 5, 4, and 3◦C, forflow rates of 3.79, 5.68, 7.57, and 9.46 l/min, respecti-vely.

Fig. 10. Heat fluxes across location 2 in theX-direction.

Fig. 9. Insert temperatures vs. water flow rates.

3.1.2. Effect of water flow rates on heat fluxTemperature gradients in the die insert cause conductive

heat transfer to occur in the die insert. To understand theeffect of water flow rates on heat transfer away from the dieinsert surface, Fourier’s law mentioned inSection 2.3.1isemployed to compute the heat flux present in the die insert.

3.1.2.1. Heat flux in the X-direction.The locations of threethermocouples have been described inFig. 2. Because, thedistances along theX-direction are much larger than theZ-direction, it is reasonable to ignore theZ-direction dis-tances between locations 1–3. It is also reasonable to assumethat there are local isothermal surfaces around these location[10]. The temperature distribution is assumed to be linearalong the direction from locations 1 to 2. Instantaneous heatflux q′′

2(t) across the isothermal location 2 in theX-directioncan be estimated by using the following equation:

q′′2,X(t) = −k

T1(t) − T2(t)

x1 − x2= −k

T1−2(t)

x(3)


Fig. 11. Heat fluxes across location 3 in theX-direction.

whereq′′2,X(t) is the heat flux across location 2 at instant

time t in theX-direction,k the steel H13 thermal conductiv-ity (W/mK), T2(t) the absolute temperature at location 2 atinstant timet, T1(t) the absolute temperature at location 1at instant timet, and x is the distance between location 1and 2 in theX-direction.

Instantaneous heat fluxq′′3(t) across location 3 in the

X-direction can be determined by using the equation:

q′′3,X(t) = −k

T2(t) − T3(t)

x2 − x3= −k

T2−3(t)

x(4)

whereq′′3,X(t) is the heat flux across location 3 at instant

time t in the X-direction,T3(t) the absolute temperature atlocation 3 at instant time, and x is the distance betweenlocation 2 and 3 in theX-direction.

Figs. 10 and 11show the instantaneous heat fluxesq′′2,X(t),

andq′′3,X(t) when the water flow rate varies. The analysis of

Fig. 12. Insert temperatures at 3.79 l/min (1 GPM).

the experimental results, after cooling water is applied, theheat fluxq′′

2,X(t) increases in the first 10 s and then staysnearly constant, while heat fluxq′′

3,X(t) gradually increases.Increasing the water flow rate increases the heat flux alongthe cooling waterline (X-direction).

3.1.2.2. Heat flux in the Z-direction.Experiments are con-ducted to examine the heat flux across location 3 in theZ-direction. Thermocouple 2 was moved back so that it waslocated at the same depth as thermocouple 3. Experimentswere conducted following the same procedures as for theprevious experiments.Fig. 12shows the temperatures ofT3and T2 when 3.79 l/min (1 GPM) water flow rates are ap-plied during the cooling stage. The temperatures ofT3 andT2 are given inFig. 13when 7.57 l/min (2 GPM) water flowrates are applied. The temperature difference betweenT3andT2 is caused by the distance in theZ-direction between


Fig. 13. Die insert temperatures at 7.57 l/min (2 GPM).

thermocouples 3 and 2. Heat flux across location 3 in theZ-direction can then be estimated by the following equation:

q′′3,Z(t) = −k

T2(t) − T3(t)

z2 − z3= −k

T2−3(t)

z(5)

where q′′3,Z(t) is the heat flux across location 3 in the

Z-direction (J/s m2), k the die H13 thermal conductivity(W/mK), T2(t) the absolute temperature of thermocouple 2(K), T1(t) the absolute temperature of thermocouple 3 (K), z is the distance between thermocouples 2 and 3 in theZ-direction (m).

Fig. 14 depicts the heat fluxes across location 3 inthe Z-direction when 3.79 l/min (1 GPM) and 7.57 l/min(2 GPM) water flow rates are applied to the internal coolingline.

The heat flux in theZ-direction is a function of time. Itincreases rapidly and reaches a maximum value in less than10 s, and then decreases gradually. It is evident fromFig. 17that the cooling water flow rates influence the heat flux in the

Fig. 15. Inlet and outlet water temperatures at 3.79 l/min.

Fig. 14. Heat fluxes across location 3 in theZ-direction.

Z-direction. Experimental results show that, an increase inwater flow rates enhances the heat flux in theZ-direction dur-ing the first 10 s of cooling. The time for the heat flux to reachits maximum value decreases with high water flow rates.


Fig. 19. Effect of flow rates on outlet flow temperature.

3.2. Experiment B

3.2.1. Transient thermal response of outlet waterFigs. 15–18depict the temperatures of the water inlet and

outlet when 3.79 l/min (1.0 GPM), 5.68 l/min (1.5 GPM),7.57 l/min (2.0 GPM), and 9.46 l/min (2.5 GPM) flow ratesare applied. The experimental data show that the tempera-tures at the inlet flow are approximately the same during theexperiments. After cooling water is applied, the outlet flowtemperatures drop rapidly and then gradually reach steadyvalues.

Fig. 19depicts the water outlet temperature for differentwater flow rates. The experimental data show that the time

Fig. 20. Heat removal rates with varying flow rates.

for outlet water to reach a steady-state temperature becomesshorter as the water flow rate increases. It also shows thatthe steady-state temperature of the outlet water decreasesthe water flow rate increases.

3.2.2. Effect of water flow rates on heat removal ratesThe instantaneous heat removal rates with different water

flow rates are shown inFig. 20. It shows that the total heatremoval rate of the internal waterline varies with coolingtime. It decreases rapidly during the first 3 s, then, gradu-ally decreases with further cooling time. Heat removal rateis also a function of cooling water flow rate. It is worth not-ing that in first 3 s of cooling, heat removal rates are sig-


nificantly high when the high water flow rates are applied.This may be primarily attributed to the enhanced convec-tion occurred in the water cooling line as the cooling waterflow rates increases. However, the significance of this factdecreases with an increase in cooling time as heat removalrates approach a saturated value. The saturation of heat re-moval rates indicates that a transfer balance between heatremoved by the cooling water, and heat transferred from theheating furnace to the die insert may take place during theexperimental time. Further studies on mechanisms of fluidflow and heat transfer inside the cooling water line needs tobe carried.

4. Conclusions

The following conclusions can be drawn from the presentstudy.

(1) An increase in internal cooling water flow rates morerapidly reduces the temperatures of die casting dies.

(2) The internal water flow rates positively influence theheat flux along (X-direction) and toward (Z-direction)the cooling waterline. The heat flux is a function ofcooling time and internal water flow rates. The higherthe applied water flow rate, the less time the die inserttakes to reach maximum heat flux.

(3) High water flow rates reduce the time for temperature ofthe outlet water to reach steady-state. The steady-statetemperature of the outlet water tends to be higher whena lower water flow rate is applied.

(4) The total heat removal rate of the waterline varies withcooling time and water flow rates. In the early stageof die insert cooling, the heat removal rates increasesignificantly with the applied water flow rates. However,these higher heat removal rates decrease with an increasein cooling time.

Acknowledgements

The authors would like to take this opportunity to thankthe Natural Sciences and Engineering Research Council ofCanada and Ryobi Die Casting (USA) for supporting thiswork.

References

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Effect of cooling water flow rates on local temperatures and heat transfer of casting dies

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