ELECTRICALLY OPERATED LIQUID DISPENSING APPARATUS AND METHOD FOR DISPENSING VISCOUS LIQUID

Abstract: An electrically operated liquid dispenser (10) to selectively dispense viscous liquid, such as an epoxy (18), into a substrate (19), such as a printed circuit board. The dispenser (10) has a dispenser body (12) with a first interior portion and a second interior portion (22). The first interior portion (20) includes a valve seat (32) and communicates with an outlet (34). An armature (60) is disposed within the second interior portion (22), and a valve stem (26) is mounted within the first interior portion (20) for movement with the armature (60) between closed and opened positions. A coil (70) is disposed about the armature (60) and is adapted to selectively generate an electromagnetic field for moving the armature (60) and is adapted to selectively generate an electromagnetic field for moving the armature (60) and the valve stem (26) between the closed and opened positions. A liquid inlet passageway (46) is in fluid communication with the first interior portion (20) and is adapted to connect to a source (47) of liquid for supplying the liquid to the first interior portion (20). To prevent liquid in the first interior portion (20) from leaking into the second interior portion (22), a seal member (52) is mounted between the first and second interior portions (20, 22) to sealingly engage the valve stem (26).

Description: ELECTRICALLY OPERATED LIQUID DISPENSING APPARATUS AND METHOD FOR DISPENSING VISCOUS LIQUID Field of the Invention The present invention generally relates to apparatus for dispensing liquid and, more specifically, to electrically operated apparatus for dispensing viscous liquids, such as hot melt adhesives or epoxies. Background of the Invention Pneumatic and electric dispensers have been developed for dispensing applications requiring precise placement of a viscous liquid. One common application for these dispensers involves placing solder, flux or epoxy onto a circuit board during its manufacture. In many situations, electrically operated dispensers are preferred over pneumatic dispensers since electrically operated dispensers may be controlled more precisely during a manufacturing operation. Generally, electrically operated dispensers include an electromagnetic coil that produces an electromagnetic field. The electromagnetic field may be selectively controlled to open and close a valve stem which is operatively associated with an armature or acts as an armature itself. More specifically, the forces of magnetic attraction between the armature and a pole move the valve stem into an open and/or closed position. The liquid passes through a valve seat when the valve stem is in the open position. In typical devices of this type, viscous liquid is introduced at a pressure of about 500 psi - 1 500 psi through the coil and over the armature. These elevated pressure levels are often required to circulate the viscous liquid through a small space, such as a radial gap between the armature and the coil. However, the associated high pressure components increase the cost and complexity of the dispensing system. In current electrically operated dispensers, the armature may be specifically configured to allow for the flow of viscous liquid to the outlet. For example, a generally cylindrical armature may contain channels or grooves to allow for liquid flow. These channels or grooves reduce the mass of the armature and can therefore adversely affect the performance of the armature in the electromagnetic field. In this regard, an armature of greater mass will perform better in the electromagnetic field by moving more quickly to the open and/or closed position. Other manners of promoting the flow of viscous liquid past an armature include reducing the diameter of the armature to provide a relatively large radial gap between the armature and the electromagnetic coil. Channels in the armature or valve stem can have a similar effect. The problem with these configurations is that the armature operates most effectively when it is positioned close to the coil. Therefore, a large radial gap or channels will promote greater flow, but will also reduce the effectiveness of the electric actuation. In addition to the problems noted above with respect to the use of high pressures and the configurations of the armature and the coil, prior electric dispensers generate undesirable shear forces on the viscous liquids moving past the armature. As the armature moves between the open and closed positions, shear forces act on the liquid as the armature moves relative to the walls of the adjacent liquid passageway. These shear forces may have detrimental effects on the physical properties and characteristics of the viscous liquid, for example, by generating additional heat which is transferred to the liquid. In one application epoxies containing a catalyst are dispensed for bonding components to a circuit board. When excessive shear forces and heat are generated in the electrical dispenser, the catalyst can break down and this can adversely affect the curing properties of the epoxy. In fact, the epoxy may cure while still in the dispenser and this cured or hardened epoxy can completely prevent operation of the dispenser. In view of these and other problems in the art, it would be desirable to provide an electrically operated viscous liquid dispenser in which the viscous liquid may be maintained at a low pressure out of contact with the armature and in which the armature is configured for highly effective operation with a coil. Summary of Invention An electrically operated liquid dispenser in accordance with the invention includes a dispenser body with a first interior portion and a second interior portion. The first interior portion communicates with an outlet for discharging the liquid. An armature is disposed within the second interior portion, and a valve seat is disposed typically adjacent the outlet. In addition, a valve stem is mounted within the first interior portion and is connected for movement with the armature between a closed position in which the valve stem engages the valve seat to prevent liquid flow from the outlet and an opened position in which the valve stem is retracted away from the valve seat to allow liquid flow from the outlet. The dispenser further includes a coil disposed about the armature. The coil is adapted to selectively generate an electromagnetic field for moving the armature and the valve stem between the closed and opened positions. A liquid inlet passageway is in fluid communication with the first interior portion and is adapted to connect to a source of liquid for supplying the liquid to the first interior portion. A seal member is mounted between the first and second interior portions and sealingly engages the valve stem to prevent liquid in the first interior portion from leaking into the second interior portion. More specifically, the valve stem may be an integral shaft which extends within both the first and second interior portions of the dispenser body. Preferably, the armature is coaxially aligned with and directly connected to the valve stem. The armature is sized and shaped such that it fills substantially the entire space between the shaft and an inner surface of the second interior portion. Accordingly, the armature has a greater mass compared to similarly-shaped armatures having liquid bypass grooves or channels. As such, the electromagnetic field of the coil will have a more forceful effect on the armature. The present invention also contemplates a method for dispensing viscous liquid with an electrically operated dispenser. Generally, viscous liquid is supplied to the first interior portion from the liquid inlet passageway. Preferably, the liquid is supplied to the first interior portion at a pressure ranging from about 1 0 to about 1 5 psi. More generally, the pressure may be less than about 1 00 psi and may be a minimum of about 3 psi. Therefore, high pressure components are not required as in the past. The liquid is prevented from entering the second interior portion and contacting the armature by dynamically sealing the valve stem between the first and second interior portions. Next, the coil is energized in order to move the valve stem between the opened and closed positions. Typically, energization will move the valve stem to the opened position to allow the liquid to flow from the first interior portion through the outlet. The coil can then be de-energized so that the valve stem returns to the closed position under the force of a spring. Preferably, the high viscosity liquid contemplated by the method of the invention is a single component heat cure surface mount epoxy. In accordance with the invention, the viscous liquids have a minimum viscosity of about 1 00 centipoise, however, the invention is particularly advantageous for liquids with viscosities above about 1 ,500 centipoise and even above about 1 0,000 centipoise. Various additional advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon consideration of the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings. Brief Description of Drawings Fig. 1 is an axial cross-sectional view of an electrically operated liquid dispenser constructed according to the invention; and Fig. 2 is a cross-sectional view of the liquid dispenser of Fig. 1 taken along line 2-2. Detailed Description of Preferred Embodiments Referring first to Fig. 1 , an electrically operated dispenser 1 0 of the preferred embodiment includes a dispenser body 1 2, a liquid dispensing nozzle body 1 4, and an electrical lead 1 6 for supplying electric current for electromagnetic actuation purposes. The dispenser 1 0 is specifically adapted for dispensing high viscosity liquids, such as single component heat cure surface mount adhesives or epoxy, but other liquid dispensers can benefit from the invention as well. Such other liquids include soldering fluxes, thermal greases, heat transfer compounds, and solder pastes. Furthermore, the dispenser 1 0 is adapted to dispense liquids in discrete amounts, preferably as droplets or dots, but alternatively in continuous beads. As shown in Fig. 1 , the dispenser body 1 2 used in conjunction with the liquid dispensing nozzle body 1 4 is particularly constructed to dispense droplets 1 8 of heat cure epoxy onto a substrate 1 9, such as a printed circuit board. The invention is most advantageous for liquids having viscosities above about 1 500 centipoise and for epoxies or other liquids employing catalysts or other components which are adversely affected by heat and shear during the dispensing operation. The dispenser body 1 2 has a first or lower interior portion 20 and a second or upper interior portion 22. These portions 20, 22 are generally coaxially aligned with one another. In accordance with the principles of the invention, the lower interior portion 20 and the upper interior portion 22 are not in fluid communication with each other. A valve stem 26 is mounted within the lower interior portion 20 and has a shaft 28 which extends within the lower interior portion 20 and into the upper interior portion 22. A ball 30 is mounted to a lower end 28a of the shaft 28 which is shown in Fig. 1 in sealing engagement with a valve seat 32 positioned in the nozzle body 1 4. With the ball 30 sealingly engaging valve seat 32, high viscosity liquid, such as an epoxy, cannot flow through an outlet 34 in the valve seat 32. The nozzle body 1 4 also has a nozzle tip 36 with an orifice 38 aligned with the outlet 34 and flush mounted to the valve seat 32 by a threaded retaining nut 40. The nozzle tip 36 can be readily exchanged with a different nozzle tip to produce droplets of a different size and, in some cases, a different shape. A liquid inlet passageway 46 is connected to the lower interior portion 20 and is adapted to connect to a source 47 of pressurized liquid, such as epoxy. This source 47 may be a syringe supply device as shown in U.S. Patent No. 5,747, 102, the disclosure of which is hereby fully incorporated by reference. A seal member 48 is positioned at the entrance to the liquid inlet passageway 46 and is adapted to sealingly engage, a liquid supply line (not shown) which connects the liquid inlet passageway 46 to the source 47 of pressurized liquid. Generally, the liquid coming from the source 47 is pressurized at between about 3 psi and about 100 psi. More preferably, the liquid is pressurized to about 1 0-1 5 psi. Arrows 50 indicate the flow path of the liquid entering through liquid inlet passageway 46 and through lower interior portion 20. A dynamic seal member 52 held in sealing engagement with the shaft 28 of the valve stem 26 between interior portions 20 and 22. Dynamic seal member 52 is held between a washer 54 and a spring clip 56. As is clearly shown in Fig. 1 , the liquid inlet passageway enters the lower interior portion 20 below dynamic seal member 52. The dynamic seal member 52 prevents liquid entering through the liquid inlet passageway 46 and contained in lower interior portion 20 from leaking into the upper interior portion 22. The dynamic seal member 52 prevents liquid from leaking into the upper interior portion 22 not only when the valve stem 26 is stationary, but also when the valve stem 26 is actuated back and forth during operation. With reference now to Figs. 1 and 2, an armature 60 is disposed within the upper interior portion 22 and is coaxially aligned with and, preferably, formed integrally with shaft 28. Because the dynamic seal member 52 prevents liquid from leaking into the upper interior portion 22, the armature 60 operates in air and not liquid. Unlike other electrically actuated dispensers, the liquid entering the lower interior portion 20 never flows through or around the armature 60. Accordingly, the armature 60 does not require longitudinal or circumferential liquid by-pass channels to allow the liquid to flow past the armature 60. As such, the armature 60 is generally a solid piece of metal with a substantially circular cross-section without liquid by-pass channels that are frequently found in armatures of prior dispensers. Additionally, the armature 60 is sized such that in fills substantially the entire space between the shaft 28 and an inner surface 62 of the upper interior portion 22. To that end, a narrow radial gap "X" is formed between an outer surface 64 of the armature 60 and the inner surface 62 of the upper interior portion 22. Gap "t" preferably is less than about 0.010 inches and, more preferably, ranges between about 0.003 inches to about 0.005 inches. An electromagnetic coil 70 is disposed about the armature 60. Although any suitable electromagnetic coil could be used, it is contemplated that the electromagnetic coil 70 will be generally toroidal in shape. The coil 70 is contained in a housing 72 and connected to a power source (not shown) by conventional electrical leads 1 6. As such, when supplied with the requisite electrical current, coil 70 generates an electromagnetic field which actuates valve stem 26 to an open position as will be described below. A bore 80 extends into armature 60 to house a return spring 82. The return spring 82 biases the valve stem 26 and, more specifically, the ball 30 to sealingly engage valve seat 32 in a closed position. Spring 82 is more specifically a compression spring which is placed under compression within bore 80 through engagement with an electromagnetic pole 84. To achieve an opened position, the electromagnetic coil 70 must generate a sufficient electromagnetic field flowing between armature 60 and pole 84 so as to attract armature 60 and pole 84 together. Since pole 84 cannot move, armature 60 will move against the force of spring 82 until it hits pole 84. The stroke length is the distance between armature 60 and pole 84 as shown in Fig. 1 . An adjustment nut 85 threaded onto threads 86 provides a means to initially set the stroke length. More specifically, brazing 88 connects pole 84 to a tubular member 90. Tubular member 90 has a lower threaded portion 92 received within an internally threaded lower housing portion 94. A tool, such as a screwdriver, may be used to turn pole 84 and, therefore, tubular member 92 as an O-ring 96 slides against an interior surface of lower housing portion 94. This adjustment varies the distance between the lower end of pole 84 and the upper end of armature 60 or, in other words, varies the stroke length of valve stem 26. A lower donut 98 is disposed about tubular member 90 and rests against an upper side of lower housing portion 94 while an upper donut 1 00 is held against coil housing 72 by nut 85 and a lock washer 1 02. In one suitable application, the dispenser 1 0 is constructed to dispense droplets 1 8 of heat cure epoxy onto a substrate, such as a printed circuit board 1 9. Accordingly, one suitable method for dispensing the epoxy onto the substrate is described herein. First, the dispenser 1 0 is positioned over the circuit board in a desired location. In its initial condition, the electromagnetic coil 70 is not energized and the valve stem 26 is in the closed position, i.e., the ball 30 is sealingly engaging the valve seat 32. Additionally, the heat cure epoxy is contained within and fills the lower interior portion 20 and liquid inlet passageway 46 which is also connected to the source 47 of pressurized epoxy. As described above, throughout the entire operation of the dispenser 1 0, the dynamic seal member 52 prevents liquid, such as the heat cure epoxy, from leaking out of the lower interior portion 20 and into the upper interior portion 22. Next, the electromagnetic coil 70 is energized to generate an electromagnetic field passing through the armature 60 and pole 84 such that the valve stem 32 is moved from its initial closed position to an opened position. The epoxy in the lower interior portion 20 will then flow from the outlet 34. After a predetermined amount of time or a predetermined amount of epoxy flow, the electromagnetic coil 70 is de-energized such that return spring 82 will cause valve stem 26 to move from its opened position to the closed position. This movement ejects an epoxy droplet from the outlet 34. The epoxy is supplied from the source 47 to the lower interior portion 20 under relatively low pressure, such as between about 10-1 5 psi. This pressure is substantially lower than that used in other electrically operated dispensers which operate at liquid pressures ranging between 500- 1 500 psi for liquids having viscosities above about 1 0,000 centipoise. Accordingly, when the valve stem is in the open position, the flow of epoxy due to pressure alone is relative slow through the liquid inlet passageway 46 and through the lower interior portion 20. Consequently, the dispensed droplet of epoxy is produced primarily by the return action of the ball 30 exerting pressure onto the epoxy below it. In order words, as the opened valve stem 26 is returned to the closed position by the return spring 82, the ball 30 exerts sufficient pressure onto the epoxy below the ball 30 to push a portion of the epoxy through the outlet 34 and the orifice 38 with sufficient force to produce a well-defined droplet of epoxy for deposit onto the circuit board. While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in considerable detail in order to describe the best mode of practicing the invention, it is not the intention of applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. The invention itself should only be defined by the appended claims, wherein it is claimed:

Claims: 1 . An electrically operated liquid dispenser comprising: a dispenser body having first and second interior portions, said first interior portion communicating with an outlet; a valve seat disposed in said first interior portion, an armature disposed within said second interior portion, a valve stem disposed within said first interior portion and connected for movement with said armature between a closed position in which said valve stem engages said valve seat to prevent liquid flow from said outlet and an opened position in which said valve stem is retracted away from said valve seat to allow liquid flow from said outlet; a coil disposed about said armature, said coil being adapted to selectively generate an electromagnetic field for moving said armature and said valve stem between said closed and opened positions; a liquid inlet passageway in fluid communication with said first interior portion, said inlet passageway adapted to connect to a source of liquid for supplying said liquid to said first interior portion; and a seal member disposed between said first and second interior portions to prevent liquid in said first interior portion from leaking into said second interior portion. 2. The dispenser of claim 1 wherein said valve stem further comprises a shaft extending within said first and second interior portions and said armature is coaxially aligned with said shaft and fills substantially the entire space between said shaft and an inner surface of said second interior portion leaving a narrow radial gap between said inner surface and said armature. 3. The dispenser of claim 2, wherein said narrow radial gap is less than about 0.01 0 inch. 4. The dispenser of claim 1 further comprising: a source of pressurized liquid operatively connected to said liquid inlet passageway for supplying pressurized liquid to said first interior portion at a pressure below about 1 00 psi. 5. The dispenser of claim 4 wherein the liquid from said source of liquid is supplied to said first interior portion at a pressure below about 1 5 psi. 6. An electrically operated liquid dispenser for dispensing viscous liquid, the dispenser comprising: a dispenser body having first and second interior portions, said first interior portion communicating with an outlet; a valve seat disposed in said first interior portion, an armature disposed within said second interior portion, a valve stem disposed within said first interior portion and connected for movement with said armature between a closed position in which said valve stem engages said valve seat to prevent liquid flow from said outlet and an opened position in which said valve stem is retracted away from said valve seat to allow liquid flow from said outlet; a coil disposed about said armature, said coil being adapted to selectively generate an electromagnetic field for moving said armature and said valve stem between said closed and opened positions; a liquid inlet passageway in fluid communication with said first interior portion; a source of pressurized liquid having a viscosity above about 1 ,500 centipoise operatively connected to said liquid inlet passageway and supplying pressurized liquid to said first interior portion at a pressure below about 1 00 psi; and a seal member disposed between said first and second interior portions to prevent liquid in said first interior portion from leaking into said second interior portion. 7. The dispenser of claim 6, wherein said valve stem further comprises a shaft extending through said armature and extending within said first and second interior portions and wherein said armature fills substantially the entire space between said shaft and an inner surface of said second interior portion leaving a radial gap less than about 0.01 0 inch between said inner surface and said armature. 8. The dispenser of claim 6, wherein the liquid from said source of liquid is supplied to said first interior portion at a pressure below about 1 5 psi. 9. The dispenser of claim 6, wherein the pressurized liquid has a viscosity above about 1 0,000 centipoise. 10. A method for dispensing viscous liquid onto a substrate using an electrically operated liquid dispenser having a housing with first and second interior portions, a valve seat in said first interior portion, an armature disposed within said second interior portion, a coil positioned about said armature, and a valve stem mounted within said first interior portion and connected for movement with said armature between closed and opened positions relative to said valve seat, the method comprising: supplying liquid to said first interior portion at a pressure less than about 100 psi; preventing said liquid from entering said second interior portion and contacting said armature by dynamically sealing said valve stem between said first and second interior portions; and energizing said coil to move said valve stem between said opened and closed positions. 1 1 . The method of claim 10 further comprising: supplying the liquid to the first interior portion at a pressure less than about 1 5 psi. 1 2. The method of claim 1 0 further comprising: supplying the liquid to the first interior portion at a viscosity above about 1 ,500 centipoise. 1 3. The method of claim 1 0 further comprising: supplying the liquid to the first interior portion at a viscosity above about 1 0,000 centipoise.

Inventors: SAIDMAN LAURENCE B, SMITH JAMES C, SAIDMAN, LAURENCE, B., SMITH, JAMES, C.
Applicants: NORDSON CORP, NORDSON CORPORATION
Application year: 2000
Main classification: B05C11
IPC codes: B05C5, B05C11, B05D7, F16K31, H05K3