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
Screens can be tilted for printing onto tapered containers, such as cosmetic jars (Figure 3), but this is not practical when the taper angle of the items is greater than 12°. Containers with taper angles greater than 12° must be treated like conical containers. These containers must be tilted, and specially shaped screens are required to compensate for wide images. A straight screen may be used for narrow images, such as those that are printed onto the outside of cereal bowls.
Facilitating the process with additional motors
Adding a second servo motor to a fixture facilitates tilting of the fixture. Such a configuration is used successfully for printing onto cereal bowls and serving bowls (Figure 4). Movement synchronization of five servo axes during printing enables the decoration of oblong bowls.
Servo motors can accelerate and decelerate quickly—a feature that can be used advantageously when printing onto square containers or onto oblong bowls. The printing speed does not need to be constant during the printing cycle. Printing around corners with a small diameter must happen slowly, but printing along the sides be done faster. Optimizing the printing speed, dependent on the shape of a section of the contour, ensures high print quality and maximum production throughput (Figures 5 and 6).
Other benefits of servo motors
The extensive use of servo motors for driving the fixtures also enables the optimization of other stages of the de-coration process in the screen-printing machine and simplifies the optical positioning of a container prior to printing. The fixture need not be disengaged from a main drive; therefore, accuracy is not lost when the fixture is again engaged with the main drive. An optical positioning system detects the target on a quickly rotating container, overshoots the target, and stops the container. The container immediately turns slowly in reverse direction and is stopped accurately when the target is detected again.
Optimizing the rotation of an item under a UV lamp also is possible. A container can be rotated not just once, but twice or three times. Servo motors also can improve surface treatments. A container under a flame can either be turned slowly once or fast twice. The speed profile can be selected to ensure a uniform treatment around the container’s circumference.
Did you enjoy this article? Click here to subscribe to the magazine.