Patent Publication Number: US-6334553-B1

Title: Anti-float plunger for pneumatically actuated syringe

Description:
FIELD OF THE INVENTION 
     This invention relates to pneumatically actuated syringes, and more particularly to the plungers used in such syringes. 
     BACKGROUND OF THE INVENTION 
     Pneumatically actuated syringes are used in dispensing controlled amounts of encapsulants, adhesives, including epoxies or other materials onto circuit boards or integrated circuits during the manufacture thereof, as shown and described in U.S. Pat. No. 5,927,560. The liquid filled syringe supplies, or feeds, liquid to a dispensing pump. 
     In a conventional set up, the dispensing material comes in standard size syringes. The syringe operatively connects to a valve and pump arrangement to dispense the material onto a substrate in precise amounts. The liquid is forced from the syringe to the pump via a plunger. More specifically, fluid pressure applied to the backside of a piston, or plunger, residing within the syringe forces the plunger toward a liquid outlet located on the frontside of the plunger. Movement of the plunger toward the outlet forces liquid out of the outlet, to the dispensing pump. 
     In the manufacture of circuit boards or integrated circuits, the syringe plunger must drive the liquid material from the syringe outlet in a consistent and precise manner, to help assure that the dispensed amounts, typically called “shots,” will be uniform in size and weight throughout the dispensing procedure. That is, to some extent the precision of the syringe plunger is necessary to achieve a high degree of repeatability with respect to the ultimate dispensing of shots of uniform size and weight. 
     These liquid dispensing materials are quite expensive, typically costing more than $150.00 per ounce. To assure minimum waste of these expensive liquid dispensing materials, and to avoid running out of dispensing liquid during a dispensing procedure, it would be helpful to know with certainty the fluid level in the syringe. One way this could be done is by monitoring the position of the plunger within the syringe. However, this is typically not done in practice. 
     One reason this is not usually done is because in some instances the plunger position gives an inaccurate indication of fluid level. This may occur as a result of air becoming entrapped between the plunger and the liquid in the syringe. In this condition the plunger is said to “float” above the liquid. In this floating condition, the position of the plunger does not provide an accurate indication of the amount of liquid left in the syringe. Thus, an operator must guess as to how much liquid remains in the syringe in order to avoid running out. Moreover, with this uncertainty, the operator cannot effectively run the apparatus until the liquid level is known to be empty. As a result, it is inevitable that at least some syringes which still have a usable amount of liquid will be discarded prematurely, representing a waste of this relatively expensive liquid. 
     Another adverse effect of entrapped air within the syringe is its negative impact on dispensing quality. Air pressure cycling causes volume fluctuations in any air that is entrapped in the syringe. Unlike dispensing fluids, which are incompressible liquids, air contracts under high pressure and expands when pressure is reduced. In some cases, particularly where a continuous fluid path exists between the syringe and the dispensing pump outlet, these volume fluctuations may result in inadvertent drooling, or dripping, of liquid from the dispensing pump outlet. This inadvertent drooling, or dripping, results in wasted liquid and/or liquid being deposited onto undesired areas of the substrate. Furthermore, these volume fluctuations may adversely effect repeatability with respect to dispensing shots of uniform size and weight. 
     One way air becomes entrapped within the syringe, thereby to “float” the plunger, results from the outward bulging of the syringe walls when air pressure is applied. For instance, in a pressurized condition, such as 80 psig for the backside air pressure, the walls of the syringe tend to bulge outwardly. Because of this bulging, a circumferential air gap forms between the plunger and the inside surface of the syringe. Typically, these components are made of hard plastic, such as polyethylene, and so there is not an airtight seal therebetween. 
     Thereafter, when the backside air pressure is reduced to zero psig, the syringe walls contract inwardly. Because there is no circumferential airtight seal at the front end of the plunger, where the plunger contacts the liquid, at least some of the air that has been introduced from this cylindrical air gap moves in front of the plunger, between the plunger and the dispensing fluid. Thus, the syringe wall contraction entraps the air between the plunger and the dispensing liquid, thereby causing the plunger to rise, or float. In some instances where air pressure is frequently cycled, the floating distance may accumulate to be as much as ¼″ or even ½″, for a syringe of 1.6″ diameter. 
     It is therefore an object of the present invention to eliminate the occurrence of a “floating” plunger condition in pneumatically actuated syringes. 
     It is another object of the present invention to reduce the possibility of air entrapment between the plunger and the liquid in a pneumatically actuated syringe, thereby to promote repeatability in liquid dispensing and to avoid drool. 
     It is still another object of the present invention to reduce the amount of liquid which is wasted during the use of liquid filled syringes, by enhancing the accuracy of liquid level sensing within the syringe. 
     SUMMARY OF THE INVENTION 
     The present invention achieves the above-stated objects via an anti-float syringe plunger having an outer flexible wiper which maintains an airtight seal with the inside surface of the syringe and a forward deformable membrane which deforms to alleviate residual pressure in the syringe when the walls return from their bulged condition. 
     The flexible wiper maintains a circumferential airtight seal with the inside surface of the syringe. So even when the syringe walls bulge under pressure, the wiper prevents formation of an air gap between the plunger and the inside walls of the syringe. Since an air gap does not form, air should not become entrapped between the piston and the liquid when the syringe walls return from the bulged condition. Thus, the invention eliminates the occurrence of entrapped air which would otherwise cause the plunger to float. Stated another way, the invention eliminates the floating plunger condition. 
     One result of this airtight seal is an increase in the residual pressure which results from the contraction of the syringe walls when air pressure is released at the backside, In conventional syringe plungers there is no excessive residual pressure buildup due to syringe wall contraction because there is such a loose seal. In contrast, with the plunger/syringe of this invention the residual pressure buildup could otherwise cause liquid drool, or dripping, at the syringe outlet, particularly when used for direct dispensing. However, because of the deformable membrane at the forward end of the plunger, contraction of the syringe walls causes neither inadvertent drooling nor undesired upward movement of the plunger when the syringe walls contract. Rather, the residual pressure which results from contraction of the syringe walls will be alleviated, or absorbed, by deformation of the membrane. 
     Also, because the plunger of this invention does not float, due to the flexible seal and the deformable membrane, the sensing of plunger position gives an accurate indication of the liquid level. This leads to reduced waste in dispensing liquid. 
     According to a preferred embodiment of the invention, the plunger includes a rigid cylindrically shaped ring, preferably of plastic, with an outer cover, preferably of silicon rubber, extending thereover. The inner ring may have an inner shelf to support a magnet for sensing purposes. The cover is preferably made in a unitary manner by molding an elastomeric material into a cylindrical shape which is closed at one end. The inner ring is inserted within the cover. The cover includes a radially outwardly extending flexible wiper traversing its entire circumference. Preferably, the cover includes two spaced flexible wipers, a first located at the forward end and a second located at the rear end of the plunger. The second wiper, however, does not extend completely around the cover and does not form a circumferential airtight seal with the syringe walls. Primarily, the second wiper helps to maintain the plunger centered within the syringe. 
     The closed end of the cover includes the deformable membrane, which is arcuately shaped in cross section. The cover remains in direct contact with the liquid in the syringe regardless of the backside pressure, due to its flexibility and its shape. The present invention may be used advantageously in dispensing various types of liquids, including but not limited to adhesives, sealants, encapsulants, coatings, epoxy, thermal grease, etc. 
     These and other features of the invention will be more readily understood in view of the following detailed description and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an anti-float plunger for a pneumatically actuated syringe, in accordance with a first preferred embodiment of the invention. 
     FIG. 2 is a perspective view, similar to FIG. 1, but with a portion of the plunger shown in cut-away view. 
     FIGS. 3A,  3 B and  3 C are longitudinal cross-sectional schematic views which illustrate, in sequence, advantageous aspects of the anti-float plunger of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an anti-float plunger  10  for a pneumatically actuated syringe in accordance with a first preferred embodiment of the invention. More specifically, FIG. 1 shows an inner ring  12  residing within a cover  14 . Preferably, the inner ring  12  is made out of a rigid plastic, such as nylon, whereas the cover  14  is preferably made out of an elastomeric material such as silicon rubber, by molding. The cover  14  has a cylindrical shape with one end closed and one end open. Both ends of ring  12  are open. 
     To form the plunger  10 , the cylindrical ring  12  is inserted within the open end of the cover  14 . The cover  14  includes a deformable, arcuately shaped membrane  16  located at a forward end of the plunger  10 , which resides in contact with the liquid to be dispensed. The cover  14  also includes a flexible wiper  18  which extends around the entire circumference. The wiper  18  is relatively thin and flexible. A second flexible wiper  20  extends partially but not completely around the cover  14  at the open end thereof. The second wiper  20  helps to center the plunger  10 . The second wiper  20  has two discontinuities, or notches  21 . 
     FIG. 2 shows the anti-float plunger  10  in partial cut away view, to expose a center shelf  22  formed as part of the ring  12 . This shelf  22  may be used to support a magnet (not shown), for magnetic position sensing of the plunger  10 , specifically the position of the shelf  22 . Alternatively, to achieve the same purpose, i.e. magnetic position sensing, the entire ring  12  may be made of magnetic material, such as a metal, or of a plastic embedded with a magnetic material. With that construction, the upper or lower edge would be sensed, and preferably made with a slightly increased thickness. 
     FIGS. 3A,  3 B and  3 C illustrate the advantageous features of the anti-float plunger  10  of this invention, with respect to a liquid filled syringe  24 . The syringe  24  includes an outlet  26 , which in these Figures is shown at the bottom. Also, in these Figures the outlet  26  is closed off with a cap  28 . However, it is to be understood that this cap  28  is not part of the dispensing system and is removed prior to attaching or connecting the syringe  24  to a dispensing pump, or system. The syringe  24  may be used for example in the type shown in U.S. Pat. No. 5,927,560, which is expressly incorporated by reference herein, in its entirety, as well as other systems. 
     With reference to FIGS. 3A,  3 B and  3 C, and relative to the plunger  10  residing within the syringe  24 , there is a fluid filled frontside  30  and a backside  32 . Liquid flow through outlet  26  normally occurs via application of pressurized air to the backside  32  of the plunger  10 , thereby moving the plunger  10  toward the outlet  26  and forcing liquid from the frontside  30  through the outlet  26 . 
     FIG. 3A shows an initial condition wherein the air pressure at the backside  32  of the plunger  10  is zero. The wiper  18  is slightly compressed, to maintain a circumferential airtight seal with the syringe  24 . The frontside  30  has no entrapped air, and the membrane  16  is in an outermost position. 
     In FIG. 3B, the backside  32  of the plunger  10  is pressurized, as for example with an air pressure of 80 psig. This pressurization of the backside  32  causes the walls of the syringe  24  to bulge outwardly. This outward bulging lowers the fluid level. The plunger  10  follows the fluid level by moving downwardly, a distance shown by directional arrows  36 . Because the flexible wiper  18  extends circumferentially around the cover  14 , the plunger  10  maintains an airtight seal with the walls of the syringe  24 , even with high pressure air applied to the backside  32 . For a syringe  24  with an inside diameter of 1.6″, the uncompressed diameter of the wipers  18  and  20  is preferably about 1.7″. 
     FIG. 3C shows a depressurized condition for the syringe  24 , wherein the air pressure on the backside  32  has been returned to zero psig. The release of the air pressure on the backside  32  causes the walls of the syringe  24  to contract. This raises the fluid level within the frontside  30 . As a result, the membrane  16  deforms in an upward direction away from outlet  26 , to accommodate the raising of the fluid level. Thus, the membrane  16  deforms to alleviate residual pressure within the frontside  30  of the syringe  24  upon the release of backside  32  air pressure. When air is again supplied to backside  32 , to deliver liquid from the outlet  26 , the membrane  16  will return to its outermost position. 
     With the present invention, the flexible wiper  18  maintains an airtight seal with the inside surface of the walls of the syringe  24 . Thus, air from the backside  32  is not able to move between the plunger  10  and the syringe  24 , nor is it able to become entrapped between plunger  10  and the liquid on the frontside  30  of the syringe  24 . The notches  21  in second wiper  20  allow the air pressure in the volume residing between cover  14  and syringe  24  to be equal to the air pressure at the backside  32 , at all times. 
     The present invention eliminates inaccuracies in liquid dispensing which could otherwise result from air entrapment on the frontside  30  of the syringe  24 . Because the invention eliminates air entrapment between the plunger  10  and the liquid in the frontside  30  of the syringe  24 , the present invention makes liquid level sensing more accurate. The sensed position of the plunger  10  more accurately provides an indication of the volume remaining within the frontside  30  of the cylinder of the syringe  24 . With increased certainty in sensing the level of the liquid remaining in the syringe  24 , an operator can use up the entire contents of the syringe without running the risk of going dry. This also avoids unnecessary or premature changing of syringes  24  which have not been completely emptied. 
     These advantageous features can be achieved with this inventive anti-float plunger when it is used with a syringe to supply liquid to a dispensing pump, such as, for example those of U.S. Pat. No. 5,927,560, an Asymtek® DP3000 dispensing system manufactured by Asymptotic Technologies, Inc. (Asymtek), or other syringe based system. The advantageous features are also achieved when the invention is used with any other dispensing set up, such as direct deposit of liquid from the syringe to a substrate. 
     While the present application describes a preferred embodiment of the invention, it is to be understood that variations may be made thereto without departing from the scope of the invention. For instance, those skilled in the art will readily appreciate that the particular structural details shown and described could be varied. For example, the flexible membrane which acts as an accumulator may be substituted with a bladder or a bellows which will change in size in order to accommodate the pressure differences. Alternatively, a slidable element carried in the plunger, like a movable pin or piston could be employed. Therefore, the inventors do not intend this detailed description to be limiting relative to interpretation of the following claims, but rather exemplary of the presently preferred embodiment.