For Printers
There are various trade names for this type of quick drying ink, all of which imply the characteristic of resistance to drying on the fountain system, press rollers and blankets while drying quickly on the substrate. The advantage is that the printer can leave the ink in the ink fountain without the worry of "skinning" (over) but the same ink will quickly dry on the printed substrate. Such a panacea!
Producing a quick drying ink on a substrate with resistance to drying in open air conditions involves the incorporation of higher boiling-point petroleum distillates and possibly volatile anti-oxidants.
The degree of "press-stability" or "quick set" varies from one ink manufacturer to another, as manufacturers have their own ideas as to what works best.
How does it all work you ask? Well, Oil-based inks use a combination of four main processes to dry which include:
- Evaporation of the solvents
- Absorption into the substrate
- Oxidation oxygen reacting with the varnishes, etc. to harden
- Polymerization of the varnish/oils, etc. to harden
In the older, more conventional inks, all four processes played a more or less equal part however, in the modern generation of quick set inks, there is slightly more emphasis put upon the absorption process, with less oxidation and evaporation.
These new quick set inks work by adding a greater percentage of higher boiling-point distillates (which evaporate more slowly) and anti-oxidants. This retards the drying rate of the inks on the in the ink fountain, rollers, etc., but, as the inks are printed down on an absorbent surface (i.e. papers and boards), the low viscosity distillates and antioxidants soak quickly into the substrate, leaving the remaining vehicle to quickly surface harden.
Quick Set Inks
This "quick setting" or skinning takes place within minutes of being printed, however, the internal drying of the ink takes a much longer time as the oxidation and evaporation parts of the drying mechanism are much slower compared to the more conventional types of inks. Another draw back with this type of ink is that when the ink has finally dried, the printed film tends to be "softer" or less abrasion-resistant. This is due to the resin systems used to produce these inks.
For optimum performance, these inks require a moderately porous surface, so that the fluid portion of the ink can separate from the top of the ink film, in a controlled fashion, into the fibrous layer below the coating. (fiber base sheet)
These should then gradually evaporate, bringing about development of hardening in the ink film.
What are the advantages of these inks:
Well, sheetfed presses today run at much higher speeds than in the past. Due to this fact, inks tend to dry on the rollers and blankets very rapidly. As productivity is a key, these new quick set inks were developed by the ink industry to meet this demand.
The changes mentioned above have resulted in:
- Reduced evaporation of distillates, which produce inks that have a lower tendency to "skin" in the can.
- Reduced tendency to tack up on rollers and blankets, (either at make-ready stage or during the print run).
- Minimized need for blanket and plate wash-ups during short stops
- Shortened turn-around and handling times, (as superficial setting of the inks is, generally, extremely fast).
What are the disadvantages of these inks:
Coated papers have good absorbency levels thus the distillates can remain tapped in the coating which results in slow evaporation of the solvents. Because of this fact, coated papers have some potential problems when printing with these inks such as:
- Set off or sheets "sticking" or "blocking" together in the printed stack
- Slower ink drying - not faster
- Chemical ghosting
- Because of the slower ink drying, potential dangers with subsequent UV (ultra-violet) varnishing or lamination.
Some of quick-set inks actually will "back up" off the "ball" fountain roller. The press operator will know right away as that unit will stop printing! The ink manufacturer can make adjustments to the ink for this reaction.
Higher tack can build due to the rapid solvent penetration resulting in a higher incidence of paper contamination.
Substrates (coated and uncoated) with very high ink holdout, low porous substrates, a high proportion of the ink solvents will remain on the surface. This means the printer will again have the potential for marking, slow drying, etc. (as mentioned above).
Many of the various ink manufacturers are making these quick-set inks today. Because of this, each manufacturers' ink carries some differences. When printing with these new inks, it is advantageous for the printer to submit a sample of the substrate to be printed to their ink supplier for the best recommendation on usage. Thus, each ink/paper combination must be evaluated on its own merit.
For more information or questions, contact Jim Kohler at jim.kohler@ipaper.com
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