Abstract:
Process for continuously printing a plastic film includes the steps of applying to the back of the plastic film a temporary anti-sticking support, heating the assembly of the plastic film and the temporary support to a temperature such that the plastic film reaches its softening point to stick temporarily to the temporary sport, transferring by sublimation a design constituted by a sublimable ink pre-printed on a printing paper, by contact under predetermined temperature and pressure with the front of the plastic film therm-stuck to the temporary support, and cooling the assembly of the thus-printed plastic film and of the temporary support to a temperature permitting unsticking the temporary support.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for continuously printing a flat plastic film, flocked or not, the device for practicing the process and the printed plastic film obtained by this process. 
     The invention also relates to a process for continuously printing a colored or uncolored motif on a flat plastic film, flocked or not, adapted to be thermoformed to a predetermined non-planar surface, as well as the flat printed plastic film and the thermoformed plastic film obtained by this process. 
     Transfer printing by sublimation is already known as such and consists, in the first instance, in printing on a support paper designs from inks constituted by sublimable coloring agents which are adapted to vaporize at a certain temperature and to be fixed permanently on synthetic materials, for example polyester or polyamide polymers, the printing process of the support paper being adapted to be anything at all, for example offset printing, heliographic or flexographic printing, or again serigraphic printing by flat plate or rotary plate. Then, in the second instance, the designs in sublimable coloring agents, preliminarily imprinted on the support paper, are transferred by sublimation, by placing in contact, under a regulated pressure and at a predetermined temperature, generally with the aid of hot rolls, the pre-printed paper with the textile support to be printed, generally for a duration of 5 to 40 seconds. The hot rolls can comprise, in the case of printing in formats, a hot press with horizontal plates, or in the case of continuous printing from rolls of printed paper and of synthetic material to be printed, a rotating heated cylinder associated with a belt rolling under tension. 
     However, transfer printing by sublimation is not adapted to print plastic films, for the following reasons: 
     plastic films, and more particularly extruded films, have errors of flatness, which renders difficult their continuous printing, from a roll of plastic film, 
     plastic films, and more particularly plastic films adapted to be thermoformed, such as polyvinyl chloride (PVC) films or polystyrene films, have a thermal sensitivity such that they melt and thus lose all cohesion, when they are subjected to impression by transfer by sublimation at a temperature of about 200° C., which gives rise to random dimensional fluctuation of the plastic film during printing whilst the pre-impregnated impression paper undergoes no deformation, thereby giving rise to blurred and irregular printing, 
     the melting of the plastic film during printing also gives rise to sticking of the plastic film on the conveyor belt of the printing machine, 
     the melting of the non-flocked plastic film also leads to sticking of the plastic film to the pre-printed printing paper. 
     This printing process by transfer by sublimation can be used to print certain plastic materials, if there is first applied a layer of glaze on the plastic material to be printed, or first treating the pre-printed printing paper so as to render it non-adhesive. This printing method is particularly used to print pieces of plastic that are injected or shaped, such as bottles, skis, pens, helmets, and in this case, the printing is carried out by means of a heating tool matching the shape of the plastic piece to be printed. 
     Another direct printing method for plastic films consists in using known processes of the flexographic or serigraphic type, but in this case, the ink does not penetrate the plastic material, such that the printing designs have low resistance to scraping and friction. Moreover, these printing processes do not permit obtaining the most detailed graphic effects, for example for color printing of the photographic type. 
     Still another known method to print plastic films consists in thermobonding on the surface to be printed of the plastic film a pre-printed support, for example polyester film printed by heliography, then hot counter-adhered to the plastic film at the outlet of the extruder. 
     On the other hand, when plastic films to be printed are adapted to be thermoformed, for example to form protective blocks, pieces for the automobile industry, elements for packing, printed designs on the flat plastic film subject the deformation of the plastic film on a non-planar surface, giving rise to a modification of the contours and of the configuration of the printed design, as well as a variation of the shade of the colors, because of the stretching of the plastic film during its thermoforming. When the design which must be given to the non-flat thermoformed surface is a geometric design, letters or numerals, the deformations of the design can be unacceptable. 
     SUMMARY OF THE INVENTION 
     The invention has for its first object to provide a continuous printing process of a plastic film, flocked or not, with transfer by sublimation, whilst having high quality, detail and sharpness of printing, for example of the four-color type, and avoiding any sticking between the plastic film and the conveyor belt or the pre-printed printing paper. 
     A second object of the invention is to provide a printing process for design on a flat plastic film, flocked or not, adapted to be thermoformed into a predetermined non-planar surface, which permits compensation for the deformations of the design and/or the variations of shade of the colors on the non-planar thermoformed surface. 
     To achieve the first-mentioned object, the invention has for a first object a continuous printing process of a plastic film, flocked or not, characterized in that it consists in: 
     applying on the back of the plastic film a temporary anti-adherent support, for example of paper, 
     heating the assembly of the plastic film and of the temporary support to a temperature such that the plastic film reaches its softening point to stick temporarily to the temporary support, such that this latter ensures the dimensional stability of the plastic film during the following thermal printing step, 
     transferring by sublimation a design constituted by a sublimable ink pre-printed on a printing paper, by contact under pressure and at a predetermined temperature with the front of the plastic film thermo-held on the temporary support, 
     cooling the assembly of the plastic film thus printed and of the temporary support to a temperature permitting the unsticking of the temporary support. 
     Preferably, the temperature to thermo-stick the plastic film to the temporary support is comprised between 100 and 200° C., preferably between 170 and 180° C. The temperature for transfer by sublimation can be comprised between 170 and 230° C. The temperature at which the assembly of the printed plastic film and the temporary support is cooled can be of the order of 20 to 30° C. 
     In one embodiment, the process of the invention consists, for a non-flocked plastic film, in pre-printing the printing paper with an ink containing sublimable coloring agents, with a low content of softening or anti-adherent agents, to avoid sticking between the plastic film and the impression paper, during the printing step by transfer by sublimation. The agents can be selected from silicones, organic fatty acids and chromium stearates. 
     In another embodiment, the process consists in preparing a flocked plastic film, by applying to the front of the plastic film an adhesive layer and flocking fibers of white color on said adhesive layer. Preferably, the adhesive layer is a polymer resin in aqueous emulsion or organic solution, and the flocks are preferably polyamide or polyester fibers, with a length comprised between 0.3 and 3.0 mm and a titre comprised between 0.5 and 20 Dtex (0.5 and 20.10 −7  Kg.m −1 ). 
     The plastic film can be a calendered, extruded or co-extruded film, selected from polymer resins of polyvinyl chloride base, polystyrene, polypropylene, acrylic butyl styrene (ABS), polyurethane, polyester, polycarbonate, polyamide or any mixture of these latter. 
     The printing paper can be printed by offset, flexographic, heliographic, serigraphic printing or by ink jet. 
     The invention also relates to the device for practicing the process described above, characterized in that it comprises: 
     an unwinding station, comprising a reel for a strip of plastic film, flocked or not, and a reel for a strip of temporary support, 
     a heating station at the inlet of which the strip of temporary support is applied against the back of the strip of plastic film, the assembly constituted by said superposed strips coming into contact via the temporary support strip, against a heating drum driven in rotation, so as to deliver at the outlet a composite strip constituted by the plastic film thermo-stuck to the temporary support, 
     a thermo-printing machine, comprising a conveyor belt under tension which comes into contact with the temporary support of said composite strip, a heating calender driven in rotation, and at least one reel for a strip of paper pre-printed with a design constituted by a sublimable ink, said strip of pre-printed paper being applied to the front of the plastic film of said composite strip, upstream of the heating calender, such that the conveyor belt can press with predetermined pressure said composite strip covered with pre-printed paper against the heating calender, to transfer by sublimation the sublimable ink to the front of the plastic film, the printing paper being separated from the printed composite strip downstream of the heating calender, 
     a cooling station to cool the composite printed strip to a temperature such that the temporary support strip can be unstuck from the strip of printed plastic film, 
     a reeling station at the inlet of which the temporary support strip is unstuck from the strip of printed plastic film, each strip being wound on a respective wheel which is driven in rotation. 
     Preferably, the cooling station comprises two double thickness cylinders, each cylinder comprising a circulation of a cooling agent, for example water, within the double thickness, such that said composite printed strip comes into contact with the first cylinder with its temporary support, then with the second cylinder with its strip of printed plastic film. 
     The invention also has for its object a strip of printed plastic film, characterized in that it is directly obtained by the mentioned process. 
     To achieve the second mentioned object, the invention has for a second object a continuous printing process of a design, colored or not, on a flat plastic film, flocked or not, adapted to be thermoformed into a predetermined non-planar surface, characterized in that it consists in: 
     geometrically dividing into elemental regions a portion of the flat plastic film to be thermoformed, 
     determining for each elementary region of the film, the amount of local stretching of the film upon its thermoforming into the non-planar surface, 
     determine for each elementary region of the film, a portion of the deformed design to be printed on the surface of the flat plastic film, this portion of the deformed design being defined by transformation by anamorphosis of the final design to be provided on the thermoformed non-planar surface, by taking account of the amount of local stretching of the plastic film by which one passes from the deformed portion of the design to the final portion of the design, 
     printing on the surface of the portion to be thermoformed of the flat plastic film, the deformed design defined by the assembly of the portions of the deformed design determined by the preceding step. 
     Preferably, the process consists in determining, for each elementary region of the film, the shade of color of the portion of the deformed design, as a function of the shade of color desired in the final design and of the amount of local stretch of the film by which one passes from the portion of deformed design to the portion of final design. 
     According to another characteristic, the process consists in printing simultaneously on the portion of the flat film other than that which will be thermoformed, a reference mark to locate precisely the portion to be thermoformed of the flat plastic film. Said reference mark to be printed can comprise a line extending along one edge of the flat plastic film and a mark extending transversely from this line. 
     Preferably, the mentioned printing step of the flat plastic film consists in: 
     applying on the back of the flat plastic film a temporary anti-adherent support, 
     heating the assembly of the flat plastic film and of the temporary support to a temperature such that the plastic film reaches its softening point to stick temporarily to the temporary support, such that this latter assures dimensional stability of the plastic film upon the succeeding thermal printing step, 
     transferring by sublimation the pre-printed deformed design in reverse with a sublimable ink, onto a printing paper, by placing in contact under pressure and at a predetermined temperature said printing paper, with the front of the plastic film thermo-stuck to the temporary support, 
     cooling the assembly of the flat plastic film thus printed and the temporary support to a temperature permitting unsticking the temporary support. 
     In one embodiment, the process of the invention consists, for an unflocked plastic film, in pre-printing the printing paper with an ink containing sublimable coloring agents, with a low content in softening or anti-adherent agents, to avoid sticking between the plastic film and the printing paper, during the step of printing by transfer by sublimation. These agents can be selected from silicones, organic fatty acids and chromium stearates. 
     In another embodiment, the process consists in preparing a flocked plastic film, applying on the front of the plastic film an adhesive layer and flocking fibers of white color on said adhesive layer. Preferably, the adhesive layer is a polymer resin in aqueous emulsion or in organic solution, and the flocks are preferably fibers of polyamide or polyester, with a length comprised between 0.3 and 3.0 mm and a titre comprised between 0.5 and 20 Dtex (0.5 and 20.10 −7  Kg. m −1 ). 
     The plastic film can be a calendered, extruded or co-extruded film, selected from polymer resins with a polyvinyl chloride base, polystyrene, polypropylene, acrylic butyl styrene (ABS), polyurethane, polyester, polycarbonate, polyamide or any mixture of these latter. 
     The printing paper can be printed by offset, flexographic, heliographic, serigraphic printing or by ink jet. 
     In the final step, the process of the invention consists in positioning laterally and transversely the portion of the surface to be thermoformed of the flat plastic film on the surface of which is printed the deformed design, face to face with the shape of the thermoforming tool, by means of the mentioned printed reference mark, and in thermoforming with said thermoforming tool alone said portion into a non-planar surface on which the final desired design appears. Preferably, the process consists in optically detecting the longitudinal line of the reference mark to guide laterally the flat plastic film and the transverse mark of the reference to stop, in the longitudinal direction, the flat plastic film with its portion to be thermoformed facing the contour of the thermoforming tool. 
     Preferably, the process consists in calculating the portions of the deformed design by computer and in pre-printing the impression paper by an ink jet printing machine controlled by said computer. 
     Printing by ink jet permits providing digitized printing under optimum conditions, to the extent it is not necessary to provide specific printing tools, such as cylinders, offset plates, serigraphic frames, and in which printing by ink jet can be directly controlled by the computer provided with software for computing deformations of the design. 
     The invention also relates to the printed plastic film with a thermoformed non-planar surface, characterized in that it is directly obtained by the mentioned process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, details, characteristics and advantages of the invention will be described in greater detail in the detailed description which follows, of a particular embodiment now preferred, of the invention, given solely by way of illustration and not limitation, with reference to the accompanying schematic drawings, in which: 
     FIG. 1 is a schematic elevational view of an example of embodiment of the device for practicing the continuous printing process, according to the first object of the invention; 
     FIG. 2 is a fragmentary plan view of a strip of pre-printed paper with a reverse design, colored or not, constituted by a sublimable ink, adapted to be used in the process according to the first object of the invention; 
     FIG. 3 is a fragmentary plan view of a strip of plastic film printed by transfer by sublimation from the pre-printed paper strip of FIG. 2; 
     FIG. 4 is a view in transverse cross-section on the line IV of FIG. 5, showing the thermoforming of the printed strip of plastic film of FIG. 3 in a thermoforming tool with a spherical cavity; 
     FIG. 5 is a fragmentary plan view in the direction of the arrow V on FIG. 4, showing the strip of thermoformed plastic film with a deformed design on its spherical surface; 
     FIGS. 6 and 7 are views similar to FIGS. 2 and 3 respectively, but with a pre-deformed design for thermoforming, according to the second object of the invention; and 
     FIGS. 8 and 9 are views similar to FIGS. 4 and 5 respectively, with a non-deformed design on the thermoformed strip of plastic film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A particular example of embodiment of the device for practicing the continuous printing process of a plastic film by transfer by sublimation, will now be described with reference to FIG.  1 . 
     This device comprises an unwinding station  1  which supports rotatably a reel  2  from which is unwound a strip of temporary support  3  for example of paper, and above the reel  2 , a reel  17  from which is unrolled a strip of plastic film  4 , flocked or not. The temporary support is of a material that is little sensitive to heat, so as to ensure a dimensional stability to the plastic film, during the ultimate operation of sublimation, this material having an anti-adhesive characteristic at ambient temperature, to facilitate the stripping off of the plastic film after thermoprinting, as well as the ability to adhere when hot to the plastic film when this latter is subjected to mounting, during ultimate operations of heating and transfer by sublimation. 
     The mentioned strips  3  and  4  then pass through a heating station  5  which comprises three guide rollers  6 , arranged upstream of a rotatable heating drum  7 , which is driven in rotation in the clockwise direction indicated by the arrow F in FIG.  1 . 
     At the entry of the heating station  5 , the strips  3  and  4  pass below a first roller  6 , the front of the strip of plastic film  4  coming into contact with the roller  6  and the temporary support strip  3  coming into contact with the front of said strip  4 . The assembly of the strips  3  and  4  thus proposed is brought horizontally to a second roller  6  against which continuously bears the front of the strip of plastic film  4 , said strips  3  and  4  effecting a turn of  900  upwardly about this second roller  6 . Then the superposed strips  3  and  4  carry out a turn of 180° about the third roller  6  which is located adjacent the heating drum  7 , such that the temporary support strip  3  comes into contact with the heating cylinder  7 , to avoid sticking by softening of the plastic film on the heating drum  7 . The assembly of the strips  3  and  4  then passes about the heating drum  7 , through an angle of about 270°, such that the strip of plastic film  4  will be brought to a temperature of about 200° C. when the assembly leaves the heating drum  7 . At the output of the heating drum  7 , the strip of plastic film adheres, because of its fusion, to the temporary support strip  3  which ensures dimensional stability to said strip of plastic film  4 , the assembly thus forming a composite strip  18  whose two layers are temporarily stuck together. 
     Said composite strip  18  is carried horizontally downstream of the heating drum  7  toward a thermoprinting machine  8 . This thermoprinting machine  8  comprises at its input a guide roller  9  against which bears the temporary support of the composite strip. Then, the composite strip is conveyed by a conveyor belt  10  about a rotatable heating calender  11  which is driven in rotation in the counterclockwise direction in FIG.  1 . The conveyor belt  10  is constituted by an endless strip which rolls about a plurality of rollers  12 , certain of these rollers being displaceable to adjust the tension of the belt  10  and hence the pressure exerted by the belt  10  against the heating calender  11 . The composite strip comes into contact with the conveyor belt  10  with its temporary support, to avoid any sticking between the plastic film and the belt. 
     The machine  8  moreover comprises at least one reel  13 , for example two reels, from which is unrolled a strip  14  of paper pre-printed with the design constituted by a sublimable ink. The strip of pre-printed paper  14  is guided by a roller  15  upstream of the belt  10  and of the heating calender  11 . The machine  8  comprises two other reels  16  arranged downstream of the heating calender  11  and of the conveyor belt  10 , on which reels  16  is rolled up the paper  14 , after the transfer by sublimation of the ink onto the plastic film of the composite strip. The pre-printed paper  14  is sandwiched between the heating calender  11  and the front of the plastic film of the composite strip which is pressed by the conveyor belt  10  against the heating calender  11 . The design constituted by a sublimable ink is pre-printed on the side of the paper  14  which is adapted to come into contact with the front of the plastic film of the composite strip, so as to permit the transfer by sublimation. By way of example, the transfer by sublimation takes place about the heating calender  11 , over an angular path of about 300°, at a temperature comprises between 180 and 230° C., under a regular pressure generally less than 40 kPa exerted by the conveyor belt  10 , and for a duration of about 5 to 40 seconds. 
     When the plastic film used is non-flocked, the sublimable ink of the pre-printed paper  14  contains softening and anti-adhering agents, to avoid sticking of the pre-printed paper  14  onto the plastic film. On the other hand, when the plastic film has a flocked surface, adapted to be printed, it is not necessary to provide such agents. 
     The structure of the printing machine  8  will not be described in greater detail, because it is known per se. In this machine  8 , there are provided two reels  13  and two reels  16  for the pre-printed paper  14 , so as to avoid any interruption of the process, when one of the reels is completely empty, the other then taking over during replacement of the completely empty reel. 
     At the outlet of the machine  8 , the composite printed strip  19  is brought to a cooling station  20  which comprises two rotating cylinders  21  and  22  vertically offset relative to each other. The upper cylinder  21  is driven in rotation in the clockwise direction, whilst the lower cylinder  22  is driven in the counterclockwise direction. In a manner known per se, each cylinder  21  or  22  comprises a double wall on its side wall, to permit circulating within this double wall water, for example at 10° C., so as to cool the composite printed strip  19 , to a temperature generally comprised between 20 and 30° C. 
     The composite strip passes first about the upper cylinder  21 , coming into contact with its temporary support, so as to avoid the sticking of the printed plastic film against this cylinder. Then, the composite strip already cooled passes about the lower cylinder  22 , with its printed plastic film in contact therewith, which no longer is in danger of sticking to the cylinder  22 . The path of the composite strip about the two cylinders  21  and  22  has a substantially S shape. At the output of the cooling station  20 , the previously printed plastic film no longer adheres to the temporary support strip, because of the anti-adherent properties of this latter. 
     Finally, the two superposed strips pass through a winding station  23  which comprises a reel  24  about which is rolled up the strip of printed plastic film  104  and a reel  25  about which is rolled up the strip of temporary support  3 . At the inlet of the rolling up station  23 , the two superposed strips  3  and  104  pass between two rollers  27  downstream of which the two strips are separated in the direction of their respective reel. The reels  24  and  25  and the rollers  27  are driven in rotation. 
     The operation of the device of FIG. 1 will now be briefly described. 
     The temporary support strip  3  is first unrolled from the reel  2  over all the path of the interior of the device up to the rolling up reel  25 , over a total length of about 20 to 30 meters. Then, the strip  4  of plastic film is unrolled from the reel  17 , for a length of about 2 to 3 meters up to the heating drum  7 , because from this place the strip of plastic film  4  adheres to the temporary support strip  3 , which permits its driving through all of the device. It should be noted that the temporary support strip  3  which is rolled up on the reel  25 , could be reused for further printing of the new strip of plastic film. 
     In FIG. 2, a checkerboard design  30  is pre-printed on the strip  14  of printing paper, the checkerboard  30  comprising in its central square an equilateral triangle whose summit is directed to the left. On this strip of pre-printed paper  14  is also printed a reference numeral constituted by a longitudinal line  31  and a transverse dash  32 , to locate on said strip  14  the pre-printed design  30 . 
     With the device shown in FIG. 1, there can be obtained at the outlet a strip of flat plastic printed film  104  comprising a checkerboard  130  which is exactly the reverse image of the design  30  of the pre-printed paper  14 , the summit of the equilateral triangle being in this case turned toward the right in FIG.  3 . 
     In FIG. 4, there is shown the step of thermoforming a portion of this strip of printed plastic film  104 , in a thermoforming tool  50  comprising a spherical cavity  51  and holes  52  connected to a source of external suction, which permits pressing the plastic film  104  softened by heating against the walls of the tool  50 , this thermoforming tool being known per se. We have not shown the heating die, whose shape can be complementary to the cavity  51 . A flat portion of the plastic film strip  104  is heated, then thermoformed to a non-planar spherical surface  114  by the thermoforming tool  50 , this deformation giving rise to stretching of the checkerboard design  131 , as shown in FIG.  5 . 
     There is indicated in FIG. 4 by broken arrows the transformation of several elementary zones  130   a  to  130   d  of the flat plastic film into deformed elementary regions  131   a  to  131   d  on the thermoformed spherical surface  114 . The assembly of the deformed elementary regions thus forms the deformed design  131  visible in FIG.  5 . 
     To compensate for this undesirable deformation of the design before it appears on the thermoformed portion of the plastic film, there is determined, by reverse transformation, by anamorphosis, the pre-deformed design  40  which is pre-printed in reverse on a portion  114  of the printing paper  14 , this portion  114  corresponding to the portion adapted to be thermoformed, as shown in FIG.  6 . This deformation is calculated by a computer with the help of suitable software. 
     The strip of plastic film  104  is printed with pre-deformed design  140  by transfer by sublimation of the pre-deformed design  40  of the printing paper  14 , as seen in FIG.  7 . 
     During thermoforming of the portion  114 , sa shown in FIG. 8, each elementary region  140   a  to  140   d  of the flat portion of the plastic film is transformed into an elementary region  141   a  to  141   d  on the non-planar surface  114  obtained by thermoforming, as indicated by the broken arrows in FIG.  8 . 
     There is thus obtained on the portion  114  of the strip of thermoformed plastic film, a design  141  which corresponds to the desired design, without final deformation. 
     During the step of thermoforming of the strip of printed plastic film  104 , said strip is brought sequentially above the thermoforming cavity  51 , so as to position precisely the portion  114  to be thermoformed above said cavity  51 . The precise positioning of the portion  114  is obtained by optically reading the longitudinal and transverse position of this portion  14 , with the help of the mentioned reference marks  31  and  32 . 
     Although not shown in the drawings, the deformation by thermoforming of the portion  114  is also accompanied by a variation in the shades of color as a function of the extent of local stretching of the plastic film, which also requires pre-compensating these color variations, during the pre-printing of the printing paper  14 . 
     Although the invention has been described in connection with a particular embodiment, it is of course evident that it is not in any way limited and that it comprises all the technical equivalents of the described means as well as their combinations if these enter into the scope of the invention.