CA1280284C - Paint spraying shroud - Google Patents

Abstract

A paint spraying shroud has a relatively narrow front opening, an air exhaust opening or openings co-extensive with the front opening and disposed on a rear side of the shroud directly opposite the front opening, and a relatively wide intermediate portion. The air flow through the shroud may be substantially entirely uniformly parallel to the horizontal, so that a more uniformly applied coating of paint is obtained on vertically oriented elongated items which are continuously passed through the shroud.

Description

12~30284 The present invention relates to a paint spraying shroud and to paint spraying apparatus employing the shroud.

Known paint spraying apparatus, used for the continuous painting of elongated articles such as extrusions, shelving, racking components, building panels and the like, comprises a spinner, driven at a high speed of rotation, to which the paint is fed. The spinner is reciprocable along a vertical axis. An overhead conveyor extends toward and away from the spinner and in a partial loop around the axis of the spinner. The articles to be painted are suspended from the conveyor and passed in succession therealong. Usually, an electrical potential difference is maintained between the spinner and the articles so that the paint, which is spun off from the spinner in the form of a fine mist ofpain-tdroplets, is attracted electro-statically to the articles. Normally, in order to collectpaint mist which is not deposited on the articles, i.e. the so-called "overspray", a paint mist collecting shroud is disposed adjacent the paint spinner. The conventional form of shroud has side walls which extend in a partial arc around the axis of the paint spinner leaving a gap at the front through which the conveyor and the articles suspended therefrom enter and leave the shroud. An air plenum extends around the base of the side walls on their outer side and this communicates with the interior of the shroud through openings formed through the lower ends of the side walls.

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A fan at the rear of the plenum exhausts air from the plenum and passes it to a stack connected to the plenum adjacent the rear of the shroud. The overspray paint mist tends to gravitate downwards, and is drawn downwards by the current of air drawn inwardly through the openings in the plenum. Usually, the openings are fitted with filters which trap the paint droplets to reduce build-up of paint on the inside of the plenum, or on the fan blades, and to reduce emissions of paint mist up the stack.

The applicants have observed that the known apparatus is inefficient, as the downward current of air within the shroud tends to distribute the paint mist non-uniformly on the articles to be painted, and tends to result in the lower portions of the articles receiving a larger proportion of the paint, while the upper portions tend to receive a smaller proportion, so that there is a difference in the paint coating thickness from top to bottom. This, therefore, increases the consumption of paint required to achieve a desired coating thickness.
Further, with the conventional form of shroud, the cross-sectional area of the plenum is too small to permit uniformsuction on all of the filters, and the air flow through the filters ad~acent the front of the shroud tends to be less than that through the filters adjacent the rear. This results in considerable quantities of the paint mist tending to escape through -the front opening of the shroud and to be deposited on adjacent floor or other surfaces, necessitating frequent clean-ups, which are inconvenient, time-consuming and expensive. The loss of paint mist through the front opening also renders the application of the paint less efficient, as it reduces the quantity of paint available for coating on the articles.

The present invention relates to a shroud whereby the above-noted disadvantages can be avoided, and provides a paint ~ 3 ~ 12 8~ 2 8 ~

spraying shroud comprising vertically extending side walls defining an enclosure having a vertically extending relatively narrow front opening, a cover over the top of the enclosure, means defining an air exhaust opening on a rear side of the enclosure opposite the front opening, the air exhaust opening extending parallel to and to approximately the same vertical extent as the front opening, and the enclosure narrowing in its width from a relatively wide region intermediate the front and the air exhaust openings toward said relatively narrow front opening.

The means defining an air exhaust opening on the rear side of the shroud may comprise a series of closely vertically adjacent exhaust openings. With the present shroud, substantially uniform suction can be applied to each of the exhaust openings, or over substantially the entire area of other means defining the air exhaust opening at the rear side of the shroud, using, for example, a plenum at the rear of the shroud co-extensive with and in communication with the air exhaust opening, and with air being exhausted from the upper end of the plenum. With this arrangement, substantially the entire air flow through the shroud takes place in a direction transverse to the vertical line of reciprocation of the paint-dispensing spinner, and the paint droplets entrained by the air flow tend to be distributed substantially uniformly over the entire length of the articles to be painted.
Further, the air entering the filters is, with the present arrangement, incident on the front face of the filter at an angle of substantially 90 with respect to the plane of the front face. Applicants have found that this can greatly increase the filtration efficiency, resulting in considerably less penetration of paint droplets through the filter and into the air exhaust plenumt as compared with the conventional arrangement, where the air containing the overspray is incident at an oblique angle on most of the filters.

~802~

In the preferred form, the shroud has a front opening of width which is less than about half the width of the enclosure at its intermediate region. As a result, the velocity of the air entering through the constricted front opening is relatively high, thus reducing or eliminating escape of overspray paint mist through the front opening. In the interior of the shroud, the air velocity drops as the flow passes through the relatively wide region toward the rear exhaust opening. The relatively low air velocity in the intermediate region is conducive to uniform distribution of the paint spray over the articles to be painted.

In the accompanying drawings, which show, by way of example only, one form of paint spraying shroud in accordance with the invention:

Figure 1 shows a partially fragmentary perspective view of a shroud equipped with a conveyor, reciprocating paint spinner, an exhaust plenum and an air exhaust system;

Figure 2 shows a horizontal section along the line 2-2 of Figure 1 through the shroud and plenum; and Figure 3 shows a partially fragmentary perspective view of a preferred form of air filter used in the shroud of Figure 1.

Referring to the drawings, the shroud 10, as best seen in plan in Figure 2, comprises an enclosure 11 which is symmetrical about a longitudinal median. The enclosure has two intermediate planar side panels 12 and 13. Forward planar side panels 14 and 16 incline inwardly from a front edge of the panels 12 and 13. Each of the panels 14 and 16 continues forwardly in a flange 18, the flanges 13 being parallel to one another, and terminates in a lip, 19 or 20, - 5 _ ~2~28~

directed outwardly, and connected at its end to a vertically extending box-section front support 22. The 'lanyes 18 define between them a front opening which is desirably less than about half the width of the intermediate region of the 5 enclosure 11 defined between the panels 12 and 13. In the example illustrated, the width between flanges 18 is about 0.49 the width between the panels 12 and 13.

A rear side panel 23 or 24 inclines inwardly from the rear edge of each panel 12 and 13, and the panels 23 and 24 are each connected at their rear edges to a rear panel 26 and 27, respectively. As will be seen from Figure 2, -the general planes of the front faces of the panels 26 and 27 incline at a relatively shallow angle with respect to the transverse direction, as compared with the panels 14 and 16 or 23 and 24, which incline at a relatively sharp angle.

The adjacent edges of the panels 12, 13, 14, 16, 23, 24, 26 and 27 are each formed with a short outwardly directed edge flange 28, and these edge flanges are connected by mechanical fasteners e.g. nuts and bolts 29.

Each rear panel 26 and 27 is formed with a series of closely vertically adjacent rectangular openings 31. A paint aroplet-entrapping air filter 32, shown in more detail in : Figure 3, is connected on the rear face of each panel 26 and 27, in register with each opening 31. As shown, each filter 32 may have a front edge flange 33 which may be removeably connected, by a releasable fastening, not shown, in air-tight engagement to the rear face of the panel 26 or 27, permitting the filters to be replaced when necessary.

Adjacent the rear of the shroud, there are a pair of vertically extending box section rear supports 34. A
connector plate 36 is connected to the inner side of each support 34 and is connected to the enclosure 11 through the - 6 - ~Z~OZ8~

bolts used to bolt together the edge flanges at the connection between the adjacen-t rear side panel 23 or 24 and the rear panel 26 or 27.

An air exhaust plenum chamber 36 is provided on the rear side of the shroud, in communication with the openings 31. The chamber 36 comprises side walls 37, connected at their front edges to the supports 34, a rear wall 38, and a roof 39. One side wall 37 is provided with a door 41 through which access may be had to the rear of the filters 32, for the purposes of inspecting and replacing the fil~ers when necessary. The roof of the chamber has a circular opening 42 to which is connected in air-tight fashion, a duct section 43 housing a fan 44 which can be actuated to exhaust air from the chamber 36 and pass it to a stack 46 connected to the section 43, which vents to the atmosphere. The lower edges of the chamber 36 and of the enclosure 11 are in substantially air--tight contact with the floor or other surface on which the shroud is supported.

The upper end of the enclosure 11 is substantially entirely closed by a cover in the form of a rectangular plate 47, which has openings through which the supports 22 and 34 pass, and which rests in air-tight fashion on the upper edges of the forward side, rear side, and rear panels 14, 16, 23, 24, 26 and 27. The side edges of the plate 47 are each spaced inwardly a small distance from the adjacent intermediate side panel 12 or 13, thus defining a narrow elongate opening 48 at each side of the upper end of the intermediate region of the enclosure 11.

The shroud 10 is used in association with an endless conveyor track which passes through a loading sta-tion, where articles to be painted are suspended from the track, through one or more paint-spraying shrouds of the type shown in the drawings, through a drying oven where the paint coating or ~ 7 ~ ~28~Z~

coatings are dried, and to an unloading station where the painted and dried items are unloaded, and then returns to the loading station. A portion of the track 49 passing through the paint-spraying shroud 10 is shown in Figure 1. In the example shown, the track 49 is of inverted T-section. The vertical limb of the track may be suspended from the underside of the plate 47 by connectors (not shown). The portion of the track shown comprises an arcuate and part semi-circular loop 51 passing around the centre of the intermediate region of the enclosure 11 and merging with straight sections 52 passing outwardly and parallel to one another through the entrance opening defined between the flanges 17 and 18.
Carriers 53 are disposed in rolling engagement on the track 49. Each carrier has a pair of upper arms, each supporting a roller. The rollers engage on the horizontal limbs of the track 49 on opposite sides of the vertical limb, respectively.
For the sake of clarity of illustration, only a few carriers 53 are shown in Figure 1. ~s will be appreciated, in practice, a carrier 53 is supported at each few inches on the track 49 forming an endless line of carriers extending around the endless track 49. Each carrier 53 is connected to a flexible endless chain 54, and has a downwardly depending hook 56 connected to it. Items to bepainted may be suspended from the hook 56. For example, elongate panels 57 to be painted may be provided with a hole through them adjacent one end, the hook 56 being passed through the hole to connect the panel 57 to the mobile carrier 53. In use, the chain 54, together with the carriers 53 attached to it, is normally driven continuously longitudinally relative to the track 49, so that items to be painted and suspended from the carriers 53 are passed continuously through the shroud 10. Periodically the drive to the chain 54 will be interrupted to permit painted and dried items to be removed from the carriers 53 at the unloading station and to permit fresh items to be painted to be attached to the carriers at the loading station.

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In use, paint is applied to the panels 57, or other items to be painted, from a paint dispensing head 58 which is recip-rocable along a vertical axis aligned with the centre of the intermediate region of the enclosure 11 and coinciding with the centre of the semi-circle which the track 49 describes around the rear portion of the enclosure. Thus, as seen in Figure 2, the items 57 pass in a semi-circular arc around the rear of the enclosure 11, and receive a paint coating at least predominantly on the side facing toward the paint dispenser 58.
Usually, the paint spraying line includes two shrouds each similar to the shroud 10 illustrated, but each disposed oppositely to the other, and with one being disposed with its paint dispenser 58 on the inner side of the endless loop formed by the conveyor and the other with its dispenser 58 on the outer side thereof, so that items receive a coating of paint predominantly on one side in one shroud and pre~ominantly on the opposite side in the other.
In the example illustrated, the paint dispensing structure comprises a frame connected to the upper ends of front and rear supports 22 and 34, the frame comprising a pair of longitudinally extending members 59, each connected at each end to a support 22 or 34, and a pair of spaced transversely extending members 61 connected to the members 59. On the upper side of the members 61 is connected, e.g. by bolts 62, a plate 63, to which is connected e.g. by bolts 64, a pressure cylinder 66. The axis of the cylinder is aligned with the centre of the intermediate region of the enclosure 11. A piston disposed in the cylinder is connected to a vertically-extending rod 67 which can be extended downwardly relative to the cylinder 66 or retracted upwardly relative thereto by controlling a flow of operating fluid, e.g. hydraulic fluid or compressed air, to the cylinder 66. The rod 67 passes with some clearance through a hole 68 in the plate 47 and is connected at its lower end to the paint dispensing head 58. This comprises a circular plate 69, or spinner, connected to a spindle which is connected to a motor 71, whereby the spinner can be rotated at high speed about the 9 ~281~Z~f.~

axis of the rod 67. As shown, the motor 71 is connected to flexible lines 72, which enter the enclosure 11 between the sections 52 of the track 49 at the entrance to the shroud, and which supply power to the motor, e.g. compressed air to a turbine constituting the motor 71, paint to outlet or outlets therefor adjacent the upper side of the spinner 69, and an electrical conductor which applies an electrostatic potential to the head 58, so that there is a potential difference between the head 58 and the items 57 to be painted.

In operation, the spinner 69 may be rotated, for example at about 20,000 r.p.m., while paint is fed along one of the lines 72 to the upper surface of the spinner 69 and is flung off therefrom as a fine mist of paint droplets. These tend to be attracted toward the items 57 to be painted by reason of electrostatic charge which the droplets assume when they leave the electrostatically charged head 58, and the electrostatic field created by the potential difference between the head 58 and the items 57. The piston rod 67 together with the head 58 are reciprocated up and down with controlled stroke and with controlled frequency of reciprocation. The stroke is such that at the extended or lower position, as shown in broken lines in Figure 1, the head 58 is at approximately the level of the lower edge of the items to be painted, and in its retracted or upper position, at approximately the level of the upper edge of the items. The frequency of reciprocation is matched to the speed at which the conveyor chain 54 draws the items along the conveyor rail 49 through the enclosure, so that, during the passage of each item through the enclosure, the head 58 is reciprocated upwardly and downwardly a sufficient number of times to provide the items with an adequate paint coating.

Attached to the underside of the plate 47 there may be a shield in the form of a vertically-extending plate 73 which is spaced inwardly from and is curved -to conform to the shape of - 10 - l~a~2s~

the track 49. The profile of this plate 73 is shown in dashed and dotted lines in Figure 2. The plate 73 shields the track 49 from the head 58 when the latter is in an upper, retracted position, to avoid accumulations of paint on the track 49 which would interfere with free rolling of the carriers 53 along the track.

As will be seen from Figure 2, where various arrows are drawn to illustrate flows of air through the shroud, the draft of the fan 44 results in air being drawn in through the entrance of the enclosure 11. Some air will be drawn into the enclosure through the elongate openings 48 at each side of the cover plate 47 and through the circular opening 68 through which the piston rod 67 passes. The openings 48 need only be sufficient in area to provide a definite down draft adjacent the upper portion of the sides of the enclosure 11, since it is found that this prevents the tendency for a mist of very fine paint droplets to rise toward the upper end of the enclosure and to become deposited there or to escape through any fine openings between the plate 47 and the enclosure 11.
Typically, however, the combined area of the openings 48 and 68 is less than about 10% of the entrance opening defined between the flanges 18 and the plate 47, and thus substantially all the demand of the fan 44 is met by air drawn in through the front opening of the enclosure 11. Since the front opening is relatively constricted as compared with the intermediate region of the enclosure defined between the side panels 12 and 13, the velocity of the air passing through the front opening is relatively high as compared with the air velocity in the intermediate region. Merely by way of example, it may be mentioned that in one embodiment in which the draft of the fan 44 is 7600 cu. ft./min., and the cross-sectional area of the front opening is about 66.5 sq. ft., and that of the inter-mediate region, between the panels 12 and 13, about 140 sq. ft., the air velocity through the front opening will be about 114 ft./min., and that through the intermediate region about 54 2~3~)213~

ft./min. The relatively high velocity inflow of air through the entrance opening eliminates or at least considerably reduces any tendency for the overspray paint mist to escape from the front opening of the enclosure. Loss of paint mist through the front opening is further reduced with the preferred form of enclosure 11, wherein the enclosure 11 extends, in plan, through a relatively large arc about the centre of the intermediate region i.e. the axis of the rod 67 and head 58, and the front opening subtends a correspondingly small arc at said centre. Preferably the enclosure 11 subtends an angle of at least about 310, more preferably at least about 320 about said centre. Within the enclosure, particularly in the relatively wide intermediate region, the velocity of the air flow drops considerably and this is conducive to flow of the paint mist, dispensed from the head 58, in a direction generally transverse of the air flowJ so that there is increased opportunity-for the electrostatically-charged paint mist to become deposited on the items to be painted. Because of the relatively low air velocities within the enclosure 11, the so-called "wrap" or extent of deposition of the paint on the transverse edges and on the rear side of the items is improved, and there is better penetration of the paint into recesses of the items to be painted. In the preferred form, the front portion of the enclosure, defined between the forward panels 14 and 16, narrows continuously to the front opening so that the sides of the enclosure are somewhat streamlined and there is little or no turbulence in theair flow between the front opening and the intermediate region.

With the shroud shown in the drawings, the exhaust openings 31, provided with the filter elements 32, are disposed directly opposite and in a closely-packed array substantially co-extensive with the front opening of the enclosure 11. Thus, except in the region immediately adjacent the openings 48 and 68 and above the level of the upper edge of the uppermost of the openings 31, the air flow through the enclosure 11 tends to ~28~Z8~L
be substantially uniformly parallel to a horizontal plane, transverse to the axis of reciprocation of the paint dispensing head 58. Further, the arrangement shown is readily adapted to apply substantially equal suction to each of the openings 31 and hence to each of the filter elements 32. In the arrangement shown, where suction is applied by the fan 44 to one end, in the example shown the upper end, of the plenum chamber 36, the transverse cross-section of the chamber can be made relatively large in relation to its length, so that the pressure gradient existing along the length of the chamber 36, if any, is negligible, so that substantially equal suction can be applied at each filter 32. The uniforrnly transverse air flQw t~us created tends to distribute the paint mist uniformly over the articles to be painted, thus achieving coatings with desired uniformity of coating thickness, with substantially reduced consumption of paint.

A preferred form of paint mist entrapping air filter is shown in Figure 3. The filter 32 has a rectangular housing 74, a forward edge of which provides the flange 33. Within the housing is mounted a three-layer paint arresting unit comprising three spaced sheets 76, 77 and 78 of stiff paper, card or the like. The front sheet 76 has rearwardly extending edges 79 which provide a box-like enclosure within which the other two sheets 77 and 78 are secured. Each sheet is formed with an array of uniform and uniformly spaced rectangular holes 81. The holes in the middle layer 77 are disposed out of register with those in the first layer 76, preferably such that opposite each hole 81 in the first layer 76, there extends an area of the unperforated paper or card material of the middle layer 77, and, similarly, each hole 81 in the rear layer 78 is out of register with the holes 81 in the middle layer. A three layer paint arresting box-like structure of this kind is in itself known and is available, for example, under the trade mark CORR AIR, from Corr-Craft, Inc., Sanborn, New York. It is highly effective in trapping fine paint mists on its surfaces. Rearwardly of the paint arresting unit, and spaced therefrom by a rectangular - 13 12~2~

spacer 82 extending around the inner side of housing 74 are disposed two filter elements 83, each having a rectangular frame 84 received snugly within and secured within the inner side of the housing 74 and each comprising a rectangular porous filter mat 86, which may be formed of coarse fibrous material e.g.
shredded paper, mat formed into a coherent mat e.g. with adhesives. While other forms of filter may be employed as the fillers 32, those shown are highly preferred for their excellent efficiency in trapping paint mist droplets, small pressure drop and high capacity for receiving paint droplets before requiring replacement as a result of impaired collection efficiency or pressure drop characteristics.

One advantage of the present arrangement is that, as will be appreciated from consideration of Figure 2 and the air flow arrows shown therein, a substantial component of the air flow incident on the front faces of the filter elments 32 is incident at an angle perpendicular to the said front faces.
The applicants have found that this markedly increases the paint droplet collection efficiency of the three layer paint arresting unit comprising the spaced perforated sheets 76, 77 and 78, as compared with arrangements in which the paint droplet-laden air flow is incident on the front faces at an oblique angle.

Over a prolonged period of use, paint droplets become deposited on the inner surfaces of the panels forming the enclosure. In order to collect and to prevent unwanted spreading of the paint thus deposited, which tends to run down the panels, a trough 87 may be provided around the inner side of the enclosure adjacent its lower end. As shown, the trough may comprise a lip extending inwardly and upwardly from each panel 12, 13, 14, 16, 23, 24, 26 and 27 and being secured e.g.
by welding to the inner side of the panel, the ends of the adjacent lips co-operating to form a continuous trough. The ends adjacent the flanges 18 at the front opening of the - 14 _ ~2~2~

enclosure may be closed by end pieces 88. The trough can also serve to collecting washings in the event that solvents, thinners or the like are applied, e.g. through the dispensing head 58 in order to clean the inner surfaces of the shroud. I.iquid accumulating in the trough 87 may be drawn off periodically or continuously through a pipe placed in the trough or through an opening formed through one of the panels in the region of the trough.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A paint spraying shroud comprising vertically extending side walls defining an enclosure having a vertically extending relatively narrow front opening, a cover over the top of the enclosure, means defining an air exhaust opening on a rear side of the enclosure opposite the front opening, the air exhaust opening extending parallel to and to approximately the same vertical extent as the front opening, and the enclosure narrowing in its width from a relatively wide region intermediate the front and the air exhaust openings toward said relatively narrow front opening.

2. A shroud as claimed in claim 1 wherein the enclosure comprises an intermediate section of uniform width and a forward section narrowing in width continuously to the front opening.

3. A shroud as claimed in claim 1 wherein the enclosure narrows in width rearwardly from the intermediate region to said exhaust opening.

4. A shroud as claimed in claim 1 wherein the exhaust opening means comprise a series of closely vertically adjacent exhaust openings.

5. A shroud as claimed in claim 4 wherein each exhaust opening is provided with an air filter for removing air-entrained paint mist.

6. A shroud as claimed in claim 1 wherein the enclosure is in plan symmetrical about a longitudinal median line and comprises intermediate planar parallel side panels on opposite sides of the enclosure at said intermediate region, a forward planar side panel inclining inwardly from a front edge of each intermediate panel to the front opening, a rear side panel inclining inwardly from a rear edge of each intermediate panel to the exhaust opening means, said exhaust opening means comprising two panels each having closely vertically adjacent openings therethrough and inclining rearwardly from a rear edge of a rear side panel to said longitudinal median line.

7. A shroud as claimed in claim 1 wherein the width of the front opening is less than about half the width of the enclosure at its intermediate region.

8, A shroud as claimed in claim 1 wherein the enclosure extends in plan through an arc of at least about 310° about the centre of the relatively wide intermediate region.

9. A shroud as claimed in claim 1 wherein said arc is at least about 320°.

10. A shroud as claimed in claim 1 including a plenum on the rear side of the enclosure co-extensive with and in communication with the air exhaust opening, and means extracting air from the plenum.

11. A shroud as claimed in claim 10 wherein the air extracting means extract air from the top of the plenum.

12. Continuous paint spraying apparatus comprising in combination a shroud as claimed in claim 1, a rotary electro-static paint sprayer reciprocable along a vertical axis extending through the mid point of said intermediate region, a conveyor track extending beneath said cover along a line extending through the upper end of the front opening and in an arc around the axis of the paint sprayer and carriers supported on and movable along the track for suspension therefrom of articles to be painted.