Abstract:
A conveyor oven has two insulated cabinets, each cabinet having two plenums for conducting heated air toward a printing plate that rests on a conveyor. The two plenums in each cabinet face each other and are substantially identical. Each plenum has a supply and return duct assembly located above the conveyor, and is supplied by a fan and heater arrangement located below and underneath the conveyor. An insulated intermediate chamber is disposed between the exit of the first upstream cabinet and the second, downstream cabinet. With this arrangement, the conveyor carries a printing plate through the first cabinet, where the plate is heated, then through the intermediate insulated chamber, where it is maintained at a heated temperature, and then into the second cabinet where it is again heated. It is then conveyed out of the second cabinet and out of the oven by the conveyor. The system is compact—so compact, in fact, that it permits a trailing portion of the printing plate to be heated in the first cabinet at the same time a middle portion is in the intermediate chamber, and a leading portion is heated in the second cabinet.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to conveyor ovens and, more particularly, to ovens in which a plate or the like is baked by directing hot air downwardly onto the upper surface of the plate from above so as to heat the upper surface of the plate uniformly. The invention is particularly useful as a pre-bake oven in a print plate imaging and processing system. The invention additionally relates to a method of using a pre-bake oven. 
     2. Discussion of the Related Art 
     So-called conveyor ovens are well known for baking plates and other relatively flat articles. Conveyor ovens are characterized by an oven having an opening through which extends a conveyor. The conveyor transports the article to be baked through the oven at a designated rate such that the article is heated to a desired temperature as it is conveyed through the oven. Conveyor ovens are used in a variety of applications. 
     For example, in direct print plate imaging and processing systems, conveyor ovens are used to heat print plates prior to development in order to render the background areas of the image soluble in the downstream alkaline developer of the system while simultaneously rendering the image areas insoluble. Precise and consistent heating of the print plate is essential. If the pre-baking or pre-heating step results in more than about a 2° C. temperature variation across the print plate&#39;s surface, adverse effects will occur. For instance, if any portions of the plate are overheated, a thermal fog, having an appearance similar to so-called “light fog” found in conventional plates, will form in the overheated areas. Conversely, if uneven or imprecise heating leads to unacceptably low temperatures on portions of the plate, polymers in the portions of the plate which are insufficiently heated will fail to cross-link sufficiently, resulting in a weakened or removed image. Many conveyor ovens which were heretofore available did not provide adequate precision and uniformity of heating to operate acceptably as pre-bake ovens. 
     Conveyor ovens are also widely used in other applications such as post-bake ovens in print plate imaging and finishing systems. One such oven is manufactured by Wisconsin Oven Corporation of East Troy, Wis. and marketed as the SPC-HTS/109 Series. This oven works quite well as a post-bake oven but exhibits a relatively high profile because the heating elements, blower, and associated ductwork are all located above the conveyor. In addition, the configuration of the ductwork linking the heat source to the conveyor is less than optimal for height minimization purposes. As a result, this oven and others of its type have an overall height on the order of 74″ or more. The relatively high profiles exhibited by these ovens render them somewhat unattractive in applications in which space constraints mandate ovens having the lowest-possible profile. 
     Many conveyor ovens which were heretofore available also were somewhat inefficient because they employed little or no air recirculation such that all or at least a substantial portion of the air used to bake the subject article was heated from ambient temperature to the working temperature. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore a primary object of the invention to provide a conveyor oven which is capable of precisely and uniformly heating an article to be baked as that article is conveyed through the oven at a designated speed. 
     Another object of the invention is to provide a conveyor oven which is well-suited for use in applications where space constraints mandate an oven with a relatively low profile. 
     Another object of the invention is to provide a plurality of insulated oven cabinets permitting multiple and substantially discrete regions of temperature control. 
     Still another object of the invention is to provide a conveyor oven that recirculates its working air and which therefore is relatively efficient to operate. 
     In accordance with a first aspect of the invention, these and other objects are achieved by providing a conveyor oven comprising a plurality of cabinets, each having at least one supply/return duct assembly, at least one source of heated air, and a conveyor extending through two cabinets. Each of the plurality of cabinets includes a plurality of sidewalls and a top wall bridging the sidewalls, an entrance being formed in a first one of the sidewalls, and an exit being formed in a second one of the sidewalls. Wherein the exit of an upstream cabinet is disposed adjacent to, and feeds, the entrance of another cabinet. The conveyor extends from the entrance of the upstream cabinet to the exit of the downstream cabinet and has an upper surface along which travels an article to be baked. Each supply/return duct assembly is positioned above the conveyor and has a lower surface which faces the upper surface of the conveyor. Each duct assembly includes a plurality of supply ducts and a plurality of return ducts. Each of the supply ducts has (a) a heated air inlet in fluid communication with the source of heated air and (b) a plurality of downwardly-opening discharge orifices formed in the lower surface. Each of the return ducts has at least one wall formed by a wall of an adjacent one of the supply ducts and has (a) a lower inlet which faces the upper surface of the conveyor and (b) an upper outlet which is in fluid communication with the source of heated air. Each cabinet is equipped with at least one, and preferably at least two facing supply/return duct assemblies. These facing assemblies define a heater source space for each cabinet. 
     Preferably, in order to facilitate assembly, promote uniform and efficient airflow, and render the oven more compact, the oven further comprises a plenum which houses at least part of the source of heated air. The plenum has an upper portion formed by the duct assembly, a supply passage assembly being formed within the plenum for conveying heated air from the source of heated air to the inlets of the supply ducts, and a return passage assembly being formed between the plenum and the cabinet for conveying air from the outlets of the return ducts to the source of heated air. 
     In a particularly preferred configuration, each supply passage assembly comprises a first supply passage extending at least generally in parallel with a first one of the sidewalls of the cabinet. A second supply passage is disposed opposite the first supply passage on the opposing side of the conveyor and extends at least generally in parallel with a second one of the sidewalls of the cabinet upward and around the opposing side of the conveyor. The source of heated air includes a blower having an axial inlet, a first radial outlet in fluid communication with the first supply passage, and a second radial outlet in fluid communication with the second supply passage. 
     Seals are preferably disposed at the interfaces between each plenum and its associated cabinet and at the entrance and exit of the cabinet so that ingress of ambient air is minimized and most of the air used to bake the articles in the oven is recirculated in a closed loop, thereby rendering the oven more efficient and increasing uniformity of heating. 
     The supply duct discharge orifices are preferably generally H-shaped to further promote uniform air distribution and to reduce whistling noises that might otherwise occur during oven operation. 
     Still another object of the invention is to provide an improved print plate imaging and processing system employing an improved pre-bake oven. 
     In accordance with another aspect of the invention, this object is achieved by providing a print plate imaging and processing system that includes a thermal imaging unit, a pre-bake oven, a developer unit, and a finishing assembly. In the thermal imaging unit, an image is thermally imposed on selected areas of the print plate to create image areas and non-image areas on the print plate. The pre-bake oven is located downstream of the thermal imaging unit. The print plate is heated in this oven sufficiently to partially cross-link polymers in the non-image areas of the print plate. In the developer unit, the pre-baked print plate is immersed in an aqueous alkaline developer. The finishing assembly includes a rinse/gum unit in which baking residues are removed from the print plate and in which a gum finisher is applied to the print plate. The pre-bake oven includes two cabinets, at least one source of heated air in each cabinet, at least one supply/return duct assembly in each cabinet, and a conveyor. The oven includes a plurality of sidewalls and a top wall bridging the sidewalls, an entrance being formed in a first one of the sidewalls, and an exit being formed in a second one of the sidewalls. The conveyor (a) has an upper surface along which the print plate travels, (b) receives the print plate from the thermal imaging unit, (c) conveys the print plate through the oven, and (d) forwards the print plate towards the developer unit. Each supply/return duct assembly (a) is positioned above the conveyor, (b) receives heated air from the source of heated air, (c) directs heated air downwardly onto the upper surface of the conveyor and the print plate so as to heat uniformly the print plate with less than a 2° C. temperature variation across the surface of the print plate, and (d) directs return air upwardly from the print plate and back to the source of heated air. 
     Preferably, each duct assembly of the pre-bake oven (a) has a bottom surface which faces the upper surface of the conveyor and (b) includes a plurality of supply ducts, each of which has (i) a heated air inlet in fluid communication with the source of heated air and (ii) a plurality of downwardly-opening discharge orifices. Each duct assembly further includes plurality of return ducts, each of which has at least one wall formed by a wall of an adjacent one of the supply ducts. Each of the return ducts has a lower inlet which faces the conveyor and an upper outlet which is in fluid communication with the source of heated air. 
     Yet another object of the invention is to provide an improved method of baking an article as it is conveyed through an oven. 
     In accordance with another aspect of the invention, this object is achieved by conveying the plate into a first, upstream cabinet of the oven using a conveyor extending into the first cabinet in the oven, then heating air via a source of heated air located within the first cabinet and beneath the conveyor. The heated air is then directed onto the plate, from a supply/return duct assembly which is located above the conveyor and which is in fluid communication with the source of heated air, so as to uniformly heat the plate. Return air then flows upwardly from the plate, through the duct assembly, then downwardly around the conveyor, and then back to the source of heated air. The plate is then conveyed out of the upstream cabinet using the conveyor. 
     The plate is then conveyed into a second, downstream, oven cabinet using the conveyor, which extends into the second cabinet, then heating air via a second source of heated air located within the second cabinet and beneath the conveyor. The heated air is then directed onto the plate from another supply/return duct assembly which is located above the conveyor and which is in fluid communication with the second source of heated air, so as to uniformly heat the plate. 
     Preferably, the above steps of directing heated air onto the plate comprises forcing the heated air radially from two radially-opposed outlets of a blower of the source of heated air, then forcing the heated air upwardly around opposed transverse edges of the conveyor and into opposed longitudinal ends of supply ducts of the duct assembly, and then forcing the heated air downwardly through discharge orifices in the supply ducts so as to impinge evenly on an entire upper surface of the plate. 
     The step of forcing the heated air downwardly through discharge orifices preferably comprises forcing air through H-shaped discharge orifices. 
     The air preferably is heated in the source of heated air, forced onto the plate, and returned to the source of heated air in a closed loop with essentially no heated air being exhausted from the oven and with essentially no ambient air being drawn into the oven. 
     Other objects, features, and advantageous of the present invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
     FIG. 1 schematically represents a print plate imaging and processing system employing an oven constructed in accordance with the preferred embodiment of the present invention as a pre-bake oven of the system; 
     FIG. 2 is a perspective view of the pre-bake oven of the system of FIG. 1; 
     FIGS. 3 a  and  3   b  are side sectional elevation views of the oven of FIG. 2; 
     FIG. 3 c  is a sectional elevation view of the oven taken generally along the line  3   c — 3   c  of FIG. 3 a;    
     FIG. 4 is an end view of the oven of FIGS. 2 and 3 showing the entrance of the oven; 
     FIG. 5 is a partially cut-away sectional end-elevation view of the oven of FIGS. 2-4, taken generally along the line  5 — 5  in FIG.  3  and omitting the heating elements and portions of the cabinet for the sake of convenience; 
     FIG. 6 is a sectional end elevation view of the oven of FIGS. 2-5, taken generally along the lines  6 — 6  in FIG. 3 a  and omitting the heating elements and portions of the cabinet for the sake of convenience to show details of plenum  36   d  that are common to all the other plenums; 
     FIG. 7 is a perspective view of any one of the plenums of the oven of FIGS. 2-6; 
     FIG. 8 is a fragmentary side sectional elevation view of a portion of cabinet  32   b  of FIGS. 2-6, illustrating supply and return airflow therethrough (the airflow and construction of cabinet  32   a  and the other plenums are the same); 
     FIG. 9 is a top plan view of a plenum of the oven of FIGS. 2-6 with the surrounding cabinet being illustrated in phantom lines the figure showing details common to all plenums; 
     FIG. 10 is a sectional top plan view of a plenum, taken through the supply/return duct assembly thereof and shows details common to all plenums; 
     FIG. 11 is a fragmentary perspective view of a typical supply/return duct assembly showing details common to all plenums; and 
     FIG. 12 is a fragmentary sectional side elevation view of one of the discharge orifices of all the plenums&#39; supply/duct return assemblies. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1. Resume 
     Pursuant to the invention, a conveyor oven is provided which is capable of heating precisely and uniformly an article to be baked as the article is conveyed through the oven at a designated speed. Precise and uniform heating is promoted by 1) a plurality of combination supply/return duct assemblies positioned above the conveyor and configured to promote uniform airflow towards the upper surface of the conveyor, wherein each of the supply/return duct assemblies is disposed in a separate oven cabinet, and, 2) discharge orifices configured to further promote uniform airflow from the supply ducts without generating whistling or other unpleasant noises. The arrangement of the supply/return duct assemblies, incorporating both supply and return ducts in the same plane, also promotes a low profile oven—an significant consideration in applications in which minimizing space is a priority. The profile of the oven is reduced further by placing the heating element beneath the conveyor and by configuring supply and return passages to circulate air between the heat source and the supply/return duct assembly using minimal space. This air recirculation, preferably enhanced by seals at appropriate locations within the oven, also significantly increases the oven&#39;s thermal efficiency and its ability to distribute heat uniformly. The oven is especially well suited for use as a pre-bake oven in a print plate imaging and processing system. 
     2. System Overview 
     The inventive conveyor oven is usable in virtually any application in which an article to be baked is heated from above as it is conveyed through the oven. It is particularly well-suited for use in print plate imaging and processing systems which require 1) precise and uniform heat transfer to the print plate and, 2) a relatively low profile to meet space constraints. Print plate imaging and processing systems of this type are gaining widespread acceptance in the industry because they offer reduced make-ready, faster turnaround, and improved quality when compared to prior imaging and processing systems. One such print plate imaging and processing system is illustrated schematically in FIG.  1  and is designated generally by the reference numeral  20 . The print plate being acted upon by the system  20  is a pre-sensitized, fully photopolymer aluminum plate which can be imaged digitally using an infrared laser source or conventionally using film negatives. The illustrated system  20  comprises, as its major components, a thermal imaging unit  22 , a pre-bake oven  24 , a developer  26 , and a finishing assembly including a rinse/gum unit  30  and possibly a post-bake oven  28 . 
     The thermal imaging unit  22  may comprise either a digital imaging device or a conventional imaging device using UV energy. In either event, energy is delivered to the plate&#39;s upper surface to create the image and to partially cross-link the polymers in the image areas. The energy takes the form of heat in digital systems and light in conventional systems. Both systems create a latent image on the print plate that is extremely stable. 
     After receiving the image, the print plate is heated as it is conveyed through the pre-bake oven  24 . Pre-baking further cross-links the polymers in the image areas of the print plate and partially cross-links polymers in the non-image areas, thereby making the background soluble in the downstream developer while simultaneously rendering the image areas insoluble. 
     After leaving the pre-bake oven  24 , the print plate is cooled to or near room temperature. It is then conveyed to the developer  26  where it is immersed in a developer tank containing an aqueous alkaline developer solution. The solution dissolves non-image areas on the print plate, and polymers in these areas are then removed by action of a scrub roller or the like located within the tank. After the print plate is removed from the tank, water is applied to the plate using a spray bar or the like to remove any remaining background polymer particles and developer residues. 
     The purpose of the post-bake oven  28  is to completely cross-link the partially cross-linked polymers in the image, thereby increasing the durability or long-run capability of the image. Post-baking, if incorporated into the process, requires that a pre-bake solution be applied to the print plate, preferably at the outlet of the developer  26 . This solution protects the image and the background from contaminants such as dirt within the oven  28 , as well as from byproducts generated from baking the coating itself. 
     Whether or not the print plate is post-baked, it should be subjected to finishing in the rinse/gum unit  30  or the like. In this unit  30 , water is first applied to the print plate with a spray bar-type system to remove pre-bake solution and any baking residues from the plate. A gum finisher is then applied to the print plate with a spray-bar-type system or the like to protect the background areas from adverse handling and to permit the plate to come to impression faster, i.e., to permit the image to take ink and background shedding ink faster. 
     The oven  24  could be used as either a pre-bake oven or a post-bake oven in the system  20  or in any other applications requiring conveyor ovens. It is particularly well suited, however, for use as the pre-bake oven because optimal pre-baking requires precise and uniform heat application to the entire upper surface of the print plate. If there is more than about a 2° C. temperature variation across the plate surface, any overheated areas of the print plate exhibit an undesired “thermal fog”, and any underheated areas exhibit a weakened image because the polymers of these areas will not be sufficiently cross-linked. The conveyor oven  24  is ideally suited for these purposes and, when used in combination with other conventional components  22 ,  26 ,  28 , and  30  of the system, provides an improved thermal imaging and processing system  20 . The conveyor oven  24  oven will now be detailed. 
     3. Description of Conveyor Oven 
     Turning now to FIGS. 2-11 and initially to FIGS. 2-6, a low-profile conveyor oven  24  is illustrated that can be used in a wide variety of applications, including as the pre-bake oven in the print plate imaging and finishing system  20  of FIG.  1 . The oven  24  includes two cabinets  32   a  and  32   b . Cabinet  32   a  is the “upstream” cabinet since it receives the article to be baked before cabinet  32   b , which is therefore called the “downstream” cabinet. The oven also includes four sources of heat  34   a  and  b , and  34   c  and  d , that are fluidly coupled to supply/return duct assemblies and are located within cabinets  32   a  and  32   b , respectively. It also includes four plenums  36   a  and  b , and  36   c , and  d  which are located within cabinets  32   a  and  32   b , respectively. Plenums  36   a  and  36   b  are located within cabinet  32   a  and plenums  36   c  and  36   d  are located within cabinet  32   b . The oven also includes a conveyor assembly  38  that extends through both cabinets. Both cabinets  32  are encased in a decorative and protective metal facade  40 , and the entire assembly is mounted on a support frame assembly  42 . 
     Each cabinet  32  preferably comprises an insulated chamber commonly used in conveyor ovens of this type. Each chamber includes a front sidewall  44 , a rear sidewall  46 , a pair of opposed transverse sidewalls  48  and  50 , a top wall  52  bridging the tops of all of the sidewalls to enclose the top end of each cabinet  32 , and a bottom wall  54  bridging the bottoms of all of the sidewalls to enclose the bottom of each cabinet  32 . Each of the sidewalls, the top wall, and the bottom wall is formed from an outer shell, an inner shell, and a layer of insulation disposed between the inner and outer shells. The shells are typically formed from interconnected sheet-metal panels fastened to one another by suitable fasteners. The construction of the walls  44 ,  46 ,  48 ,  50 ,  52 , and  54 , per se, forms no part of the present invention and, accordingly, will not be detailed. 
     An entrance opening  56  is formed through the front sidewall  44  of each cabinet  32 , and an exit opening  58  is formed through the rear sidewall  46  of each cabinet in the same horizontal plane as their entrance openings  56 . As can best be seen in FIGS. 4-6, the width of the resulting conveyor opening  60  is substantially less than the width of the oven chamber in order to accommodate ductwork for the flow of supply air and return air around the conveyor opening  60  as detailed below. 
     An insulated intermediate chamber  33  is disposed between cabinets  32   a  and  32   b . This chamber surrounds the portion of conveyor  38  extending between cabinets  32   a  and  32   b . This chamber has an insulated top wall  35 , two insulated sidewalls  37  and an insulated bottom wall  39  constructed essentially as described above regarding the insulated structure of each cabinet. By providing this insulated intermediate chamber, the article being heated in the upstream cabinet can be conveyed to the downstream cabinet with limited heat loss during the transition from one cabinet to another. 
     The conveyor assembly  38  may comprise any known conveyor assembly capable of conveying plates or other articles through the oven  24  at a designated rate. Referring to FIGS. 2-4, the illustrated conveyor assembly  38  includes a slide bed  62  and an endless conveyor  64 . The slide bed  62  is mounted on the floor  118  of the conveyor opening  60  and includes 1) a pair of laterally opposed side braces  66  and  68  and 2) a grid of interconnected support rods  70  linking the side braces  66  and  68  to one another. A drive sprocket assembly  72  is mounted at the front of the slide bed  62  and is driven by an electric motor (not shown). A guide sprocket  74  assembly is mounted at the rear of the slide bed  62 . The conveyor  64  is driven by the drive sprocket assembly  72  and guided by the support rods  70  and the guide sprocket assembly  74 . The conveyor  64  preferably comprises a conventional wire belt conveyor formed from a mesh of interconnected steel wires. 
     Each plenum  36  serves several functions. First, it incorporates a supply/return duct assembly  76  at its upper end. Second, it presents a housing  78  at its lower end which at least partially houses the heat source  34 . Third, the interior portion of the conveyor opening  60  is formed through it. Fourth, it cooperates with the sidewalls  48  and  50  of its corresponding cabinet  32  to recirculate air between the heat source  34  and the supply/return duct assembly  76 . All of these functions are achieved using a remarkably compact structure. 
     Each supply/return duct assembly  76  is positioned vertically between the conveyor  64  and the top wall  52  of the cabinet  32  and is characterized by the presentation of both supply and return ducts in the same horizontal plane. Each duct assembly  76  is formed from sheet metal and shares many of its walls with walls of other portions of the plenum. Duct assembly  76  extends transversely with respect to the conveyor opening  60  and is rectangular in transverse cross section and in longitudinal cross section. Each has a lower surface or wall  84  facing the upper surface of the conveyor  64 , an upper surface or wall  86  facing the top wall  52  of the cabinet  32  to define a return air chamber  88  therebetween, and presents a plurality of interleaved or alternating supply ducts  80  and return ducts  82 . First and second longitudinally-opposed transverse end walls  90  and  92  each define the inner edge of a respective supply passage  94 ,  96 . Each of these walls  90  and  92  is notched in a saw-toothed fashion to form inlets of the supply ducts  80  while closing-off the ends of the adjacent return ducts  82 . Third and fourth longitudinally-opposed transverse end walls  98 ,  100  are located longitudinally beyond the first and second end walls  90  and  92 , respectively. Each of these end walls  98 ,  100  defines an outer edge of a supply passage  94 ,  96  and an inner edge of a corresponding return passage  102 ,  104 . First and second transversely opposed edge walls  106 ,  108  extend longitudinally from the third end wall  98  to the fourth end wall  100  and define outer walls of the outermost return ducts  82 . A plurality of intermediate walls  110  extend longitudinally from the first end wall  90  to the second end wall  92  such that each wall  100  defines a transverse edge of both a supply duct  80  and an adjacent return duct  82 . Hence, each of the supply ducts  80  is flanked by a pair of return ducts  82 . Each of the return ducts  82  of the resulting structure has a lower inlet which faces the upper surface of the conveyor  64  and an upper outlet opening into the return air chamber  88 . 
     Each of the supply ducts  80  has a plurality of downwardly-opening discharge orifices  112  formed in the bottom surface  84  of the duct assembly  76 . The discharge orifices  112  are carefully constructed to maximize uniform distribution of discharged air. Various configurations of discharge orifices were investigated with varying degrees of success. It was discovered that providing a large number of round orifices promoted somewhat uniform air distribution during oven operation but resulted in an unpleasing whistling noises. Other discharge orifice configurations were rejected because they did not provide the requisite uniformity of air distribution. 
     The preferred orifices comprise a pattern of H-shaped orifices  112  formed in the bottom wall  84  of the duct assembly  76  as best seen in FIGS. 10-12. These orifices  112  are formed by slitting the bottom wall  84  in an “H” pattern and by punching the resulting tabs  114  upwardly as best seen in FIGS. 11 and 12. H-shaped orifices, used in other applications such as the relatively large oven disclosed, for example, in U.S. Pat. No. 5,303,660, were initially rejected as an orifice option because it was thought that such discharge orifices would not provide sufficiently uniform airflow distribution for use in a pre-bake oven. However, it has been discovered that properly sized and arranged H-shaped discharge orifices  112  meet the uniformity requirement while avoiding the whistling problems associated with some other orifices. Use of this H pattern also was found to increase spreadability i.e., to increase distribution from the supply ducts  80 . Orifices having a length of about 2″, a width of about 1″, and a density of about 25 orifices per square foot proved optimal. 
     A breaker  116  (FIGS. 6 and 10) extends transversely across an intermediate longitudinal section of each of the supply ducts  80  so as to essentially prevent airflow therepast. These breakers  116  promote turbulence within the supply ducts  80  and hence improve uniform air distribution from the supply ducts. Each of the breakers  116  is preferably formed from a piece of sheet metal attached to the walls  110  of the duct  80  in which the breaker  116  is located. 
     The interior portion of the conveyor opening  60  includes a floor  118 , a ceiling formed by the bottom surface  84  of the duct assembly  76 , and a pair of opposed sidewalls formed from the walls  90  and  92  of the duct assembly  76 . All of the walls extend from the entrance  56  of the cabinet  32  to the exit  58 . The supply passages  94 ,  96  and return passages  102 ,  104  extend vertically between the sidewalls  90 ,  92  of the conveyor opening  60  and the corresponding transverse sidewalls  50 ,  52  of the cabinet  32 . 
     The housing portion  78  of each plenum  36  forms a heated air chamber  122  bounded at its lower end by a bottom wall  124  of the plenum  36 , at its rear end by the first edge wall  108 , at its longitudinal ends by walls formed by extensions of the third and fourth end walls  98  and  100  of the duct assembly  76 , at its upper end by the floor  118  of the conveyor opening  60 , and at its front end by a vertical wall  126 . 
     Heated air chamber  122  is in direct fluid communication with the inlets of the first and second supply passages  94  and  96  which, as discussed above, are in turn in direct fluid communication with the inlets of the supply ducts  80 . 
     In each cabinet, two plenums face each other to define a heater element chamber  128 . FIG. 3 shows the facing arrangement of plenums  36   c  and  36   d  of cabinet  32   b  and plenums  36   a  and  36   b  of  32   a . Chambers  128   a  and  128   b  are located between the heated air chambers  122  of the facing plenums in each cabinet. Chamber  128  is bounded at its rear end by the wall  126  of plenum  36   d , at its upper end by floor  118  of the conveyor opening  60 , and at its front end by wall  126  of plenum  36   c . A triangular opening is provided to chamber  128  in which the heating elements are inserted. These triangular sections are defined by the inwardly and upwardly slanting portions of the edge walls  106  of plenums  36   c  and  36   d.    
     Heater element chamber  128  is in direct fluid communication with the first and second return passages  102  and  104  or each plenum which, as discussed above, are in turn in direct fluid communication with the return air chambers  88  of each plenum and hence the outlets of the return ducts  82  of each plenum. 
     Measures are preferably taken to prevent ingress of ambient air as much as practically possible so that essentially the same air mass is continuously recirculated through the oven  24 . This closed-loop recirculation reduces energy expenditure and also promotes more uniform heating. In order to promote this closed-loop recirculation, the edge walls  106  and  108  of each plenum are sealed to their corresponding front and rear sidewalls  44  and  46  of their containing cabinet, a seal is similarly provided between. 
     Referring to FIGS. 9 and 10, the seals preferably comprise “tadpole” seals  130  and  132  of known configuration. These seals also preferably extend across the bottom edge of the entrance of cabinet  32   a  and exit of cabinet  32 . In addition, “profile curtains”  134  and  136 , taking the form of fiberglass gaskets, are mounted at the upper portions of the entrance and exit of cabinets  32   a  and  32   b . These gaskets extend downwardly to a position closely adjacent the upper surface of the conveyor  64  as best seen in FIG. 3 so as to permit passage of the conveyor  64  and of the articles to be baked while minimizing inflow of ambient air. 
     The source of heated air  34  for each cabinet could comprise any assembly capable of heating air and of recirculating the heated air between the source and the supply/return duct assembly  76 . The preferred and illustrated assembly comprises a direct drive blower assembly  140  associated with each plenum, and a heater plug assembly  142  associated with each cabinet to preferably provide four blower assemblies and two heater plug assemblies per oven  24 . 
     The blower assemblies  140  comprise electrical motors  144  mounted at a front (or rear) wall of the cabinets  32 , and a blower  146  associated with each motor and disposed within the heated air chamber  122 . The front and rear walls of each cabinet each has a blower motor in the preferred embodiment, with two motors  144  disposed in a side-by-side arrangement between the upstream and downstream cabinets, and two motors disposed on the downstream end wall of the downstream cabinet and the final motor mounted on the upstream end wall of the upstream cabinet. 
     Each motor  144  has an output shaft  148  that extends through the cabinet wall on which it is positioned, and through the plenum wall of its associated plenum. This shaft is coupled to its blower to drive the blower and circulate heated air through the system. 
     Each blower  146  has an axial inlet  150  that opens into its associated heater element chamber  128 . Thus, for each cabinet there are two longitudinally opposed blowers with facing blower inlets. 
     Each blower also has at least one, and preferably two, opposed radial outlets  152  and  154  opening into the heated air chamber  122  of its associated plenum. The illustrated two-outlet configuration is preferred because it maximizes air distribution uniformity by providing an outlet associated with each end of supply ducts  80 . 
     Each heater plug assembly  142  comprises a plurality of electrical coils or heater elements  156  disposed within chamber  128  between the blower inlets of that cabinet. There are preferably two heater plug assemblies per oven—one for each of the cabinets. Each of the heater elements  156  is mounted on an associated support panel  158 . One panel  158   a  forms a portion of the side wall of the upstream cabinet, and the other panel  158   b  forms a portion of the side wall of the downstream cabinet. 
     In some applications, such as in a print plate imaging and processing system, it is desirable that the oven  24  incorporate measures to cool the baked articles as they exit the oven. Such cooling, if provided, should be controlled to adequately cool the article to be baked without overcooling and without blowing cold air back into the oven. Cooling is achieved in the illustrated embodiment using a cooling fan assembly  160  located outside the facade  40 . 
     Finally, a control panel  166  (FIG. 2) is mounted on the facade  40  to permit individual control of the various components of the oven  24 . 
     Control panel  166  is electrically coupled to control circuit disposed outside the downstream cabinet, but inside facade  40 . The control circuit, in turn is electrically coupled to each of the four blower motors and to the conveyor motors. 
     Control panel  166  includes a conveyor speed control that is adjustable by the operator to selectively vary the speed of the conveyor motors. Similarly, the control panel also includes a blower speed control that is adjustable by the operator to selectively vary the speed of the blower motors. Control panel  166  also includes a temperature controller which sets and monitors the temperature of the oven  24 . Panel  166  also includes ON-OFF switches for the blower motors, heater plug assembly  142  and conveyor  64 , and an over-temperature alarm. 
     The temperature controller comprises a suitable dial or the like to set a temperature and suitable displays which display the current temperature and the set temperature. A separate conveyor speed control dial is also provided to permit the operator to vary the speed at which articles are conveyed through the oven  24 . 
     4. Operation of Conveyor Oven 
     In operation, an article to be baked such as a print plate  170  is mounted on the upper surface of the conveyor  64  and is conveyed into the entrance  56  of the oven  24  and thence through the oven in the direction of the arrows  172 . 
     Air is heated in heater element chamber  128  by heater elements  156 . This heated air is then drawn into the inlets of blowers  146   a  and  146   b . The hot air is discharged from radial outlets  152  and  154  of those blowers and into heated air chambers  122  of plenums  36   a  and  36   b.    
     The air flows up through the supply passages  94  and  96  of those two plenums, upwardly around the conveyor opening  60  through the supply passages, and then into the inlets of the supply ducts  80  of those two plenums as best seen by the arrow  174 . 
     Since each plenum has a supply passage disposed on each side of the conveyor, the two plenums define four individual and discrete hot air carrying paths, two paths disposed on each side of the conveyor in a fore-and-aft arrangement. 
     The hot air then flows through the supply ducts  80  in each plenum, across the top of the conveyor and is forced downwardly through the H-shaped discharge openings  112  so that it impinges on the upper surface of the article  170  being baked. 
     The distribution of the discharged air is extremely precise for at least two reasons. First, uniform airflow within the ducts  80  is promoted by the flow of air into the ducts  80  from both ends and by the turbulence-promoting action of the breakers  116 . Second, uniform discharge of air onto the entire upper surface of the article  170  is assured by the configuration, distribution, and location of the H-shaped orifices  112 . As a result, the entire upper surface of the print plate or other article  170  being baked is uniformly heated with less than a 2° C. temperature variation thereacross. 
     After impinging on and heating the upper surface of the article  170  being baked, the air flows upwardly through the return ducts  82  to the return air chamber  88  located above the supply/return duct assemblies  76  of each plenum. Air flows from this chamber  88 , downwardly through the return passages  102  and  104 , and into the heater element chamber  128 , where it is reheated by heater elements  156 , and the process begins again. 
     Once the article  170  has been baked in the first upstream cabinet as described above, conveyor  38  carries it out the exit of the first cabinet and into the insulated intermediate chamber  33  disposed between the two cabinets. 
     The air in insulated intermediate chamber  33  is essentially still and is preferably neither heated, cooled or vented to the outside atmosphere in chamber  33  itself. It provides a transition zone between the first and second cabinet, each of which are thereby substantially thermally isolated from the other, permitting separate control of each cabinet at different temperatures if so desired. 
     As conveyor  38  pulls article  170  forward, the article is drawn completely through insulated chamber  33  and into cabinet  32   b  through its entrance. The operation of the blowers and plenums of cabinet  32   b  is substantially the same as that of cabinet  32   a , as described below. 
     In cabinet  32   b , air is heated in heater element chamber  128  by heater elements  156 . This heated air is then drawn into the inlets of blowers  146   a  and  146   b . The hot air is discharged from radial outlets  152  and  154  of those blowers and into heated air chambers  122  of plenum  36   c  and  36   d.    
     The air flows up through the supply passages  94  and  96  of those two plenums, upwardly around the conveyor opening  60  through the supply passages, and then into the inlets of the supply ducts  80  of those two plenums as best seen by the arrow  174 . 
     Since each plenum has a supply passage disposed on each side of the conveyor, the two plenums define four individual and discrete hot air carrying paths, two paths disposed on each side of the conveyor in a fore-and-aft arrangement. 
     The hot air then flows through the supply ducts  80  in each plenum, across the top of the conveyor and is forced downwardly through the H-shaped discharge openings  112  so that it impinges on the upper surface of the article  170  being baked. 
     The distribution of the discharged air is extremely precise for at least two reasons. First, uniform airflow within the ducts  80  is promoted by the flow of air into the ducts  80  from both ends and by the turbulence-promoting action of the breakers  116 . Second, uniform discharge of air onto the entire upper surface of the article  170  is assured by the configuration, distribution, and location of the H-shaped orifices  112 . As a result, the entire upper surface of the print plate or other article  170  being baked is uniformly heated with less than a 2° C. temperature variation thereacross. 
     After impinging on and heating the upper surface of the article  170  being baked, the air flows upwardly through the return ducts  82  to the return air chamber  88  located above the supply/return duct assemblies  76  of each plenum. Air flows from this chamber  88 , downwardly through the return passages  102  and  104 , and into the heater element chamber  128 , where it is reheated by heater elements  156 , and the process begins again. 
     Once the article  170  has been baked in the second cabinet as described above, conveyor  38  carries it out the exit of second cabinet  32   b  and out of the oven. 
     The temperature to which the article  170  is heated or baked in the oven depends upon 1) the temperature and flow rate of the air recirculating through each of the cabinets of oven  24 , and 2) the speed at which the article  170  is conveyed through the oven. 
     In a typical mode of operation, the air will be discharged from blowers  146  at a temperature of between 300° F. and 500° F. and at a flow rate of 1700 cubic feet per minute. This temperature can be selectively varied in each of the cabinets by varying the temperature setting of each of the cabinets. 
     The belt conveyor  64  normally moves at a speed of about 2-3 feet per minute. As a result, the print plate or other article  170  is heated to approximately 240° F. to 260° F. by the time it exits the oven, at which time it is cooled by the action of the cooling fans  164 . 
     It can thus been seen that the configuration of and cooperation between the plenums  36   a - 36   d , the cabinets  32   a  and  32   b , and the heat sources  34   a  and  34   b  maximize uniformity of air distribution while minimizing the height of the oven  24 , thereby providing a low-profile oven which provides precise and uniform heating of the articles being baked. Many changes and modifications could be made to the oven design without departing from the spirit of the invention. The scope of these changes will become apparent from the appended claims.