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
An optical item-orientation system in a high-speed screen-printing machine, for example, can position an item accurately and index the product to the printing stations for precision decoration. An item is positioned in two stations with its fixture disengaged from the main drive. The press rotates the item quickly in the first orientation station. Therefore, the container cannot be stopped immediately when the sensor detects the target. The item overshoots, but the approximate position of the target is known. The item stays in this position while it is indexed to the second orientation station, where the product is rotated very slowly and can be stopped immediately once the sensor detects the target. Any angular advance required for positioning an image is set at the press’s control panel. The item is rotated by the set angular advance before the fixture is again engaged with the main drive.
Another example of the need to disengage a fixture from the main drive while the fixture is in a machine station is the required movement of a fixture in a print-inspection station. An item has to rotate further than a full revolution to enable the software of the print-inspection system to identify a complete image before it can check the image for print defects.
Other issues arise when mechanically driven, multicolor systems print at speeds of more than 90 cycles/min. For instance, each screen has to move forward and backward when decorating round items. A screen follows a specific speed profile. The screen has to accelerate, move at speed, decelerate, stop, and then accelerate in the reverse direction. The speed of the screens during printing dictates the rotational speed of the fixtures because the surface speed of the rotating items at any moment must be equal to the linear speed of the screens. A mechanical drive ensures that a fixture’s speed profile with acceleration and deceleration is the same as the speed profile of the screen—but the fixture’s profile will remain the same whether the fixture is in a printing station, in a surface treatment station, or in a UV curing station.
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