Abstract:
A system ( 799 ) and method for continuous web sublimate dye transfer printing uses platens ( 800, 805 ) which also act as air bearings. The platens are heated by resistive heating elements ( 825 ) or other means. A sandwich of air-impermeable dye-image donor tissue ( 1100 ), the medium to be printed ( 1140 ), and air-impermeable backup tissue ( 1115 ) are fed through the heated air bearing where the dye transfer step occurs. The tissues and medium are supplied on supply rolls ( 1105, 1120,  and  1145 ) which are restrained by braking mechanisms (not shown). Take-up rolls ( 1110, 1160,  and  1135 ) are driven by motor-and-clutch mechanisms (not shown) so that the tissues and medium to be printed move through the heated region between the platens without sliding past one-another. The platens are forced together on either side of the tissue-medium sandwich with sufficient pressure to prevent the sublimate dye gas from migrating sideways through the medium being printed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of provisional patent application, Ser. No. 60/490,370, filed Jul. 26, 2003. 

   FEDERALLY SPONSORED RESEARCH 
   None 
   SEQUENCE LISTING 
   None 
   BACKGROUND 
   1. Field of Invention 
   This invention relates generally to heat transfer presses, and in particular to a heat transfer press for transferring sublimate dye images to textiles and films. 
   2. Prior-Art 
   Flat-Bed Heat Transfer Presses—FIGS.  1 - 7   
   Image transfer through dye-sublimation printing is an old and well-established art. A reverse, sublimate dye image  100  is first applied to a paper or tissue carrier or donor sheet  105  by a printing method such as a rotogravure or offset press (not shown), as indicated in  FIG. 1 . Sublimate dye formulations are well-known by those skilled in the art of dye-sublimation printing. Image  100  will be transferred from sheet  105  to a receiving medium  110  ( FIG. 2 ) in the following steps. Medium  110  can comprise textiles, plastic film, wood, and many other image-receivers. 
   The dye-bearing side of donor sheet  105  is placed against a surface of medium  110  to which image  100  is to be transferred. Sheet  105  and medium  110  are then placed in a heat press, indicated schematically by planar platens  115  and  120  in  FIG. 3 . Platens  115  and  120  typically range in size from 10 cm to over 1.50 m on a side. They can be square, rectangular, and even circular in shape. Platen  115  is typically maintained at a temperature of 200 degrees Celsius (C.). Platen  120  is generally not heated, but comprises a metal plate with a thin layer, typically 0.7 cm thick, of high-temperature rubber padding on its top surface. 
   In  FIG. 4 , image  100  is transferred from sheet  105  to medium  110  by forcing the two into intimate contact between platens  115  and  120  while heating them for a predetermined period of time, called the dwell time. Platens  115  and  120  are forced together as indicated by arrows  125  for a typical dwell time between  10  and  60  seconds in order to effect image transfer. During this time, the dye in image  100  sublimes, or passes from a solid state to a gaseous state. The resulting dye gas (not shown) is absorbed by medium  110 , permanently marking it. The magnitude of force indicated by arrows  125  is typically sufficient to cause a pressure of between 0.1 and 10 kg/cm 2  between the platens. 
   After the dwell time, platens  115  and  120  are separated, as shown in  FIG. 5 . Sheet  105  and medium  110  are removed from the press, separated, and sheet  105  is discarded. As shown in  FIGS. 6 and 7 , the reverse of image  100  has been transferred from sheet  105  to medium  110 . 
   Flat-bed heat transfer presses of the type described above are manufactured by Adams International Technologies, of Ball Ground, Ga., U.S.A. 
   While flat bed presses of the type described above work well in many applications, they are not well-suited to continuous-web manufacture since they must be opened for insertion and removal of goods and closed for a period during image transfer. 
   Flat-Bed Heat Transfer Presses for Continuous Use 
   In U.S. patent application publication No. US 2002/0148054 A1, Drake teaches a belt-type heat transfer press in which the dye donor sheet and receiving medium are transported between moving belts. The belts move the sheet and medium through heat transfer stations on a continuous basis. 
   While Drake&#39;s heat transfer press offers certain advantages of prior-art platen presses, it requires two moving belts, each comprising a low-friction surface on one side, and a high-friction surface on the other. The low-friction surface allows the belts to slide along a support surface, while the high-friction side holds the donor sheet and receiving medium firmly in place. These belts are moved by pulleys attached to a frame. Despite the presence of the low-friction surface, significant frictional forces must be overcome to move the belt. In addition, wear of the belt is a consequence of its sliding along its supports. 
   Transport Mechanism with Rolling Belts Supported by Air Bearings 
   In U.S. Pat. No. 4,495,021 (1985), Goldsworthy teaches a system for maintaining pressure on a length of laminate as it moves through a processing station. The laminate is squeezed between two moving belts. Force is applied to the back of each belt by an air bearing. Large forces can be applied by an air bearing, yet little frictional force results since the supported surface rides on a film of air. 
   While the air bearings provide an improvement by reducing friction due to belt motion, Goldsworthy&#39;s system merely applies pressure to a laminate by squeezing it between two belts. 
   Transport Mechanism with Carrier Tape Supported by Air Bearings 
   In U.S. Pat. No. 4,594,129 (1986), Bok teaches a floating transport mechanism in which substrates are attached to a tape belt and moved through a plasma discharge processing station within a vacuum chamber. The belt and substrates are supported by nearly-frictionless air bearings. Cold gas passed through the bearings is also used to cool the belt and substrates after processing of the substrates in a plasma discharge. 
   While air bearings are used to provide cooling, support, and nearly frictionless motion, they are not directly involved in the processing of the wafer. 
   Transport Mechanism for Continuous Honeycomb Panel Molding Method 
   In U.S. Pat. No. 5,037,498 (1991), Umeda teaches a method and apparatus for the continuous production of a honeycomb panel laminated with a prepreg material. As part of the curing and finishing process, he uses two opposed, pre-loaded air bearings which apply heated air to the assembled honeycomb sandwich. The air bearings are 120 cm square and are pre-loaded with a force of 800 kg, resulting in a pressure at the work surface of 55.6 g/cm 2 . Air at 130 deg. C. flowing through each bearing both flattens and post-cures the materials in his honeycomb sandwich. 
   While this system doesn&#39;t print dye-sublimation images on Umeda&#39;s panels, it does show the use of air bearings to provide heated air and a low-friction processing step. 
   Sublimatic Printing Machine 
   In U.S. Pat. No. 3,949,574 (1976), Glover teaches a system which transfers a sublimate dye image by heating a donor sheet and receiving medium in a flat platen press. Both platens of the press are porous and supplied with air flow. Air passes from the heated platen through the donor sheet, carrying the gaseous phase of the dye into the receiving medium, typically a rug or carpet. The second platen is optionally connected to a vacuum source, further drawing the sublimed dye into the receiving medium. The result is deep penetration of the dye into the medium. 
   While Glover&#39;s system accomplishes improved dyeing, it does not perform on a continuous basis. His platens must be separated to introduce a new donor sheet and receiving medium for each piece to be printed. 
   Rotary Heat Transfer Presses 
   Belt-and-drum, rotary heat transfer presses are well-known to those skilled in the art of dye-sublimation printing of textiles and films. Similar presses are taught by Miller in U.S. Pat. No. 4,710,271 (1987) and U.S. Pat. No. 4,889,048 (1989), Haigh in U.S. Pat. No. 3,319,352 (1967), and many others. While their end use as taught may be different, the structure of all these is similar to a dye-sublimation heat transfer press. 
   In these presses, a large, rotating drum is typically filled with hot oil. A thick fabric belt is wrapped around most of the circumference of the drum, then passes over rollers which guide the web around the back side of the drum. 
   A sandwich of fabric to be printed and a previously-printed donor sheet are fed into the nip between the drum and the fabric web as the drum rotates. The two are held at a high temperature for a dwell time determined by the rate of rotation of the drum. As they emerge from the other side of the drum, the fabric and donor sheet are separated and the dye-transfer printing is complete. 
   Such rotary dye-sublimation transfer printing presses have been in use for many years. Drawbacks to their use include significant initial equipment cost, and the cost and labor associated with replacing the belt. In addition, a significant amount of heat is removed from the drum by the belt and lost to the ambient atmosphere as the belt travels around its path and back to the drum. Additional heat is lost by the exposed surface of the drum adjacent the nip where the fabric and donor sheet are introduced, and the point at which they exit contact with the drum. 
   Thus while such transfer presses perform their intended task, they are expensive, large, and inefficient. 
   OBJECTS AND ADVANTAGES 
   Accordingly, several objects and advantages of the present invention are to provide an improved sublimate dye-transfer-printing system which can print a continuous web without the interruption of multiple transfer operations, which does not employ a belt wrapped around a drum to provide dwell time at an elevated temperature, which is simple in construction and low in cost, and which employs air bearing technology to reduce friction thereby reducing mechanical drive requirements to move the fabric and donor sheet through the heat transfer zone. Other objects and advantages are to utilize the heat gained during pressurization of the air for the air bearings so that only supplemental heating of the air bearing platens is required, resulting in a thermally efficient system. 
   Additional objects and advantages will become apparent from a consideration of the drawings and ensuing description thereof. 
   SUMMARY 
   In accordance with the present invention, a method, apparatus, and system are provided for producing a low-cost dye-sublimation transfer printing press. A donor sheet a receiving medium, and a backup sheet are maintained in intimate contact as they are effortlessly drawn through the apparatus as a continuous web, and forced together by opposing air bearings which also provide heat. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 through 7  show a prior-art platen press, donor and receptor sheets, and images before and after transfer. 
       FIG. 8  shows two opposed, heated platens with air flow as contemplated in the present invention. 
       FIG. 9  shows a platen with air distribution by a plenum. 
       FIG. 10  shows a platen with air distribution by piping. 
       FIG. 11  shows a transfer press according to the present invention in the open on. 
       FIG. 12  shows the transfer press of  FIG. 11  in the closed position. 
       FIG. 13  shows a plan view of the press shown in  FIGS. 11 and 12   
   

   DRAWING FIGURE REFERENCE NUMERALS 
   
     
       
             
             
             
             
           
         
             
                 
             
           
           
             
               100 
               Dye image 
               105 
               Donor sheet 
             
             
               110 
               Receiving medium 
               115 
               Platen 
             
             
               120 
               Platen 
               799 
               Transfer Press 
             
             
               800 
               Platen 
               805 
               Platen 
             
             
               810 
               Hole 
               815 
               Air flow indicating arrow 
             
             
               820 
               Unspecified barrier 
               825 
               Resistive heating elements 
             
             
               830 
               Radiant heater 
               900 
               Plenum 
             
             
               902 
               Air source 
               1000 
               Pipes 
             
             
               1100 
               Donor sheet 
               1105 
               Donor sheet supply roll 
             
             
               1107 
               Roller 
               1108 
               Roller 
             
             
               1110 
               Donor sheet take-up roll 
               1115 
               Medium 
             
             
               1120 
               Medium supply roll 
               1125 
               Roller 
             
             
               1130 
               Roller 
               1135 
               Medium take-up roll 
             
             
               1140 
               Backup tissue 
               1145 
               Roller 
             
             
               1150 
               Roller 
               1155 
               Roller 
             
             
               1160 
               Backup tissue take-up roll 
               1300 
               Pipe 
             
             
                 
             
           
        
       
     
   
   DETAILED DESCRIPTION 
   Preferred Embodiment—FIGS.  8  THROUGH  10   
   In accordance with a preferred embodiment of the invention a heat transfer press, indicated generally by the dashed lines at  799 , is provided which comprises two opposed platens which further comprise air bearings. In  FIG. 8 , each of platens  800  and  805  is supplied with one or more holes  810  through which air is forced, as indicated by air flow indicating arrows  815 . Holes  810  are preferably between 1 and 5 mm in diameter. 
   Platens  800  and  805  are preferably planar, between 1 and 3 cm thick, and of any required extent in orthogonal directions perpendicular to their thickness, typically several tens of cm. 
   Platens  800  and  805  are preferably steel, but can be made of any other metal, including aluminum. They can be solid or made of a porous material such as sintered bronze. 
   Air from an air source  902  is delivered to platens  800  and  805  through a plenum  900 , shown in  FIG. 9 . A plenum is generally required if platens  800  and  805  are made of a porous material. Alternatively, air is delivered to platens via individual pipe connections  1000  to each hole, as indicated in  FIG. 10 . 
   Air source  902  is widely available. An example is a rotary screw compressor Model ASD37, manufactured by Kaeser Kompressoren of Coburg, Germany. 
   In the presence of a barrier such as platen  805  ( FIG. 8 ), or other unspecified barrier  820  (dashed lines in  FIGS. 9 and 10 ), air flow as indicated by arrows  815  leaves the region separating platen  800  and barrier  820  by flowing laterally into the region outside the platen and barrier. 
   Platens  800  and  805  are optionally heated by any of a variety of means including resistive heating shown by elements  825  in intimate contact with them, high-pressure steam passed through pipes (not shown) also in intimate contact with plenums  800  and  805 , and radiant heaters  830 . Some heat is also available from compression of the air being delivered by air source  902 . Platens  800  and  805  may be kept at different temperatures. One of them may even be cooled, if it is desired to impose a large thermal gradient from one to the other. The temperatures of platens  800  and  805  are preferably regulated by temperature controllers (not shown). 
   Operation—Preferred Embodiment—FIGS.  11  through  13   
   In the discussion to follow, it is presumed that platen  800  is heated by one of the aforementioned means. Platen  805  may also be heated in a similar fashion, or maintained at a lower temperature as dictated by the requirements of the particular sublimation printing process employed. 
   The air supply indicated by arrows  815  is initially turned OFF. The heat sources for platens  800  and  805  are optionally also turned OFF. 
   Platens  800  and  805  are then separated by a distance sufficient to permit an operator (not shown) to load the press assembly. Pre-printed sublimate-dye-bearing donor tissue  1100  is threaded from supply roll  1105  over roller  1107  to roller  1108  and to take-up roll  1110 . Tissue  1100  is oriented so that its dye-printed surface faces medium  1115  to be printed, such as a textile or film. Tissue  1100  is presumed to be air-impermeable so that it will block air indicated by arrows  815  from contacting medium  1115 . Medium  1115  is threaded from supply roll  1120 , over roller  1125 , to roller  1130 , and to take-up roll  1135 . Backup tissue  1140  is threaded from supply roll  1120 , over roller  1150 , to roller  1155 , and to take-up roll  1160 . 
   Tissue  1140  is available from a variety of sources including Beaver Paper Company, of Atlanta, Ga., U.S.A. It is called “thermal transfer tissue” and is sold under the mark Pro-Tex. 
   Rollers  1107 ,  1125 ,  1150 ,  1108 ,  1155 , and  1130  are positioned so that tissue  1100 , medium  1115 , and backup tissue  1140  are in intimate contact. The centroid of the sandwich is coincident with a line drawn between platens  800  and  805  when they are forced together during printing, as explained below. The above-mentioned rollers can be either cylindrical or crowned. 
   During dye-transfer printing, rolls  1105 ,  1120 , and  1145  are allowed to rotate, but prevented from rotating freely by a braking arrangement (not shown). Rolls  1110 ,  1160 , and  1135  are caused to rotate in order to move tissue  1100 , medium  1115 , and tissue  1140  from left-to-right through the region between platens  800  and  805 . Rolls  1110 ,  1160 , and  1135  are driven in concert so that tissue  1100 , medium  1115 , and tissue  1140  move at exactly the same rate and do not move relative to one-another during transfer printing. This is accomplished by well-known motor-and-clutch mechanisms (not shown). 
   To perform the dye-transfer printing operation, platens  800  and  805  are first brought into contact with the above-described sandwich comprising tissue  1100 , medium  1115 , and tissue  1140 , as shown in  FIG. 12 . 
   Platens  800  and  805  are further forced together, or “preloaded”, as described above. The preloading force required is determined by the requirements of the particular transfer printing operation. It is typically sufficient to cause a pressure of at least 100 g/cm 2  between the platens. 
   Next air from source  902  is turned ON and flows as shown by arrows  815  and as described above. Tissues  1100  and  1140  are impermeable to air flow and therefore are forced together by a force determined by the preloading force described above. This force acts nominally over the entire surface of platens  800  and  805 . 
   Next, heat sources  825  are energized and platens  800  and  805  are brought to their operating temperature, typically 200 degrees C. 
   Finally, motive power is applied to take-up rolls  1110 ,  1135 , and  1160  and the braking mechanism for rolls  1105 ,  1120 , and  1145  is activated, as described above. The sandwich comprising tissue  1100 , medium  1115 , and tissue  1140  is thus forced together, and drawn through the region between platens  800  and  805 . The sandwich moves through this region virtually without friction because of the action of the air bearings formed between platen  800  and the top surface of tissue  1100 , and platen  805  and the bottom surface of tissue  1140 . 
   The dwell time, or time at an elevated temperature when the dye transfer printing step takes place, is determined by the length of the press in the process (printing) direction and the rate of motion of the web through the press. This is typically between 10 and 60 seconds. 
     FIG. 13  shows a plan view of the heat press assembly with a sandwich of dye donor tissue  1100 , medium  1115 , and backup tissue  1140  moving therethrough. The direction of motion is indicated by the arrow at the upper left of the figure. This embodiment communicates air from air source  902  through pipe  1300 , and to plenum  900 , as shown in  FIG. 9 . Plenum  900  is affixed to platen  800 . Platen  805  lies beneath platen  800  and is not visible in this view. Supply, take-up, and guide rollers have been eliminated from this view for clarity. Heating elements lie out of view beneath plenum  900 . 
   CONCLUSIONS, RAMIFICATIONS, AND SCOPE 
   Thus it is seen that I have provided a simple, low-cost system and method which can transfer sublimate dye images from a donor sheet to a receiving medium. The press assembly comprises only an air source, a heat source, two flat plates with one or more holes in each, and supply and take-up rolls for the materials to be passed through the press. No expensive rollers or fabric belts are required. 
   Some of the heat required to elevate the platens to operating temperature can be provided by the compressed air source, since compressing air causes its temperature to rise. 
   Recirculating the air which exhausts from the press back through the press can also scavenge some heat which would otherwise be lost. If this air is too contaminated by reaction products produced within the press, a heat exchanger can be used to extract heat from the exhaust gases. Finally, the remaining heat required to raise the platens to their proper operating temperature is obtained from resistive heating elements, steam heat, radiant heat, or a combination of these. Thermal insulating material covering heated parts of the press will further increase operating efficiency. 
   Air exhausted from the press can be captured and processed to remove any undesirable vapors arising from the heat transfer operation. 
   Instead of air, another gas or a mixture of gases can be used. 
   In the event air-permeable sublimate dye donor paper is used, a second layer of back-up thermal transfer tissue is positioned above this paper and adjacent the top platen in order to cause platen  800  to apply pressure to the donor paper, instead of allowing the air to pass through it. This assures that the sandwich of tissues and medium is firmly compressed and that dye gases do not flow in a direction parallel to the plane of the medium being printed. 
   A chamfer can be added to the platens along the edge where the tissue-fabric sandwich enters. This will provide smoother entry into the transfer area. 
   Rollers can be added ahead of the edge where the tissue-fabric sandwich enters the press. These can flatten knots and bumps in the fabric which otherwise might get caught at the entrance to the press. 
   While the above description contains many specificities, it will be apparent that the invention is not limited to these and can be practiced with other parameters and materials. A smooth or a lightly textured platen surface can be used. A non-stick substance can be applied to the platen surfaces. Different relief shapes can be machined into the platen surfaces, as is well known in the art of air bearing design. 
   Under some circumstances, an electric potential can be applied between the top and bottom platens. This creates an electric field which encourages normal migration of charged dye molecules into the substance being dyed. 
   The surfaces of the two platens can be curved or wavy in shape, provided their shapes are complimentary. 
   Accordingly the scope of this invention should be determined, not by the embodiments illustrated, but by the appended claims and their legal equivalents.