1. Field of the Invention
The invention relates to a novel printing screen assembly having a shield formed by a flexible, nonporous sheet of material mounted on the frame of the screen assembly and positioned parallel to the mesh screen to form an enclosed ink containment area. The invention further relates to a method for making such a printing screen assembly and a process of screen printing on hard non-absorbent materials, such as glass.
2. Summary of Related Art
Screen printing is an important process in making automotive glass and other glass components, for making printed circuit boards, and for a number of other applications where a specific pattern is printed on a hard substrate surface. In the glass manufacturing process, screen printing is used to print shaded bands around the edges of the window glass and to print the electrically conducting networks for heater arrays on back lights. Screen printing presses are also used for package printing applications. Presses print on both flat and curved surfaces and are used in a variety of industries.
In the various manual and automatic screen printing apparatus, a flood bar and a squeegee blade are mounted on a carriage in the printing press. The press includes a control system to control both vertical and horizontal motion of the squeegee blade assembly and the carriage. A screen with the desired pattern is placed in contact with the object being printed, and the ink or paint is placed on top of the screen. The flood bar is used to distribute ink across the screen in a first pass over the screen. The flood bar is raised and the squeegee blade is lowered and drawn across the screen to force the ink through the screen onto the workpiece to be printed upon in a pattern determined by art work formed on the screen. Ink is forced through any openings in the emulsion coating on the screen such that ink is deposited on the glass or other solid substrate adjacent the openings in the screen.
In some presses, the drive carriage will include only a single squeegee blade. The squeegee blade is pressed across the screen in one direction to force ink through the screen in any of the apertures in the screen coating. A return run of the squeegee blade back across the screen forces the ink onto the surface of the workpiece.
In order to achieve pressure on the working edges of the squeegee on a screen, without ripping the screen, the elasticity of the rubber material in the squeegee blade is utilized. The elastic rubber material of the squeegee blade extending from the face plates develops an even pressure over the entire length of the working edge during the printing process.
Various printing screens structures and manufacturing methods are known in the art. The typical printing screen for automotive glass starts by preparing the original pattern as a positive film, and then transferring the positive image on a positive film to a full-size photosensitive transparency. A screen is prepared by coating the screen with a photosensitive emulsion and placing the transparency on the screen. A light source on the transparency side of the screen is switched on for the required exposure time. The screen is separated from the transparency and the non-exposed areas of emulsion are washed away to form the desired pattern in the screen.
The thickness of the layer of ink which is deposited on the glass or other substrate depends on a number of factors, including size of the mesh in the screen, the type of ink or paint, the thickness of the hardened emulsion formed in the screen, the pattern of the emulsion in the deposit areas, and other similar factors. The thickness of the emulsion depends on the thickness of wet emulsion applied to the screen and the photo cure time to harden the emulsion. In many cases, the emulsion must be applied in several steps to harden the emulsion to the desired thickness.
Once the screen is formed, the screen is mounted in the printing press for production use. In automotive glass applications, a single screen can be used to print several thousand pieces of glass. The screens are replaced on a regular basis and production quality is monitored to detect defects caused by screen problems such as a tear in the screen, deterioration of the emulsion, or clogging of the mesh openings of the screen.
One of the problems encountered in the manufacture of automotive glass and in other printing processes used in production operations is contamination of the paint. In most applications, the ink is exposed to the production environment about the printing press such that dust, dirt, glass chips, and other contaminants may settle in the paint. Contaminants in the ink may block openings in the screen such that the ink does not pass through the screen and the printing on the glass does not meet acceptable quality standards.
The contamination of ink has recently become a greater concern due to changes in the type of ink used in production. In automotive glass applications, the ceramic ink (paint) used for the obscuration band printing had typically been a lead based ink. Because of the documented environmental problems with lead ink and lead paint, glass manufacturers are using water-based inks which do not contain lead. The water-based inks are not as opaque as the lead inks. Consequently, more water-based ink must be applied to the glass to provide the desirable obscuration band. In order to achieve a heavier deposit of water-based ink, thicker mesh fabric has been used on the obscuration bands.
Problems have occurred with the use of water-based ink in the printing process with the clogging of the mesh openings in the screen. The water-based ink, which is exposed to ambient conditions on top of the screen, is sensitive to the temperature and humidity in the production environment. The optimum operating conditions occur when the temperature and humidity are maintained within a specific range. Ink will thicken and clog the openings in the screen at a low temperature, but low temperatures are not usually a problem in production applications. At high temperatures or low humidity, evaporation is a problem with the water-based ink such that the ink will block openings in the screen. The ink cures and hardens in the mesh of the screen to block opening and prevent ink from passing through the mesh to the glass substrate.
Keeping contaminants out of the ink reduces the undesirable clogging of the mesh openings in the screen. Maintaining the water-based ink in a humid atmosphere is also desirable when using the ink in a printing process. The water-based ink maintains the desired viscosity and passes through the mesh openings in the screen. Relative humidities in the range of 80%-100% are preferred for water-based inks.
A number of U.S. patents have disclosed various screen assemblies used for production operations such as automotive glass production. U.S. Pat. Nos. 3,851,581 and 3,852,564 to Baum et al disclose a method of making a silk screen and the manufacture of electrically heated windows. The references teach a screen having a uniform thickness and that the uniform thickness may be varied by depositing successive layers of emulsion.
U.S. Pat. No. 4,958,560 to Collins teaches a method of screen printing a patterned ink layer using a screen with a patterned coating on its surface facing the substrate. The reference discloses that the thickness of a patterned ink layer screen printed on a glass surface can be increased in selected areas by providing a pattern of emulsion dots for local support between the screen surface and the glass in the selected areas. U.S. Pat. No. 4,379,737 to Mearig and U.S. Pat. No. 4,791,006 to Galvagni et al disclose additional screen assemblies and methods for providing variable thickness ink build up.
A printing screen configuration and method for printing a variable thickness pattern are disclosed in U.S. Pat. Nos. 5,388,509 and 5,390,595 issued to applicant. A screen is formed by applying a first coat of emulsion to the mesh material of the screen to define open areas. A second coat of emulsion of different thickness is applied to the screen, the second layer being in the form of a dot pattern. The difference of thickness in the emulsion dot pattern produces a thicker pattern of ink during the screen printing process.
Screen assemblies and printing processes are also disclosed in a number of U.S. patents relating to squeegee holder assemblies and printing presses. U.S. Pat No. 5,078,063 to Johansen et al. discloses a squeegee holder with a levelness control and a pressure control for the squeegee blade.
An alternative squeegee arrangement is shown in U.S. Pat. No. 4,989,512 to Lindstrom et al. The squeegee is maintained at an angle and includes a plurality of defined support positions. U.S. Pat. No. 4,841,854 to Bubley shows a squeegee assembly with two face plates and a center bar.
U.S. Pat. No. 4,779,529 to Ericsson discloses a squeegee system which maintains the squeegee blade at an obtuse or an acute angle, irrespective of the direction of relative squeegee movement. A plurality of independent squeegee holder members are shown in U.S. Pat. No. 4,638,733 to Schneider et al. The screen assemblies disclosed in the prior art provide for a layer of ink disposed on the screen and direct contact between the squeegee and the ink.
In order to control the environment of the ink to prevent contamination and maintain the desired humidity, one option is to build a "clean room" type of enclosure around the printing press in the production area. The enclosure is expensive to build and maintain. Placing the printing press in an enclosure may adversely effect operator access to the printing area for production operations, troubleshooting, and routine maintenance.
A significant problem in using a clean room type enclosure for the printing operations is maintaining a higher humidity in the enclosure to ensure the desired results in transferring the ink through the open mesh areas of the screen. When moisture is added to the atmosphere in the enclosure to maintain a high relative humidity, the additional moisture in the enclosure has an adverse effect on the equipment in the enclosure. At a relative humidity of near 100%, moisture will form on the printing press, the conveyors, and other equipment in the enclosure. Such equipment is not typically designed for operation in a high moisture environment, such that the moisture will cause deterioration problems with both the electrical components and the mechanical components in the enclosure.
In addition, moisture forms on the glass substrate and the screen such that the screen sticks to the glass during production operations. The sticking problem often causes delays in, or even a shut down of, the production line.
Automotive glass manufactures and other users of screen assemblies for printing processes in production operations have a need for a low cost and convenient means for preventing contamination of the ink and for maintaining water-based inks in a moist environment. The preferred screen assembly would be usable on the existing printing press without modifying the equipment, the building, or the process used to print the glass.