Lithographic printing

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By Debbie Anderson | Aug 20, 2020
Lithographic print sample at GMN

Lithographic printing, an offset printing technique, is based on the basic principle that oil and water do not mix. It is a process in which ink is transferred from a photographic plate to a rubber blanket, which then presses the image onto the printing surface.

Off-set lithographic printing process

The first step in lithographic printing is creating the artwork on a photographic plate through a chemical process. Similar to the process of developing photographs, lithographic printing also requires the creation of a “negative” and a “positive” image. First, a thin aluminum plate is coated with a hydrophobic material so that it attracts oil (ink) and repels water. The plate is then selectively exposed to light, thereby curing the hydrophobic coating only in areas comprising the artwork. Finally, the coating from the remaining areas is chemically stripped off and the plate is ready for use.

The lithographic printing press consists of a series of rollers laid next to each other. The foremost roller transfers water, placed in a tray beneath it, to the photographic plate. The ink is manually applied to the second roller with a putty knife, which then wets out to the entire cylindrical surface as the roller spins and transfers the ink to the plate. The photographic plate, carrying the artwork, is wrapped around the next roller in the series. Given the immiscibility of ink and water, the ink adheres only to the artwork on the photographic plate, while the water adheres only to the remaining background.

On the other side of the photographic plate are two other rollers called the blanket roller and the impression roller respectively. The blanket roller simply acts as a medium to transfer the artwork from the plate to the substrate. When printing, a stack of printing sheets is placed on the top tray of the machine. A gripper grabs one sheet at a time and wraps it around the impression roller. The artwork on the photographic plate is imprinted on the rubber blanket roller, which in turn transfers it to the substrate on the impression roller. Then, the ink is cured by exposing the printed sheet to UV light. The process is repeated for every unique color in the image.

Advantages and disadvantages of lithographic printing

Ideal for high-volume manufacturing, a lithographic printer can run from 3,000 to 6,000 impressions per hour, with the largest sheet size being 18”x24”. It can produce detailed and intricate artworks with half-tones, gradients, or a four-color process (a four-color process uses a CMYK color module to create four separate dotted patterns, which when printed on top of each other, yield the required color). However, lithographic printing comes with its own limitations. It can only accommodate substrates with thickness ranging from 0.003” to 0.020”. The printing system is not compatible with metallic or conductive inks. Also, the ink used is very transparent, and typically requires extra layers of printing for opacity.

Off-set lithographic printing services at GMN

While lithographic printing is often performed on standard substrates such as paper and vinyl, GMN walks the unbeaten path by predominantly using materials including polycarbonate, polyester, and aluminum. Allowing for extremely tight tolerances and consistently high-quality images, GMN’s sheet-fed lithographic process is frequently utilized for printing graphic overlays and labels.

Lithographic printing at GMN utilizes UV-cured inks. GMN also offers custom color matching services to help customers achieve their exact specifications and uphold brand consistency. To formulate a custom color, the different colored inks are meticulously weighed, poured over a flat glass surface in carefully measured proportions, and mixed together using a putty knife. With projects requiring custom color matches, a small sample sheet is first tested on a machine called the orange proofer. This counter-top machine is simply a condensed version of the actual printing press, that allows GMN to test the color and make necessary alterations before the final production run.

Printing solutions at GMN

As multiple factors go into consideration before selecting a suitable printing technique, GMN works closely with each customer to understand the project needs and requirements to determine the best printing solution for every unique application. To learn more about our flexo, screen, and digital printing technologies, visit our blog on GMN’s printing capabilitiesTo kick-start a discussion with a GMN expert, request a free design consultation.