RPM's Article Resource Center For Screen Printing, Buying, Tips and How-to Articles for the Screen Printing Industry. These screen printing articles have been featured in major screen printing industry publications and are authored by Rick Fuqua owner of Real Performance machinery. Read about Rick Fuqua's Background in the screen printing industry at his bio page on our site.
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For the past several installments, we've looked at indexing systems as the hearts of their printing machines. This time, we'll take a look at the limbs—more specifically, the arms —of those machines.
In the human body, the heart must work harder when the brain calls for the arms and legs to move—or, for that matter, when the body in question is overweight. And, just as the human body's systems, including its heart, can be damaged if the brain continues to overwork them, the same is true of overworking a printing machine's systems and individual components
Accordingly, to avoid excess wear, slow speed or out-and-out breakdown, the body or mass of the indexer table or chain/platen structure that is being indexed must not be overweight in relation to the size and strength of the indexing system that makes it move. It is for these basic reasons that equipment manufacturers often find it challenging to accomplish the same fast speeds of smaller machines when building the large-format or multi-station (14 and bigger) presses currently growing in popularity.
However, if low weight was the only master to be served, the design problem could be easily solved. But it's not, for if lightweight materials and/or fewer structural supports are used to avoid extra wear on the drive unit when indexing, then the platens are very liable to bend or deflect under the relentless pressures of squeegees and flood bars.
Bend me, shape me
Naturally, platen deflection is more noticeable out on the end of the print arm, at the farthest distance from the machine's center. Picture the platen-arm structure as a diving board, bending downward when squeegee pressure is placed upon its very end. The amount of
deflection normally is greatest on the end, and reduces as the squeegee travels toward the center of the press where there is more support.
The problem with platen deflection is that it allows the screen to stretch and distort, causing more off-contact in the front than the back of the screen. This changes the resulting image from front to back—in both size and shape—and it undermines the consistency of ink-deposit thickness. There are presses in use that seem to turn out decent work, despite actually deflecting and distorting in this manner. This is because, often (and fortunately for their owners), the screens on these presses all distort somewhat equally, depending on the consistency of their mesh tension. Still, such is a prescription for trouble, and a printer should seek equipment designed to keep deflection to a minimum.
(Note: To observe and assess the deflection of your press, try the simple experiment described in the sidebar on page 76.)
To prevent the high cost of huge indexer-drive units—ones capable of moving heavy, deflection-proof platen structures—or of frequently servicing slow or undersized drive systems constantly pushed beyond their limits, many design solutions have been put forth with varying degrees of success.
And as complicated as some of these design solutions may seem, the evaluation of what works well and what doesn't is easy. If you are looking to buy a new press or evaluating a used one, have the seller set it up to print the largest image area you plan on printing. With this setup, the machine will have the largest platens you'd be using, and therefore the most weight—heavier platens being easier to deflect—along with the widest squeegees, and therefore the maximum squeegee pressure.
If all-over prints are in your future, pay close attention to whether the machine can resist the deflection required to force ink in and around the seams of garment sleeves and necks. During the print stroke, observe the area at the end of the platen for deflection.
And for you serious analysts, measure deflection by holding a ruler under the screen or by placing a dial indicator under the platen. If this seems a little much, remember, many manufacturers adhere to the specifications provided by screen-printing experts, including recommended off-contact distances of 1/16" or less; you may, in fact, even find these set-up recommendations in their machine manuals. But does the equipment have the fortitude to resist deflection to a degree equal to such a tolerance? My point is, why worry about such exact off-contact settings if the machine's inherent deflection is greater than the tolerances you're setting up to?
To address problems such as those outlined above, some machine designers simply put platen-tip supports at each station, instead of over-building the arm structure to prevent downward deflection. So why don't all machine makers supply such supports? The answer depends on who you talk to. The normal reply from a machine builder who does not use outside supports is likely to be something like "They get in the way, things catch on them and they prevent the printing of closed-ended items such as bags." However, a printer who uses a platen-tip supported machine may well claim he has little or no problem with things catching and, for those somewhat-rare occasions when he prints closed-ended items, he simply removes the platen tip supports.
Regardless of who is right, one real issue is that outside support comes at a cost. The hardware necessary to hold up the platen tip can be fairly substantial and, by some users, considered unsightly. A press with fewer individual hardware components can often appear more streamlined (that's right, a little sexier to those real equipment lovers). But don't forget what we're here for. Yes, it's possible that the better looking machine may be the better designed one—but it's not a given. That perfect print may just as likely be produced on the less-attractive machine if you give it a chance.
The final issues that effect our platen-arm structure designs are those of access, adjustability and accessories. Loading garments onto a platen should be easy and require little attention to avoiding parts that may catch the garment.
Simple as this may sound, though, it's not easy to accomplish. Most carrousel-type machines have arms that are bolted on during installation and, therefore, start out at different heights at the end of the platen—this is true not only immediately after installation, but is also common after they've been in use for some time. Thus, to satisfy the critical requirement that all platens arrive at each print station at the same height and level—both when the press is new and after years of use—each arm must have some contrivance to accomplish leveling and height adjustment. It is normally these components that interfere with the loading of garment
If a designer places leveling and height-adjustment hardware directly under the platen, it must be made very thin to allow small child-sized garments to be loaded around it without catching. Other designs locate such hardware further back away from the platen area; this avoids the hazard of catching the garment during loading/unloading, but it can also sometimes allow the platen to deflect, if not supported elsewhere (such as out on the tip).
Some carrousel designers have addressed the problem by machining very accurate arms which do not need individual leveling adjustments. In the field, though, such presses can sometimes be found to have shims driven under the platens to make up for small height inconsistencies (which shims must be readjusted whenever platens are changed). Thus, it becomes questionable whether the added cost of the precision arm is justified, if the shims are still required.
Other presses, oval-shaped equipment in particular; have no arm structures at all. Instead, the platens rest on rails that can themselves be leveled, leaving only the concerns of consistency and individual platen flatness from one to the next, since all platens are treated the same.
Finally, the ease with which a platen can be exchanged for another size, or for an attachment, is perhaps the most important feature to many machine buyers, as it determines downtime between jobs. Changing platens can be a major time consumer and, as a result, most manufacturers now tout "tool less" changeover. While some systems are obviously faster than others, they may not treat all platens or accessories with the same finesse and speed. Overall, you must look at the total picture to determine which is best for you. For example, is getting platens on and off your machine quickly as important as the quality result you can expect from a strong, level, non-deflecting platen? The best means of making such a determination is to apply the above criteria to assessing a machine's potential for deflection, then decide how directly such potential may be tied to its changeover-speed concessions.
Real Performance Machinery L.L.C.
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