[0001]     The present invention relates generally to the application of curable coatings to articles, and more particularly to a curing apparatus utilizing ultraviolet radiation for curing coatings applied to articles, wherein the coating is cured in an atmosphere of inert gas that has the property of being heavier than air, wherein the inert gas is delivered to the apparatus in a volume sufficient to displace oxygen in a curing chamber.  
       BACKGROUND OF THE INVENTION  
       [0002]     Radiation curing uses a variety of sources to polymerize a reactive coating material. Ultraviolet light (UV) is the radiation source most frequently used to cure coatings. UV curing is a photochemical process by which monomers having photoinitiators undergo curing (polymerization or cross-linking) upon exposure to ultraviolet radiation.  
         [0003]     Methods and apparatus relating to the use of CO 2  gas when curing certain coatings with UV radiation has been described in German patent DE19957900A1 to Beck et al., U.S. Pat. No. 3,956,540 to Laliberte et al., U.S. Pat. No. 4,436,764 to Nakazima et al., U.S. Pat. No. 4,862,827 to Getson, and U.S. Pat. No. 6,620,251 to Kitano.  
         [0004]     The present invention relates generally to the application of UV curable coatings to articles, and more particularly to a curing apparatus utilizing ultraviolet radiation for curing coatings applied to articles, wherein the coating is cured in an atmosphere of inert gas that has the property of being heavier than air, wherein the inert gas is delivered to the apparatus rapidly in large volume via a vaporizing means or heat evaporating means.  
         [0005]     Radiation curing has become an established and important commercial process and has benefited from the trend away from environmentally unfriendly products such as solvent based thermally cured coatings. Since many radiation curable coatings can cure in seconds, they quickly find their way to applications where continuous processing and the need for higher production speeds are essential in making a financially viable product.  
         [0006]     Radiation curing uses a variety of sources to polymerize a reactive coating material. Ultraviolet light (UV) is the radiation source most frequently used to cure coatings accounting for a majority of the volume and market. UV curing is a photochemical process by which monomers having photoinitiators undergo curing (polymerization or cross-linking) upon exposure to ultraviolet radiation. A specially formulated monomer with photoinitiators will polymerize when exposed to ultraviolet radiation. This UV “curable” monomer includes a photoinitiator which absorbs UV energy and initiates a polymerizing reaction therein. The rate or speed of curing will depend on at least the chemical compound, the thickness of the coating, and the amount of UV intensity per unit surface.  
         [0007]     The chemical compound itself affects the speed of curing. Each monomer cures at a different rate, depending on its composition and the type and amount of sensitizer, pigment or filling material used. In formulating the UV curable compound, the manufacturer must consider the physical properties of the finished product as well as curing speed.  
         [0008]     A general summary of application, markets and products where radiation cured coatings have found commercial success include, but are not limited to, graphic arts coatings, inks, wood, plastics, metal coatings, optical fibers, electrical/electronics and automotive. Some of the more important coating applications are found in every day products such as hardwood flooring, metal and wood furniture, electrical wire and cable, release papers, beverage cans, magazine covers, packaging, leather finishes and computer magnetic media. Although the final properties of radiation cured coatings are often superior to other systems, the reason for their popular growth has been primarily due to improvements in productivity, ability to coat heat and solvent or water sensitive substrates, and environmental emission considerations.  
         [0009]     Some of the advantages of processes for curing coatings with radiation include low solvent emissions; low fire hazard; low usage of hazardous solvents, reduced down time, waste, and cleanup; low temperature, solvent free cure process suitable for temperature or chemical sensitive substrates; very fast production, rapid curing, in-line processing capability and minimal manufacturing steps; a variety of different acceptable chemistries and formulations for the coating and inert atmosphere.  
         [0010]     Favorable coating and excellent performance properties result, such as high gloss depth, abrasion resistance, chemical resistance, and hardness; smooth finish (unlike powder and some spray coatings).  
         [0011]     Machines have been developed for use in the curing process. However, machines do not effectively provide proper lighting geometries. In a proper lighting geometry, the photoinitiator must be exposed to the UV light to effectively cure the coating. In addition to solving the lighting geometries, current machines are able to cure thicker coatings on the product.  
         [0012]     Radiation technologies can be used to cross-link or cure organic resins into durable coatings. These durable coatings have excellent physical properties with high chemical and temperature resistance. Radiation curing technology involves at least four considerations; type of radiation source, organic polymer to be irradiated, mechanisms of physical and chemical interaction, and final properties associated with the cured product. Radiation curing coatings react through unsaturation sites on oligomers and monomers. These active sites (double bonds) are capable of reacting to form larger polymers and cross-linked, three dimensional network structures. Reference to  FIG. 11  shows the interaction of UV radiation with a linear polymer (a) to develop a cross-linked networked structure (b).  
         [0013]     The organic resins useful in the invention include those with a radiation hardendable connections used as bonding agents. These are connections with radical or cation polymerizable chemical groups. In the preferred embodiment, examples of the organic resins include vinylether, vinylamide with maleic acid or fumaric acid and styrene as reactive solvents. In the preferred embodiment, examples are polyester(meth)-acrylates, polyether(meth)acrylate, urethane(meth)acrylate, epoxi(meth)acrylate, silicon(meth)acrylate. Concentrations preferred are 40 mol percent to 60 mol percent radiation hardenable per (meth)acrylate group. Other reactive groups include melamin, isocyanate, epoxy, anhydride, alcohol, groups of carbonic acids for additional thermal hardening. Chemical reaction hardening can also be used in part by substitution of alcohol, carbonic acid, amine, epoxy, anhydride, isocyanates and other methyl groups contained in a binary cure process.  
         [0014]     It is widely known that atmospheric oxygen reduces both the rate and extent of UV induced polymeric cross-linking. This oxygen inhibition creates the need for high concentrations of photoinitiator—the most expensive component of UV curable inks, coatings and adhesives. Carbon dioxide (CO 2 ) inert gas technology eliminates the inhibiting effects of oxygen. The effect of radiation dose on adhesion and cohesive properties of the coating is that as the dose increases, the tack decreases and the cohesive strength of the coating increases. Temperature and chemical resistance are also generally increased by a greater radiation dose. The exact curing window for a product must be determined for every formulation and for each thickness. Ultraviolet light is one of the main sources of energy for curing coatings by radiation. UV light can provide instantaneous curing of coatings that polymerize from a liquid to a solid when irradiated.  
         [0015]     During UV curing in air, the presence of oxygen, known as oxygen inhibition, can have a detrimental effect on the cure response for certain coatings. Oxygen reacts with the free radical and forms peroxy radicals by reaction with the photoinitiator, monomer or propagating chain radical. The reactivity of the peroxy radical is insufficient to continue the free radical polymerization process, leading to chain termination and resulting in an under cure system.  
         [0016]     One method of overcoming oxygen inhibition is curing the free radical system under an inert gas atmosphere. The inert gas should be heavier than air. The molar weight of the gas should be larger than 28.8 grams per mol and preferably larger than 32 grams per mol (oxygen and 80% nitrogen correspond in the molecular weight of a gas mixture of 20%, for instance). An inert gas atmosphere comprised of noble gases such as argon, hydrocarbon and halogen gases is also acceptable. Carbon dioxide is particularly suitable for use in providing an inert gas atmosphere to overcome oxygen inhibition. It is known that liquid CO 2  can be very conveniently stored and transported in metal cylinders at normal room temperature. It can be easily stored in liquid form due to its inherent nature to be more compact than the gaseous form.  
         [0017]     The use of CO 2  gas when curing certain coatings using UV radiation has been described in PCT application PCT/EP00/11589 to Beck, et al., titled “Light Curing of Radiation Curable Materials Under a Protective Gas”. As described in that application, articles are coated with and are placed in an enclosure filled with CO 2  gas. The coating is cured by UV radiation while the article is surrounded by CO 2 . The process described by Beck, et al., however, is not easily adapted to a high volume, production environment. Beck, et al. also does not provide for efficient use and conversion of liquid CO 2 .  
         [0018]     It is desirable that oxygen be eliminated from the curing area to the extent possible. A curing environment that is completely filled with CO 2  provides the best results for the curing process to occur. Articles having coatings that are to be cured are lowered into an airtight enclosure, wherein the enclosure is filled with a heavier than air gas. The article is cured and is then lifted from the enclosure.  
         [0019]     The reflector and/or reflective surfaces are important components in a UV curing apparatus system since they directly affect the amount of UV energy that encounters the curing surface. During production, various deposits can accumulate on reflectors and reflective surfaces that will greatly lessen cure efficiency. While some systems do not permit reflectors and/or reflective materials to be changed or cleaned, scheduled interior cleaning helps maintain consistent performance in the curing process. Reflectors, lamps, reflective surfaces, and other interior surfaces that become permanently contaminated, pitted, scratched or that have lost their  10  reflective quality, should be replaced. Currently, most systems, such as Beck, et al., do not provide for easy replacement of these or other internal components.  
         [0020]     What is needed, therefore, is a curing system apparatus, and method used for hardening UV curable coatings in an inert gas, such as CO 2 , while also having the capability of maintaining high production volumes.  
         [0021]     It is further desirable for a curing system and apparatus to permit the operator to easily access the light emitting elements/systems and reflective surfaces for quick and easy change out, cleaning, and/or general interior chamber maintenance. This quick-change, easy internal chamber access feature significantly reduces downtime while also allowing the operator to maintain optimum interior surface reflectivity and system performance all at minimal cost with little production downtime.  
         [0022]     It is further desirable for a curing system, apparatus, and method to be adapted provide for efficient storage and conversion of liquid CO 2  to gaseous CO 2  via a vaporizing means or heat evaporation means for use in the curing process and to provide rapid delivery of a large volume of the converted gas to the apparatus via a gas distribution means.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The features characteristic of the present invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects, and advantages thereof, are best understood by reference to the following detailed description when read in conjunction with the accompanying drawings, wherein:  
         [0024]      FIG. 1  is an isometric view of the top and front of the curing apparatus.  
         [0025]      FIG. 2  is a detail view of the frame and track of the apparatus.  
         [0026]      FIG. 3  is a top view of the apparatus showing certain portions of the support frame and track in ghost as dashed lines.  
         [0027]      FIG. 4  is side view of the curing apparatus showing the positioning of the track, products and lamp caddies relative to the housing of the curing apparatus.  
         [0028]      FIG. 5  is an isometric view showing the top and back of the curing apparatus.  
         [0029]      FIG. 6   a  and  6   b  are detail views showing the placement of two light caddies with respect to a portion of the track and suspended product.  
         [0030]      FIG. 7  is a schematic diagram of the electrical architecture of the control unit of the curing apparatus.  
         [0031]      FIG. 8  is a flow chart of the steps carried out by the controller of the curing apparatus.  
         [0032]      FIG. 9  is a plan view of the manifolds of the apparatus.  
         [0033]      FIG. 10  shows a schematic representation of the interaction of UV radiation with a cross-linking linear polymer.  
     
    
     DETAILED DESCRIPTION  
       [0034]      FIG. 1  shows and isometric view of curing apparatus  10 . Curing apparatus  10  includes support frame  20 . Support frame  20  provides the internal structure which supports the operating components of the apparatus and the products which are cured by it. Support frame  20  is shown in detail in  FIG. 2 . Support frame  20  includes a plurality of upward supports  21 , a plurality of upper longitudinal supports  23  and  15 , lower longitudinal support members  26 ,  27 ,  28 , and  29 , lower latitudinal support members  11  and  12  and upper lateral support members  28  and  29 . Support frame  20  also includes entry supports  18  and exit supports  19 . In the preferred embodiment, the support frame is constructed of 2-inch box aluminum channel. Other rigid materials can be used for the frame such as steel.  
         [0035]     A generally oval track  30  is suspended from upper lateral support members  28  and  29 . Track  20  is supported in four places, one support (shown as  13 ); by upper lateral support member  28  and three (shown as  14 ) by lateral support member  29 . The track is attached to the support frame by spacers  31  which provide for leveling of the track.  
         [0036]     Turning to  FIG. 4 , track  30  is shown from a side view. Track  30  comprises an upper section  22 , a lower section  23  and a descending/ascending section  24 . Upper section  22  is connected to descending/ascending section  24  through a curved radius of approximately 4 feet. Descending/ascending section  24  forms a radius also of approximately 4 feet. Descending/ascending section  24  is connected to lower section  23  by a radius of curvature of approximately 3 feet. Lower section  23  is connected to upper section  22  through elevated ascent  25 , having a radius of curvature of approximately 2 feet. The distance between upper section  22  and lower section  23 , vertically, is approximately 8 inches. The distance between upper section  22  and the lowest point of descending/ascending section  24  is approximately 4 feet. In the preferred embodiment, the aspect ratio of track  30  as seen from the top ( FIG. 3 ) is approximately 4 to 1. Of course, each of the dimensions of the track is subject to engineering choice and can be modified within the scope and spirit of the invention.  
         [0037]     Track  30  is made up of aluminum tubing with a round cross-section, having a linear slot of approximately ½ inch centered at the bottom side of the tubing. A continuous chain (not shown) resides within track  30 . The continuous chain is comprised of cylindrical rollers. The cylindrical rollers are sized to fit within the track at alternating angles of 45° and 135° from the vertical axis of the diameter of the track. The cylindrical rollers are spaced at intervals of approximately 6 inches and are connected by a continuous chain. Properly lubricated, the continuous chain and rollers can be moved to traverse the inside of the track in a continuous loop.  
         [0038]     Hangers  15  are attached at regular intervals on the chain through the slot in the bottom side of track  30 . In the preferred embodiment, the hangers are designed to support a load between 5 and 20 pounds. The bottom of each hanger  15  is designed to support a product  90  and to articulate or swing to allow movement by the suspended product. If the weight of product  90  exceeds the capacity of a single hanger  15 , additional hangers can be added to the continuous chain to support the additional weight.  
         [0039]      FIG. 1  shows drive motor  70 , reduction gear box  75  and cog drive  80 . Cog drive  80  is connected to the continuous chain through an opening (not shown) in the top of track  30 . The cog drive is driven by reduction gearbox  75 , which is in turn driven by drive motor  70 . The cooperation of drive motor  70 , reduction gearbox  75  and cog drive  80  in the preferred embodiment provides a travel speed in the chain of approximately 5 to 10 feet per minute. In another embodiment, travel speeds in the chain can range from about 1 to about 30 feet per minute. In the preferred embodiment, drive motor  70  is an adjustable speed, fractional horsepower 110-volt AC motor. Reduction gearbox  75 , drive motor  70  and cog drive  80  are mechanically supported by upper lateral supports  15 . In the preferred embodiment, the track, motor and drive components are provided by Pacline Corporation of Canada.  
         [0040]     Support frame  20  and track  30  are partially enclosed by housing  40 . Housing  40  includes a front side  42 , a back side  43 , a top  44  and a bottom  45 . The front and back sides, top and bottom are made of aluminum panels fitted to the frame and sealed in place to form a generally airtight box. Housing  40  forms a gas basin  202 . In the preferred embodiment, the gas basin is approximately 3 feet deep and comprises a volume of approximately 60 cubic feet. Of course, in other embodiments, the depth of the gas basin and its volume can be raised or lowered to accept smaller or larger products, respectively.  
         [0041]     Housing  40  has 4 openings, an entrance  50 , an exit  60 , a front curing chamber access portal  55  and a back curing chamber access portal  65  (shown best in  FIG. 4 ). Entrance  50  and exit  60  are positioned to block light emitted from the curing basin. Entrance  50  is loosely covered by curtain  55 . Exit  60  is loosely covered by curtain  65 . Curtains  55  and  65  are comprised of strips of transparent polypropylene in the preferred embodiment. In other embodiments, other curtain materials may be effectively utilized which give way to regular sized objects easily. Front curing chamber access portal  55  is sealed by a front access panel  66 . Front access panel  66  is designed to slightly overlap front curing chamber access portal  55 . Front access panel  66  has a around its perimeter a rubber seal  67 . In its closed position, rubber seal  67  forms a fluid tight seal between front access panel  66  and front side  42 .  
         [0042]     As seen in  FIG. 5 , rear access panel  68  is designed to overlap the back of curing chamber access portal  65 . Rear access panel  68  is provided with a peripheral rubber seal  69  around its inside edge. When in its closed position, rear access panel  68  forces rubber seal  69  against back side  43 , providing a fluid tight seal. Front access panel  66  is mechanically attached to front side  42  through machine screws. Similarly, rear access panel  68  is attached to curing chamber access portal  65  through machine screws. Of course, other removable attachment devices can be used.  
         [0043]     The front access panel and the rear access panel, respectively, provide access to front light caddy  100  as seen in  FIG. 1  and rear light caddy  102  as seen in  FIG. 5 . Front light caddy  100  is mounted on sliding track  105 . Sliding track  105  allows front light caddy  100  to be extended outward through front curing chamber access portal  45  and removed. Rear light caddy  102  is supported by siding track  106 . Siding track  106  allows rear light caddy  102  to be moved linearly out of back curing chamber access portal  65 . In use, each sliding track is used to remove the light caddies for cleaning and maintenance. In other embodiments, the sliding track may be replaced by a remotely actuated linear actuator such as a hydraulic or pneumatically driven piston and cylinder. In other embodiments, the linear slide may be replaced by a set of roller bearings or linear wheels on opposed sides of the light caddy to enable easy movement with respect to the curing basin. In yet another embodiment, the light caddies may be hinged to pivot out of the access portal facilitating access to the reflectors and rack lights.  
         [0044]     In the preferred embodiment, only two light caddies are employed. In alternate embodiments, additional light caddies can be employed to add additional light intensity. The light caddies provide an optimum geometry for generation and delivery of light to the product through a combination of angled light support panels and cooperating parabolic reflectors.  
         [0045]     The structure of each light caddy can best be seen from  FIGS. 6   a  and  6   b.  The light caddies are mirror images of each other and therefore, only one will be described for brevity. Front light caddy  100  includes a reflector support frame  110 . Reflector support frame  110  is made up of aluminum channel in a box-like structure including bottom support rails  112 , upright support rails  114  and light support panel  116 . Light support panel  116  supports rack light  130 . Rack light  130  is positioned in light support panel  116  to shine through and be directed toward product  90 . The light support panel in the preferred embodiment forms an additional 45° angle with the upright support racks and serves to direct the light of the rack light toward the center of the product. The interior of reflector support frame  110  is covered with a flexible reflector  120 . In the preferred embodiment, flexible reflector  120  is polished to a reflectivity of between 80 and 90% and forms a parabolic curve along the interior of support frame  110 . The parabolic reflector has a focal point to direct light from each of the rack lights toward the center of the product. When assembled, the parabolic reflections of both light caddies completely irradiate the bottom and sides of the product with light from the opposing rack lights. Flexible reflector  120  in the preferred embodiment is made of stainless steel available from Superior Company of in a size of 44 inches by 23 inches, part no. 020-00183. In alternative embodiments, flexible reflector  120  is made of polished aluminum or reflectorized flexible plastic.  
         [0046]     Rack light  130  in the preferred embodiment houses mercury filled ultraviolet lamps having an arc length of approximately 6 inches drawing approximately 200 watts of current. The lamps have a warm-up time of approximately 3 to 5 minutes in order for the arc to create sufficient plasma to generate ultraviolet light. In the preferred embodiment, rack light  130  is model number MC6-200, manufactured by Ultraviolet Systems, Ltd. of Houston, Tex., USA. In alternate embodiments, ultraviolet lamps having low, medium or high pressure gas can be used as well as doped lamps including amalgam, gallium or iron. Each of the rack lights has an integral cooling fan of sufficient power to maintain the rack light at an acceptable continuous operating temperature. In other embodiments, other power ranges and gas mixtures can be utilized to cure different coatings.  
         [0047]     As shown in  FIGS. 6   a  and  6   b , when used in combination, front light caddy  100  and rear light caddy  102  illuminate product  90  from all sides with high intensity ultraviolet light.  
         [0048]     The relationship in position between track  30  and light caddies  100  and  102  is also shown in  FIGS. 6   a  and  6   b . Descending/ascending section  24  of track  30  is centrally placed between light caddies  100  and  102 . Additionally, the lowest juncture of descending/ascending section  24  occurs such that hangers  15  and product  90  extend below and into the light generated by light caddies  100  and  102 .  
         [0049]     Returning to  FIG. 4 , gas manifolds  175  and  176  can be seen at the base of gas basin  202  in housing  40 . In the preferred embodiment, gas manifolds  175  and  176  are placed in the bottom of housing  40  and are held in place through attachments to lower lateral support members  26  and  27  of frame  20 . A top view of gas manifolds  175  and  176  is shown in  FIG. 10 . Gas manifold  175  is comprised of ¾ inch aluminum tubing having a threaded opening  177  and a sealed end  178 . Likewise, gas manifold  176  is a threaded opening  179  and a sealed end  180 . Both gas manifolds  175  and  176  are perforated on their interior with multiple gas orifices. In the preferred embodiment gas orifices are drilled with a No. 20 drill at about a 120 degree angle from the top of each manifold. Drilling the gas orifices at this angle provides for a gas flow out of the manifolds at a downward angle toward bottom  45  of housing  40 .  
         [0050]     Returning to  FIG. 1 , threaded opening  177  and  179  of gas manifolds  175  and  176  respectively, are connected to an evaporator  200  through hoses  195  and  196 . In the preferred embodiment, evaporator  200  is available from Carbo Tech, Inc. of Monroe, Ga., USA. In the preferred embodiment, the evaporator can produce a flow rate of up to 720 lbs/hr at 70° F. Evaporator  200  is connected to CO 2  cylinder  210  through hose  205 . In the preferred embodiment, a pressure regulator  215  and an electrically variable gas valve  216  control the flow of CO 2  from CO 2  cylinder  210  to evaporator  200  and in turn, to gas manifolds  175  and  176 . In the preferred embodiment, the pressure regulator is part no. SR-310 500 PSIG available from Victor Company of Denton, Tex., USA  
         [0051]     In an alternate embodiment, CO 2  cylinder  210  and evaporator  200  can be replaced by a CO 2  vaporizer. In this alternative embodiment, an adequate CO 2  vaporizer is sold under the brand name MV6, of 150 lbs/hr capacity, sold by Cryogenic Experts, Inc., of Oxnard, Calif., USA.  
         [0052]     Gas level sensor  732  is located within housing  40  at a height approximately equal to the lowest level of descending/ascending section  24  of track  30 . In the preferred embodiment, gas level sensor  732  is comprised of an oxygen sensor connected to an oxygen analyzer. Gas level sensors are known in the art. In the preferred embodiment, the gas level sensor is set to detect when less than 5% O 2  by volume is present in the curing basin.  
         [0053]     The functions of the apparatus are controlled by a controller  700  mounted by upper lateral supports  15 . The controller is connected to the pressure regulator  215 , the variable gas valve dial  216 , drive motor  270 , each rack light, a start switch  738 , a stop switch  736 , front access panel indicator switch  218  and rear access panel indicator switch  219  through appropriate wiring (not shown).  
         [0054]      FIG. 7  shows the logical arrangement of controller  700 . Controller  700  is operated by a programmable logic controller  710 . In the preferred embodiment, programmable logic controller  710  is a “PICO” type programmable logic controller available from Allen-Bradley of Milwaukee, Wis., USA. Of course, other controllers such as a personal computer can be used in other embodiments. In the preferred embodiment, ladder logic programming is used to instruct the programmable logic controller how to carry out its functions. Programmable logic controller  710  is connected to relay block  722 . Relay block  722  includes circuitry to convert digital signals from programmable logic controller  710  into analog signals with sufficient current to drive the various peripheral devices required by the apparatus. Relay block  722  is connected to variable gas valve  216  through a connection  724 . Relay block  722  is connected to drive motor  70  through a connection at  726 . Connection  726  includes a motor controller capable of altering the speed of drive motor  70  and applying sufficient current for that purpose.  
         [0055]     Relay block  722  is also connected to each of the rack lights through a connector at  727 . Relay block  722  is also connected to the cooling fans directly adjacent to the lamps that operate to lower lamp temperature during operation and reduce temperature after operation of the lamps to room temperature.  
         [0056]     Programmable logic controller  710  is also connected to input connector block  730 . Input connector block  730  is capable to accepting analog signals from the various peripheral devices required by the apparatus and converting them into digital signals accepted by programmable logic controller  710 .  
         [0057]     Input connector block  730  is connected to gas level sensor  732 .  
         [0058]     Gas level sensor  732  provides a variable voltage output representing the amount of oxygen present. The oxygen analyzer is connected through an RS232 port to input connector block  730  which in turn is connected to programmable logic controller  710 .  
         [0059]     Input connector block  730  is connected to front access panel indicator switch  218  and rear access panel indicator switch  219  through a connector  734 . The access panel indicator switches each produce a binary output which can be interpreted as “door open” or “door closed”. In the preferred embodiment, each of these switches is a pressure switch located between the access panel and housing  40 .  
         [0060]     Input connector block  730  is also connected to an analog stop switch  736 , an analog start switch  738 , and a numerical keypad  742  for entry of digital data, as required by the programmable logic controller to perform its functions.  
         [0061]     Programmable logic controller  740  is also connected to durable memory  728 . In the preferred embodiment, durable memory  728  is a battery backed up RAM. Of course, in other embodiments, durable memory  728  can be peripheral memory, magnetic or optical disk drives or network memory connected to the programmable logic controller through a network connection.  
         [0062]     In operation, programmable logic controller  710  initiates a program, the steps of which are shown in  FIG. 9 , to operate the functions of the curing apparatus.  
         [0063]     Referring then to  FIG. 8 , the program is initiated at start block  805 . As a first step, the program requires input to determine if it should enter run mode  817  or program mode  809 . Upon entry into program mode  809 , several parameters are required to be set for the operation of the curing apparatus. Initially, a timer is set to delay startup until the lamps have reached operation temperature. Program mode  809  then requires an input of the cool-down time for the rack lights at step  813  other parameters such as the speed of the continuous chain, rate of heavy gas flow, curing time and automatic start and stop times can be programmed in other embodiments. Upon proper entry of the required data parameters, the program returns to mode selection  807 .  
         [0064]     Upon entry into run mode at  817 , the program loads the parameters previously input in program mode. If the parameters are not present, the program returns to mode selection  807 . If program parameters are present, the program activates the apparatus by first activating gas valve  216  to initiate gas flow at step  821 . When gas flow is activated, CO 2  gas from CO 2  cylinder  210  is admitted to evaporator  200  through gas valve  216 , pressure regulator  215 , and hose  205 . CO 2  cylinder  210  provides liquid CO 2  to the evaporator. Evaporator  200  converts the liquid CO 2  into gaseous CO 2 . The gaseous CO 2  travels into pressure regulator  215  to gas manifolds  175  and  176  through threaded openings  177  and  179  and into the gas basin through the orifices.  
         [0065]     Once the gas enters gas manifolds  175  and  176 , the gas is distributed through the manifolds and enters housing  40  at its lowermost point. Referring to  FIG. 4 , it can be seen that housing  40  provides a gas basin  202 . The CO 2  gas, being heavier than air, fills gas basin  202  to a predetermined level. Consequently, the CO 2  gas displaces the oxygen and other gases present in gas basin  202  before operation of the apparatus. Gas basin  202  therefore provides an oxygen free environment within housing  40 .  
         [0066]     In other alternate embodiments, other noble gases such as nitrogen, argon, hydrocarbons,  10  or halogenated hydrocarbons can be used to provide an oxygen free environment within gas basin  202 .  FIG. 4  also shows, by a dashed line, gas level  207 .  
         [0067]     At step  823 , the program activates rack lights  130 . Depending on the type of lamp used, a warm-up period may be required. A delay is instituted as programmed in the parameters to allow the rack lights to rise to operating temperature. Upon activation of the rack lights at step  823 , the program also activates the cooling fans. Once at operating temperature, rack lights  130  produce an intense ultraviolet light which is reflected from each of the flexible reflectors  120  resulting in a high ultraviolet light intensity between the light caddies and below the surface of the heavy gas.  
         [0068]     At step  825 , the program activates drive motor  70 . Its speed is adjusted by its controller  726  to correspond with the desired speed of the continuous chain. Drive motor  70  in turn activates reduction gearbox  75  and cog drive  80  to motivate the continuous chain. Simultaneously, the program sends a message through programmable logic controller  710  to display  720  to indicate a “run” condition indicating that the curing apparatus is functioning.  
         [0069]     In use, one or more products  90  are attached to hangers  15 . The products are moved by the continuous chain in a counterclockwise fashion along upper section  22  of track  30 . Product  90  is supported and moved by continuous chain  35  around track  30 , through curtain  55  and into entrance  50  in curing apparatus  10 . Hangers  15  and product  90  follow track  30  into descending/ascending section  24  of track  30 . Upon entering the descending/ascending section of the track, the products enter gas basin  202  and fall below gas level  207 , as can be seen in  FIG. 4 . Product  90  passes between front light caddy  100  and rear light caddy  102  and underneath rack lights  130 .  
         [0070]     While in gas basin  202  and between the front and rear light curing caddies, the ultraviolet sensitive coating on the product cures. In the preferred embodiment, the product is immersed in gas basin  202  and resident between the curing caddies for approximately 1 to 2 minutes. Of course, this time can be adjusted by adjusting the speed of drive motor  70  or adding a delay to the motor of the chain.  
         [0071]     Hangers  15  and product  90  then ascend the track to lower section  23 . During the product&#39;s movement from upper section  22  to lower section  23 , the weight of the product contributes energy to the continuous chain because its potential energy while at upper section  22  is greater than its potential energy at lower section  23 . The additional energy provided by gravity acting on the product reduces the amount of drive power necessary from drive motor  70  and further reduces friction and wear on the continuous chain  35  within track  30 . Additionally, the lower height of lower section  23  allows for a shorter overall track length by eliminating an added section of track required to return to the height of upper section  22 .  
         [0072]     Returning to  FIG. 8 , the program enters a loop after step  825 , starting at step  829  by checking gas level sensor  203  to determine if gas basin  202  is indeed completely filled with heavy gas. The variable voltage output of gas level sensor  732  is used by programmable logic controller  710  to proportionately open or close gas valve  724 . If the oxygen reported by gas level sensor  732  is high, programmable logic controller  710  opens variable gas valve  216  proportionately to allow more CO 2  to enter gas basin  202 . As the level of oxygen drops, gas level sensor  732  proportionately reduces voltage to input connector block  730  and programmable logic controller  710 . If not, an alarm display is sent to display  720  by programmable logic controller  710  at step  831 . If the gas level is sufficient, then the program checks to assure that front access panel indicator switch  218  and rear access panel indicator switch  219  are closed. If not, an alarm is sent from programmable logic controller  710  to display  720  at step  835 . If both access panels are indeed closed, the program proceeds to step  837 . At step  837 , programmable logic controller  710  polls the input connector block  730  to determine if stop switch  736  has been activated. If not, the loop returns, repeating step  829  and following, allowing continuous function of the curing apparatus.  
         [0073]     After leaving gas basin  202 , product  90  supported by hanger  15  is moved by guide chain  35  through curtain  65  at exit  60 , leaving curing apparatus  10 . During the process, the uncured UV coating on product  90  becomes cured and hardened. The product is then removed from hangers  15 , completing the process.  
         [0074]     If the stop switch  736  has been activated, then a cool-down procedure is initiated at step  839 . Upon initiating cool-down, gas flow is terminated at step  840  by deactivating solenoid valve  216 . At step  841 , drive motor  70  is deactivated through a gradual slowing of its speed to zero to avoid an instantaneous stop of all products supported by the continuous chain. Once the motor has been deactivated, the program deactivates the rack lights at step  843 . At step  845 , programmable logic controller  710  sends a cool-down display message to be displayed by display  720 . The cooling fans are allowed to run for the time indicated by the parameters  819  as set by step  813  in program mode  809 . At step  849 , the display is sent a “stop” message indicating, indicating a stop condition of the apparatus and the program terminates at step  851 . After step  851 , a loop is entered, checking for start condition  738  which will then return the program to step  805 .  
         [0075]     In an alternate embodiment, the steps carried out by programmable logic controller  710  in program  800  can be accomplished manually. In this process, the drive motor and gas are manually activated. Gas level  207  is maintained in gas basin  202  by a hand held device suitable for monitoring CO 2  levels or alternately, the lack of oxygen levels.  
       EXAMPLE 1  
       [0076]     A preferred example of the radiation hardenable curing an inert gas composition and process parameters are given below.  
         [0077]     Irradiation hardenable coating comprising 35 weight % Laromer™ LR 8987 (available from BASF Corporation of Germany), 20 weight % urethane acrylate hexandioldiacrylate, 38.5 weight % Laromer™ LR 8863, (available from BASF Corporation of Germany), 3.5 weight % polyetheracrylate Iragucure™ 184 (Ciba Corporation).  
         [0078]     0.5 weight % of a Photoinitiator Lucirin™ TPO (available from BASF Corporation).  
         [0079]     2 weight % Tinuvin™ 400 (Ciba Special Chemistry), 1.5 weight % UV absorber Tinuvin™292.  
         [0080]     In this example, each rack light had a power rating of approximately 500 watts placed a distance of approximately 6 inches from the product. The travel speed for the conveyor was approximately 15 feet per minute.  
         [0081]     In this example, the gas level was approximately 30 inches. In this example, the gas level was CO 2  with less than 5% oxygen present in the curing basin. The resulting finish on the product was clear lacquer and was highly scratch resistant.  
         [0082]     This invention is susceptible to considerable variation in its practice. Accordingly, this invention is not limited to the specific exemplifications set forth herein above. Rather, this invention is within the spirit and scope of the appended claims, including the equivalents thereof available as a matter of law.  
         [0083]     The patentees do not intend to dedicate any disclosed embodiments to the public, and to the extent any disclosed modifications or alterations may not literally fall within the scope of the claims, they are considered to be part of the invention under the doctrine of equivalents.
A system, apparatus, and method is provided for curing ultraviolet (UV) curable coatings on articles using UV lamps while the article is immersed in an atmosphere of inert gas heavier than air. One example of a UV curable coating includes 35 weight % Laromer™️ LR 8987, 20 weight % urethane acrylate hexandioldiacrylate, 38.5 weight % Laromer™️ LR 8863, 3.5 weight % polyetheracrylate Iragucure™️ 184, 0.5 weight % of a Photoinitiator Lucirin™️ TPO, 2 weight % Tinuvin™️ 400 and 1.5 weight % UV absorber Tinuvin™️ 292. Examples of the inert gas used in the process disclosed include carbon dioxide, nitrogen, argon, hydrocarbon and halogen gases. An example of an apparatus provided by the invention includes a suspended track system; a housing, wherein the housing comprises an internal portion of the suspended track system and a curing chamber having highly reflective surfaces of favorable geometry. Further provided is a plurality of UV lamps, wherein the lamps are disposed on a slidably removable curing caddy system, wherein the slidable curing caddy enables lamp replacement and general interior maintenance of the apparatus. Further provided is an evaporator and alternatively, a vaporizer for providing a heavy gas supply. Further provided is a controller and software to coordinate the functions of the apparatus disclosed.