Coding and marking is required in every industry regardless of what product is being assembled or manufactured. Continuous Inkjet (CIJ) printers offer several advantages over other coding technologies in applications such as food & beverage, dairy, pharmaceutical, and aerospace. One of the biggest advantages is the wide array of inks available which allow CIJs the ability to print on virtually any type of product or packaging substrate.
With the multitude of applications and substrates to print on, how does one narrow all those choices down to the right one? The first step is to identify the core elements:
What industry are you in?
Different industries may have unique requirements. For example, the Non-Direct Food Contact (n-DFC) needs of a food & beverage producer are not needed in the aerospace industry or MIL-SPEC applications where adherence and contrast specifications are the primary concern.
What happens to the product before, during, and after printing?
Are there any events within the processing of the item that may affect the printing? For example, coding on the dry fiberboard package of an electronic device may use a general-purpose ink. Conversely, if printing on a beverage can that has condensation—where the ink must penetrate the moisture and adhere to the aluminum—may need a distinct, fast-dry ink.
Does the product go through a retort process? Thermochromic ink is perfectly suited to be printed on pouches or metal cans beforehand. The code is printed as one color and changes to a different color (e.g., black to red) after a successful retort adding an inherent process and quality control mechanism.
What color is the substrate?
If the substrate is clear or light colored, the choice is easy. However, there are many different packaging colors such as a dark gray confectionary wrappers or amber vitamin bottles that the standard black or white may not offer the proper level of contrast. CIJ inks come in several other colors such as yellow, red, orange, green, gray, or blue.
Occasionally the code is not meant to be seen such as brand protection and anti-counterfeiting applications. In those instances, CIJ printers use a clear ink that is not visible without a specific wavelength UV light source.
What is the substrate being marked?
CIJ inks are formulated to print on many different substrates. They can vary from papers, plastics, and metals to glass, wood, and eggshells. (Yes, eggshells.) Some substrates are easier to print on than others. In other words, some surfaces are harder to adhere to than others.
Proper Ink Adhesion
Selecting the proper ink for the substrate is vital to proper adhesion and legibility. Some substrates may be more difficult to print on than others. Three factors that contribute to the adhesion are surface roughness, foreign substances, and surface energy.
- Surface roughness: The difference between non-porous and porous packaging is their ability to hold ink. Some substrates, such as foils, medical tubing, and blister packs are non-porous and may be more prone to print smudging than corrugated boxes and chipboard packaging. However, keep in mind, some porous materials may be coated with resin, varnish, or wax which could also make them more difficult to print.
- Foreign substances: Some substrates may become contaminated with dust, dirt, grease, water, or processing oils during processing. While some operations account for this and wash the substrate prior to printing, others do not and must be considered when determining the proper ink to use. CIJ printers have inks that have been specially formulated to cut through water, processing fluids, and other particulates.
- Surface Energy: Surface energy is the strength of a substrate’s molecular bond to itself and other materials measured in Dynes. Surface energy is a good indicator of how easily an ink will adhere to a surface; the higher the surface energy, the more likely it will readily accept the ink.
Surface Energy Explained
Surface energy is calculated using water as the base liquid. A water droplet applied to any surface will bead to some degree. If the surface energy were zero, the droplet would remain a perfect sphere. However, if the surface energy approached infinity, the droplet would become uniformly flat with the surface. While neither of those are practical, the reality is somewhere in between. Based on their surface energy level, substrates can be classified as high, medium, or low.
- High Surface Energy (300 – 1000+ Dynes/cm): These materials readily come to mind for their strength in our day-to-day lives. Glass and metals such as Zinc, Aluminum, and stainless steel have the strongest molecular bonds. Thus, their higher energy levels allow inks to stick to them effortlessly.
- Medium Surface Energy (40 – 300 Dynes/cm): Items in this category are typically natural materials such as concrete, stone, and wood, as well as many grades of plastics such as PVC, ABS, polyester, and nylon.
- Low Surface Energy (<40 Dynes/cm): Engineered materials such as polypropylene, polyethylene, and PTFE were designed to be low density, water/liquid resistance materials. Hence, what give them superior characteristics for their intended purposes also makes them more difficult to print on.
While inks have been developed for use on broad range of high, medium, and low surface energy products, there are some circumstances—materials with surface energy less than 36 dynes/cm—the material may need to be pretreated to permit coding. This is typically accomplished by either flame or plasma.
Formulating CIJ Inks
CIJ inks are comprised of five primary components: the base solvent (commonly Acetone or MEK), the polymers which provides basic ink performance, additives provide enhanced ink performance, colorants, and conductivity salt which is charged by the CIJ and allows proper ink droplet deflection.
Unfortunately, there is not one type of ink that works in every application. Chemists must balance the components’ concentrations depending on the specific characteristic trying to be achieved. Acetone-based inks may have faster dry times that MEK-based ones, however, MEK-based inks have a broader range of materials they work with. Additionally, dye colorants are soluble in solvents but are easier to migrate than pigmented colorants. Pigmented colorants, however, deliver better light fastness and opacity, but are not soluble in solvents and require occasional agitation.
Easier than it Sounds
While it may seem a daunting task to determine the proper ink for the application, it really isn’t that complicated. Determine the substrates that need coding - are they the same type of material, similar color, and are they all processed the same? Are some items heated while others are frozen? If there are variations, you may need a different color and/or solvent-based ink to meet the requirements.
Regardless, a sample should be run on every substrate used within a facility to ensure proper adhesion, contrast, and rub/scratch resistance is achieved. With the number of ink combinations Domino has, we can find solutions to all your printing demands. Contact your local Domino Account Manager, or connect with me on LinkedIn or email directly (eric.corzine@domino-na.com).
Also, have you ever wondered what it would be like working as a formulation chemist? Take a look at Domino Senior Chemist, Mariam Khalfey’s vlog.