Making the Most of Pin Registration
Discover how to refine each step of the prepress workflow so you can realize the benefits of pin registration.
Few things improve the overall productivity and efficiency of a screen-printing operation better than a good pin-registration system. The advantages of such systems are consistent placement of all screens on press and a dramatic reduction in press-setup time. Yet I'm amazed by the number of printers who've attempted to implement pin registration and complain that it didn't work for them.
While it is true that a poorly implemented system will slow you down more than if you used a manual system, this doesn't have to be the case. You need only pay some basic attention to the progressive stages of prepress, and any reliable pin system will perform almost flawlessly. This month, I'll touch on the essential areas of pin registration that typically cause trouble.
The first step is creating artwork within one or more software applications. Working within a standardized system is important whether you use vector images, raster images, or composites of both. Here, it means you must place artwork into digital templates. Drawing programs, such as Adobe Illustrator or Macromedia Freehand, and page-layout programs, such as Quark XPress, support template creation and application. The key is to use the same template for all jobs, carefully positioning register and printers' marks in the same locations each time.
Adjust template size based on the size of the image, but locate the final printing image within the same digital-coordinate layout. This is very important, especially if your imaging device does not prepunch the final film.
There are several ways of precisely locating your image each and every time. The two most common are to use the digital coordinates provided by the info pallet as a reference or to use the auto-centering command within a predefined picture window. Your software documentation describes the exact steps you need to take to benefit from this very precise positioning approach.
Creating uniform digital registration marks is as important as using templates. I strongly suggest that you create marks with crosshair thickness of no more than 1/2 pt, which is about 0.007 in. Very thin marks aren't popular because they're easy to overexpose and, consequently, impossible to wash out. Thick marks may wash out more easily, but make accurate film positioning very difficult. Keeping the stroke thin ensures greater image accuracy later in the process.
You'll find yourself in trouble if you create new layouts for every job, or if you try to be too conservative with film use. In both cases, the results are randomly generated positives that must be dealt with on an individual basis. This lack of repeatability in your system plays a huge role in misregistration on press. Each job is new and different, which makes it challenging to spot problems in advance. As a result, mistakes flow through unnoticed and typically show up much later in the process where they are most expensive to fix.
Film-positive generation is the next step. The ideal method would be to utilize prepunched film within a drum imagesetter. Machine tolerance determines film-to-film accuracy, and drum systems usually are accurate to ±0.0002 in. This is much, much finer than you can print, but it is stable and repeatable. Prepunched film is a huge bonus because you can use the punched holes to further guarantee extremely accurate image transfer in latter stages of the process.
Barring punched film, you are stuck with capstan or roll-fed film. These are the materials used with just about every imagesetting device out there, be it inkjet, thermal, toner, or laser-based. The important factors to consider are the accuracy of the image-to-image placement for separations and the flatness of the final imaged film. If film separations don't accurately line up at this stage, they will not align on the screen or on press.
How close an alignment is good enough depends on your preferences. For me, it is not a visual factor. I insist that separations must appear aligned when viewed under 10x magnification because accuracy degrades at each stage of the process. You must be much more critical at the front end of the process because things only get worse as you move along.
Film curl causes big problems. The curl creates tension within the film (and the carrier base, if you use one). This tension wants to resolve itself, which usually means something is going to move. The movement often happens during the vacuum phase as the layers of film and screen come together. The curled film wants to scoot around and can be very difficult to control.
Progressing from the imaged film to a carrier base is typically the next step. Regular film is usually 0.004 in. thick, compared to the carrier base, which is 0.007 in. thick and more stable than conventional film. The carrier base is much less sensitive to changes in size caused by heat and pressure, both of which are quite common during screen exposure. Most carrier bases come prepunched to correspond with the pin-registration system you are using, which is helpful if you don't own a film punch.
In the ongoing quest to save money, you may be inclined to strip images off the carrier sheets and use the carrier sheets over and over. But remember that each time you mount the carrier on the pin bar, you slightly enlarge the punched holes. While the enlargement isn't too much of a problem at the top of the sheet, where the pins are, it is at the bottom because very small errors magnified over a distance create what is known as a <I>radial error</I>.
Picture a slice of pie. The error at the pointed end of the pie is minimal. However, by the time you get out to the edge, there is a huge transmitted error on either side of the fixed point. This flex means that you will see all kinds of variation at the tail end of the sheet.
Trapped air under the carrier is another common source of image-placement errors. To reduce the potential for trapped air, position the film on the pins and then carefully squeeze the air out, moving from the pins to the tail end of the sheet. A good tool for this is a 1-in. diameter hardwood round dowel, available at a hardware store. Wrap the dowel in several layers of cheesecloth to prevent damage to the carrier base or the film. After you force the air out, place a small strip of tape at the tail end of the carrier to keep it from moving.
Now is the time to apply the imaged film to the carrier. Those thin registration marks come in handy here.
Position your image according to any layout grid you use. Make sure to look at the positioning under at least 10x magnification. When you position the film, tape the top of the film nearest the register pins first. Then, using your dowel, carefully squeeze the air out from under the positive. Tape the tail, and be careful to apply slight downward tension onto the tape and positive, away from the pins. Doing so helps to avoid any film buckle from shifting during exposure.
Apply a second carrier sheet over the first. Repeat the dowel process. Very carefully position the next film separation over the initial positive. Make sure to use magnification to assure accuracy of placement--do not trust your naked eye! Tape the head first, nearest the register pins. Dowel out the air and check the tail-register marks under magnification. Make whatever small adjustments are necessary for perfect registration and tape the film down to the base.
When you complete this step, remove the second carrier from the pins and replace it with a new one. Repeat the process until all colors register with the base positive.
When you are done, stack all of the positives up on the pins and check for collective alignment. Inspect the alignment under magnification and be sure to look exactly straight down. If you position yourself at any angle to the loupe, you introduce a <I>parallax error</I>, which prevents precise registration.
The next step is exposure. Many bad things can happen during the process because the amount of force exerted on the screen and positive is huge, and the force of vacuum drawdown moves anything if given the slightest chance. But you can minimize the threat of registration error at this stage.
First, make sure that your screens are absolutely flat. The frame must be free of wobble and rack. If you lay the screen on the exposure glass and there is any movement at the corners when you press on them, your image is guaranteed to be out of register.
If you experience the problem with static frames, get new ones. Make sure that all corners are ground perfectly flat and that there is no interference from tape, glue, welds, or other sources of contamination. If you use retensionable frames, retension to flatten the frame. Retensioning on a stretching table--preferably pneumatic--is best. Check to make sure the screen is absolutely solid and flat. I cannot stress enough the importance of flat screens.
Keep an eye on screen tension. For me, the lower limit is 20 N/cm. But consistency is more important than the amount of tension. Try to keep the tension of all screens within ±2 N/cm of one another, preferably around 23-25 N/cm. Tension variation beyond ±2 N/cm leads to trouble. Higher tensions are more desirable, but I find most shops have difficulty maintaining consistent, high tension while handling screens in a typical environment.
Vacuum drawdown is the next big problem because it's usually uneven and incomplete and because drawdown variability pushes and pulls the screen and positive inside the frame. Always use bleeder cords under the blanket to ensure even and uniform draw from the center of the screen to the edge of the frame. Uniformity is one of the keys to success, and is as valuable as the repeatability of the carrier base.
After exposure and washout, check your positive to the image on the screen. Your goal is a perfect fit. Perform the inspection under backlit conditions and use a magnified loupe. If the image does not fit, you can be sure that the problem is either with screen flatness or uneven vacuum drawdown. This inspection is your last chance to catch a shift before you go to press.
Alignment and printing
The final step is alignment on press. You can attribute any shifting at this point to either clamping or press mechanics. Clamping can be a problem, especially if you have twist clamps. If that's the case, make many small adjustments and work in opposite directions--front to back and side to side. Small adjustments mean small shifts.
The preferred method is to use air clamps for securing the screens on press. Clamp closest to the pins or register locks first, then at the tail end of the screen. If you use a three-point registration system, be careful not to press too hard on the screen when positioning it on the guides. Applying too much pressure causes errors.
So now it's time to pull a print. If you executed the processes correctly, you'll see few problems, if any, and your marks will be perfectly aligned, even under magnification. The need to make one or two very small adjustments is typical, but if you find more are necessary, then the press is most likely not level and flat. As a result, all of your hard-earned progress goes right down the drain.
Pinning down profits
A properly designed and maintained pin-registration system can reduce your setup times to 15-30 sec per color. Total setup time per color should be considerably less than two min, and that figure accounts for mounting squeegees and flood bars and inking the screen. Proper use of a pin system can ensure consistent results in prepress and on press, allowing you to focus on the other register--the one in which you'll record increased profits.