Inks and Substrate Performance Perfect
By Tom Kerchiss, RK Print Coat Instruments
What would the label and packaging printer like from their ink supplier? To begin with they’d like an ink that runs at faster press speeds; they’d like an ink that adheres to the substrate, to filmic materials, laminates, cartons, pressure sensitive materials. They’d like to be able to use ink that runs at varying speeds without losing colour strength; ink that is weather resistant, rub resistant and chemical resistant. Some might say that their customers are interested in inks that are of any colour as long as they are ‘green,’ that is that they should have minimal environmental impact and that there are no components, no chemistries that can leach out after packaging disposal. Still other would say that there should be no risk of inks or coating components migrating from the pack into the food or other item contained within the pack.
Although ink formulators and component producers pull out all the stops to make flexographic and other ink systems as versatile as possible the fact remains that everyone prints under different conditions and for different purposes. Ink adjustment and customisation is ongoing as there are so many variables to contend with. That’s not to say that progress hasn’t been made. Ink producers now formulate inks that work at higher densities with finer screens and which are increasingly able to run on new generation flexographic presses that are capable of speeds of as much as 600 metres or more per minute. The development and manufacture of inks has also taken a different turn with inks formulated for LED UV cure and speciality inks such as thermo chromic and photo chromic inks which require heat and light for colour optimisation.
Printing for labels and packaging is often far from straight forward. Some issues can be resolved on press; others require the assistance of the ink supplier and other supply chain providers including the substrate supplier and manufacturer, etc. Close cooperation between ink producers, additive suppliers, paper, film, foil manufactures and agents and the printer/converter are essential. Quality control, colour communication devices, proofing and systems and equipment have a role to play in helping to identify the source of a problem and assist in determining a course of action.
Inks need to adhere to a wide range of substrates with differing surface structures. These surfaces may be porous or non-porous. Some surfaces must be made ink receptive such as film, but even this does not always go as expected. Films may have different types of lubricants or varying levels of a lubricant that can affect ink performance. It is imperative therefore that ink and substrate compatibility is evaluated prior to processing. RK Print Coat Instruments FlexiProof 100, FlexiProof UV and FlexiProof LED UV can be used to determine how an ink and substrate interact over time and also how the component parts of the ink, additives and different resins, etc., perform.
Printers and converters have to deal with many ink/substrate related issues. Filmic substrates such as polyethylene incorporate slip agents. While slip agents do an admirable job of modifying the co-efficient of friction, if they migrate to the surface of the substrate the greasy nature of these substances can result in ink adhesion difficulties and print defects. Some inks may also contain slip additives which when combined with the filmic slip additives softens the ink and increases the tendency for blocking to occur. An additional complication is that when combined the slip additives adversely affect the co-efficient of friction.
Corona treatment of the film on press may burn off the excess additive, provide for better wettability and allow the ink a better chance of adhering to the substrate. There are other options though. For instance: printers working in conjunction with the ink supplier may instead opt for an alternative ink system to resolve the problem. Another option, though one that’s not often up for consideration is to switch to a substrate.
Consistency of colour is essential, but one must remember that many factors including the chosen substrate can make it difficult to meet colour targets. When we consider colour, how it appears, we’re looking at more than transparency, opacity and gloss, we’re looking at the subtleties: hue or shade, the strength or saturation (chroma) of a colour along with how light or dark a colour appears. With certain inks even the size and shape of pigments can impact on how a colour looks. The amount of pigment used can affect colour strength, while the colour of the substrate and the drying or absorption properties also impact on print quality. The pigment is essentially any particulate solid, coloured, black, white, fluorescent, etc., that is able to alter the appearance of a subject material or object by the process of selective absorption and/or a scattering of light.
Let’s look at a quality issue associated with pigments. Ink density correlates closely to the percentage of pigment contained in the ink film. This pigment loading accounts for most of the density, with, taking paper as an example, absorption playing a lesser role. Ink film thickness rather than ink density can determine the gloss appearance of the ink. Two ink films printed to the same ink thickness but with different levels of pigment loading will produce the same level of gloss but will differ in density. How does this concern the printer in a production environment? Well, if a printer comes up against a rough paper stock, they could try to achieve a smooth and full-bodied appearance by reducing the pigment loading for the ink. Film thickness will be increased and appropriate density levels can be obtained.
With paper-based substrates ink receptivity can be affected by a papers clay coating or by other ways in which it is finished. If the fibres are well interlinked and bedded thoroughly within the base stock, the more likely the ink or other coating flow will be smooth and desired thickness levels will be easily obtainable. Conversely the risk of ink and coating inconsistencies will be greater when fibres are oriented horizontally or are randomly fashioned.
When providing a customer with a colour match it is important to use the same substrate that will be used during a commercial run. Printing on Kraft or on mottled sheets that may from the viewers perspective seem to have the same surface appearance may well once printed yield a wide range of shades using the same coloured ink. The reason for this is that substrates with a high level of absorption will exhibit a weaker looking colour as the colour dives into the structure. On the other hand, a lower level of absorption will result in stronger colour reproduction. It’s worth noting that with clay coated papers some inks may not even penetrate into the surface.
Colour matching, determining printability on differing substrates, resolving ink and substrate related issues and assessing the merit or demerit of an ink system; trialling new formulations and running a job that ran well with one grade of material but refuses to do so with a new batch of purportedly the same material can be costly in terms of waste, is time consuming and can result in missed deadlines, excessive production machine downtime and can be a drain on profitability. Devices that enable users to determine printability issues such as gloss, flexibility, rub resistance, chemical resistance; devices that can be used by printers and converters and by ink and substrate producers and by the formulators of additives, resins and other consumable elements to identify issues and bring order to processing are becoming well established in many sectors of production including flexography.
One such device for all flexographic pre-press applications is the FlexiProof. The FlexiProof functions effectively the same as a conventional flexographic press. The device can be used for a variety of procedures from batch testing to colour matching and from trialling new or unfamiliar inks and materials to assessing gloss, durability, rub resistance, chemical resistance and other printability elements. The FlexiProof UV and FlexiProof LED UV differ from the original FlexiProof 100 in that they are equipped with a miniaturised UV curing system so that printing/proofing can be undertaken seamlessly and without the problem typically associated with proofing UV products and that is detecting pinholes and many other blemishes.
RK Print Coat Instruments Ltd
Litlington, Royston, Hertfordshire SG8 0QZ
www.rkprint.com sales@rkprint.com
Source: RK Print Coat Instruments Ltd
