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Squeegee Is What Squeegee Does

(May 2003) posted on Wed Jun 18, 2003

Tamas S. Frecska counters Professor Steven Abbott's "Confessions of a Squeegee Heretic" with his own perspectives on the role of the squeegee in screen printing.

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By Tamas S. Frecska

To an outsider, screen-printing technology must appear as the perfect Rube Goldberg process. With screen printing, the simple task of "stenciling" a design onto a surface somehow turns into a ridiculously complex procedure that requires fabrics with a dozen physical and processing parameters and screen frames with various profiles and strengths. The process needs stencil materials that come in the form of coatings or films that are directly or indirectly applied to the mesh and have numerous physical and photochemical properties. It requires squeegees with dozens of parameters and inks with specific rheological and finished characteristics that are compatible with the substrate, as well as the printing process. Add to this the variety of printing equipment and the slew of ancillary pre- and post-press machines and gadgets used today, and we have the perfect subject for an industrial cartoon. Unlike some of the printing processes developed in the latter half of the 20th century, screen printing did not have a "blue print" to follow in order to mature into a smooth, sophisticated technology. The simplicity of its goal--transferring ink in the form of a specific image--was often blurred by additional requirements to handle a wide variety of substrates, surfaces, and shapes. The process also suffered from a piecemeal correction of technical problems that often led to new problems that had to be corrected ad infinitum. Any engineer who takes a look at the process today cannot help but wonder, "Why are we doing this? There's got to be an easier way!" Perhaps this was the question that prompted Prof. Steven Abbott to examine the squeegee's contribution to the screen-printing process and conclude that the process would be better off without the squeegee ("Confessions of a Squeegee Heretic," Screen Process, Nov. 2002, page 18). What struck me most about his discussion was his statement that "it's easy to screen print without a squeegee." He proceeded to explain that while printing without a squeegee is "not a practical method," all we really need is a "scraping device" that does not participate in the printing process. Excuse me? What is a squeegee if not a scraping device? And how could it not participate in printing once it ends up in the middle of the screen? Far from ideal The squeegee (Figure 1) is by no means an ideally simple component in the screen-printing process. But neither is the screen/stencil printing plate. Ideal mesh threads would be infinitely thin in the horizontal direction and adjustably thick vertically. This way, threads would stop interfering with image edges and would allow for specific ink-thickness deposits. The ideal screen would also be able to release all the ink it contains. Of course, a mesh with these characteristics would not be practical because, first, there is no infinitely thin material in one direction only, especially one strong enough to tension. Second, if the material were inert enough to release all the ink (like Teflon) we could not adhere a stencil to it. Looking only at the stencil, the ideal stencil would be one in which thickness could be controlled with sub-micron accuracy. It would have an absolutely flat print-side surface with an Rz value that ensures perfect gasketing around the image and still allows for the release of the stencil from the substrate (minimum vacuum effect). It would also have a resolution that is comparable to photographic films. To the best of my knowledge, no such stencil exists. I could list most screen-printing materials and printing components and find similar discrepancies between what is ideal and what is practical. Abbot is not the first to complain about all the problems the squeegee causes in screen printing. In fact, I summarized the situation 19 years ago, when I wrote that "the ideal squeegee--as any screen printer would agree--is no squeegee at all." (See "Linear, Multipoint, Polymer Pressure Activator: Introducing the Squeegee," Screen Printing, April 1984, page 100). Since then, dozens of technical articles have been written in various publications that expound one theory or another and explain why the squeegee is a wonderful tool or a necessary evil. At least half a dozen different US patents exist for screen-printing methods and equipment that tried--mostly through the use of vacuum--to eliminate the need for squeegees in screen printing. Some of the articles and patents are listed in the bibliography at the end of this article. To the best of my knowledge, there are only two or three squeegeeless printing methods in use today. One is used in electronics to fill via-holes in circuit boards and the other is the Hix cap printer for textile-based materials. Both of these methods use vacuum to print (although the electronics printer also uses a "scraper"). The third method is an experimental electrostatic deposition of powders through a screen (a proprietary and not well-known process). All the rest of the squeegeeless printing methods have quickly faded into obscurity after their initial novelty period. The real question printers should be asking is, "Why haven't we found a replacement for squeegees after 30 or more years of searching?" If screen printing were used solely as a graphic arts process printing on, let's say, porous flexible sheet materials only, every press today would be a vacuum-printing press. Alas, no such uniformity exists for screen-printed products. Although graphic arts is the major market for screen printing, the substrates used in the process include paper, wood, textiles, polymers, metals, glass, and even food items. The designs themselves break down to halftones, line art, surface coatings, and functional designs (e.g., electrical conductors, gaskets, etc.). Because each of the substrates and each of the designs require a different ink (chemistry) to fulfill the print's purpose, the printing process must be modified to match these requirements. As rational as it might seem to eliminate the squeegee from the process, the fact that screen printing is not solely a graphic-arts process used on a narrow range of substrates puts squeegeeless printing into the same class of technology as personal antigravity flying machines. For the last hundred years, the screen-printing industry had choices to make. Should the manufacturers of equipment and inks develop product-specific tools that would enhance specialization or should they make universally applicable products that can be modified for different requirements? Given the size of the industry and the variety of decorating markets it serves, both manufacturers and printers chose the latter. What gave the squeegee such prominence in screen printing was the universality it imparted to the process. Yes, certain things could be done better without a squeegee. But with it, all decorating problems could be solved with some degree of success. While a certain amount of specialization did develop in screen printing (e.g., industrial vs. graphic arts, textile vs. electronics), the elements of the process that connected all these applications were the screen and, especially, the squeegee. So, how does the squeegee accomplish this incredible feat that apparently no squeegeeless process can? To transfer a stencil image to a substrate, we need a method to fill a stencil with ink and a method to bring the stencil into intimate contact with the substrate. Vacuum printing processes accomplish this by simply keeping a lot of highly viscous ink on top of the screen and evacuating air below the substrate to bring the ink through the screen and onto the material. Unfortunately, to achieve a uniform image using this method, the substrate must be porous and the ink must have very precise rheological properties. These two requirements alone would leave 80-90% of screen-printed products unprintable. However, with a squeegee, we can--in a single step--fill the stencil with ink, remove the excess ink, and bring the screen into contact with the substrate just long enough for ink-transfer to occur. There are no special requirements for substrate porosity, size, and surface uniformity, and ink rheology becomes a lot less critical. Of course, this universal printing capability of the squeegee comes at a price; namely, everything the squeegee is and does affects the print. Even if there were only a few squeegee parameters (there are probably more than a dozen), the way these parameters interact with the ink and the screen creates a complex system that is difficult, but not impossible, to understand. Today we know significantly more about the screen-printing process than we did even 25 years ago. Significant advances were made in mesh, stencil, and ink technologies, and the manufacturers of these products made sure that we knew about them. Similar advances were also made in squeegee technology, but these advances did not seem as important simply because the squeegee was never considered an integral part of the process. For some reason, the squeegee is always assumed to be part of a printing press not of the printing process. Normally, a considerable amount of forethought and design go into the preparation of screens and stencils because everyone agrees that they determine the success of printing. Squeegees, on the other hand, are an afterthought: "Oh yeah, we'll also need a squeegee to print." To demonstrate how much the squeegee is an integral part of the printing process, we can perform a small "thought experiment." First of all, we must remember that the squeegee has a number of physical and dynamic attributes that cannot help but affect the printing process. The most important of these are listed in Table 1. The significant thing about this list is not how many items appear on it, but how they are related. If we have to use a squeegee--and I do not see how we can avoid it--these are the simple physical rules that determine how our squeegee is going to act. The decision we have to make is whether to completely ignore them or study them and make them part of our printing process.
,br> Table 1 Relationship between physical and dynamic squeegee characteristics


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