by Mike Young
A view of a typical windshield/backlite screenprinting machine with computer-controlled edge-to-edge printing capability. Photo courtesy of Ianua of the Americas, Inc.
It is not every day in the world of screenprinting that one sees the launch of a new printing technique, particularly when it comes to printing flat glass. The development I'm referring to is known as "edge-to-edge" printing. It has the ability to successfully print (frequently also called paint) the black protective band around the glass edge without a clear image margin, as opposed to printing conventionally with one. What makes the introduction of this edge-to-edge feature even more stunning is that the technique was designed specifically for the automotive glass marketplace.
The means to screenprint to a substrate's edge (image is effectively allowed to bleed to the outer edge without a working/processing margin) is nothing new in graphic art printing, but has proven not to be a feasible practice with glass due to dimensional variations in substrate size. In graphic arts, paper, board, plastics, etc, are usually sheeted or guillotined accurately to size and have only one edge to be concerned withfrom a three-dimensional perspective. Flat glass, on the other hand, has two edges from a screenprinting point of viewand there lays the bulk of the problem regarding print registration as seen by process engineers. Glass has both a surface edge as well as a mass edge, regardless of whether the edge is raw or has been ground. This "double-edge" creates two additional tolerances that the typical printing machine has a hard time dealing with: the plus/minus of cutting (surface edge and the part where the image is printed) and the plus/minus of grinding (body/substrate's mass edge), Figure 1.
From a registration standpoint, an image printed on a piece of glass is seen by the viewer as a two-dimensional entity, one that has height and width, which can then be held to tight tolerances in printing for acceptable repeatability. Yet, registration and positioning (repeatability) of that image are determined entirely by a three-dimensional meansin this casethe edge of the glass. As such, if tolerances of both cutting (surface) and grinding (mass) are not controlled jointly within the overall OEM's print registration requirements, printing very close to the edge and holding tolerances are all but impossible. In conventional screenprinting, registration using the glass edge (Figure 2) typically will produce rejects and serious interruptions to production. This is because some "oversize" glass will result in an image with a thick edge (risking drying or undercure problems), while "undersize" pieces will likely have ink running down the sides from excessive ink buildup under the screen (risking total rejection). Understandably, process and production engineers want to reduce these problems since paradoxically, screenprinting, even as a low-tech reproductive processing method, is often a more exacting and constant process than high(er)-tech cutting and grinding in everyday production.
An alternative but rarely used method to the standard approach to edge-locating is to "surface" locate by means of a CCD camera vision system to register the glass surface optically. However, even this high-tech method causes register differentials no better (or worse) than the edge methodas perceived by the viewer (Figures 3 and 4). Therefore, the optical system does not resolve the real issue since the so-called "print error" would still be there, in some form, as judged by specifiers and engineers.
Let's reflect on the history of printed automotive glass for a moment. Image clearance from the glass edge was originally rather graceful, in part to compensate for the idiosyncrasies of the printing process, erratic nature of large size screening equipment used then and, not least, printed glass was an added but totally unknown feature for the auto industry. As equipment and the process came under better control, designers pushed the outer edge of the "image" (the black obscuration print) further towards the glass edges for a cleaner look. Despite the precarious nature of screenprinting as it was seen by some, it has always been more accurate than glass processing from a dimensional control standpoint. However, it was only a matter of time before process/production engineers began to realize the envelope had been pushed too far for their own operations' registration-holding capabilities. The image marginthat is, the clearance between the edge of the printed image to the edge of glasswas getting smaller for each model, year after year.
Today, an increasing number of designers and specifiers desire no margin at all. They want the black band to be printed right up to the edgeeven though they have little idea of the complex challenges that must be overcome to meet this new requisition. Consequently, the ability to print edge-to-edge means there is no longer a realistic engineering concern nor critical mechanical limitations to print registration from a physical operation viewpoint. In a nutshell, adopting the technique to print edge-to-edge is seen as a true answer for trouble-free automotive glass printing, in terms of higher constant quality, lower operation cost and greater productivity. The solution to the problem is be found with a printing machine that has been designed specifically to print edge-to-edge, as well as having conventional printing capabilities. This means the printer must have an integrated system to remove excess ink without fear of print damage, interruptions or rejects.
By default, such equipment would also have glass centerline capability, in X' and Y' axes, as part of the integrity package. Therefore, employing this new technique essentially requires two functions: an appropriate image size in the screen, and the means to remove excess ink/paint from the underside of the screen automatically.
Taking the former first, the screenmaker will enlarge the OEM's original print image according to the maximum glass surface size the operation can deliver and control constantly, plus an allowance for the printing process, if any. (The idea here is not to deliberately print more excess ink than is actually necessary.) Since printing equipment fitted with edge-to-edge capability usually centerlines the glass, the enlarged image would then accommodate the largest glass, tolerance-wise, equally.
The second function calls for a device that removes the excess ink/paint that will buildup under the screen. When several smaller glass parts are printed, ink has nowhere to go except to collect in an undesirable placethe outer edge of the non-printed area of the image. Unfortunately, the ink leaves either a "thick" edge when the image is printed on a larger glass part or will eventually run down the sides of smaller parts. Neither situation is acceptable. Although several ink-removing systems have been devised, such as ink-suction (from below), scraping (the screen) and even printing blotting paper, by far the most promising one appears to be ink-removal (suction) from above the screen. By this system, once the squeegee has made its print, a suction-head immediately follows behind it, in both the X' and the Y' axes, removing any ink left in the screen image that was not transferred (printed). Effectively, this procedure prevents ink building up on the under side of the screen, thus ensuring a perfect print each time. The screen is then flooded, as normal, as in conventional printing.
Unlike the other devices mentioned, one of the undeniable advantages of a system that removes ink from above the screen is that it does not add production time to the print cycle. This is an important distinction, since other edge-to-edge printing methods can add six or more seconds to each print cycle, seriously damaging production output. Otherwise, the extra time added to the cycle for "cleaning-on-the-fly" can well translate into a net loss of more than 100 windshields/backlites in an hour.
While the ability to print edge-to-edge automatically confers a superior all-around printing operation, the ability and necessity to print up to the glass edge should not cut into productivity. It anything, it should do nothing less than enhance shippable yields.
A number of machinery manufacturers have released information about products that address the needs of glass and metal fabricators. Machinery news and a sampling of some of the newest products available follow.
Elumatec USA, Inc. of South Bend, IN, offers the AF 221 end milling machine for profile end milling in aluminum and vinyl sections. According to the company, horizontal and vertical pneumatic clamps offer flexibility and the manual feed of the milling unit has linear bearing guides. The AF 221 features an adjustable support fence that can be swiveled to the left and right to 60 degrees, an integrated support with an exhaust hose, an enclosed cutter set using a safety hood and a regulator with an air filter.
Tampere, Finland-based Tamglass Engineering Oy's complex bending and tempering system CBTS is designed for medium-sized backlite and sidelite production. According to the company, the bending depth has increased to approximately 200 mm, while making a bending radii of 200 mm possible. The company also says CBTS makes installation and maintenance easy, with installation times cut to four to five weeks and a control mechanism that allows simplified, fast control.
AISA S.p.A. of Ticengo, Italy, offers complete glass processing lines for automotive, architectural and decorative glass, with different handling and process machinery, before and after the bending machinery and tempering lehr. Included among the products are corner grinding machines, cutting and breaking lines, screenprinting machines, IR or UV dryers and coolers for ink curing, inspection systems, registration devices, automatic loading and unloading systems and packaging systems.
Uniglass Engineering Oy of Tampere, Finland, has announced a new automated tempering line concept for the market depending on maneuverability and consistent quality. In addition, Uniglass has announced that it is developing its own safety glass operation for the Finnish market.
TO.M.A.S. s.r.l. of Torino, Italy, offers the Tom 16 PV, an automatic, vertical, straightline mitering grinding machine. The machine features 16 spindles, with electronic management, for grinding and polishing by liquid cerium of flat edge, 45-degree front and rear arises of the zero to 45-degree plane.
Standards Drafts European standards drafts for a range of glass machinery have been sent to members of the European Committee for Standardization (CEN) for comment and inquiry. The products for which European and international standards are being considered include eight machines and plants for manufacture, treatment and processing of glass including those for stacking and destacking, treatment and processing of flat glass and production of insulating glass. CEN's primary purpose is to develop standards.
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