Felsberg tackles Challenge No. 3—press speed versus ink drying speed. “Typically, as the speed of the press increases, ink drying speed needs to decrease. As air turbulence on the anilox and plate increases at a higher speed, more heat is generated and shear increases, causing the ink to dry at shorter intervals. To compensate for the higher air flow and higher heat generation, the drying speed of the inks needs to be reduced.”
Additional details that he voices indicate, “As with the anilox volume and ink drying speed, the most important thing to understand is that the ink needs to be formulated to match a given press speed. Most new presses are designed to run at roughly 1,600 fpm, with many approaching or exceeding 2,000 fpm.”
Even if the press speed matches the ink drying speed, Felsberg decrees, “Solubility can also affect the overall ink performance. As the ink is carried by the anilox and delivered to the substrate, the inks will dry. The key is using a solvent that allows the inks to resolubilize, and thereby not plug the anilox.” He tells package printers, “At Sun Chemical, we ask questions at the outset to achieve the optimal press speed by ensuring the proper ink formulation is used. Additionally, we sample inks at pre-determined time intervals and run a gas chromatograph to understand how solvency changes over time. We can then make pre-adjustments or recommendations on which thinning solvents to use throughout the process.”
Rahmeh offers similar observations relative to Challenge No. 4—solubility and stability. “To make an ink, resin is solubilized into a solvent, and then a pigment is dispersed in the resulting resin solution to attain the desired color. In this case, solubility refers to how well the resin is dissolved into the solvent. A variety of resins and solvents can be used in this process. For example, polyamide is a resin that is solubilized in alcohol, nitrocellulose is a resin that is solubilized in acetate and polyurethane can be solubilized in an alcohol/acetate blend. The accompanying solvents—alcohols, acetates and glycols—also have an effect on the drying speed of the ink.”
Cautionary advice comes into play here. Rahmeh says, “The challenge is that resins must be blended to obtain the specific desired properties of the ink, such as resistance, print speed, adhesion, gloss and more. Various resins have different solubility parameters that must be able to handle the mix of resins added to an ink. This is complicated even further because ink properties can change over time, due to factors such as drying speeds, so the right resin blends much be chosen.”
According to the technical team member, “Sun Chemical does pre-work and research and development to identify the optimal mix of resins and solvents when formulating a specific ink for a given application. For example, in the flexible packaging market, there are multiple film substrates that may be used and an ink must be able to perform with each.”
His promise: “To solve this, Sun Chemical collaborates with internal teams and suppliers to get the chemistry right up front. Using all the resins to obtain the desired properties of the ink, we then determine the required solubility to identify the appropriate mix of solvents.”
Finally, Felsberg addresses Challenge No. 5—plate ink interaction. “Solvent inks are applied to the printing plate directly from the anilox. Ideally, they are applied to the top of the plate surface with minimal impression, so the plate is not over-inked. Even though the inks are on the top of the plate, solvents in the ink can influence the overall properties of the plate, including swell and degradation. Swelling is when the plate expands and distorts, causing the dots holding the ink to grow in size or thickness.”
He claims that, “Alcohol has minimal influence on plates, and there’s no general limit to the amount that can be used.” Meanwhile, this expert says, “Some glycols can cause moderate issues and produce an interaction with the photopolymer to swell the plates. Some inks contain 80 percent to 90 percent glycol, but the thickness of the ink plays a major role in plate swelling, so the ink manufacturer must select glycols that minimize plate interaction.”
In addition, Felsberg maintains, “Acetates will cause plate swelling when used at high levels—15 percent or more acetate in the total solvent requires careful monitoring and 20 percent is the maximum level that can be tolerated before massive plate swelling occurs.”
He credits extensive research and development with allowing Sun Chemical to determine the appropriate mix of ink solvents needed to meet solubility requirements and avoid plate swelling and, of course, providing customers with the market leading ink solutions they’ve come to expect.
In conclusion, Rahmeh reiterates the opening point: “While each of these five variables can affect the success or failure of the print, the most important thing to understand is that each variable is linked. Each desired result can be attained by implementing careful consideration at the ink design stage.”
Color: The Critical Concern
“Run fast! Run clean!” Package printers around the globe sound the common refrain and Mark Lewis, label market manager, and his team of color experts at Wikoff Color Corp take it to heart and make it their mission by assisting in warding off color shift, ensuring press to proof matches, and accounting for press speed and its impact on color laydown.
In presenting their list of the five most common ink challenges faced by, and present in, nearly every pressroom, they offer a brief preamble. “It’s best to fingerprint press anilox rollers with a 1-color plate showing solid ink densities and screen values from 1 percent to 99 percent. The fingerprint results of each anilox roller needs to be recorded into a binder showing solid ink density and screen values of that roller.”
Elaborating on the point, the team stresses, “Prior to the press operator setting up the job, he or she should read densities and screen values of the sample or ink drawdowns with a proper spectrophotometer or densitometer. Then he or she should select the proper anilox to match density, color and screen values at startup and prior to signoff.”
After issuing the call for standard operating procedures, the discussion turns to situations and circumstances ripe for their application. According to the team, color shifting during the pressrun is something to target. Why? “Color can shift due to ink viscosity changing and not being properly maintained during the run.”
Similarly, they point to exactly what just might be up, when color does not match the sample or proof. “Color can have the right L*a*b* values, but may look off due to the wrong anilox selection.” They insist, “It’s important to read the density of the sample supplied and match the sample reading to select the correct bcm anilox prior to run.”
“Prior to the press operator setting up the job, he or she should read densities and screen values of the sample or ink drawdowns with a proper spectrophotometer or densitometer. Then he or she should select the proper anilox to match density, color and screen values at start-up and prior to signoff.”
On an obvious note, Wikoff’s experts list out fast checks to roll out when dealing with the challenge of ink spitting during a pressrun:
- The doctor blade needs a backer blade to prevent ink blow by
- Doctor blade is installed wrong
- If press has chill rolls, chiller may be set too low, causing ink to not flow out
In similar fashion, Lewis and the group maintain that when ink is misting, the likely culprit is readily apparent. “Anilox roller bcm is too heavy, with the doctor blade causing ink to mist.” They caution, “If a heavy coat weight/density is needed, it’s best to run a 60 percent screen of the same color, prior, and use a lighter bcm over the 60 percent screen to achieve proper coat weight and density. The 60 percent screen creates nice pockets, allowing the final coat of the same color to bond, too.”
The final challenge they comment on again involves anilox bcm. When color matches at idle, but not production speed, they issue specific advice: “Change your anilox bcm to match the color at your production speed. Do not adjust or tip your inks. By adjusting your ink to match speed, you are changing the ink L*a*b* values. It’s a density problem, not a color issue.”
Mitigate the Cause
“Way too many of our industry’s printers continue to use the press as a giant, expensive proofing device!” proclaims Ed Dedman of Zeller+Gmelin. “When you’re using the press to adjust color, you’re burning time that could be used running salable product.” That’s what prompts him to list color matching as perhaps the most persistent ink-related issue in package printing. He stands passionate about taking the necessary steps to attain an exact match.
Specifically, Dedman explains, “First and foremost is the challenge of getting ink to press that is matched correctly for the job conditions. Second is the amount of time needed to get the color to match. Third is the challenge of matching between jobs run with different ink types/chemistries.”
Launching into an analysis of each sub-challenge in matching color, he begins, “This first challenge is an issue across all aspects of flexographic printing—wide web, narrow web, corrugated, envelope, etc. I am amazed when I walk into so many flexo printers who don’t match/make/check the inks before sending them out to the press. If they don’t have an in-house inkroom, many operations don’t work closely enough with their ink suppliers to make sure they have the correct information needed—anilox volume, substrate, color target, color approval method, etc.—to assure the color is right when it’s received.”