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Best Practice in Screen Printing

(January 2001) posted on Thu May 31, 2001

The authors discuss the most commonly overlooked variables in the screen-printing process and how to control them.


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By Carol Swift, Peter Kiddell

It is vital that the frame be robust enough to resist distorting under the forces of the tensioned mesh, or the printing process will suffer. A frame measuring 1 m (39 in.) per side holding mesh tensioned to 20 N/cm have approximately 200 kg (440 lbs) of force exerted on each beam. Also be aware that temperature has a considerable effect on mesh tension. Screens need to have time to stabilize when brought from an environment with a different ambient temperature.

Floodbar

The purpose of the floodbar is to charge the mesh openings with ink before the ink is displaced by the action of the squeegee. The floodbar is basically an inhibited squeegee. Angle, profile, speed, contact or off contact with mesh, are all key issues in determining how much the floodbar prefills the mesh, and as a result, how much ink is deposited by the squeegee. By controlling the amount of prefill, the floodbar indirectly influences the required drying time for the print.

The floodbar is a much-misunderstood part of the process. Some printers don't even use the floodbar and, instead, rely on ink sticking to the squeegee to feed the wave on the next pass. With a manual press, this is to be expected since the squeegee is used both as a squeegee and a flood coater. In manual printing, it takes a skilled printer with greater dexterity to manipulate the squeegee and achieve the desired results.

For most applications on an automatic or semiautomatic machine, the floodbar should only slightly contact the stencil during the flood stroke. The shape of the contact edge is a critical factor here. Sharp-edged coaters prefill the mesh with less ink, whereas rounded coaters prefill the mesh with more ink. Using the floodbar off-contact from the screen means you are relying on the ink to flow into the mesh openings on its own, and you essentially give up all control.

The press



Every printing machine (even the same make and model) will have a slightly different print characteristic. In all but the most precise applications, these variations are irrelevant. On press, 80% of all production problems are caused by improper setup.

Attempting to remember press settings or use the \"magic touch\" to tweak the machine into submission is a recipe for disaster. Consistent setup is crucial, and the only way to get it is through accurate recordkeeping. Production control cards detailing all the parameters of press setup are the foundation of good process control.

A simple rule is to record enough information so that any printer familiar with the machine controls can use your information to set up a job and achieve the required print characteristics without having to ask for more information. In a situation where multiple colors are being printed on the same machine, or several machines are arranged as an inline printing system, the setup characteristics should ideally be the same. Variations will result in changes in image size and ink deposit.<P>Having material on hand to support the production run is a related press-setup concern. Make sure to gather everything you need for a print run before you start. This results in minimal down time and far fewer rejects.

Finally, keep in mind that continuous operation is the most effective way of running a press. Stopping to clean the stencil alters the consistency of ink flow, and, if you clean it without care, you shorten stencil life and degrade printed-edge definition. If possible, you should avoid touching the print side of the stencil at all. Preventing squeegee wear (which can lead to an increased ink deposit) is another way to keep the press running. Applying low squeegee pressure reduces the potential for wear.

Other concerns

Beyond these areas, you also need to make sure that the substrates you print are clean and uncontaminated and that you maintain a proper printing environment with controlled temperature and humidity. By taking these steps and striving to reduce the previously mentioned variables, you'll see rejects decline, press productivity rise, and the overall quality of your printed products improve.
 


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