Screen printing is at least as old as the Great Wall of China, having evolved from primitive stenciling techniques.

Modern day screen printing consists of six basic components: stencil, screen fabric, screen frame, squeegee, ink and substrate. The squeegee is used to force ink through image areas of the screen fabric onto the substrate. The substrate may vary from textile materials such as a T-shirt to paper media such as a large full color wall poster.

While this type of printing has evolved from ancient art, there's been very little change in how the process occurs. Finding better components for the printing process only one avenue of change for screen printing. The biggest change has been with the introduction of automation strategies.

Types of Automated Screen Printing Presses
The screen printing industry offers three different configurations for automated screen printing presses for textile printing: belt, ovals and carousels.

The most common type of automated textile screen printer is carousel. Carousel screen printing presses have multiple pallets and print heads that rotate around a central shaft. Substrates are manually loaded onto the pallets. The pallets are indexed into position, followed by each print head being lowered into position automatically. The number of printing heads can go as high as 16. At least two pallet positions are used for loading and unloading of the substrates, so you can have up to 18 stations.

Motion Control in Screen Printing
There are several areas where motion control is an important factor in machine design. Servo-drives and programmable logic controllers generally handle the indexing needs. However, cost and size have been prohibitive factors in using drives to control the squeegee movement. So, pneumatic pistons are the norm.

The problem with pneumatically controlled pistons is that the speed of the squeegee across the screen is not constant. It starts out slow and becomes faster as the piston nears full extension. This results in an inconsistent flow of ink.

Replacing the Pneumatics for Squeegees with Drives
For years, carousel manufacturers have been trying to find a practical solution for replacing the pistons. Because electronic drives can provide constant speed motion control for the squeegee, engineers have been trying to find a way to use them to achieve their replacement goals.

Until recently, servo-drives were viewed as impractical because the drive and motor combination is cost prohibited. Remember too, that carousel presses may have as many as 16 print heads. That means a large carousel would need 16 drives and motors. That's a lot of equipment and a great deal of capital investment

On the other hand, engineers attempted to use AC induction motors. As the size of these drives became smaller, AC drives using induction motors seemed to be pointing the way to a practical solution. Until now.

Finally a Solution
A few years ago, the drive industry unveiled vector-controlled drives capable of drawing very few amps during heavy cycling applications, while providing both constant speed and torque capability. One of the first to do this was Siemens. The Micro Master Vector® line from Siemens offers vector control at a unit cost that was very palatable.

M&R tested the line and found that the vector-controlled drives easily handled the load requirements of the automated carousels. Thus, M&R recently began offering its Challenger® II carousel models with this newer technology installed. The unit cost per head for using the 6SE32 vector-controlled drive from Siemens is very low, making it a very practical replacement for the pneumatic control. The added quality due to more consistent ink coverage is outstanding.

Some Added Benefits
The decision by M&R to use the vector drives had some added benefits that were quite unexpected. First off were the energy savings. The electrical current used with the vector drives is very small in comparison to other type drives, i.e. voltage-to-frequency drives. This adds up to as much as 34 percent in operating energy costs.

A second benefit has to do with the LCD panel on the Siemens drive and the multitude of layered parameters that can be reported and setup through this visual interface. M&R is using this medium to aid customers in synchronizing the squeegee speed among the various heads.

Conclusion
AC vector drive technology has shown itself to be a very reliable, performance-enhancing alternative for pneumatic motion control of the squeegee in carousel screen printing processes. As additional technologies become available and size and economics continue to change positively, the look and performance of screen printers may continue to change proportionally. Moreover, the traditional substrates, such as T-shirts, are now displaying images and typography with clarity and brilliance never before thought possible.