Printing Techniques

Offset lithography is an analog printing process still the most common way to produce printed materials on paper today. Offset refers to the technique in which the inked image is transferred (or 'offset') from a plate to a rubber blanket, then to the paper surface. Lithography is based on the repulsion of oil and water. Offset lithography uses oil-based inks. The plate on which the image is carried obtains ink from the ink rollers, while the non-printing area attracts a water-based film (called 'fountain solution'), keeping the non-printing areas ink-free. Offset lithography prints images in four colours: Cyan, Magenta, Yellow and Black (CMYK). Pre-press - specifically colour separations, the converting of RGB images to CMYK - is an essential part of the offset lithography process. Offset lithography is optimal for economically producing large volumes of high quality prints. In terms of quality, offset lithography is regarded as the benchmark, with digital printing processes ranked in accordance with their ability to achieve offset like results. Today, with continued improvements in digital printing, the benchmark for quality is shifting, with digital being at least equal to litho, or considered on its own terms for a specific look or finish that is not achievable with offset lithography.

Digital printing is the process of printing from a digital image directly onto a variety of media. Digital printing uses inkjet or dry toner technology, electrically controlling the location of ink or toner particles with laser technology. Digital printing can be used for short run or high volume printing. The higher cost per page for digital printing is countered by avoiding the cost of making plates required for offset printing. There is also a considerable time saving compared to offset lithography, where plate changes are required during the printing process. Based on the volume of a print run, there is a threshold at which offset lithography becomes more economical than digital printing, although increasingly, as digital technology develops, this threshold is getting higher. Digital printing is a 'clean hands' process, only requiring ink cartridges to be changed, while offset requires ink to be added to the printer manually. A fundamental difference between digital and offset is that digital uses binary code, as opposed to the analog plates used in offset lithography, to produce an image. This means that the image can be instantly changed during the digital printing process without incurring any costs. This capacity is referred to as variable data printing.

Inkjet printing reproduces a digital image by propelling droplets of ink through a nozzle onto a variety of media. There are two types of inkjet printing systems: multi-pass printing and single-pass printing. In multi-pass printing, the print head moves laterally over the print media, making several passes to complete the image printing process. In single-pass printing, the print head remains in a fixed position and completes image printing in a single pass while the print material passes underneath. Due to its high productivity and speed, single-pass printing is used for high volume print runs. Multi-pass printing is used for low volumes and large format print applications. There are two types of colorants used in inkjet inks - dye and pigment. Dyes are fully dissolved in the carrier fluid while pigments, based on microscopic particles of colour, do not fully dissolve. Pigment based inks are often referred to as 'archival inks' as under the right conditions they can keep their colour for 80 to 100 years. This is dependent on using an inkjet compatible media i.e. specially treated paper.

Electrophotography, or Xerography as it became know, is a dry photocopying technique. The development of Xerography into a commercial process led to the founding of the Xerox company, a market leader in digital printing today. Xerography was originally an analog process. The image to be printed is exposed onto a photosensitive drum using a combination of flash lights and electrostatic charge. Toner, a dry powder, is used to develop the image. Through a combination of pressure and electrostatic charge the toner image is developed on the drum before being transferred onto paper. The toner image is permanently fixed to the paper using either a heat mechanism (hot roll fuser) or a radiant fusing technology (oven fuser) to melt and bond the toner particles into the paper. Xerography became digital with the introduction of laser to the printing process. In a laser or LED printer, modulated light is projected directly onto the drum's surface to expose the image. Xerography is recognisable for the shiny finish of the printed image that results from the fusing of the toner using heat.

Laser printing is an electrostatic digital printing process. It produces printed material by repeatedly passing a laser beam back and forth over a negatively charged cylinder called a 'drum'. In accordance with a digital file the laser defines a differentially charged image on the drum. The drum then selectively collects electrically charged ink (powdered ie xerography or wet ink ie inkjet and Indigo), and transfers the image to the media.

Digital offset is a term coined by HP to describe their digital printing process. Although of all the digital printing process, the printed output of HP presses most closely resemble offset lithography, the printing process is fundamentally different, having more in common with electrophotography. Instead of printing from metal plates, the HP presses use lasers to create a latent image on the Photo Imaging Plate or PIP drum through the use of an electrostatic charge. This negatively charged area then attracts the positively charged ElectroInk (an HP Patent) which in turn is transferred to the ITM drum or 'blanket' and then again from the blanket to the media. Like toner, ElectroInk contains pigment particles that can be electrically charged. ElectroInk differs from toner as the pigment is carried by a liquid. ElectroInk is supplied as a concentrated paste that is loaded into the press in tubular cartridges in a 'clean hands' operation. Inside the press it is automatically fed into ink supply tanks and diluted with oil, to form a fluid mixture ready for printing. As is the case with Litho, ElectroInks can be premixed before being loaded onto the press to produce accurate Pantone emulations. HP presses print on a wide range of media. A wide range of papers are produced specifically for HP presses, but standard offset papers can also be used by applying an inline primer.

Risograph is a brand of digital duplicators manufactured by the Riso Kagaku Corporation in Japan. The Riso printer was developed for high-volume photocopying and printing. When printing or copying more than 20 reproductions RISO is less expensive per page than a conventional toner photocopier, laser printer or inkjet printer. The underlying technology of RISO is very similar to a mimeograph, or stencil printer. The original artwork is scanned through the machine. A master is created from the scan by means of tiny heat spots on a thermal plate burning voids that correspond to image areas into the master sheet. This master is then wrapped around a drum and ink is forced through the voids in the master that acts as a stencil. The Riso process uses oil based inks similar to offset printing inks. The printing process does not require heat or a dryer to fix the image on the paper as is the case with Inkjet, Xerography and Digital Offset. As a result, like Offset, Riso can handle a variety of papers. Riso printers typically have interchangeable colour inks and drums allowing for printing in different colours or using spot colour in one print job. Due to difficulties with colour alignment, it is difficult to print full colour images on Riso. Riso is often used precisely for this misaligned colour effect, which resembles screen printing in its finish.