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Oct 1, 2010 12:00 AM
So often, in this industry, we use terms to describe print issues and think everyone uses them in the same way. I hope to bring a little clarity to the terms “image fit,” “grids” and “register.”
Grids | Grids have been used over the years for many purposes. The first use I saw was for striking in pinning systems for plate register on a press. Today, the most popular use is to see if lines and dots can “stack” on a multicolor press.
The most popular grid sold is the Printing Industries of America Register Test Grid. This grid, designed by Gene Bulinski and John Peters of GATF, is quite clever. It uses lines of two widths and dots that, when imaged at 150 line, are 0.002 inch in diameter. With dots of precisely this size, it is easy to use the eye to determine the degree that you are out of fit. The product is available in film as well as digital format.
Fit | When a printer uses a grid on a press, it usually is for the determination of image fit. Image fit is the ability of a press and prepress system to place one image on top of another image in precise position.
The first step is to ensure the prepress system can accurately place the grid in the same position every time. Next, for a multicolor press, you hang a set of plates, pull up to standard density and then look at the sheet with a 100x magnifier. (Note that the dots must stack; no rosette pattern.) This represents the first pull. Then, strike in the grid at the gripper and lead-to-tail at the center. Now look at the lead, center and tail of the sheet — this is, then, the fit of the press. What is good and what is poor is a subject of great discussion. If 100-lb. coated stock is used, generally we would like to see the dots grow to no more than 0.004 inch in size.
Register | Once you have fit, then you want to see if you can keep it. This is register. In essence, will the dots stay in the same place from sheet to sheet to sheet?
Register can be measured in several ways, with some rather specialized equipment available though the press manufacturers.
Raymond J. Prince, NAPL partner consultant, is a leading expert in pressroom technical and operational issues. Contact him at (605) 941-1492 or firstname.lastname@example.org.