What is “off-contact?” Off-contact is the gap between the bottom of the screen and the platen.
In the screen printing process, the screen is filled with ink and the squeegee is pulled across the screen pushing the ink through the stencil to produce a printed image. When the screen is lowered onto the substrate (a fancy term for the item being printed), you must have a slight gap between the screen mesh and the substrate, otherwise the ink will most likely smear onto the substrate.
Moreover, without off-contact you will experience difficulty lifting the screen from the substrate because the screen or ink may stick to it causing further problems. Printing with off-contact eliminates those problems by keeping the screen just slightly above the substrate. Once the squeegee is pulled across the mesh, off-contact immediately lifts the screen mesh away from the substrate as the squeegee is moved the across the stencil, resulting in a good print.
USING A SHIM FOR “OFF-CONTACT”
How do I adjust off-contact if my press does not have a built-in off-contact adjustment? There are several makeshift off-contact adjustments that you can use, we'll show you a few that we have used with success. All of these involve the use of a “shim.”
A “shim” is any type of material added to the bottom of the screen frame to slightly lift the mesh from touching the substrate during the printing stroke. It's purpose is to add a light extra thickness at the end of the screen frame to raise the screen up slightly.
You can add a single shim to the bottom front section of the screen frame or to both sides of the frame. Adding one shim to the bottom front section of the screen frame is usually all that is needed.
A shim can made from a piece of reasonably thick, stiff piece of corrugated cardboard or a piece of plastic like a ruler. "Popsicle" or tongue depressor sticks are also used. Just cut the material that you intend to use as a shim to a size that is a few inches shorter than the bottom front section of the screen frame. See photo. Using a tape that is strong enough to keep the shim attached for the entire print run, tape the shim securely to the bottom of the frame. That's it. Lower your screen and “feel” the mesh. Is there a slight “springy” feel between the mesh and platen? If so, you have created off-contact.
A good shim can be made from a single “slat” of a plastic, roll-up blind. If you have an old set of plastic blinds laying around or can “borrow” a “slat” from an existing set, you'll be in business. Most plastic blind “slats” are “bowed” which makes them deflect a little when the weight of the printing stroke is applied. They work great as a shim-type off-contact device because that bowing makes them deflect slightly and snap back. The best advice regarding the use of shims as an off-contact solution is to experiment. You should be able to find something that works well for you.
HOW DO CREATE “OFF-CONTACT?”
If you have a press with an “off-contact” adjustment, it is a simple matter to set it precisely at the height that you want. Just turn the knob and watch the screen raise or lower. You can “feel” the amount of off-contact by pressing down on the mesh.
If your press does not have a built-in off-contact adjustment, you will have to use a “shim.”
PRESSES WITH “BUILT-IN OFF-CONTACT”
Having an off-contact adjustment knob on a screen printing press is a very desirable feature and should be considered a top priority when purchasing a press, as you will use this feature daily. The best presses have off-contact adjustments that can be made without tools for fast, easy adjustment.
HOW MUCH “OFF-CONTACT” IS NECESSARY?
No matter what method you choose to create off-contact, there is an optimum distance. Based on this author's experience of screen printing a range of products (both apparel and hard goods), a general rule of thumb to use for setting off-contact height is to set it to a height that is about the thickness of a dime.
If you are printing sweatshirts and other thick, heavy apparel, the thickness of a nickel should be sufficient. Keep in mind that these off-contact heights are guidelines only. You may find your ideal settings to differ from these depending on the type of products that you are printing. Experiment.