Abstract:
An apparatus and method for the production of small, uniform sized droplets. The apparatus is a droplet generator including a housing defining a chamber for holding a material to be ejected therefrom, an inlet and a droplet outlet communicating with the chamber. The housing is coupled to a pressurizing system connected to the inlet for applying pressure pulses to the chamber. The housing includes a vent in communication with the chamber for relieving pressure in the chamber. The vent has an effective size so that during application of the pressure pulse the chamber is pressurized to a pressure effective to eject a droplet of the material therefrom and thereafter the chamber is vented through the vent at a rate sufficient to prevent further discharge of droplets. The droplet generator produces molten metal or alloy droplets and is particularly suitable for generating single droplets on demand in manufacturing techniques using droplet deposition. It is also useful for production of spherical microspheres and uniform sized powders, and dispensing of precise quantities of materials such as adhesives and pharmaceuticals.

Description:
CROSS REFERENCE TO RELATED U.S. APPLICATION 
     This application relates to United States Provisional patent application, Ser. No. 60/122,271, filed on Mar. 1, 1999, entitled APPARATUS AND METHOD FOR GENERATING UNIFORM SIZED DROPLETS. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for the production of droplets of liquid at high temperatures, such as a molten metal or alloy, and more particularly the present invention relates to a method and apparatus for controlled generating of droplets on demand in manufacturing processes. 
     BACKGROUND OF THE INVENTION 
     Free standing metal objects can be manufactured by the deposition of individual droplets of molten metal, using a computer to manipulate the droplet generator and the substrate, as described in U.S. Pat. No. 5,340,090 to Orme et al. and U.S. Pat. No. 5,746,844 to Sterett. Droplet deposition has also been used in U.S. Pat. No. 5,229,016 to Hayes et al. to dispense small amounts of solder at precisely determined locations on a circuit board prior to attaching an integrated circuit chip to it. Such manufacturing techniques require the ability to generate, on demand, small droplets of a molten metal. Consequently several designs for such droplet generators have been developed. Typically such generators consist of a heated chamber filled with molten metal. A droplet is formed by applying a pressure pulse to the pool of metal, ejecting a small quantity of liquid through a nozzle. Several different techniques have been used to apply this pressure pulse, including piezoelectric crystals, mechanical plungers, acoustic waves, and magneto-hydrodynamic (MHD) forces discussed herebelow. 
     Piezoelectric droplet generators are widely used for ink-jet printing. They have a chamber containing liquid, one wall of which is made from piezoelectric material. Applying a voltage pulse to the piezo-electric crystal makes it flex, sending a pressure pulse through the liquid in contact with it and forcing out a droplet. In U.S. Pat. No. 4,828,886 to Hieber, liquid solder is supplied through a glass tube around which an annular piezoelectric transducer is mounted. Application of a voltage to the transducer makes it contract and compress the glass tube, emitting solder from the tube. Use of such transducers is restricted to low melting point metals, because they lose their responsive properties above the Curie temperature (about 350° C. for most piezoelectric materials). 
     Mechanical systems of levers and plungers have been used to form droplets of high temperature metals. Chun et al. (U.S. Pat. No. 5,266,098) and Yuan et al. (U.S. Pat. No. 5,609,919) used a reciprocating plunger to periodically apply impulses to a liquid metal and force it through an array of holes in the bottom of the container. In U.S. Pat. No. 5,598,200 to Gore single droplets are ejected on demand by positioning a plunger over an orifice in a chamber containing a liquid, and rapidly moving the plunger towards the orifice. Mechanical actuators allow droplet generators to be used at high temperatures, but increase their complexity and restrict the frequency with which droplets can be produced. 
     Acoustic radiation pressure can be used to eject metal droplets from the free surface of a pool of molten metal by directing towards the surface bursts of energy from an acoustic source located at the bottom of the pool (U.S. Pat. No. 5,722,479 to Oeftering). Magneto-hydrodynamic (MHD) forces can also be used (U.S. Pat. No. 4,919,335 to Hobson et al.) to form a fine spray by passing an electric current through the molten metal and simultaneously applying a magnetic field perpendicular to the direction of the electric current. The resultant MHD force is used to force molten metal through a nozzle, forming droplets. Acoustic and MHD droplet generators are useful in producing sprays, but it is difficult to precisely control the size of droplets produced by these devices. 
     A stream of droplets can be produced by vibrating a liquid jet issuing from an orifice, inducing capillary instabilities that break the stream into uniform sized droplets. The excitation force can be applied to the jet using either an acoustic source (as in U.S. Pat. No. 5,445,666 to Peschka et al.) or a mechanical actuator (as in U.S. Pat. No. 5,810,988 to Smith Jr. et al.). This technique is useful in forming metal micro-spheres, but cannot be used to generate droplets on demand. 
     It would be very beneficial to provide a method and device for reproducibly producing individual droplets of a chosen size. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and device for producing individual or multiple droplets of a chosen size on demand. 
     In one aspect of the invention there is provided a method of producing droplets, comprising pressure pulsing a chamber with a gas, the chamber holding a material to be ejected as droplets, the chamber being pressurized for a sufficient time to build up a pressure sufficient to forcefully eject at least one droplet of said material through an outlet and thereafter relieving the pressure sufficiently rapidly to avoid ejection of further droplets from the chamber. 
     In another aspect of the invention there is provided an apparatus for generating and ejecting droplets therefrom, comprising: 
     a) a housing enclosing a chamber for holding a material to be ejected therefrom, a gas inlet and an outlet passageway communicating with said chamber; 
     b) pressurizing means connected to said gas inlet for pressure pulsing the chamber with a gas for forcefully ejecting at least one droplet through said outlet passageway; and 
     c) pressure relief means for relieving pressure in said chamber sufficiently rapidly to avoid ejection of further droplets and to provide control of a number of droplets ejected from said chamber through said outlet passageway. 
     In this aspect of the invention the pressure relief means may include a vent in communication with the chamber for relieving pressure in the chamber, the vent having an effective size so that during application of a gas pulse the chamber is pressurized to a pressure sufficient to eject a droplet of material therefrom and thereafter the chamber is vented through the vent at a rate sufficient to prevent further discharge of droplets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The device for producing droplets constructed in accordance with the present invention will now be described, by way of example only, reference being had to the accompanying drawings, in which: 
     FIG. 1 is a schematic drawing showing the droplet generator assembly; 
     FIG. 2 is a diagram showing the components of the chamber in which molten metal is contained; 
     FIG. 3 shows two spherical tin droplets, 200 μm in diameter; 
     FIG. 4 shows an electron microscope image of a single tin particle; 
     FIG. 5 shows the size distribution of droplets produced using a 0.003″ diameter nozzle and a gas pressure of 21 psi; 
     FIG. 6 shows the size distribution of droplets produced using a 0.003″ diameter nozzle and a gas pressure of 30 psi; 
     FIG. 7 shows the variation of droplet diameter with nozzle diameter for tin and bismuth; 
     FIG. 8 shows 16 tin droplets, each 300 μm in diameter, deposited in a square grid spaced 3 mm apart on a stainless steel plate; 
     FIG. 9 is a cross sectional view of a droplet generator constructed with multiple nozzles; and 
     FIG. 10 is a cross sectional view of another embodiment of a droplet generator for rapid cooling of metal droplets. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a method of producing droplets, comprising pressurising a chamber with a gas, the chamber holding a material to be ejected as droplets, the chamber being pressurized for a sufficient time to build up a pressure sufficient to eject at least one droplet of the material through an outlet and thereafter relieving the pressure sufficiently rapidly to avoid ejection of further droplets from the chamber in order to allow control over the number of droplets ejected during processing. 
     Referring to FIG. 1 an apparatus for generating droplets shown generally at  10  uses compressed gas  12  to deliver a pressure pulse to high temperature liquid  14  contained in a chamber  32  forming part of droplet generator  16 . Briefly the pressure increase forces a small amount of liquid through a nozzle  42  in the wall of the chamber, thereby ejecting a liquid droplet  18 . The gas in the chamber  32  is then vented through a small orifice  22 , relieving the pressure and preventing any further liquid from being ejected. Each pressure pulse to the droplet generator  16  may therefore be used to produce a single droplet on demand. 
     Referring to both FIG.  1  and FIG. 2, droplet generator chamber  16  includes a cylindrical stainless steel housing  26  heated by means of a band heater  30  (seen only in FIG. 1) wrapped around the housing. Housing  26  incloses a central chamber  32  to contain the liquid  14 . Housing  26  includes a lid  36  placed over the top of the housing to seal chamber  32  thereby permitting it to be pressurised. Housing  26  includes an outlet passageway  40  drilled through the bottom of the housing through which the liquid droplets are ejected. 
     A commercially available synthetic sapphire nozzle  42  is placed at the exit of passageway  40  and is held fixed in a recess  46  located in a retainer plate  48  fastened to the bottom of the housing  26 . Nozzles  42  are cylindrical, with an outer diameter of 0.0785″ and a length of 0.034″ in one embodiment. Liquid is forced through a hole  50  drilled in the centre of the nozzle: hole diameters ranging from 0.003″ to 0.022″ were used for testing purposes. In another embodiment nozzles were formed by directly drilling holes in the stainless steel plate with a laser instead of using removable sapphire nozzles. 
     Referring again to FIG. 1, the chamber  32  is filled with high temperature liquid  14  which is typically a molten metal. Housing  26  is heated by means of a band heater  30  to a temperature above the melting point of the liquid by a temperature controller  52 . Chamber  32  is pressurised using nitrogen gas supplied through stainless steel tubing  56  from a compressed gas cylinder  58 . Other inert gases may be used as long as they do not react with the molten metal being discharged. The pressure at which gas is supplied is controlled by a pressure regulator  60 . Flow of nitrogen is controlled by a normally closed solenoid valve  62  that is opened for a period of time determined by an electronic timing circuit  64 . 
     Droplets are formed by forcing liquid through the synthetic sapphire nozzle  42  sitting in retainer plate  48  at the bottom of housing  26 . Nitrogen gas at a pressure of 20-40 psig is supplied to the cavity in which the liquid is contained. The cavity is rapidly pressurised by opening the solenoid valve  62  for 5-10 ms. This is sufficient to force a small droplet  18  through the sapphire nozzle  42 . The pressure in the chamber  32  then drops as the nitrogen escapes through vent hole  22  drilled in a T-junction  70  in the coupling  72  connecting the gas line  56  to the droplet generator  16 . The sudden decrease in pressure prevents any more metal droplets being ejected through the nozzle  42 . The location and size of the vent hole  22  is important to the operation of the droplet generator. Hole  22  must be small enough to allow gas to accumulate in the chamber  32  and increase the pressure adequately to force a droplet out. However, hole  22  must also be large enough that the gas escapes quickly and relieves the pressure in the chamber  32  by the time a single droplet has escaped. If the pressure in chamber  32  does not drop with sufficient rapidity a jet of liquid issues out of the nozzle rather than a single droplet. 
     This design of the droplet generator  16  disclosed herein offers several advantages over previous designs. First, there is no inherent restriction on the operating temperature. Piezo-ceramic crystals fail at temperatures above 250° C., restricting their use to metals with melting points lower than this. Typically they have been used for depositing solder balls on printed circuit boards. The present system is extremely simple in that there are no moving parts in contact with the metal. This is advantageous in scaling up the system. Use of metal plungers greatly increases the complexity of the system, and makes it much more prone to clogging. 
     Droplet generator  16  is very advantageous because it may be used to produce a single droplet on demand. Most previously developed droplet generation systems work in a continuous mode and they cannot form just a single droplet when triggered. Another significant advantage of the present droplet generator is there is great control over droplet size. The droplet size is a function of the gas pressure, pulse duration, nozzle size, size and location of the relief vent. Some of these parameters, such as the pressure and duration of the gas pulse, can be altered during operation. It will be therefore possible to change the droplet size without dismantling the system and replacing the nozzle. Another significant advantage of the present system is repeatability of droplet size. Tests have shown that the droplet diameter produced is extremely repeatable. Other mechanically driven atomisation techniques used to produce droplets typically yield a very large range of particle sizes. 
     It will be understood that there are other ways of relieving the pressure in the chamber in addition to using the vent. For example, solenoid valves may be used wherein a gas outlet passageway includes the solenoid valve which is opened as required to relieve the pressure in the chamber after a droplet has been ejected. Pressure relief valves may also be connected to the chamber and designed to open at a pre-set pressure threshold thereby rapidly relieving pressure in the chamber. Whatever the mechanism for relieving the pressure it should be sufficiently rapid to prevent discharge of further droplets. The following is a non-limiting example of the invention disclosed herein. 
     EXAMPLE 
     Using the apparatus of FIG. 2, tin droplets were formed. Molten tin was held in the chamber at a temperature of 245°C., above the melting point of tin (which is 232° C.). A synthetic sapphire nozzle with an opening 0.003″ in diameter was installed in the bottom of the chamber. Nitrogen gas was supplied to the chamber through ¼″ stainless steel tubing. A ¼″ Swagelok T-junction was used to connect the tubing to a threaded hole drilled in the lid of the chamber. The open branch of the T-junction was covered with a steel disk in the centre of which was drilled a 0.125″ vent hole, which provided a vent for gas to escape from the chamber. 
     A pulse of nitrogen gas was supplied to the chamber by opening the solenoid valve for 6 ms. Gas pressures were varied from 20 psig to 40 psig. Droplets ejected from the generator fell into a tube filled with nitrogen and solidified while in free-fall. Solidified tin particles were captured in a dish and examined under a microscope. They were found to be spherical and fairly uniform in diameter. FIG. 3 shows two tin particles, 200 μm in diameter, formed by the droplet generator. FIG. 4 shows a scanning electron microscope image of a single tin particle. FIG. 5 is a graph showing the size distribution of 8 spheres, formed using a gas pressure of 21 psig. The average droplet diameter was approximately 250 μm. FIG. 6 shows the size range when the gas pressure was increased to 30 psig; the droplet diameter is smaller, with an average value of approximately 200 μm. 
     Tests were done with a range of different nozzle sizes from 0.003″ to 0.022″ to produce particles of both tin and bismuth. FIG. 7 shows the relationship between the diameter of the droplet produced and the nozzle diameter. Droplet diameters increased linearly with nozzle diameter. 
     By moving a substrate under the droplet generator droplets could be deposited in a predetermined pattern. FIG. 8 shows 16 tin droplets, each 300 μm in diameter, deposited in a square grid spaced 3 mm apart. The substrate was a polished stainless steel plate mounted on computer-controlled positioning stages so that it could be moved under the droplet generator. 
     Uniform sized particles comprising metal powder were produced by releasing droplets into an inert atmosphere and letting them freeze as they fall. Referring to FIG. 9, a droplet generator  80  includes a housing  82  defining an interior chamber  86  and multiple droplet discharge outlets  90 ,  92 ,  94  and  96  so that when chamber  86  is pressurised several uniformly sized droplets are ejected. Tests were done with 4 to 16 nozzles in a single droplet generator. Uniform sized powder particles have many applications, in plasma spraying it is useful to have a uniform powder size distribution because the trajectory and solidification rate of particles in a thermal spray depends on the size of the particles. Small particles may not have enough momentum to land on the substrate, or they freeze before impact and do not bond with the substrate. Therefore, having a very narrow size distribution permits much better control of the deposition process and reduces wastage of the powder. Similarly, when spray painting, dispensing adhesives, or spraying pesticide using conventional pressure atomiser, small droplets are prone to being blown away thereby missing the substrate also causing wastage and producing a major pollution hazard. The use of the present multiple droplet generator  80  permits production of mono-sized sprays thereby offering significant enhanced control over the spraying process. 
     However it will be understood that the different outlets  90  to  96  may be of different sizes relative to each other for applications requiring more than one size of particle. 
     Typically, in order to produce metal spheres from large (greater than 1 mm in diameter) droplets, the droplets have to fall through a 10-20 m height in air to solidify completely thus requiring large drop towers. Referring to FIG. 10, the present invention avoids this problem by immersing the end of the housing containing the nozzle  42  in a fluid bath comprising a fluid  98  having sufficient density and viscosity to slow and solidify the droplets  18  as they fall so that they freeze before hitting the bottom of the container. When oil was used as the fluid the droplets freeze within a distance of 5-10 cm of the nozzle after being ejected. In one example the container was filled with vegetable oil to a depth of 15 cm and tin and bismuth spheres were produced having diameters from 0.8 mm to 2.0 mm by letting molten metal droplets freeze as they fell in oil. This technique also has the advantage of eliminating any oxidation of the metal, so that oxide free spheres can be produced. 
     The apparatus and method disclosed herein is of significant utility for generating single droplets on demand in manufacturing techniques using droplet deposition such as in microelectronics manufacturing and processing. This invention also has utility in processes requiring spherical microspheres and uniform sized powders, and dispensing of precise quantities of materials such as adhesives and pharmaceuticals. 
     The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.