CA1256750A - Method for electrostatic coating threaded fasteners with a thermoplastic resin - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for electrostatically applying a thermoplastic resin coating on a threaded portion of a fastener to provide a self locking element thereon. A plurality of fasteners are moved by a spaced pair of endless belts passed a spray station where a stream of powdered resin is directed toward the threaded ends of the fasteners. The fasteners and resin are polarized to opposite polarities at the spray station whereby the powdered resin is attracted to and clings to the threaded ends of the fastener and extends completely about the circumference of said ends. Air jet nozzles located downstream of the spray station may direct streams of compressed air against certain portions of the powdered resin to remove the resin therefrom to provide uncoated lead threads and to regulate the distance that the resin extends circumferentially about the threaded ends to provide a coating extending between 180 degrees and 360 degrees about the threaded ends. The powder coated fastener then pass through a heating zone which melts the powdered resin on the threaded ends forming a permanent bond therebetween and providing the self locking element thereon. The fastener also may be heated before the stream of polarized powdered resin particles is directed toward the fastener to assist in bonding the resin to the fastener and to enable a greater buildup of thermoplastic resin to be applied thereto.

Description

j7~Q

ME~HOD FOR ELECTROSTATIC COATING THREADED FASTENERS
WITH A THERMOPLASTIC ~ESIN

TECHNICAL FIELD

The invention relates to self locking fasteners having a coating of a thermoplastic resin material applied thereto and particularly to a method of applying resin to the fastener.
More particularly, the invention relates to an improved method o~ applying the resin coating electrostatically throu~hout a predetermined circumferential portion of the fasteners preferably between 180 degrees and 360 degrees. __ BACKGROUND ART

The use o~ self locking fasteners has increased considerably in recent years for a variety of fastening applications wherein the integrity of the fastened members is ~o critical. One common form of providing a sel~ locking fastener i~ by the use o~ a thermoplastic resin coatin~ which i8 applied to portions of the threaded bolt end or internal threads of a nut. Various methods have been developed for applying the thermopla~ic material to the threaded area oP the fastener.
One type of method use~ a patch of a soft pliable thermoplastic material which i~ pressed or melted into the threaded area.
Some examples o~ this method and resulting fastener are shown in Patent Nos. 3,0g3,17~; 3,263,726; 3,294,139; 3,313,01~;
3,416,492; 3,43~,541; 3,552,46~; 3,56~,~46; and 3,634,5~.
~o Another common method i8 to heat the bolt and then direct a spray of powdered resin against the heated bolt whereby the resin will melt and adhere to the threaded areas of the bolt.
Examples of such coating methods are shown in Paten~ Nos.

;6~

3,766,584; 3,~31,724; 3,579,684; 3,787,222 and 4,120,993. In another method, the fastener has a wetting agent applied over a predetermined area and the powdered thermoplastic resin applied to the wetted area after which the fastener i5 heated to bond the resin to the bolt.
Although many of these known methods provide a satisfactory procedure ~or applying the thermoplastic resin material to the bolt, one of the disadvantages is that it is difficult to regulate the size of the circumferential area covered by the resin in a simple and inexpensive manner, and in particular to form a resin coated fastener wherein the coating i~ applied over a circumferential area of bPtween 180 and 360 degrees instead of the 180 degrees area provided by as most known spraying operations. The application of the resin by spraying of the powdered material i8 believed to be the most economical and satisfactory but does not provide for the satisfactory coating of a circumferential area of the bolt greater than 180 degrees.
Patent No. 4,428,981 shows a method for applying a resin

2~ coating covering approximately 360 degrees about a bolt by spinning the bolt as it passe~ through the coating or spray area. Although thi~ method provides a bolt having a resin coating of more than 180 degrees, it requires a dual drive mechanism for spinning the bolts. An alternative means of coating the bolt throughout a full 360 degrees is by the use of a pair of spray nozzles, one on each side of the moving bolt to attempt to provide the 360 degree coatlng.
Another problem when applying a resin coating to a threaded fastener is tha~ it is dif~icult to achieve a relatively thick coating of the resin on the fastener without the use of multiple spray nozzles since it is difficult to achieve a sufficient buildup oP the resin part~cles with only a single nozzle.
Therefore, the need has existed for an improved method for ~ ~ S 6 ~ 5 ~

coating fasteners and in particular bolts, with a self locking thermoplastic resin in which the coating can be applied throughout a predetermined circumferential area of between 180 degrees and 360 degrees of the threaded area in an extremely simple, efficient and economical procedure.

DISCLOSURE OF THE INVENTION

Objectives of the invention include providing an improved method for coating fasteners with a self locking thermoplastic resin by electrostatically applying the resin to the fas-teners by spraying the fasteners as they move past a spray station with a powdered resin electrically charged at an opposite polarity to the charge placed on the fasteners whereby the particles are attracted to the fasteners by the electric field surrounding the fasteners. In a specific embodiment of the invention there is provided an improved method in which the fasteners may be at ambient temperature when the spray of powdered resin particles is directed toward the fasteners whereby certain portions of the electrostatically adhered resin particles may be removed by subsequent air jet nozzles providing a predetermined pattern of resin particles adhered to the fastener prior to the fasteners passing through a heating zone to firmly bond the desired pattern of resin particles to the threaded area.
In still another embodiment of the invention there is provided an electrostatic coating procedure in which the fas-teners upon entering the spray zone are contacted by a plurality of rnetal wires or brushes which are connected to ground to form an induced ground, and in which a positive polarity is applied to the powdered resin particles by high voltage electrodes which extend into the path of the particles as they are sprayed i7~

through a nozzle by pressurized air. In a further embodiment there is provided an improved method in which the spray nozzle assembly is formed of a dielectric material except for an outer flared nozzle end, which end is subjected to a lower voltage of the same polarity as the electrodes which are located upstream in the path of the moving powdered material to maintain the polarity on the powder without presenting a problem of flashover with the adjacent metal fasteners which are at the opposite potential.
In a further embodiment of the invention there is provided an improved method in which a vacuum duct is positioned on an opposite side of the fasteners from the resin spray nozzle assembly at the spray station to recover excess powder, and in which air jet nozzles may be located downstream of the resin spray station to remove portions of the adhered powdered resin from certain portions of the fasteners to obtain the desired circumferential coated distance and to dress and clean the ends of the threaded area to provide an uncoated area enabling the fasteners to be started easily with their mating elernent before encountering the self locking resin coated area. In still 2~ another embodiment there is provided an improved method in which a specially designed vacuum duct having an air defuser as a part thereof confines the powered resin particles to a generally specific area on the bolt shank instead of completely covering the bolt due to the attraction of the particles to the bolt of opposite polarity. In another embodiment there is provided an improved method which provides an extremely simple, inexpensive and highly eEfective procedufe for coating threaded fasteners and in particular bolts, with a predetermined circumferential area of resin material.
In yet another embodiment of the invention there is provided an improved me-thod in which the fasteners may be heated before entering the spray station whereby a relatively thick coating of -5- ~ ~ 5~7~'~

resin material can be deposited on the heated fasteners without requiring multiple spray nozzles and without reducing the linear Qpeed of the fasteners as they move through the spray station.
These objectives and advantages are obtained by the improved method of the invention, the general nature of which may be stated as a method for electrostatically coating a threaded fastener with a thermoplastic resin extending around a predetermined circumferential threaded portion of fastener including the steps of moving a threaded fastener along a predetermined path having a spray zone and a heating zone;
polarizing the fastener at the spray zone; and polarizing a stream of powdered resin to an oppos~te polarity than the fastener and directing the resin towards a port~n~of the threaded end of said fastener whereby the polarized resin is attracted to and oling~ to said threaded portion forming a coating of powdered resin about ~a~d threaded end.

BRIEF DESCRIPTION OF THE~ DRAWIN~S
A preferre~ embodimen~ of the invention, illustrative of the best mode in which applicants' have contemplated applying the princlple~, i8 set forth in the following description and i8 ~hown in the drawings and i8 particularly and distinctly pointed out and set forth i~ the appended claims.
FIG. 1 i~ a perspectlve view showin~ a type of fastener with the thermopla~tic resin applied to a predetermined area thereof by the ~teps of the improved method;
FIG. 2 is an enlarged view with portion~ broken away and ln seGtion showing certain of the threads of the fastener FIG. 1 with the resin applied thereto;
FIG. 3 is a diagramatic top plan view showing a type of ~ZS~i7~;0 apparatus for carrying out the steps of the improved method;
FIG. 4 is a diagrammatic side elevational view of the apparatu shown in FIG. 3;
FIG. 5 i8 an enlarged fragmentary sectional view taken on line 5-S, FI~. 4;
FIG. 6 is an enlarged fragmentary sectional view taken on line 6-6, FIG. 4;
FIG. ~ is a fragmentary diagrammatic top plan view showing a type of spray nozzle for applying the resin to a fastener of the type shown in FIG. 1;
FIG. 8 is a fragmentary diagrammatic side elevational view of the spray nozzle of FI¢. 7;
FIG. 9 is an enlar~ed fragment~ry sèctional v~ew taken on lin~ 9-9, FIG. 8; and FIG. 10 is a perspective view w1th portions broken away and in section showing the vacuum duct for recovering spent resin particles.
Similar numerals refer to similar parts throughout the drawings.
BEST MODE FOR CARRYING OUT THE INVENTION

The improved method i de-~cribed below in relationship to applying a powdered thermoplastic resin coating on a bolt although the improved method could be used for applyins similar material~ onto other types of threaded fastener~ whether having internal or external threads or other arrang~ents than a threaded area at the end of a ~hank, and need not be limited to the fastener bolts shown in the drawin~ and described below.
FIG. 1 shows a bolt indicated generally at 1, consisting o~
a shank 2 having a lower threaded area 3, a smooth upper area 4, and a head 5. The individual threads consi~t of vall~ys ~
and peaks 8 as shown in ~IG. 2. The resin coating is indicated at 9 and is shown in FIG. 2 being bonded to the bolt and providing a desired uniform coating of valleys 7 preferably not extending above peaks 8.
The apparatus for carrying out the steps of the improved method i5 shown particularly in FIG. 3 and includes a pair of pulleys 11 and 12 which are power driven preferably by an electric motor and a single shaft (not shown) connected to the pulley shafts 13 whereby the pulleys will rotate at the same speed. Other types of drive mechanisms can b~ used if desired.
Pulleys 11 and 12 are double grooved pulleys in which a pair of endless flexible belts 14 and 15 are located which preferably are formed of nylon or similar flexible inexpensive material.
Belts 14 and 15 could al~o be formed of other typ~s of pla6tlc or dielectric materials.
A spray station, indicated generally at 1~, is located generally ad~acent pulley 11 and consists principally of a spray nozzle assembly indicated at 18, a vac~um recovery duct 19, and a grounding a~sembly indicated yenerally at 20.
Grounding assembly 20 consists of a plurality of metallic strands 22 preferably formed of copper to provide good conductivlty. Strands 22 extend downwardly from a supporting bar 23 which is connected to ground 24 to form an induced ground. The resiliency and thickness of metallic strands 22 may vary depending upon the particular size and weight of bolts being ~oved past the spray station by belts 14 and 15 since the larger more rigid strand3 22 could affect the movement of the bolts in contrast to a thinner more flexible strand. Other types o~ grounding strands or brush arrangement can be util~zed from that chown in FIG. 3 to provide a grounding affect on the bolts as they pass through the ~pray station without affecting the concept of the invention.
Spray nozzle assembly 18 i8 shown particularly in FIGS.
and 8 and .~ncludes a mixing block 26 formed of a dielectric -8~ 75~

material such a~ nylon or other type of plastic. A passage 27 extends completely through the nozzle block, one end of which is connected to the end of a hose 28 by a connector 29, with hose 28 being connected to a source of compressed air. A
powder inlet opening 30 extends vertically into block 26 from top surface 31 and terminates in passage 27. Preferably passage 2~ and inlet opening 30 are of the same uniform diameter size.
A feed ~unnel or hopper 33 i5 mounted on upper surface 31 and contains a supply of resin powder 34 which is fed by gravity through opening 30 into passage 27 where the incoming supply of compressed air carrie~ the resin powder through passage 27 and through a nozzle supply sleeve 35 which iB
connected to block 26 and then to a flared defusing nozzle 3~.
Sleeve 35 and feed funnel 33 also are formed o~ a dielectric material with flared nozzle 37 preferably being formed of copper or other metallic material.
In accordance wlth the invention, high voltage electrodes 39 and 40 extend through supply sleeve 35 and across sleeve bore 41. ~ore 41 preferably is tapered outwardly as shown in FIG. 8 to reduce the velocity o~ the powdered resin being carried by the pres~urized air stream along the ~leeve upon approachlng nozzle 3~ for discharge onto bolt l. This enables sufficient air pres~ure to be used for carrying the powdered resin material discharged from funnel 33 through sleeve 35 and out of nozzle end 37 w1thou~ too great a velocity being imparted ~o the powdered material as it approaches bolt l.
Another electrode 42 preferably is connected to defusing nozzle

3~.
Electrodes 39 and 40 pre~erably are section~ of tungsten wire and extend across sleeve bore 41 whereby the particles moving past the electrodes will have a positive D.C. charge applied thereto. In the preferred embod1ment, electrodes 39 3~25~7~

~nd 40 will be at a potential of approximately 20 kv positive D.C. voltage with nozzle electrode 42 being at a considerable reduced voltage such as 3 kv. Electrodes 39, 40 and 42 are connected to a usual D.C. transformer (not shown) which i5 well known in the art.
Vacuum recovery duct l9 preferably i8 mounted directly opposite nozzle 3~ to recover any powdered re~in particles which do not adhere to bolt 1. Recovery duct l9 ~FIG. 10~
includes an air defuser housing indicated generally at 55 which is connected to a source of suction by a hose 56. Housing 55 includes a ~enerally rectangularly shapad front portion and a conical shaped rear portion wi~h the front portion being formed with an inlet end 57 which converges into a narrow elongated opening 58. The air entering inlet opening 5~ indicated by arrow3 A, flows through a hollow rectangular-shaped passageway or chamber 59 and into a tapered area 60 where it flows out of a round outlet 61 and into hose 56. An aiF defuser mechanism indicated generally at 62 is formed a~ a part of housing 55 and extends upwardly therefrom having a rectangularly shaped air lnlet openin~ 63 which communicates with conical shaped passage 60. A baffle 65 1~ pivotally mounted within air defu~er 62 and is ad~ustable by a nob 56 to regulate the amount of air flowing into the air defussr indicated by arrow B.
The particular configuration and construction of vacuum recovery duct 19 enables ~he exces~ powder resin particles not attracted to the bolt to be collected and removed from the ~pray area. More lmportantly the air stream which it creates moving across the bolt prevents the particles from completely covering the bolt and provides for a general uniform pattern of the resin particle~ in a specific area of the bolt shank. The use of defuser 62 greatly aids and assists in maintaining the powered resin particles in a defined area on the bolt instead of completely covering the bolt in its entirety with resin -10~ ;75(~

particles due to the natural tendency of the particles to adhere to all surfaces of the metal bolt due to the opposite polarities of the balt and particles and by eliminating turbulence in the ~oving stream of pressurized air.
As shown in FIG. 5, the powdered resin particles 34 upon being ~ischarg~d through nozzle 37 are directed toward a predetermined area of threaded end 3 of bolt shank 2. The electric field established by the opposite polarities of the bolt and resin particles cause the particles to be attracted to the bolt whereby these particle~ which normally flow past the bolt are captured by the side and rear area~ of the bolt to provide the 360 degree~ circumferential coverage of the threaded end of the shanX. The axial height or length of the powdered coated area 43 ia determ~ned by the height of dl~charge opening 44 of nozzle 37 and vacuum duct 19. Thus in accordance with the concept of the invention, the charging of the res~n particles and ~olt at opposite polarities enables the heretofore passing particle~ to adhere to the side and rear ,`_ portions of the bol~ with the front portions being covered by /i the normally directed contacted particles being sprayed ~rom the nozzle.
The powder particles impinge against the surface of the bolt and ln the thread valleys and cling sufficiently to the threads until the bolt passes through a heating zone lndlcated generally at 4~ where th~ bolt i8 heated above the fusion point of the resin material whereby the resin melts and bonds to the sur~aces of the thread valleys to form the deslred thermoplastic coating 3.
However, be~ore the powder coated bolts reach the heatlng zone, they are passed throu~h a control zone indicated generally at 4~, having one or more air ~et nozzles 48.
Nozzles 48 direct streams of pressurized air 49 against predetermined areas of the threaded ~hank to remove certain of the powdered resin particles as shown in FIG. 6 especially at the extended end of the threaded shank. Removal of the powdered resîn at the end of shank 2 provides a predetermined uncoated length of lead thread3 which enables the threads to have a start with a mating nut or threaded hole before the mating thread~ encounter the resin coated threads.
If desired, other air nozzles 51, only one of which is shown in FIG. 3, may be positioned at control zone 4~ or further downstream therefrom and direet against a stream of pressurized air against predetermined portions of the bolt, for : example, the back surface thereof, in order to remove the -powdered resin from certain areas thereof. Baffilo also may be -- ~used to direct this pressurized air to the desired areas of the powder coated threads. For example, the powdered resin can be removed from the rear 180 degree surface or portion of the bolt to provlde a circumferential powdered coating of between 180 degrees and 360 degrees. If a full 360 degree circumferential coating is desired, air nozzles 51 will not be used. Nozzles 51 provide the means for varyin~ the size of the circu~ferential resin coating which will be firmly bonded to th~ threaded area of the bolt at heating zone 46. Since the bolt i8 at aobient temperature and the resin particles are still in the powdered form, they are removed easily by the ~ets - of air fro~ noz~les 48 an~ 51 to provide the desired pattern of the ultimate thermoplastic coated area 9.
Heating zo~e 46 preferably l~cludes an induction heater 53 which rai~e~ ~he te~perature of the metal bolt passlng therethrough to a point above the fusion or melting point of the thermopla~tic powdered material. After the bolt leaves heating zone 46 it i9 cooled whereby the resln forms a firm bond with the bolt threads. Other types of heaters can be used ~or heating either the bolt or resin particles for adhering the particles to the bolt than the induction heater shown in the -12- ~ Z 5 ~7 ~

drawing After the coated bolts,leave the heating zone, they are cooled, either by the ambient air or preferably by depositing the bolts into a bath of oil or water, from which they are removed for subsequent stora~e and/or shipment. The heat of fusion o~ a type of nylon resin found suitable for coating the threaded bolts ends i~ approximately 450 to 500 degrees Farenheit.
Although the above described method indicates that the powdered resin particles are sprayed onto the moving fastener before it passes through the heating zone, it may be desirable for certain applications ~o heat the fastener before it moves through the spray zone. Thi~ enables a considerably larger deposit o~ resin particles to be formed on the fastener and provides a strong bond between the powdered resin and fastener ~ince the resin immediately melts upon contacting the heated fastener with additional resin particles than being attracted to the fastener to provide a thicker coating than believed heretofore pnssible with known nonelectrostatic coating methods.
The improved method steps of the invention consist broadly of moving one or a plurality of fasteners along a predetermlned linear path pa~t a spray station, at w;~ich station the fastener~ have a certain polarity applied thereto and an OppO8~ te polarity i8 lmparted to particles of thermoplastic resin which are sprayed toward the fasteners from a nozzle whereby the resin particle~ which heretofore would pass by the fa teners are attracted to and clin~ to the sides and rear portions of the fastener shank to provide a powder coatlng -' completely about the shank. Next the powder coated fasteners may pa88 by a control zone in which one or more nozzles remove the powdered particles from selected areas of the fasteners by dlrecting a stream of compressed air toward the selected bolt areas after which the fasteners pass through the heating zone where the powder material i8 firmly bonded thereto. The -~3~

~asteners after leaving the heating zone are cooled either by the a~bient air or external means at which time the resin material is firmly bonded to the threads to provide the self locking effect desired thereof. If desired, the fasteners are moved through the heating zone before movlng through the spray zone.
The above sequence of steps is carried out in a continuous movement of the fasteners by equipment which i5 extremely simple to operate and relatively inexpensive to manufacture, in contrast to equipment heretofore used ~or coating fasteners with such a thermoplastic material, and in which the improved method is able to provide a re~in coatin~ between 180 degrees and 360 degrees which i8 difficult to achieve economically by ~nown method~ and `~~ a~paratus. Although the electrostatic coating of metal ob~ects has been performed ~or years, it has never been applied nor used with the ther~oplastic coating of fasteners, and in particular bolts, which enables the powdered resin material to be applied to the bolts when either ln a heated or unheated state whereby the powder will adhere completely about the circumference of the bolt enabling ~he final resin material to be bonded throughout this 2Q . 360 degree area, i~ d~sired, and whlch permits the easy removal ~ of the powderod resin partlcle~ from selected bolt areas by ~ets - of air if app led before the bolt i8 heated and the resin : material fused to the threaded area, thereby enabling a re~in - coatlng to be applied to a circum~arential thread0d area between 180 degrees and 360 degrees in an sxtremely ~imple and economic manner.
In production lot~ the heads of the bolts u~ually will be closely ad~acent or in abutme~t with each other as they move down the coating line by belts 14 and 15 with one or more bolt ~hank~ being sprayed with the resin powder upon passing spray nozzle 37. Also the polarity applied to the bolts and powdered resin could be reversed from that described above and -qhown in the drawings. Likewise, other apparatus can be used for -14- ~25~75~
., applying the opposite polarities to the bolt and resin particles than that shown without affecting the concept of the invention. Preferably, the powdered resin is applled uniformly 360 degrees about the bolt shank to provide a final uniform coating of resin material 9 as shown in FIG. 2. Also, it is readily understood that the bolts will be cleaned with a solvent to remove all ~rease and oil therefrom be~ore being placed on belts 14 and 15 as i9 customary in most resin coating procedure~.
Accordingly, the improved method is simplified, provides an e~fective, safe, inexpensive, and efficient procedure which achieves all the enumerated ob~ectives, provide3 for eliminating difficulties encountered wlth the prior .neth~ds, and solves problems and obtain~ new re~ults in the art.
In the foregoin~ descriptlon, certain terms have been used for brevity, clearness and understanding; but no unnecessary limitations are to be implied therefrom beyond the requirements o~ the prior art, because such terms are used for descriptive purposes and are intended to be broadly construed.
~0 Moreover, the description and illustration of the invention is by way o~ example, and`the ~co~e of the in~Q~tion i8 not limited to the exact details qhown or described.
Havin~ now de~cribed the features, discoverle~ and principles of the invention, the manner in which the improved method ~or electrostatic coating threaded fasteners with a thermoplastic re~in i~ carried out, the characterl~tics of the improved method, and the advanta~eous, new and useful results obtained; and the structures, device-~, elements, arrangementc, part~, and combinations thereof for carrying out the stPp~ of the invention are set ~orth in the appended claims.

Claims (20)

WHAT IS CLAIMED IS:

1. A method for electrostatic coating a threaded fastener with a thermoplastic resin extending around a predetermined circumferential portion of a threaded portion of the fastener, including the steps of:

a) moving a threaded fastener along a predetermined path having a spray zone and a heating zone;
b) polarizing the fastener at the spray zone; and c) polarizing a stream of powdered resin to an opposite polarity than the fastener and directing the resin towards a threaded portion of said fastener whereby the polarized powdered resin is attracted to and clings to said threaded portion forming a coating of powdered resin thereon.

2. The method defined in Claim 1 including the step of passing the threaded fastener through the heating zone before moving the fastener through the spray zone.

3. The method defined in Claim 1 including the step of passing the powdered resin coated fastener through the heating zone to soften the powdered resin coating causing it to firmly adhere to said threaded end to provide a self-locking resin deposit about a predetermined circumferential portion of said threaded end.

4. The method defined in Claim 1 in which the threaded fastener is a bolt having a shank and a head; and in which the bolt is suspended by the head between a spaced pair of endless horizontally moving belts with both of said belts moving at the same linear speed.

5. The method defined in Claim 4 in which the belts extend about a pair of spaced pulleys at least one of which is power driven.

6. The method defined in Claim 1 in which the heating zone has an induction heating element.

7. The method defined in Claim 1 in which the spray zone includes a spray assembly having a mixing block and a delivery tube formed of a dielectric material; in which a spray nozzle is mounted on an extended end of the delivery tube; and in which high voltage electrodes are attached to the delivery tube and extend into the path of the powdered resin which is discharged from the nozzle by pressurized air.

8. The method defined in Claim 5 in which the electrodes are connected to a source of D.C. voltage.

9. The method defined in Claim 7 in which the nozzle is formed of metal and is connected to a source of D.C. voltage lower than the voltage applied to the electrodes in the delivery tube.

10. The method defined in Claim 7 in which the delivery tube is formed with a bore the diameter of which increases towards the spray nozzle.

11. The method defined in Claim 1 in which a plurality of flexible metal strips are polarized at a certain polarity and slidably contact the moving fastener at the spray station for polorizing said fastener.

12. The method defined in Claim 9 in which the metal strips are connected to ground; and in which the powdered resin has a positive polarity.

13. The method defined in Claim 11 in which the powdered resin is subjected to a 20 K.V. positive D.C. charge.

14. The method defined in Claim 1 including the further step of recovering a portion of the powdered resin which does not adhere to the fastener at the spray zone by a vacuum system.

15. The method defined in Claim 1 including the further step of directing a jet of air against a certain area of the powdered resin coating on the threaded portion of the fastener to remove the powdered resin therefrom before the fastener enters the heating zone.

16. The method defined in Claim 15 in which the powdered resin is removed from an outermost portion of the threaded end of the fastener by the air jet to provide uncoated lead threads on the fastener.

17. The method defined in Claim 15 in which the powdered resin is removed from a predetermined circumferential portion of the threaded portion to provide a resin coated area extending less than 360 degrees about said threaded end.

18. The method defined in Claim 12 in which the vacuum system includes a vacuum duct formed by a hollow housing having a pair of inlet openings which communicate with interior flow chambers which terminate in an outlet opening; and in which the outlet opening is adapted to be connected to a vacuum source.

19. The method defined in Claim 18 in which the housing has a rectangularly shaped front portion and a conical shaped rear portion which terminates in the outlet opening; and in which one of the inlet opening is formed in an end of the rectangularly shaped front portion of the housing with the other of the inlet openings being formed in the conical shaped rear portion of the housing.

20. The method defined in Claim 19 in which said other inlet opening is formed by a rectangularly shaped duct having a baffel mounted therein which controls the flow of air and captured resin particles entering said one inlet opening.