Aqueous coatings are bigger than ever in today’s printing market. An aqueous coater is often built directly into a printing press because it is ideally applied immediately after the ink. The coating itself has numerous benefits including a beautiful glossy shine to the finished product and improved durability for jobs that might be going through the mail. Benefits on the production side are quick dry-times that allow the job to be shipped out right away or flipped over and printed again in a short amount of time. There are many misconceptions in the world of aqueous coatings that we will address in this article.
Many variables exist in aqueous coatings. Differences in infra-red dryers, anilox application rollers, coating formulas, and mixes all play into the behavior and final look of the coating. All reputable coating suppliers offer guidelines for their coatings providing specific details and instructions to follow.
In addition to manufacturer specifications, there are many broader guidelines and general rules to follow. First we’ll start with the coating itself. Plan ahead; when shooting for a high gloss, large amounts of coating must be used. This will limit your options later on. Jobs with a heavy coating applied for high gloss can only be coated on one side whereas lighter coatings can be done on both sides of a job.
Coatings are applied to the sheet by means of an anilox roller. An anilox roller is designed specifically for this purpose and has a special surface covered with a large number of tiny pockets or cells. The cells act as buckets to evenly disperse the coating on the sheet. Cell count is measured in LPI or lines per inch which is a count of the rows of cells per inch around the diameter of the roller. Cell depth is a specification exclusively for anilox rollers. Cell depth determines how much and how thick the coating applied to the sheet is.
The viscosity will cause the look and behavior of your coating to vary. Viscosity is a measurement of how runny or thick your coating is. This is controlled by adding water. Viscosity is measured with a tool called a Zahn cup, which is essentially a bucket with a hole in it. Measurement is made by filling the Zahn cup with coating and using a stopwatch to measure how long it takes for the coating to run through. Viscosity will also vary based on the environmental temperature around the press. A cool coating will be thick and a warm coating will be runny. Most suppliers suggest an environment between 66 and 78 degrees Fahrenheit for an optimum performance. You should consult your supplier for mixing guidelines to control the viscosity of your coating.
In regards to drying your coating, it is generally recommended to maximize the airflow in the dryer rather than simply turning up the heat. Aqueous coatings dry by evaporation, and while heat accelerates evaporation, without proper airflow the surrounding air becomes saturated and moisture has nowhere to go. Excess heat in the dryer can cause problems like curling or in extreme cases can cause fire. As a general rule, it is recommended that coatings are dried at 95 degrees Fahrenheit on the first side and 90 degrees on the other side. Most manufacturers recommend that your heater never exceeds 105 degrees Fahrenheit. Adjusting the speed at which your press is running also has an impact on drying because of the simple fact that a slower sheet will spend more time inside the dryer. Work and Turn printing offers the best dry times.
When considering foil stamping, ink jet, laser printing, or ink over coating to name a few specialty applications, consult your supplier for specific information.