- Email: [email protected]
Producing molten metal droplets smaller than the nozzle diameter using a pneumatic drop-on-demand generator
Experimental Thermal and Fluid Science 47 (2013) 26–33
Contents lists available at SciVerse ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Producing molten metal droplets smaller than the nozzle diameter using a pneumatic drop-on-demand generator A. Amirzadeh a,b,⇑, M. Raessi b, S. Chandra a a b
Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 Department of Mechanical Engineering, University of Massachusetts-Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747, USA
a r t i c l e
i n f o
Article history: Received 23 August 2012 Received in revised form 9 December 2012 Accepted 10 December 2012 Available online 20 December 2012 Keywords: Metal droplet Drop-on-demand Droplet formation Liquid metal jet
a b s t r a c t A pneumatic droplet generator to produce molten metal droplets smaller than the nozzle diameter is described. The generator consists of a heated cylinder in which a cavity is machined. A nozzle is fit into a stainless steel nozzle holder and attached to the bottom plate of the generator. The system is connected to a gas cylinder through a solenoid valve. Opening the valve for a preset time creates a pulse of alternating negative and positive pressure in the gas above the surface of the molten metal, and a droplet is ejected through the nozzle. The effect of various parameters such as the ejection frequency, nozzle diameter, pulse width and secondary gas flow on droplet formation is investigated. This method made it possible to produce droplets as small as 60% the nozzle diameter. An approximate analytical method is studied to understand the liquid behavior within the nozzle, estimate the droplet size, and investigate the effect of the secondary gas flow pressure on droplet diameter. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Techniques to produce molten metal droplets on demand are of great interest in research laboratories or in industrial applications such as circuit-board manufacturing, 3-D printing and solder jetting. One of the principal challenges has been to reduce the droplet size, which improves the accuracy and resolution with which components can be made. Reducing the diameter of nozzles through which droplets are formed decreases their diameter, but also increases the risk of nozzles being blocked by contaminants in the liquid. One of the most common used methods to produce liquid droplets on demand is the piezoelectric droplet generator [1]. The shape of the voltage pulse used to excite the piezoelectric element strongly affects the size of produced droplets. Switzer [2] used a piezoelectric droplet generator and produced droplets on demand with diameters the same size as the nozzle diameter. The droplet diameter could be changed by varying the voltage pulse and liquid pressure. Similarly, Ulmke et al. [3] studied the formation of single droplets from distilled water and a mixture with 50 wt% glycerin. Furthermore, Sakai [4,5] and Chen and Basaran [6] applied alternating negative and positive pressure pulses to reduce the diameter of droplets using a piezoelectric droplet generator. First, a ⇑ Corresponding author at: Department of Mechanical Engineering, University of Massachusetts-Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747, USA. Fax: +1 508 999 8881. E-mail addresses: [email protected] (A. Amirzadeh), [email protected] (M. Raessi), [email protected] (S. Chandra). 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.12.006
negative pulse drew fluid inward into the nozzle, and while it was withdrawing, the voltage was reversed. Fluid in the central region reversed its motion and emerged from the nozzle to detach as a droplet, while viscous forces in the vicinity of the nozzle wall prevented the flow direction to change as rapidly. The droplet produced was therefore smaller than the nozzle diameter. In addition to aqueous liquids, molten metal droplets have been produced using piezoelectric droplet generators. Orme and Bright [7] produced 200 lm diameter aluminum droplets at a rate of 20,000 per second by vibrating a jet emerging from a nozzle. Orme et al. [8] produced solder (63Sn–37Pb) droplets from a 100 lm diameter nozzle. Yim et al. [9] used a piezoelectric droplet ejector to investigate the break-up behavior of a laminar molten tin jet. Also, Haferl and Poulikakos [10] produced solder droplets and studied the solidification phenomena that occur during deposition of droplets. Yamaguchi et al. [11] produced molten metal droplets to fabricate three-dimensional microstructures using a fusible alloy (Bi–Pb–Sn–Cd–In). Droplets 200 lm in diameter, the same size as the nozzle diameter, were ejected. Lee et al. [12] produced solder droplets larger than the nozzle diameter (50 and 100 lm) using a piezoelectric droplet generator and investigated the effect of different experimental parameters such as chamber pressure and operating frequency on droplet diameter. They also derived an equation to show the relation between the droplet volume and design parameters. Similarly, Jiang et al. [13] investigated the optimal conditions for accurate droplet production. They conducted experiments with water (using a 150 lm diameter nozzle) and a tin-lead alloy (using a 100 lm
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
27
Nomenclature R Ps
r a
a l q Oh u z t
nozzle radius, m capillary pressure, Pa surface tension, N/m radius of curvature, m liquid-solid contact angle dynamic viscosity, kg/ms liquid density, kg/m3 Ohnesorge number, dimensionless liquid velocity, m/s axial location, m time, s
diameter nozzle) and generated droplets with diameters twice the nozzle diameter. Also, Gao et al. [14] generated uniform tin droplets 180 lm in diameter from a 100 lm diameter nozzle using a piezoelectric droplet generator. Piezoelectric droplet generators have typically been used to produce droplets of solder with a melting point of approximately 180 °C. It is difficult to use them for materials with higher melting points since typical piezoceramic materials fail at temperatures much above 200 °C, and have to be insulated from the high temperature melt, which is often not practical. Alternatives to piezoelectric technology exist: Sohn and Yang [15] used a ceramic rod, driven by a solenoid, to push molten metal (Sn–Pb37 wt%) through a 130 lm diameter nozzle. When the solenoid was activated, a pulse was transmitted to the molten metal above the nozzle, and droplets 300 lm in diameter were ejected. Pneumatic droplet-on-demand generators [16], use gas pressure to force droplets out of a nozzle. They do not have any moving part in contact with the liquid and can be used at high temperatures, making them robust in operation and relatively easy to construct. The generator consists of a heated cylindrical chamber containing liquid at the bottom of which a small nozzle is located. The chamber is connected to a nitrogen cylinder through a solenoid valve. Rapidly opening and closing the valve applies a gas pressure pulse to the surface of the liquid, forcing a thin liquid jet out of the nozzle. As the gas inside the chamber escapes (creating negative pressure), the jet is withdrawn into the chamber; its tip detaches and forms a droplet. Pneumatic generators have been used to produce droplets of tin, indium, bismuth, zinc and aluminum alloys [17,18], with diameters more than twice that of the nozzle. Luo et al. [19] applied the same method to produce solder droplets (Sn-40 wt% Pb) with diameters at least 2.5 times that of the nozzle (100–300 lm) from which they emerged. They used a numerical model to study the mechanism of droplet formation. De Almeida et al. [20] used a pneumatic drop-on-demand generator to produce solid kernels of yttrium-stabilized zirconia and soft gel micro spheres of iron hydroxide. Two dispensers, 150 and 330 lm in diameter were used, which produced 1000 lm diameter droplets. Tropmann et al. [21] also used a pneumatic drop-on-demand generator and produced 210 lm in diameter metal droplets from a 183 lm diameter nozzle. Chao et al. [22] produced metal droplets (Sn60–Pb40) about 1.5 times the nozzle diameter using nozzle diameters 100–250 lm to build metal components. Similarly, Zhong et al. [23] used a pneumatic drop-on-demand generator to produce copper droplets from a 600 lm diameter nozzle and investigate the droplet spread diameter after impact on a heated substrate. Furthermore, Cao and Miyamoto [24] produced molten aluminum droplets to build three-dimensional parts. By applying a pulse pressure of argon gas, molten metal was forced through a graphite nozzle 300 lm in diameter. The droplets generated at the rate of 1–5 droplets per second were
P Pp L s d Q v Psf dj l
liquid pressure, Pa peak pressure, Pa nozzle length, m liquid displacement, m droplet diameter, m flow rate, m3/s jet velocity, m/s secondary flow pressure, Pa liquid jet diameter, m liquid jet length, m
600–750 lm in diameter. Luo et al. [25] produced solder droplets (Sn-40 wt% Pb) 350 lm in diameter using a pneumatic drop-on demand generator to investigate the effect of droplet velocity on spread of the solder droplet. Li et al. [26] investigated the spreading, cooling and solidification process of molten aluminum droplets deposited on a moving substrate using a numerical 3D model and experiments, in which droplets were produced using a pneumatic droplet generator from a 600 lm diameter nozzle. Though droplets produced by a pneumatic generator are typically larger than the diameter of the nozzle, Cheng [27] reported that under some settings of the pressure pulse amplitude and duration droplets smaller than the nozzle diameter were produced. The mechanism by which this occurred was not understood. Amirzadeh and Chandra [28,29] conducted an experimental study, using water/glycerin mixtures and developed an approximate analytical model of droplet formation to study the mechanism of small droplet formation in pneumatic droplet generators. They concluded that small droplets were formed using a process similar to that earlier described for piezoelectric droplet generators [6]. As none of the previous studies investigated the formation of metal droplets smaller than the nozzle diameter, this study was carried out to show that it is possible to reduce the droplet size without reducing the nozzle diameter and produce single molten metal droplets. Experiments were done using tin and Zamak 3 (a zinc alloy) and by modulating the gas pressure in the droplet generator droplets smaller than the nozzle diameter were produced, and the effect of various experimental parameters on droplet diameter was studied. Furthermore, an approximate analytical model was also developed to estimate the diameter of produced droplets based on experimental parameters. 2. Experimental method The main body of the generator consisted of a 3.8 cm diameter and 5.8 cm long stainless steel cylinder through which a 1.5 cm diameter hole was machined. Figs. 1 and 2 show a photograph and a schematic diagram of the droplet generator. To melt the metal, four 1/4 inch diameter cartridge heaters (Model CIR-1021/ 120V, Omega Engineering Inc., Stamford, CT, USA) were inserted into the body of the droplet generator (Fig. 2). To minimize heat loss from the sides of the droplet generator an insulating layer of high temperature cement (Omega Bond 400) was used around it. Fig. 3 shows the schematic diagram of the experimental apparatus. Commercially available sapphire nozzles (Swiss Jewel Company, Philadelphia, PA, USA) ranging from 75 to 375 lm were press-fit into a stainless steel nozzle holder, which was attached to the bottom surface of the main body through a stainless steel supporting plate. The top of the droplet generator was connected to a cross-junction (Fig. 3) with one of its outlets connected to a nitrogen gas supply line (O2 less than 5 ppm). The second outlet
28
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
Fig. 1. Photograph of the droplet generator used to produce molten metal droplets.
Fig. 2. Schematic diagram of the droplet generator.
was attached to a pressure measurement system (Type 601B1, Kistler Instrument Corporation, New York, USA) to record gas pressure variations inside the droplet generator. The last outlet was used as an exit vent to which a stainless steel tube was connected. Initially, the nozzle holder was filled with metal particles. To keep the nozzle holder filled with molten metal during operation more particles were added from the top vent of the droplet generator. Experiments were done using either pure tin, with a melting point of 232 °C or a zinc alloy, Zamac 3 (Zn-4%Al-0.04%Mg), with a melting range of 380–387 °C. The droplet generator temperature was maintained constant using a temperature controller (CN9112A, Omega Engineering Inc., Stamford, CT, USA) at 320 °C
when producing molten tin droplets and at 410 °C when working with Zamac 3. To prevent oxidation of droplets emerging from the nozzle, two different techniques were used: releasing droplets into a chamber filled with an inert gas, or directing a flow of nitrogen to the nozzle. In the first method, the chamber consisted of a 15 cm long Pyrex glass tube with an inner diameter of 4.4 cm. The oxygen content was maintained below 150 ppm [27], and an oxygen analyzer (model 911, Illinois Instruments, IL, USA) was used to measure the oxygen content. To decrease the oxygen concentration in the test chamber, the chamber was pressurized with nitrogen gas up to 50 kPa. Then, the gas was released through a non-return valve to the atmosphere. This process was repeated several times to reduce the oxygen level to 30 ppm. During experiments, the pressure inside the test chamber was maintained at atmospheric pressure. As an alternative to the glass chamber, a cap was machined and placed over the bottom of the droplet generator, leaving a small space that was flooded with nitrogen gas (secondary gas flow). The secondary gas flow was supplied through two tubes, placed on either side of the nozzle (Fig. 2), creating an inert atmosphere around the emerging droplets. A 1 mm diameter hole was drilled into the bottom surface of the chamber (cap) through which the molten metal droplets emerged. The droplets were collected on a surface about 40 cm below the nozzle location and photographed using an optical microscope. The diameter of droplets was measured using image analysis software. The heater was switched on till the tin temperature reached 320 °C. When the solenoid valve was opened for a controlled duration, gas flowed into the droplet generator, increased the pressure inside the generator and forced liquid out of the nozzle that resulted in droplet formation due to fluid instability. To photograph
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
29
Fig. 3. Schematic diagram of the experimental apparatus.
12
pulse width 9
droplet starts emerging 6
3
0 -12.5
-6.25
0
6.25
12.5
Time (ms) Fig. 4. Gas pressure variation within the droplet generator when single droplets are ejected from a 102 lm diameter nozzle, pulse width: 4.1 ms, and exit vent: closed ball valve connected to a 12 cm long tube.
droplet formation, a high speed CCD camera (Type 370 KF, OPTIKON Corporation, Kitchener, ON, Canada) was used, which was capable of recording up to 30 frames per second with a resolution of 1280 1024 pixels and an exposure time as low as 100 ns. A 150 W light bulb was placed close to the test chamber to provide illumination for photography. LabVIEW control software controlled timing of signals for the camera, solenoid valve and pressure transducer. 3. Results and discussion To produce single tin droplets the droplet generator with a 102 lm nozzle was mounted above the glass tube that acted as a controlled atmosphere chamber. The droplet generator was connected to a nitrogen supply kept at 276 kPa. Previous experiments [29] have demonstrated that the amplitude and frequency of pressure oscillations within the chamber can be controlled by varying the length of a tube attached to the exit vent, and also the size of the opening at the exit of the tube. Experiments showed that single droplets could be produced with a 12 cm long tube whose exit was completely closed by a ball valve. Opening and closing the solenoid valve for 4.1 ms produced oscillations with a frequency of 535 Hz and peak gauge pressure a little over 7 kPa (Fig. 4). A single droplet emerged from the nozzle approximately 7 ms after the solenoid valve was closed. Time t = 0 corresponds to when the droplet was detached from the nozzle after few oscillations. Droplets were collected on a surface placed far enough below the nozzle to allow
Fig. 5. Single droplets produced from a 102 lm diameter nozzle, pulse width: 4.1 ms, exit vent: closed ball valve connected to a 12 cm long tube, and average droplet diameter: 73 lm.
them to solidify before impact (Fig. 5). The average droplet diameter was 73 lm, less than that of the nozzle (102 lm). When the pulse width was increased to 4.5 ms a longer jet of liquid emerged from the nozzle that broke into multiple droplets (Average droplet diameter: 92 lm). Table 1 shows the distribution of droplet diameter: over 94% of the droplets had diameters lying between 40 and 80 lm. Droplets were also generated with the solenoid valve operating at higher frequencies (ejection rates) using a function generator, and the valve at the exit vent was kept fully open. At 0.5 Hz, all single droplets were smaller than the nozzle diameter, and for multiple droplets about 55% (Table 1). At 5 Hz, the pulse width was gradually increased until droplets were produced at a pulse width of 6 ms, which was 4.9 ms for 10 Hz. At 5 and 10 Hz at least 90% of droplets ranged from 40 to 100 lm. The maximum operating frequency of the droplet generator is limited, because (1) it takes about 10 ms for the solenoid valve to fully open, and (2) in most cases, it took about 50 ms for pressure oscillations inside the droplet generator to damp out each time a pressure pulse was applied. We did experiments by increasing the frequency beyond 10 Hz and adjusting the pressure pulse to produce singe droplets, however, only a stream of molten metal emerged from the nozzle. Using a controlled atmosphere chamber made it difficult to operate the droplet generator since it was not possible to access the produced droplets without removing the chamber. Therefore, experiments were also done with the secondary gas supply shown in Figs. 1 and 2, where inert gas was directed at the exit of the
30
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
Table 1 Percentage of droplet size distribution and mean diameter, nozzle diameter: 102 lm. Diameter range (lm)
40–79.9 80–99.9 100–139.9 Mean diameter (lm) Standard deviation (lm)
Case Single Multiple droplets droplets Rate: 0.5 Hz Rate: 0.5 Hz Rate: 5 Hz Rate: 10 Hz Pulse: 4.1 ms Pulse: Pulse: Pulse: 4.5 ms 6 ms 4.9 ms 94.3 5.7 – 73 5
11 43.5 45.5 86 12
74.6 15.9 9.5 75 17
42.5 52.3 5.2 84 12
ondary flow was kept at 1.6 kPa and a 15 ms duration pulse was applied, droplets with a mean diameter of 475 lm (standard deviation 13 lm) were produced (Fig. 7a). However, the secondary gas pressure was found to strongly affect droplet size. When the secondary gas pressure was increased to 3.8 kPa, the pulse width had to be increased to 20 ms to overcome the increased resistance at the nozzle exit. However, the mean droplet diameter was greatly reduced to 164 lm (standard deviation 20 lm) as seen in Fig. 7b. To estimate the size of droplets produced by the pneumatic generator and to understand the effect of the surrounding gas pressure an approximate analytical method is used. Fig. 8 shows a liquid meniscus emerging from a nozzle of radius R. The capillary pressure Ps can be estimated from the following relation:
Ps ¼
2r a
where r and a are the surface tension and radius of curvature, respectively. The radius of curvature is:
a¼
R sinða 90 Þ
where a is the liquid–solid contact angle. The liquid flow inside the nozzle is assumed to be incompressible, and the radial pressure is small enough to be ignored compared to its gradient along the nozzle axis. Since the order of magnitude of the Ohnesorge number, Oh ¼ plffiffiffiffiffiffiffi (l: dynamic visqrR
cosity, q: density), for molten tin at 300 °C is 103 (considering the nozzle sizes used in this work) it is assumed that viscous forces are negligible compared to surface tension forces. Neglecting body forces, the continuity and Navier–Stokes equations will be Fig. 6. Single tin droplets when the pulse width was set to 8.25 ms, Nozzle diameter: 150 lm, liquid: molten tin at 300 °C, secondary gas flow pressure: 0.8 kPa, exit vent: 7 cm long tube connected to an open ball valve, and average droplet diameter: 134 lm.
@u ¼0 @z
continuity :
z-momentum : Table 2 Statistics on tin droplet diameter, liquid: molten tin at 300 °C, secondary gas flow pressure: 0.8 kPa, exit vent: 7 cm long tube connected to an open ball valve, and sample size: 35. Nozzle diameter (lm)
Pulse (ms)
Mean diameter (lm)
Standard deviation (lm)
Droplet diameter (analytical model) (lm)
150
8.25 11 25
134 146 130
8 12 15
129 140 144 (2 Droplets) 126 (3 Droplets)
125
10 15 25
112 108 104
4 7 7
107 83 88 (2 Droplets)
@u 1 @P ¼ @t q @z
ð1Þ
ð2Þ
The gas pressure is assumed to rise linearly from zero, and after time tp, reach a peak value Pp, which is large enough to overcome the capillary pressure Ps (reached after time ts) at the nozzle exit and eject a droplet. The pressure variation within the liquid can be presented as:
Pðz; tÞ ¼ FðzÞ
Pp ðt ts Þ þ Ps tp
ð3Þ
where F(z) is a function to be determined. Substituting Eq. (3) into (2) gives
uðtÞ ¼
2 1 @FðzÞ Pp t þ ts t þ c q @z tp 2
ð4Þ
Now, applying the continuity equation, F(z) will be droplet generator. The gas supply pressure and ejection rate were set to 207 kPa and 0.5 Hz, respectively. Fig. 6 shows single tin droplets (average diameter of 134 lm) produced from a 150 lm diameter nozzle. Additional tests were done with a 125 lm diameter nozzle, and single spherical droplets with diameters about 85% that of the nozzle were produced. Table 2 lists a range of other combinations of nozzle sizes and pulse widths that were used to produce tin droplets smaller than the nozzle diameter. The droplet generator was also used successfully to produce droplets of a higher temperature melting material such as the zinc alloy, Zamac 3 (Zn–4Al–0.04Mg), with a melting range of 380– 387 °C. The molten metal temperature was kept at 410 °C, and a 7 cm long tube connected to a 1/3-closed ball valve was attached to the exit vent. Fig. 7 shows single droplets produced from a 102 lm diameter nozzle. When the pressure of the gas in the sec-
FðzÞ ¼ c1 z þ c2
ð5Þ
Considering that the liquid is initially at rest u(ts) = 0, the axial velocity becomes
uðtÞ ¼
2 c1 P p t t2 þ ts t s q tp 2 2
ð6Þ
The upstream (z = 0) and downstream (z = L) pressure boundary conditions are used to obtain c1 and c2 (Fig. 9). Using the upstream boundary condition, P(0, tp) = Pp, F(0) = c2 = 1. From the downstream boundary condition P(L, tp) = Ps, F(L) = 0 and c1 ¼ 1L , so the maximum velocity becomes
umax ¼ uðtp Þ ¼
1 Pp ðtp ts Þ2 2qL tp
ð7Þ
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
31
Fig. 7. Effect of the secondary gas flow pressure on Zamac 3 droplets produced from a 102 lm diameter nozzle, (a) pulse width: 15 ms, secondary gas flow pressure: 1.6 kPa, and (b) pulse width: 20 ms, secondary gas flow pressure: 3.8 kPa.
smax ¼ sðtp Þ ¼
1 Pp ðt p ts Þ3 6qL tp
ð9Þ
Assuming that the maximum displaced liquid (smax) detaches and forms a single droplet, the droplet diameter d can be estimated from the following relation
4 3
pR2 smax ¼ p
3 d ) d ¼ ð6R2 smax Þ1=3 2
ð10Þ
For molten tin at 300 °C, r = 0.56 N/m, q = 6950 kg/m3, and a = 110° [27]. For a 102 lm diameter nozzle, the maximum pressure Ps will be 7.5 kPa, so the peak pressure Pp (average value of 7.8 kPa from experiments) is large enough to push the liquid out of the nozzle. Substituting L = 104 m, tp = 0.001 s, and ts = 0.00077 s (corresponding to the largest peak pressure, Fig. 4) gives d = 70 lm, which is in relatively good agreement with the experimental result (73 lm, Fig. 5). During the first three peak pressures (Fig. 4), due to the quickly changing pressure, the liquid jet oscillates at the nozzle tip before it detaches at some later time and forms a droplet at t = 0. Since the pressure remains positive at all times, the liquid jet is not pushed far back into the nozzle during oscillations. Experimental results show that when multiple droplets are produced, tp and ts are 0.001 and 0.00045 s, respectively, and Pp = 10.75 kPa. In this case, smax = 428 lm, which is the maximum jet length corresponding to 5–8 droplets produced in each ejection, so the droplet diameter ranges from 90 to 105 lm (compare with average diameter of 92 lm from experiments). To include the effect of the secondary gas flow on droplet formation, the jet diameter dj close to the chamber hole (Fig. 10) is first approximated. Using the maximum displacement (Eq. (9)), the average molten metal flow rate can be calculated.
Fig. 8. Radius of curvature a.
Q¼
pR2 smax tp ts
ð11Þ
Assuming that the pressure of the metal jet close to the chamber hole is the same as the gas pressure, the jet velocity will be Fig. 9. Upstream and downstream pressure boundary conditions.
The liquid displacement (length of liquid jet emerging out of the nozzle) can be estimated as follows:
sðtÞ ¼
Z
1 Pp t3 t2 t2 uðtÞdt ) sðtÞ ¼ t s þ s t þ c3 qL tp 6 2 2
ð8Þ
Considering that s(ts) = 0, the maximum liquid displacement will be
v¼
sffiffiffiffiffiffiffiffiffi 2Psf
q
ð12Þ
where Psf is the pressure of the secondary flow, so the jet diameter can be calculated as follows:
sffiffiffiffiffiffiffiffiffi 14 2 dj Q Pp 1 p ¼ ) dj ¼ 2Rðtp ts Þ 4 v 6Ltp 2qPsf
ð13Þ
32
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
Fig. 10. Schematic diagram of the molten metal jet.
Table 3 Effect of the secondary gas flow pressure on formation of tin droplets, nozzle diameter: 150 lm, pulse width: 25 ms, liquid: molten tin at 300 °C, exit vent: 7 cm long tube connected to an open ball valve, and sample size: 35. Gas pressure (kPa)
Mean diameter (lm)
Standard deviation (lm)
Droplet diameter (analytical model) (lm)
0.4 0.8
168 130
12 15
182 144 (2 Droplets) 126 (3 Droplets)
It is assumed that the jet reaches a length of l after the time period tp ts; then, it detaches and forms a droplet with a diameter of dD.
l ¼ v ðt p t s Þ
p
3 1 4 dD 3 2 3 l¼ p dj l ) dD ¼ 4 2 3 2
2 dj
ð14Þ
ð15Þ
Table 2 compares the droplet diameter obtained from experiments and analytical model for different pulse widths and two nozzle diameters. The results were in relatively good agreement. The most important role of the secondary gas flow was to prevent oxidation during droplet formation. The effect of increasing the pressure of the secondary gas flow was also investigated using a 150 lm diameter nozzle. At 0.4 kPa, single droplets 1.1 times that of the nozzle diameter were produced. Increasing the pressure to 0.8 kPa made it possible to reduce the average droplet diameter by 23% (Table 3). In this case, 2 or 3 droplets were produced during each pulse. Using equation 13 for two different flow pressures gives
1 ðPsf Þ2 4 ðdj Þ1 ¼ ðdj Þ2 ðPsf Þ1
ð16Þ
which shows that increasing the secondary flow pressure reduces the jet diameter, and therefore, formation of smaller droplets is expected. 4. Summary and conclusions This paper presents an experimental study to produce molten metal droplets using a pneumatic drop-on-demand generator and investigate the effect of various parameters on reducing the droplet size. The required pressure oscillation was generated attaching a tube with a specified length to the exit vent. To prevent oxidation of droplets as they emerged from the generator two dif-
ferent methods were used. First, a controlled atmosphere chamber was made from a glass tube that was evacuated and filled with nitrogen. Alternately, nitrogen gas was injected around the nozzle to surround it with an inert atmosphere. Using a controlled atmosphere chamber, tin droplets with an average diameter of 70% that of the nozzle diameter (102 lm) were produced. For single droplets produced at a frequency of 0.5 Hz from one nozzle, more than 99% of the droplets were ranging from 60 to 100 lm in diameter. For multiple droplets produced at higher frequencies, 5 and 10 Hz, more than 90% of droplets were smaller than the nozzle diameter. Using a secondary flow of nitrogen around the nozzle single tin droplets about 85% that of the nozzle diameter were produced from nozzles as small as 125 lm in diameter. Increasing the pressure of the secondary gas flow decreased the droplet diameter. Also, single droplets were produced from a zinc alloy (Zamac 3) using a 102 lm diameter nozzle, and increasing the pressure of the secondary gas flow made it possible to decrease the droplet diameter by 65%. Finally, an approximate analytical model was described to explain the fluid flow behavior within the nozzle and estimate the droplet diameter. Experimental results showed good agreement with those obtained from the model. The analysis shows that the droplet diameter depends on the nozzle size, applied pressure pulse, pressure of the secondary gas flow and molten metal density. Since predictions of the analytical method were in relatively good agreement with the experimental results for molten tin, we should expect acceptable results for different alloys. The equations show that the only affecting physical property of the molten metal is density. However, testing different alloys under the same experimental conditions and with nozzle sizes much smaller than 200 lm in diameter might be challenging. References [1] E.R. Lee, Microdrop Generation, CRC Press, Stanford Linear Accelerator Center, Stanford University, 2003. [2] G.L. Switzer, A versatile system for stable generation of uniform droplets, Rev. Sci. Instrum. 62 (11) (1991) 2765–2771. [3] H. Ulmke, M. Mietschke, K. Bauckhage, Piezoelectric single nozzle droplet generator for production of monodisperse droplets of variable diameter, Chem. Eng. Technol. 24 (2001) 69–70. [4] S. Sakai, Recording method by inkjet recording apparatus and recording head adapted for said recording method, US Patent No. 5,933,168, 1999. [5] S. Sakai, Dynamics of piezoelectric inkjets printing systems, IS & T’s NIP 16, in: International Conference on Digital Printing Technologies, IS & T: The Society for Imaging Science and Technology, 2000, pp. 15–20. [6] A.U. Chen, O.A. Basaran, A new method for significantly reducing drop radius without reducing nozzle radius in drop-on-demand drop production, Phys. Fluids 14 (2002) L1–L4. [7] M. Orme, A. Bright, Recent advances in highly controlled molten metal droplet formation from capillary stream break-up with applications to advanced manufacturing, TMS Annual Meeting, 2000. [8] M. Orme, J. Courter, Q. Liu, J. Zhu, R. Smith, Charged molten metal droplet deposition as a direct write technology, MRS Spring Meeting, San Francisco, 2000. [9] P. Yim, J. Chun, N. Saka, J.C. Rocha, The effect of oxygen concentration on the break-up behavior of laminar, liquid metal jets, Liq. Met. Atomization: Fundam. Practice, The Miner., Met. Mater. Soc. (2000) 169–181. [10] S. Haferl, D. Poulikakos, Transport and solidification phenomena in molten microdroplet pileup, J. Appl. Phys. 92 (2002) 1675–1689. [11] K. Yamaguchi, K. Sakai, T. Yamanaka, T. Hirayama, Generation of threedimensional micro structure using metal jet, Precis. Eng. 24 (2000) 2–8. [12] T.M. Lee, T.G. Kang, J.S. Yang, J. Jo, K.Y. Kim, B.O. Choi, D.S. Kim, Drop-ondemand solder droplet jetting system for fabricating microstructure, IEE Trans. Electron. Packag. Manuf. 31 (2008) 201–210. [13] X.S. Jiang, L.H. Qi, J. Luo, H. Huang, J.M. Zhou, Research on accurate droplet generation for micro-droplet deposition manufacture, Int. J. Adv. Manuf. Technol. 49 (2010) 535–541. [14] S. Gao, Y. Yao, C. Cui, Vibrating breakup of jet for uniform metal droplet, J. Mater. Sci. Technol. 23 (2007) 135–138. [15] H. Sohn, D.Y. Yang, Drop-on-demand deposition of superheated metal droplets for selective infiltration manufacturing, Mater. Sci. Eng. A 392 (2005) 415–421. [16] S. Chandra, R. Jivraj, Apparatus and method for generating uniform sized droplets, US Patent No. 6,446,878, 2002.
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33 [17] S.X. Cheng, T. Li, S. Chandra, Producing molten metal droplets with a pneumatic droplet-on-demand generator, J. Mater. Process. Technol. 159 (2005) 295–302. [18] V. Foutsis, Producing small molten metal drops with a pneumatic generator, University of Toronto, MASc Thesis, 2004. [19] J. Luo, L.H. Qi, J.M. Zhou, X.H. Hou, H.J. Lee, Modeling and characterization of metal droplet generation by using a pneumatic drop-on-demand generator, J. Mater. Process. Technol. 212 (2012) 718–726. [20] V.F. de Almeida, R.D. Hunt, J.L. Collins, Pneumatic drop-on-demand generator for production of metal oxide micro spheres by internal gelation, J. Nucl. Mater. 404 (2010) 44–49. [21] A. Tropmann, N. Lass, N. Paust, T. Metz, C. Ziegler, R. Zengerle, P. Koltay, Pneumatic dispensing of nano-to picoliter droplets of liquid metal with the starjet method for rapid prototyping of metal microstructures, Microfluid. Nanofluid 12 (2012) 75–84. [22] Y. Chao, L. Qi, Y. Xiao, J. Luo, J. Zhou, Manufacturing of micro thin-walled metal parts by micro-droplet deposition, J. Mater. Process. Technol. 212 (2012) 484– 491. [23] S. Zhong, L. Qi, J. Luo, Y. Xiao, Parameters study on generation of uniform copper droplet by pneumatic drop-on-demand technology, J. Adv. Mater. 430– 432 (2012) 781–784.
33
[24] W. Cao, Y. Miyamoto, Freeform fabrication of aluminum parts by direct deposition of molten aluminum, J. Mater. Process. Technol. 173 (2006) 209–212. [25] J. Luo, L.H. Qi, S.Y. Zhong, J.M. Zhou, H.J. Li, Printing solder droplets for micro devices packages using pneumatic drop-on-demand (DOD) technique, J. Mater. Process. Technol. 212 (2012) 2066–2073. [26] H. Li, P. Wang, L. Qi, H. Zuo, S. Zhong, X. Hou, 3D numerical simulation of successive deposition of uniform molten Al droplets on a moving substrate and experimental validation, Comput. Mater. Sci. 65 (2012) 291–301. [27] S. X. Cheng, Development of a molten metal droplet generator for rapid prototyping, University of Toronto, MASc Thesis, 2002. [28] A. Amirzadeh, S. Chandra, Small droplet formation in a pneumatic drop-ondemand generator: experiments and analysis, Exp. Therm. Fluid Sci. 34 (2010) 1488–1497. [29] A. Amirzadeh Goghari, S. Chandra, Producing droplets smaller than the nozzle diameter by using a pneumatic drop-on-demand droplet generator, Exp. Fluids 44 (2008) 105–114.
Contents lists available at SciVerse ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Producing molten metal droplets smaller than the nozzle diameter using a pneumatic drop-on-demand generator A. Amirzadeh a,b,⇑, M. Raessi b, S. Chandra a a b
Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 Department of Mechanical Engineering, University of Massachusetts-Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747, USA
a r t i c l e
i n f o
Article history: Received 23 August 2012 Received in revised form 9 December 2012 Accepted 10 December 2012 Available online 20 December 2012 Keywords: Metal droplet Drop-on-demand Droplet formation Liquid metal jet
a b s t r a c t A pneumatic droplet generator to produce molten metal droplets smaller than the nozzle diameter is described. The generator consists of a heated cylinder in which a cavity is machined. A nozzle is fit into a stainless steel nozzle holder and attached to the bottom plate of the generator. The system is connected to a gas cylinder through a solenoid valve. Opening the valve for a preset time creates a pulse of alternating negative and positive pressure in the gas above the surface of the molten metal, and a droplet is ejected through the nozzle. The effect of various parameters such as the ejection frequency, nozzle diameter, pulse width and secondary gas flow on droplet formation is investigated. This method made it possible to produce droplets as small as 60% the nozzle diameter. An approximate analytical method is studied to understand the liquid behavior within the nozzle, estimate the droplet size, and investigate the effect of the secondary gas flow pressure on droplet diameter. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Techniques to produce molten metal droplets on demand are of great interest in research laboratories or in industrial applications such as circuit-board manufacturing, 3-D printing and solder jetting. One of the principal challenges has been to reduce the droplet size, which improves the accuracy and resolution with which components can be made. Reducing the diameter of nozzles through which droplets are formed decreases their diameter, but also increases the risk of nozzles being blocked by contaminants in the liquid. One of the most common used methods to produce liquid droplets on demand is the piezoelectric droplet generator [1]. The shape of the voltage pulse used to excite the piezoelectric element strongly affects the size of produced droplets. Switzer [2] used a piezoelectric droplet generator and produced droplets on demand with diameters the same size as the nozzle diameter. The droplet diameter could be changed by varying the voltage pulse and liquid pressure. Similarly, Ulmke et al. [3] studied the formation of single droplets from distilled water and a mixture with 50 wt% glycerin. Furthermore, Sakai [4,5] and Chen and Basaran [6] applied alternating negative and positive pressure pulses to reduce the diameter of droplets using a piezoelectric droplet generator. First, a ⇑ Corresponding author at: Department of Mechanical Engineering, University of Massachusetts-Dartmouth, 285 Old Westport Road, North Dartmouth, MA 02747, USA. Fax: +1 508 999 8881. E-mail addresses: [email protected] (A. Amirzadeh), [email protected] (M. Raessi), [email protected] (S. Chandra). 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.12.006
negative pulse drew fluid inward into the nozzle, and while it was withdrawing, the voltage was reversed. Fluid in the central region reversed its motion and emerged from the nozzle to detach as a droplet, while viscous forces in the vicinity of the nozzle wall prevented the flow direction to change as rapidly. The droplet produced was therefore smaller than the nozzle diameter. In addition to aqueous liquids, molten metal droplets have been produced using piezoelectric droplet generators. Orme and Bright [7] produced 200 lm diameter aluminum droplets at a rate of 20,000 per second by vibrating a jet emerging from a nozzle. Orme et al. [8] produced solder (63Sn–37Pb) droplets from a 100 lm diameter nozzle. Yim et al. [9] used a piezoelectric droplet ejector to investigate the break-up behavior of a laminar molten tin jet. Also, Haferl and Poulikakos [10] produced solder droplets and studied the solidification phenomena that occur during deposition of droplets. Yamaguchi et al. [11] produced molten metal droplets to fabricate three-dimensional microstructures using a fusible alloy (Bi–Pb–Sn–Cd–In). Droplets 200 lm in diameter, the same size as the nozzle diameter, were ejected. Lee et al. [12] produced solder droplets larger than the nozzle diameter (50 and 100 lm) using a piezoelectric droplet generator and investigated the effect of different experimental parameters such as chamber pressure and operating frequency on droplet diameter. They also derived an equation to show the relation between the droplet volume and design parameters. Similarly, Jiang et al. [13] investigated the optimal conditions for accurate droplet production. They conducted experiments with water (using a 150 lm diameter nozzle) and a tin-lead alloy (using a 100 lm
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
27
Nomenclature R Ps
r a
a l q Oh u z t
nozzle radius, m capillary pressure, Pa surface tension, N/m radius of curvature, m liquid-solid contact angle dynamic viscosity, kg/ms liquid density, kg/m3 Ohnesorge number, dimensionless liquid velocity, m/s axial location, m time, s
diameter nozzle) and generated droplets with diameters twice the nozzle diameter. Also, Gao et al. [14] generated uniform tin droplets 180 lm in diameter from a 100 lm diameter nozzle using a piezoelectric droplet generator. Piezoelectric droplet generators have typically been used to produce droplets of solder with a melting point of approximately 180 °C. It is difficult to use them for materials with higher melting points since typical piezoceramic materials fail at temperatures much above 200 °C, and have to be insulated from the high temperature melt, which is often not practical. Alternatives to piezoelectric technology exist: Sohn and Yang [15] used a ceramic rod, driven by a solenoid, to push molten metal (Sn–Pb37 wt%) through a 130 lm diameter nozzle. When the solenoid was activated, a pulse was transmitted to the molten metal above the nozzle, and droplets 300 lm in diameter were ejected. Pneumatic droplet-on-demand generators [16], use gas pressure to force droplets out of a nozzle. They do not have any moving part in contact with the liquid and can be used at high temperatures, making them robust in operation and relatively easy to construct. The generator consists of a heated cylindrical chamber containing liquid at the bottom of which a small nozzle is located. The chamber is connected to a nitrogen cylinder through a solenoid valve. Rapidly opening and closing the valve applies a gas pressure pulse to the surface of the liquid, forcing a thin liquid jet out of the nozzle. As the gas inside the chamber escapes (creating negative pressure), the jet is withdrawn into the chamber; its tip detaches and forms a droplet. Pneumatic generators have been used to produce droplets of tin, indium, bismuth, zinc and aluminum alloys [17,18], with diameters more than twice that of the nozzle. Luo et al. [19] applied the same method to produce solder droplets (Sn-40 wt% Pb) with diameters at least 2.5 times that of the nozzle (100–300 lm) from which they emerged. They used a numerical model to study the mechanism of droplet formation. De Almeida et al. [20] used a pneumatic drop-on-demand generator to produce solid kernels of yttrium-stabilized zirconia and soft gel micro spheres of iron hydroxide. Two dispensers, 150 and 330 lm in diameter were used, which produced 1000 lm diameter droplets. Tropmann et al. [21] also used a pneumatic drop-on-demand generator and produced 210 lm in diameter metal droplets from a 183 lm diameter nozzle. Chao et al. [22] produced metal droplets (Sn60–Pb40) about 1.5 times the nozzle diameter using nozzle diameters 100–250 lm to build metal components. Similarly, Zhong et al. [23] used a pneumatic drop-on-demand generator to produce copper droplets from a 600 lm diameter nozzle and investigate the droplet spread diameter after impact on a heated substrate. Furthermore, Cao and Miyamoto [24] produced molten aluminum droplets to build three-dimensional parts. By applying a pulse pressure of argon gas, molten metal was forced through a graphite nozzle 300 lm in diameter. The droplets generated at the rate of 1–5 droplets per second were
P Pp L s d Q v Psf dj l
liquid pressure, Pa peak pressure, Pa nozzle length, m liquid displacement, m droplet diameter, m flow rate, m3/s jet velocity, m/s secondary flow pressure, Pa liquid jet diameter, m liquid jet length, m
600–750 lm in diameter. Luo et al. [25] produced solder droplets (Sn-40 wt% Pb) 350 lm in diameter using a pneumatic drop-on demand generator to investigate the effect of droplet velocity on spread of the solder droplet. Li et al. [26] investigated the spreading, cooling and solidification process of molten aluminum droplets deposited on a moving substrate using a numerical 3D model and experiments, in which droplets were produced using a pneumatic droplet generator from a 600 lm diameter nozzle. Though droplets produced by a pneumatic generator are typically larger than the diameter of the nozzle, Cheng [27] reported that under some settings of the pressure pulse amplitude and duration droplets smaller than the nozzle diameter were produced. The mechanism by which this occurred was not understood. Amirzadeh and Chandra [28,29] conducted an experimental study, using water/glycerin mixtures and developed an approximate analytical model of droplet formation to study the mechanism of small droplet formation in pneumatic droplet generators. They concluded that small droplets were formed using a process similar to that earlier described for piezoelectric droplet generators [6]. As none of the previous studies investigated the formation of metal droplets smaller than the nozzle diameter, this study was carried out to show that it is possible to reduce the droplet size without reducing the nozzle diameter and produce single molten metal droplets. Experiments were done using tin and Zamak 3 (a zinc alloy) and by modulating the gas pressure in the droplet generator droplets smaller than the nozzle diameter were produced, and the effect of various experimental parameters on droplet diameter was studied. Furthermore, an approximate analytical model was also developed to estimate the diameter of produced droplets based on experimental parameters. 2. Experimental method The main body of the generator consisted of a 3.8 cm diameter and 5.8 cm long stainless steel cylinder through which a 1.5 cm diameter hole was machined. Figs. 1 and 2 show a photograph and a schematic diagram of the droplet generator. To melt the metal, four 1/4 inch diameter cartridge heaters (Model CIR-1021/ 120V, Omega Engineering Inc., Stamford, CT, USA) were inserted into the body of the droplet generator (Fig. 2). To minimize heat loss from the sides of the droplet generator an insulating layer of high temperature cement (Omega Bond 400) was used around it. Fig. 3 shows the schematic diagram of the experimental apparatus. Commercially available sapphire nozzles (Swiss Jewel Company, Philadelphia, PA, USA) ranging from 75 to 375 lm were press-fit into a stainless steel nozzle holder, which was attached to the bottom surface of the main body through a stainless steel supporting plate. The top of the droplet generator was connected to a cross-junction (Fig. 3) with one of its outlets connected to a nitrogen gas supply line (O2 less than 5 ppm). The second outlet
28
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
Fig. 1. Photograph of the droplet generator used to produce molten metal droplets.
Fig. 2. Schematic diagram of the droplet generator.
was attached to a pressure measurement system (Type 601B1, Kistler Instrument Corporation, New York, USA) to record gas pressure variations inside the droplet generator. The last outlet was used as an exit vent to which a stainless steel tube was connected. Initially, the nozzle holder was filled with metal particles. To keep the nozzle holder filled with molten metal during operation more particles were added from the top vent of the droplet generator. Experiments were done using either pure tin, with a melting point of 232 °C or a zinc alloy, Zamac 3 (Zn-4%Al-0.04%Mg), with a melting range of 380–387 °C. The droplet generator temperature was maintained constant using a temperature controller (CN9112A, Omega Engineering Inc., Stamford, CT, USA) at 320 °C
when producing molten tin droplets and at 410 °C when working with Zamac 3. To prevent oxidation of droplets emerging from the nozzle, two different techniques were used: releasing droplets into a chamber filled with an inert gas, or directing a flow of nitrogen to the nozzle. In the first method, the chamber consisted of a 15 cm long Pyrex glass tube with an inner diameter of 4.4 cm. The oxygen content was maintained below 150 ppm [27], and an oxygen analyzer (model 911, Illinois Instruments, IL, USA) was used to measure the oxygen content. To decrease the oxygen concentration in the test chamber, the chamber was pressurized with nitrogen gas up to 50 kPa. Then, the gas was released through a non-return valve to the atmosphere. This process was repeated several times to reduce the oxygen level to 30 ppm. During experiments, the pressure inside the test chamber was maintained at atmospheric pressure. As an alternative to the glass chamber, a cap was machined and placed over the bottom of the droplet generator, leaving a small space that was flooded with nitrogen gas (secondary gas flow). The secondary gas flow was supplied through two tubes, placed on either side of the nozzle (Fig. 2), creating an inert atmosphere around the emerging droplets. A 1 mm diameter hole was drilled into the bottom surface of the chamber (cap) through which the molten metal droplets emerged. The droplets were collected on a surface about 40 cm below the nozzle location and photographed using an optical microscope. The diameter of droplets was measured using image analysis software. The heater was switched on till the tin temperature reached 320 °C. When the solenoid valve was opened for a controlled duration, gas flowed into the droplet generator, increased the pressure inside the generator and forced liquid out of the nozzle that resulted in droplet formation due to fluid instability. To photograph
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
29
Fig. 3. Schematic diagram of the experimental apparatus.
12
pulse width 9
droplet starts emerging 6
3
0 -12.5
-6.25
0
6.25
12.5
Time (ms) Fig. 4. Gas pressure variation within the droplet generator when single droplets are ejected from a 102 lm diameter nozzle, pulse width: 4.1 ms, and exit vent: closed ball valve connected to a 12 cm long tube.
droplet formation, a high speed CCD camera (Type 370 KF, OPTIKON Corporation, Kitchener, ON, Canada) was used, which was capable of recording up to 30 frames per second with a resolution of 1280 1024 pixels and an exposure time as low as 100 ns. A 150 W light bulb was placed close to the test chamber to provide illumination for photography. LabVIEW control software controlled timing of signals for the camera, solenoid valve and pressure transducer. 3. Results and discussion To produce single tin droplets the droplet generator with a 102 lm nozzle was mounted above the glass tube that acted as a controlled atmosphere chamber. The droplet generator was connected to a nitrogen supply kept at 276 kPa. Previous experiments [29] have demonstrated that the amplitude and frequency of pressure oscillations within the chamber can be controlled by varying the length of a tube attached to the exit vent, and also the size of the opening at the exit of the tube. Experiments showed that single droplets could be produced with a 12 cm long tube whose exit was completely closed by a ball valve. Opening and closing the solenoid valve for 4.1 ms produced oscillations with a frequency of 535 Hz and peak gauge pressure a little over 7 kPa (Fig. 4). A single droplet emerged from the nozzle approximately 7 ms after the solenoid valve was closed. Time t = 0 corresponds to when the droplet was detached from the nozzle after few oscillations. Droplets were collected on a surface placed far enough below the nozzle to allow
Fig. 5. Single droplets produced from a 102 lm diameter nozzle, pulse width: 4.1 ms, exit vent: closed ball valve connected to a 12 cm long tube, and average droplet diameter: 73 lm.
them to solidify before impact (Fig. 5). The average droplet diameter was 73 lm, less than that of the nozzle (102 lm). When the pulse width was increased to 4.5 ms a longer jet of liquid emerged from the nozzle that broke into multiple droplets (Average droplet diameter: 92 lm). Table 1 shows the distribution of droplet diameter: over 94% of the droplets had diameters lying between 40 and 80 lm. Droplets were also generated with the solenoid valve operating at higher frequencies (ejection rates) using a function generator, and the valve at the exit vent was kept fully open. At 0.5 Hz, all single droplets were smaller than the nozzle diameter, and for multiple droplets about 55% (Table 1). At 5 Hz, the pulse width was gradually increased until droplets were produced at a pulse width of 6 ms, which was 4.9 ms for 10 Hz. At 5 and 10 Hz at least 90% of droplets ranged from 40 to 100 lm. The maximum operating frequency of the droplet generator is limited, because (1) it takes about 10 ms for the solenoid valve to fully open, and (2) in most cases, it took about 50 ms for pressure oscillations inside the droplet generator to damp out each time a pressure pulse was applied. We did experiments by increasing the frequency beyond 10 Hz and adjusting the pressure pulse to produce singe droplets, however, only a stream of molten metal emerged from the nozzle. Using a controlled atmosphere chamber made it difficult to operate the droplet generator since it was not possible to access the produced droplets without removing the chamber. Therefore, experiments were also done with the secondary gas supply shown in Figs. 1 and 2, where inert gas was directed at the exit of the
30
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
Table 1 Percentage of droplet size distribution and mean diameter, nozzle diameter: 102 lm. Diameter range (lm)
40–79.9 80–99.9 100–139.9 Mean diameter (lm) Standard deviation (lm)
Case Single Multiple droplets droplets Rate: 0.5 Hz Rate: 0.5 Hz Rate: 5 Hz Rate: 10 Hz Pulse: 4.1 ms Pulse: Pulse: Pulse: 4.5 ms 6 ms 4.9 ms 94.3 5.7 – 73 5
11 43.5 45.5 86 12
74.6 15.9 9.5 75 17
42.5 52.3 5.2 84 12
ondary flow was kept at 1.6 kPa and a 15 ms duration pulse was applied, droplets with a mean diameter of 475 lm (standard deviation 13 lm) were produced (Fig. 7a). However, the secondary gas pressure was found to strongly affect droplet size. When the secondary gas pressure was increased to 3.8 kPa, the pulse width had to be increased to 20 ms to overcome the increased resistance at the nozzle exit. However, the mean droplet diameter was greatly reduced to 164 lm (standard deviation 20 lm) as seen in Fig. 7b. To estimate the size of droplets produced by the pneumatic generator and to understand the effect of the surrounding gas pressure an approximate analytical method is used. Fig. 8 shows a liquid meniscus emerging from a nozzle of radius R. The capillary pressure Ps can be estimated from the following relation:
Ps ¼
2r a
where r and a are the surface tension and radius of curvature, respectively. The radius of curvature is:
a¼
R sinða 90 Þ
where a is the liquid–solid contact angle. The liquid flow inside the nozzle is assumed to be incompressible, and the radial pressure is small enough to be ignored compared to its gradient along the nozzle axis. Since the order of magnitude of the Ohnesorge number, Oh ¼ plffiffiffiffiffiffiffi (l: dynamic visqrR
cosity, q: density), for molten tin at 300 °C is 103 (considering the nozzle sizes used in this work) it is assumed that viscous forces are negligible compared to surface tension forces. Neglecting body forces, the continuity and Navier–Stokes equations will be Fig. 6. Single tin droplets when the pulse width was set to 8.25 ms, Nozzle diameter: 150 lm, liquid: molten tin at 300 °C, secondary gas flow pressure: 0.8 kPa, exit vent: 7 cm long tube connected to an open ball valve, and average droplet diameter: 134 lm.
@u ¼0 @z
continuity :
z-momentum : Table 2 Statistics on tin droplet diameter, liquid: molten tin at 300 °C, secondary gas flow pressure: 0.8 kPa, exit vent: 7 cm long tube connected to an open ball valve, and sample size: 35. Nozzle diameter (lm)
Pulse (ms)
Mean diameter (lm)
Standard deviation (lm)
Droplet diameter (analytical model) (lm)
150
8.25 11 25
134 146 130
8 12 15
129 140 144 (2 Droplets) 126 (3 Droplets)
125
10 15 25
112 108 104
4 7 7
107 83 88 (2 Droplets)
@u 1 @P ¼ @t q @z
ð1Þ
ð2Þ
The gas pressure is assumed to rise linearly from zero, and after time tp, reach a peak value Pp, which is large enough to overcome the capillary pressure Ps (reached after time ts) at the nozzle exit and eject a droplet. The pressure variation within the liquid can be presented as:
Pðz; tÞ ¼ FðzÞ
Pp ðt ts Þ þ Ps tp
ð3Þ
where F(z) is a function to be determined. Substituting Eq. (3) into (2) gives
uðtÞ ¼
2 1 @FðzÞ Pp t þ ts t þ c q @z tp 2
ð4Þ
Now, applying the continuity equation, F(z) will be droplet generator. The gas supply pressure and ejection rate were set to 207 kPa and 0.5 Hz, respectively. Fig. 6 shows single tin droplets (average diameter of 134 lm) produced from a 150 lm diameter nozzle. Additional tests were done with a 125 lm diameter nozzle, and single spherical droplets with diameters about 85% that of the nozzle were produced. Table 2 lists a range of other combinations of nozzle sizes and pulse widths that were used to produce tin droplets smaller than the nozzle diameter. The droplet generator was also used successfully to produce droplets of a higher temperature melting material such as the zinc alloy, Zamac 3 (Zn–4Al–0.04Mg), with a melting range of 380– 387 °C. The molten metal temperature was kept at 410 °C, and a 7 cm long tube connected to a 1/3-closed ball valve was attached to the exit vent. Fig. 7 shows single droplets produced from a 102 lm diameter nozzle. When the pressure of the gas in the sec-
FðzÞ ¼ c1 z þ c2
ð5Þ
Considering that the liquid is initially at rest u(ts) = 0, the axial velocity becomes
uðtÞ ¼
2 c1 P p t t2 þ ts t s q tp 2 2
ð6Þ
The upstream (z = 0) and downstream (z = L) pressure boundary conditions are used to obtain c1 and c2 (Fig. 9). Using the upstream boundary condition, P(0, tp) = Pp, F(0) = c2 = 1. From the downstream boundary condition P(L, tp) = Ps, F(L) = 0 and c1 ¼ 1L , so the maximum velocity becomes
umax ¼ uðtp Þ ¼
1 Pp ðtp ts Þ2 2qL tp
ð7Þ
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
31
Fig. 7. Effect of the secondary gas flow pressure on Zamac 3 droplets produced from a 102 lm diameter nozzle, (a) pulse width: 15 ms, secondary gas flow pressure: 1.6 kPa, and (b) pulse width: 20 ms, secondary gas flow pressure: 3.8 kPa.
smax ¼ sðtp Þ ¼
1 Pp ðt p ts Þ3 6qL tp
ð9Þ
Assuming that the maximum displaced liquid (smax) detaches and forms a single droplet, the droplet diameter d can be estimated from the following relation
4 3
pR2 smax ¼ p
3 d ) d ¼ ð6R2 smax Þ1=3 2
ð10Þ
For molten tin at 300 °C, r = 0.56 N/m, q = 6950 kg/m3, and a = 110° [27]. For a 102 lm diameter nozzle, the maximum pressure Ps will be 7.5 kPa, so the peak pressure Pp (average value of 7.8 kPa from experiments) is large enough to push the liquid out of the nozzle. Substituting L = 104 m, tp = 0.001 s, and ts = 0.00077 s (corresponding to the largest peak pressure, Fig. 4) gives d = 70 lm, which is in relatively good agreement with the experimental result (73 lm, Fig. 5). During the first three peak pressures (Fig. 4), due to the quickly changing pressure, the liquid jet oscillates at the nozzle tip before it detaches at some later time and forms a droplet at t = 0. Since the pressure remains positive at all times, the liquid jet is not pushed far back into the nozzle during oscillations. Experimental results show that when multiple droplets are produced, tp and ts are 0.001 and 0.00045 s, respectively, and Pp = 10.75 kPa. In this case, smax = 428 lm, which is the maximum jet length corresponding to 5–8 droplets produced in each ejection, so the droplet diameter ranges from 90 to 105 lm (compare with average diameter of 92 lm from experiments). To include the effect of the secondary gas flow on droplet formation, the jet diameter dj close to the chamber hole (Fig. 10) is first approximated. Using the maximum displacement (Eq. (9)), the average molten metal flow rate can be calculated.
Fig. 8. Radius of curvature a.
Q¼
pR2 smax tp ts
ð11Þ
Assuming that the pressure of the metal jet close to the chamber hole is the same as the gas pressure, the jet velocity will be Fig. 9. Upstream and downstream pressure boundary conditions.
The liquid displacement (length of liquid jet emerging out of the nozzle) can be estimated as follows:
sðtÞ ¼
Z
1 Pp t3 t2 t2 uðtÞdt ) sðtÞ ¼ t s þ s t þ c3 qL tp 6 2 2
ð8Þ
Considering that s(ts) = 0, the maximum liquid displacement will be
v¼
sffiffiffiffiffiffiffiffiffi 2Psf
q
ð12Þ
where Psf is the pressure of the secondary flow, so the jet diameter can be calculated as follows:
sffiffiffiffiffiffiffiffiffi 14 2 dj Q Pp 1 p ¼ ) dj ¼ 2Rðtp ts Þ 4 v 6Ltp 2qPsf
ð13Þ
32
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33
Fig. 10. Schematic diagram of the molten metal jet.
Table 3 Effect of the secondary gas flow pressure on formation of tin droplets, nozzle diameter: 150 lm, pulse width: 25 ms, liquid: molten tin at 300 °C, exit vent: 7 cm long tube connected to an open ball valve, and sample size: 35. Gas pressure (kPa)
Mean diameter (lm)
Standard deviation (lm)
Droplet diameter (analytical model) (lm)
0.4 0.8
168 130
12 15
182 144 (2 Droplets) 126 (3 Droplets)
It is assumed that the jet reaches a length of l after the time period tp ts; then, it detaches and forms a droplet with a diameter of dD.
l ¼ v ðt p t s Þ
p
3 1 4 dD 3 2 3 l¼ p dj l ) dD ¼ 4 2 3 2
2 dj
ð14Þ
ð15Þ
Table 2 compares the droplet diameter obtained from experiments and analytical model for different pulse widths and two nozzle diameters. The results were in relatively good agreement. The most important role of the secondary gas flow was to prevent oxidation during droplet formation. The effect of increasing the pressure of the secondary gas flow was also investigated using a 150 lm diameter nozzle. At 0.4 kPa, single droplets 1.1 times that of the nozzle diameter were produced. Increasing the pressure to 0.8 kPa made it possible to reduce the average droplet diameter by 23% (Table 3). In this case, 2 or 3 droplets were produced during each pulse. Using equation 13 for two different flow pressures gives
1 ðPsf Þ2 4 ðdj Þ1 ¼ ðdj Þ2 ðPsf Þ1
ð16Þ
which shows that increasing the secondary flow pressure reduces the jet diameter, and therefore, formation of smaller droplets is expected. 4. Summary and conclusions This paper presents an experimental study to produce molten metal droplets using a pneumatic drop-on-demand generator and investigate the effect of various parameters on reducing the droplet size. The required pressure oscillation was generated attaching a tube with a specified length to the exit vent. To prevent oxidation of droplets as they emerged from the generator two dif-
ferent methods were used. First, a controlled atmosphere chamber was made from a glass tube that was evacuated and filled with nitrogen. Alternately, nitrogen gas was injected around the nozzle to surround it with an inert atmosphere. Using a controlled atmosphere chamber, tin droplets with an average diameter of 70% that of the nozzle diameter (102 lm) were produced. For single droplets produced at a frequency of 0.5 Hz from one nozzle, more than 99% of the droplets were ranging from 60 to 100 lm in diameter. For multiple droplets produced at higher frequencies, 5 and 10 Hz, more than 90% of droplets were smaller than the nozzle diameter. Using a secondary flow of nitrogen around the nozzle single tin droplets about 85% that of the nozzle diameter were produced from nozzles as small as 125 lm in diameter. Increasing the pressure of the secondary gas flow decreased the droplet diameter. Also, single droplets were produced from a zinc alloy (Zamac 3) using a 102 lm diameter nozzle, and increasing the pressure of the secondary gas flow made it possible to decrease the droplet diameter by 65%. Finally, an approximate analytical model was described to explain the fluid flow behavior within the nozzle and estimate the droplet diameter. Experimental results showed good agreement with those obtained from the model. The analysis shows that the droplet diameter depends on the nozzle size, applied pressure pulse, pressure of the secondary gas flow and molten metal density. Since predictions of the analytical method were in relatively good agreement with the experimental results for molten tin, we should expect acceptable results for different alloys. The equations show that the only affecting physical property of the molten metal is density. However, testing different alloys under the same experimental conditions and with nozzle sizes much smaller than 200 lm in diameter might be challenging. References [1] E.R. Lee, Microdrop Generation, CRC Press, Stanford Linear Accelerator Center, Stanford University, 2003. [2] G.L. Switzer, A versatile system for stable generation of uniform droplets, Rev. Sci. Instrum. 62 (11) (1991) 2765–2771. [3] H. Ulmke, M. Mietschke, K. Bauckhage, Piezoelectric single nozzle droplet generator for production of monodisperse droplets of variable diameter, Chem. Eng. Technol. 24 (2001) 69–70. [4] S. Sakai, Recording method by inkjet recording apparatus and recording head adapted for said recording method, US Patent No. 5,933,168, 1999. [5] S. Sakai, Dynamics of piezoelectric inkjets printing systems, IS & T’s NIP 16, in: International Conference on Digital Printing Technologies, IS & T: The Society for Imaging Science and Technology, 2000, pp. 15–20. [6] A.U. Chen, O.A. Basaran, A new method for significantly reducing drop radius without reducing nozzle radius in drop-on-demand drop production, Phys. Fluids 14 (2002) L1–L4. [7] M. Orme, A. Bright, Recent advances in highly controlled molten metal droplet formation from capillary stream break-up with applications to advanced manufacturing, TMS Annual Meeting, 2000. [8] M. Orme, J. Courter, Q. Liu, J. Zhu, R. Smith, Charged molten metal droplet deposition as a direct write technology, MRS Spring Meeting, San Francisco, 2000. [9] P. Yim, J. Chun, N. Saka, J.C. Rocha, The effect of oxygen concentration on the break-up behavior of laminar, liquid metal jets, Liq. Met. Atomization: Fundam. Practice, The Miner., Met. Mater. Soc. (2000) 169–181. [10] S. Haferl, D. Poulikakos, Transport and solidification phenomena in molten microdroplet pileup, J. Appl. Phys. 92 (2002) 1675–1689. [11] K. Yamaguchi, K. Sakai, T. Yamanaka, T. Hirayama, Generation of threedimensional micro structure using metal jet, Precis. Eng. 24 (2000) 2–8. [12] T.M. Lee, T.G. Kang, J.S. Yang, J. Jo, K.Y. Kim, B.O. Choi, D.S. Kim, Drop-ondemand solder droplet jetting system for fabricating microstructure, IEE Trans. Electron. Packag. Manuf. 31 (2008) 201–210. [13] X.S. Jiang, L.H. Qi, J. Luo, H. Huang, J.M. Zhou, Research on accurate droplet generation for micro-droplet deposition manufacture, Int. J. Adv. Manuf. Technol. 49 (2010) 535–541. [14] S. Gao, Y. Yao, C. Cui, Vibrating breakup of jet for uniform metal droplet, J. Mater. Sci. Technol. 23 (2007) 135–138. [15] H. Sohn, D.Y. Yang, Drop-on-demand deposition of superheated metal droplets for selective infiltration manufacturing, Mater. Sci. Eng. A 392 (2005) 415–421. [16] S. Chandra, R. Jivraj, Apparatus and method for generating uniform sized droplets, US Patent No. 6,446,878, 2002.
A. Amirzadeh et al. / Experimental Thermal and Fluid Science 47 (2013) 26–33 [17] S.X. Cheng, T. Li, S. Chandra, Producing molten metal droplets with a pneumatic droplet-on-demand generator, J. Mater. Process. Technol. 159 (2005) 295–302. [18] V. Foutsis, Producing small molten metal drops with a pneumatic generator, University of Toronto, MASc Thesis, 2004. [19] J. Luo, L.H. Qi, J.M. Zhou, X.H. Hou, H.J. Lee, Modeling and characterization of metal droplet generation by using a pneumatic drop-on-demand generator, J. Mater. Process. Technol. 212 (2012) 718–726. [20] V.F. de Almeida, R.D. Hunt, J.L. Collins, Pneumatic drop-on-demand generator for production of metal oxide micro spheres by internal gelation, J. Nucl. Mater. 404 (2010) 44–49. [21] A. Tropmann, N. Lass, N. Paust, T. Metz, C. Ziegler, R. Zengerle, P. Koltay, Pneumatic dispensing of nano-to picoliter droplets of liquid metal with the starjet method for rapid prototyping of metal microstructures, Microfluid. Nanofluid 12 (2012) 75–84. [22] Y. Chao, L. Qi, Y. Xiao, J. Luo, J. Zhou, Manufacturing of micro thin-walled metal parts by micro-droplet deposition, J. Mater. Process. Technol. 212 (2012) 484– 491. [23] S. Zhong, L. Qi, J. Luo, Y. Xiao, Parameters study on generation of uniform copper droplet by pneumatic drop-on-demand technology, J. Adv. Mater. 430– 432 (2012) 781–784.
33
[24] W. Cao, Y. Miyamoto, Freeform fabrication of aluminum parts by direct deposition of molten aluminum, J. Mater. Process. Technol. 173 (2006) 209–212. [25] J. Luo, L.H. Qi, S.Y. Zhong, J.M. Zhou, H.J. Li, Printing solder droplets for micro devices packages using pneumatic drop-on-demand (DOD) technique, J. Mater. Process. Technol. 212 (2012) 2066–2073. [26] H. Li, P. Wang, L. Qi, H. Zuo, S. Zhong, X. Hou, 3D numerical simulation of successive deposition of uniform molten Al droplets on a moving substrate and experimental validation, Comput. Mater. Sci. 65 (2012) 291–301. [27] S. X. Cheng, Development of a molten metal droplet generator for rapid prototyping, University of Toronto, MASc Thesis, 2002. [28] A. Amirzadeh, S. Chandra, Small droplet formation in a pneumatic drop-ondemand generator: experiments and analysis, Exp. Therm. Fluid Sci. 34 (2010) 1488–1497. [29] A. Amirzadeh Goghari, S. Chandra, Producing droplets smaller than the nozzle diameter by using a pneumatic drop-on-demand droplet generator, Exp. Fluids 44 (2008) 105–114.