Offset Printing
Offset printing is a widely used printing technique where the inked image is transferred (or “offset”) from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a film of water, keeping the non-printing areas ink-free. Currently, most books and newspapers are printed using the technique of offset lithography.
Offset printing is a widely used printing technique where the inked image is transferred (or “offset”) from a plate to a rubber blanket, then to the printing surface. When used in combination with the lithographic process, which is based on the repulsion of oil and water, the offset technique employs a flat (planographic) image carrier on which the image to be printed obtains ink from ink rollers, while the non-printing area attracts a film of water, keeping the non-printing areas ink-free. Currently, most books and newspapers are printed using the technique of offset lithography.
Flexography Printing
Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. Flexography is an updated version of letterpress that can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid colour).
Flexography (often abbreviated to flexo) is a form of printing process which utilizes a flexible relief plate. Flexography is an updated version of letterpress that can be used for printing on almost any type of substrate including plastic, metallic films, cellophane, and paper. It is widely used for printing on the non-porous substrates required for various types of food packaging (it is also well suited for printing large areas of solid colour).
Letterpress printing
Letterpress printing is a term for the relief printing of text and image using a press with a “type-high bed” printing press and movable type, in which a reversed, raised surface is inked and then pressed into a sheet of paper to obtain a positive right-reading image. It was the normal form of printing text in the west from its invention by Johannes Gutenberg in the mid-15th century until the 19th century and remained in wide use for books and other uses until the second half of the 20th century. In addition to the direct impression of inked movable type onto paper or another receptive surface, the term Letterpress can also refer to the direct impression of inked printmaking blocks such as photo-etched zinc “cuts” (plates), linoleum blocks, wood engravings, etc., using such a press. In the 21st century, commercial Letterpress has been revived by the use of “water-wash” photopolymer plates which are adhered to a near-type-high base to produce a relief printing surface typically from digitally-rendered art and typography.
Letterpress printing is a term for the relief printing of text and image using a press with a “type-high bed” printing press and movable type, in which a reversed, raised surface is inked and then pressed into a sheet of paper to obtain a positive right-reading image. It was the normal form of printing text in the west from its invention by Johannes Gutenberg in the mid-15th century until the 19th century and remained in wide use for books and other uses until the second half of the 20th century. In addition to the direct impression of inked movable type onto paper or another receptive surface, the term Letterpress can also refer to the direct impression of inked printmaking blocks such as photo-etched zinc “cuts” (plates), linoleum blocks, wood engravings, etc., using such a press. In the 21st century, commercial Letterpress has been revived by the use of “water-wash” photopolymer plates which are adhered to a near-type-high base to produce a relief printing surface typically from digitally-rendered art and typography.
Digital printing
Digital printing is the reproduction of digital images on a physical surface. It is generally used for short print runs, and for the customization of print media.
The process differs from lithography, flexography, gravure, and letterpress printing in several ways:
Digital Printing is used for personalized printing or variable data printing (VDP or VI), for example personalized children's books, which are customized with the specific child's name and images. Print on Demand (POD) systems also use digital printing, for short run books of varying page quantities, and binding techniques. Digital prints can also be done on photographic paper, exposed with RGB laser lights from computer files, and processed in photographic developers and fixers. These prints are continuous tone images, and have the dyes imbedded in emulsion layers within plastic coatings. They can be very archival.
Digital printing is the reproduction of digital images on a physical surface. It is generally used for short print runs, and for the customization of print media.
The process differs from lithography, flexography, gravure, and letterpress printing in several ways:
- Every print can be different, because printing plates are not required, as in traditional methods.
- There is less wasted chemical and paper, because there is no need to bring the image “up to colour” and check for registration and position.
- The ink or toner does not permeate the substrate, as does conventional ink, but forms a thin layer on the surface and may in some systems be additionally adhered to the substrate by using a fuser fluid with heat process (toner) or UV curing process (ink).
Digital Printing is used for personalized printing or variable data printing (VDP or VI), for example personalized children's books, which are customized with the specific child's name and images. Print on Demand (POD) systems also use digital printing, for short run books of varying page quantities, and binding techniques. Digital prints can also be done on photographic paper, exposed with RGB laser lights from computer files, and processed in photographic developers and fixers. These prints are continuous tone images, and have the dyes imbedded in emulsion layers within plastic coatings. They can be very archival.
Inkjet
An inkjet printer is a type of computer printer that reproduces a digital image by propelling variably-sized droplets of liquid or molten material (ink) onto a page. Inkjet printers are the most common type of printer and range from small inexpensive consumer models to very large and expensive professional machines.
The idea of inkjet printing dates back to the 19th century and the technology was first developed in the early 1950s. Starting in the late 1970’s, inkjet printers that could reproduce digital images generated by computers were developed, mainly by Epson, Hewlett-Packard and Canon. In the worldwide consumer market, four manufacturers account for the majority of inkjet printer sales: Canon, Hewlett-Packard, Epson, and Lexmark.
The emerging ink jet material deposition market also uses ink jet technologies, typically piezoelectric crystals, to deposit materials directly on substrates. There are three main technologies in use in contemporary inkjet printers: thermal, piezoelectric, and continuous.
Thermal inkjets
Most consumer inkjet printers from companies including Canon, Hewlett-Packard, and Lexmark, use print cartridges with a series of tiny electrically heated chambers constructed by photolithography. To produce an image, the printer runs a pulse of current through the heating elements causing a steam explosion in the chamber to form a bubble, which propels a droplet of ink onto the paper (hence Canon's trade name of Bubble Jet for its inkjets). The ink’s surface tension as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir.
The ink used is known as aqueous (water-based inks using pigments or dyes) and the print head is generally cheaper to produce than other inkjet technologies. The principle was discovered by Canon engineer Ichiro Endo in August 1977. Note that thermal inkjets have no relation to thermal printers, which produce images by heating thermal paper, as seen on older fax machines, cash registers, ATM receipt printers, and lottery ticket printers.
Piezoelectric inkjets
Most commercial and industrial ink jet printers and some consumer printers (Epson) use a piezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape or size, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. This is essentially the same mechanism as the thermal inkjet but generates the pressure pulse using a different physical principle. Piezoelectric (also called Piezo) ink jet allows a wider variety of inks than thermal or continuous ink jet but the print heads are more expensive. Piezo inkjet technology is often used on production lines to mark products - for instance the use-before date is often applied to products with this technique; in this application the head is stationary and the product moves. Requirements of this application are a long service life, a relatively large gap between the print head and the substrate, and low operating costs. There is a drop-on-demand process, with software that directs the heads to apply between zero and eight droplets of ink per dot and only where needed. As of June 2009, the fastest cut-sheet inkjet printer on the market is the RISO ComColour 9050, which prints 146 US Letter and 150 A4 full-colour pages per minute in both one-sided and two-sided printing modes.
Continuous ink jet
The continuous ink jet method is used commercially for marking and coding of products and packages. The idea was first patented in 1867, by Lord Kelvin and the first commercial devices (medical strip chart recorders) were introduced in 1951 by Siemens.
The continuous ink jet method is used commercially for marking and coding of products and packages. The idea was first patented in 1867, by Lord Kelvin and the first commercial devices (medical strip chart recorders) were introduced in 1951 by Siemens.
In continuous ink jet technology, a high-pressure pump directs liquid ink from a reservoir through a gun body and a microscopic nozzle, creating a continuous stream of ink droplets via the Plateau-Rayleigh instability. A piezoelectric crystal creates an acoustic wave as it vibrates within the gun body and causes the stream of liquid to break into droplets at regular intervals - 64,000 to 165,000 drops per second may be achieved. The ink droplets are subjected to an electrostatic field created by a charging electrode as they form, the field varies according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each droplet. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighbouring droplets.
The charged droplets pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the receptor material (substrate), or allowed to continue on undeflected to a collection gutter for reuse. The more highly charged droplets are deflected to a greater degree. Only a small fraction of the droplets are used to print, the majority being recycled.
Continuous ink jet is one of the oldest ink jet technologies in use and is fairly mature. One of its advantages is the very high velocity (~50 m/s) of the ink droplets, which allows for a relatively long distance between print head and substrate. Another advantage is freedom from nozzle clogging as the jet is always in use, therefore allowing volatile solvents such as ketones and alcohols to be employed, giving the ink the ability to “bite” into the substrate and dry quickly.
The ink system requires active solvent regulation to counter solvent evaporation during the time of flight (time between nozzle ejection and gutter recycling) and from the venting process whereby air that is drawn into the gutter along with the unused drops is vented from the reservoir. Viscosity is monitored and a solvent (or solvent blend) is added in order to counteract the solvent loss.
Inkjet Inks
The basic problem with inkjet inks is the conflicting requirements for a colouring agent that will stay on the surface and rapid disbursement of the carrier fluid.
Desktop inkjet printers, as used in offices or at home, tend to use aqueous inks based on a mixture of water, glycol and dyes or pigments. These inks are inexpensive to manufacture, but are difficult to control on the surface of media, often requiring specially coated media. Aqueous inks are mainly used in printers with thermal inkjet heads, as these heads require water in order to perform. While aqueous inks often provide the broadest colour gamut and most vivid colour, most are not waterproof without specialized coating or lamination after printing. Most Dye-based inks, while usually the least expensive, are subject to rapid fading when exposed to light. Pigment-based aqueous inks are typically more costly but provide much better long-term durability and ultraviolet resistance. Inks marketed as “Archival Quality” are usually pigment-based. Some professional wide format printers use aqueous inks, but the majority in professional use today employ a much wider range of inks, most of which require piezo inkjet heads and extensive maintenance:
An inkjet printer is a type of computer printer that reproduces a digital image by propelling variably-sized droplets of liquid or molten material (ink) onto a page. Inkjet printers are the most common type of printer and range from small inexpensive consumer models to very large and expensive professional machines.
The idea of inkjet printing dates back to the 19th century and the technology was first developed in the early 1950s. Starting in the late 1970’s, inkjet printers that could reproduce digital images generated by computers were developed, mainly by Epson, Hewlett-Packard and Canon. In the worldwide consumer market, four manufacturers account for the majority of inkjet printer sales: Canon, Hewlett-Packard, Epson, and Lexmark.
The emerging ink jet material deposition market also uses ink jet technologies, typically piezoelectric crystals, to deposit materials directly on substrates. There are three main technologies in use in contemporary inkjet printers: thermal, piezoelectric, and continuous.
Thermal inkjets
Most consumer inkjet printers from companies including Canon, Hewlett-Packard, and Lexmark, use print cartridges with a series of tiny electrically heated chambers constructed by photolithography. To produce an image, the printer runs a pulse of current through the heating elements causing a steam explosion in the chamber to form a bubble, which propels a droplet of ink onto the paper (hence Canon's trade name of Bubble Jet for its inkjets). The ink’s surface tension as well as the condensation and thus contraction of the vapor bubble, pulls a further charge of ink into the chamber through a narrow channel attached to an ink reservoir.
The ink used is known as aqueous (water-based inks using pigments or dyes) and the print head is generally cheaper to produce than other inkjet technologies. The principle was discovered by Canon engineer Ichiro Endo in August 1977. Note that thermal inkjets have no relation to thermal printers, which produce images by heating thermal paper, as seen on older fax machines, cash registers, ATM receipt printers, and lottery ticket printers.
Piezoelectric inkjets
Most commercial and industrial ink jet printers and some consumer printers (Epson) use a piezoelectric material in an ink-filled chamber behind each nozzle instead of a heating element. When a voltage is applied, the piezoelectric material changes shape or size, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. This is essentially the same mechanism as the thermal inkjet but generates the pressure pulse using a different physical principle. Piezoelectric (also called Piezo) ink jet allows a wider variety of inks than thermal or continuous ink jet but the print heads are more expensive. Piezo inkjet technology is often used on production lines to mark products - for instance the use-before date is often applied to products with this technique; in this application the head is stationary and the product moves. Requirements of this application are a long service life, a relatively large gap between the print head and the substrate, and low operating costs. There is a drop-on-demand process, with software that directs the heads to apply between zero and eight droplets of ink per dot and only where needed. As of June 2009, the fastest cut-sheet inkjet printer on the market is the RISO ComColour 9050, which prints 146 US Letter and 150 A4 full-colour pages per minute in both one-sided and two-sided printing modes.
Continuous ink jet
The continuous ink jet method is used commercially for marking and coding of products and packages. The idea was first patented in 1867, by Lord Kelvin and the first commercial devices (medical strip chart recorders) were introduced in 1951 by Siemens.
The continuous ink jet method is used commercially for marking and coding of products and packages. The idea was first patented in 1867, by Lord Kelvin and the first commercial devices (medical strip chart recorders) were introduced in 1951 by Siemens.
In continuous ink jet technology, a high-pressure pump directs liquid ink from a reservoir through a gun body and a microscopic nozzle, creating a continuous stream of ink droplets via the Plateau-Rayleigh instability. A piezoelectric crystal creates an acoustic wave as it vibrates within the gun body and causes the stream of liquid to break into droplets at regular intervals - 64,000 to 165,000 drops per second may be achieved. The ink droplets are subjected to an electrostatic field created by a charging electrode as they form, the field varies according to the degree of drop deflection desired. This results in a controlled, variable electrostatic charge on each droplet. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighbouring droplets.
The charged droplets pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the receptor material (substrate), or allowed to continue on undeflected to a collection gutter for reuse. The more highly charged droplets are deflected to a greater degree. Only a small fraction of the droplets are used to print, the majority being recycled.
Continuous ink jet is one of the oldest ink jet technologies in use and is fairly mature. One of its advantages is the very high velocity (~50 m/s) of the ink droplets, which allows for a relatively long distance between print head and substrate. Another advantage is freedom from nozzle clogging as the jet is always in use, therefore allowing volatile solvents such as ketones and alcohols to be employed, giving the ink the ability to “bite” into the substrate and dry quickly.
The ink system requires active solvent regulation to counter solvent evaporation during the time of flight (time between nozzle ejection and gutter recycling) and from the venting process whereby air that is drawn into the gutter along with the unused drops is vented from the reservoir. Viscosity is monitored and a solvent (or solvent blend) is added in order to counteract the solvent loss.
Inkjet Inks
The basic problem with inkjet inks is the conflicting requirements for a colouring agent that will stay on the surface and rapid disbursement of the carrier fluid.
Desktop inkjet printers, as used in offices or at home, tend to use aqueous inks based on a mixture of water, glycol and dyes or pigments. These inks are inexpensive to manufacture, but are difficult to control on the surface of media, often requiring specially coated media. Aqueous inks are mainly used in printers with thermal inkjet heads, as these heads require water in order to perform. While aqueous inks often provide the broadest colour gamut and most vivid colour, most are not waterproof without specialized coating or lamination after printing. Most Dye-based inks, while usually the least expensive, are subject to rapid fading when exposed to light. Pigment-based aqueous inks are typically more costly but provide much better long-term durability and ultraviolet resistance. Inks marketed as “Archival Quality” are usually pigment-based. Some professional wide format printers use aqueous inks, but the majority in professional use today employ a much wider range of inks, most of which require piezo inkjet heads and extensive maintenance:
- Solvent inks: the main ingredient of these inks is volatile organic compounds (VOCs), organic chemical compounds that have high vapor pressures. Colour is achieved using pigments rather than dyes for excellent fade-resistance. The chief advantage of solvent inks is that they are comparatively inexpensive and enable printing on flexible, uncoated vinyl substrates, which are used to produce vehicle graphics, billboards, banners and adhesive decals. Disadvantages include the vapour produced by the solvent and the need to dispose of used solvent. Unlike most aqueous inks, prints made using solvent-based inks are generally waterproof and ultraviolet-resistant (for outdoor use) without special over-coatings. The high print speed of many solvent printers demands special drying equipment, usually a combination of heaters and blowers. The substrate is usually heated immediately before and after the print heads apply ink. Solvent inks are divided into two sub-categories:
- Hard solvent ink offers the greatest durability without specialized over-coatings but requires specialized ventilation of the printing area to avoid exposure to hazardous fumes.
- Mild or “Eco” solvent inks, while still not as safe as aqueous inks, are intended for use in enclosed spaces without specialized ventilation of the printing area. Mild solvent inks have rapidly gained popularity in recent years as their colour quality and durability have increased while ink cost has dropped significantly.
- UV-curable inks: these inks consist mainly of acrylic monomers with an initiator package. After printing, the ink is cured by exposure to strong UV-light. The advantage of UV-curable inks is that they “dry” as soon as they are cured, they can be applied to a wide range of uncoated substrates, and they produce a very robust image. Disadvantages are that they are expensive, require expensive curing modules in the printer, and the cured ink has a significant volume and so gives a slight relief on the surface. Though improvements are being made in the technology, UV-curable inks, because of their volume, are somewhat susceptible to cracking if applied to a flexible substrate. As such, they are often used in large “flatbed” printers, which print directly to rigid substrates such as plastic, wood or aluminum where flexibility is not a concern.
- Dye sublimation inks: these inks contain special sublimation dyes and are used to print directly or indirectly on to fabrics which consist of a high percentage of polyester fibres. A heating step causes the dyes to sublimate into the fibers and create an image with strong color and good durability.
Comparison of printing methods
| printing process |
transfer method |
pressure applied |
drop size | dynamic viscosity | thickness of ink on substrate | notes | cost-effective run length |
|---|---|---|---|---|---|---|---|
| Offset printing | rollers | 1 MPa | - | 40-100 Pa⋅s | 0.5-1.5 µm | high print quality | >5,000 (A3 trim size, sheet-fed) >30,000 (A3 trim size, web-fed) |
| Rotogravure | rollers | 3 MPa | - | 0.05-0.2 Pa⋅s | 0.8-8 µm | thick ink layers possible, excellent image reproduction, edges of letters and lines are jagged | >500,000 |
| Flexography | rollers | 0.3 MPa | - | 0.05-0.5 Pa⋅s | 0.8-2.5 µm | moderate quality | - |
| Letterpress printing | platen | 10 MPa | - | 50-150 Pa⋅s | 0.5-1.5 µm | slow drying | - |
| Screen-printing | pressing ink through holes in screen | - | - | - | <12 µm | versatile method, low quality | - |
| Xerography | electrostatics | - | - | - | 5-10 µm | thick ink | - |
| Inkjet printer | thermal | - | 5-30 pl | 1-5 Pa⋅s | <0.5 µm | special paper required to reduce bleeding | <350 (A3 trim size) |
| Inkjet printer | piezoelectric | - | 4-30 pl | 5-20 Pa⋅s | <0.5 µm | special paper required to reduce bleeding | <350 (A3 trim size) |
| Inkjet printer | continuous | - | 5-100 pl | 1-5 Pa⋅s | <0.5 µm | special paper required to reduce bleeding | <350 (A3 trim size) |