Best Practice in Screen Printing
The authors discuss the most commonly overlooked variables in the screen-printing process and how to control them.
Companies that use screen printing as part of a broader manufacturing sequence tend to treat it as an engineering process. Their aim is to understand where problems are likely to occur so that the problems can be avoided. As consultants, we encourage this approach to screen printing, because without it, printers tend to forget the fundamentals and lose control of the process. This month, we'll look at several important fundamentals as they relate to key variables in screen printing.
Ink has perhaps the greatest influence on success or failure in screen printing, namely because of the number of ink-related variables printers face. Some of these variables include viscosity, shear, surface tension, pigment particle size, speed of solvent evaporation, and temperature. Of these variables, one that printers often overlook is the evaporation speed of solvents in the ink, which, among other things, determines the likelihood of ink drying in the stencil.
If you print with conventional UV-curable ink, evaporation is not as important an issue, but shielding the ink from excessive amounts of ambient ultraviolet radiation (from sunlight, fluorescent tubes, etc.) is important. Water-based UV ink is best treated as a solvent-based ink system, since on-screen evaporation of water and co-solvents is constantly occurring, and it can alter your ability to print the ink successfully.
Regardless of what type of ink you use, a simple but effective rule is to add little ink to your screen at any one time, but add it often to maintain the correct working conditions in the ink. Besides concerns about ink drying in the stencil, large amounts of ink on the screen can lead to deflection of the screen, altering its off-contact height, and consequently, its snap-off characteristics. It is also very expensive to put too much ink on the stencil, as unused ink will lose solvents or pick up contamination.
The purpose of the squeegee is to bring the stencil into contact with the substrate and cause the ink to release from the mesh. Any other effects of the squeegee are likely to have a negative impact on the printing process.
The squeegee is the power plant of the process. Essentially, it creates a wave in the ink as it passes across the screen. A wave is a pressure system, so as the ink wave moves over the stencil in front of the squeegee, the pressure causes it to fill mesh openings and push through the stencil until it makes contact with the substrate.
Once the ink is in contact with the substrate, another mechanism takes effect. This is the relationship between the surface energy of the substrate and the surface tension of the ink. If the surface energy of the substrate is higher than the surface tension of the ink, the ink will be attracted to the substrate and will be pulled out of the mesh openings as the screen snaps off the material. This, in simple terms, is the wetting of the substrate by the ink. Note that if the substrate is greasy or naturally has a low surface tension, wetting may not occur and ink transfer will be either incomplete or non-existent.
This wetting effect will only occur efficiently if the tension of the mesh is sufficient to overcome the tackiness of the ink. Should the mesh lie on the ink film for an extended period after the squeegee has passed, ink transfer will only be partial, and much of the ink will be retained in the mesh, resulting in a mottled-looking print. But with proper tension, snap off will occur immediately after the screen has moved on, resulting in a complete transfer of ink from the top of the mesh to the substrate.
The parameters to consider when selecting and using a squeegee include durometer (hardness), flexibility, profile, length, unsupported height, angle, speed, and pressure. Looking solely at durometer, we recommend using harder rather than softer squeegees because they are more stable and last longer. Softer squeegees tend to absorb more solvents from the ink, which causes them to distort. Use of soft squeegees is only advised when the surface to be printed is uneven, and a harder squeegee will not maintain stencil contact with the substrate.
If you sharpen the squeegee, do so with great care. The edge provided by the manufacturer is better than what you'll be able to obtain with any but the most effective squeegee-sharpening equipment. The practice of sharpening a squeegee immediately after it's removed from the press also should be avoided because it produces an unstable, and often uneven, printing edge that is more open to attack by solvents in your inks. We recommend allowing squeegees to rest for 24 hours before sharpening them.
Screen and stencil
he stencil defines the image by providing a boundary for the ink and metering how much ink is deposited by the mesh. The accuracy of image replication depends on a stencil's ability to form a gasket-like seal between image and non-image areas during the squeegee stroke.
Stencil thickness and flatness (Rz), as well as the bridging properties of the emulsion across the mesh openings, affect the accuracy of edge definition in your printed images. If the stencil is too smooth (very low Rz), the stencil will tend to stick to the substrate, resulting in reduced print quality. Mesh geometry (mesh count, thread diameter, etc.) determines how much ink will be metered through the mesh. Stencil thickness only effects the ink film thickness on the edge of the image. If the image consists of fine lines or fine dots, stencil thickness can have a considerable effect on the ink deposit. But with large printing areas, stencil thickness only affects edge definition.
Tension in the mesh provides the hidden energy of the process. A screen should be tensioned within the range recommended by the mesh manufacturer. As the tension is increased, snap off can be reduced. Reducing the snap off also reduces stretching of the image and improves image quality. Tension gives stability to the image. However, too much tension can make the screen more fragile and can take the mesh close to or past its elastic limit, resulting in irrecoverable tension loss. For multicolor work, each screen used in the job should have a tension level that falls within 1 N/cm of the other screens for that job.
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.
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.
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.
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.