Consistent results in flexography are to some degree dependent upon how much ink, whether thick or thin is laid down on the plate and on the substrate. Anilox, doctor blades and plate mounting tapes cannot be left out of the flexo quality equation; neither can devices that enable colour, text and graphics to be monitored and adjustments made.
Given the right circumstances, there are quality and performance advantages to be gained when using a minimal amount of ink. For example, if a press has less than perfect dryers, applying less ink speeds drying. Speedier drying prevents bottlenecks and reduces the risk that product shipment or additional converting processes are delayed because of wet ink. Wet or insufficiently dried ink can produce many problems. Ink that is still wet may lift from the substrate and depending on press set up may transfer to the plate in the next deck. As the transferred ink builds up the printed image will become progressively dirtier.
Depth and visual acuity
With the smooth and controlled lay down of a thinner ink film, using the appropriate anilox roll and doctor blade, not only do large solids appear even and consistent but also the rest of the line work improves as well. Type prints are sharper because the controlled ink layer on the surface of the plate does not build up on the shoulders as readily. Reverse type benefits the most; less ink allows for smaller point sizes to be reproduced with less risk of fill-in. The aim of the flexo printer is to have as many sizes of dots as possible in order to reproduce an image with depth and visual acuity. By printing smaller, low-percentage dots the printer is able to reproduce lighter or more defined highlight areas. In addition, when a high percentage of dots are open, there is better shadow definition.
By laying down a thinner ink film there is less variability and a reduced risk of quality problems arising. There is more latitude in plate pressure setting and impression settings are more forgiving because there is less ink to skew the separation. The smaller amount of ink on the tip of the dot is also more difficult to distort.
A high line count anilox roller is an option; lower volume anilox uses a smaller cell with a shallower engraving depth. Shallower engraving depths hold less ink, which in turn produces a thinner ink film.
Communication, protection and decoration
Since inks are primarily used for their communicative, protective and decorative attributes, great importance is given to the subject of colour communication. Typically colour variability is defined as the Delta E of the inks when measured on a spectrophotometer. A Delta E between 1 and 2 is generally acceptable, but there are applications where the Delta E must be kept lower. In order to meet colour targets, colour-matching capabilities must be precise and there must be good ink room to press correlation. Printers want colour matches and targets that they can actually print.
There are many variables that affect colour reproduction; the absorptivity of the stock, not only varies among stocks even within the same category, but also from batch to batch within the same stock. This can affect the gloss of the ink film by letting more or less of the vehicle portion of the ink film penetrate into the stock. The more the vehicle penetrates into the stock, the less varnish there is available for the top of the printed ink film, which means that gloss will be less.
Converters that print must be aware that ink film thickness limits the pigment/ratios needed for optimum performance on press. For example, the customer wants the converter to run a job that contains a very bright deep red. A slight absorption increase in the stock will make the high concentration of pigment particles protrude through the surface varnish layer, causing the print to look less glossy and lighter.
Threat of overcompensation
Printers and converters that print sometimes then overcompensate by increasing the ink film, which may already be close to maximum. This could result in line patterns, mottling and the possibility of offsetting on the backside.
A slightly lighter red will allow less pigment loading and thinner ink films to be printed. This allows the print operator to make colour adjustments during the run without compromising on quality. If, as is often the case, the customer has specified the problematic deeper, darker red, then the operator can try making two impressions, wet on wet.
Improved ink mileage and lower ink costs are typically associated with printing thin. Pay close attention to colour matching and determine the optimal printing ink thickness by running various test rolls with several engraving specifications or bands across the roll.
The FlexiProof 100 colour communication or proofing system and variants FlexiProof UV and FlexiProof LED UV can be supplied with ceramic anilox rollers, each having two engraved bands. These rollers range from those engraved with 55 cells per linear inch with a volume CM³/M² up to and including 1,000 cells per linear inch with a Cbm/inch² volume of 2.0. Rolls engraved with other screens can be supplied.
FlexiProof
The FlexiProof can be used for colour-matching purposes and to determine how inks and substrates interact or perform over time. A scaled-down but component-critically exact version of a flexographic production press, this colour communication system saves time and material waste by enabling unfamiliar materials to be run and trials to be undertaken without recourse to taking a production machine out of line.
For multi-print processing converters, the K Printing Proofer enables users to match colour and bring process variables under control, off-press, minimising on-press waste, and improving workflow and quality. It proofs using flexo, gravure and gravure-offset inks. The unit is so designed that two or more inks may be printed simultaneously for comparison purposes; registration is included for overprinting. Wet or dry laminating samples can also be produced on the K Printing Proofer using the gravure head with K-Lam laminating samples.
Written by Tom Kerchiss, Chairman of RK Print Coat Instruments Ltd