Offset Printing

Offset printing, an indirect printing method, is named after early lithography, which initially used stone plates. In early lithography, stone blocks were ground flat and used as plates. Later, this method evolved to use metal plates made of zinc or aluminum, but the principle remained the same.

In offset printing, there is no height difference between the printing and non-printing areas; they are both flat. It utilizes the principle of the immiscibility of water and oil to maintain a layer of oily film on the image area, while the non-image area of the plate can absorb water. The idea is that after inking the plate, the image area repels water and accepts ink, while the non-image area absorbs water and repels ink. This method of printing is called "offset printing."

Offset printing originated from the transfer method of early lithography, where images were transferred onto transfer paper and then onto the plate as a reversed image, which was then printed onto paper as the correct image. However, the pressure exerted during printing caused the ink to spread beyond the intended lines due to the originally flat nature of the plate, resulting in poor line quality. This led to improvements, known as the "offset printing process," where the plate surface is made as the correct image, transferred onto a rubber roller as a reversed image during printing, and then onto paper as the correct image, thus improving the elasticity of printing pressure.

Early offset printing was flat-bed or flat-bed rotary, later evolving into two types: flat-bed rotary and rotary rotary. Flat-bed rotary machines were mostly used for special printing purposes such as proofing machines, while rotary rotary machines were used for printing on paper and other materials. In flat-bed rotary printing, the printing plate is placed flat, and the pressure is applied by cylindrical rollers. This method is similar to flat-bed rotary machines in letterpress printing. In rotary rotary printing, the printing plate is wrapped around a cylinder called the plate cylinder, while another cylinder on the machine, wrapped with rubber, called the blanket cylinder, applies pressure. Machines using these three basic cylinder configurations are called "offset printing presses."

Advantages of offset printing include easy plate-making process, low cost, accurate color registration, ease of plate duplication, smooth and soft color tones, and the ability to handle large print runs.

Disadvantages include reduced color fidelity and vibrancy due to the influence of water and ink, thin ink coverage on the plate (only utilizing about 70% of its capability, requiring double-sided printing for posters to enhance color), and limited applications for special printing needs.

In 1798, the Austrian (now Czech in Prague) composer Alois Senefelder (1771-1834) invented lithography, a printing method that directly prints on a flat surface, thus initiating the technique known as planography. He also developed a wooden lithographic press. Senefelder initially studied law but later pursued composing, although with little success. Facing financial difficulties, he turned to printing his compositions for sale, but printing costs were high and profits meager. He then apprenticed at a printing house but lacked the capital to innovate on his own. While living near the Solnhofen Quarry near Munich, where there was an abundance of limestone, used for grinding copper plates or as a substitute for copper plate engraving, he observed his mother washing clothes and accidentally wrote on a limestone slab with greasy ink, which he found could not be washed away after several days. This led him to experiment with greasy ink on limestone, first writing text in reverse on the stone, then applying a mixture of nitric acid and gum arabic followed by ink, which ultimately succeeded. He referred to this as "chemical printing." The term "lithography" derives from the Greek words "stone" (litho) and "writing" (graphein), now synonymous with planography.

In 1834, Senefelder became the Inspector of the Bavarian Royal Print Works, but he passed away in February of the same year.

In 1810, a German, F. Weishaept, assisted Senefelder in completing an iron hand-operated lithographic press.

In 1817, Senefelder attempted to use zinc plates instead of limestone for lithography, but without success.

In 1826, a Frenchman, Niepce, invented the heliographic process.

In 1832 (Qing Dynasty, 11th year of Daoguang), the British missionary W.H. Medhurst established a lithographic press in Macau for printing Chinese books.

In 1837, a Frenchman, G. Engelmann, invented chromolithography.

In 1840, a British inventor, M. Ponton, invented the albumin process.

In 1868, a German, J. Albert, invented collotype.

In 1869, a British scientist, D. Hauron, invented trichromatic color lithography using subtractive color mixing.

In 1876 (Qing Dynasty, 2nd year of Guangxu), a Frenchman, Father Vénard, and a Chinese man, Qiu Ziangong, established the Tushehuan Lithographic Press in Shanghai for printing church magazines.

Offset printing is based on the principle of water and oil repulsion. The printing process proceeds as follows:

Firstly, the inked image portion is formed on the offset plate. The image can be drawn directly on the plate with an oily pencil or created using a photographic method.

Secondly, the plate is dampened with water. Because water and oil repel each other, water covers the non-image areas of the plate, which are rejected by the image.

Thirdly, the entire plate is covered with ink. Since water and ink repel each other, the ink is rejected by the dampened areas of the plate, adhering only to the oily image areas.

Finally, the paper is pressed onto the surface of the plate, transferring the inked image onto the paper.

Offset printing is also commonly referred to as lithography. Unlike letterpress and gravure printing, where the printing image is raised or recessed on the plate, in offset printing, the printing image is merely flat on the plate surface. During the printing process, as the paper rubs against the image, the image quickly wears off, especially when the plate is used in high-speed printing presses. To reduce wear on the image areas of the plate, the image is first transferred onto a rubber blanket.

According to available records, offset printing was invented by the German Alois Senefelder in 1798, who directly printed oily patterns on paper from a heavy limestone block. Various types of offset plates are used for high-speed and small offset printing presses, which produce more output than any other printing method.

Offset printing, invented by Alois Senefelder of Bohemia in 1798, was the first new printing process since the invention of letterpress in the 15th century. In the early days of offset printing, smooth limestone was used, hence the English name "lithography" or "lithos," derived from the ancient Greek word for "stone." After oil-based images were applied to the stone, acid etched beneath the stone's surface, and then a gum arabic solution was applied, which adhered only to the non-oily areas and sealed. During printing, water adhered to the gum arabic surface but not to the oily parts, while the oily ink used during printing adhered to the oily image areas.

In the years following the invention of offset printing, this printing process was used for multi-color images, known in the 19th century as chromolithography. Many exquisite chromolithographic prints and publications from the 19th century are still preserved in museums across the United States and Europe. Each color used a separate stone plate. The main challenge was accurate registration of the images.

Today, aluminum plates are used in offset printing. The plates are brushed with a grain texture, or "graining," and then coated with a smooth layer of photosensitive emulsion. The photographic negative of the desired image is placed on the plate and exposed, creating a positive image on the emulsion. After chemical processing, the unexposed emulsion is removed. The plate is mounted on a roller on the printing press, and a dampening roller passes over the plate, applying water to the rough areas or non-image areas of the plate. Then an ink roller passes over the plate, applying ink only to the smooth areas or image areas of the plate.

If this image were directly transferred to paper, it would produce a reversed image, but the paper would become wet. The method used is to pass a roller covered with rubber cloth over the plate surface, which squeezes out the water and transfers the ink. (In reality, the rubber cloth is elastic and better at transferring the image, while still transferring water, although in smaller amounts, which can be ignored.)

The roller passes over the paper, transferring the ink onto the paper. Because the image is first transferred to the rubber cloth roller, this process is also called "offset printing." This term originates from the offset transfer of the image from the plate to the rubber roller and then onto the paper.

Over the years, this process has seen many innovations and technological improvements, including multi-color printing presses that can print several colors on a single pass, Dahlgren ink systems that eliminate the need for damping (incorporating damping into the ink delivery process).

At the input end, advances in desktop publishing have enabled almost anyone to produce professional-quality layouts, and the development of typesetting machines has allowed printing houses to bypass the intermediate step of phototypesetting; typesetting machines can directly produce film from computer-generated images. Since the 21st century, direct plate-making machines have eliminated the need for film and can directly image onto the printing plate.

Waterless Printing Principle

Toray System

The Toray waterless printing system consists of three main components: waterless plates, specially formulated waterless printing inks, and printing equipment equipped with a temperature control system.

The structure of Toray waterless plates is multi-layered. The bottom layer is an aluminum base, coated with a layer of photosensitive polymer as the middle layer, and topped with a 2-micron layer of silicone rubber resin. Depending on the type of Toray waterless plate, its run length ranges from 150,000 to 600,000 impressions, based on standard copper plate paper. However, if the paper surface is rough, the run length may decrease. Toray waterless plates are recyclable, similar to traditional recyclable PS plates. They are suitable for use on various sheet-fed and web offset printing presses.

Exposure of these plates can be achieved using traditional vacuum frame exposure units and light sources, with exposure times similar to those of traditional plates. During exposure, UV light intensity is controlled based on the density of the film, allowing light to penetrate the silicone resin layer and expose the polymer layer underneath. The light reaction on the polymer layer is very precise, resulting in high plate resolution of up to 175 lines per inch, with a dot range of 0.5% to 99.5%. The diagram below illustrates the imaging mechanism of traditional wet plates compared to waterless plates.

After exposure, the plates are developed, a simple process involving specialized chemicals and mechanical equipment for waterless plate processing. The processed plate has a silicone resin layer in the non-image area, which repels ink, while in the image area, the silicone resin layer is removed, leaving behind the polymer material that attracts ink. During plate design, the plate can be selectively engineered to attract or repel ink without the use of water or alcohol.

The main difference between waterless ink and traditional ink lies in the type of resin and binder used. The binder in waterless printing inks is selected based on its rheological properties and is slightly more viscous than binders in traditional inks. The principle of waterless printing relies on the silicone resin layer in the non-image area of the plate having lower surface energy. If the ink viscosity is high, the molecular attraction within the ink itself is greater than its affinity with the silicone resin, resulting in strong ink repellency in those areas.

Temperature is a significant factor affecting viscosity. Since water is not involved in the waterless printing process, there is no substance to cool the plate surface. Due to friction, the temperature of the plate cylinder surface continuously rises during printing. Additionally, the high viscosity of waterless ink and the grinding of the ink on the rollers also contribute to an increase in surface temperature.

This is why precise temperature control is essential in waterless printing equipment. The most popular systems utilize vibration cooling technology, where hollow vibration cooling rollers extract coolant during the cooling process. This structure has been used for many years in high-speed web offset presses. Following improvements, it has also been applied to sheet-fed printing equipment. Nearly all sheet-fed press manufacturers offer hollow ink vibration rollers for temperature control. The diagram below illustrates a typical temperature control system in a waterless printing press. The number of ink vibration rollers affects the effectiveness of temperature control.

The function of the temperature control system is to circulate the coolant along the trajectory of the rollers, removing the heat generated by the mechanical movement of the printing unit.

It is worth noting that this type of system is not designed to cool (or refrigerate) the ink rollers, but merely to maintain the inherent ambient temperature of the press during normal operation. Maintaining a constant temperature is crucial for optimal ink viscosity. The diagram below illustrates the optimal temperature task window in a waterless printing temperature control system.

Proofing in waterless printing can be directly accomplished using film. However, not all analog systems can reproduce the low dot gain of waterless printing accurately. Some experienced waterless printing companies provide high-precision proofs, while others utilize calibrated digital proofing systems to achieve good results.

Presstek System

The Presstek PEARLdry printing system consists of two main components: printing plates and specially formulated waterless inks, which are very similar to the inks used by Toray.

According to Presstek, the installation of a temperature control system is optional for printing companies. However, to achieve higher run lengths, it is recommended to install a temperature control system. The maximum run length of PEARLdry plates is 50,000 impressions. PEARLdry plates are also recyclable, with a maximum plate size of 23.75" x 29.4" (four-up). The diagram below illustrates the PEARLdry ABL negative plate used in waterless printing.

Similar to Toray plates, the top layer of PEARLdry plates is a silicone resin layer that repels ink. The middle layer is photosensitive, capable of absorbing ink while maintaining plate stability.

Imaging and exposure of the plates are done without the need for film and plate development. The imaging technology used is ablative imaging, which employs a set of IR (infrared) laser diode arrays to remove the silicone resin image layer, exposing the aluminum or polyester layer underneath for ink absorption. During imaging, lasers rapidly heat the image layer, causing gas expansion that separates the image and the upper silicone resin layer from the plate. Once the plate is imaged, it is stripped and mounted on the press. This system allows for direct plate imaging on the press (such as the 8-up A3 format GTP-DI or Quickmaster DI), or offline plate imaging using Presstek's PEARLsetter exposure unit. Offline imaging sizes include 8-up A3 (15.75"x20") and 4-up A2 (23.75"x29.4"). For larger formats up to 40 inches (102cm), imaging of large-format plates can be achieved using thermal direct imaging machines. With the emergence of various PostScript systems, waterless printing can be proofed using various types of digital proofing systems, such as Iris inkjet printers and 3M Rainbow.

Advantages

Environmentally friendly: Eliminates the use of fountain solution, making it highly beneficial for the environment.

Consistent colors: Without a dampening system, color variations on the printed materials are minimized during the printing process.

Good color saturation: Waterless printing achieves approximately 20% higher color density than SWOP standards (Specifications for Web Offset Publications, a commonly used ink set in the United States).

Dot gain: Excellent reproduction of details, with waterless printing exhibiting low dot gain, allowing for printing of high line screen jobs with excellent detail reproduction. Even with a line screen of 175 lines per inch, waterless printing produces excellent results.

Wide substrate range: Waterless printing can be used on papers and substrates that traditional printing cannot, such as coated papers. Without a dampening system, many paper-related issues in traditional printing are eliminated.

Short printing preparation time: Approximately 40% shorter than traditional printing preparation time.

Environmental Impact

Breakthrough in environmental protection.

Fresh air, abundant water sources, dense forests, and safe working conditions are environmental goals pursued by countries worldwide. However, in some areas, the printing industry has caused serious environmental damage. Waterless printing can make significant contributions to environmental protection.

Elimination of volatile organic compounds (VOCs).

Waterless printing eliminates the use of VOCs found in fountain solutions. Some VOCs can deplete the ozone layer and contribute to global warming. By using water-based inks, waterless printing bids farewell to VOCs.

Revolution in ink technology eliminates the need for organic solvents when cleaning rubber blankets, greatly reducing the emission of VOCs. Wash-up solutions consist of 93% water and 7% harmless surfactants (such as soap), making them water-based cleaning agents.

Protection of water resources.

In 1995, 92% of the global population had access to sufficient water sources. However, if water resources continue to be consumed at the current rate, by 2050, only 58% of the global population will have access to sufficient water. In the future, some countries may even go to war over water resources.

Waterless printing can protect water resources. For example, a Swiss printing company that introduced the world's first generation of waterless web offset presses saves 250,000 liters of water annually. Most of this water would otherwise be drawn from a nearby lake, which serves as a water source for nearly ten thousand people.

Protection of forest resources.

Producing one ton of paper requires the use of 17 large trees, enough to provide paper for 20 people to read newspapers for a year.
While the destruction of forests provides raw materials for paper production, it also destroys the natural habitats of countless flora and fauna, severely disrupting ecological balance.
Due to the rapid ink-setting process in waterless printing, sheets can be immediately registered and color consistency maintained during printing, resulting in a 30% to 40% reduction in paper consumption compared to traditional printing. This not only protects forests but also significantly reduces production costs.

A cleaner environment and healthier humans.

Printing workers who inhale gases containing toxic organic compounds all day significantly damage their health, affecting kidney and lung function, causing dry skin, and even leading to dermatitis and damage to the nervous system, among other issues.

The most serious health damage occurs when workers use high-volatility solvents to clean rubber blankets.
Clearly, waterless printing, which eliminates the use of fountain systems, removes toxic substances from blanket cleaning solvents, protecting not only air, soil, and water but also human health.

Comparison with Traditional Printing

Brighter and more vivid colors. The comparison of prints between waterless and traditional printing shows that waterless prints have greater contrast, with darker shadow areas and brighter highlights. Waterless printing can achieve printing quality that traditional printing cannot.

More accurate color reproduction. Waterless prints have higher ink density, allowing for richer and more realistic images compared to traditional prints.

Better resolution. Waterless prints appear sharper and more vibrant compared to traditional prints. Waterless printing uses a line screen of 300 lines per inch, while traditional printing uses 175 lines per inch.

Larger color gamut. Waterless printing achieves better print quality. Due to its low dot gain, fine details in shadow areas can be clearly reproduced. In particular, the higher ink density in waterless prints expands the color gamut, resulting in more saturated colors.

The diagram below shows the color gamut spaces of three types of printing measured using an SPM100 densitometer. The color gamut space of traditional printing is significantly smaller than that of waterless printing.

Due to the high ink density, the color gamut perimeter of waterless sheet-fed printing is 21.3% longer than that of traditional web offset printing, and 13% longer than that of traditional sheet-fed printing.

Posted on 2024-04-15 by ZXC Print