Prepress Printability Assurance
By Tom Kerchiss, RK Print Coat Instruments
The term printability is frequently banded about but it’s not as easy to define as one would think. How you interpret it depends very much upon whom you ask. Perhaps one definition would be to say that its associated with the ease with which ink is transferred in an even layer over the total image carrier so that a faithful reproduction of the image is obtained without any inconsistencies or blemishes that reduce image quality. But printability is associated with much more than this; it’s the ability to produce a commercially saleable product; one that looks goods, meets the expectations of the print buyer and complies with health, safety and environmental requirements.
If you were to ask a paper producer, they may say that printability is how well the paper performs when going through the various zones of a printing machine. Variables include mottling, random, patterned or stripes, which is sometimes due to unevenly coated paper or board. Others may cite porosity or lack of it in the material to be printed on as of concern. Printability issues are occasionally directly or often indirectly related to material properties: gloss, smoothness, whiteness and opacity.
Unevenness of print density, colour and pick resistance; uniformity of colour; uniformity of ink transfer, rate of setting and drying may be mentioned by ink producers and printers and converters when the subject of what can affect printability arises.
The ink technologist and others may agree with much of what has gone before but might add that material topography, compressibility, ink receptivity and hold out; ink vehicle absorption and dimensional stability to some extent determined printability.
Physical and chemical inconsistencies many affect printability. If the rheological properties of ink are not balanced correctly issues such as ghosting can affect printability and quality. Too low a viscosity and tack may be at the root of the problem. Inks with poor flow or insufficient wetting can produce a ghost image. In this instance a faint unwanted image appears in a printed area as a result of ink starvation on the transfer roll from the previous impression.
In flexo printing the properties of the ink, that is the viscosity, chemistry and/or solvent blend together with mechanical and processing properties sometimes result in ghosting. Arguably the combined effects of high press speed, ink distribution in the chambered doctor blade, fluidity of the ink or the inability of the anilox cell to achieve the optimum level of fill are contributing factors.
Surface energy instability, pH and disturbances associated with the humidity, temperature, dust and on- machine processing conditions influence how well a job is printed. A good example of how on machine processing conditions impacts on quality and speed of delivery is inadequate or inappropriate tension control that can cause registration problems, an inability to run at speeds and much more besides.
Advancements in colour matching, ink technology and complimentary technology now make it far easier, or so the theory goes to accurately achieve colour and printability targets. In the real world the reality is somewhat different.
Many variables affect the printer or converter’s ability to reproduce colour accurately and blemish free. Sometimes the stock that’s being printed upon has a bearing on how colour, graphics and text appear. The absorptivity of the stock can vary not only amongst material of the same group or class of materials but also from different batches within the same stock of materials bought at the same time from the same producer or supplier. Variations can affect the gloss of the ink film by letting more or less of the vehicle portions of the ink film penetrate into the stock. The more the vehicle dives into the stock, the less varnish there is available for the top of the printed ink film. The overall result is that gloss is reduced.
A factor that should be taken into account but sometimes doesn’t get the attention it deserves is lighting conditions. Colour judgement is affected by the quality of the light and the manner in which different people perceive light/colour. A lighting box or area shielded from ambient light but illuminated by a light source conformable to ISO 3664 is generally recommended. The immediate background should be neutral in colour and capable of reflecting 20 percent of light that falls on it.
Meeting the colour standards set out by a brand owner and print or packaging buyer can be challenging. Colour can vary during the course of a run for many reasons; colour can appear too strong as a result of selecting the wrong anilox cylinder; poor set up of the doctor blade and too higher ink viscosity. The opposite can occur, a flexor colour can appear weak or washed out. Again, this could be due to an incorrectly specified anilox; a worn anilox and plugged cells, etc.
Today it is possible to approach and manage colour in many ways. Various digital sampling technologies provide the means whereby colour can be created digitally, but this is not without its problems.
Digitised information and the way it is displayed on screen depends upon careful calibration; moreover, the print process (and as already outlined) the material to be printed upon, the ink, the way in which the ink or coating interacts with the substrate and even processing components, the plates, anilox rolls, etc., can influence colour representation, resulting in high levels of waste, reworking, product rejection and the evoking of penalty clauses when deadlines are missed.
Even where colour measurement is employed the colour image and other visual elements must be considered in their entirety. Numerical representation does not necessarily impart any meaningful notion to an individual of what colour is about, how it reacts with other colours and if it ticks all the right boxes on a subconscious or emotional level. It’s what we perceive as a person, as an individual that counts.
Much more than a colour matching device the pre-press FlexiProof 100 and variants FlexiProof UV and FlexiProof LED UV enables printers, converters, ink producers, coating and substrate manufactures, or others connected in some way with the flexographic process to run product trials, test new formulations, determine printability issues such as gloss, scuff resistance, rub resistance and, durability and to resolve the many issues associated with colour matching and with the compatibility of inks/substrates and consumables/flexo components over time.
The compact, bench top FlexiProof incorporates flexo press critical performance components such as doctor blade and anilox roller provides many benefits not least of which is that by conducting printability tests away from the production machine, press downtime is reduced; waste is minimised and production machine uptime and output is maximised.
The FlexiProof UV and FlexiProof LED UV are equipped with an integral miniaturised UV system allowing for printing/proofing and curing to be undertaken seamlessly and much more accurately than was previously possible. In the past, and because a print had to be taken to an off line UV conveyor, chemical changes occurred that made it impossible to detect pin holes and other defects.
The latest FlexiProof LED UV is equipped with LED lamps that offer a tailored output at the 385 or 395 nm UV wavelength. LED UV lamps do not require a warm up time; fully operational at a flick of a switch, energy/heat output is low; they are ozone free and only need a small power unit.
RK Print Coat Instruments Ltd
Litlington, Royston, Hertfordshire SG8 0QZ
www.rkprint.com sales@rkprint.com
Source: RK Print Coat Instruments Ltd
