Conveyor oven usable as pre-bake oven in a print plate imaging and processing system and method of using same

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.

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.degree. C. temperature variation across the print plate'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.degree. 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.

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'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'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.degree. 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 32a
 and 32b. Cabinet 32a is the "upstream" cabinet since it receives the
 article to be baked before cabinet 32b, which is therefore called the
 "downstream" cabinet. The oven also includes four sources of heat 34a and
 b, and 34c and d, that are fluidly coupled to supply/return duct
 assemblies and are located within cabinets 32a and 32b, respectively. It
 also includes four plenums 36a and b, and 36c, and d which are located
 within cabinets 32a and 32b, respectively. Plenums 36a and 36b are located
 within cabinet 32a and plenums 36c and 36d are located within cabinet 32b.
 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 32a and
 32b. This chamber surrounds the portion of conveyor 38 extending between
 cabinets 32a and 32b. 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 36c and 36d of
 cabinet 32b and plenums 36a and 36b of 32a. Chambers 128a and 128b 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
 36d, at its upper end by floor 118 of the conveyor opening 60, and at its
 front end by wall 126 of plenum 36c. 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 36c and 36d.
 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 32a 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 32a and 32b. 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 158a forms a portion
 of the side wall of the upstream cabinet, and the other panel 158b 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 146a and 146b. The hot
 air is discharged from radial outlets 152 and 154 of those blowers and
 into heated air chambers 122 of plenums 36a and 36b.
 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.degree. 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 32b through its entrance.
 The operation of the blowers and plenums of cabinet 32b is substantially
 the same as that of cabinet 32a, as described below.
 In cabinet 32b, air is heated in heater element chamber 128 by heater
 elements 156. This heated air is then drawn into the inlets of blowers
 146a and 146b. The hot air is discharged from radial outlets 152 and 154
 of those blowers and into heated air chambers 122 of plenum 36c and 36d.
 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.degree. 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 32b 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.degree. F. and 500.degree. 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.degree. F. to 260.degree. 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 36a-36d, the cabinets 32a and 32b, and the heat sources 34a and
 34b 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.