Printing – Stenciling – Processes
Reexamination Certificate
2001-07-03
2003-01-14
Evanisko, Leslie J. (Department: 2854)
Printing
Stenciling
Processes
C101S115000, C101S490000, C101S128400, C358S001900
Reexamination Certificate
active
06505554
ABSTRACT:
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'clock” angle would be 90 degrees and an “11 o'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 28
Evanisko Leslie J.
Jansson & Shupe & Munger Ltd.
LandOfFree
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