Lawson Screen & Digital Products co-owner, David Landesman, answers some general knowledge questions about screen printing plastisol ink in this article. Learn how to store ink, save ink, and minimize “bleeding”.
Is Water-based ink safer than plastisol?
No it is not. This is a common myth about water-based inks. Depending on the formulation, some water-based inks can contain components that are known carcinogens, or elements that pose other health hazards. Compare safety data sheets to learn the details, you may be surprised to learn that most plastisols are equally safe, or safer, than water-based inks.
Should I stir plastisol ink before using?
Yes. By stirring the ink, this action creates “sheer” and this breaks down the “false body” inherent in the ink when it is idle. By stirring the ink you create heat which loosens the molecules of the ink and makes the plastisol ink “creamier.” Stirring plastisol ink prior to use will also show the viscosity of the ink. Never modify the ink without thoroughly stirring the plastisol first. Printers too frequently will add a viscosity reducer when they don’t need to. A simple solution is to use a stainless steel ink spatula to manually stir the ink.
Where should I store my plastisol ink?
Plastisol ink should be stored away from heat and sun. Never store, or keep near dryers or flash cure units. Always keep the lid on the plastisol ink bucket for a wide variety of reasons, including contamination, and because some plastisol formulations can absorb humidity in the air. Also, remember that “cold” plastisol gets thicker and stiffer and does not print well.
What causes plastisol ink to “build-up” on the back of screens?
There are several reasons why some plastisols “build-up”. And to some degree, all plastisol ink will build-up on successive screens. Minor build-up does not cause a problem and, sometimes, it can actually enhance the look and brilliance of the print. So we will concentrate in unwanted or excessive build-up.
Here are some of the most common reasons, and what you can do to “fix” this issue.
- A common cause of too much ink build-up is because too course of a screen mesh is being used for the color or print sequence. Another way of saying this is that you are laying down too much ink. Simply select a finer screen mesh.
- Loose screen tension is another common problem. A loose screen causes “ink smear” can also lead to build-up on the next screen.
- Color sequencing is another build-up issue. For example, if black is being printed as an interior color, this frequently leads to build-up.
- Sticky ink also causes a problem. If an ink is too sticky (the reasons for this are many, including the ink formulation) and does not release from the screen easily, this will lead to excessive build-up. Modify the ink with a detackifier or flash before the sticky ink is printed.
- Artwork design. Too frequently the artwork does not lend itself to good multi-color printing. Many printers still “trap” colors, and this contributes to build-up.
- On-contact printing presents build-up problems, as the mesh has no opportunity to “snap-back” and contributes to the “ink smear” problem. You should be printing t-shirts, hoodies and other garments with off-contact. You want about ¼ inch (the thickness of a quarter) gap between your screen printing press platen and aluminum frame.
- Excessive off-contact can also contribute to ink build-up, as excessive squeegee pressure needs to be used to overcome the off-contact gap created by too much distance between the mesh and the garment.
- Squeegee and Flood speeds are not matched to the ink, mesh tension and substrate.
How to minimize “bleeding”, more accurately described as dye migration?
Dye migration (“bleeding”) is caused by the dye in the polyester fabric “gassing out” during the curing process, and embedding itself in the plastisol layer of ink. There are basically only 3 ways to control dye migration (bleeding):
- Don’t print on a garment where the dyes easily migrate under heat. Not all garments are created equal. Some manufacturer’s garments bleed less than others, because of the dyes they use, and the curing process that is used to “set” the dyes.
- Use a plastisol ink specifically designed to control dye migration – called low-bleed polyester inks. There are significant differences between the various brands of low-bleed polyester plastisol inks…so do your due diligence and test prior to using.
- Cure at lower temperatures, and use jet-air as this is a more “gentle” and consistent type of heat. Avoid using a dryer this is entirely quartz as quartz tubes are extremely hot and not as controllable as I.R. panels and hot-air.
Why does my ink sometimes crack?
Cracking ink is almost always under-cured. Of course if you are printing on a substrate that has has enormous stretch, and the plastisol ink you are using does not have equal stretch characteristics, then the ink could also crack. Add a “stretch additive” or “soft-hand extender” to the ink and this will frequently solve this cracking issue.
Crocking is not to be confused with cracking. Crocking is a condition where the ink actually can be rubbed off. Of course first make sure the plastisol is thoroughly cured. If high-grade pigments are used by the plastisol manufacturer, crocking is usually not a problem. Red plastisol is generally the problem, as some manufacturers use a lesser-grade pigment – automotive grade red pigments are expensive, so spend the extra money for the good plastisol.
How to calculate how much ink is needed for a print job?
The most accurate method of determining ink cost is to weigh an un-printed shirt, then print your design on it, then weigh it again. This weight will exactly tell you – given your particular print environment (i.e. mesh count, emulsion thickness, pressure, etc.) – how much ink you are using. Then simply determine your cost of ink per gram, and there is your cost per print.