Screen Printing Lines: “Roll-to-roll flatbed screen printing systems”; Professional Printer; July-August 2008

Advanced digital control systems, improved engineering, and better material-handling technology characterize the latest generation of automated screen-printing systems for wide-format web substrates. Learn about the ease of operation, precision, and high throughput they can deliver in industrial and electronics printing environments.

Winding Up Productivity with Roll-to-Roll Screen-Printing Technology

Reinhard Zimmerman
Spartanics-Systec

The latest generation of roll-to-roll screen-printing systems marks a significant evolutionary step in the press technology that has dominated since the early 1900’s. Now, the incorporation of more sophisticated electronic controls reduces the need for time-consuming manual adjustments that typify earlier and less sophisticated equipment. In this discussion, we will consider how state-of-the-art electronic controls and software integration are changing the capabilities of roll-to-roll screen-printing systems, including both flatbed models and cylinder-based equipment.
The underlying electronics and software engineering that now have bearing on screen-printing technology are cut from the same cloth as the electronic controls we now see in a wide swatch of industrial equipment. At the core are programmable logic controllers (PLCs)— computers geared to provide stability in real-world manufacturing conditions and able to handle multiple inputs and outputs. PLCs, originally developed for the automotive industry, are now programmed with more sophisticated instructions than when they were originally introduced, enabling expert programmers to fine tune industrial controls to an unprecedented extent. This includes today’s PLCs that can fully automate the steps in roll-to-roll screen printing.

Job set up and tool-free changeovers
Adjustments to the screen clamps, as well as squeegee and floodbar pressures, can now be achieved with touch screens operating pneumatic controls via the system’s PLC. This eliminates a good deal of time that had previously been spent by operators making mechanical adjustments to balance these components for each job. On today’s roll-to-roll presses, no tools are required to make these adjustments—they are all done electronically and with greater precision and repeatability than what human operators can typically muster. If and when fault conditions arise, they can also be corrected with a few keystrokes.
Going from a manually sheet-fed printing process to an automated web process can speed throughput by 50% or more. The actual speed improvement varies, and is dependent on the size of the job, among other factors.
Once a skilled operator has set up all the required job parameters for a particular application—web speed, web transport settings, squeegee angle, screen height, dryer settings, etc.—the PLC will automatically store all these parameters. For repeat runs of the same job, a far less skilled operator is able to quickly initiate and set up the system by recalling those parameters.
The greater precision electronic controls also are key to enabling many specialty graphics companies to expand into electronics applications that have requirements for exact ink thicknesses and minimal screen stretch that non-automated presses cannot consistently achieve. Since membrane switches and other printed electronic products are a growing segment of the specialty graphics industry, we can expect that the capacity and sophistication of PLC-enabled control systems will become more and more a determining factor of where the industry can go.
More precisely, it is the software engineering that makes the real difference between one screen printing system and another. The best-in-class roll-to-roll systems use standard industrial components that one can source worldwide, and customize the software for the precise application requirements of a particular specialty graphics company. These systems typically use higher capacity PLCs that are able to finely control all the inputs and outputs of a web printing line, including unwinding, web tension, material positioning, printing head position, web transport, drying systems, and rewinder.

Large format considerations
In recent years, the growing demand for large format work has required the use of custom-configured screen printing systems to achieve maximum efficiency. In these applications, the generic 400 mm wide flatbed systems are inefficient or just unworkable. Fortunately, along with improved controls, today’s roll-to-roll presses can be engineered to be virtually any size dimensionally. Flexibility is now the overriding design principle. Dimensions are custom-configured to job requirements, as opposed to putting the onus on screen printers to adapt to job formats feasible in the common 400-mm machines. Now roll-to-roll systems can be designed to support jobs as large as 2.3 x 4.2 m, which allows artwork layouts that provide higher yields and faster job throughput.
A change in physical dimensions that might not be as apparent is in the screen holder designs. These had been the same for as long as anyone can remember. Newer systems have redesigned screen holders to allow as little as 1 mm distance between the screen and the printing table. That means significantly less screen stretch, which is one of the reasons why roll-to-roll flatbed systems are playing a bigger role in high precision electronics applications.
Larger press sizes also create the need for more sophisticated methods of controlling how material is held and advanced through the systems. High-end roll-to-roll flatbed printing systems now feature an automated and synchronized vacuum release on the printing table that is especially important to speeding the throughput of large format jobs. On cylinder style roll-fed machines, tension of the substrate on the cylinder also is controlled via vacuum.
Use of vacuum for material positioning has a clear advantage in helping to deliver a scratch free product. Indeed, if you are printing any length over 400 cm, using a vacuum positioning method to secure material has proven to be the only successful method. On flatbed machines, the use of synchronized vacuum release means that as the squeegee leaves a section the vacuum is released such that the transport system is able to pull on the material. If one does not release the vacuum in sections in this manner, there is a considerable reduction of throughput. Here too, it is the ability of the PLC electronic controls to precisely coordinate the squeegee advance and the vacuum release that makes this throughput efficiency possible.

Electronic controls and quality
In a simple screen-printing system, one needs to put pre-printed material in position on a printing table and then search for points of alignment and make manual adjustments before printing the next element. This is not only inefficient but also relatively inexact compared to the precise positioning capabilities of the newer generation electronically-controlled roll-to-roll systems. These systems have advanced algorithms in the software controls that can not only precisely register the material positioning but also can automatically compensate for screen stretch. The days of operators needing to look at every single print are a thing of the past. The quality advantage even enables these systems to handle the very high-precision requirements of security products, such as holographs. Here too, it means that far less skilled operators are able to run a job and that the throughput advantage is considerable.
The servomotors that drive the squeegee assembly are another hallmark of the newer generation of roll-to-roll presses. A servomotor drive is an automatic device that uses error-sensing feedback to improve performance. This electronic feedback on the squeegee positioning enables much greater precision in squeegee advance and thus the smaller gaps between first and second prints. Today’s systems can achieve a 1-mm gap between first and second prints. Earlier generation roll-to-roll flatbed systems used traditional motors to drive squeegees and could not achieve that standard.

High-tech inks and dryer controls
The recent proliferation of specialty inks is creating new capabilities for high-tech products such as printed circuits with precisely controlled electrical resistance characteristics. These new inks are also increasingly preferred for products that need to weather radiation and other environmental impacts. Handling these newer high-tech inks has called for the development of new technology, including electronically-controlled contact dryer technology that is able to very precisely maintain proper temperature and air flow, which affect ink reactions.
For those not familiar with contact drying technology, it consists of an adjustable heated vacuum plate and conveyor carrying the material being dried combined with adjustable hot air from the top. In this type system the wet ink dries from the substrate upward toward the print surface. Even for thick coated inks, drying times can be expected to be a fraction of that required in conventional heated oven systems. For example, a substrate that takes two hours to dry with traditional heated air methods now takes 30 seconds.
With certain high-tech inks, there is a particle in the ink makeup that requires a very controlled temperature and air flow to properly cure. The ink reaction is not simply a process of drying, but rather an unfolding of a chemical reaction that requires strictly controlled environments to proceed as designed. The contact dryers are controlled electronically and can be set for the precise drying requirements of particular solvents and high-tech inks. In fact, the better systems also have feedback mechanisms that automatically shut the dryer off if and when solvent concentrations become dangerously high and potentially explosive.

Other components
Modular design principles are now firmly established for the latest generation of roll-to-roll screen-printing systems. This makes them flexible for adding multiple print stations, drying systems, die cutters, laminators, slitting and sheeting systems, or other modules as needs arise. This has allowed many screen-printing shops equipped with this newer technology to move into new markets more easily, because equipment is more readily adapted to new job requirements.
Moving into new markets is what the latest technological improvements are all about. Whether it’s lightweight airplane cabin interior panels, RFID components, transfers for industrial textiles, printed luminescent faceplates, or intricate membrane-switch designs, the proliferation of modern uses for screen-printing technology continues. Many of the products made with the latest generation of web screen-printing systems simply did not exist a decade ago. Many of those who design and build screen printing equipment now devote considerable R&D resources to keeping pace with these changing market requirements.

Get business rolling
Roll-to-roll screen-printing systems are now the best fit technology for high-volume production of many screen-printed products for automotive and electronics industry applications, and especially those using the newer high tech inks. Since such products are likely to become a larger part of the specialty graphics market, now is a good time to pay attention to the developments in roll-to-roll technology and what it could mean for your company.

Reinhard Zimmermann
Reinhard Zimmermann is General Manager of Systec and a director of the Spartanics-Systec partnership, which manufactures Spartanics-Systec Fineprint Screen Printing Systems.

 

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