Ink used for lithographic offset presses is a paste, rather than a fluid. That means it is thick and tacky.It is typically delivered in canisters. The image below shows Quattro offset printing inks from 4S Graphics.
Like all printing inks, offset lithography ink consists of three substances:
- The ink vehicle is the fluid part of the ink that transports the pigment onto the substrate. This is typically an oil based material.
- The pigment is the part of the ink that imparts gloss, color, texture, and other characteristics to the printed image. Pigments can be black, white (either opaque or transparent), and colored. The colored pigments are usually produced from either mineral sources or from derivatives of organic coal tar. Other materials such as metallic powders are used for specialty inks.
- Various additives influence the characteristics of the ink. Driers speed up the drying of inks. Bodying agents increase the viscosity. Waxes help prevent printing defects like ink setoff and blocking and they increase the scuff resistance of the ink. Other materials are added to reduce the ink’s tack, improve its distribution on the printing plate, prevent oxidizing and enhance other properties of the ink.
Web offset inks tend to be more fluid and have less tack than sheetfed lithographic inks. Most offset inks dry rapidly without the need for additional equipment, but there are also inks optimized for infrared, ultraviolet or electron beam curing. All offset inks contains water-repellent materials, needed because offset lithography is based on the fact that oil and water do not mix.
Manufacturing offset inks
Since offset presses can print on a wide variety of surfaces, a large number of inks are available for the process. This YouTube movie shows how it’s manufactured.
Low-energy UV inks
In the last few years UV-curable inks have gained in popularity. These inks which dry faster when exposed to ultraviolet light were already in use in the packaging and label printing industry for decades. Now that cheap low-energy LED lights can be used to cure ink, the technology has also taken off in the commercial print market.
UV-curable inks offer several advantages:
- The ink dries faster, so presses can run faster and there is less risk of offset (wet ink transferring from one sheet to the sheet on top or underneath).
- There is less absorption of ink in the substrate, so colors are more vibrant and glossy.
- Lower emissions of organic solvents.
- UV inks are more suited for printing on plastic and other non-porous substrates.
- Compared to traditional UV drying which consumes a lot of power and an ozone extraction system, the newer low-energy UV systems are simpler and require far less energy.
There are obviously also disadvantages:
- You need a press with UV curing. New presses include the technology but bars with UV LED lights can also be retrofitted on existing presses. According to a Graphic Arts Magazine article 70% of all new Japanese presses are now low-energy UV presses.
- The use of these inks affects the entire printing process, so you may need to use other blankets and press chemicals. The blanket washes and plate cleaners that are used with UV-inks are more aggressive, which may affect the life span of the printing plates. If these wear down earlier, an additional sets of plates may be needed for long print runs. The wear down affects small dots first, so the pressman may see loss of highlight details after a few (ten) thousand impressions.
- UV-inks are significantly more expensive.
- Due to shortages in some chemicals required for the production of UV-inks, there have been supply issues in 2019. These have driven up prices even more.
Special offset printing inks
Regular process inks or spot color inks can have unique properties. They can be extra glossy, resistant to abrasion, low-odor, dry fast under UV light, etc and of course, they are usually optimized for a specific printing process and/or type of substrate. There are however also special inks. These are often used to make advertising material, packaging and labels stand out. Some of them are used for security purposes.
- Thermochromic inks
These inks either change color or become transparent when temperatures increase or decrease. There are inks that will keep changing color or transparency and others that will only change once. Thermochromic inks are used in toys, battery indicators and some types of thermometers. They are also used frequently in packaging, to show commercial messages or to indicate if the packaged product is warm or cold enough.
- Metalic inks
These inks have a metal-like character. They contain metallic particles that rise to the surface after printing, reflecting light and creating a metallic sheen that simulates zinc, copper, gold or silver. Inks that aim to create a metallic sparkle effect are called glitter inks.
- Mirror inks
Mirror inks are a special kind of metallic inks. When printed on the reverse side of a transparent substrate like glass, polyester or poly-carbonate you achieve a mirror effect. This can be done with offset but also with screen printing.
- Pearlescent inks
The metal oxide based pigments in these inks give a characteristic gloss to the printed image that is comparable to the iridescent gloss of pearls.
- Watermark imitation inks
As the name implies these colorless inks imitate a watermark.
- Scented inks
These inks give off a scent when the printed surface is scratched. The inks contain tiny nano-sized particles that encapsulate a fragrance. Scratching the printed surface tears open the capsules so that the scent is released.
- Scratch-off inks
Instant win lottery tickets have a thick layer of scratch-off ink applied. Scratch off ink is also used to hide secrets codes for contests on beverage cans or other types of packaging. First a release varnish or UV coating is printed and then the scratch-off ink layer is printed on top. This can be done with screen printing as well as offset lithography. In offset you may have to print multiple layers to get a good result.
- Magnetic inks
These inks are used as a security measure on checks and other high-value documents. They contain particles of a magnetic substance whose presence can be detected by magnetic sensors. A counterfeit copy produced using regular inks will not pass a validation by a MICR (magnetic ink character recognition) reader.
- Luminescent inks
These have two completely different uses: Luminescence security inks, which are sometimes called marking inks or UV/IR excitation inks, light up when exposed to ultraviolet or infrared light. They are used as a security measure on banknotes and other high value printed documents. A bit more frivolous are photo luminous glow-in-the-dark inks, which temporarily emit a weak light in the dark.
Other sources of information
More information on inks is available on this page.