Printing – Stenciling – Stencils
Patent
1993-04-02
1994-11-22
Burr, Edgar S.
Printing
Stenciling
Stencils
101127, B05C 1706
Patent
active
053658400
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to screen printing utilizing a printing frame having a tensioned fabric thereon. Generally, the present invention contemplates the tensioning of the fabric or screen material with a roller type frame. The improved screen material permits higher tensions to be achieved so that various improvements are accomplished in the printing process. Moreover, the present invention relates to a method of using higher screen tensions and to the improved screen material so as to obtain these advantages.
BACKGROUND OF THE INVENTION
In the process of screen printing, the result achieved by the printing equipment is no better than the screen material utilized therewith. Even if a frame is stable and does not change during operation, inaccurate results can be achieved if the fabric screen material fails or stretches during the printing process.
New low elongation fabrics have been developed in recent years. However, even these fabrics begin to relax soon after the squeegee is first applied to transfer ink during the printing process. This is analogous to the tuning of a guitar with new guitar strings. The process is thus complicated by this initial relaxation during use. Loss in screen tension during any part of the printing process can result in image distortion, mis-registration, blurred edges, color shifts, ink penetration through porous or semi-porous substrates (such as uncoated paper or cloth garments), and ink build-up on the bottom of the screens. Manifestly, these changes in tension will require ink, squeegee and press adjustments during the printing operation. The result of this change in tension is a loss in quality, productivity and control over the printing process.
Roller type frames, such as those manufactured by Stretch Devices, Inc. of Philadelphia, Pa., may be utilized to meet the rigidity and stability requirements for accurate screen printing results. These roller type frames permit the retensioning of the fabric so as to maintain a constant tension over the entire printing operation. In a roller type frame, retensioning is performed by rotating (at least) one roller within the frame. This retensioning is performed after the fabric has received the stress exerted by the squeegee and has reacted to all of the inks and chemicals in the processes. This retensioning tends to restress or shock the polymer chains in the screen fibers and is essentially a work-hardening type process. During retensioning, the molecular chains become more and more highly oriented in the direction of the fibers while developing even greater bond strength. The more the screen is used and retensioned (restressed) after reclaiming, the stronger and more stable it becomes. Manifestly, screens that have been worked for an extended period of time will have consistency and repeatability in their printing.
Higher tensions in a screen can result in a faster squeegee speed during the printing operation. The squeegee speed is typically dictated by the speed at which the screen lifts off the substrate. In the lifting of the substrate, a "snap"-action or force is desired to assist in ink shear. Ink shear is important in order to provide accurate lines during the printing process. High tension gives a quicker and/or more forceful snap of the screen off of the substrate and thus a higher ink shear. This results in the ability to use a faster squeegee speed during printing, and thus a faster printing process.
Higher tensions also result in a more uniform application of the ink on the screen by the flood bar prior to the squeegee moving across the screen. Because of the higher tension, there is no build-up at the center of the screen material due to a downward bowing caused by the weight of the ink on the screen material. Moreover, because of the tension in the screen, the flood bar can move more quickly and at a greater force without resulting in the ink being pushed through to the opposite side of the screen material.
In the operating machinery, the off contact distance between the screen a
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Burr Edgar S.
Raciti Eric P.
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