Use of High Resolution Definition in Flexo - Part One

One of the most important factors to be taken into consideration in FLEXO printing, is the cell volume of an anilox roll. The higher the volume, the higher the transfer that produces higher Solid Ink Density(SID) in print. The problem comes when one has to print high resolution screen of 175, 200 to 250 lpi on plate where the anilox screen used is 1200 lpi or higher. As the screen ruling increases the cell opening reduces along with the reduced cell depth and the cell volume comes down and so al

28 Oct 2014 | By R S Bakshi

In an attempt to get a higher SID, increase in cell volume becomes mandatory, to print the solid together with the fine screen on the same plate. Generally the higher cell volume resolves the issue of the required SID but this adds to the dot gain on press. This problem can be overcome by modifying the image suitably depending on the dot gain on press. The problem of the dot gain in the highlights cannot be resolved, as the highlight dots cannot be reduced in the manner as the dots of the mid-tone and shadow areas, in the same screen ruling. However this can be resolved by printing a higher screen ruling where the highlight dot sizes are smaller. 

It is difficult to achieve this with the conventional anilox rolls technology where the cell opening is very small and engraving depth is not enough to provide the SID. thereby limiting the volume of ink that it can transfer. While it can enable to print a finer screen, it cannot improve the SID beyond a limit. 
Consistency from one roll to another roll is a critical issue with finer conventional anilox rolls. In other words, not every 1400 lpi anilox will deliver the same amount of ink density, even when they are having the same cell volume. Finer aniloxes are more prone to scoring and lifetime is relatively short as well. This is where the new Genetic Transfer Technology(GTT) rolls have been able to achieve the objective of using a finer engraving and enabling the SID to be maintained at the required Level while enabling the consistency to be maintained between repeat orders of the same rolls, within a very fine tolerance. 

Printing Specs : Nilpeter  Label Press Model : FB33078   
Paper : Avery Dennison semi glossy fascoat extra
UV Ink : Flint
Press Speed : 70 MPM
Plates : MacDermid Lux 1.14 mm
DSA Tape : Tesa 52015
Sequence : YMCK
Anilox : 1400 lpi from Harper and GTT* XS from Apex Europe

                 Anilox     Volume    Density
                 Lpi           BCM
cyan     1400               2.1        1.12
Mag      1400              1.15       1.29
Yellow  1400              2.1         1.15
Black    1200             1.20        1.18

                   Anilox    Density
cyan      GTT -XS        1.39
mag       GTT-XS         1.58
yellow   GTT-XS         1.23
Black     GTT-XS        1.28
Genetic Transfer Technology, the new technology metering rolls that uses an open slalom channel engraving provides the higher SID and also enables the flexo printer to print the finer screens. The higher SID with the GTT is possible due to its wavy channel engraving with a flat bottom with lower engraving depth, where the transfer of ink is much better. This transfer is aided by the use of a harder and much lower porosity ceramic that improves the surface tension of the roll surface, thereby improving the ink transfer. 

1995 - more porus

1998 - less porus

2000-micro porus

2002 - not porus
Fig 3 -  Development of Ceramics
The effort taken in developing the ceramic and the engraving shows the judicious manner in which these rolls are produced. To produce rolls that give higher SID with less ink, where the screen ruling required is less than 133 lpi leading to a reduction in the cost of ink with other added benefits. Or even enable print finer screen rulings such as 225 to 250 lpi with sufficient SID. The GTT metering roll meets all these requirements, as compared to the conventional anilox roll. 
The GTT rolls uses a technology for engraving that enables rolls to be engraved with much more consistency in the cell volumes than that used for  engraving the conventional rolls. With the GTT engraving rolls on the same press and between those ordered earlier and in repeat orders. This consistency is of great value in maintaining the quality of the printed results. GTT engraving has a much better tolerance (<3%) than standard engraving(+/- 5% or higher), which means from the point of view of repro, they can be very accurate on getting better on consistency in print. The higher SID enables the production of a larger range of Pantone shades, thereby reducing the necessity to print additional special colours. 
A direct comparison of the cell volumes of the conventional and the GTT rolls is not recommended as: 
1. The type and quality of ceramic used in both are different. 
2. The engraved cell structure of both the rolls are different.
3. The surface tension of the ceramics are different and so all the stress levels of the ink   on both the rolls are different.
4. Both these lay down the ink differently. 
However a comparison of the printed results taken from both the rolls is possible. This is clearly visible when the single colour printed sheets taken with the 1400 lpi and the GTT XS roll, as shown in my article published in Nov 2013, are compared. Suffice to say that the printed results with the 1400 lpi/cell vol 1.4 BCM anilox roll from Harper and the GTT XS roll from Apex show that the SID of the printed result with the GTT XS is as required while that from the 1400 lpi roll is lower. Even the image reproduction shows a better contrast with the GTT. Suffice to say that the ink consumption with the GTT comes down benefiting the flexo printer where solids are to be printed and a little higher where more is demanded when printing the high screen counts of 175 lpi to 250 lpi. 
Another advantage of the GTT is that you can have the same roll on all the print stations and use the same screen ruling for all the colours, as being done in gravure printing without sacrificing the SID. It also means that the same roll can be used to print most jobs eliminating the necessity to change rolls on press, reducing press down time. It eliminates the need to keep a track of which roll was used for which job. It also reduces the make-ready time and the wastage associated with this. 
The addl. colours(over the process CMYK) used for printing on the addl. Print stations with the same screen ruling,  enable you to use these to improve the colour gamut in print, just as in gravure. 
This means the GTT metering technology enables flexo to compete with gravure on an equal footing. Hitherto this was not possible with flexo. Having a uniform ink supply with flexo as compared to the uneven cell volumes in gravure could be an advantage considering that there is reduced ink consumption with GTT rolls in printing of solids. It is a known fact that gravure solid depth is achieved with a higher cell volume.
There are many other advantages with the use of the GTT technology. Some of these are: 
1. There are no angles to engraving. This eliminates the problem of moire normally associated with cell angles on anilox rolls. 
2. The press speed can increase as ink spitting is not seen, increasing your production.
3. The frequency of cleaning of rolls is reduced thereby reducing down time and increasing production.
4. The lower cell depth reduced the cell volume but improved the ink lay for printing solids, while reducing the energy required in drying of the solid colour.
This improved ink transfer reduces the drying of the ink in the cell of the GTT roll leading to reduced cleaning frequency of anilox roll, reducing the press down time.
5. In an anilox cell, the weakest part and where most friction takes place, is across the top and the bottom cell walls which run at 60 degrees to the rotational direction. GTT has no top and bottom walls. This increases the life of the doctor blade and that of the ceramic roll.
6. GTT ceramic has 80% chromium oxide and 20% Titanium Oxide compared to conventional anilox made from 100% chromium oxide. The ceramic used on the GTT rolls is harder and more durable.
7. Inventory of rolls reduces thereby reducing the investment in rolls that lie ideal on the shelf. Ideal rolls on the shelf are a dead investment.
8. Leaving the same roll on the print station for printing of all jobs reduces press down time leading to more production time on press.
9. Reduction in the inventory of Pantone inks is a big saving in ink costs.
10. Pre-press work is reduced and becomes easier with the use of the GTT rolls as the solids are achieved easily and highlights print better with finer screen.
With time listing out some more advantages with the use of the GTT rolls will be possible. Judiciously these are a big advantage to the flexo printer.