Setting & Drying Inks
It’s all about the setting and drying of conventional sheet fed offset inks. Anti-set-off spray powder is used to prevent set-off, the unwanted transfer of wet ink to an adjacent sheet, in a press end pile. Spray powder not only provides a physical spacing between sheets of substrate, but also allows the circulation of oxygen containing air necessary to the drying (oxidation) of inks. Sheetfed offset lithography is a printing process that feeds sheeted substrate into the press one sheet at a time for printing. After printing the sheets are stacked in the press end delivery section one sheet upon another in a pile.
This places the printed surface in direct contact with the back of the sheet next to it in the delivery pile of sheets. Substrates vary from paper, paper-board, lightweight corrugated board, plastic laminated paper substrates, and stiff plastic films that can be fed effectively through a press.
Oil & Water Do Not Mix
The offset lithography printing process is based on the principal that oil and water do not mix. Oil based inks are attracted to the plates image area, and fountain solution to the plates water loving non-image area. Inks are generally formulated to be heavily pigmented and strong in color because of the very thin film of ink that is deposited.
Conventional litho ink vehicle systems may be petroleum-based or vegetable oil based. Ink properties, including setting, drying, gloss, and rub resistance are all known to be affected by the vehicle oil type used. Vegetable oil based inks are slower drying than linseed oil based inks, while soybean oil based inks are faster drying than some semi-drying vegetable oils.
Setting
Setting is a physical process that occurs after printing when conventional litho ink’s viscosity is increased by evaporation, and porous substrate absorption of the vehicle. It’s said that ideally, conventional litho ink set time on press should be less than 30 minutes, and dry time on glass should be 4-6 hours. Some very fast drying, quickset and super quickset inks are formulated using mineral oils that produce inks that set as fast as 20 minutes and 10 minutes respectfully on press.
Conventional litho inks dry by an oxidation-polymerization reaction process. Oxidation depends on the presence of oxygen from air. Inks formulated for optimized drying utilizing cobalt and manganese driers can dry in as little as four hours. Slower drying formulations have been known to take days to dry. The effectiveness of driers can be reduced by low pH from acids, and excessive moisture, which can come from high moisture content paper, or over emulsification of inks.
Set-off. Bricking. Blocking.
Set-off describes the problem of printed wet conventional inks sticking or transferring to the adjacent sheet in a pile (load) at press end before or during the ink set and dry process. Other terms used to describe wet-ink set-off are offsetting, bricking, and blocking. The micro-transfer of wet-ink to the backside of the adjacent sheet in the pile is described as light ink setoff or offset. Bricking or blocking refers to a pile of printed sheets that are stuck together (essentially glued together) by wet-ink or over coating under pressure.
Conventional sheetfed offset ink chemistry can be varied somewhat by formulation to optimize (speed up) setting, and oxidative drying. However, the combined processes are still too slow to prevent offsetting, or set off in the load, especially considering the use of very high speed presses. Extended delivery providing extra drying time is an advantage at any press speed.
A Necessary Evil?
Anti-set-off spray powders are accepted as a necessary evil by the sheetfed offset printing industry. Judicious use of them is considered to be the only practical means of preventing printed sheets in the delivery pile from being spoiled by wet inks transferring to an adjacent sheet, or causing sheets to stick together. Anti-offset powders prevent these problems by creating an air gap (spacer) between piled printed sheets. Essentially, this not only creates a physical gap or (spacer), but it also allows air circulation and exposure to oxygen necessary to the ink drying oxidation process.
Anti-set-off spray powder is made from food grade starches derived from corn, potato, tapioca, rice or wheat depending on the particle diameter sought.
Spray powder is available in a range of particle sizes (grades) measured in microns, (μm, or .001 mm). Sizes (grades) are referred to generically such as, fine for paper, medium for labels and carton stock, and coarse for heavy board. Spray powders are supplied as natural (uncoated), or with a waterproof silicone coating (coated). Coated powders might be preferred because they lay down smoother, offer more latitude in particle size, and the amount of powder needed, but they cannot be used for multiple pass printing, or when foil stamping or laminating.
The key to success is to be mindful that the powder type and the particle size must be a perfect match for the print job with no more than what is absolutely necessary used. Meeting these criteria is essential to producing a final print that does not have the feel of sand paper.
The over use of spray powder and/or too large a particle size can also lead to problems when laminating, coating or varnishing.
Aqueous coating in-line is a great alternative for printers with press mounted coaters. These coatings may be final top coats or primers to facilitate off-line UV coating. Aqueous coatings eliminate, or require only the very minimum of spray powder. These coatings seal printed wet inks, which will dry under the oxygen porous coating. Coated water repellent, hydrophobic spray powders are recommended when aqueous coating.
When UV coating off-line over conventional inks dried (16-24 hrs), spray powder use must be closely monitored to eliminate surface roughness that will negatively affect coating smoothness and gloss. UV coating in-line, over UV hybrid inks or UV inks also eliminates the need for spray powder.
Subject to New Regulations
New EU legislation limiting very small particles classified as dust is creating a new generation of spray powders. Ultra low-dust grades are recommended for high speed new presses with spray heads that can be very accurately adjusted to the substrate sheet.
The selection of the powder grade (incorrect particle size) is very important as it can cause a high percentage of wasted spray powder, housekeeping problems, and increased wear on delivery chains and gripper bars.
Contact our Coating Experts for any questions you may have regarding your use of Cork coatings with spray powders.
