Abstract:
A process of screen printing images on a substrate which effectively diminishes the Moire effect and other screen printing process distortions. The process utilizes specific screen angles for printing each process ink color, the application of UV light to set the ink on the substrate and the automated transfer of the substrate between printing and drying stations.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the screen printing of inks onto substrates, and in particular, to a process of screen printing which diminishes or eliminates the Moire effect and other visual discrepancies between the original image and printed reproductions. 
     BACKGROUND OF THE INVENTION 
     The screen printing process is a process of forcing ink through unblocked areas of a metal, synthetic or silk fiber screen by spreading the ink onto the screen and passing a squeegee over the screen. This process is typically used in the production of posters, signs, decalcomania, etc. Most screens are comprised of polymeric or metal threads. 
     It is known to screen print images on substrates through the successive application of single tones of ink. This printing process achieves tone variation by separating each process color, typically cyan, magenta, yellow and black, into fine dots of differing size on a halftone screen grid. Typically, the dots are closely spaced and arranged in parallel lines. Image coloring can be varied by superimposing the dots of the process colors. In an ideal embodiment of the printed reproduction, human vision integrates the dots into an accurate impression of the original image. 
     However, the use of four separate colors printed successively through a screen often results in the presence of Moire effects. A Moire effect is a repetitive interference pattern caused by overlapping symmetrical grids of dots or lines having a differing pitch or angle. Such effects are often seen as waves, shimmering or rosettes in the reproduced image. 
     It is understood that Moire effects are introduced by the application of the dot pattern to the screen which causes the dot shapes to be resized by the screen structure, depending on the size of the dot and the screen cell. The interference between the screen structure pattern and the dot pattern will be repeated for each color and results in Moire effects. 
     In order to avoid Moire effects, it is known to vary the dot arrangements for each color. For instance, U.S. Pat. No. 5,778,091 to Shibazaki et al. discloses the printing of cyan with a dot pattern axis of 15 degrees, magenta with a dot pattern axis of 45 degrees, yellow with a dot pattern axis of 0 degrees and black with a dot pattern axis of 75 degrees. While these screen angles do diminish Moire effects, discrepancies between the original image and the reproduced image still exist due to Moire. 
     At least some of these discrepancies are caused by positional shifts or deviations caused by the mechanical factors of the screen printer and the ink setter. Specifically, heat storage effects of these devices cause deformation in the ink receiving substrate. For each ink printing onto the substrate and setting or drying in position, the substrate is heated and cooled several times. Typically, the ink is passed under several heating/drying elements which can cause the substrate to deform or shrink by an ⅛inch each pass. After an ink is set, the substrate must be realigned and re-registered with another screen for the printing of other process colors. The deformation of the substrate can significantly impair this realignment and cause dramatic positional shifts or deviations. 
     An improved screen printing process which addresses these problems of known screen printing processes would be an important advance in the art. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide a screen printing method overcoming some of the problems and shortcomings of prior art screen printing processes. 
     Another object of the invention is to provide a screen printing method which diminishes or eliminates interferences between dot and screen patterns. 
     Another object of the invention is to provide a screen printing method which reduces Moire effects to a minimum. 
     Another object of the invention is to provide a screen printing method which utilizes the same standard screens for each ink application. 
     Another object of the invention is to avoid Moire effects in screen printing without utilizing expensive, specially made screens. 
     Still another object of the invention is to provide a screen printing method which reduces ink-receiving substrate deformation. 
     Still another object of the invention is to provide a screen printing method which reduces the need for repeated registration of the substrate during the printing process. 
     Yet another object of the invention is to provide a precise screen printed image which results from the novel process herein described. 
     How these and other objects are accomplished will become apparent from the following descriptions and the drawings. 
     SUMMARY OF THE INVENTION 
     The new screen printing process is intended to result in an extremely precise and accurate screen printed reproduction of an original image. The process utilizes novel screen angles and ink drying processes which are able to reduce dot and screen pattern interference, position shifts and substrate deformation. The method of screen printing the image onto a substrate comprises the steps of (1) separating the original image into cyan, magenta, yellow and black components at a predetermined line count, (2) converting each color component into linear dot patterns on respective process films with axes at respective angles relative to the vertical, (3) transferring the linear dot patterns from the respective process films to respective screens, (4) securing the substrate in a fixed position relative to an automated substrate-guide or gripper which is able to automatically transport the substrate between ink-receiving and ink-drying positions, (5) transferring the substrate to an ink-receiving position while retaining the fixed position relative to the substrate-guide, (6) aligning a screen with the substrate, (7) applying ink to the screen so that the ink flows through the dot pattern on the screen onto the substrate, (8) transferring the substrate to an ink-drying position while retaining the fixed position relative to the substrate-guide, (9) setting the ink on the substrate through the application of light, and (10) repeating steps 5-9 for each desired process color. 
     The preferred substrate is a smooth material such as vinyl, other polymeric materials typically used for posters and the like, or paper coated with such material. 
     The precision of this screen printing process demands that the substrate be extremely smooth and able to retain ink so that the image can be properly reproduced. 
     It is preferred that the linear dot patterns are positioned on the process films at specific angles which have been determined to be proficient at minimizing Moire effects. One set of these angles is 82.5 degrees for cyan, 142.5 degrees for magenta, 7.5 degrees for yellow and 22.5 degrees for black. It is to be understood that all angle measurements disclosed and claimed in this patent are relative from the vertical axis, such that a “3 o&#39;clock” angle would be 90 degrees and an “11 o&#39;clock” angle would be 330 degrees. It is further understood that the effectiveness of these angles is due to their relationship with the geometry of the screen. Orthogonal screens have threads which run vertically and horizontally, creating rectangular screen cells. The angle measurements are effective when they are measured relative to the vertical screen threads. It is contemplated that any angular rotation of the screen threads would result in “different” angle measurements which would minimize Moire. In accordance, the angles claimed in relation to the “vertical” refer to the vertical axes of the screen threads. 
     Another preferred set of angles determined to be extremely effective at minimizing Moire effects is 75 degrees for cyan, 135 degrees for magenta, 0 degrees for yellow and 15 degrees for black. 
     Each set of preferred angles utilizes the same angular displacement between pairs of dot patterns. Specifically, the angular displacement between the cyan dot pattern axes and the magenta dot pattern axes is 60 degrees. The angular displacement between the cyan dot pattern axes and the yellow dot pattern axes is 285 degrees. The angular displacement between the cyan dot pattern axes and the black dot pattern axes is 300 degrees. The angular displacement between the magenta dot pattern axes and the yellow dot pattern axes is 225 degrees. The angular displacement between the magenta dot pattern axes and the black dot pattern axes is 240 degrees. The angular displacement between the yellow dot pattern axes and the black dot pattern axes is 15 degrees. 
     The dot pattern of each color is created in order to properly represent the final reproduced image. A preferred dot pattern comprises either 24, 36, 45, 60, 62, 70 or 82 dot-positions per inch. A dot-position is a rectangular area in which a single dot is placed. 
     If the image color at a specific position does not require the presence of a certain process color, then the dot-position for that process color at that position will be empty. Conversely, if the image color at a specific position requires the full density of a process color then the dot-position for that process color will be completely filled by ink and the dot will be a rectangle. As the size of the dot increases from an empty dot-position to filled dot-position it transforms from a small substantially circular dot to a diamond shape, or ellipse. When the dot covers approximately 30-41% of the dot-position it merges with the proximate dot in the linear pattern. When the dot covers approximately 65-70% of the dot-position it merges with the proximate dot perpendicular to the linear pattern. 
     The shape of the dots utilized in the invention are preferably elliptical or rugby-shaped when medium sized. Rugby-shaped refers to the shape of a rugby football, which is similar to an elliptical shape though flatter at each end. Medium sized refers to the range of dot size when the dot covers approximately 20-50% of the dot-position. As the dot size moves from 20% to 50% the dot merges or joins its neighboring dot in the linear pattern. 
     The preferred screens for use in the claimed invention are flat screens as are known in the art. The screens are preferably orthogonally arranged such that the screen threads pass vertically and horizontally across the screen, intersecting at right angles. The resulting screen cells are rectangular, and typically square. The screen typically includes a rectangular frame which keeps the threads under tension. 
     During the transfer of the dot pattern to the screen from the process film, the screen cells correlating to the dot pattern on the film are unblocked and the other screen cells are blocked so that an area corresponding to the dot pattern is created through which ink is able to pass. 
     It is preferred that the dot pattern is transferred from the process film to the screen using photo emulsion. Typically during photo emulsion the screen is coated with emulsion that is dried. The film is aligned next to the screen so that the dot pattern angle on the film is properly oriented with respect to the vertical threads of the screen. Light is passed through the film onto the emulsion covered screen for an extended period of time. During this period the emulsion which light touches hardens and blocks screen cells. The emulsion which is shaded by the film&#39;s dot pattern does not receive light and stays unhardened and soluble in water. The film is removed from the screen and water is sprayed over the screen, removing the emulsion which was shaded by the film&#39;s dot pattern. In this manner, unblocked screen cells and partially unblocked screen cells substantially duplicate the dot pattern of the process film. The dot pattern can be better duplicated by screens in which the threads are thin and spaced closely together. Screens with synthetic threads are typically able to provide superior duplication since synthetic threads can be made much thinner than silk or metal threads. The screens utilized in the invention typically have 380 to 420 threads per inch. 
     After the film&#39;s dot pattern is transferred to the screen, the screen is placed next to the ink receiving substrate which is fixed in position relative to a substrate-guide, preferably a gripper. It is preferred that the screen is aligned with the substrate using register marks. This is particularly important for the screens which are utilized after the first color printing. For these screens, the register mark from the first printing is aligned so that the subsequent inks are printed in the correct relationship to the first ink. The substrate need not be registered in the ink-receiving position since it is secured to the substrate-guide or gripper which automatically aligns itself precisely with respect to the ink-receiving position. 
     After ink is printed onto the substrate, the ink must be set or dried. The substrate-guide or gripper transfers the substrate from the ink-receiving position adjacent to the screen to an ink-drying position. The drying process is preferably performed using UV light. The application of UV light results in much less heat being absorbed by the substrate than in most prior art ink setting processes. In addition, the application of UV light can be accompanied by heat transfer away from the substrate by water-cooling the table on which the substrate lies. Furthermore, the drying process using UV light can be performed more quickly than prior art processes. 
     Once the most recently printed ink is set, the substrate guide or gripper transfers the substrate to an ink-receiving position where the next screen is positioned adjacent to the substrate for printing the next ink. This process is repeated for each ink which is necessary to create a satisfactory reproduced image. It is preferred that cyan is printed first, followed by magenta, yellow and black. However, this order is not compulsory in achieving satisfactory results. Often the process requires cyan, magenta, yellow and black. However, it is understood that some images may require only one, two or three of these process colors. 
     Because the substrate-guide or gripper is able to transfer the substrate between each ink-receiving position each ink-drying position the substrate does not need to be re-registered each time it is transferred between these positions. This allows significantly more precision than in the prior art process of manually transferring the substrate to and from each device and re-registering the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a magnified view of an orthogonal screen used in the preferred method of printing. 
     FIG. 2 is a magnified view of a parallel dot patterns typical of the dot patterns used in the preferred method of printing. 
     FIG. 3 is a magnified view of a process film with a single linear dot pattern overlying an orthogonal screen. 
     FIG. 4 is a magnified view of a screen with unblocked cells representing a single linear dot pattern. 
     FIG. 5 is a schematic of a single vertical thread from an orthogonal screen with four sets of parallel dot patterns positioned in relation to the thread approximately at the preferred angles. 
     FIG. 6 is a schematic representation of steps used in the inventive method of screen printing which minimizes Moire effects. 
     FIG. 7 is a plan view of a preferred arrangement of a single ink-receiving position and ink-drying position. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the figures, details of the screen printing process with diminished Moire effect will be set forth. FIG. 1 depicts a magnified view of a screen  10 . The screen  10  is comprised of parallel vertical threads  12  and parallel horizontal threads  14  which run perpendicularly to the vertical threads  12 . The vertical and horizontal threads  12 , 14  define screen cells  16 . In their original state all the screen cells  16  are open and allow fluids to pass through the screen  10 . 
     In the preferred embodiment, about 380-420 vertical threads  12  pass through each square inch of screen  10 . Likewise, in the preferred embodiment, about 380-420 horizontal threads  14  pass through each square inch of screen  10 . Therefore, in the preferred embodiment, there are between about 144,400 and 176,400 screen cells  16  per square inch of screen. 
     FIG. 2 depicts a magnified view of a parallel linear dot patterns  20  typical of the dot patterns used in the preferred method of printing. Each of the dot patterns  20  in FIG. 2 runs from the bottom or right edge of the figure to the left or top edge. For example dot pattern  20   a  runs from position  20   b  to position  20   c . It can be seen that the dots  22   a  of dot pattern  20   a  grow in size as the pattern moves from  20   b  to  20   c.    
     Each dot  22  is positioned in a dot-position (not shown). In the preferred embodiment there are between about 62 and 82 dot-positions per inch. Towards the bottom of the figure the dots  22  cover less than 10% of their respective dot-positions. Towards the top of the figure the dots  22  cover more than 70% of their respective dot-positions. It is understood that the coverage of dot-positions ranges from 0-100% and is dependent upon the color, tone and shade of the image to be reproduced. 
     The axis  24   b  of dot pattern  24   a  is parallel to all other dot pattern axes  24  in the figure. As will be discussed in below, these axes  24  are positioned in specific angular relationships with the vertical threads  12  of the screen  10  when the dot patterns  20  are transferred to the screen  10 . 
     FIG. 3 depicts the magnified view of a process film with a single linear dot pattern  320  overlying an orthogonal screen  310 . It is preferred that during the transfer of the dot pattern  320  from the film to the screen, the orthogonal screen has been coated with emulsion. The dots  322  cover approximately 25-45% of their respective dot-positions. The axis  324  of the dot pattern  320  crosses the vertical threads  312  at an angle of approximately 82.5 degrees as is preferred for the cyan dot patterns. 
     When the dot pattern  320  of the process film is aligned over the screen  310  correctly, the process film and screen  310  are held in position and light is applied to the composite for an extended period of time allowing the exposed emulsion to harden. After a sufficient period of time the film is removed to reveal unhardened emulsion covered areas on the screen  310  which correspond to the dot pattern  320  of the process film. 
     FIG. 4 depicts the screen  410  after the process film has been removed and the unhardened emulsion has been washed away. The screen cells which were exposed to light have become blocked screen cells  417 . The screen cells which were covered by the dot pattern of the process film are unblocked screen cells  418 . Unblocked screen cells  418  will allow fluids to pass through the screen  410 . The blocked screen cells  417  will not allow such passage. Certain cells are partially blocked and partially unblocked. These partial blocked cells  419  allow fluids to pass through the unblocked portions of the cell. 
     During printing ink is able to pass through the unblocked screen cells  418  and to contact the substrate so that an ink image in the shape of the screen dot pattern  430  is transferred to the substrate. 
     FIG. 4 depicts a single dot pattern in unblocked cells  418  and portions of partially blocked cells  419 . The areas of screen outside of the blocked cells  419  which appears to be unblocked are not unblocked. Instead they are simply excess screen which in ordinary use would be covered by other dot patterns or blocked cells. 
     FIG. 5 depicts the printed image shown by parallel dot patterns  520  for each of the four process colors at approximately preferred angles. Cyan dot pattern  520   a  has an axis  524   a  which passes through the vertical thread  512  at an angle of approximately 82.5 degrees. Magenta dot pattern  520   b  has an axis  524   b  which passes through the vertical thread  512  at an angle of approximately 142.5 degrees. Yellow dot pattern  520   c  has an axis  524   c  which passes through the vertical thread  512  at an angle of approximately 7.5 degrees. Black dot pattern  520   d  has an axis  524   d  which passes through the vertical thread  512  at an angle of approximately 22.5 degrees. 
     FIG. 5 depicts only several parallel dot patterns for clarity. However, in the preferred embodiment of the invention, there would be between substantially continuous dot patterns for each color. These dot patterns would contain dots of varying sizes depending on the color of the image to be printed. The overlapping of differing dot patterns results in the ability to reproduce virtually any color, shade or tone using only four process colors. 
     The angles depicted in FIG. 5, i.e., cyan at 82.5 degrees, magenta at 142.5 degrees, yellow at 7.5 degrees, and black at 22.5 degrees, are preferred in the inventive process when using line screens of 60 or 62 (60 or 62 dot-positions per inch). For lines screens of 70 or 82 (70 or 82 dot-positions per inch), the angles of 75 degrees for cyan, 135 degrees for magenta, 0 degrees for yellow and 15 degrees for black are preferred. These angles are each 7.5 degrees less than the corresponding angle measurement in the first preferred embodiment using lines screens of 60 or 62. 
     The angles depicted in FIG. 5 are necessary to avoid Moire effects which plague prior art screen printed images. It has been discovered through extensive research that these angles, along with the disclosed method of printing, are best suited to minimizing Moire effects. 
     FIG. 6 is a schematic representation of steps used in the inventive method of screen printing which minimizes Moire effects. In step  1001 , an original image is analyzed and is separated into process color components, preferably cyan, magenta, yellow and black, at a predetermined line count. In step  1003 , each color component is converted into a dot pattern for each line. The dot pattern comprises dot-positions which are filled to varying degrees by dots depending on the level of each particular color needed at the dot-position. Preferably there are about either 24, 36, 45, 60, 62, 75 or 82 dot-positions per inch. The number of dot-positions per inch is also referred to as line screen numbers. The dot patterns for each color are preferably printed or otherwise applied to a process film. 
     Step  1005  involves the transfer of the dot pattern from the process film to a screen in preparation of screen printing. Preferably this transfer is achieved utilizing photo emulsion. During this process the screen is coated with emulsion. Then the process film is positioned adjacent to the screen such that the dot pattern can be negatively transferred to the screen. When the screen and film are properly aligned, light is applied through the process film onto the emulsion covered screen. The light acts to harden the emulsion which it contacts. The emulsion which is shaded by the dot pattern of the process film remains unhardened and water soluble. After light is applied to the screen to sufficiently harden the exposed emulsion, the film is removed. The screen is then washed or otherwise treated by fluid so that the unhardened emulsion is stripped from the screen. This process results in the transfer of the dot pattern from the process film to the screen, albeit in a rectilinear shape. The dot pattern is depicted by unblocked screen cells which are bounded by blocked screen cells. 
     Step  1007  involves securing the substrate to the gripper. The gripper is an elongate U-shaped member. The substrate is slid into the cavity of the gripper where it is secured. In step  1009  the gripper positions the substrate properly in the ink-receiving position. It is noted that for the initial ink application, the gripper may be in the proper position when the substrate is fed into it, so that the gripper does not necessarily initially move to position the substrate. 
     Step  1011  involves the alignment of the screen with the substrate. Preferably the screen and substrate are aligned using register marks. This means that during the first ink printing onto the substrate a register mark, typically a target-type mark, is printed on the substrate outside of the image area. The mark is preferably created during the dot pattern transfer step on each screen. Thus the mark can be realigned with in subsequent printings. 
     In step  1013  ink is passed through the unblocked cells of the screen onto the substrate. This step preferably involves the pouring of ink onto the screen and forcing the ink through the unblocked cells through the use of a squeegee. Ink cannot pass through the blocked cells during this step, ensuring that the rectilinear dot pattern is reproduced on the substrate. 
     Step  1015  involves the transfer of the substrate from the ink-receiving position to the ink-drying position. This step is preformed by the gripper. The gripper is able to travel linearly between the screen printing device and the ink dryer. As it moves, it pulls the substrate along with it. The gripper precisely finds its “home” position within each device so that the substrate is repeatedly precisely positioned within each device. Thus, manual adjustment of the substrate is unnecessary. 
     Step  1017  involves setting the ink in position on the substrate. Preferably this step is performed using UV light application. Preferably during UV light application the substrate is passed under UV light emitters. At the same time the substrate is cooled so that the heat from the light causes insubstantial deformation. Step  1019  involves repeating steps  1009  though  1017  for each process color used in the printing process. After each process color has been printed on the substrate and set, the reproduction of the image is complete. 
     FIG. 7 depicts the arrangement of a single ink-receiving position  710  and a single ink-drying position  720 . In the preferred method, there is a pair of ink-receiving positions and ink-drying positions for each color to be printed. The ink-receiving positions and ink-drying positions are preferably arranged linearly. 
     For a single ink application, a substrate  740  is secured to the substrate-guide or gripper  730 . Gripper  730  is preferable an elongate U-shaped member which is able to receive an end of substrate  740  and clamp on or otherwise secure the substrate  740 . The cavity of gripper  730  faces upstream so that substrate  740  can be pulled downstream (as indicated by the arrows) as gripper  730  travels downstream. A screen (not shown) is positioned above the substrate  740  and is aligned for printing as is known in the art. After ink has been forced through the screen onto substrate  740 , gripper  730  pulls substrate  740  out of ink receiving position  710  and into a drying unit  750 . Gripper  730  automatically positions substrate  740  into proper ink-drying position  720 . UV lights within drying unit  750  dry the ink on substrate  740 . While UV light is applied to substrate  740 , the lower portion  760  of drying unit  750  transfers heat away from substrate  740 , typically through the use of cooled water passing immediately beneath the table on which the substrate rests. In this manner, substrate  740  experiences minimal deformation. 
     After sufficient drying, gripper  730  pulls substrate  740  out of ink-drying position  720 . For the application of additional inks, gripper  730  pulls substrate  740  into another ink-receiving position and the process is repeated. Because gripper  730  transfers substrate  740  between devices automatically, substrate  740  does not require re-registering with each subsequent device. This allows for much greater precision than is currently performed in the art.