1. Field of the Invention
The present invention generally relates to screen printing. More specifically, the invention relates to a screen printing apparatus for printing on three dimensional surfaces.
2. Description of Related Art
Screen printing is a versatile printing process that can be used to print images on a variety of substrates. Some of the more common substrates include fabrics, metals, glass, plastics, paper and paperboard, and some common products from the screen printing industry include clothing, glass and plastic bottles, labels, decals, signs, electronic circuit boards and windows. One particular application in the automotive industry to which screen printing has been applied is the applying of masks around the border of automotive windows.
As suggested by the above listing of products, one advantage of screen printing is that machines can be used to print on substrate having a variety of shapes, thicknesses and sizes. As a result of the development of automated and rotary screen printing machines, improved dryers, and UV curable inks, the utilization of screen printing has increased because of the simplicity of the application process. A wide range of inks and dyes can be used in screen printing. (For convenience, hereafter only the term “ink” is used.)
A machine for carrying out screen printing may be of a single or multiple table design, the latter often being seen as a rotary table style of machine. Generally, the machine includes as its primary components a screen, as substrate support, a squeegee and a mechanism for drawing the squeegee across the screen. As further mentioned below, the machine might also include a flood bar as well as a mechanism for dispensing ink onto the screen.
The screen is a porous mesh stretched tightly in a frame made of wood or metal. In order to assure proper dispensing of the ink through the mesh, proper tension on the mesh, via the frame, is required. The mesh itself is constructed of a porous fabric or stainless steel. A stencil is produced on the mesh (by either a manual or photochemical process) to define the image that is to be printed on the substrate.
After the substrate has been loaded into the machine, ink is applied onto the top of the screen and may be spread across the screen by the flood bar. With the screen being held down onto the substrate, the squeegee is drawn across the screen, applying pressure and thereby forcing the mesh to the substrate and the ink through the openings of the mesh in the areas where no stencil is applied. As a result, ink is transferred to the substrate according to the image defined by the stencil.
Many factors contribute to the quality of the image transferred to the substrate. One factor relating to the amount of ink transferred through the screen is the diameter and thread count of the thread forming the mesh. Regarding the squeegee, the hold angle, pressure, draw speed, size, hardness/durometer and material composition are all factors. While squeegee blades have typically been made from various rubbers, polyurethane has recently become one of the materials of choice.
Screen printing machines themselves are generally known to be of three basic varieties. The most used variety is the flat bed screen printing machine. Generally, in a flat bed machine a single printing station exists and the squeegee is draw across the screen, which is being held down on flat substrate. Another type of printing machine is the cylinder screen printing machine. With such a machine, the substrate is laid out in a cylindrical shape beneath a flat screen. The substrate is rotated while the screen is translated past the squeegee in order to imprint the image on the substrate. A third type of screen printing machine is the rotary machine. In this latter type of machine, a series of flat beds are provided around an indexing table and the beds are successively rotated through a loading station where a substrate is loaded onto the bed, a printing station where a screen is laid over the substrate and a squeegee drawn thereacross, and a drying station where drying or curing of the ink occurs, and a take-out station where the substrate now containing the printed image is removed from the machine.
As seen from the above, machines and components exist for screen printing images onto flat and cylindrical substrates. These technologies are well developed and result in high quality images being printed on the substrates. However, as the shapes of the substrates vary into more complex three dimensional shapes, such as those associated with automotive windows, the ability of these prior types of machines to lend themselves to the printing on three dimensional substrates is limited. Substrates having a multiplicity of curvatures across its surface are therefore a unique problem in the industry.
One problem with printing on such surfaces is maintaining the proper tension in the screen and holding the screen at a proper off-contact distance from the substrate. “Off-contact”, as that term is known in the industry, is the distance by which the mesh of the screen is held away from the substrate immediately prior to and after the squeegee is drawn thereover, the squeegee forcing the mesh into contact with the substrate. Proper off-contact distances allows for precise and highly detailed images to be applied. Another problem associated with printing on multi-curvature, three dimensional surfaces is maintaining a consistent pressure across the length of the squeegee itself.
In view of the above, it is apparent that there exists a need for a screen printing apparatus or machine specifically adapted for printing on complex three dimensional surfaces.