WO2002034416A1

WO2002034416A1 - Exhaust duct for coating devices of the type which provide coatings on one or opposite surfaces of a substrate

Info

Publication number: WO2002034416A1
Application number: PCT/US2001/042800
Authority: WO
Inventors: Michael Kreiberg; Doug Edwards
Original assignee: Material Sciences Corporation
Priority date: 2000-10-27
Filing date: 2001-10-29
Publication date: 2002-05-02
Also published as: AU2002211924A1

Abstract

An exhaust duct (110) is provided for, among other things, use in coating devices of the type which provide coatings on one or opposite surfaces of the substrate. The exhaust duct facilitates removal of excess coating substances from a coating device while keeping the coating substance associated with one surface of the substrate away from the coating substance which is associated with an opposite surface of the substrate. The exhaust duct (110) also facilitates withdrawal of a coating substance from a dispersion of the coating substance in a substantially uniform manner. The exhaust duct (110) comprises an elongated triangular body which is hollow to define a cavity (114) in the elongated body (112), an entrance (116) into the cavity (114) for the coating substance, and an exit port (118) in the elongated body (112). The entrance (116) extends longitudinally along the elongated body (112). The cavity (114) of the elongated body (112) has a cross-sectional area which progressively decreases toward longitudinal ends of the elongated body so that, when pressure at the exit port is less than atmospheric pressure, the entrance exhibits a pressure reduction relative to atmospheric pressure which is substantially uniform along a length of the entrance. This, in turn, facilitates withdrawal of the coating substance into the cavity through the entrance in a substantially uniform manner.

Description

EXHAUST DUCT FOR COATING DEVICES OF THE TYPE WHTCH PROVIDE COATINGS ON ONE OR OPPOSITE SURFACES OF A SUBSTRATE

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an exhaust duct for coating devices that provide coatings on one or opposite surfaces of a substrate and, more specifically, to an exhaust duct which efficiently and effectively removes excess coating substances from opposing volumes adjacent the substrate.

2. Description of Related Art

Coating devices of the type which provide coatings on opposite surfaces of a substrate are generally known. Although the present invention is not limited to use in conjunction with devices that electrostatically apply powder coatings, the use of the present invention with such coating devices is particularly advantageous. Examples of electrostatic coating devices which coat a substrate with a powder are disclosed in U.S. Patent Nos. 4,582,718; 4,795,339; and 5,279,863; which are hereby incorporated herein by reference.

Electrostatic coating device may produce an aerated dispersion of powder. The powder is electrostatically attracted to the substrate to provide a desired coating. If excess powder is not withdrawn uniformly about the substrate, the dispersion is negatively influenced aerodynamically from its initial state of aeration. The resulting distortion of the powder dispersion prevents the powder from being deposited uniformly on the surface of the substrate. The coating which is provided as a result of the coating process therefore lacks the uniformity which is highly desirable in most coating arrangements. It is therefore important to withdraw the excess powder uniformly about the substrate.

The task of providing uniform withdrawal of the powder, however, is especially difficult in applications where wide webs are being coated. In the coil coating industry, for example, it is not unusual for the webs to range in width up to 84 inches and, in some instances, up to 108 inches. Despite the widths of such webs, certain efficiencies are realized by providing a single outlet port to a reclamation apparatus and by providing this single outlet port with a compact cross-sectional area. These efficiencies are difficult to realize, however, without compromising the uniformity of the powder's withdrawal. Typically, a reduction in the cross-sectional area of the single outlet port or an increase/decrease in the number of ports will have an adverse effect on the uniformity of powder withdrawal. The difficulty in balancing the reduction in cross-sectional area of the outlet port with the uniformity of powder withdrawal is exacerbated if collections of resettled powder occur within the duct through which the powder is withdrawn. Collections of powder change the duct's effective internal geometry and thus affect the uniformity of airflow through the duct. Also, settled powder inside the duct adversely affects time for a color change-over by requiring cleaning of the duct.

To avoid these problems, it is desired that all collected powder remain entrained in the exhaust airflow, and that it does not collect in any portions of the duct, including the lower portions of the duct toward which the powder tends to be settled by gravity. An additional problem concerns application of differently colored powder to the surfaces. It is important that the powder coating one surface not be drawn over onto the opposite surface and thereby mar its color.

There is consequently a need in the art for an exhaust duct which facilitates withdrawal of excess coating substances or powder from around a substrate in a substantially uniform manner while keeping the coating substance associated with one side of the substrate away from the coating substance associated with an opposite side of the substrate.

SUMMARY OF THE INVENTION A primary object of the present invention is to provide an exhaust duct and exhaust duct system for coating devices of the type which provide coatings on one or opposite surfaces of a substrate and, more specifically, an exhaust duct and exhaust duct system which facilitate removal of excess coating substances from a coating device (or from a dispersion of the coating substance in the coating device) while keeping the coating substance associated with one surface of the substrate away from the coating substance associated with an opposite surface of the substrate.

Yet another object of the present invention is to provide an exhaust duct and exhaust duct system capable of uniformly withdrawing excess powder from a powder dispersion along the width of a substrate, so that distortion of the powder dispersion's initial state of aeration is avoided and the powder is deposited uniformly on the surface of the substrate.

Another object of the present invention is to provide an exhaust duct which, in use, keeps all collected powder entrained in the exhaust airflow and thus prevents the powder from collecting in the duct, including the lower portions of the duct where the powder tends to collect by gravity forces.

Still another object of the present invention is to provide a coating device which provides an interface between two adjacent coatings that is substantially straight and extends longitudinally along the middle of each lateral edge of the substrate, with little or no overlapping of the adjacent coatings.

Yet another object of the present invention is to provide an exhaust duct and exhaust duct system in a coating apparatus which do not physically contact the moving substrate and therefore are not subjected to wear which otherwise results from such contact.

To achieve these and other objects, the exhaust duct of the present invention comprises an elongated body which is hollow to define a cavity in the elongated body, an entrance into the cavity for the coating substance, and an exit port in the elongated body. The entrance extends longitudinally along the elongated body. The exit port is generally located substantially equidistant from longitudinal ends of the elongated body. The cavity of the elongated body has a cross-sectional area which progressively decreases toward longitudinal ends of the elongated body so that, when negative pressure is applied at the exit port, the entrance exhibits a pressure reduction relative to atmospheric pressure which is substantially uniform along the length of the entrance. This, in turn, facilitates withdrawal of the coating substance into the cavity through the entrance in a substantially uniform manner.

In one embodiment of the invention, a single exhaust duct is disposed along each opposite lateral edge of the substrate. In a second embodiment, the exhaust duct system comprises a first exhaust duct for placement laterally along the dispersion of the coating substance and a second exhaust duct for placement along an opposite side of the dispersion from the first exhaust duct.

In the preferred embodiment, the elongated body defines a substantially equilateral triangle. The above and other objects and advantages will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a fragmentary cross-sectional view of an exhaust duct system according to one embodiment of the present invention.

Figure 2 is a fragmentary cross-sectional view of one of the pair of exhaust ducts illustrated in Figure 1. Figure 3 is a perspective view of one of a pair of exhaust ducts according to another embodiment of the present invention.

Figure 4 is a fragmentary perspective view of one of the exhaust ducts illustrated in Figure 3.

Figure 5 is a plan view of an exhaust duct system according to a preferred embodiment of the present invention.

Figure 6 is a side elevational view of the exhaust duct system illustrated in Figure 3.

Figure 7 is a schematic view of a coating device having an exhaust duct system, according to a preferred embodiment of the present invention, taken along line V-V of Figure 6.

Figure 8 is a cross-sectional view of the pair of exhaust ducts illustrated in Figure 3.

DETAILED DESCRIPTION OF THE INVENTION As best shown in Figure 1 , a pair of exhaust ducts 10 is provided along opposite lateral edges la of substrate 1 for facilitating withdrawal of first and second coating substances deposited on substrate 1. Substrate 1 may be a continuous length of strip material, such as steel, aluminum, or the like, although semi-conductive polymeric materials, glass, cloth, wood and the like may be coated through the electrostatic coater in combination with the disclosed invention. The ducts 10 withdraw first and second coatings substances or powders from first and second aerated dispersions 2, 4 of the coating substances in a substantially uniform manner. The coating substances preferably are powder paint, although various thermosetting, thermoplastics, adhesives and the like including corn starch or similar substances, may be applied. Each respective duct 10 comprises an elongated body 20 which is hollow to define a cavity 28 in the exhaust duct 10, an entrance 21, as best shown in Fig. 2, for the substrate into the cavity 28, and an exit port 22 in the elongated body 20. The entrance 21 extends longitudinally along the elongated body 20 to accommodate a lateral edge la of a substrate 1 so that the elongated body 20 straddles the lateral edge la without contacting the edge la of substrate 1. The entrance 21 is sufficiently wide to accommodate the deflections which substrate 1 may exhibit as the substrate 1 moves through the electrostatic (or other equivalent) coater. The exit port 22 is generally located substantially equidistant from longitudinal ends of the elongated body 20 or where the maximum density of aerated material occurs.

The elongated body 20 has a generally triangular cross-section, with an apex of the generally triangular cross-section being truncated to define the entrance 21. The elongated body 20 further comprises a substantially vertical plate 25 which extends upwardly from a top of the entrance 21 to prevent clumps of coating substance which may accumulate on a top surface of the elongated body 20 from falling onto the substrate 1.

As best shown in Fig. 2, the elongated body 20 of each exhaust duct 10 preferably comprises a first substantially planar wall 32, a second wall 34, and a third wall 36. Because of gravity, the coating substance in the top dispersion 2 is drawn more easily toward the substrate 1 than the coating substance in the bottom dispersion 4. The bottom dispersion 4 therefore is produced with more energy than the top dispersion 2, and has a higher "dispersion velocity" than the top dispersion 2. In some instances, if compensation is not provided for the differences between the top and bottom dispersions 2, 4, a small amount of "wraparound", typically of the top surface substance onto the bottom surface of the substrate 1, may occur. Compensation may be achieved by off-setting the exit port 22 toward the second wall 34 or top dispersion 2. For example, the exit port 22 illustrated on the right side of Figure 1 may be offset toward the upper (second) wall 34 whereby the distance 'x' is shorter than the distance 'y'. The second embodiment described below also helps eliminates the aforementioned wraparound effect.

The exhaust duct system illustrated in Figures 1 and 2 is a simplified embodiment, to the extent that a single exhaust duct 20 is located at each lateral edge la of the substrate 1 to facilitate simultaneous withdrawal of coating substances from both dispersions 2, 4. During such withdrawal, the exhaust duct system illustrated in Figures 1 and 2 advantageously facilitates the application of coatings not only on the opposite surfaces of the substrate 1 but also on the lateral edges la of the substrate 1. The longitudinal placement of the exhaust ducts in the embodiments of

Figures 1 and 2, as well as the embodiments set forth below, advantageously keeps the geometry of the exhaust ducts biased slightly such that the pick up area of the ducts is located in the most dense region of the powder dispersions, versus the otherwise central location. Further, the generalized shaped of the body 20 is similar in form to an equilateral triangle, which is a preferred shape of the exhaust systems set forth in this invention.

The various exhaust ducts described herein may be manufactured using any suitable materials. When the coating process being carried out includes electrostatic application of the coatings substance(s) or coating powder(s), the exhaust ducts preferably are made of a non-conductive material, such as plastic. A preferred plastic is polycarbonate. Since many other materials would suffice, it is understood that the present invention is not limited to the particular materials or groups of materials which are expressly described in this disclosure.

Figures 3-8 illustrate an exhaust duct 110 according to a second preferred embodiment of the present invention. The exhaust duct 110 includes an elongated body 112. The elongated body 112 is hollow and therefore defines a cavity 114 (see Fig. 4). An entrance 116 into the cavity 114 is provided so that a coating substance can be drawn into the cavity 114 from an aerated dispersion volume of the coating substance located adjacent to the entrance 116. The entrance 116 extends longitudinally along the elongated body 112.

The exhaust duct 110 also includes an exit port 118 in the elongated body 112. The exit port 118 may be located substantially equidistant from longitudinal ends 120, 122 of the elongated body 112 or where the most densely aerated volume is located. Therefore, the elongated body 112 defines an equilateral triangle in the preferred embodiment.

With reference to Figure 4, the cavity 114 has a cross-sectional area which progressively decreases toward the longitudinal ends 120, 122 of the elongated body 112. In particular, the cross-sectional area decreases from a maximum at the central portion of duct 110 to a minimum at the opposite longitudinal ends 120, 122. Thus, when pressure at the exit port 118 is less than atmospheric pressure, the entrance 116 exhibits a pressure reduction relative to atmospheric pressure which is substantially uniform along a length of the entrance 116. This, in turn, facilitates withdrawal of the coating substance into the cavity 114 through the entrance 116 in a substantially uniform manner. Such withdrawal therefore is performed without significantly disrupting the uniform distribution of coating substance in the dispersion.

With reference to Figure 4, the elongated body 112 preferably includes a first substantially planar wall 124, a second wall 126, and a third wall 128. The first wall 124 is connected at an acute angle to the second wall 126 at a first interface 130. The first wall 124 has a first free edge 132 opposite to the first interface 130. The third wall 128 is likewise connected at an acute angle to the second wall 126 at a second interface 134. The third wall 128 has a free edge 136 opposite to the second interface 134. Preferably, the free edge 136 extends substantially parallel to the first free edge 132 of the first wall 124.

The first and second interfaces 130, 134 converge toward the longitudinal ends 120, 122 of the exhaust duct 110 to provide the progressively decreasing cross- sectional area which decreases progressively toward the longitudinal ends 120, 122. The first free edge 132 is spaced apart from the free edge 136 of the third wall 128 to define the entrance 116 therebetween. In the illustrated embodiment, since the free edge 136 extends substantially parallel to the first free edge 132 of the first wall 124, the entrance 116 has a constant width over its entire length. Of course, certain variations may be possible without departing from the spirit and scope of this invention. Figure 5 is a top view showing substrate 142 travelling in the direction of arrow 172 between exhaust ducts 110, 110' disposed on opposite edges of the substrate 142. The ducts 110, 110' are essentially the same in configuration symmetrically disposed on opposite edges of the substrate 142. As illustrated in Figure 5, a first duct system 150 for facilitating withdrawal of a coating substance from a dispersion of the coating substance in a substantially uniform manner is provided by locating one of the exhaust ducts 110 of Figures 3 and 4 on each lateral side 146 of the substrate 142. The resulting duct system 150, when provided with a pressure which is less than atmospheric pressure at the exit port 118, withdraws the coating substance uniformly from opposites sides of the dispersion.

Referring to Figures 5, 6 and 7, the first wall 124 of the first exhaust duct 110 is substantially coplanar with the first wall 124 of the second exhaust duct 110', and is spaced apart therefrom to accommodate the substrate 142 therebetween (see Fig. 7). The elongated bodies 112 of the ducts 110 are arranged with respect to one another and spaced so that placement of the substrate 142 between the first walls 124 of the exhaust ducts 110 causes the third wall 128 of each exhaust duct 110 to overlap a respective lateral side 146 of the substrate 142. Preferably, the first walls 124 are substantially horizontal so that, when the substrate 142 is located horizontally therebetween, the third walls 128 overlap the respective lateral sides 146 of the substrate 142 and the acute angle 144 is formed between the substrate 142 and the plane 140 at each lateral side 146 of the substrate 142. This acute angle 144 and the overlapping cause the coating substance to be drawn at an oblique angle out and away from the substrate 142 and generally prevents the coating substance from "wrapping around" the substrate 142. The surface of the substrate 142 which faces away from the dispersion therefore is not contaminated by the coating substance on the dispersion-facing surface.

As used in the present disclosure, the term "overlap" or "overlapping" is not limited by which of the overlapping elements is above the other. In this regard, the expression "A overlapping B" encompasses not only arrangements where A is at least partially over B but also arrangements where B is at least partially over A. In addition, the term "overlap" or "overlapping" is not limited to arrangements where there is contact between the elements which are described as being overlapped. Thus, there may be some separation between "overlapping" elements.

Notably, contamination of the surface which faces away from the dispersion is avoided without requiring the substrate 142 to touch the ducts 110 and without the substrate 142 having to physically bear against any type of partition as it is conveyed. When different coatings are applied to opposite surfaces of the substrate 142, a dispersion of coating substance is provided on each surface of the substrate 142. It is important in such an arrangement to keep the different coatings from "wrapping around" the lateral edges of the substrate 142 and cross-contaminating one another. The different coatings may, for example, include coatings of different colors, coatings of different powder types, coatings of different density, etc.

As illustrated in Figure 6, such cross-contamination can be avoided using an exhaust duct system 152 according to a second embodiment of the present invention. The exhaust duct system 152 facilitates withdrawal of coating substances in a substantially uniform manner from respective dispersions of the coating substances located on opposite surfaces of the substrate 142 to be coated. The exhaust duct system 152 includes first and second exhaust ducts 110 for placement laterally along the respective dispersions. The exhaust ducts 110 preferably are coextensive with one another.

The cavity 114 of one exhaust duct 110 is divided from the cavity 114 of the other by a partition 154. The first walls 124 are defined by opposite, parallel surfaces of the partition 154. Preferably, the partition 154 is substantially horizontal and one of the exhaust ducts 110 is arranged above the other exhaust duct 110 as a mirror image of the top exhaust duct 110 taken along the partition 154.

As illustrated in Figures 7 and 8, the partition 154 has a lateral partition edge 156, which accommodates a lateral edge 148 of the substrate 142. When the lateral edge 148 of the substrate 142 is brought close to the lateral partition edge 156, the third walls 128 of the top and bottom exhaust ducts 110 overlap a lateral portion 146 of the top and bottom surfaces, respectively, of the substrate 142. Each of the planes 140 forms the acute angle α with the substrate 142 when the lateral edge 148 of the substrate 142 is brought close to the lateral partition edge 156.

The resulting overlap of each angled entrance 116 with a respective surface of the substrate 142 causes withdrawal of the coating substances from each of the dispersions to occur obliquely out and away from the respective surfaces. The different coating substances therefore do not cross-contaminate or overlap one another on opposite surfaces of the substrate 142. Instead, a substantially uniform interface between the coating substances is created medially along the lateral edge 158 of the substrate 142. As shown in Figure 7, all of the advantages of the previous embodiments can be realized by providing a pair of exhaust ducts 110 of the type illustrated in Figures 6 and 8 at each lateral side 146 of the substrate 142. The exhaust duct system illustrated in Figure 7 facilitates withdrawal of coating substances in a substantially uniform manner from respective airborne dispersions of different coating substances located on opposite surfaces of the substrate 142 to be coated.

Each pair of entrances 116 of the exhaust duct pairs overlaps opposite surfaces of the substrate at the lateral portions 146 when the substrate 142 is horizontally arranged between the pairs of exhaust ducts 110. Withdrawal of the coating substances therefore occurs obliquely out and away from each of the opposite surfaces of the substrate 142, and a substantially uniform interface of coating substances is created medially along the lateral edges 148 of the substrate 142.

The illustrated arrangement advantageously facilitates withdrawal of excess coating substances or powder from around the substrate 142 in a substantially uniform manner while realizing the efficiencies associated with a single exit port 118 of relatively small cross-sectional area. Notably, such uniform withdrawal is provided by the illustrated arrangement even if the substrate 142 is thick.

The equilateral shape of each exhaust duct's cavity 114 also facilitates formation of a stable vortex of airborne powder which, in turn, provides a relatively high airflow velocity near the walls 124, 126, 128 of the duct 110 and which effectively prevents settling of the coating substance or powder in the duct 110. By providing the stable vortex that prevents settling of the coating substance, the system prevents unwanted shape change in the duct and reduces the color-change cleanout requirement discussed above. When used in conjunction with a powder coating device, the exhaust duct 110 advantageously produces a substantially uniform static pressure distribution across its entrance 116 so that withdrawal of the powder into the exhaust duct 110 does not disturb the uniformity of the powder dispersion in the coating device. Because the exhaust duct systems of the present invention avoid cross- contamination without requiring physical contact between the substrate 142 and the partition 154, both surfaces of the substrate 142 can be coated simultaneously without experiencing many of the problems which arise with other coating and exhaust techniques. There is no need for the different coatings to be applied successively, nor is there a need for an intermediate curing step.

The resulting four-duct arrangement shown in Figure 7 better maintains the uniformity of both dispersions and therefore tends to provide more uniform coatings on the substrate 142. The four-duct arrangement also advantageously provides a straight and uniform interface between the different coatings on the medial portion 148 of the substrate's lateral edges 148 and prevents cross-contamination of each surface of the substrate 142 with coating substances pertaining to the other surface. Although the present invention can be practiced using the same coating substance on both surfaces of the substrate 142, it is noteworthy that the present invention can be practiced using different coatings without experiencing the problems typically associated with the use of different coating substances (e.g., cross- contamination, successive coating steps, an intermediate curing step, and the like).

The illustrated arrangement advantageously has separate ducts 110 for the top and bottom surfaces of the substrate 142, as well as for the lateral sides of the substrate 142. Since the reduction in pressure applied to each of the exit ports 118 can be varied independently, it is possible to control the rate of withdrawal through each of the ducts 110 independently of the others. Should cross-contamination or uneven coatings begin to appear on or between the surfaces of the substrate 142, the air pressure reduction at each exit port 118 can be varied until the cross-contamination is sufficiently reduced or altogether eliminated and uniformity of the coatings is again achieved. The present invention thereby provides compensation for the types of cross- contamination problems which could arise, for example, because of physical differences in the coating substances (or powders) or because of the differences in gravitational effect between the coating process on the top surface of the substrate and the coating process on the bottom surface.

Since the entrance 116 of each duct 110 spans substantially the entire distance covered by the powder dispersions longitudinally along the substrate 142, the powder is uniformly withdrawn from substantially the entire length of each powder dispersion. When placed on opposite lateral sides 146 of the substrate 142, each exhaust duct 110 can be used to laterally delimit a powder coating chamber and to define the lateral sides of the coating device.

The preferred exhaust ducts illustrated in some drawings generally have outside shapes which resemble the cross-sectional shapes of their respective cavities. The present invention, however, can be practiced using baffles which extend longitudinally through the inside of the ducts to define the cross-sectional shape of the cavities and/or provide the progressively decreasing cross-sectional area, e.g. FIGS. 1 and 2. In such embodiment, the exterior appearance of the exhaust ducts would not necessary resemble the cross-section shape of their respective cavities. While this invention has been described as having a preferred design, it is understood that the invention is not limited to the illustrated and described features. To the contrary, the invention is capable of further modifications, usages, and/or adaptations following the general principles of the invention and therefore includes such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the central features set forth above, and which fall within the scope of the appended claims.

Claims

CLAIMSWe claim:

1. An exhaust duct for facilitating withdrawal of a coating substance from a dispersion of said coating substance in a substantially uniform manner, comprising: a hollow body defining a cavity in the exhaust duct; an entrance into said cavity for said coating substance, said entrance extending in a longitudinal direction along the hollow body; and an exit port in the hollow body, wherein said cavity defines a generally triangular cross-sectional area transverse to said longitudinal direction.

2. The exhaust duct of claim 1 , wherein said exit port is located substantially equidistant from longitudinal ends of the hollow body.

3. The exhaust duct of claim 1, wherein the cavity of said hollow body defines a second cross-sectional area which progressively decreases toward longitudinal ends of the elongated body so that, when pressure at said exit port is less than atmospheric pressure, said entrance exhibits a pressure reduction relative to atmospheric pressure which is substantially uniform along a length of said entrance, to facilitate withdrawal of said coating substance into the cavity through said entrance in a substantially uniform manner.

4. The exhaust duct of claim 1, wherein said hollow body comprises: a first substantially planar wall; a second wall connected at an acute angle to the first wall, to define a first interface between the first and second walls, said first wall having a first free edge opposite to said first interface; and a third wall connected at an acute angle to said second wall, to define a second interface between the second and third walls, said third wall having a free edge opposite to said second interface and extending substantially parallel to the first free edge of the first wall, wherein said first free edge is spaced apart from the free edge of the third wall to define said entrance.

5. The exhaust duct of claim 4, wherein said first and second interfaces converge toward longitudinal ends of the exhaust duct to provide said cross-sectional area which progressively decreases toward longitudinal ends of the hollow body.

6. The exhaust duct of claim 4, wherein an apex of the generally triangular cross-section is truncated at the first free edge and at the free edge of the third wall, to define said entrance.

7. The exhaust duct of claim 4, wherein said exit port is located through said second wall.

8. The exhaust duct of claim 1, wherein an apex of the generally equilateral triangle is truncated to define said entrance.

9. The exhaust duct of claim 1 , wherein the cross-sectional area of said hollow body remains substantially constant through a medial portion of said hollow body and progressively decreases outside of said medial portion toward said longitudinal ends.

10. The exhaust duct of claim 1, wherein said entrance is sized to receive and partly surround overlap an edge of a substrate being treated by said dispersion of said coating substance along said longitudinal direction.

11. An exhaust duct system for facilitating withdrawal of a coating substance from a dispersion of said coating substance in a substantially uniform manner, comprising: at least one first exhaust duct for placement laterally along said dispersion of the coating substance; at least one second exhaust duct for placement along an opposite side of said dispersion from said first exhaust duct, wherein each of said first and second exhaust ducts includes: an elongated body which is hollow to define a cavity in each of the first and second exhaust ducts; an entrance into said cavity for said coating substance, which extends in a longitudinal direction, along the elongated body; and an exit port in the elongated body, said exit port being located opposite said entrance, and wherein each cavity defines a generally equilateral triangle in a cross-section taken transverse to said longitudinal direction.

12. The exhaust duct system of claim 11 , wherein said exit port is substantially equidistant from longitudinal ends of the elongated body.

13. The exhaust duct system of claim 11, wherein the cavity of each elongated body has a cross-sectional area which progressively decreases toward longitudinal ends of the elongated body so that, when pressure at said exit port is less than atmospheric pressure, said entrance exhibits a pressure reduction relative to atmospheric pressure which is substantially uniform along a length of said entrance, to facilitate withdrawal of said coating substance into the cavity through said entrance in a substantially uniform manner.

14. The exhaust duct system of claim 11 , wherein each elongated body comprises: a first substantially planar wall; a second wall connected at an acute angle to the first wall, to define a first interface between the first and second walls, said first wall having a first free edge opposite to said first interface; and a third wall connected at an acute angle to said second wall, to define a second interface between the second and third walls, said third wall having a second free edge opposite to said second interface, wherein said first free edge is spaced apart from the free edge of the third wall to define said entrance.

15. The exhaust duct system of claim 14, wherein the first wall of said first exhaust duct is spaced apart from the first wall of said second exhaust duct to accommodate a substrate therebetween.

16. The exhaust duct system of claim 15, wherein the elongated body of said first exhaust duct and the elongated body of said second exhaust duct are arranged with respect to one another so that, when said substrate is located between said first wall of the first exhaust duct and said first wall of the second exhaust duct, said third wall of the first exhaust duct overlaps a first lateral side of the substrate and said third wall of the second exhaust duct overlaps an opposite lateral side of the substrate.

17. The exhaust duct system of claim 14, wherein said first wall of the first exhaust duct and said first wall of the second exhaust duct are substantially horizontal so that, when the substrate is located horizontally therebetween, said third wall of the first exhaust duct overlaps the first lateral side of the substrate and said third wall of the second exhaust duct overlaps the opposite lateral side of the substrate.

18. The exhaust duct system of claim 14, wherein each elongated body of the first and second exhaust ducts has a generally triangular cross-section with an apex of the generally triangular cross-section being truncated at the first free edge and at the free edge of the third wall, to define said entrance.

19. The exhaust duct system of claim 11, wherein the cross-sectional area of said elongated body of said first and second exhaust ducts remains substantially constant through a medial portion of said elongated body and progressively decreases outside of said medial portion toward said longitudinal ends.

20. The exhaust duct system of claim 11 , wherein at least two exhaust ducts are provided along opposite sides of said dispersion.

21. A combination exhaust duct and electrostatic coater for facilitating withdrawal of a coating substance from a dispersion of said coating substance for coating a substrate in a substantially uniform manner, comprising: an electrostatic coater; an elongated substrate; an exhaust duct; a hollow body defining a cavity in the exhaust duct; an entrance into said cavity for said coating substance, said entrance extending in a longitudinal direction along an elongated edge of the substrate; and an exit port in the hollow body, wherein said cavity defines a generally triangular cross-sectional area transverse to said longitudinal direction.

22. The exhaust duct of claim 21, wherein said exit port is located substantially equidistant from longitudinal ends of the substrate.

23. The exhaust duct of claim 21 , wherein the cavity of said hollow body defines a second cross-sectional area which progressively decreases toward longitudinal ends of the elongated body so that, when pressure at said exit port is less than atmospheric pressure, said entrance exhibits a pressure reduction relative to atmospheric pressure which is substantially uniform along a length of said entrance, to facilitate withdrawal of said coating substance into the cavity through said entrance in a substantially uniform manner.

24. The exhaust duct of claim 21, wherein said hollow body comprises: a first substantially planar wall; a second wall connected at an acute angle to the first wall, to define a first interface between the first and second walls, said first wall having a first free edge opposite to said first interface; and a third wall connected at an acute angle to said second wall, to define a second interface between the second and third walls, said third wall having a free edge opposite to said second interface and extending substantially parallel to the first free edge of the first wall, wherein said first free edge is spaced apart from the free edge of the third wall to define said entrance.

25. The exhaust duct of claim 24, wherein said first and second interfaces converge toward longitudinal ends of the exhaust duct to provide said cross-sectional area which progressively decreases toward longitudinal ends of the hollow body.

26. The exhaust duct of claim 24, wherein an apex of the generally triangular cross-section is truncated at the first free edge and at the free edge of the third wall, to define said entrance.

27. The exhaust duct of claim 21 , wherein said entrance overlies and substantially circumscribes an edge of said substrate.

28. The exhaust duct of claim 21, wherein an apex of the generally equilateral triangle is truncated to define said entrance, said substrate being disposed partially within said entrance.

29. The exhaust duct of claim 21 , wherein the cross-sectional area of said hollow body remains substantially constant through a medial portion of said hollow body and progressively decreases outside of said medial portion toward said longitudinal ends.

30. A method of withdrawing a first coating substance from a first dispersion of said first coating substance located at a first surface of a substrate, said method comprising the steps of: reducing pressure below atmospheric pressure at a portion of an exhaust duct where an exit port of said exhaust duct is located, to generate airflow through a cavity of the exhaust duct toward the exit port; and drawing said first coating substance, using said airflow, into an entrance of the exhaust duct disposed longitudinally between longitudinal ends of the exhaust duct, wherein said step of drawing is performed at an acute angle with respect to said first surface of the substrate so that, outside of said cavity, said first coating substance flows from the first dispersion into said entrance in a generally oblique direction out and away from said first surface of the substrate to prevent said first coating substance from coating the substrate beyond a first-surface side of a medial portion of a first lateral edge of the substrate.

31. The method of claim 30, wherein said method further provides withdrawal of a second coating substance from a second dispersion of said second coating substance located at a second surface of the substrate, said method further comprising the steps of: reducing pressure below atmospheric pressure at a portion of a second exhaust duct where an exit port of said second exhaust duct is located, to generate airflow through a cavity of the second exhaust duct toward the exit port thereof; and drawing said second coating substance, using said airflow through the cavity of the second exhaust duct, into an entrance of the second exhaust duct disposed longitudinally between longitudinal ends of the second exhaust duct, wherein said step of drawing said second coating substance is performed at an acute angle with respect to said second surface of the substrate so that, outside of said cavity of the second exhaust duct, said second coating substance flows from the second dispersion into said entrance of the second exhaust duct in a generally oblique direction out and away from said second surface of the substrate to prevent said second coating substance from coating the substrate beyond a second-surface side of said medial portion.

32. The method of claim 31, further comprising the steps of: reducing pressure below atmospheric pressure at a portion of a third exhaust duct where an exit port of said third exhaust duct is located, to generate airflow through a cavity of the third exhaust duct toward the exit port thereof, said third exhaust duct being located along an opposite lateral edge of said substrate from said first lateral edge; drawing said first coating substance, using said airflow through the cavity of the third exhaust duct, into an entrance of the third exhaust duct which extends longitudinally between longitudinal ends of the third exhaust duct, reducing pressure below atmospheric pressure at a medial portion of a fourth exhaust duct where an exit port of said fourth exhaust duct is located, to generate airflow through a cavity of the fourth exhaust duct toward the exit port thereof; and drawing said second coating substance, using said airflow through the cavity of the fourth exhaust duct, into an entrance of the fourth exhaust duct which extends longitudinally between longitudinal ends of the fourth exhaust duct, said fourth exhaust duct being located along said opposite lateral edge of said substrate; and wherein said step of drawing said first coating substance into the third exhaust duct is performed at an acute angle with respect to said first surface of the substrate so that, outside of said cavity, said first coating substance also flows from the first dispersion into said entrance of the third exhaust duct in a generally oblique direction out and away from said first surface of the subsfrate to prevent said first coating substance from coating the substrate beyond a first-surface side of a medial portion of the opposite lateral edge of the substrate, and wherein said step of drawing said second coating substance into the fourth exhaust duct is performed at an acute angle with respect to said second surface of the substrate so that, outside of said cavity of the fourth exhaust duct, said second coating substance flows from the second dispersion into said entrance of the fourth exhaust duct in a generally oblique direction out and away from said second surface of the substrate to prevent said second coating substance from coating the substrate beyond a second-surface side of the medial portion of said opposite lateral edge.