Nozzle assembly and system for applying powder to a workpiece

• US 5,520,735 A
• Filed: 09/20/1994
• Issued: 05/28/1996
• Est. Priority Date: 06/30/1992
• Status: Expired due to Fees

• Current Assignee: Nordson Corporation
• Original Assignee: Nordson Corporation
• Inventors: Mulder, Douglas C.
• Primary Examiner(s): ENGEL, JAMES J
• Application Number:
  US08/309,427
• Time in Patent Office: 616 Days
• Field of Search: 118/308, 118/319, 118/326, 118/620, 118/622, 118/624, 239/3, 239/290, 239/291, 239/690, 239/693, 239/697
• US Class Current: 118/308
• CPC Class Codes: B05B 1/06 in annular, tubular or hollow conical form
• CPC Class Codes: B05B 1/265 the liquid or other fluent material being symmetrically deflected about the axis of the nozzle
• CPC Class Codes: B05B 12/004 comprising sensors for monitoring the delivery, e.g. by displaying the sensed value or generating an alarm B05B12/08 takes precedence; registering or indicating the condition or the working of machines or other apparatus in general G07C3/00
• CPC Class Codes: B05B 12/06 for effecting pulsating flow nozzles, spray head or outlet with means for generating a discharge of pulsating nature B05B1/08
• CPC Class Codes: B05B 13/0242 the objects being individually presented to the spray heads by a rotating element, e.g. turntable
• CPC Class Codes: B05B 13/0609 the hollow bodies being automatically fed to, or removed from, the machine
• CPC Class Codes: B05B 14/10 the excess material being particulate for spray booths B05B14/48
• CPC Class Codes: B05B 14/30 comprising enclosures close to, or in contact with, the object to be sprayed and surrounding or confining the discharged spray or jet but not the object to be sprayed
• CPC Class Codes: B05B 15/55 using cleaning fluids
• CPC Class Codes: B05B 5/032 for spraying particulate materials
• CPC Class Codes: B05B 5/047 using tribo-charging
• CPC Class Codes: B05B 7/1454 comprising means for supplying collected oversprayed particulate material spray booth with arrangements for collecting oversprayed material B05B14/40
• CPC Class Codes: B05B 7/1486 for spraying particulate material in dry state
• CPC Class Codes: Y02P 70/10 Greenhouse gas [GHG] capture, material saving, heat recovery or other energy efficient measures, e.g. motor control, characterised by manufacturing processes, e.g. for rolling metal or metal working

Abstract

An apparatus is operable to sequentially spray powder onto each workpiece of a plurality of workpieces. The powder flows from an improved nozzle assembly as a series of pulses. Accumulation of powder on surfaces in the nozzle assembly is retarded by conducting a flow of fluid (air) from a source of fluid pressure through openings in the surfaces in the nozzle assembly. Thus, a plurality of openings are formed in a side wall of the nozzle assembly to enable fluid to flow through the openings in the side wall into the flow path of air and powder through the nozzle assembly to retard accumulation of powder on an inner side surface of the side wall. An inner deflector has an end surface which faces toward the workpiece and on which powder tends to accumulate. A flow of fluid is conducted through openings in the end surface of the inner deflector to retard the accumulation of powder on the end surface of the inner deflector.

Claims

1. A nozzle assembly for use in directing a flow of air and powder toward a workpiece, said nozzle assembly comprising a side wall which at least partially defines a flow path of air and powder from an inlet to said nozzle assembly to an outlet from said nozzle assembly, said side wall having an inner side surface along which the flow of air and powder is conducted and on which powder tends to accumulate, a plurality of openings formed in said side wall at locations between the inlet to said nozzle assembly and the outlet from said nozzle assembly, and a conduit connected in fluid communication with a source of fluid pressure and with the openings in said side wall to conduct fluid which flows from the source of fluid pressure through the openings in said side wall into the flow path of air and powder through said nozzle assembly to retard accumulation of powder on the inner side surface of said side wall, said plurality of openings in said side wall include a plurality of slots which extend along said side wall from a location adjacent to an inlet portion of said nozzle assembly to a location adjacent to an outlet portion of said nozzle assembly.

2. A nozzle assembly as set forth in claim 1 wherein said inner side surface of said side wall has a circular cross-sectional configuration and flares radially and axially outward in a direction away from the inlet to said nozzle assembly, each slot of said plurality of slots having longitudinally extending side surfaces which are skewed relative to radii of the inner side surface of the side wall which extend through the slot, said side surfaces of said slots being effective to direct fluid flow from the source of fluid pressure into the flow path of air and powder through said nozzle assembly in a direction which is generally tangential to the inner side surface of said side wall to obtain a swirling fluid flow along the inner side surface of said side wall.

3. A nozzle assembly as set forth in claim 1 wherein each of said slots has a linear longitudinal central axis.

4. A nozzle assembly as set forth in claim 1 wherein each of said slots has a curving longitudinal central axis.

5. A nozzle assembly as set forth in claim 1 further including a manifold wall extending around said side wall and cooperating with said side wall to at least partially define a manifold chamber which is connected in fluid communication with said slots in said side wall and with said conduit.

6. A nozzle assembly as set forth in claim 5 further including an excess powder collector housing at least partially enclosing said manifold wall and cooperating with said manifold wall to at least partially define an excess powder collector chamber, said excess powder collector housing at least partially defining an excess powder collector chamber opening disposed adjacent to and extending around the outlet from said nozzle assembly, a second conduit connected in fluid communication with a source of reduced fluid pressure and with said excess powder collector chamber to induce a flow of air and excess powder into said excess powder collector chamber through the excess powder collector chamber opening.

7. A nozzle assembly as set forth in claim 1 further including control means for establishing a flow of air and powder along the flow path through said nozzle assembly toward the workpiece and for interrupting the flow of air and powder along the flow path through said nozzle assembly, said control means being operable to establish a flow of fluid from the source of fluid pressure through said conduit and the slots in said side wall into the flow path of air and powder through said nozzle assembly when the flow of air and powder through said nozzle assembly has been interrupted.

8. A nozzle assembly as set forth in claim 1 further including a deflector which is at least partially enclosed by said side wall and which cooperates with said side wall to at least partially define the flow path of air and powder from the inlet to said nozzle assembly to the outlet from said nozzle assembly, said deflector having an end surface which faces toward the workpiece, and a second conduit for conducting a flow of fluid to said end surface on said deflector to retard accumulation of powder on said end surface of said deflector.

9. A nozzle assembly as set forth in claim 8 wherein said deflector includes a body portion and a porous screen which cooperate to at least partially define a chamber in said deflector, said porous screen having an outer side surface area which at least partially defines said end surface of said deflector, said second conduit being connected in fluid communication with said chamber in said deflector to enable fluid to flow into said chamber in said deflector and from said chamber in said deflector through said porous screen.

10. A nozzle assembly for use in directing a flow of air and powder toward a workpiece, said nozzle assembly comprising a side wall which at least partially defines a flow path of air and powder from an inlet to said nozzle assembly to an outlet from said nozzle assembly, said side wall having an inner side surface along which the flow of air and powder is conducted and on which powder tends to accumulate, a plurality of openings formed in said side wall at locations between the inlet to said nozzle assembly and the outlet from said nozzle assembly, a deflector which is at least partially enclosed by said side wall and which cooperates with said side wall to at least partially define the flow path of air and powder from the inlet to said nozzle assembly to the outlet from said nozzle assembly, said deflector having an end surface which faces toward the workpiece, a first conduit connected in fluid communication with a source of fluid pressure and with the openings in said side wall to conduct fluid which flows from the source of fluid pressure through the openings in said side wall into the flow path of air and powder through said nozzle assembly to retard accumulation of powder on the inner side surface of said side wall, and a second conduit for conducting a flow of fluid to said end surface on said deflector to retard accumulation of powder on said end surface of said deflector.

11. A nozzle assembly as set forth in claim 10 wherein said deflector includes a body portion and a porous screen which cooperate to at least partially define a chamber in said deflector, said porous screen having an outer side surface area which at least partially defines said end surface of said deflector, said second conduit being connected in fluid communication with said chamber in said deflector to enable fluid to flow into said chamber in said deflector and from said chamber in said deflector through said porous screen.

12. A nozzle assembly as set forth in claim 10 further including an excess powder collector at least partially defining an excess powder collector chamber having an opening disposed adjacent to and extending around said end surface on said deflector, a third conduit connected in fluid communication with a source of reduced fluid pressure and with said excess powder collector chamber to induce a flow of air and powder away from said end surface on said deflector and into said excess powder collector chamber through the excess powder collector chamber opening.

13. A nozzle assembly as set forth in claim 10 further including control means for establishing a flow of air and powder along the flow path through said nozzle assembly toward the workpiece and for interrupting the flow of air and powder along the flow path through said nozzle assembly, said control means being operable to establish a flow of fluid from the source of fluid pressure through said second conduit to said end surface on said deflector when the flow of air and powder through said nozzle assembly has been interrupted.

14. A nozzle assembly for use in directing a flow of air and powder toward a workpiece, said nozzle assembly comprising a side wall having an inner side surface, a deflector which is at least partially enclosed by said side wall and which cooperates with the inner side surface of said side wall to at least partially define a flow path along which a flow of air and powder is conducted, said deflector having an end surface which faces toward the workpiece and on which powder tends to accumulate, and a conduit connected in fluid communication with a source of fluid pressure and said end surface of said deflector to conduct fluid which flows from the source of fluid pressure to said end surface of said deflector to retard accumulation of powder on said end surface of said deflector.

15. A nozzle assembly as set forth in claim 14 wherein said end surface of said deflector is disposed on a porous screen through which a flow of fluid from said conduit is conducted to retard accumulation of powder on said end surface of said deflector.

16. A nozzle assembly as set forth in claim 14 wherein said deflector includes a chamber which is disposed within said deflector and which is connected in fluid communication with said conduit, said deflector having a plurality of passages through which fluid flows from said chamber in said deflector to said end surface of said deflector.

17. A nozzle assembly as set forth in claim 14 wherein said deflector includes a body portion connected with said side wall by a plurality of supports which extend between said side wall and said body portion of said deflector, said conduit including a first passage section which extends through one of said supports and a second passage section which is at least partially disposed within said body portion of said deflector, said second passage section extending from said first passage section to a chamber disposed adjacent to said end surface of said deflector, and a plurality of passages extending from said chamber to said end surface of said deflector.

18. A nozzle assembly as set forth in claim 14 further including a plurality of openings formed in said side wall and a second conduit connected in fluid communication with a source of fluid pressure and with the openings in said side wall to conduct fluid which flows from the openings in said side wall into space between said side wall and said body portion of said deflector.

19. An apparatus for use in sequentially applying powder to each workpiece of a plurality of workpieces, said apparatus comprising a spray device which sprays powder on each of said workpieces in turn while said workpiece is at a work station, said spray device including components which produce pulses of air with powder entrained therein, a body section connected with said components which produce pulses of air and in which a flow of pulses of air with powder entrained therein is conducted, charging elements which are connected with said body section and electrostatically charges powder entrained in the flow of pulses of air in said body section, a nozzle connected with said body section to spray powder onto each of said workpieces in turn at said work station, said nozzle being effective to spray powder in one of said pulses of air with powder entrained therein onto one of said workpieces and to spray powder in a pulse of air with powder entrained therein next succeeding said one pulse of air with powder entrained therein onto a workpiece next succeeding said one workpiece, said nozzle having an inner side surface along which the pulses of air with powder entrained therein are conducted and on which powder tends to accumulate, a plurality of openings formed in said inner side surface of said nozzle, a conduit connected in fluid communication with a source of fluid pressure and with the openings in said inner side surface of said nozzle to conduct fluid which flows through openings in said inner side surface of said nozzle into a flow path of pulses of air with powder entrained therein through said nozzle, and a diverter located upstream of said nozzle and connected with said body section to divert away from said nozzle a portion of at least some of the pulses of air with powder entrained therein and to induce a flow of fluid with powder suspended therein from said nozzle into said diverter between pulses of air with powder entrained therein.

20. An apparatus for use in sequentially applying an annular band of powder to a surface of each can lid of a plurality of can lids, said apparatus comprising a conveyor which is operable to move each of said can lids in turn to and from a work station, and a spray device disposed adjacent to said conveyor, said spray device being operable to spray an annular band of powder on one surface of each of said can lids in turn while each of said can lids is held at said work station by said conveyor in a spaced apart relationship with said spray device, said spray device including a body section in which a flow of air with powder entrained therein is conducted, a charging apparatus connected with said body section to electrostatically charge powder entrained in said flow of air in said body section, a nozzle connected with said body section and spaced apart from said one surface of each of said can lids during movement of each of said can lids in turn to and from said work station and during spraying of an annular band of powder onto each of said can lids at said work station, said nozzle including an inner deflector element which has an outer side surface which flares outwardly in the direction of flow of the stream of air with electrostatically charged powder entrained therein and which is engaged by the flow of the stream of air with electrostatically charged powder entrained therein, said nozzle including a side wall having an inner side surface which flares outwardly in the direction of flow of the stream of air with electrostatically charged powder entrained therein and which circumscribes at least a part of said outer side surface of said inner deflector element and is engaged by the flow of the stream of air with electrostatically charged powder entrained therein, said outer side surface of said inner deflector element and said inner side surface of said side wall cooperating to define a flow path which has an annular cross sectional configuration and which flares outwardly in the direction of flow of the stream of air with electrostatically charged powder entrained therein, said inner deflector element having an end surface which faces toward the work station and on which powder tends to accumulate, a conduit connected in fluid communication with a source of fluid pressure and said end surface of said inner deflector element to conduct fluid which flows from the source of fluid pressure to said end surface of said inner deflector element to retard accumulation of powder on said end surface of said inner deflector element, and an excess powder collector at least partially enclosing said nozzle to draw a flow of excess powder away from said one surface of a can lid held at said work station by said conveyor and into said excess powder collector and to draw a flow of fluid with powder suspended therein away from said end surface of said inner deflector element and into said excess powder collector, said excess powder collector being spaced apart from said one surface of said can lid at said work station during flow of the stream of air and powder through said space between said nozzle and said one surface of said can lid at said work station and during a flow of excess powder away from said one surface of said can lid at said work station into said excess powder collector.

21. An apparatus as set forth in claim 20 further including a diverter located between said charging apparatus and said nozzle and connected with said body section to divert a flow of air and electrostatically charged powder being conducted in said body section away from said nozzle to interrupt a flow of powder from said nozzle toward said can lid at said work station while the flow path between the outer side surface of the inner deflector element and the inner side surface of the side wall remains open to facilitate spraying an annular band of powder on a next succeeding can lid.

22. A method comprising the steps of utilizing a nozzle assembly to direct a flow of air with powder entrained therein toward a workpiece, said step of utilizing a nozzle assembly to direct a flow of air with powder entrained therein toward a workpiece includes conducting a flow of air with powder entrained therein along a flow path which extends between a deflector having an end surface which faces toward the workpiece and an inner side surface of a side wall which extends around the deflector, retarding accumulation of powder on a surface in said nozzle assembly, said step of retarding accumulation of powder on a surface in said nozzle assembly includes conducting a flow of fluid from a source of fluid pressure through openings in the surface in the nozzle assembly, said step of conducting a flow of fluid from a source of fluid pressure through openings in the surface of the nozzle assembly includes conducting a flow of fluid through openings in said side wall and swirling the fluid along the inner side surface of the side wall, and conducting a flow of fluid from a source of fluid pressure through openings in the end surface of the deflector to retard accumulation of powder on the end surface of the deflector.

23. A method comprising the steps of utilizing a nozzle assembly to direct a flow of air with powder entrained therein toward a workpiece, said step of utilizing a nozzle assembly to direct a flow of air with powder entrained therein toward a workpiece includes conducting a flow of air with powder entrained therein along a flow path which extends between a deflector and a side wall which extends around the deflector, and retarding accumulation of powder on a surface in said nozzle assembly, said step of retarding accumulation of powder on a surface in said nozzle assembly includes conducting a flow of fluid from a source of fluid pressure through openings in the surface in the nozzle assembly, said step of conducting a flow of fluid from a source of fluid pressure through openings in the surface in the nozzle assembly includes conducting a flow of fluid through openings in the end surface of the deflector.

24. A method as set forth in claim 23 wherein said step of conducting a flow of fluid through openings in the surface in the end surface of the deflector includes conducting fluid pressure through a porous screen in the deflector.

25. A method comprising the steps of utilizing a nozzle assembly to direct a flow of air with powder entrained therein toward a workpiece, said step of utilizing a nozzle assembly to direct a flow of air with powder entrained therein toward a workpiece includes conducting a flow of air with powder entrained therein along a flow path which extends between a deflector and a side wall which extends around the deflector, and retarding accumulation of powder on a surface in said nozzle assembly, said step of retarding accumulation of powder on a surface in said nozzle assembly includes conducting a flow of fluid from a source of fluid pressure through openings in the surface in the nozzle assembly, said step of conducting a flow of fluid from a source of fluid pressure through openings in the surface in the nozzle assembly includes conducting a flow of fluid from the source of fluid pressure to a chamber in the deflector, and conducting a flow of fluid from the chamber in the deflector through a plurality of passages extending from the chamber in the deflector through an end surface of the deflector.

26. An apparatus for use in sequentially applying powder to each workpiece of a plurality of workpieces, said apparatus comprising a spray device which sprays powder on each of said workpieces in turn while said workpiece is at a work station, said spray device including components which produce pulses of air with powder entrained therein, a body section connected with said components which produce pulses of air and in which a flow of pulses of air with powder entrained therein is conducted, charging elements which are connected with said body section and electrostatically charges powder entrained in the flow of pulses of air in said body section, a nozzle connected with said body section to spray powder onto each of said workpieces in turn at said work station, said nozzle being effective to spray powder in one of said pulses of air with powder entrained therein onto one of said workpieces and to spray powder in a pulse of air with powder entrained therein next succeeding said one pulse of air with powder entrained therein onto a workpiece next succeeding said one workpiece, said nozzle having an inner side surface along which the pulses of air with powder entrained therein is conducted and on which powder tends to accumulate, a plurality of openings formed in said inner side surface of said nozzle, a conduit connected in fluid communication with a source of fluid pressure and with the openings in said inner side surface of said nozzle to conduct fluid which flows through openings in said inner side surface of said nozzle into a flow path of pulses of air with powder entrained therein through said nozzle, and a diverter located upstream of said nozzle and connected with said body section to divert away from said nozzle a portion of at least some of the pulses of air with powder entrained therein and to induce a flow of fluid with powder suspended therein from said nozzle into said diverter between pulses of air with powder entrained therein, said components which produce pulses of air with powder entrained therein includes a source of powder, a pump connected with the source of powder to induce a flow of powder from said source of powder toward said nozzle, said pump being operable between an active condition in which said pump is effective to induce a flow of powder from said source of powder and an inactive condition in which said pump is ineffective to induce a flow of powder from said source of powder, and control means for effecting operation of said pump between said inactive condition and said active condition, said diverter being operable between an active condition in which said diverter is effective to divert a flow of air and powder away from said nozzle and an inactive condition in which said diverter is ineffective to divert a flow of air and powder, said control means being operable to effect operation of said diverter between said inactive condition and said active condition in a predetermined relationship with operation of said pump between said inactive condition and said active condition to enable said diverter to sequentially divert the same portion of each of said pulses of air with powder entrained therein.

27. An apparatus as set forth in claim 26 wherein the portion of each of said pulses which is diverted is a trailing end portion of each of said pulses.

28. An apparatus as set forth in claim 26 further including an excess powder collector at least partially enclosing said nozzle and operable to induce a flow of excess powder away from said workpieces into said excess powder collector and to induce a flow of fluid with powder suspended therein from said nozzle into said excess powder collector between pulses of air with powder entrained therein.

29. An apparatus for use in sequentially applying powder to each workpiece of a plurality of workpieces, said apparatus comprising a spray device which sprays powder on each of said workpieces in turn while said workpiece is at a work station, said spray device including components which produce pulses of air with powder entrained therein, a body section connected with said components which produce pulses of air and in which a flow of pulses of air with powder entrained therein is conducted, charging elements which are connected with said body section and electrostatically charges powder entrained in the flow of pulses of air in said body section, a nozzle connected with said body section to spray powder onto each of said workpieces in turn at said work station, said nozzle being effective to spray powder in one of said pulses of air with powder entrained therein onto one of said workpieces and to spray powder in a pulse of air with powder entrained therein next succeeding said one pulse of air with powder entrained therein onto a workpiece next succeeding said one workpiece, said nozzle having an inner side surface along which the pulses of air with powder entrained therein is conducted and on which powder tends to accumulate, a plurality of openings formed in said inner side surface of said nozzle, a conduit connected in fluid communication with a source of fluid pressure and with the openings in said inner side surface of said nozzle to conduct fluid which flows through openings in said inner side surface of said nozzle into a flow path of pulses of air with powder entrained therein through said nozzle, and a diverter located upstream of said nozzle and connected with said body section to divert away from said nozzle a trailing end portion of at least some of the pulses of air with powder entrained therein and to induce a flow of fluid with powder suspended therein from said nozzle into said diverter between pulses of air with powder entrained therein.

30. An apparatus for use in sequentially applying an annular band of powder to a surface of each can lid of a plurality of can lids, said apparatus comprising a conveyor which is operable to move each of said can lids in turn to and from a work station along an arcuate path, and a spray device disposed adjacent to said conveyor, said spray device being operable to spray an annular band of powder on one surface of each of said can lids in turn while each of said can lids is held at said work station by said conveyor in a spaced apart relationship with said spray device, said spray device including a body section in which a flow of air with powder entrained therein is conducted, a charging apparatus connected with said body section to electrostatically charge powder entrained in said flow of air in said body section, a nozzle connected with said body section and spaced apart from said one surface of each of said can lids during movement of each of said can lids in turn to and from said work station and during spraying of an annular band of powder onto each of said can lids at said work station, said nozzle including an inner deflector element which has an outer side surface which flares outwardly in the direction of flow of the stream of air with electrostatically charged powder entrained therein and which is engaged by the flow of the stream of air with electrostatically charged powder entrained therein, said nozzle including a side wall having an inner side surface which flares outwardly in the direction of flow of the stream of air with electrostatically charged powder entrained therein and which circumscribes at least a part of said outer side surface of said inner deflector element and is engaged by the flow of the stream of air with electrostatically charged powder entrained therein, said outer side surface of said inner deflector element and said inner side surface of said side wall cooperating to define a flow path which has an annular cross sectional configuration and which flares outwardly in the direction of flow of the stream of air with electrostatically charged powder entrained therein, said inner deflector element having an end surface which faces toward the work station and on which powder tends to accumulate, said end surface on said deflector element being disposed on a porous screen through which a flow of fluid from said conduit is conducted to retard accumulation of powder on said end surface of said inner deflector element, a conduit connected in fluid communication with a source of fluid pressure and said end surface of said inner deflector element to conduct fluid which flows from the source of fluid pressure to said end surface of said inner deflector element to retard accumulation of powder on said end surface of said inner deflector element, and an excess powder collector at least partially enclosing said nozzle to draw a flow of excess powder away from said one surface of a can lid held at said work station by said conveyor and into said excess powder collector and to draw a flow of fluid with powder suspended therein away from said end surface of said inner deflector element and into said excess powder collector, said excess powder collector being spaced apart from said one surface of said can lid at said work station during flow of the stream of air and powder through said space between said nozzle and said one surface of said can lid at said work station and during a flow of excess powder away from said one surface of said can lid at said work station into said excess powder collector.

31. An apparatus as set forth in claim 30 wherein said inner deflector element includes a chamber which is disposed within said inner deflector element and which is connected in fluid communication with said conduit, said inner deflector element having a plurality of passages through which fluid flows from said chamber in said inner deflector element to said end surface of said inner deflector element.

32. An apparatus as set forth in claim 30 further including a plurality of openings formed in said side wall and a second conduit connected in communication with a source of fluid pressure and with the openings formed in said side wall to conduct fluid which flows from the openings in said side wall into the flow path of the stream of air with electrostatically charged powder entrained therein.

33. A nozzle assembly for use in directing a flow of air and powder toward a workpiece, said nozzle assembly comprising a side wall which at least partially defines a flow path of air and powder from an inlet to said nozzle assembly to an outlet from said nozzle assembly, said side wall having an inner side surface along which the flow of air and powder is conducted and on which powder tends to accumulate, a plurality of openings formed in said side wall at locations between the inlet to said nozzle assembly and the outlet from said nozzle assembly, a conduit connected in fluid communication with a source of fluid pressure and with the openings in said side wall to conduct fluid which flows from the source of fluid pressure through the openings in said side wall into the flow path of air and powder through said nozzle assembly to retard accumulation of powder on the inner side surface of said side wall, and control means for establishing a flow of air and powder along the flow path through said nozzle assembly toward the workpiece and for interrupting the flow of air and powder along the flow path through said nozzle assembly, said control means being operable to provide a flow of fluid from the source of fluid pressure through said conduit and the openings in said side wall into the flow path of air and powder through said nozzle assembly when the flow of air and powder through said nozzle assembly has been interrupted.

34. An apparatus for applying powder to workpieces, said apparatus comprising a pump operable to produce a flow of air with powder entrained therein, a charging apparatus connected in fluid communication with said pump to electrostatically charge powder in the flow of air with powder entrained therein, a nozzle assembly connected in fluid communication with said charging apparatus to direct the flow of air with powder entrained therein toward a workpiece, means for retarding an accumulation of powder on a surface in said nozzle assembly by conducting a flow of fluid to said surface in said nozzle assembly, and means for interrupting a flow of fluid to said surface in said nozzle assembly during a flow of air with powder entrained therein from said charging apparatus to said nozzle assembly and for establishing a flow of fluid to said surface in said nozzle assembly upon interruption of a flow of air with powder entrained therein from said charging apparatus to said nozzle assembly.

35. An apparatus for applying powder to workpieces, said apparatus comprising means for producing a flow of pulses of air with powder entrained therein, a charging means for electrostatically charging powder in the flow of pulses of air with powder entrained therein, a nozzle assembly connected in fluid communication with said means for producing a flow of pulses of air with powder entrained therein to direct the flow of pulses of air with powder entrained therein toward a workpiece, and means for retarding accumulation of powder on said nozzle assembly, said means for retarding accumulation of powder on said nozzle assembly including a conduit connected in fluid communication with a source of fluid pressure and with said nozzle assembly and control :

means for enabling fluid to flow through said conduit to said nozzle assembly between flow of pulses of air with powder entrained therein in said nozzle assembly and for blocking fluid flow through said conduit to said nozzle assembly during flow of pulses of air with powder entrained therein in said nozzle assembly.

36. An apparatus as set forth in claim 35 wherein said nozzle assembly includes a side wall having a plurality of openings connected in fluid communication with said conduit and through which fluid flows between flow of pulses of air with powder entrained therein in said nozzle assembly to retard accumulation of powder on said side wall.

37. An apparatus as set forth in claim 35 wherein said nozzle assembly includes an end surface which faces toward the workpiece, said end surface having a plurality of openings connected in fluid communication with said conduit and through which fluid flows between flow of pulses of air with powder entrained therein in said nozzle assembly to retard accumulation of powder on said end face.

38. A method comprising the steps of conducting a series of pulses of air with powder entrained therein along a flow path which extends from an inlet to a nozzle to an outlet from the nozzle, directing each of the pulses of air with powder entrained therein through the outlet from the nozzle toward a different workpiece of a plurality of workpieces, directing a flow of fluid into the flow path of the pulses of air with powder entrained therein at a location between the inlet to the nozzle and the outlet from the nozzle during an interval between flow of a first pulse in the series of pulses and flow of a next succeeding pulse in the series of pulses to suspend in the fluid powder which remains in the flow path after the first pulse of air with powder entrained therein has been conducted through the nozzle, and conducting at least a portion of the fluid and the powder which is suspended in the fluid from the nozzle.

39. A method as set forth in claim 38 further including the step of conducting a continuous flow of fluid through a surface of the nozzle during performance of said step of conducting a series of pulses of air with powder entrained therein along a flow path which extends from the inlet to the nozzle to the outlet from the nozzle to retard accumulation of powder on the surface of the nozzle.

40. A method as set forth in claim 38 wherein the flow path of the pulses of air with powder entrained therein from the inlet to the outlet of the nozzle is at least partially disposed between a surface on an inner deflector and a surface on a side wall which extends around the inner deflector, said step of directing a flow of fluid into the flow path of the pulses of air with powder entrained therein includes conducting fluid from openings in one of the surfaces into the flow path of the pulses of air with powder entrained therein during the interval between flow of the first pulse in the series of pulses and flow of the next succeeding pulse in the series of pulses between the surface on the inner deflector and the surface on the side wall.

41. A method as set forth in claim 38 wherein said step of conducting a series of pulses of air with powder entrained therein along a flow path which extends from the inlet to the nozzle to the outlet from the nozzle includes conducting the series of pulses of air with powder entrained therein in a first direction along the flow path, said step of conducting at least a portion of the fluid and the powder which is suspended in the fluid from the nozzle includes conducting the fluid and the powder which is suspended in the fluid in a second direction along the flow path, said second direction being opposite to said first direction.

42. A method as set forth in claim 38 further including the step of conducting a portion of each of the pulses of air with powder entrained therein into a diverter conduit to more sharply define an edge portion of each of the pulses of air with powder entrained therein, said step of conducting at least a portion of the fluid and the powder which is suspended in the fluid from the nozzle includes conducting fluid and the powder which is suspended in the fluid into the diverter conduit.

43. A method comprising the steps of producing a flow of pulses of air with powder entrained therein, utilizing a nozzle assembly to direct the flow of pulses of air with powder entrained therein toward workpieces, and retarding accumulation of powder on a surface of the nozzle assembly, said step of retarding accumulation of powder on the surface of the nozzle assembly includes conducting a flow of fluid from a source of fluid pressure through the surface of the nozzle assembly between each of the pulses of air with powder entrained therein.

44. A method as set forth in claim 43 wherein said step of utilizing the nozzle assembly to direct the flow of pulses of air with powder entrained therein toward workpieces includes conducting the flow of pulses of air with powder entrained therein along a flow path having a portion which is disposed between a deflector and a side wall which extends around the deflector, said step of conducting a flow of fluid from a source of fluid pressure through the surface of the nozzle assembly includes conducting the flow of fluid through openings in a surface on the side wall which extends around the deflector.

45. A method as set forth in claim 44 further including the step of interrupting the flow of fluid through the openings in the surface on the side wall which extends around the deflector as each of the pulses of air with powder entrained therein moves along the portion of the flow path which is disposed between the side wall and the deflector.

46. A method as set forth in claim 43 wherein said step of utilizing the nozzle assembly to direct the flow of pulses of air with powder entrained therein toward workpieces includes conducting the flow of pulses of air with powder entrained therein along a flow path which extends past an end of the nozzle assembly, said step of conducting a flow of fluid from a source of fluid pressure through the surface of the nozzle assembly includes conducting the flow of fluid through openings in a surface on the end of the nozzle assembly.

47. A method as set forth in claim 46 further including the step of interrupting the flow of fluid through openings in the surface on the end of the nozzle assembly as each of the pulses of air with powder entrained therein moves past the end of the nozzle assembly.

48. A method as set forth in claim 46 further including the step of maintaining the flow of fluid through openings in the surface on the end of the nozzle assembly as each of the pulses of air with powder entrained therein moves past the end of the nozzle assembly.

49. A method comprising the steps of conducting a series of pulses of air with powder entrained therein in a first direction along a flow path which extends through a nozzle, leaving powder in the nozzle after passage of one of the pulses of air with powder entrained therein through the nozzle, suspending the powder left in the nozzle in fluid after passage of the one pulse of air with powder entrained therein through the nozzle, said step of suspending the powder left in the nozzle in fluid includes directing a flow of fluid into the nozzle after the conducting of the one pulse of air with powder entrained therein through the nozzle in the first direction and prior to conducting a next succeeding pulse of air with powder entrained therein through the nozzle in the first direction, and conducting at least a portion of the fluid with powder suspended therein from the nozzle in a second direction along the flow path which extends through the nozzle, the second direction in which the fluid with powder suspended therein is conducted from the nozzle being opposite to the first direction in which the one pulse of air with powder entrained therein is conducted through the nozzle.

50. A method as set forth in claim 49 wherein said step of conducting the series of pulses of air with powder entrained therein in the first direction along the flow path which extends through the nozzle includes increasing the fluid pressure at an inlet to the nozzle as each pulse of air with powder entrained therein enters the nozzle, said step of conducting fluid with powder suspended therein from the nozzle in the second direction includes decreasing the fluid pressure at the inlet to the nozzle while performing said step of directing a flow of fluid into the nozzle.

51. A method as set forth in claim 49 wherein the flow path of the pulses of air with powder entrained therein through the nozzle is at least partially disposed between a surface on an inner deflector and a surface on a side wall which extends around the inner deflector, said step of directing a flow of fluid into the nozzle includes conducting fluid through openings in one of the surfaces into the flow path of the pulses of air with powder entrained therein.