One of the great challenges that flexographic printing presents is to obtain an excellent transfer of color in the final result, it is a rigid point in the giant universe of flexographic printing, since the quality of the prints we obtain depends on it. The inks of the future should provide more color power and be more soluble to obtain clearer prints. As Krstuli says. “Tomorrow’s inks will have to be stronger and have more solubility for cleaner printing, have better press stability and they will need to have better acceptance and release the anilox roll, printing plate and substrates used.”
Currently on the market, we are going to find various types of inks and formulations, with a common denominator based on the transfer of color pigments. Ink manufacturers seek to increase the pigment load as much as possible, minimizing the size of the particle to its maximum expression, seeking to set the resistance to breakage and gloss. Their evolution has been very fast, always focused on them being more organic and causing less impact on the environment, which means that when used they are more delicate and require knowledge prior to handling.
The ink is basically composed mostly of organic pigments such as carbon (black), diarylide (yellow), pyrazolene (orange / red), disazo (yellowish green, red, orange), naphthol (green, blue), quinacridone (Red). And for inorganic pigments we have Titanium dioxide (white), iron oxides (yellow, red, brown), metallic (aluminum) (silver, gold), clay (green, red, yellow, brown). There are also inks on the market that contain natural or synthetic resins, the most widely used being those formulated with polyamides, nitrocellulose, polyurethane and water-soluble ones; solvents, where we have the presence of alcohols, esters, hydrocarbons, ethers, glycols and ketones; And varnish, known as the pigment vehicle, results from the dissolution of resins with solvents.
We must bear in mind that the selection of the ink will depend on the type of printing machine or the substrate to be printed and its formulation, so we can classify the Most Common Inks into three groups:
Water Based Inks: As its name indicates, its dominant component is water, which evaporates during the printing process, which is why they are also called ecological inks, they have the characteristic of slow drying, so their PH levels must be constantly monitored, as easily being out of range could cause poor quality prints. A neutral PH would be between 0 – 7 and very alkane would be 7 – 14.
Another important characteristic of water-based inks, is their sensitivity to sunlight and direct exposure could discolor the printing, contact with moisture is not recommended as it could dilute the pigments already printed, for this reason they require the application of varnishes.
Solvent Based Inks: Unlike the previous one, these inks are composed of petrochemical solvents in their formulation, the stronger the solvent the ability to adhere the pigment to the substrate is greater, since they tear or sensitize the printing surface which consequently a better adherence is achieved. Its formulation makes it resistant to humidity, since they are not soluble in the presence of water, in addition to them they are resistant to sunlight or exteriors and as if that were not enough, they do not require varnishes or laminates.
The most important disadvantage of this type of inks is that they evaporate at the time of drying and can present a risk to the human resource that operates the machine, so it is recommended to have good chemical gas extraction systems.
UV inks (Ultraviolet or Monomeric): Their main characteristic is that they are inks cured in UV radiation by special lamps inserted in the printing machine, their base does not evaporate and polymerizes on the substrate, leaving a very good quality impression. Unlike solvent-based inks, UV inks do not penetrate into the material and their drying is immediate, their disadvantage is when printing very flexible materials that have thick thicknesses, since they can run the risk of cracking and can release particles.
Another very important point is based on the Viscosity of the ink and its measurement, so we have:
Gravity: It is the driving force that causes a liquid in a viscosity cup to flow through an orifice, causing a high-density material to flow in less time than a low-density material of the same viscosity.
Stoke: We can define it as the poise divided by the density; 100 centistokes = 1 stoke. Measurements are made in different glasses, the most common being the zahn glass.
The Centistoke: That it is a reference unit in all measuring cups, and Seconds (time) is the measure that the operator of a printer handles.
Poise: It is the fundamental unit of viscosity defined as the resistance to flow of a liquid where gravity is not a factor; 100 centipose = 1 poise.
It is also important that we address the types of preparation of the substrate to be printed, since the porosity of the surface of the substrate will be decisive in the printing quality to be obtained. Non-porous surfaces are found in most plastics, such as polypropylene, polyethylene and polyester, chemically inert and have a low surface tension which causes that this type of surface is not receptive to anchor printing inks, coating and adhesives. Therefore, the treatment that is given to them will increase their adhesion for printing, lamination and any other conversion process. Clarified this we have:
Corona Treatment: Substrates exposed to this type of treatment are exposed to ionized air in order to increase the surface tension.
Plasma Treatment: This acts in the same way as corona in terms of electrical ionization of a gas, only that it differs by using a lower voltage, producing controlled chemical reactions to make the surface more controllable.
Flame Treatment: This type of treatment is given by burning and consists precisely in burning of surface contaminants by means of a light gas mixture.
In the market there are numerous companies that are dedicated to the manufacture of inks for flexographic printing, whether they are water-based inks, UV inks or solvent-based inks tale, here are some:
As can be seen, there are inks for all kinds of needs, the selection will depend on multiple factors, printer machine, substrate, personal taste of the printer, but all in turn have a common denominator and that is to obtain “prints with excellent color transfer”.