The present invention relates to printing apparatus, and more particularly to an improved apparatus and method which facilitates production of quality images upon repetitive printing operations.
The printing of multicolored and other complex images on articles (such as sheets of textiles, plastic or the like) is commonly accomplished by screen printing machines. Generally, these machines are provided with a conveyorized printing blanket driven over a pair of spaced apart rotating drums. The article is placed flat on the blanket and indexed to each in a series of printing stations. At each station, a printing head is lowered onto the article and a printing operation is performed. For example, a print squeegee is stroked across the surface of a horizontal screen in registry with the article so as to force printing ink through the screen and onto the article, thereby effecting printing. In this manner, each in a series of printing steps are performed on the article to obtain a desired complex image.
To assure quality, it has been found desirable that the printing head be in precise registration with the article. However, manufacturing imperfections inherent in printing blankets, i.e., the nonuniform center or neutral plane within, have been found to inhibit precise registration. In particular, upon each indexing movement of the blanket (and associated angular displacement of the drums), the distance traveled by the blanket circumference (over the drums) varied with the neutral plane of the blanket. While these variations were small initially, they compounded upon repeated printing operations, visibly offsetting the blanket from a desired position relative to the printing head. This resulted not only in overlapping images and other printing inaccuracies, but also complicated set up by requiring numerous adjustments to obtain acceptable, though seldom precise, registration.