Economics of Screen Ink and Color Control
Written October 17, 2019
This article is intended to focus on screen printing ink and how the many variables in application create problems in the duplication of color and the basic rules/regulations that should be followed to assure color consistency and cost efficiency. When evaluating a new screen printing ink series, printers often get excited when the new product seemingly will offer improved performance or cost savings, however, they have second thoughts when they look at their present ink inventory. It is in the ink room that the printer reveals his tremendous inventory, followed by a statement, “I have too much ink now, how can I possibly look at taking on another line?”
Now we are all aware of the need to better control our inventory, however, here we need to define this customer’s inventory. Looking through the shelves, one usually does not see a lot of standard colors, i.e., red, blue, black and white. What one does see are many partial gallons of special colors or special products. Furthermore, most of these containers have been setting on the shelves for several years. Oh yes, the customer says, “One day we will use those colors up. I just can’t throw them away.” And under my breath I say, “Sure you will, and also save the company a lot of money.” It is the leftover, odd lots of unused products—that is where your ink costs are.
The cost of unused ink further escalates when you now have to pay to have it properly disposed. To better understand the ink and what leads to some of the variables that create excessive time and money in color control, let’s take a moment to review basic chemistry in screen inks. Most screen printing inks are manufactured from resins that are generically known to most of us, i.e., nitrocellulose, ethylcellulose, acrylic, vinyl, polyester, styrene, alkyd and epoxy, just to name a few.
These resins are made into printable viscosities with additives of solvents, monomers or water and other ingredients. To this mass are added the pigments, which are usually ground in by means of a three-roll mill to assure uniform pigment particle size and smoothness of the printed color. Only a few years ago, most screen printing inks were manufactured with between 25% and 40% solids.
This means that 25% to 40% of the formula consisted of resin and pigment, while the balance of the formula was solvent or volatile compounds. After the ink was printed, you ended up with a dry film thickness of between 25% to 40% of what you originally deposited. The rest of the formula (solvent and/or volatile) went up into the air. Many ink formulations today are manufactured with higher solids content in an attempt to decrease the amount of expendable and volatile solvents, increase the screen stability of the ink in the printing mode and allow the use of finer meshes, which are capable of greater mileage figures, and to enhance the print quality further.
To further illustrate variables in color control, look at the 100% solids UV ink. If the manufacturer claims 78 to 86 square meters per liter through a 154 threads/cm mesh, you can see this would be approximately 13 microns (0.5 mil) of deposit. With the variables in the process as described, it is easy to deposit 20 microns (0.8 mil) of deposit through the same 154 threads/cm 390 threads/inch) mesh. This not only poses increased ink cost with less yield, it will dramatically change the color. The higher solids formulations are usually more expensive because the solvents have been replaced with more expensive resins and other additives.
Although these higher solids yield greater coverage per gallon (or per liter), they are much more costly to have in inventory as unused or scrap ink. Now let’s look at the variables in the application of screen inks that create problems and excessive cost. With all of the improvements in the printing process, including the inks, why does one continually hear of problems in controlling color and overall cost of ink? Go back to the lower solids of a few years ago and look at the meshes these colors were screened through.
They range from 61 to 91 threads/cm (175 to 230 threads/inch) mesh or equivalent. The inks were lower solids, but had a fairly high pigment load using the old leaded colors that demonstrated excellent opacity. With these deposits of ink and/or amount of wet thickness laid down, the print could tolerate a lot of variables, such as poor squeegee, uneven printed surface and/or poor grind in the ink. Remember, these colors were usually not the clean bright shades that we see in demand today. Those were the days when we could print yellow on black or orange over blue and not show a trap.
Today we are using meshes between 102 and 185 threads/cm (260 and 470 threads/inch) requiring a better quality ink and allowing less tolerance in variables to achieve a quality lay-down of color. With the combination of finer meshes and cleaner colors comes less opacity, therefore personifying all the variables that relate to poor squeegees, non uniform printing surfaces, poor substrates, etc.
With all the variables in substrates, types of printing, equipment, etc., the manufacturer has a difficult time trying to adjust the ink for application. It is the ink manufacturer’s responsibility to produce a certain formula or color as close to consistency as he (or a color machine) will allow from batch-to-batch. It is not uncommon for the ink manufacturer to match a color through a 102 threads/cm (260 threads/inch) mesh with optimum print techniques yielding 7.6 micron (mm) (0.3 mil) and learn that the customer finds the color too dark.
This is usually a case of laying down more ink in a production mode using the same 102 threads/cm (260 threads/inch) mesh. Remember, an ink film thickness of 12.7 μm (0.5 mil) with a clean color will not match the same color at a deposit of 7.6 μm (0.3 mil). It is up to the screen printer to adjust either the color or the process to arrive at the end result desired. The first step in addressing color control is the proper selection of ink formulation. There are obviously many formulations to select from, several of which will, under certain conditions, work on the substrate you select.
The ink manufacturer will certainly assist in the type of ink, recommended meshes, viscosity parameters and dry times; however, it is the printer that makes the determination of ink types and handling characteristics, based on specific needs. Once a given formulation has been selected and approved, you now must look at specific mesh, optimum viscosity, squeegee types and coverage, or yield, of this ink. Before consistency of color, cost or usage can be plugged in, you will have to work with a few jobs and keep specific records. You are looking for:
- Through a specific mesh—What is the best viscosity on a given printing press?
- With proper viscosity—What is the square meter per liter (square foot per gallon) or equivalent yield?
- How can I get consistency of color with these facts?
If you have three different types or sizes of presses, all printing the same ink, with the same mesh, on the same stock, then you have a good chance of getting three different shades of color. When using the same mesh for a small printing area on a small press and a large print area on a large format press, you will find that the larger press or printing area will deposit more ink, therefore changing the color. This is due to the larger area or mass of ink being laid down (more ink by volume in the screen for proper flood and print) and the fact that there will be more flood time which will increase the amount of deposit.
The ink manufacturer’s most difficult task in matching a special color is guessing that you will get the same amount of ink through the mesh (film thickness) as was done in the laboratory. If that differs, the color will change, especially a translucent or transparent color. Remember, the higher the solids content of the ink and the finer the mesh, the more any variables will show up and the faster the color will shift. For an example, normally a 154 to 180 threads/cm (390 to 455 threads/inch) mesh is recommended for general purpose ultraviolet curable inks (100% solids). It is not uncommon to see film thickness vary 300% through the same 390 mesh.
A translucent or transparent color with that kind of film thickness variable will change the color dramatically. The following steps will assist you in controlling color from the ink mixing room through to production. All color matching should be done with the use of a good set of scales. As the weight per gallon (liter) varies in all colors, you can never be accurate or consistent with formulas blending by volume. With the higher solid inks of today and their respective cost, you want to be as proficient as possible and only use the amount of ink necessary to print the job. Start color matching using a small gram scale.
Using a small non-waxed, clean container or paper cup, weigh out a few grams of color at a time. Let’s say for example a special corporate blue will require some ultra blue, white and yellow. If after three adds of color you see that the color is drifting off, throw the cup and ink away and start again. You always want to keep the formula as simple as possible, hopefully not over three pigments or colors. Never start with large amounts of ink when matching colors, as you will probably end up with more ink than you can use or even work off in the production batch of another color.
To proof the color, always use a clean screen, stretched properly and the same durometer and type of squeegee you will be using to produce the color on the press. The proof screen should have a large enough opening to allow proper viewing. Remember that the color produced by the proof screen in the lab must correspond to the production set-up on the press. (Remember also that very often a small print area will not necessarily display the same deposit or color as a larger print area on a large press.) You may find that a coarser mesh should be used in the proof screen to duplicate the production color through the production mesh. Some of this will be initial trial and error until you have arrived at the proper combinations.
You will not learn much if you do not have good formulas and keep good records. 1. With a specific mesh and on a certain press, the proper mechanics of the squeegee* press and ink viscosity must be determined. The selection of the squeegee is important not only for its sharpness, but its overall resistance to a specific ink formulation for consistent printing. The press must obviously be set to allow uniform flood control and print quality. Some trial and error may be required to achieve optimum settings. To achieve proper print viscosity along with the mechanics, the ink must be at proper viscosity.
Proper viscosity on a given machine will probably vary from one ink line to another. The dictionary defines viscosity as the property of fluids that causes them not to flow easily because of friction of their molecules. All screen inks are manufactured with certain viscosity standards, however, both the viscosity in the container and the effective printing viscosity will vary considerably in many formulations. Some inks may print very well as they are packaged in the container, while others may require the addition of up to twenty-five percent (25%) or more of a viscosity modifier.
The ink manufacturer should offer suggestions as to proper print viscosities with a given formulation using a given mesh on a certain type of printing press. When a modification is necessary, it is usually that of reducing the viscosity with the addition of reducers. These low molecular weight (thin) reducers are petrochemical solvents in solvent based inks, monomers in UV inks and water in water-based inks. There are occasions when the viscosity needs to be increased (heavier body) for fine reverse printing. This is done with additions of body agents, sharp printing compounds or heavy body bases. Now, how do we determine the proper viscosity on a particular ink/press combination? Go back to the manufacturer and ask for his suggestions for a proper starting point for viscosity.
Let’s say the manufacturer recommends a 15% reduction with thinner for a specific solvent based ink. This reduction should be done by weight and not by volume. After trying this on your particular press, the results indicate a non-uniform finish of the ink; then you may need to reduce the color by another 5%. With any ink, proper color will not be developed until the ink has completely flowed out to a smooth uniform finish. A color that displays bubbles, craters or orange peel will not display the proper color shade or density.
If, after re-printing the color with the addition of another 5% (total 20% reduction), you are getting the results desired, it is then safe to assume that the formula or viscosity is correct. The ink manufacturer’s responsibility is to manufacture a given color in a given series of ink with a consistent viscosity, therefore allowing you to use a consistent 25% solvent reduction to achieve repeatability in production with this specific color. To assure that you are going to press with the same repeatable viscosity, you may choose to control or check the actual viscosity before going to press.
The ink manufacturer uses a viscometer that will yield a certain centipoid reading. These are obtainable, however rather expensive. You may option for a less expensive method employing a Zahn cup. This is a stainless steel cup with a hole in the bottom attached to a long handle. After you have reduced the ink the 20%, you lower the cup into the ink. With the use of stop watch, pull the cup out of the ink. Count how many seconds it takes for the ink to run completely out of the hole. If this is, let’s say, 24 seconds, all of your subsequent colors may be adjusted in viscosity to achieve the 24 second time. This method is a good guide as some colors may need more variations to achieve proper print characteristics.
There are also the variables in the type of print, large open area with a small reverse copy, halftone dots and/or combinations of any of the above. Again, the ink manufacturer will supply you with the viscosity readings of a given ink series and offer suggestions on how to control viscosity or recommend the best way to modify viscosity where needed for a particular production situation.
Now that you have determined the proper mechanics and ink viscosity for producing a fully developed color on the press, take the production color into the ink room and see if the color matches through a small color match screen of 102 threads/cm (260 threads/inch) mesh. For this exercise let’s assume they match. 2. Color Matching — Now you need a way of color matching that you can reproduce the first time on the production press. We will say, for example, the production job will require a 102 threads/cm mesh screen.
Let’s assume that you have to match a particular corporate blue. You say, “No problem, we match colors by eye all the time, in fact, we have a man in color matching that will get it dead on.” Now, this may be true, but, how much time was spent in actual color matching; how much press down time in getting it okayed and then how much ink is left over (with no formula)? Now let’s match the color:
Converting this to the percentages would be as follows: 40 grams Ultra Blue = 78.43 percent (of 51) 10 grams White = 19.60 percent (of 51) 1 gram Yellow = 1.96 percent (of 51) Now the formula is broken down into parts. Looking back at our results from step one, we conclude that the production run on a particular press will utilize a 102 threads/cm monofilament screen. Remember, we determined this mesh when a given formulation of ink at a given viscosity performed to our expectations.
Between the ink manufacturer’s recommendations concerning coverage and some history of actual usage of the particular ink line, you should know approximately how many square feet per gallon (square meters per liter) that you will yield using a 102threads/cm screen on a particular press. By looking at the artwork, you should he able to approximate the total coverage of this particular “corporate blue” per sheet. Multiply this figure by the number of sheets to be printed, to give a total usage of blue. Knowing also approximately how many square feet per gallon (square meters per liter) we will get from this particular ink line on a given press.
We should be able to arrive at the amount of “corporate blue” needed for the job. This series of ink through a 102 threads/cm mesh screen will yield approximately 39 square meters/liter (1600 square feet per gallon). The job calls for 2500 sheets (69 cm x 102 cm stock) with a print area of 64 cm x 97 cm 25” x 38”. Formula: Sheet Size: 27" x 40" ( or 69 cm x 102 cm) Print Area: 25 x 38 = 950 sq. inches per sheet (64 x 97 = 613 sq. centimeters per sheet) If we use an average coverage of this particular ink at 39 square meters/liter (1600 sq. feet/gallon), we end up with a need for 39 liters or 10.3 gallons. Our lab formula now reads: Ultra Blue 78.43 percent of the total formula White 19.60 percent of the total formula Yellow 1.96 percent of the total formula Using 46.8 kg (103 lb.) of total color then the formula will be: Let’s assume the job has now been completed to everyone’s expectations.
There are about 1.89 liters (1/2 gallon) of ink left over which will be put into a well-sealed container with the proper label attached. You will also maintain a printed sample of this blue along with the original formula. This is placed in the proper customer file for future reference. I also recommend keeping another sample of this color in a color file. This will go in the “blue section” according to shade.
This can be used for quick formula reference when matching another shade of blue. If you are lucky, the customer will re-order this job, allowing you to use the leftover blue. If, however, you do not get the re-order in a reasonable period of time, you will still have the color chip and formula. It is much easier to use leftover color in the formulation of another color when you know what colors were used, and in what percentages. In the future, there will be changes in both the inks and materials we print on.
For every new synthetic material to print on, there will most likely be a new ink formulation with which to print. Some of these changes in technology and chemistry may necessitate some change in application techniques. For us to better handle and control the finished product, we must learn to control each step through the process. There are variables, so let us learn to recognize and control them.
Most ink manufacturers are controlling color as it is manufactured with the aid of a color computer. These same color computers can look at a color with their color scanners and give you the formula for making production, provided you have a program that is based on the given ink system. However, the always present variables will continually be there, which is the actual film thickness or deposit of color.
Today we print with colors that are clean and bright and are usually more transparent; as well as higher solids content inks which are printed through finer meshes. All of these conditions necessitate better quality control throughout the process to maintain consistent uniformity of color. The screen printing ink is a unique blend of chemistry designed specifically for applications through finely woven fabrics with the use of a squeegee. Each manufacturer’s ink series is formulated to render specific performance characteristics on a given substrate(s). Screen ink is expensive when compared to house paint, however, it is much more cost effective in both performance and yield, if used properly.
Proper steps and procedures in assuring control in color matching and production printing will result in improved efficiency in both ink cost and costly down-time in production. A perfect color match is all for naught if you cannot effectively reproduce it in a production mode. The cost of the ink is usually a small part of the finished piece if you effectively use it—all of it. The economics of ink and effective consistency of color control are totally dependent on implementing proper techniques and handling procedures related to its use. * See the Technical Guidebook article, “The Squeegee, Our Basic Tool.”