Driers

Sheet fed Inks dry by oxidation of the vehicles or varnishes carrying the pigment in the ink. These varnishes made from Linseed Oil, or Tung oil will oxidize slowly at room temperatures. To assist in drying all inks have some drying catalysts added in the form of an inorganic salt, Cobalt, Manganese, and Lead (rarely).
When ink is applied to a non-porous surface or where accelerated drying is required an extra boost of these catalysts can be applied by adding Cobalt driers to the ink. The accelerated drying also takes place on the rollers and in the ink duct so inks with added driers must be processed quickly or they set in the ink train.

On non-porous surfaces getting enough Oxygen to the ink may be a problem so Graphodriers, (Calcium Perborate), can be added to release Oxygen into the ink. GATF advise "they work particularly on less porous substrates". The more ink the slower the drying and some colours dry slower than others.

Cobalt acetate is sometimes added to the fountain dampening solution to accelerate drying. It is difficult to control but eliminates the problem of faster setting in the ink duct.

Ink drying will be retarded by cold temperature, low Ph (acid), and high Relative Humidity. Fortunately with neural or alkaline sizing slow drying due to low PH paper is a thing of the past. High humidity and high levels of fountain water emulsification in ink will slow ink drying. Keeping fountain water to a minimum and using alcohol to reduce the film thickness is important on non-porous surfaces.

If a customer complains of slow ink drying check it is drying not scuffing, the touch with unprinted paper tests drying, rubbing tests scuffing. If it's drying check what type of ink being used, what if not all colours are wet and how thick is the ink where it's not dry. Some surfaces, Stardream, Gilclear may take 2/3 days to dry if the ink film is thick and water has not been kept to a minimum.


Setting and Drying

Setting and drying occur simultaneously when an ink film is printed. Setting is caused by solvent being evaporated or absorbed leading to an increase in the viscosity of the film such that the sheet can be backed up without marking. Coated papers with their fine capillaries absorb solvent easily but are able to leave the vehicle on the surface, uncoated dissipate the solvent and vehicle more evenly so the ink film spreads and "dies back". Surface treatment applied to Mohawk Inkswell products and Cyberstar inhibit absorption so slow setting and drying compared tom untreated uncoated products.
Drying refers to the chemical process where the vehicle in the film oxidizes. This process takes longer to complete than setting. Drying is normally complete overnight.
Quick set inks are available where rapid backing up is required, they work by rapid absorption of a solvent into the substrate, and they will not work on non-porous materials.
Some ink such as newsprint dry by absorption only, but the binding of the pigment is poor, notice how newsprint ink comes off onto your hands or clothes.

Radiation Drying

UV
Radiation polymerization occurs in a fraction of a second. Under UV or electron beam radiation the ink film, containing acrylate monomers polymerizes into a dry film. These inks are more expensive than conventional inks and have been linked with health warnings. UV is usually to magazine covers, cartons, and screen-printed spot on covers. UV litho inks are chemically incompatible with conventional drying ink, but an IR coating after conventional four-colour printing can then be UV coated.
Infrared
Infrared radiation as a source of intense heat promotes absorption evaporation and oxidative polymerization. IR units are usually a last colour deck putting an overall water based varnish on the sheet that dries almost instantaneously after exposure through the IR unit at the end of the press. The coating being water based cannot be spot printed by the offset process so is normally a "film" coating over the entire sheet except the extreme edges. To spot a job the blanket has to be cut out like a flexography plate (relief).

The surface temperature of the coating rises above 100degrees Celsius, Peter Elliot from Monocure told me that they like a temperature up to 120degrees maximum. What this does to the moisture content of the sheet I don't know.

Non-Absorbent stocks
Inks for printing on plastic or plastic coated stocks are sometimes called collodion. If possible UV or IR works well. If conventional offset printing is required quickset inks should not be used and water should be at a minimum. Oxidising inks should be used with dries and the sheets "winded" top allows more air ingress. Inks and coatings must have the necessary surface tension to properly wet and adhere to the substrate.

Overprint Varnishes
Overprint varnishes are similar to ink without pigment. Some of the component ratios will be changed to enhance gloss, increase or decrease slip and give various levels of scuff protection. Adherence to the previous ink is important. Varnish can be wet trap, applied on the last unit on the press onto the non-dry ink film or dry trap applied, as a separate pass after the initial ink film is dry. Wet trap is cheapest and often used to assist in further processing of dull or matt-coated stocks. Dry trap gives better protection and higher gloss, but being an extra process is more expensive. The varnish can be overall or spot, applied to parts of the image only. Conventional varnishes dry at about the same speed as conventional ink.

Water based varnishes dried by IR are becoming more common; they can be gloss, satin or matt and give excellent protection. They may be applied by coating tower or specially designed dampening systems and can carry a heavier coating weight. Printers desiring to work and turn short run matt or silk stocks find satin IR varnishing eliminates scuffing and set off.

The use of an overall Matt IR coating while assisting processing of Dull, Silk or Satin coatings decreases the contrast between the printed and unprinted area and the coating takes away the startling whiteness of the unprinted Novatech Satin surface. To retain the contrast and whiteness a spot varnish has to be used.