Patent Publication Number: US-6984830-B2

Title: Apparatus for limited-heat curing of photosensitive coatings and inks

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application claims the benefit of U.S. Provisional Application Ser. No. 60/297,811, filed Jun. 13, 2001, which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates to curing of inks and coatings. More particularly, the invention relates to curing of photosensitive inks and coating using ultraviolet radiation. 
   BACKGROUND OF THE INVENTION 
   Photosensitive inks and coatings are formulated to react to radiant energy in the ultraviolet range (250 to 400 nm) for accelerated curing. The inks and coatings are applied, in a printing press for example, to moving webs or sheets. The webs or sheets are then directed through a beam of radiant energy generated by a curing device to subject the inks and coatings to ultraviolet rays. Curing devices typically include a high intensity source of radiant energy to generate sufficient amounts of ultraviolet radiation for rapid curing of the photosensitive inks and/or coatings applied to the moving substrate. Curing devices typically include a reflector positioned adjacent the lamp to redirect a portion of the radiant energy to form a focused beam. 
   The radiant energy generated by the high intensity light source, however, includes heat generating rays of infrared radiation and visible light rays in addition to the desired ultraviolet rays. If left untreated, the amount of heat contained in the infrared and visible light rays could damage many substrates, such as heat shrinkable labeling used for food and beverage containers, for example. U.S. Pat. No. 4,864,145 discloses a curing device having a high intensity, medium pressure, mercury vapor lamp and a liquid cooled reflector. The beam is directed through a liquid filled filtering chamber to remove infrared radiation from the beam. The beam is then redirected, through a filtering pane, by an angled reflector. U.S. Pat. No. 5,321,595 discloses a curing device having liquid filled tubes for filtering infrared radiation from a radiant energy beam. 
   It is sometimes necessary to stop a printing press to make adjustments, for example. Prolonged exposure to the radiant energy from a curing device during a stoppage could be damaging to many substrates. U.S. Pat. No. 5,722,761, discloses a curing device having reflector members that can be pivoted to impinge on a portion of the radiant energy beam thereby preventing passage of the beam portion to the substrate. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus for curing photosensitive material such as inks and coating, for example. The apparatus includes a lamp generating radiant energy containing ultraviolet radiation. The apparatus further includes a filter body having an open interior positioned adjacent the lamp to receive at least a portion of the radiant energy generated by the lamp. The apparatus further includes first and second panes transmissive to ultraviolet radiation on opposite sides of the filter body to enclose the open interior forming a chamber. The apparatus includes an inlet and an outlet communicating with the chamber that are connectable to a fluid circulation system for circulating a coolant through the chamber. The apparatus also further includes a solid filter transmissive to ultraviolet radiation positioned in the chamber between the first and second panes. The solid filter is capable of removing substantially all radiation above approximately 700 nm from the radiant energy received by the solid filter such that the radiant energy is cooled to provide for limited-heat curing of a photosensitive material. 
   The invention also provides a printing apparatus. The printing apparatus includes at least one print stand capable of applying photosensitive inks or coatings to a substrate. The printing apparatus further includes a lamp adjacent the print stand generating radiant energy containing ultraviolet radiation for curing the photosensitive inks or coatings applied to the substrate. The printing apparatus also includes a filter assembly positioned between the lamp and the substrate to receive radiant energy directed toward the substrate from the lamp. The filter assembly includes a body defining an open interior and opposite panes enclosing the interior of the body to form a chamber. The filter assembly further includes an inlet and an outlet for circulating a fluid through the chamber. The panes and the solid filter are each transmissive to ultraviolet radiation. 
   The invention further provides a system for filtering a beam of radiant energy. The system includes a body defining an open interior and a pair of panes secured to opposite sides of the body to define an enclosed chamber. Each of the panes is transmissive to at least a portion of the radiant energy beam. The system includes an inlet and an outlet communicating with the chamber for connection of the chamber to a circulation system for circulating a liquid coolant through the chamber. The system includes a shutter system in which a plurality of opaque particles are suspended in the liquid coolant such that the opaque particles can be circulated through the chamber with the liquid coolant. The shutter system also has a trap system for selectively removing the opaque particles from the circulating liquid coolant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
       FIG. 1  is a schematic side view of a portion of a sheet fed printing press according to the present invention having an apparatus for curing a photosensitive material; 
       FIG. 2  is a perspective view of a curing apparatus according to the present invention; 
       FIG. 3  is a sectional view taken along the lines  3 — 3  in  FIG. 2 ; 
       FIG. 4  is a sectional view of a curing apparatus according to the present invention having multiple solid filters; 
       FIGS. 5 and 6  are sectional views each showing a curing apparatus according to the present invention having an infrared generating device upstream of an ultraviolet generating device; 
       FIG. 7  is a schematic view of a filtering system according to the present invention; and 
       FIG. 7A  is a schematic view of a portion of an alternative filtering system according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, where like numerals identify like elements, there is shown an apparatus  10  for curing photosensitive inks and coatings used in web fed and sheet fed printing presses, for example. Referring to the schematic illustration of  FIG. 1 , the apparatus  10  is shown installed on a sheet fed printing press  12  adjacent to a print stand  14 . The print stand  14  includes a transfer cylinder  16  and an impression cylinder  18  in a lower portion of the stand. The transfer and impression cylinders  16 ,  18  of print stand  14  contact the transfer and impression cylinders of adjacent print stands to form a series of interconnected cylinders for directing sheets  20  through the press  12 . The print stand  14  further includes a plate cylinder  22  and a blanket cylinder  24  in an upper portion of the print stand  14 . The plate cylinder  22  and blanket cylinder  24  supply a photosensitive ink to the sheet  20  that is applied to the sheet  20  as it is directed between the blanket cylinder  24  and the impression cylinder  18  of print stand  14 . 
   The apparatus  10  is shown in the schematic illustration of  FIG. 1  supported by an interdeck housing  26  having perpendicular top and side plate portions  28 ,  30 . The apparatus  10  may be mounted within the interdeck housing  26  in any suitable manner such as by bracketing (not shown). The interdeck housing  26  is connected to a main press housing  32  such that the apparatus  10  is enclosed within the press  12  by the main housing  32  and the interdeck housing  26 . The support of the apparatus  10  in this manner positions the apparatus  10  adjacent the impression cylinder  18  of print stand  14  in the angled orientation shown to direct ultraviolet radiation to a sheet  20 . The connection between the interdeck housing  26  and the main housing  32  preferably provides for removal of the apparatus  10  from the enclosed condition shown in  FIG. 1  for maintenance of the apparatus  10 . The interdeck housing  26  could, for example, be pivotably secured to the main housing  32 , in the manner described in U.S. Pat. No. 5,832,833, to provide for access to the apparatus  10 . Alternatively, the interdeck housing  26  could be completely removable from the main housing  32  using a tab and slot connection, for example. 
   Referring to  FIG. 2 , the apparatus  10  is shown removed from the printing press  12 . The apparatus includes a high intensity lamp  34  providing the source of radiant energy containing ultraviolet radiation for curing of photosensitive material such as the photosensitive ink applied to sheet  20  in printing press  12 . The lamp  34  is preferably a medium pressure, mercury vapor lamp, per se known in the art. Such lamps have power requirements ranging from approximately 5,000 to 25,000 watts. An example of such a high intensity lamp is the air-cooled medium pressure, mercury vapor lamp, described in U.S. Pat. No. 4,864,145 the description of which is incorporated herein by reference. Such lamps produce radiant energy that includes ultraviolet and infrared radiation as well as visible light. 
   The apparatus  10  further includes a reflector  36  having a parabolic curved surface  38 . The apparatus  10  includes lamp support collars  40  secured to opposite sides of the reflector  36 . Each of the support collars  40  includes an opening  42  for receipt of an end fitting of the lamp  34  such that the lamp  34  extends parallel to the reflector  36  and spaced from a center line of the parabolic surface  38 . The reflector  36  defines a hollow interior  44  for circulation of water, or a water-based coolant, through the interior  44  to cool the reflector  36 . Liquid cooled reflectors are known, as described in U.S. Pat. No. 4,864,145, the description of which is incorporated herein by reference. The reflector  36 , positioned in this manner with respect to the lamp  34 , functions to redirect a portion of the radiant energy emitted by lamp  34 . The portion redirected by the reflector  36  is joined with a directly emitted portion to form a focused beam of radiant energy. 
   The apparatus  10  further includes a filter assembly  46 , shown in greater detail in  FIG. 3 . The filter assembly  46  is secured to the reflector  36 , in the manner described in greater detail below, such that the focused beam of radiant energy will be directed from the reflector  36  through the filter assembly  46 . The filter assembly  46  is transmissive to ultraviolet radiation in the focused beam but filters out undesirable radiation that generates heat in the focused beam. 
   The filter assembly  46  includes a body  48  having side walls  50  and end walls  52  forming an open interior. Recesses  56  formed in the body  48  receive panes  58 , transmissive to ultraviolet radiation, to enclose the open interior of body  48  to form a chamber  54 . The panes  58  are preferably made from material that is resistant to elevated temperatures. The filter assembly  46  includes fittings  60  in each of the end walls  52  of the body  48 . The fittings  60  provide for connection between the filter assembly  46  and a circulation system for directing a liquid coolant  62 , such as water or a mixture of water and glycerol, through the chamber  54 . As shown in  FIG. 1 , the apparatus  10  is mounted to the interdeck housing  26  such that the apparatus  10  is oriented at an angle with respect to the press  12 . The angled orientation of the apparatus facilitates targeting of a sheet  20  carried by impression cylinder  18  by the filtered beam of the apparatus  10 . The apparatus  10  is preferably mounted such that the fittings  60 , located on the same side of the body  48 , will be upwardly located with respect to the chamber  54 . This construction and orientation of the filter assembly  46  is less likely to create air pockets within the chamber  54  of filter assembly  46  than would an orientation in which the fittings  60  are downwardly located with respect to the chamber  54 . 
   The apparatus  10  further includes a solid filter  64  positioned within the chamber  54  of the filter assembly  46 . The solid filter  64  is received in notches  68  formed in support plates  66  that are located within the chamber  54  adjacent the side walls  50  of body  48 . The filter assembly  46  further includes a retainer plate  70  at each of opposite sides of the body  48  to secure the panes  58  to the body  48  with the solid filter  64  and the associated support plates  66  positioned within the chamber  54  between the panes  58 . The retainer plates  70 , each having a central aperture  72 , are secured to the body  48  of filter assembly  46  by threaded fasteners  74 . A gasket  76  is positioned between the recesses  56  of the body  48  and the panes  58  to seal the chamber  54  to provide for circulation of the liquid coolant  62 . The enclosed chamber  54  of the filter assembly  46  provides for surrounding of the solid filter  64  by the liquid coolant  62  circulated through the chamber  54 . The construction of the filter assembly  46  facilitates access to the chamber  54  for maintenance or for removal and replacement of the solid filter  64 . 
   The solid filter  64  removes unwanted heat producing radiation, such as infrared radiation, from the focused beam while permitting the desired ultraviolet radiation to pass through the filter. Such materials, sometimes referred to as “band-pass” or “UV-pass” filter materials, are per se known. The solid filter  64  is preferably capable of filtering substantially all radiation above approximately 700 nm from the focused beam. 
   The addition of a glycerol to the liquid coolant  62  circulated through the chamber  54  will also provide for some filtering of the heat-producing radiation from the energy beam. The panes  58 , providing an ultraviolet transmissive enclosure for the chamber  54 , may also provide an additional filtering effect for reducing heat producing radiation from radiant energy beam. The placement of the solid filter  64  within the circulating liquid coolant  62  in chamber  54  will remove heat from the solid filter  64  caused by the filtered radiant energy above 700 nm. 
   The apparatus  10  includes connectors  78  securing the reflector  36  to the filter assembly  46 . Each connector  78  includes opposite first and second end portions  80 ,  82 . The first end portion  80  includes a notch  84  in which the filter assembly  46  is received. The connectors  78  are secured to the reflector  36  by fasteners (not visible) received in holes  86  in the second end portions  82  of the connectors. Threaded members  88 , received by the notched first end portions  80  of the connectors  78 , positions the filter assembly between opposite connectors  78  as shown in  FIG. 3 . Connection of the filter assembly  46  to the reflector  36  could be made by other means. For example, the apparatus  10  could include angled bracket secured to the sides and top, respectively, of the reflector  36  and the filter assembly  46 . 
   As described previously, the lamp  34  and reflector  36  of apparatus  10  produces a beam of radiant energy containing the desired ultraviolet radiation as well as infrared radiation and visible light rays. Passage of the beam through the filter assembly  46  removes heat-producing rays of infrared radiation and visible light. The resulting cooled beam that exits from the filter assembly  46  consists almost entirely of ultraviolet radiation as well as radiation in the purple-blue portion of the visible spectrum. The provision of such a cooled beam of radiant energy is highly desirable for printing on heat sensitive substrates such as heat shrink polymers used for container labeling. The cooled beam is also desirable where multiple curing cycles may be required for one substrate such as for multiple-color applications. 
   The combination of the solid filter  64  within the liquid cooled chamber  54  of filter assembly  46  provides for a highly compact device for forming the cooled beam containing ultraviolet radiation. Such space saving efficiency is highly desirable and leads to greater applicability of the apparatus in devices, such as the new generation of digital printing presses, in which compactness is required. 
   Some printing presses are adapted to cut power to the lamp during slowdowns or stoppages to limit heating of the printing press components and to then re-strike the lamp when the substrate is sufficiently moving again. While this is theoretically possible, in practice, the voltage required to strike a “hot” arc, before re-condensing is in the order of 5 to 10 times the operating voltage. For safety and reliability this is not a practical solution. 
   In extended exposure of a press cylinder to the cooled beam of the present invention, the temperature of the cylinder was increased only 5 degrees Fahrenheit after 40 minutes of exposure. Limited heating of the press cylinder is desirable as heat absorbed by the cylinder could be transferred to the substrate. The apparatus  10  is highly desirable for printing on very thin substrates as well as for printing on heat sensitive material such as heat-shrinkable materials now commonly used for labeling on containers. The cooled beam provided by the apparatus  10  also facilitates multi-colored printing applications where the substrate may be subjected to multiple exposures to the radiant energy beam following the application of each color. 
   Referring to  FIG. 4 , there is illustrated an alternative apparatus  90  according to the present invention having a pair of spaced solid filters  64  positioned within the body  48  of the filter assembly  46 . The spaced filters  64  could be adapted to define separate compartments  92  in which liquid or gaseous materials having varying opacity could be circulated to provide adjustability in the radiant energy transmission characteristics. It should be added, that variation in the transmission properties of the filter assembly are also possible by varying the relative thickness of the compartments as required by the suitable materials. 
   There are certain uv coatings for which a controlled amount of heating is actually desirable for optimal curing. A controlled amount of heating is also desirable for curing uv coatings on closed substrates such as polycarbonate, polyester, and styrene where heating during the reaction can increase the adhesion characteristics of the materials to the substrate. This is especially true when these materials have a coating applied before the ink to enhance the dyne level of the substrate. Such a “pre-coating” bonds better with the top ink or coating when heated above ambient temperatures. Variations in the photo-polymer chemistry can sometimes reduce the amount of heat needed, but this is not always possible or practicable. Therefore, the addition of a controlled amount of heat by the curing device would be desirable in such applications. 
   Referring to  FIG. 5 , a heating device  94  such as an IR emitter is located upstream of apparatus  10  to provide the controlled heating of the substrate prior to exposure to the radiant energy beam. Alternatively, as shown in  FIG. 6 , a curing apparatus  96  includes filter assembly  46  and additionally incorporates an IR heating device  98  upstream of a lamp  100  and reflector  102  to apply a dose of the infrared energy immediately upstream of the cooled ultraviolet beam. 
   By use of the proper IR emitting device, very finely controlled temperature parameters can be achieved. One way to achieve this is to include a short wave IR device which has a low thermal inertia, and the ability to infinitely vary the amount of heat generated by control means known to those skilled in the art. The IR emitter is tuned to produce the proper amount of heating effect and because of the low thermal inertia, whenever the machinery or substrate is stationary, the device can be immediately switched off. It is also possible that suitable control means using temperature-sensing means in a closed loop system could provide for proportional control of UV and/or heating device parameters for constant substrate temperature. Such control would be highly desirable during variable speed operation, for example. 
   The present invention is not limited to the embodiments shown in  FIGS. 5 and 6 . The curing apparatus could include multiple heating devices prior to the general location of the UV curing device to achieve a predetermined temperature of the substrate for optimum curing, without damage to the substrate or deleterious effects on the equipment and environment close to the UV device. 
   Referring to  FIG. 7 , there is illustrated a system for filtering a beam of radiant energy according to the present invention. The filtering system includes a shutter system  104  that provides for optional additional filtering during slow-downs or stoppages of a substrate, for example, to limit excessive exposure of the substrate to the radiant energy beam. The shutter system  104  includes a plurality of opaque particles  106  that are inert to a circulating liquid coolant and capable of suspension in the liquid coolant. The suspension of the particles  106  in the liquid coolant provides for circulation of the particles to a filter assembly  110  of a curing apparatus  108  to provide for an additional filtering of the beam to that otherwise provided by the filter assembly  110  absent the suspended particles  106 .  62  and capable of suspension utilizes opaque particles  106  that are inert to the liquid coolant. The opaque particles  106  are preferably made from a magnetically attractable material, such as a ferromagnetic material, to provide for their removal from the circulating coolant, in the manner to be described, when the additional filtering by shutter system  104  is not needed. 
   The shutter system  104  is incorporated into a circulation system  112  for the liquid coolant that includes a supply tank  114  and a pump  116 . The shutter system  104  further includes a magnetic trap  118  for removing the opaque particles  106  from the circulating liquid coolant. The trap  118  includes an electromagnet  120  for generating a magnetic field having a sufficient strength to attract and hold the opaque particles  106  thereby preventing their circulation to the filter assembly  110 . The trap system  118  includes inlet and outlet vessels  122 ,  124  adjacent the electromagnet  120  and connected to the circulating system  112  upstream and downstream, respectively, of the filter assembly  110  of apparatus  108 . The inclusion of separate inlet and outlet vessels  122 ,  124  facilitates more rapid removal of the opaque particles  106  from the circulating coolant. 
   Additional shuttering could also be provided by including separate compartments  126  within the filter assembly  110  and circulating a more opaque liquid or gas in one of the chambers. A solid filter device capable of being selectively transmissive or opaque to the radiant energy, such as in response to electric current, could also provide the additional filtering. 
     FIG. 7A  illustrates an alternative filtering system according to the present invention. The filtering system includes a shutter system  130  having a circulating system  132  for directing a liquid coolant to a first filter assembly  134 . The first filter assembly  134  includes a filter body  136  and ultraviolet transmissive panes  138  defining a chamber  140  for receiving the circulating liquid coolant. The shutter system  130 , similar to shutter system  104 , includes a plurality of opaque particles  142  in suspension in the liquid coolant for circulation through the chamber  140  of the first filter assembly  134 . The shutter system  130 , also similar to shutter system  104 , includes a trap system (not shown) having an electromagnet for removing the suspended particles  142  from circulation to the first filter assembly  134  when additional filtering of the radiant energy beam is not needed. 
   The filtering system of  FIG. 7A  further includes a second filter assembly  144  positioned adjacent the first filter assembly  134 . The second filter assembly  144  includes a filter body  146  and opposite panes  148  defining a chamber  150  in a similar manner to the first filter assembly  134 . The second filter assembly  144  is connected to a circulation system  152  for receipt of a liquid coolant in the chamber  150 . A solid filter  154 , similar to solid filter  64 , is positioned within the chamber  150  of the second filter assembly  144 . The use of separate filter assemblies  134 ,  144  connected to separate circulating systems  132 ,  152  prevents contact between the opaque particles  142  of the shutter system  130  and the solid filter  154 . The separation of the solid filter  154  from the circulating particles  142  serves to prolong the life of the solid filter  154  by preventing abrasion that could otherwise occur if the circulating particles  142  and solid filter  154  contained in the same chamber. 
   The second filter assembly  144  is positioned between the lamp/reflector assembly  156  and the first filter assembly  134 . In this manner, the radiant energy beam generated by the lamp/reflector assembly  156  is directed first through the second filter assembly  144  and then through the first filter assembly  134  of the shutter system  130  before being directed to the substrate  158 . 
   As previously discussed, this invention relates to curing materials on various substrates. The limited-heat curing of the present invention has application beyond the graphics industry to any application where heat generated during curing would have a deleterious effect on either the equipment in which the curing device is mounted, or on the substrate that is being cured. Examples may be found in the floor covering and in the electronics related industries for curing of CD and DVD discs having UV curable material. 
   While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather should be construed in breadth and scope in accordance with the recitation of the appended claims.