1. Field of the Invention
The present invention relates to a screen printing apparatus that can adjust the position of a material to be printed on mounted on a table and that of a screen relative to each other to achieve proper alignment and to a method of adjusting the printing position.
2. Description of the Related Art
Screen printing apparatuses are known for applying an ink (or a printing paste) on a material (hereinafter referred to as printing material) such as paper and substrates. In such a printing apparatus, a squeegee is slid from a printing start side toward a printing end side on a screen on which an ink is loaded to apply the ink onto the printing material mounted on a printing table. After completion of printing, a scraper is slid from the printing end side toward the printing start side to scrape the residual ink on the screen back to the printing start side. By repeating the procedures described above, a plurality of printing materials are designed to be printed successively.
However, when a printing material mounted on the printing table is printed as described above, it is necessary to carry out accurate alignment between the screen and the printing material. For such purpose, the screen and the printing material are generally provided with alignment marks for bringing them in proper printing positions.
In a double-table type screen printing apparatus, alignment of the screen and the printing material with each other using the alignment marks provided on both of them and actual printing are often carried out at different places. In such case, the screen and the printing material get out of position relative to each other when they are transferred from the aligning place to the printing place, leading to poor printing accuracy.
There is another method, in which the alignment marks provided on the screen are detected by alignment mark detectors located above the screen, and the alignment marks provided on the printing material are detected through the screen by the alignment mark detectors located above the screen to adjust the position of a table unit based on these detection results in the X, Y and theta (.theta.) directions.
However, in this case, due to the structure of the screen printing apparatus where the screen is arranged above the printing material mounted on the table, the alignment mark detectors are spaced away from the screen so as to detect the alignment marks provided on the printing material through the screen.
Accordingly, the screen hinders accurate detection of the marks, particularly the pattern of the alignment marks provided on the printing material. Therefore, in order to accurately detect the pattern of the alignment marks on the printing material, it is necessary, for example, to increase the detection sensitivity of the alignment mark detectors or to apply a filtering treatment through an electric circuit so as to remove the screen from the image detected.