Printing – Stenciling – Processes
Patent
1993-02-12
1994-10-18
Eickholt, Eugene H.
Printing
Stenciling
Processes
118654, 118656, 427468, B41M 112
Patent
active
053557948
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the dry printing of printing substrates, especially in screen printing and tampon printing, and to an apparatus for its use. The invention further relates to a process for the manufacture of printing powder for use in such a process and such an apparatus.
In the dry screen printing process, printing powder is moved through a screen printing stencil having the desired pattern in its fine-meshed stencil material. The printing powder reaches the printing substrate to be printed via the screen printing stencil.
A dry screen printing process is known from U.S. Pat. No. 3,285,167 in which printing powder particles are pressed through a screen printing stencil using a brush having flexible mechanical fibres. The screen printing stencil is located above and spaced from the printing substrate to be printed. An electrode arranged beneath the latter and the screen made of electrically conductive material are connected to the terminals of a high voltage source and in this manner an electrostatic field the field lines of which are substantially perpendicular to the plane of the printing substrate is produced at the location of the printing substrate. By means of that field the printing powder particles having dielectric material are moved towards the printing substrate once they have been pressed through the meshes* of the screen printing stencil by the fibres of the brush. Rotating brushes are also proposed as brushes. The application of the printing powder from a reservoir is carried out using transfer rollers which dip into the reservoir.
A similar dry screen printing process and an apparatus that can be used for it are described in U.S. Pat. No. 3,202,092. That specification also proceeds on the assumption that movement of the printing powder particles takes place in two phases: a mechanical pressing of the printing powder particles through the meshes of the screen printing stencil using a brush and subsequently movement of the dielectric printing powder particles in an electrostatic field, a space again being maintained between the screen printing stencil and the printing substrate.
The procedure according to U.S. Pat. No. 3,285,167 and U.S. Pat. No. 3,202,092 is satisfactory so long as large areas of the printing substrate are printed with the same colour. It has been found, however, that when printing small details and especially in the case of half-tone screen printing the colour reproduction is not entirely satisfactory.
EP-A-197 242 describes special toner particles and the manufacture thereof. These consist of a basic body of synthetic resin having an average particle diameter of from 0.1 to 1000 .mu.m and modifier particles adsorbed onto those basic bodies. The modifier particles consist, for example, of a material having an anionic or cationic group. In this manner it is possible to make the surface properties and optical properties of the composite particles different from the corresponding properties of the basic bodies. The diameter of the modifier particles is 1/5 to 1/20 of the diameter of the basic bodies.
The invention provides a dry printing process in which a screen printing stencil is arranged over the printing substrate that is to be printed. Printing powder is then distributed over the screen printing stencil, an electrostatic voltage difference being applied between the powder and the printing substrate. This assists in attracting the printing powder towards the substrate. In this manner, an image of the pattern of the screen printing stencil is obtained on the printing substrate.
It is not necessary to maintain a significant air gap between the screen printing stencil and the printing substrate if a voltage difference is applied between the printing powder and the printing substrate. Preferably, according to the invention it is even preferred to work with the screen printing stencil and the printing substrate almost in contact or in very intimate contact with each other. Images are thus obtained with good resolution since it is thus possible to m
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Eickholt Eugene H.
Philpitt Fred
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