Pocono Mt. Screen Supply, Inc.

Controlling Off-Contact For Successful Printing

Question

I have been told by many people that controlling off-contact is the key to success in printing. But how does it really affect my results?

Solution

Off-contact is the distance between the substrate and the screen. This distance is needed to keep the screen from staying in contact with the substrate during printing and to control proper snap-off. While off-contact is not the only variable in the ocean of possible problems, it is a decent size fish as far as press variables go. The off-contact distance can affect your printing in any number of ways. Before you tackle this problem, however, you must make sure your screen has proper tension. Check your mesh manufacturer's tension recommendations.

Assuming that your screens are at the proper tension levels, we will begin by covering the problems caused by too little off-contact. Poor release: if the fabric sticks to the substrate after the squeegee has passed, and the screen is lifted, you'll notice it takes a percentage of the ink with it and leaves mesh marks. You then have two options; either raise the off-contact, or slow down the print speed to accommodate this poor release. These are common but incorrect solutions. If you make no adjustments, double images can appear, as well as image distortion. Not to mention the poor ink coverage that will result.

High off-contact, which is as common, causes a myriad of other problems. (Again, we will assume that your screen tension is correct.) Registration problems: these occur due to the excessive fabric deflection brought on by high off-contact. Inadequate squeegee pressure: As the off-contact distance increases, the squeegee pressure must be increased to force the fabric into contact with the substrate. This causes slower press speeds, image distortion, squeegee wear, and screen failure. Faster snap-off: Increasing the off-contact and squeegee pressure creates a faster snap-off. This can cause print defects such as bubbles, smearing, incomplete images, mis-registration and print voids.

Until recently, there were only a few ways to determine the off-contact distance. The most accurate method was by using a mechanical gauge. However, this process could take valuable time away from production, so it was usually abandoned and left to the operator to "eyeball" the distance. Now, the newly developed Positector 6000 off-contact gauge, with an easy-to-read digital display, can provide measurements automatically in minutes. Thus repeatability can be built into this print variable. Keep in mind however, that this measurement provides the actual off-contact distance, not a determination of what is the proper off-contact distance. It is up to you, through testing, to determine the proper distance required for each press, screen tension, and format size. Having this tool to measure the distance, and maintain uniformity, makes the process more repeatable.

Note: a good rule of thumb is to have your screen as close as possible to the substrate, while maintaining the proper snap-off.