Working with Screen Printing Inks
inkHave you ever opened a can of ink to find it thick and gummy like peanut butter? Such ink will not pass easily through the mesh and will wear you out printing.
Friendly ink
These inks can be converted quickly to a friendly ink that is easy to print. First, it will be helpful to understand how inks are formulated so that we know how to modify them. Stir and print soft hand (also known by terms like base, extender base and other terms). There is no pigment in the ink, and the ink stirs and prints very easily.
Next, stir and print a process ink. Again, the ink is easy to work with, but offers some resistance. General purpose inks offer a little more resistance. The opaque inks for printing on dark garments, such as athletic inks, polyester inks and inks specifically formulated for dark garments are often very stiff and difficult to stir or print. So the ease of printing depends on the amount of pigment in the ink.
Stiff inks
In your frustration dealing with the stiff inks, you might wonder why the ink manufacturers make such inks. An automatic press printing say 500 shirts per hour is moving the ink 1000 times an hour, and all that movement breaks ink down like the ink mixing machines displayed at trade shows. If the ink does not have sufficient body at the beginning of a job, the ink will become watery and unstable during the print run.
Stir thoroughly
A screen printer using a manual press or starting a job on an automatic press has four ways to make the ink friendly. First, of course, is stirring thoroughly like those ink mixing machines displayed at trade shows. A variable speed electric drill with a spade drill bit will accomplish the same result. However, if you use a drill, rotate the spade drill bit as slowly as possible, and cover the top of the can with cardboard, or you will be flipping ink all over the room.
Blending
Second, and my favorite, is blending. Blending is taking 75-80% of the ink required from the heavily opaque ink can, and 20-25% from another can of the same color, but medium opaque ink. This dilutes the pigment percentage without changing the colors, or upsetting the chemical balance of the ink. Blending will make ink friendly. Do not exceed 25%, because the ink film could lose the opacity you are looking for.
Mixing
The third option is to mix the 20-25% using soft hand clear with the heavily pigmented ink, but this procedure reduces opacity. So this option should be used for light colored garments only, and not dark garments.
Curable reducer
Fourth and clearly the last option to employ, is curable reducer. Reducer is plasticizer. Plasticizer is clear like water, and pours like light oil. Reducer is very potent, and will have unfavorable results when used to excess. So the manufacturers add some PVC that is white plastic to reduce the chance of having an ink disaster. An ink manufacturer might tell you to add 10-15% of curable reducer by weight, but we recommend only 2-3% at most. Actually, only a few drops per screen should be added per screen, and the curable reducer should be very thoroughly mixed into the ink and only after following steps 1, and 2 or 3 above. The curable reducer helps ink to shear from the screen.
The danger of curable reducer is ink loses its cohesive characteristic. Curable reducer should not be used with process inks, because half tone dots break down and dot gain is the result. Fine lines and details spread out. The ink settles into the garment rather than sitting up on the garment, and the color of the garment can now be seen through the thinner ink film. As half tone dots flatten out, the image color might exhibit a muddy hue.
Curable reducer also promotes dye migration. Dye migration is the color of the garment tinting the ink color. Dye migration occurs with synthetic fabrics and synthetic blends like 50% cotton and 50% polyester shirts. So don’t use curable reducer on synthetics, and especially red, green and maroon garments. Those colors are high risk for dye migration.
“Low bleed”
Synthetics should be printed with “low bleed” inks which are also known as ‘non-migrating” inks. These inks typically are stiff and unfriendly. Stirring and blending are recommended to make these inks friendly. Since dye migration is initiated by heat, extra protection against dye migration can be achieved by running the garment through the conveyor at a hotter temperature and for a longer dwell time than later when ink is on the garment.
When inks are stiff and unfriendly, the ink suppliers respond with comments about changing the viscosity. Viscosity is a flow concept. Their recommendation invariably is to add curable reducer which in fact will promote the flow of ink.
Printing off-contact
When printing off-contact the mesh should pop off the surface of the garment leaving the ink on the garment. Tight screens are always preferred over loose or soft screens, because a tight screen helps to shear, or cut, the ink. The ink should transfer from the screen to the garment without leaving an ink residue under the screen.
Test
A good test we employ on every job before putting ink in the screen is to pull the stir stick straight out of the ink while looking at how quickly the ink in the can separates from the ink on the stick. We want the experience to be similar to pulling a spoon out of yogurt. If the stick gets to 6” above the can, and there is a string of ink from the stick to the can, the ink is not shearing. We want to avoid the taffy pulling experience.
The screen is typically only 1/32” above the garment when printing on shirts. The ink in the screen must shear, or be cut from, the ink deposited on the garment. Taffy-like ink will not shear. Inks that have been modified by the four steps above will shear more easily regardless of viscosity. In fact, a loose ink might be unstable and therefore undesirable.
A friendly ink shears easily, holds the shape of the image in the stencil and completely transfers from the screen to the garment. So now you know how to convert peanut butter like ink into friendly ink.
Questions and comments contact Roger@screenprinting.com