Screen printing has proven itself as an effective method of decorating three-dimensional items and unusually shaped products at high production speeds. This article introduces the printing systems used for such jobs and the innovations they feature to accommodate challenging products.
By Harald Gavin
Servo-based screen-printing machines overcome the disadvantages and potential problems of mechanically driven machines. About a decade ago, servo technology had developed to a point where screen-press manufacturers could utilize this technology at an economic price. The price for servo technology accounts for not only the hardware, but also for the required programming of the man-machine interface and the drives for the servomotors. The prices for servo hardware and servo programming tools suitable for screen-printing machines have come down, driven by the demand from packaging-machine OEMs. The number of packaging machines manufactured annually is several times that of screen-printing presses; nevertheless, manufacturers of screen systems took advantage of these price developments.
Modern, servo-based screen lines use an indexing table or indexing ring to carry items to the machine stations. Individual servo motors drive the fixtures on the indexing ring. The servo motors can receive new parameters during each indexing cycle. The parameters can be different for each servo motor; therefore, the profiles of the rotations of the fixtures can differ from each other. As a result, the rotation of each fixture is optimized to meet the requirements of the specific machine station where the fixture rotates.
Solving the problem of driving each fixture with its own servo motor facilitated the redesign of servo-based screen-printing presses. These machines have an individual servo motor for each screen, in addition to a servo motor for each fixture. A sophisticated motion controller synchronizes the movements of all the servo axes simultaneously, especially the movements of two axes in each printing station (screen and fixture) during printing onto cylindrical containers. The individual servo motors allow the printing movements in a printing station of a multicolor, servo-based screen-printing system to be independent of the movements in the other stations.
If necessary, it is possible to stretch or compress an individual color, either by changing the rotational speed of the fixtures when they are in a specific printing station or by changing the horizontal speed of the screen in the specific printing station. The image will be shorter when the surface speed of the container is slightly slower than the speed of the screen. Conversely, the image will be longer when the surface speed of the container is slightly faster than the speed of the screen.
Synchronization for oval and square objects
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