Whether printing simple dates and time codes or complex ingredients lists, allergen warnings and traceability information, coding is a critical part of a packaging operation, consumer safety and product branding. Determining the ideal inks and ribbons to use for printing on a particular flexible packaging type should involve a basic understanding of your printer and support from an expert who can provide sample testing and application-specific solutions. A well-chosen ink or ribbon of good quality can optimize the adhesion, durability, and quality of printed codes, text and graphics.

For flexible packaging such as BOPP, PET, PE, PVC and metallized films, coding may be performed on the packaging line before or after the film is formed and the product is packaged. To help identify an ink or ribbon that will perform well on a particular flexible packaging material, it is important to consider the packaging conditions and environment:

  1. If there are any surface coatings or contaminants from the manufacturing process present on the product either before or after coding
  2. If there is any surface color variation and the requirement for visual or machine-readable code contrast
  3. The printer’s operating environment and the temperature extremes that the product will experience and that the code needs to endure
  4. Whether any components will come into contact with the ink code immediately after printing

Equipped with this information, an ink or ribbon specialist can help to quickly narrow down the selection of inks or ribbons to a handful of suitable options.

While various printing and marking technologies are available for flexible packaging, let’s take a look at two that are used throughout the flexible packaging industry due to their versatility and print clarity: continuous inkjet (CIJ) and thermal transfer overprinting (TTO).

CIJ printing and ink selection

Continuous inkjet (CIJ) printing is a versatile coding method that can produce clear, readable alphanumeric codes on the irregular and curved surfaces of flexible packaging including pouches, bags and packs.

Producing high-quality ink codes on these surfaces goes beyond the selection of a CIJ printer. Meeting specific code requirements for contrast, adhesion, solvent content, food grade, abrasion resistance and color can be achieved by selecting CIJ inks according to their properties and compatibility with the materials to be printed.

The ink code must be able to survive the production environment’s moisture, temperature and humidity conditions to minimize the risk of ink transfer and smudging. A code’s adhesion and legibility can also be impacted by material handling components, like belts and mechanical guides, other products, rollers, sealers or even human handling that may take place immediately after printing. Within a few seconds after printing, the code will need to resist smearing from these contact points.

Most CIJ inks feature a combination of special properties that address production conditions while offering the preferred color, solvent content, odor and dry time. From inks that penetrate thin layers of condensation and withstand the pasteurization process to high-contrast pigmented inks, standard and customized formulations are available. For example, an application that requires a fast-drying, heat-resistant code may be printed with an MEK-based (methylethylketone) or acetone-based ink, depending on how the printed material with be handled after printing and whether a certain color or contrast is needed.

An ink’s adhesion is the result of the physical bond of the ink to the substrate and is influenced by the ink’s chemical makeup. Various chemicals can be used as an ink’s solvent, including MEK. While MEK is not classified as a HAP (hazardous air pollutant) nor an ODC (ozone depleting chemical), local regulations, “green” requirements and preference may limit use of MEK-based inks. For instance, ethanol and ethyl acetate-based inks are often selected for odor-sensitive PE and nylon applications and meeting environmental objectives.

Since flexible packaging may be composed of laminated layers, it is important to choose an ink that is compatible with the material’s top layer. In addition, some flexible packaging substrates contain plasticizers that may bloom to the surface under certain conditions. The ink must be able to penetrate the plasticizer if it is present at the time of printing and be resistant to the plasticizer if it comes to the surface after printing.

Thermal transfer overprinting (TTO) technology and ribbon selection

TTO is a digital contact coding technology that uses heat and pressure to melt ink from a ribbon and transfer it to the substrate. TTO technology uses standard and specialized ribbons to print high-resolution codes onto thin, flexible packaging such as films, pouches and labels made of plastic or foil.

Installed directly onto packaging machinery and labeling systems, TTO systems can print information including ingredients, logos, marketing information, barcodes, lot numbers, traceability data, pricing and dates.

TTO ribbon quality impacts the life of the thermal transfer printhead, the quality of the code and ultimately the uptime of the packaging line. Cheaply constructed ribbons can cause printheads to wear out quickly, cause a static charge to build up, or provide weak thermal conduction for ink transfer, thereby degrading the print quality. However, many higher quality ribbons feature a non-abrasive coating that protects the printhead and optimizes thermal conduction for the consistent printing of excellent codes.

A ribbon’s composition, mainly its resin content, plays a significant role in the resulting code’s adhesion to flexible packaging materials. Two key TTO ribbon types, wax/resin mix and resin-based, are well-suited for printing on many flexible films like BOPP, PET and LLPE, because of the smudge and scratch resistance they provide.

Designed for general-purpose use, wax/resin mix ribbons are available in a variety of colors and provide good print quality at a range of speeds. Resin-based ribbons, often considered higher quality than wax/resin ribbons, perform better in smudge and scratch-resistance tests and achieve excellent results at higher temperatures, but run at slower speeds.

In addition to consideration of the production environment’s impact on ribbon performance, ribbon selection should be determined by ribbon performance features like speed, bar code definition, contrast, smudge and scratch-resistance and the desired color. Excellent print performance requires the optimum match between the printer, the material to be coded, and the selected thermal transfer ribbon. A good match helps ensure fewer ribbon breaks, better print quality, optimized adhesion and improved uptime performance.

Test and proven solutions

If you have experienced poor print performance, keep in mind that an ink or ribbon that worked yesterday might not work today due to a subtle change in a manufacturing process or environment. An ink or ribbon specialist can use their unique set of tools to help diagnose a problem and recommend solutions that can help get code performance back to an optimal state. When well-tested and proven inks and ribbons are specifically matched to the printer and production process, excellent print quality with fewer line stoppages are achievable.

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