This invention relates to a method for cleaning the screen stencil of a screen printing device in which a sheet of paper is placed beneath the screen stencil for an intermediate printing and is printed by a squeegee passing over the stencil. This invention also relates to a screen printing machine for performing this method.
A cleaning method and a screen printing machine suitable for performing the cleaning method are known from DE 199 17 794 C2. With this known machine, the intermediate printing table is designed as a table that can be pivoted parallel to the guide path of the printing table and which is below the guidance of the printing table during the printing process and is pivoted upward for the purpose of cleaning beneath the screen stencil for the intermediate printing.
Machines of this type function satisfactorily but cannot prevent the disadvantage that the screen printing ink may dry out in part and clog the screen. Therefore despite the arrangement of an intermediate printing table which is used for cleaning the screen, it is still necessary with screen printing devices to stop production to clean the screen.
The object of the present invention is to perform such a cleaning operation automatically for the most part and to perform it during operation to avoid production downtime. At the same time, better working conditions are to be created and rejects are to be prevented.
To achieve this object, it is provided that before performing the intermediate printing, the bottom side of the screen stencil is wetted with a cleaning agent. Due to this measure, even ink residues that have already dried are dissolved from the screen; these residues could not have been removed from the screen stencil through the measure of an intermediate printing, which is performed by moving the squeegee mechanism onto an intermediate printing paper which is then rolled up and removed.
In another embodiment of the inventive method, wetting is performed by a rotating cylinder or brush which is brought into contact with the bottom side of the screen stencil and is immersed in a cleaning fluid. Due to the rotation of the cylinder or brush, the cleaning agent is conveyed out of the bath and brought to the bottom side of the screen stencil, where it can loosen dried-on ink residues which can then be removed in the intermediate printing.
This invention also relates to a screen printing machine for performing the cleaning method, whereby the screen printing machine is equipped with an intermediate printing device with which a sheet of paper can be introduced beneath the screen stencil for the purpose of cleaning. The new screen printing machine is characterized by a rotating body assigned to the intermediate printing device and arranged across the direction of movement of the intermediate printing device, said rotating body being coatable with a cleaning agent and coming in contact with the bottom side of the screen stencil before the intermediate printing and being guidable along the stencil.
Due to this design, the cleaning can be performed automatically and in the screen printing machine.
In an embodiment of this invention, a cylinder which is immersed in a bath of cleaning agent may be provided as the rotating body.
In a refinement of this invention, this cleaning agent may be present in a container that surrounds the bottom of the cylinder and is adapted to the shape of the cylinder, in which case the cylinder is designed with a roughened surface to retain a sufficiently thick layer of cleaning agent and to be able to bring it against the screen stencil.
In a refinement of this invention, the cylinder is driven in a rotating manner and to this end engages in a toothed rod with pinion gears arranged at the side next to the screen stencil on the printing machine.
The intermediate printing device itself is designed so that it can be raised and lowered and thus can be folded up and unfolded in which case a control device may be provided for determining the cleaning cycle so that, for example, a cleaning printing operation is performed after every second, third or fourth printing.
In an advantageous refinement, the cylinder and the container assigned to it to which the cleaning agent is supplied in circulation in a simple manner are arranged on the end of a paper impression cylinder mounted displaceably in the direction of movement of the squeegee, where said end of the cylinder faces the screen stencil.
To implement the possibility of raising and lowering the cleaning agent cylinder, the cylinder together with the container may expediently be arranged on a pair of swivel levers that can be acted upon by pneumatic cylinders so that the lateral pinion gears of the cylinder engage in the toothed rod which runs fixedly next to the screen stencil. Finally, in an essentially known manner, the intermediate impression cylinder may be provided with paper and arranged so that it can be moved at least by the length of the screen stencil, so that before the intermediate printing the entire underside side of the screen stencil is wettable with a cleaning agent. In practice, it is sufficient to wet the stencil in the printing area.
Instead of the intermediate impression cylinder with an unrolling web of paper, an endless sheeting material may also be used to accommodate the intermediate printing or a plate cylinder may be used. In both variants, continuous cleaning of the continuous loop or the plate cylinder may be ensured. Then there is no consumable material in the form of paper.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings for example.