This article first appeared in Screen Printing magazine
By Joseph Gilberti
Chief Engineer and CEO
Systematic Automation Inc.
Screen Printing Expert
Thirty thousand years ago,
one of our prehistoric ancestors chewed on some burnt wood from his fire, placed his hand against the wall of the cave he called home, and spit the mixture of carbon and saliva over his hand and onto the wall. The females in the cave reacted favorably to his creation, more artists were born, and Homo sapiens have manipulated pigments ever since.
In the millennia that have passed since this early experiment, mankind has developed hundreds of alternatives for transferring images. But have printing processes really changed that significantly? The cave dweller introduced the first stencil in the form of his hand, but we still rely on stencils as the basis for image reproduction in screen printing. Ironically, the action of spitting homemade ink onto the wall could be perceived as a crude precursor to modern inkjet printers.
The point of this retrospection is to demonstrate that screen printing is unlikely to become obsolete due to any new technology available today or on the horizon. The same physical laws that made stencils an ideal imaging tool in the dawn of history continue to ensure screen printing’s success well beyond the year 2000. There is no substitute for the thick, durable, highly pigmented, vivid images that screen printing offers or the variety of ink chemistries the process supports. Because of these ink characteristics, it’s unlikely that inkjet, thermal transfer, or electrostatic digital printing will spell the end for screen printing in the foreseeable future.
The winds of change
We can assume that screen printing is here to stay (at least for another century) and that the fundamental mechanics of the process will remain basically unchanged. Refinements to inks and substrates will continue to enhance the flexibility of screen printing, and a growing trend toward modular equipment design is already allowing printers to address specific production requirements. But it’s unlikely that we’ll see technological breakthroughs that will revolutionize the process.
Among the changes we can expect (and are already seeing), however, are enhancements that improve the efficiency and accuracy of screen printing. For screen printers, this means looking at ways to automate the process and improve product quality and turnaround.
We’ll explore some of the most common areas where such automation is taking hold. Throughout this discussion, we’ll draw examples from compact-disc (CD) printing equipment, which represents the cutting edge of automation. But keep in mind that many similar automation solutions are also being applied to presses for flat-graphic and 3-D applications.
Computerized machine control
During the past decade, screen-printing equipment manufacturers started to embrace digital technology by incorporating programmable logic controllers (PLCs) into their equipment. PLCs supplant standard, analog machine controls with solid-state digital controls based on the same type of microprocessors that drive personal computers. Generally, PLCs are hardwired into the equipment and feature user interfaces on the machine for setting operational parameters.
The next step in the evolution of screen-printing presses is already beginning as equipment manufacturers begin to move machine control out of the equipment into the most obvious external system: the personal computer (PC). Like PLCs, computers offer the ability to control machine functions with digital precision. But unlike PLCs, where instructions and programming routines are fixed when the machine is assembled, PCs have the flexibility to support a broad range of software applications. These programs drive and control press operation, allow you to schedule production time and track orders, let you analyze production statistics (often through conventional spreadsheet and database programs), and make it possible to upgrade any of this support software as new revisions and competitive products become available.
The move to PC-based production control is really not that surprising. Consider that the printing equipment shown throughout this article was designed on a PC. The milling machines and lathes used to manufacture the parts from which the printing machines were built also were controlled by PCs. Even the camera used to take many of the photos in this article was digital, and the master images reside in a PC. So it’s quite natural that PCs would eventually find their way onto the screen-printing production floor.
Current available digital press functions One of the screen-printing sectors that has pushed automation to new heights is without a doubt the CD industry. CD printers were among the first to recognize computers as a production tool to help them meet decreasing turnaround times and accommodate long- and short-run (less than 500 pieces) production. And it was apparent very early that PCs offered the means to standardize and control production while allowing printers to carefully monitor the efficiency and quality of the process through statistical process control.
The following sections detail some of the press functions and related tasks that have become automated thanks to the introduction of the computer on the production floor.
One of the first goals manufacturers sought to address with PCs in production was the ability to track the time and resources required to process a job, from the initial order through shipping. Some manufacturers offer custom production-management software that works in conjunction with programs that control press setup and operation. Third party production management packages have also been developed (See “Software for Screen Printing Management” by Tom O. Frecska, Screen Printing, Feb.’98, page 94). In most cases, these management applications allow users to record and track the following: 1. inventory levels and arrival dates for all material required to do the job 2. number of pieces to be printed 3. number of colors to be printed 4. format and specifications for original artwork (electronic file or film), including identification of PMS colors or availability of color chips, proofs, or other reference materials for color matching 5. press setup parameters (screen characteristics, squeegee floodbar settings, off-contact, etc.) 6. packaging and shipping details 7. customer invoices, credit arrangements, and related financial data
Once all the information above has been entered and verified, the job can be scheduled for production. When the job begins, its progress is tracked by a computer at each manufacturing stage, and the information is posted on a network for access at the following stage, where specifications can be checked and additional production details can be recorded.
Throughout the production process, each department can enter the number of pieces that successfully pass through that stage of production, as well as the number of rejects, and the time required to complete that particular production step. In some cases, productivity is further enhanced by including a barcode on a job jacket that allows statistics and specificatins to be called up instantly when the barcode is scanned at each stage of production. In many cases, individual employees scan in personal barcodes at the same time to indicate who was responsible for the job at that stage.
In advanced operations, production data on all jobs produced within a given period can be recorded for statistical analysis. After several months, job characteristics and productivity levels can be compared and used to accurately predict production times on future jobs. This ability to provide accurate production time estimates often means the difference between winning or losing a potential order.
Computers not only offer an easy method to assess productivity and turnaround, they also enhance the accuracy and repeatability of the printing process. Today”s printing equipment increasingly relies on servo-motors and similar digitally controlled drive mechanisms. These devices can be turned on and off with digital commands to provide a degree of control and accuracy unparalleled by mechanical controls. Let’s take a look at how some of today’s CD decorating systems use computers to control various press functions and settings.
Press setup The range of press functions that can be controlled by computers really only is limited by a shop’s printing requirements (and its ability to afford the features). Some settings continue to be manually controlled simply because it’s expensive and unnecessary to automate them. For example, even on today’s specialty presses, parameters such as screen off-contact height and squeegee floodbar angles are typically manual adjustments.
But the ability of computers to provide accurate and repeatable control does lend itself to other press functions, such as stroke length, screen registration, and press cycle speed. Some CD presses also allow you to control screen orientation in x, y, and rotational directions. And on presses used in high volume production environments, a computer can also be used to drive an automated ink feeder. Here, the operator simply programs in a time-delay between feedings, and the feeder adds a predetermined volume each time it is triggered by the computer program.
Additionally, computers are frequently used to control ancillary equipment and enhance production efficiency. CD presses, for example, commonly feature robotic loading and/or unloading systems that are controlled with the same software that drives the press. All models also feature inline UV curing between colors, and the user can program in the lamp intensity desired at each curing station based on the cycle rate of the press. This capability makes it less likely that you’ll overcure the ink and end up with a brittle or damaged image.
Optical systems One of the most significant areas of development couples the PC with one or more cameras on press. The cameras can be used for parts recognition as well as to maintain the registration accuracy of each screen on multicolor jobs.
In CD decorating, the parts-recognition function is particularly useful since a shop may find itself handling several jobs for multiple clients at the same time. The problem is that one blank CD looks like any other. But since the CDs are already recorded with data, it’s imperative to keep their identities clear. That’s why CDs are manufactured with small ID codes near the center of the “doughnut” to identify them.
And that’s where the optical recognition systems come in. If, for example, a CD press was loaded with a mix of the correct CDs, as well as CDs that belong to another job, a camera on the in-feed end of the press can “read” the ID on the CD and determine if it belongs with the job that’s currently set to print. If this code does not match the expected code, the CD is cycled through the system unprinted and unloaded into a special “reject” spindle. The operator can also program the press to halt production when a specified number of rejects are discovered.
The other primary function of optical systems is quality control. On today’s sophisticated CD presses, this means ensuring print quality by carefully comparing the printed image with a digital “template” representing the desired image characteristics. This template is based on the average image characteristics of six or 12 printed CDs that the operator deems to be acceptable, which are scanned by the optical system at the beginning of the run.
As part of the press’s digital control system, the camera, which is mounted near the out-feed end, takes a high-resolution “snapshot” of the printed surface, reading the location of individual half-tone dots of other colors. By comparing the scan against the digital template, the computer can determine which, if any of the colors, require a registration adjustment based on tolerances established by the user. The press then automatically adjusts screen registration for the problem color and the job continues without any human intervention.
Incidentally, the optical/digital system used to maintain registration accuracy during a print run is also used to line up the screens during initial setup. Here, the computer accelerates the registration process so much that all screens for a process-color CD job can be perfectly positioned within 45 sec.
Automation off the press
Although the focus of this article is primarily automation trends as they apply to presses, the discussion isn’t complete without addressing other areas of the process that have also seen increased production efficiency. At the front end of the process, a huge assortment of graphic design computer programs, including Adobe Illustrator and Photoshop, Corel-DRAW!, and others, have made art creation faster than ever.
The digital files created with such applications can be used to drive image-setters for film generation or direct screen imaging systems for those who want to avoid film. And as mentioned previously, when these digital graphics are used in conjunction with optical systems for measuring image quality, they serve as the basis from which automatic registration adjustments can be made.
While not really tools for automation, devices such as pin registration systems also streamline the front end of the process. The ability to quickly align film positives with screens prior to exposure is an advantage that can’t be understated. Also, while we’re on the subject of screen making, we shouldn’t overlook semi-automatic and automatic screen-coating machines, which can produce high-quality stencils for fine detail printing. These devices are available for virtually any screen size. And CD decorators and others who use small-format screens can often “gang” multiple screens or use cartridge systems where screens are automatically coated in rapid succession.
New direct screen imaging systems can also enhance productivity by allowing you to “print” a positive image directly onto a coated screen prior to exposure. Other models use lasers to directly expose only the image areas on a coated screen without a positive (directly printed or film). In both cases, the devices operate on digital information from the original design file.
Finally, solutions for automation can also be seen in post-press areas, including automatic screen washout and reclaiming systems. In many cases, these self-contained units recycle solvents and other potentially harmful chemicals, making them both economical and environmentally friendly.
Should you embrace automation?
Industrial and specialty applications have been, and continue to be, areas where screen printing thrives. These applications challenge screen printers to create their own niches by being resourceful and clever. And customers are prepared to pay for this creativity and skill.
But because specialty applications are often characterized by short runs, total automation may be unnecessary. In many case, standard, semi-automatic presses will handle the job perfectly, and sophisticated machine controls are not needed. However, the equipment should be flexible enough to allow easy job changeovers, even if it means change from a flat-graphics printing to printing cylindrical containers.
Automation in screen printing means automation of the entire process. Every job can be broken down into smaller tasks and parts. And each task represents an opportunity for improving quality and efficiency if you can take advantage of new technology. For screen printers, the main thrust of this new technology is computers, software, and the automated motion-control they bring to the production environment.
Just as the aerospace industry develops technology that carries forward to other industries, the CD industry has been the most significant catalyst for implementing current technology in screen printing. High volume screen printing on CDs forced the development of efficient screen-printing processes and procedures that are becoming standards for the entire industry.