Use of high resolution and high definition in flexo - Part two
In part one of this two-series article on pre-press in flexography, R S Bakshi dives deep into the technology to find solutions to reproducing rich coloured labels and saving costs as well.
29 Oct 2014 | By R S Bakshi
Use of high resolution and high definition in flexo - Part two
Flexo is known for its high dot gain, which results in improper printing of fading effects in vignettes and images. The problem becomes more acute when there is a demand to print a higher solid density or saturated colour, coupled with having a tonal image/vignette that has a fading effect. This was overcome by extending the highlight to the edge of the image/vignette, to avoid the fading effect. This problem was more visible because of the use of low screen lpi being used to match the capability of the chrome anilox roll used by flexo printers.
With time these printers realised that they had to purchase higher resolution ceramic anilox rolls to improve their capability to print a higher resolution and in the process the library of anilox rolls went up. Not because they wanted it but because they had little choice. The selection of anilox roll was basically on the advise of several sources they had access to, and was based on an algorithm too difficult to understand. It was a permutation of sorts that would confuse the most learned of printers; they simply bought the anilox rolls to be able to complete the jobs in hand. The library helped to somehow complete a job by using a particular anilox and to replace the same with another when the result could not be achieved.
This system is in practice even today with most printers and the printer is happy that he was somehow able to produce a result that his customer approves, not with-standing that this results in losses like low press speed; frequent cleaning of blocks, leading to stoppage of press and increased substrate plus ink wastage and press down time; increase in cost of printing blocks and DSA tape for printing of solids and tonal shades separately on separate print stations. This results in increased cost of makeready and register setting time. Several more points could be added but this is not the time and place to discuss this.
This system is in practice even today with most printers and the printer is happy that he was somehow able to produce a result that his customer approves, not with-standing that this results in losses like low press speed; frequent cleaning of blocks, leading to stoppage of press and increased substrate plus ink wastage and press down time; increase in cost of printing blocks and DSA tape for printing of solids and tonal shades separately on separate print stations. This results in increased cost of makeready and register setting time. Several more points could be added but this is not the time and place to discuss this.
Unstand the function
A printer would do well to understand the function of the anilox roll which is basically transferring ink with a certain volume and evenness. An anilox roll with the same specifications from different manufacturers do not perform identically. However instead of understanding this, the price of the anilox became the benchmark on which anilox roll to buy. This placed many printers into grave problems.
Once purchased the investment in the aniloxes was made and they had to make use of these as best as they could. This situation was acceptable when competition was not very strong. Things have changed and there is a price war. Printers are now finding it difficult to produce the required result as his anilox rolls do not produce the desired result.
A new solution was found with the introduction of the UV ink that provided the saturation of colour in print. It did the job for many customers, but could not delivery for designs that were more complicated. One of the reasons for this was the use of low resolution in printing of designs. Using blocks with a high resolution in combination with metering rolls that deliver the right volume of ink can help the printer to achieve the best of results and compete with offset and gravure.
For a better understanding, I am reproducing print samples taken in Chennai India (such high resolution has never been attempted in India before) from a block made from a photopolymer plate of 1.14mm thickness using a 200lpi and 225 lpi screen ruling with the following specs: Nilpeter Label press at 75 mpm, using Seigwerk UV inks, Tesa DSA foam tape 52017, opaque white PP film and using the MacDermid Lux plates. The same plate was printed using two different anilox rolls, one 1400lpi/1.14 BCM anilox from Harper and the other a GTT XS from Apex Europe.
The purpose was to see the printed result with each and compare, everything else remaining the same. The printed result is copied and reproduced alongside(below and on the next page). In addition to this several segments of the printed sheet were copied and enlarged to show the difference in the printed results which is visible very clearly. The printed images shown are arranged in the same sequence (a) taken with conventional anilox roll (b)taken with GTT metering roll and (c) the image on plate. When you look at the enlarged pictures of the segments one can clearly see that:
1 The thin lines printed with the GTT roll has a clean and smooth edge compared to the conventional anilox roll in each of the pictures.
2 The print shows that the plate prints more uniformly with the GTT metering rolls as compared to the conventional roll used.
8Since the GTT enables the flexo printer to print a higher density, the prepress will have to be modified to use this to advantage.
The purpose was to see the printed result with each and compare, everything else remaining the same. The printed result is copied and reproduced alongside(below and on the next page). In addition to this several segments of the printed sheet were copied and enlarged to show the difference in the printed results which is visible very clearly. The printed images shown are arranged in the same sequence (a) taken with conventional anilox roll (b)taken with GTT metering roll and (c) the image on plate. When you look at the enlarged pictures of the segments one can clearly see that:
1 The thin lines printed with the GTT roll has a clean and smooth edge compared to the conventional anilox roll in each of the pictures.
2 The print shows that the plate prints more uniformly with the GTT metering rolls as compared to the conventional roll used.
3 The dots of different percentages are printed more uniformly retaining their shape when printed with the GTT as compared to the dots printed with the conventional rolls.
4 The star target printed with the GTT is cleaner and smoother than that printed using the conventional anilox roll.
5 The solid ink density (SID) obtained with the GTT is 1.46 as compared to 1.27 obtained with the conventional anilox roll even though the cell volume of the conventional roll is higher.
6 Traditionally offset and gravure have been using the higher resolution of 175 and 200 lpi to get better definition. With the GTT metering technology flexo can now compete on a equal level with printing the same resolutions.
7 Traditionally gravure has been using the advantage of printing the same resolution in all cylinders for all the colours. With GTT metering technology flexo can also do the same.
8Since the GTT enables the flexo printer to print a higher density, the prepress will have to be modified to use this to advantage.
Therefore, when using GTT metering roll, the printer gets a richer colour in print compared to a similar conventional anilox roll. This enables the printer to print a halftone and better solid in a single plate on the same station instead of having to print the solid separately with a second plate on a second station. The cost of mounting the second plate is eliminated and so also having to register the two plates on the two print stations. It also reduces the need for the makeready of the two stations.
The biggest advantage comes from a reduced inventory of anilox rolls and having to remember which anilox rolls were used for printing of which job, as the printer uses the same GTT rolls for printing most of the jobs. A big savings in reduced downtime for removing and fixing the anilox rolls from job to job.
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