High speed, low temperature and pressure diazo processing method
BACKGROUND OF THE INVENTION 
     Diazo sensitized papers have been used for a long time for making duplicate copies of originals, normally by contact printing and the subsequent development of the exposed diazo paper in an aqueous ammonia vapor atmosphere. In such an application, resolution requirements and development times are not critical. More recently diazo sensitized films have received increasing attention as an ideal medium for making microfilm or microfiche masters and, perhaps more importantly, duplicates thereof because of the relatively low cost of such film, its high resolution capability, etc. For such applications, however increasingly stringent demands are made on the film developing process, particularly as to the speed with which it can be accomplished so as to enable an efficient, high volume production of diazo film copies from a master, for example. 
     In this regard, problems have been encountered in the past. In order to attain short development times for diazo film, it was heretofore thought necessary that the development takes place in a high pressure ammonia atmosphere. In general, the pressures that were considered necessary are substantially above, e.g., several times the atmospheric pressure and they ranged up to as high as 1,000 psi or more. For example, U.S. Pat. No. 3,411,906 speaks of ammonia pressures in the range of between 50 and 1,000 psi. Little attention was paid to the actual design of the developing chamber into which the ammonia is introduced other than to maintain the volume relatively small for the obvious expedient of limiting the amount of ammonia that is expended in the developing process. 
     From a practical view, however, such high pressure requirements represent severe drawbacks, particularly in connection with a continuously operating diazo film developer since the exposed film must be transported from the exterior into the high pressure atmosphere. For one, ammonia leakage is quite unacceptable because of its noxious odor and the potential health hazard it represents if present in appreciable concentrations. Further, it is notoriously difficult to seal a pressurized chamber if continuous access to its interior is required unless the ammonia in the chamber is evacuated each time a film is inserted therein or withdrawn therefrom. This, however, is not compatible with a high speed, high volume operation. 
     Although the above-referenced U.S. patent does not concern itself with the actual chamber construction and is not concerned with the above summarized difficulties of operating it, another U.S. Pat. No. 3,364,833 proposes the construction of a diazo film developing device comprising a sealed chamber defined by a base having a cavity dimensioned to receive the film and a cover that is bolted and sealed against the base. The space of the cavity is kept as small as possible and once it is sealed, the air therein is evacuated and replaced with high pressure ammonia to develop the film. Although this device no doubt assures the full and complete development of the film and, if operated along the lines suggested in the earlier referenced U.S. patent yields short development times, the insertion and removal of the film from the cavity must be manually performed and surely exceeds the development time for the film by a very large factor. Thus, the device disclosed in U.S. Pat. No. 3,364,833 may be ideally suited for developing an individual diazo film from time-to-time; but it is unsuitable for continuous, large volume operations. 
     Thus, in spite of the advantages afforded by diazo film for high resolution, high volume applications such as for the duplication of microfiche masters, there is presently no technologically feasible device for economically mass developing such film. 
     SUMMARY OF THE INVENTION 
     The present invention provides a diazo film developing method and apparatus which dramatically departs from prior art concepts for the construction and operation of diazo film developers to render them compact, efficient and easy to operate at even very high film output volumes. This is accomplished by dispensing with the high ammonia pressures heretofore thought necessary to achieve short developing times. Instead, in the method of the present invention, the film is developed in aqueous ammonia vapor at a pressure which does not substantially exceed atmospheric pressure. For purposes of the present invention, this means that the pressure is only slightly higher than atmospheric pressure, by an amount no more than that required to introduce the vapor into the developing chamber. Thus, the pressure is typically in the area of no more than a few, say 1 to 2 inches of water column above atmospheric pressure and in any event, it is substantially less than the heretofore suggested ammonia pressures and, therefore, will always be less than one atmosphere (about 14 psi) above atmospheric pressure. 
     This low pressure ammonia vapor is combined with a minimal developing chamber volume which is no greater than that required to conveniently pass a film to be developed through the chamber. The low pressure ammonia vapor and small developing chamber volume is further coupled with relatively low operating temperatures in a range of between about 150° F. to 200° F. and, preferably, between about 175° F. to 190° F. so that the emulsion layer on the film is not softened by heat. In this manner, moving contact between the film and in particular its emulsion and components of the chamber will not damage the emulsion. This facilitates the minimization of the developing chamber volume since an actual contact between the moving film emulsion and chamber walls does not adversely affect the images on the film. In fact, applicants presently believe that such a moving contact between the emulsion and chamber walls, at least so long as it takes place intermittently, enhances the development of the film by more intimately subjecting the emulsion to the ammonia vapor and removing from the emulsion any expended ammonia. 
     Thus, in general terms the method of the present invention contemplates to form a developing chamber which is defined by first and second, parallel surfaces, the spacing between them being maintained so that it only slightly exceeds the thickness of the film to enable the passage of the film between the surfaces. In a practical embodiment such as spacing is normally between about two to about eight times the film thickness, with a spacing of about 0.020&#34; being presently preferred because of the ease with which the component making up the chamber can be manufactured, the ease with which the film can be transported between the surfaces in actual use, and the relatively very low volume exhibited by a chamber having such dimensions. 
     Further, the method of the present invention contemplates that the film is advanced in a downstream direction through the chamber at a speed so that the stay time for (any part of) the film does not substantially exceed a few seconds, say five seconds and, preferably, so that the stay time is no more than about one or two seconds. Aqueous ammonia is introduced into the chamber from the chamber surface which faces the emulson side of the film. The ammonia vapor is of substantially atmospheric pressure, that is it is only slightly, e.g., a few inches of water column above atmospheric pressure. 
     The vaporization and (minimal) pressurization of the ammonia is achieved by providing a heated platen which defines the surface that faces the emulsion side of the film and forming an open conduit therein which extends through the film facing surface of the platen. The platen is heated to the above-stated developing temperature and aqueous ammonia is flowed into the conduit where it is vaporized due to the elevated temperature. This results in the above-discussed slight rise in the ammonia vapor pressure of a few inches of water column so that the vapor discharges from the conduit into the chamber. In a preferred embodiment of the invention, a metered amount of aqueous ammonia is pumped into the conduit for each film to be developed. This can be done by using an appropriately designed pump or by sensing an approaching film and intermittently actuating a metered aqueous ammonia pump in response thereto. 
     The above summarized developing method is performed with apparatus of the present invention which, generally speaking, comprises a housing that defines an upstream film intake opening and a downstream film outlet opening. First and second platens are disposed within the housing and opposing, parallel first and second surfaces of the platens are positioned to receive incoming film from the intake opening and to discharge outgoing film to the outlet opening. Means is further provided for maintaining the spacing between the platens in the above-outlined range so as to permit the uninhibited passage of the film between the surfaces while minimizing the spacing between the surfaces and, therefore, the volume of the developing chamber defined by the space between the platens. Pairs of cooperating rollers advance the film (with its emulsion side facing the first surface) in a downstream direction from the intake opening through the chamber to the outlet opening. 
     The present invention further provides means for sealing the developing chamber from the intake and the outlet openings. The sealing means comprise sets of cooperating, elongate, opposite rollers disposed adjacent and parallel to the openings for receiving and discharging the film. The rollers have resilient surfaces in mutual contact and low friction e.g., Teflon ( a registered Trademark of the Dupont de Nemours Company) strips are sealingly disposed in corresponding grooves of the housing, are arranged parallel to the rollers, and are resiliently biased thereagainst so as to form a seal therewith and prevent the escape of ammonia vapors through the openings to the exterior. Although such a seal would not be sufficient to withstand the high ammonia pressures found in prior art diazo film developing systems, it is sufficient to form a seal during the low pressure operation of the apparatus of the present invention so that no noxious ammonia odors escape. Consequently, a device constructed in accordance with the invention can be operated in closed rooms with little or no separate ventilation without representing a health hazard or creating an objectionable stench or odor. 
     Further, the apparatus of the present invention includes a heater for heating the first platen to a temperature in the above-stated range and at least one conduit is formed in the first platen, communicates with the first surface and is located proximate the upstream end thereof. Pump means supplies to the conduit aqueous ammonia and means is provided for evaporating the ammonia therein to effect a discharge of the ammonia vapor from the conduit at the above-discussed low pressure. Thus, as the diazo film advances through the developing chamber, it is contacted by the ammonia vapor and causes the formation of a turbulent ammonia vapor layer between at least portions of the emulsion and the first surface to effect the rapid developing of the emulsion at low temperatures and pressures. 
     The first surface preferably includes a transverse groove which fluidly communicates with the ammonia conduit to effect the distribution of the vapor over the full film width. Additional grooves downstream of the first mentioned groove enhance the developing speed believed to be at least in part caused by the increased turbulence in the ammonia vapor which is caused by such grooves. 
     To further enhance the developing process the film itself is preferably preheated before it enters the developing chamber. In a preferred embodiment of the invention, this is accomplished by passing the film between two platens heated to the same temperature as the first platens, and disposed upstream of the developing chamber. 
     From the foregoing summary, it is apparent that the present invention achieves the high speed developing of diazo-type film without having to rely on the high pressures required in prior art diazo developing systems. This, coupled with the small chamber dimensions enables the very rapid development of microfiche, often in less than one second. It conserves ammonia and, most importantly, it enables a very simple construction of the chamber and the film transport mechanism while assuring an effective seal to prevent the escape of ammonia into the surrounding atmosphere. Therefore, the present invention is ideally suited for incorporation in microfiche duplicators and its wide acceptance throughout the industry is fully anticipated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall side elevational view, in section, of a diazo film developer constructed in accordance with the present invention for incorporation in a microfiche copier or the like; 
     FIG. 2 is a front elevational view, in section, of the developer shown in FIG. 1 and is taken on line 2--2 of FIG. 1; 
     FIG. 3 is an enlarged, schematic side elevational view of the developing chamber shown in FIG. 1; and 
     FIG. 4 is a schematic plan view of the platen defining the side of the chamber shown in FIG. 3 which faces the emulsion side of the film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, a diazo film developer 2 may comprise part of a microfiche duplicator 4 (schematically shown only) which includes suitable means for exposing such film and for thereafter advancing it towards the developer (not separately shown in the drawings). The developer itself generally comprises a housing 6 which has upright side walls 8 and spaced apart end walls 10 and which is normally horizontally positioned. The end walls 10 define an upstream intake opening 12 through which a microfiche 14 may enter the interior of the housing. The other, opposite end wall of the housing defines an outlet opening 16 which is aligned with the intake opening. A flat cover 18 is placed over the housing and a gasket 20 seals the interior space from the exterior. Suitable closure hinges 22 retain the cover to the housing while they assure the formation of a seal between the cover and the gasket. 
     In the presently preferred embodiment of the invention the housing interior is divided into a pair of serially arranged upstream and downstream cavities 24, 26 by a first pair of cylindrical, parallel drive rollers 28 positioned at about the center of the housing interior. A second and a third pair 30, 32 of like drive rollers is disposed adjacent the intake and the oulet openings, respectively. The rollers are in mutual contact along a line aligned with the center of the intake and the outlet openings, they are constructed of a resilient material and they are biased against each other so that the rollers form an airtight seal between them. 
     A groove 34 in the housing is adjacent and parallel to each roller of the second and third roller pairs. Each such groove receives an elongated low friction, e.g., Teflon strip 36 which is biased against the periphery of the adjacent roller by a resiliently compressible member 38 such as a foam rubber pad. The sealing engagement of the rollers themselves and their sealing engagement of the low friction strips 36, seals the housing interior from the intake and outlet openings 12, 16. The rollers are further journaled in bearings (not shown) carried by the housing side walls 8 which form a seal against the end faces of the rollers. A drive, such as a chain or sprocket drive (not shown) rotates the rollers of each pair in opposite directions so that a microfiche 14 placed between the upstream, second roller pair 30 is grasped and advanced in a downstream direction into the first, upstream housing cavity 24. Roller pair 28, which may be sealed against the housing and the cover in the above described manner with suitably placed seal strips 40, thereafter grasps the downstream moving fiche and advances it towards the third roller pair which discharges such film through outlet opening 16 into a receptacle 42. It will be noted that during this transport of the film the housing interior remains fully sealed irrespective of the speed and/or frequency with which fiche is fed through the developer. 
     In the presently preferred embodiment of the invention the upstream cavity 24 is utilized for the preheating of microfiche 14 before it is developed in the downstream cavity 26. For this purpose, a pair of parallel, opposing upper and lower heating platens 44, 46, respectively are placed in the upstream cavity. The lower platen rests in a rectangular groove 48 on a raised frame 53 projecting from a bottom plate 50 of the housing and spaces the platen therefrom so that a plate heater 52 can be attached to the underside of the platen for heating it to the desired temperature as is further discussed below. Edges 54 (shown in FIG. 2 only) of the platens overlap, are raised and provided with interengaging grooves to maintain the platen in mutual alignment and at the desired spacing so that opposing platen surfaces 56, 58 are spaced apart a sufficient distance to permit the passage of a microfiche therebetween. In a typical embodiment, the spacing is approximately 0.02&#34;. Grooves 60 may be formed in the platen to prevent the adherence of the film to one or the other. Further, the edges are spaced apart so that the effective width of the opposing platen surfaces 44, 46 is just slightly larger than the width of the fiche 14. 
     A pair of generally Z-shaped leaf-springs is mounted, e.g., welded or bolted to the underside of cover 18 and, when the cover is secured to the housing, exerts a downward pressure against the upper platen 44 so as to maintain it in firm contact with the lower platen. 
     A second pair of developing platens 64, 66 is disposed in the downstream cavity 26. The construction of the developing platens is generally similar to that of heating platens 44, 46. Thus, the lower platen rests in an other rectangular groove 48 formed in a raised frame 53 so as to provide space for a heater 68 attached to the underside of the lower plate. The opposing surfaces 70, 72 of the developing platens define between them a developing chamber 85. Their spacing &#34;T&#34; is closely controlled and in the presently preferred embodiment is 0.02&#34; for accommodating microfiche 14 having a thickness &#34;t&#34; of between 0.003 to about 0.007&#34;.At the indicated dimensions microfiche is readily transported in a downstream direction, to the left as viewed in FIGS. 1 and 3, without undesirable interference from the opposing platen surfaces. 
     The lower platen includes a first groove 74 adjacent the upstream end of the platen which extends over the full effective width of the platen and which is convexly curved when viewed in a downstream direction, that is downwardly as seen in FIG. 4. Additional, similarly shaped grooves 76 and disposed in the lower platen and arranged downstream of groove 74. An open conduit 78 is formed in the lower platen 66 and terminates at about the bottom center of groove 74. Thus, it is also located adjacent the upstream end of the lower platen. The conduit is connected with an aqueous ammonia reservoir 80 via a metering pump 82 and a valve 84 so that upon the actuation of the pump aqueous ammonia is flowed into the conduit. The earlier mentioned Z-shaped leaf-springs 62 are employed to bias the upper developing plate 64 downwardly against the lower plate when cover 18 is closed. 
     Turning now to the operation of the developer 2, heater control 86 is initially actuated to energize heater 52 of the lower preheating plate 46 and heater 68 of the lower developing platen 66. In this connection, it should also be noted that a heater may also be applied to the upper platens 44 and 64 although under normal operations of the developer that is not necessary. The heater control maintains the platen temperature within the desired range, e.g., between 150° F. to 200° F. and, preferably in the vicinity of 175° F. to 190° F. 
     Metering pump 82 may be selected so that it pumps a very low volume of aqueous ammonia which is selected to provide just enought ammonia to develop the fiches at whatever rate they pass through the developing chamber. Alternatively, the metering pump may be an intermittently operating pump which is selectively activated in response to an approaching microfiche 14. For that purpose the microfiche duplicator 4 includes a sensor 88 (such as an optical sensor, for example) which is operatively coupled with the pump and activates the pump each time a microfiche approaches housing intake 12, for examle, to flow a metered amount of aqueous ammonia to conduit 78. In an alternative operational mode the sensor 88 may be coupled with the valve 84 downstream of pump 82 to temporarily open the valve to flow the desired amount of ammonia to the conduit. 
     The conduit has a configuration so that ammonia is heated to about the temperature of the platen while in the conduit. This results in the evaporation of the ammonia therein and the above-discussed slight pressure build up so that ammonia vapor escapes from the end of conduit 78 which terminates in the upstreammost groove 74. When the platen is operated at the stated temperature range and the conduit has a diameter of 1/16ths inch, a conduit length within the platen of 21/2 to 3 inches is sufficient to effect the desired evaporation of the ammonia. 
     The drive (not separately shown) for roller pairs 28, 30 and 32 may be continuously or intermittently operated, in the latter case it is suitably coupled with sensor 88. 
     A microfiche 14 to be developed which approaches intake opening 12 triggers sensor 88 and causes a metered amount of aqueous ammonia to be flowed to conduit 78 where it evaporates and escapes into the developing chamber for the film. Since there is a necessary lag between the introduction of aqueous ammonia to the conduit and its evaporation the pump is operated a short time period before the fiche arrives at the chamber. 
     Once the leading edge of the fiche is grasped by roller pair 30 the fiche is driven in a downstream direction through the space between the heating platens with its emulsion side 90 facing downwardly, that is oriented so that it faces the lower, heated platen 46. The fiche, and in particular, the emulsion carried thereon is heated to about the temperature prevailing in the developing chamber. After its leading edge issues from the downstream end of the heating platen roller pair 28 advances it into and through developing chamber 85. 
     Since the developing chamber has a height which is only slightly greater than the thickness of the fiche the downstream movement of the latter causes a great deal of turbulence in the ammonia vapor escaping from conduit 78 against the emulsion side of the film. Frequently, the emulsion side will either contact portions of the opposing platen surface 72 or pass closely adjacent to them to further increase the turbulence of the ammonia vapor. Additionally, as it moves over the surface and the grooves therein, the ammonia vapor therebetween retains its turbulence. It is believed that this action provides repeated access for fresh ammonia vapor to the film emulsion and significantly contributes to its the high speed development. The fully developed film is then discharged from the developer into receptacle 42. 
     For the above-outlined developing parameters, it has been found that developing times of as little as one second, or in some instances, even less than that, can be achieved so that the film can be moved through the developer at relatively high speeds. 
     This is achieved with a very low pressure ammonia vapor which is readily sealed so that practically no ammonia odors are noticeable even in close proximity to the developer. 
     To remove ammonia condensate from the housing interior a drainage opening 94 is preferably provided beneath developing plate 66 which is connected with a condensate receptacle 96 via a hose 98 or the like. It should further be observed that by placing the Z-shaped leaf-springs 62 over the gap between the respective ends of the developing plates and the adjacent roller pairs 28, 30 any ammonia condensation which may form on the underside of cover 18 is guided along the spring onto the top developing plate, from where its flow may be guided towards drain 94 via the sides of the platens to prevent ammonia droplets from contacting the film. Further, to evacuate all ammonia vapors from the housing interior preparatory to opening cover 18, an air blower 100 is provided to force air into the interior and to thereby correspondingly force the ammonia vapors through the drainage opening 94 into condensate receptacle 96 where it can be absorbed in a suitable liquid, e.g., water bath.