GB749008A - Improvements in or relating to spray coating of articles and apparatus for use in connection therewith - Google Patents

Abstract

<PICT:0749008/III/1> <PICT:0749008/III/2> <PICT:0749008/III/3> <PICT:0749008/III/4> <PICT:0749008/III/5> A method of electrostatically coating an article comprises establishing in a quiescent atmosphere a particle-depositing electrostatic field over at least a portion of the surface of the article, introducing a spray of finely-divided particles of liquid coating material into the field from a source spaced from the article and electrically charging the particles to move them along the lines of force of the field, for deposition, while still in the liquid state, on the article, and electrostatically modifying the arrangement of the lines of force of the field by a secondary electrostatic field, maintained adjacent the depositing field, for rearranging the pattern of deposition of the liquid particles on the article. The article may be moved relatively to the spray source. In the form shown in Fig. 1, a tube 12, preferably of electrically-conductive material, has a bevelled tip 14 and is threaded exteriorly at 17 to support adjustably a disc 27 of insulating material. Coating material is fed into the bore of the tube 12 through a pipe 20 and valve 19, and is atomized at the tip 14 by a electro <PICT:0749008/III/6> <PICT:0749008/III/7> <PICT:0749008/III/8> static field produced by connecting the tube 12 to the live terminal of a highvoltage source 21, the other terminal, and the articles 13 to be coated, being earthed. The member 27 carries a metal ring 29 at its outer edge; this functions as a control member for the electric field, and may be connected through a variable resistor 33 to the tube 12. The members 27, 29 may be replaced by a single member of conductive material, having a depending, rounded edge. The apparatus of Figs. 3 and 4 comprises an elongated chamber 340 of triangular section to which coating material is fed through a tube 312 connected to a high-voltage source. At its lower end there is an aperture 343, one of the walls having a lower projecting edge 316 from which the coating material issuing through the aperture 343 is discharged. The control electrode consists of L-shaped wires, 329, the downwardly directed portions 330 of which terminate in a plane slightly above the edge 316. In Fig. 5, sheet material 410 is carried on an earthed conveyer 413 beneath an atomizing device 415 connected to a high-voltage source 416, and comprising supports 420 carrying an elongated member 421, preferably of electrically-conductive material, which has a sharp lower edge 422. Coating material is fed to the edge 422 from a nozzle 429 on a tube 427, connected by a flexible hose 428 to a source of coating material; the tube 427 carried by a lever 425 which reciprocates in a slot 426. Field-modifying members 431 are provided at one or both ends of the member 421, and extend below its edge 422. The hand gun 510 of Fig. 6 comprises an insulating housing 511 containing an electrostatic atomizing unit 512 consisting of plates 513 and 515, tapered to sharp edges and spaced by shims. Into the elongated feed orifice 517 so produced coating material is fed from a tube 518 through passages 525, 521 and 522. The unit 512 is connected to the live terminal of a powerpack 526, the other terminal of which is earthed. The housing 510 carries a flared shield member 530 of insulating material, having an opening 531 of substantially rectangular cross-section, and an earthed outer metallic coating 532. The gun of Fig. 8 comprises an integral insulating housing 540, handle 541 and shield 542, the housing containing a rotatable bell type atomizing unit 544. This comprises a bell-shaped head 545 with a central feed opening 546, and mounted on a hollow shaft 547 which is journalled in bearings 548 and driven by an hydraulic motor unit 549, fluid being supplied to and carried away from the motor by tubes 555 and 5551. Coating material is supplied to the head 545 from a pump through a tube 533, of insulating material, passage 551 and chamber 552. The motor unit 549 is connected by an insulated wire 558 to the live terminal of a power pack 559, the other terminal of which is earthed, as is a metallic coating 570 on the outside of the handle, housing and shield unit. In the arrangement of Fig. 9, flat metal panels 610 are carried by a conveyer 611 through a coating zone an atomizing device 612 being provided at a fixed distance therefrom. The device 612 comprises a bell-shaped head 613 which is rotated through shafts and gearing in the support 621, and to the centre of which liquid coating material is supplied through a hose 622. The head 613 is connected to a source of high electrical potential 619, and a sprayshaping electrode 614, consisting of a square metal plate to which are attached fins 617, mounted on the atomizing apparatus by an insulating ring 615, is also connected thereto through a potential divider 620. The electrode 614 may be replaced by one of triangular plate form. In the device of Fig. 17, the field shaping electrode is placed within the atomizing edge of a rotatable head 683, and comprises triangular metal plates 680a, 680b, 680c and 680d mounted on heavy wires 681a, b, c and d, which are connected to a disc 682, which is mounted on a rod running through the head 683. The electrodes are maintained at a high electrical potential at or near that of the head 683. Specification 591,474 and U.S.A. Specificatio 2,559,225 are referred to.