Even after over 30 years in print, watching a press running can be fascinating. You stand at one end of the press, and see how the sheets are lifted and then moved into the first print head. You walk along the press, seeing the rollers, plate and blanket cylinders are moving around and around, and then you come to the opposite end, and watch as sheet after sheet is pulled through and dropped one on top of the other, often at speeds that make you wonder “how the HECK does it all keep in perfect register?!” Well, let’s take a little walk through the press, one section at a time.
The the far end of the press is the feeder, which is the device that feeds the paper into the press. Presses either have a feed system where the paper is in a stack, and each sheet is grabbed and pulled into the feed path, where it’s pushed against a metal “guide” that gets it in correct position. Large presses (20″ and up) have “stream” feeders, where the paper is in a lift, and then is pulled onto the stream. As it approaches the first print unit, it is pushed against the metal guide, and then is stopped for a millisecond as the “head stops” get it to be aligned precisely. This is the beginning process of registration. Most presses use a combination of air blown into the sheet which separates it from the rest of the stack, and suction cups which lift each sheet and feed it into the stream. The person at this end of the press is called the Feeder, and their job is to know paper, to know how it needs to be set up, and make sure that it goes into the press correctly.
THE PRINT UNIT
Most presses in commercial print shops have at least 4 print units, one each for Cyan, Magenta, Yellow and blacK (CMYK). Some shops may have a 2-color press, which they use for simple 1- or 2-color work. Many printers will have at least a 5-color or 6-color press, which will allow them to run 4-color with maybe a spot color, or a varnish. Each print unit has the ink fountain, where ink is dispensed through a system of rollers to the printing plate. The plate uses a mixture of water and other solutions to allow for ink to adhere only to areas on the plate that are laser-etched and then processed. The ink then is transferred to a sheet of rubber wrapped around a cylinder. That sheet of rubber is called the blanket, and the image on it is reversed. So, as the sheet of paper passes along the path, the impression cylinder in each print unit presses it against the blanket, and ink is transferred to that sheet of paper. Now, what is amazing is that this is happening to EACH SHEET OF PAPER as it goes under each print unit, in perfect alignment (or what we call registration), at a speed that can be close to 13,000 SHEETS PER HOUR! That is roughly 3 or 4 sheets EVERY SECOND running through the press!
As each sheet travels through the press, it is guided along by a row of pincers called “grippers”, and (here’s the amazing part), each print unit has, on its impression cylinder, its own set of grippers. The final set of grippers is on a bar that takes the sheet from the final print unit, pulls it along and then releases it so it gently lays down on the sheet below it. This happens so rapidly that when a pressman needs to pull a sheet to check for color and other variables, he puts his hand in a safe spot in the delivery, and just “pinches” out a few sheets, simultaneously pulling a lever that opens up the delivery so he can take the sheets out. A pressman can pull anywhere from 4-6 sheets easily in just a couple of seconds.
THE FINAL CHECK
Once the press operator pulls the sheets out, he takes them to a console that has a densitometer that reads the color bars at the rear of the sheet. Even if the press operator sets the CMYK values at standard densities during the make-ready process, they may alter the densities to match the proof, or the client and/or sales rep may ask for shifts in color. All this is done on the console, and automatically adjusted on the ink fountains. Once the densities and all settings are in place, the operator begins the run, pulling sheets out at various intervals to check color and registration.
It really is amazing how press manufacturing technology has become so precise that you can run sheets through the press at 13K per hour, and registration and color will be exact sheet after sheet. I will admit that often, when I get into the shop, I get my coffee and go out to the press room just to watch. After a few hundred sheets (we’re talking 2-3 minutes), I head back to my quiet world to start my day.