As a printer in today’s complex marketplace, selecting an ink set for your customers’ applications can be a challenging activity. Requirement for the ink can be impacted by end-use regulation, customer branding demands, product marketing campaigns or application requirements.
Those mandates need to be balanced with a printer’s capabilities, substrate compatibility, cost and productivity targets.
Process colors, spot colors and specialized coatings required for one application can get multiplied by two or even three ink sets, when driven by intended use. The result can be a sizable amount of inventory. All of that inventory ties up working capital, as well as ever-critical storage space. In a perfect world, a single ink set would be capable of doing it all. However, while different ink technologies have overlap across several factors, each has its niche where the benefits shine brightest.
Differences in packaging applications have resulted in the development of various ink technologies. Here is just a sampling:
- A variable information label with a color block requires a low-cost ink with a relatively short life expectancy
- A durable label for a global engine manufacturer that must consistently represent the brand across multiple countries and withstand exposure to chemicals, fading and abrasions presents quite a different performance expectation
- A flexible package stand-up pouch for food with migration limitations, different laminations and the heat-sealing process also comes with prime chemistry considerations
Would it be possible to print these three applications with the same ink set? Yes, of course it would. However, is it the best use of technology or the most economical solution? Does it provide end users with exactly what they are looking for? Probably not.
Let’s break this down.
The single ink set for all three applications would be over engineered for each single application, but would be capable of working all of them. The ideal ink would drive up the cost and could lead to compromises on print speed or shelf life.
Contamination & Migration
UV-cured inks printed on flexible packaging applications have requirements that differ from labels. Most notably is the potential for contamination to the contents of the package. The printed flexible package substrate becomes the container for the packaged goods, and so the potential for contamination is higher than a label application where the printed material is adhered to the container.
Contamination from any ink technology can occur when the ink film is not fully cured and unreacted components of the ink transfer to other surfaces. The freshly printed surface can contact the contents surface of the packaging material when it is rewound on the press after printing. If the ink is improperly cured, contaminants can be deposited and remain on what ends up being the inside of the package when the material is unwound. This is considered offset contamination.
The other mechanism to contaminate the contents surface of the packaging material is when the components maneuver through the material. This is considered migration contamination. Both forms of contamination are impacted by the completeness of cure but also are heavily dependent on the chemical makeup of the ink and packaging material.
Specifically in UV inks, raw materials are selected to have lower mobility, often by using higher molecular weight building blocks. Higher molecular weight raw materials typically increase viscosity and can have trade-offs on press performance in terms of ease of use, print speed and stability over time. More recent developments include polymeric photoinitiators to reduce migration potential and monomers to reduce the overall ink viscosity. These developments have resulted in low-migration UV inks that perform on press similar to standard UV inks.
Labels for outdoor durable applications do not have migration limitations. However, the label may convey safety information that needs to be clearly legible for the life of, for example, a lawnmower engine, while being exposed to extreme environments. It can be required to use inks with fade-resistant pigments and an overlaminate film or overprint varnish for added protection.
The mechanics of an overlaminate film stress the adhesion of the ink to protect the label from abrasion and contaminants like oil, gas and other chemicals. The ink needs to be fully cured, have strong adhesion to the substrate and cohesion within the ink layers. The demands on the ink will cause some systems to fail or will increase the cost to build in the robustness, which may not be economical for all applications.
As e-commerce has grown, so has the demand to label parcels. Many of the labels are die cut blank, but some contain branding information, or color blocks to highlight specific information and create visual cues to aid in efficient sorting and distribution. Supplied in small rolls of diecut labels, variable information is printed on demand using primarily direct thermal or thermal transfer printers.
Due to the commodity nature of this label value stream, cost and productivity are major factors. When compared to the flexible package and durable label example, very specific requirements are designed into the ink, which drive up the cost. That cost is something that would not be economical for these types of parcel labels. Those ink systems have features that are not needed and for which the market would not pay.
UV vs LED
An important technology development that has helped increase the overlap across applications is the expansion of UV-LED lamps. These lamps are designed to provide an intense, focused energy source. That energy at a longer light wavelength results in a particularly efficient cure through the ink film. This benefit can improve cohesive strength in thicker ink films and anchorage on difficult-to-adhere-to substrates without the additives that can increase cost. UV-LED lamps exhibit a much more consistent output over their lifespan versus a traditional UV mercury vapor bulb lamp, resulting in process control robustness.
Let’s compare these characteristics to the applications previously mentioned. In the first example, migration and lamination are important. The intense, consistent-over-time output of a UV-LED lamp will drive the reaction more completely and more consistently from job to job and week to week. It is not to say that conventional UV lamps are not capable; rather, they can require more maintenance and monitoring to ensure consistent results. As through cure promotes ink adhesion and cohesion, laminations in flexible packaging and durable labels can stand up to more rigorous testing.
An advantage of UV-LED that cannot go overlooked is how it can impact productivity. In the e-commerce label example, an important factor in ink selection is cost. Cost of the ink material should not be the only expense considered to make the decision. The productivity gain using a UV-LED ink may offset the material cost if significant enough. In this case the total cost needs to be understood and evaluated to properly make the decision.
Despite how unique these use cases are, not every application requires an independent ink set. Core UV flexo inks are built to provide excellent print quality for prime labels, robust adhesion to a variety of film substrates, the flexibility for low shrink ratio sleeve labels, and lamination for flexible packages where contents are not regulated by migration limits.
It is important to know the requirements of the end use and consult your ink suppliers, so they can support you and get you prepared with the right ink. It is possible that what is currently on your shelf will work for the application. If your current standard ink does not meet the application requirements, it is possible something new has been developed to meet those needs, even with the capability of replacing what you have been using.