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In past issues, we've looked at various press-drive designs that move platens around the machine, the focus being on how well they function with respect to speed, smoothness and overall resistance to wear and service. In this installment, we'll focus on the last segment of the index cycle, and the most important one: registration accuracy.
Since the level of registration accuracy for a machine affects the kind of printing you wish to accomplish, and the ability of the machine to maintain this accuracy affects your pocketbook by way of spoilage, downtime and service calls, press registration is a major concern to all machine owners, whether their equipment is new or used.
Without an accurate index movement, all other issues of speed and performance are meaningless. Successful multicolor printing is dependent on the substrate being delivered to the individual print station in the same exact position time after time. If not, colors within the design will be out of their intended orientation—in other words, out of register.
There are two ways a press can be out of register: consistently and inconsistently. It is very important to establish which of these two is the case. To determine this, two complete sets of prints must be run on your machine so that two prints from any given platen may be compared. If, for example, you have a 10-station press, 20 prints must be run; mark the first 10 with their respective platen numbers and the letter "A"; mark the second set of ten with their platen numbers and the letter "B."
Now, when two shirts off the same platen are in exact registration with one another (while pairs from other platens are not), you know the job was set-up correctly but the machine is not repeating its registration tolerance on all platens. If platen
lA's shirts, for example, have a color off to the left and lB's have the same color off to the right, you know you have inconsistent misregistration. In such a case, the machine's overall registration calibration may well be correct, but something is worn or moving. Typical causes of inconsistent misregistration are loose or worn bearings and/or moving parts that are not intended to move, including screen-frame supports.
If two prints from the same platen —say, 2A and 2B—show a misalignment of colors that is off in the same direction to the same degree, you're experiencing consistent misregistration, and your press's registration-calibration settings are probably in need of adjustment. Make certain, at this point, that at least one platen exhibits a perfectly registered print to prove that the artwork/screen parameters are not to blame and that there is no random movement.
To the limit
In the case of machine registration miscalibration, a service tech may be needed unless you have purchased your own calibration device from your press manufacturer. When such a device is used, you can take a location reading of each registration bearing on each arm of the press to make sure they are within proper specifications. Here, you are ensuring that all bearings are an equal distance apart—within a tolerance of *.002"—all the way around the machine. Some bearings may have moved over time due to a rough-running index motion, severe or frequent emergency stops, or just plain wear. In such cases, the press simply needs to be re calibrated back to an acceptable tolerance level.
If the machine is already calibrated within the recommended range, or if very accurate calibration cannot be achieved, your press may simply be exhibiting its registration shortcomings. In other words, the accuracy you desire may be impossible to attain or maintain for any length of time, due to actual press design. The key is to know what is possible from your machine, then not to ask too little or too much of it.
To address the issue of registration, manufacturers have to deal with the problems of cost and speed, as well as accuracy, to come up with design solutions. The basic demands are simple, as one of two things must take place: either the screen must register to the platen, or the platen to the screen. While the first scenario may seem impossible, it isn't. In fact, some high-tolerance circuit-board presses register the frame to the substrate—a very involved and costly approach to registration. A more conservative approach takes advantage of a moving platen and simply stops its travel in a very precise manner. This is simpler than adding motion to each print head, more cost effective for textile applications and, thus, is the premise from which we normally work.
Our basic goal is to register each platen to a screen. In this case, the platen should be thought of as "the press," because if all platens index to the same point each time—within a few thousandths of an inch—then the result will be a registered piece of art on all platens. It is, therefore, important to understand the need for the platens to arrive at each print station in exactly the right position every time—in other words, a condition of repeatability.
To better appreciate how manufacturers address the challenge of maintaining *.002" registration repeatability for thousands of indexes at speeds up to 72 dozen per hour or better, it is realistic to reduce the subject down to its two most basic categories: flexible (chain) and rigid transportation systems.
On a press using a flexible transportation system, the emphasis is placed on moving the platens quickly to the desired point whereby an external register system locks them into exact registration. Because the exact distance between platens cannot be accurately maintained on a flexible chain subject to stretch over time, no attempt is made to do so. Instead, a centering device is used with each platen so it may be locked into position before each print cycle.
An advantage of this system is that it is not encumbered by the tremendous weight of a rigid indexer table. By not having to overcome this massive inertia at the start of each index, such a machine is capable of running very fast, even in large print formats. While older presses of this type suffered a bad reputation for mediocre registration accuracy, the current use of front and rear platen locators has come a long way toward solving such problems.
In the rigid category, there are two basic types: those with external registration devices, independent of the drive, and those without.
The latter depend solely on the drive unit to not only index the press, but also to stop it with extreme accuracy and hold it tight during print action so position is not lost. Only a few European machines with this type of system exist; they're relatively unknown to the U.S. market. They rely on gear-drive units which, though also readily available from several domestic gear-box manufacturers, have been avoided due to their relatively high cost.
Machines with external locking/registration systems place lower demands on the indexing unit itself, in terms of accuracy. The main advantage here is a cost savings in the drive area and, to a lesser extent, more accurate registration since position is held out toward the actual print area versus the center of the press.
Because machine sizes range in diameter from 13 to 21 feet or larger, registration error can be amplified from within an acceptable tolerance in the center of the press to beyond an acceptable one at the outer diameter. The analogy I find most helpful is imagining the small amount of movement that occurs at a door's hinges when it is opened or closed, compared to that movement which occurs in the area of the door handle. A few degrees of movement at the hinge or, in our case, at the center of the press, will have a magnified effect out at the door-handle or print area.
Likewise, consider the effort required to start or stop movement of a door when that effort is applied at the hinges versus at the handle. On press, there is considerably less force required to pull the indexer table into register and hold it from moving when the registration/locking device is further out — in the door-handle area. However, providing for a locking device that far out on a large-diameter press is somewhat more involved, from a support structure standpoint. As a result, several manufacturers opt for a more central locking design.
And herein reside two more categories of rigid registration systems: inside and outside.
One configuration of the "outside" locking system employs a single locking mechanism — a V-shaped locking fork beneath the platen area—and a series of adjustable bearings (cam followers), one under each arm; these are calibrated to divide the indexer table into equal parts, much like wedges of a pie. Only one V-fork is required, because the forces necessary to pull the platens into alignment this far out on the perimeter of the press (near the "door handle") are much less than near the center (the "hinge" area). What's more, the use of a single locking site avoids the tension possible between multiple points and/or cam followers that, in turn, can lead to unwanted movement and loss of registration.
The V-shape is appropriate because such indexer tables work in a single axis; they do not go up and down. As a result, the V-shaped locking fork can more positively seat the bearing in dead-center position.
It should be mentioned that this design has been updated within the last few years to incorporate a spring-loaded fork. The use of a heavy duty spring in the fork allows the bearing heights at each arm to vary somewhat and still facilitate proper seating of the bearing. On older machines, special care had to be taken to ensure that these individual bearing heights did not vary more than a few thousandths of an inch, to ensure proper seating and locking.
Presses with inside-locking configurations are typically those with index tables or head platforms that go up and down. With these designs, the press may be more streamlined, less complicated, due to the absence of outside locking devices.
In such configurations, there are normally at least three locking devices —though often as many as there are arms on the press —located at the point at which the indexer table arms are mounted to the chassis, usually about 44 inches out from the center of the machine. (Why 44 inches? Because a 44-inch radius X 2 = 88 inches: about the widest machine you can fit into a semi-trailer without taking it apart.)
With these designs, the table is mated and locked with the head platform (or vice-versa) by one of two relatively similar means: the passage of either a bearing into a U-shaped fork, or a keel into a set of cam rolls.
In the first instance each arm of the indexer table is fitted with a single cam follower that can be adjusted side-to-side for calibration purposes. When raised together, these cam followers lock into a series of U-shaped forks. V-shaped forks are not used in such configurations as they would fight the required upward motion of the indexer table in order to force the platens into their centers. A balancing act would result in order to achieve sufficient downward pressure on the fork to direct or force the table laterally, without preventing the platens from achieving their correct upward print position. Thus, a U-shaped fork is used that can guide the cam follower into a fixed position while allowing for a variation in height to occur.
On the downside, some clearance is needed between the two side walls of the U-fork in order to allow the bearing to roll. This clearance is present in neither the V-shaped fork design nor the keel/cam-roll type (described below). For such clearance to be minimized to prevent it being a problem, the use of multiple forks comes into play. By adding additional forks, the clearance can be shared in order to distribute the side loading that occurs when the fork tries to pull a heavy indexer into position from the center of the press.
In the second instance—a keel seating into a pair of cam rolls—the cams can turn freely, with zero clearance, up and down the side of the keel during engagement/disengagement; in other words, a tight fit. While these systems are arguably better, from accuracy and wear standpoints, than the single-bearing type, they are not as quick and easy to register. However, if they stay in register longer, such a disadvantage may be worth the trouble.
As you can see, there are various ways to achieve registration. Keep in mind what it is you want to achieve. If accurate registration is your goal, you should consider these points carefully. And always remember that maintaining a few thousandths of an inch registration is different than showing it on a meter. If the dynamics of the press allow for excessive movement during operation, it will have an effect on the print now and in the future, with regards to re-registration. Re-registration is normal and necessary, but should not be required more than once or twice a year.
Real Performance Machinery L.L.C.
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