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Making the Most of Pin Registration

(May 2003) posted on Tue Jun 03, 2003

Discover how to refine each step of the prepress workflow so you can realize the benefits of pin registration.

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By Mark A. Coudray

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.




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