Web-Fed Screen Printing
The author explains how automation and flexibility make web technology a competitive option.
Web screen printing is a proven production method, yet still misunderstood by the majority of printers in the industry. It is also a versatile and efficient manufacturing technology that screen printers must consider in any production automation or expansion plans. Many printers still do not realize, however, that successful introduction of a higher level of automation requires immediate attention to other support areas--an important thought process that is an integral part of such evaluations. Automation uncovers the weakest link in any manufacturing process immediately, and often ends in disappointment and poor economic performance if not applied correctly.
Screen printers who have this mental awareness can apply web technology with greater success and profitability for short- and medium-length jobs than for those long runs with cut-throat profit margins. The break-even point for run sizes in web screen printing can be as small as 200 or 300 impressions per job, if the web technology is applied correctly. Indeed, web screen printing competes extremely favorably with sheet-fed screen printing and with the emerging digital printing technologies in certain applications.
Obviously, web technology can only be applied when the substrate exhibits enough flexibility to be processed through a web screen-printing or finishing line. These substrates include 1-mil Mylar®, 30-mil polycarbonate, 8-mil polyethylene, various transfer papers, and other materials with thicknesses of up to 0.08 in. (2 mm). An additional competitive edge is web printing's ability to print the back side of the substrate in perfect register to the front--whether the substrate is opaque or translucent. Speed is also an advantage: For example, a 30-in.-wide web press printing at rates of 50-70 ft/min, using either solvent-based or UV inks, would print 125-175 sq ft/ min, with as many colors as the press can print in line. A final benefit is, of course, the ability to select and control ink-deposit thickness--an inherent part of what makes the screen-printing process as a whole so unique. History
After promising sales in the 1980s, many of us thought that web screen printing would enjoy much greater success. Dozens of single and multicolor web lines and in- or off-line web finishing units were sold to the flexible-circuit, membrane-switch, graphic-overlay, and, to some extent, nameplate and label industries. The textile industry, whether for short, medium, or long production requirements, has strongly embraced this technology since the 1950s and '60s. Multicolor web lines with ten or 20 screen-printing heads using either flatbed or rotary screens are still being manufactured for that market segment.
Industries requiring long production runs or companies involved in "high-tech" screen-printing applications realized the many advantages of this technology early on. These companies often enjoyed resources that included manufacturing and production engineering--a very important supporting element for influencing management decisions and gaining the subsequent funding necessary for adding web technology. The ability to justify the investment--based on process-control improvements, productivity increases, overall quality enhancements, and manufacturing flexibility--allowed these companies to make significant purchases of capital equipment for web processing.
Web screen printing still triggers an erroneous feeling that this technology can only be utilized for long production runs. This is not the case. It is true, however, that you must have enough short or medium production runs to keep a web screen-printing line busy at all times, without production interruptions and with the least equipment downtime. Easier said than done. Success of web screen printing
For more than ten years, several screen-printing companies in the US and Europe have operated two and three multicolor web screen-printing lines with production requirements not exceeding 300-2000 images per job. Often, these companies employ only 40-70 employees (and sometimes fewer), depending on the products manufactured. Here are some important factors that have made all of these companies successful:
1. Standardization-- Standardization applied to any form of automation is the key to success. For single-color or multicolor web-line configurations, not more than two or three different web widths should be used. This way, only two or three different squeegee and floodbar sets are required, and the time needed for setting up the web guide systems and adjusting other equipment settings is automatically shortened. This approach also simplifies the preparation of diecutting and other finishing tools, allowing printers to standardize the purchase of web materials.
Efficiently utilizing web material is important, as is determining the most economical printing/diecutting area. For each production day, the scheduling department is responsible for selecting the following: the amount of jobs for specific web widths (different types of web material will already be slit to the standard range of web widths) the amount of jobs with similar color sequences and ink systems (to minimize changeovers and setup times) the most profitable printing/diecutting line (the tooling cost for small- and medium-size production runs is a serious item in job-cost calculations) the necessary screen frames, squeegee/floodbar sets, inks, etc. (to minimize changeovers/setup times and maximize equipment utilization)
2. Unsupported web-loop technology
Web screen-printing equipment with "unsupported web loop" technology offers sequential makeready and setup procedures. This means that individual in-line modules are set up as the beginning of the web reaches them. After the first print station is made ready, the web advances to the next print station. While the first print station still prints at a minimum speed, the second print station can be set up. During setup of the second print station, the web material being printed at the first station will accumulate into the "loop" or buffer zone between the first and second print stations. Once the second print station is made ready, the print speed on this station can be accelerated (without changing the production speed of the first print station) until the accumulated web material is withdrawn from the "loop."
3. Elimination of run-in waste
Sequential setup procedures are efficient and virtually eliminate "run-in" waste. As a matter of fact, web screen-printing equip-ment in general produces less costly waste than automated sheet-fed cylinder presses. Because web material exhibits better stability than individual sheets, handling rolls of material is much easier than handling sheets, so spoilage is reduced. Such spoilage is imminent in sheet-fed short runs where a few sheets have to be handled quickly and often.
4. Leader web material
The use of leader web material (which works much like the leader in a roll of film for your camera) is another important way of cutting the overall setup time for web screen-printing lines. For each web screen-printing or finishing line, inexpensive but stable and reusable web material is cut to the exact length from unwind to rewind station and is considered the "leader." Depending on production procedures, several of these leaders are cut to size for each web width category. The actual web printing material is spliced onto the end of the leader (web material and leader must be of the same width) and another leader is spliced to the web printing material at the end of the production run. This procedure saves costly web material, reduces run-in waste, and simplifies and reduces setup procedures considerably. Production flexibility
Designing a web production line is an exciting and very creative phase of entering web screen printing. In most cases, different processes such as screen printing and finishing should not be incorporated in one big line configuration. This would be an ideal approach if the finished product would always be processed in the same way--for example, screen printing four colors, laminating, diecutting, waste removal, slitting, edge trimming, and rewinding. If this job is all that has to be produced on that particular in-line configuration, then all processes can safely be incorporated into the same line.
However, in screen printing as we know it, this remains wishful thinking. Every day, printers face new production challenges that require intelligent production flexibility with the fastest possible output capacity.
Production flexibility can be achieved by standardization and implementation of strict scheduling procedures. The unsupported web-loop technology allows additional advantages in that respect:
1. Each module, whether it is designed for in- or off-line use, is self-contained and can be electronically incorporated into any line configuration.
2. This modular approach can be very effective when designing "custom tailored" web lines. The company's individual production requirements can already be considered in the designing phase of the web line.
3. After a company's product mix is clearly defined, selecting the appropriate screen-printing and/or finishing lines becomes easier.
4. Initial web-line configurations can always be changed, and additional modules (printing station, dryers, etc.) can be added at any time. On a single-color web line, additional colors can be printed by reintroducing the web for each subsequent color. Once the technology of web processing has been successfully "digested" by the production staff, other print stations or manufacturing modules can easily be added.
5. Individual web screen-printing lines can be operated separately or can be connected with each other to benefit from additional in-line print stations. For example, two two-color print lines can produce two separate jobs, or, if connected with each other, can print one four-color job. Connecting two separate web lines is easily accomplished by threading the web material accordingly.
6. One web screen-printing line or separate web lines can feed a separate and self-contained finishing line that can incorporate processes such as top or bottom laminating, embossing, hot-foil stamping, diecutting with steel rules or hard tools, waste removal, slitting, and sheeting. Setup of screen-printing stations takes only 5-10 min; finishing stations require setup times of 1 hr or more. This is an important reason to keep screen printing separated from finishing, if possible.
7. Sequential makeready and setup, as previously described, make these processes more efficient.
8. Registration capability and print quality is equal if not better than comparable sheet-fed, high-speed screen-printing methods.
9. Net production is faster than with comparable sheet-fed, high-speed screen-printing methods, especially when considering handling and finishing requirements.
10. One operator and one assistant are usually all that is needed to operate a single or multicolor web line.
11. The finished product can be sold in roll or sheet form--a flexibility that will never be available to sheet-fed printing. (Thus, printed roll material can be finished on either web-fed or sheet-fed finishing equipment, another important example of web flexibility.) Automating supporting processes
Introducing a higher level of automation in the form of web screen printing and finishing requires a serious look at supporting areas such as screenmaking. Under a complete process philosophy that includes process control and effective process management, making screens uniformly and consistently becomes a necessity. Automation is available for a number of other procedures, which will further enhance the automation of the screen-printing process: Automatic screen coating guarantees uniform emulsion thickness. Computer-to-screen imaging technology eliminates film positives and their related shortcomings. Automatic screen washing, reclaiming, and developing guarantees uniformity, consistency, and savings in solvent and fresh water usage. Web: The way of the future
Web screen printing, applied intelligently and correctly, will be the next wave of automation. For many applications, it provides a means of retaining screen printing as a viable production method that can successfully compete with electrostatic and other forms of digital printing. Turnaround times for web screen printing are extremely fast; the print quality is much better than comparable digital or related print technologies; climate, chemical, and mechanical resistance is unequaled; and most importantly--the production flexibility that web manufacturing offers will remain a strong sales advantage for a long time to come. With filmless screenmaking and other automated support processes, the cost factor per square foot of web screen printing will be half or less compared with electrostatic printing. This competitiveness of web screen printing offers a positive outlook for the future of our industry--at last.
About the author
Educated in Switzerland, Peter Max Weber began his screen-printing career 20 years ago as sales manager for a US distributor of screen-printing equipment. For the last 14 years, he has been president and co-owner of Fineline, Ronkonkoma, NY, a supplier of European-made screen-printing equipment. Weber specializes in all aspects of automation for both sheet- and web-fed screen printing, screenmaking/prepress procedures, and post-press processes.