Ink & Ink Dispensers Optimising ink delivery By Tom Kerchiss of RK Print Coat Instruments. I
nks and coatings tend to top the list in the blame naming game when something goes awry and the job has to be reworked. If it so proves its nothing to do with the ink the usual retort is: Well, if it’s not the ink, it must be the substrate! Sometimes there is an element of truth. It’s both the ink and the substrate. For some reason not as yet determined they’re not getting on
It’s not unusual for substrates to differ to some degree, even from batch to batch and from reel to reel. Relatively simple tests can help determine how inks and substrates interact. Colour communication devices such as the FlexiProof 100 or FlexiProof UV and FlexiProof LED UV complete with integrated miniaturised UV lamp/curing system can be used to determine the stability of the inks, the substrate and other materials and to assess elements associated with printability. That is the gloss, the durability; scuff resistance, chemical resistance and flexibility. For an ink system to be effective the formulator must be sensitive to the needs of the printing process. Sometimes issues that arise in terms of quality are due to other sources, for instance insufficient time has been allowed for a full and thorough cure to take place in a roll stacked and awaiting a further converting process.
In this day and age the converter and others in the supply chain will turn to the ink supplier as a source of knowledge, for information on regulatory issues, technical support and for guidance. The ink manufacture must continue to remain innovative and be quick to respond to ensure that their inks remain compatible with an
ever-changing variety of substrates. Converters who print flexo might want inks formulated to work at higher densities with finer screens, at faster press speeds, and where possible, the inks and components have to be environmentally friendly. On top of this ink makers must provide it all at a competitive price.
Ink performance is, to a large degree, dependent on the sum of its parts. Take additives as an example. Additives, each of which optimises ink delivery include lubricants, surfactants, cross-linkers, adhesion promoters, anti-foam agents, anti-fungal agents, alkaline materials, thickening agents, oxidising agents and plasticizers.
Amongst the most common lubricants used in inks are PE and PP waxes, silicones, fatty acid amides, and fluorocarbon waxes such as tetrafluoroethylene. These lubricants differ in their effectiveness, rate of migration, and temperature resistance.
Inks often employ chemical cross-linking agents in the ink film. Cross-linking agents chemically link themselves in the binder, improving the binder’s resistance properties to chemicals. They also help to improve block resistance and encourage heat resistance. Adhesion promoters provide excellent adhesion characteristics but have to be controlled carefully to prevent problems such as odour or discoloration. Anti-foam agents are also used, albeit in minute quantities. Without anti-foam agents the printer/converter would be unable to print successfully. Biocides or anti-fungal are added to the ink as well to prevent the growth of microorganisms over an extended
period. Again, these are added in minute quantities.
Alkaline chemicals are added to dissolve some
of the resin binders, maintain pH, and keep the ink open. Thickening agents provide the necessary rheological properties, but it’s not unusual for the ink formulator to add a thickening agent if the process requires a higher application viscosity. The other additives for our water based flexo ink example are the oxidising agents and plasticizers. The former are included when a very high alkaline resistance is required, while the latter are used to improve the flexibility of an ink film. Each and every component of an ink including additives is subject to thorough testing for quality control purposes and to ensure compatibility with other component parts.
The appearance, the colour and density of the ink and much more besides are influenced by the substrate. Ink receptivity can be affected by the clay coating of a paper stock or by other ways in which it is finished. If paper stock fibres are well bedded in and interlocked the film of ink or other type of coating will probably be fine and of a desired thickness and smoothness. If on the other hand the fibres are oriented horizontally in a random fashion then print or coat quality will be less pleasing. In corrugated applications absorption and surface tension are two key factors influencing print density, drying, trapping and dot gain. Printing one sample on Kraft and other on mottled or clay coated will result in a wide range of shades from the same colour The ink manufacturer, the additive provider and indeed all others in the supply chain involved in bringing a quality product to market depend on quality control devices and procedures. Trusting to luck and trial and error are no longer options when it comes to controlling colour and other variables. Systems needs to be in place that can be used by all suppliers so that results can be documented and results duplicated.
The FlexiProof 100 and variants are essentially scaled down but component critically an exact version of a flexographic press, which can be used for pilot runs, the processing of unfamiliar product materials, product monitoring, colour matching and for, as mentioned earlier determining printability issues.
Xwww.rkprint.com
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May 2022
www.convertermag.com
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