Peristaltic voltage blocks
This invention relates to peristaltic voltage blocks primarily for use in electrostatically aided systems for atomizing and dispensing conductive coating materials. 
     Throughout this application, the term &#34;voltage block&#34; is used to describe both the prior art and the devices of the invention. It is to be understood, however, that these devices function to minimize, to the extent they can, the flow of current. Such current otherwise would flow from a dispensing device maintained at high electrostatic potential through the conductive coating material being dispensed thereby to the grounded source of such coating material, degrading the electrostatic potential on the dispensing device. Attempts to prevent this by isolating the coating material supply from ground result in a fairly highly charged coating material supply several thousand volts from ground. This in turn gives rise to the need for safety equipment, such as high voltage interlocks to keep personnel and grounded objects safe distances away from the ungrounded coating material supply. 
     Various types of voltage blocks are illustrated and described in U.S. Pat. No. 4,878,622, U.S. Ser. No. 07/357,851 and PCT/US89/02473, and in the references cited in those disclosures. Those disclosures are hereby incorporated herein by reference. 
     It is a primary object of the present invention to provide an improved peristaltic voltage block. 
     According to the invention, a coating material dispensing system comprises an electrostatic high potential supply having an output terminal on which the supply maintains a high electrostatic potential, a source of coating material, a dispenser for dispensing the coating material, and a peristaltic device for coupling the dispenser to the source of coating material. The output terminal is coupled to the dispenser to supply potential to the coating material dispensed by the dispenser. The peristaltic device has a length of resilient conduit and means for movably contacting the length of resilient conduit at multiple contact points for substantially dividing the coating material in the peristaltic device into discrete slugs of coating material substantially to interrupt the electrical path through the coating material from the terminal to the coating material supply. According to one aspect of the invention, the peristaltic device includes an inlet end for coupling to the source of coating material. The length of resilient conduit has a first inside transverse sectional area at a first location along its length when it is filled with coating material at the first location, and a second inside transverse sectional area larger than the first at a second location along its length further from the inlet end than the first location when it is filled with coating material at the second location. 
     Illustratively, according to this aspect of the invention, the inside transverse sectional area at the first location is larger than about sixty-five (65) percent of the inside transverse sectional area of the second portion. In an illustrated embodiment of this aspect of the invention, the inside transverse sectional area at the first location is about twenty percent smaller than the inside transverse sectional area of the second portion. 
     According to another aspect of the invention, the peristaltic device comprises a housing having a wall formed from an acrylic resin against which the resilient conduit lies. The movable contacting means compresses the resilient conduit against the wall of the housing substantially to separate the coating material carried thereby into slugs. 
     According to yet another aspect of the invention, the resilient conduit is formed from a first inner layer having a first hardness and a second outer layer having a second hardness relatively less than the first hardness. 
     According to a further aspect of the invention, the means for coupling the dispenser to the source of coating material further comprises a length of compliant conduit coupled between the peristaltic device and the dispenser and a variable flow restrictor for controlling the back pressure in the length of compliant conduit. 
     Illustratively according to the invention, the length of resilient conduit is formed into multiple loops of resilient conduit, the first location being in the or a first loop of the resilient conduit, and the second location being in a subsequent loop of the resilient conduit. 
     Further, illustratively, the wall of the housing is generally right circular cylindrical in configuration and the resilient conduit lies generally within the right circular cylinder formed by the wall. 
    
    
     The invention may be best understood by referring to the following description and accompanying drawings which illustrate the invention. In the drawings: 
     FIG. 1 illustrates a diagrammatic side elevational view of a system including a peristaltic voltage block according to the present invention; 
     FIG. 2 illustrates a top plan view of a peristaltic voltage block constructed according to the present invention; 
     FIG. 3 illustrates a fragmentary sectional view, taken generally along section lines 3--3 of FIG. 2; 
     FIG. 4 illustrates a perspective view of a combination piston and cradle formed to support a contactor according to the embodiment of the invention illustrated in FIGS. 2-3; and, 
     FIGS. 5a-b illustrate fragmentary sectional views along section lines 5a--5a and 5b--5b, respectively of FIG. 2. 
    
    
     In FIG. 1, a dispensing device -0 and some of the related electrical, liquid and pneumatic equipment for its operation are illustrated. Dispensing device 10 is mounted from one end 12 of a support 14, the other end 16 of which can be mounted to permit movement of dispensing device 10 as it dispenses coating material onto an article 18 to be coated, a &#34;target,&#34; passing before it. Support 14 is constructed from an electrical insulator to isolate dispensing device 10 from ground potential. 
     The system further includes a color manifold 20, illustrated fragmentarily. Color manifold 20 includes a plurality of illustratively air operated color valves, six, 21-26 of which are shown. These color valves 21-26 control the introduction of various selected colors of coating material from individual supplies (not shown) into the color manifold 20. A solvent valve 28 is located at the head 30 of color manifold 20. A supply line 32, which is also maintained at ground potential, extends from the lowermost portion of color manifold 20 through a peristaltic voltage block 34, a length of compliant conduit 35 flowing through an air-controlled variable restrictor and 37, a gear flowmeter 39, to a triggering valve 36 mounted adjacent dispensing device 10. A feed tube 38 is attached to the output port of triggering valve 36. A coating material flowing through a selected one of color valves 21-26 flows through manifold 20 into supply line 32, through voltage block 34, compliant conduit 35, variable flow restrictor 37, flowmeter 39, triggering valve 36, feed tube 38 and into the interior of dispensing device 10. Operation of device 10 atomizes this selected color of coating material. 
     For purposes of cleaning certain portions of the interior of device 10 during the color change cycle which typically follows the application of coating material to each target 18 conveyed along a grounded conveyor (not shown) past device 10, a line extends from a pressurized source (not shown) of solvent through a tube 44 and a valve 46 to device 10. Tube 44 feeds solvent into device 10 to remove any remaining amounts of the last color therefrom before dispensing of the next color begins. 
     The coating material dispensed by device 10 moves toward a target 18 moving along the grounded conveyor due, in part, to electric forces on the dispensed particles of the coating material. To impart charge to the particles of coating material and permit advantage to be taken of these forces, an electrostatic high potential supply 48 is coupled to device 10. Supply 48 may be any of a number of known types. Although high potential supply 48 is illustrated as being coupled to device 10 by an electrical conductor, it is to be understood that high electrostatic potential can simply be supplied to the conductive coating material at the outlet end of peristaltic voltage block 34, with the electrostatic potential being supplied to device 10 through the conductive coating material. 
     In the embodiment of the peristaltic voltage block 34 illustrated in FIGS. 2-4, a resilient conduit 220 lies in planar loops 222 around the interior of a right circular cylindrical housing cartridge 224. Cartridge 224 is supported in a framework 226 including caps 228 mounted to a block 230 by cap bolts 232. The flat loops 222 are uniformly spaced axially along cartridge 224 and each loop 222 is substantially perpendicular to the axis of cartridge 224. The transfer of the largely separated slugs of coating material from one loop 222 to the next adjacent loop is achieved by threading the conduit 220 through passageways 236 provided in the sidewall 238 of cartridge 224. The transfer of coating material from each loop 222 to the next adjacent loop 222 as the coating material flows from the inlet end 240 of device 242 to the outlet end 244 thereof takes place outside of the cartridge 224 sidewall 238. 
     The rotor 246 construction illustrated in FIG. 3 is provided to speed solvent flushing of coating material from the device 242. The rollers 250 which actually contact the conduit 220 to separate the coating material in the conduit 220 into discrete slugs are rotatably mounted in elongated rectangular prism-shaped cradles 252. One long side 254 of each cradle 252 is open to receive its respective roller 250. The axles 256 of rollers 250 are rotatably mounted in the opposed short end walls 258 of cradles 252. The rotor 246 is provided with eight equally spaced longitudinally extending slots 264 (only one of which is illustrated) in its outer generally right circular cylindrical sidewall 266. Slots 264 are slightly larger in length and width than cradles 252. This permits the cradles 252 to be mounted in respective slots 264 for relatively free sliding movement radially of the axle 260 of rotor 246. Each slot 264 defines a pocket within which a respective cradle 252 is reciprocable radially of axle 260 of rotor 246. A chamber 253 is defined between the respective cradle 252 and the radially inner end, or head, 265 of its respective slot 264. An air bag 267 is provided in each slot 264. A port 273 is provided in the head 265 of each slot 264. Each port 273 communicates with a respective air bag 267. Compressed air is provided from a rotary air coupler 274 (FIG. 2) at the ground potential, or driven, end 276 of device 242. Each cradle 252 is held in the radially outer end 278 of its respective slot 264 by a cap 280 having an arcuately shaped outer surface 282 generally conforming to the contour of rotor 246. A plurality of, for example, electrically non-conductive plastic screws hold each cap 280 onto rotor 246 at the radially outer end of a respective slot 264. Each roller 250 protrudes through a longitudinally extending slot 284 in a respective cap 280. A strip 286 of compliant material having a somewhat hourglass-shaped section transverse to its longitudinal extent extends along each long edge of the outer end 288 of each cradle 252 between the outer end 288 of its respective cradle 252 and its respective cap 280. The compliant material of strip 286 illustratively is a thermosetting rubber, such as compound 215 or compound 253 available from Randolph Austin Company, Post Office Box 988, Manchaca, Tex. 78652. This material provides variable restraining force necessary to promote sufficient occlusion of the conduit 220, even when conduit 220 is somewhat worn, to block voltage. 
     The surface of each roller 250 is circumferentially scalloped at multiple locations along its length, one scallop for each loop 222. The scallops are shallow, being only five-one thousandths of an inch (0.005&#34;-0.127 mm) and help to maintain the spacing of the loops 222 within cartridge 224 during operation of the voltage block 34. 
     The loop 222 nearest the inlet end of the cartridge 224 has an inside diameter up to twenty percent (20%) smaller than the inside diameters of the remaining loops 222. Illustratively, the inside diameter of the conduit in the first loop is ten percent (10%) smaller than the inside diameter of the conduit forming the remaining loops. This configuration results in a marked improvement in the voltage blocking capacity of the cartridge 224. It is believed that the conduit 220 between the rollers 250 of the voltage block 34 is typically expanded by fluid pressure, and that a small amount of fluid therefore tends to leak or &#34;slip&#34; past the points of contact of the rollers 250 with the conduit 220, reducing the voltage blocking capacity of the cartridge. The smaller inside diameter first loop causes a slight vacuum to be induced in the subsequent, larger inside diameter loops reducing the fluid slip at the points of contact of the rollers 250 with the larger inside diameter loops, thereby improving the voltage blocking capacity at each of these points of contact. The first loop 222 could also be constructed with an inside diameter gradient between its inlet, or ground potential, end and its end adjacent the second loop 222 by extruding the first loop on a mandrel having the desired diameter gradient. 
     In addition, the use of &#34;lay-flat&#34; conduit for the loops 222 of the peristaltic voltage block 34 has previously been discussed. It should be appreciated that the cross sectional areas of such conduit at all points along its length when it is empty will be essentially zero. Therefore, when such lay-flat conduit is employed, cross sectional area gradients between various locations along its length must be measured when it is full of coating material at those locations. 
     The cartridge 224 itself is constructed from acrylic material rather than the previously employed nylon material. It is believed that, even with the same microfinish, acrylic material permits the conduit 220 in loops 222 to slip back and forth without as much elongation, adding to the life of the conduit 220. It is believed that this greater slip is permitted by the lower coefficient of friction of the acrylic material. 
     The conduit 220 which is loaded into the cartridge 224 is a coextruded conduit rather than the prior art single extrusion. The coextruded material has an approximately five mil thick inner wall of 87A Shore hardness, with the remaining wall material being 70A Shore hardness. The material used in the prior art single extrusion tubing was polyurethane. The material used in the coextruded tubing of the invention is Monsanto Santoprene™ thermoplastic elastomer or its equivalent. 
     When it is desired to employ the voltage blocking capacity of device 242, such as when an electrically highly conductive coating material is being supplied therethrough to a coating material atomizing and dispensing device maintained at high-magnitude electrostatic potential, compressed air is supplied through coupler 274 and ports 273 to air bags 267, forcing the rollers 250 outward and occluding conduit 220 between adjacent slugs of the conductive coating material. Rotor 246 divides the coating material substantially into electrically isolated slugs which move along conduit 220 peristaltically from inlet end 240 to outlet end 244 while maintaining a potential difference across ends 240, 244 substantially equal to the potential difference across the output terminals of the high-magnitude electrostatic potential supply. Compressed air is supplied to variable restrictor 37 (FIG. 1) to smooth out the pulsating effect of the passage of the slugs through compliant conduit 35. 
     When it is desired not to employ the voltage blocking capacity of device 242, such as when dispensing of an electrically conductive coating material is complete and the high-magnitude potential supply has been disconnected from the dispensing device in preparation for solvent flushing prior to a subsequent dispensing cycle with a different coating material, the compressed air source is disconnected from variable restrictor 37 and coupler 274 and the variable restrictor and coupler are vented to atmosphere. The resiliency of conduit 220 and the pressure of the solvent in conduit 220 are aided by strips 286 acting between caps 280 and cradles 252 to urge cradles 252 and their respective rollers 250 radially inwardly, permitting the free, rapid flow of solvent through conduit 220 to flush any remaining traces of the pre-change coating material from it. Compressed air can then be passed through conduit 220 to dry it in preparation for the next dispensing cycle.