When it comes to digital printing on packaging—corrugated, folding carton, flexible packaging—primers are an essential part of the process. Primers provide key benefits for end product quality and for manufacturing productivity by providing a better surface for ink adhesion and drying.
Primer is needed to enable adhesion between ink and substrate during inkjet printing, forming a strong linkage between the two. The primer also has in important role in controlling how the ink interacts with the substrate once the jetted drop of ink lands on it. The drop needs to spread to a certain size (dot gain). If dots are too large, the graphics will be blurry; too small, and it looks like not enough ink. Primer controls dot size while helping anchor and hold ink on the surface to maximize color. In this way primers can reduce the amount of ink needed to get the desired color density, maximizing the use of ink while enabling more photo quality for the graphic.
From a packaging manufacturing standpoint, primers facilitate drying in water-based systems and help improve rub resistance, which allows more packaging to be printed and stacked on an assembly line to maximize productivity.
When looking at primers, there a number of factors to consider and questions to ask:
- What is the substrate?
- The finish—gloss, matte or satin?
- The type of ink to be printed:
- Water-based or UV system?
- Dye, pigment or both?
- How will the primer be applied?
Primers should be as robust as possible to work across a broad variety of applications and substrates. In most cases, manufacturers would like to use the same primer in their printers. However, the number of types of paper and film make it difficult to make a primer which works on all substrates. Normally, a primer will be developed for a group of paper types or filmic materials.
In addition to the factors mentioned above, application method is another consideration because primers can be applied on-line/off-line via flexo/gravure or digital print methods. Packaging facilities print for a variety of customers who require a uniform look and feel for their packaging regardless of how it’s printed. Gloss substrate must maintain the consistent level of gloss whether printed digitally or via analog process. The same is true of the shade—a bright white coating must keep its white appearance.
In summary, the primer needs to deliver all of the desired print qualities but also adhesion, rub resistance, dot size, absorption, quality imaging, gloss, shade, etc.
Overcoming Even More Challenges
As if those challenges aren’t enough, primers must also be formulated for different inks. The most challenging scenario being water-based ink printing onto film. Water-based ink is 90-95% liquid, so it needs help to dry quickly to keep productivity up. Primers enable faster drying through absorption of the ink into the primer layer. Absorbent substrates have different challenges—drying alone isn’t enough—absorption needs to be controlled to keep ink from wicking. Wicking is where ink is absorbed without constraint resulting in vein-like striations in dense print areas.
The final task primers are asked to perform is to help minimize curl. When ink is applied on one side of a film or paper, it has a tendency to expand. As it dries, the ink tries to pull itself toward the side where it was coated, which can cause it to bow and not fit through printing press and lead to printer damage. Primer helps reduce this tendency to curl and thus helps to eliminate these potential problems.
Lubrizol has developed a variety of polymer technologies that are suited for use across a wide range of packaging and labeling applications and substrates. Lubrizol also has the ability to work with you to optimize primer formulations for a specific print system.
Contact us to learn more about our high quality primers for inkjet inks for digital printing in packaging.