By Henry L. Newman, Newman Printing Equipment, Inc.
Printing cylindrical and slightly tapered items is not difficult with the right equipment and technique. It is almost impossible, however, to print a cylindrical item (let’s not call it round) without a machine that can hold the squeegee and screen accurately.
Friction printing is the most basic technique for printing cylindrical objects that are solid enough to be supported on four wheels or rollers. A fixture is needed to support the item being printed, allowing it to spin or turn true, much like cutting on a lathe. Tooling also can be made to hold the ends, but adjustable roller holders don’t require special tooling.
There are three general steps to simplify any cylindrical printing application.
STEP 1- SQUEEGEE CENTERED
Once the item is turning true on the rollers or tooling, check the position of the squeegee to ensure it is centered (left and right) with the item to be printed. This should be done before the screen is sandwiched between the two, making it impossible to check. The second photo shows an 80 durometer, urethane, double bevel, often called “chisel” blade. Keep in mind, unlike flat printing, this relationship will hold through the print stroke because the screen is moving across, not the squeegee. In some cases, the squeegee will move across the screen but always with the cylindrical item being printed, thus never losing its center. Also, from the side, check the squeegee length to ensure it at least covers more than the print height (up and down). The squeegee pressure down can be adjusted once the screen and ink are in place. Too much squeegee pressure down can cause the blade to fold over – off of center – blurring the print in that direction.
STEP 2- SCREEN TO CYLINDER
Jog the machine with the squeegee retracted to position the item being printed with the screen in order to check the “off contact.” Off contact is an even space between the screen and the item being printed and normally is adjusted by moving the screen up and down. “On contact” works in flat printing, but rarely in cylindrical. Some printers use thin cardboard to check this gap. With a tightly stretched screen, the gap is a small amount, typically about 1/16 of an inch. Large, baggy screens may require more off contact when full of ink. The reason for this space is to allow the squeegee to push the screen down in a line contact, allowing a peel effect for crisp print. Too much space between the item being printed and the screen will distort the print and wear the screen out early. Screens, if made well, can last 25,000 impressions or more with good set-up. Because the screen will move across left and right, check the off contact when the screen is right and again when the screen is left. Ensuring even off contact under all four corners of the screen is important.
STEP 3- STROKE OF SCREEN
Finally, adjust the start and stop positions of the screen in relation to the artwork in the screen. This is referred to as the screen stroke. Machines are all slightly different, but similar rules apply. The stroke always will be slightly longer than the width of the image. The start point – or right margin of the screen (shown in the third photo) – reveals the stroke started about one inch before the image. It is important to set the stroke to stop immediately after the image ends in the screen to prevent rolling around more than 360 degrees and smudging the start point of the wet print. This also helps to flood the ink back to the right side start point, usually using a second flood blade to skim slightly over the screen, never touching it on the backstroke. Most machines will print in one direction and flood back to this same side start point.