Printer (computing)

In computing, a printer is a peripheral device which converts digital information to permanent marks on paper or other materials. These are most commonly used in conjunction with a desktop or laptop computer, but can in some cases be used with other external devices such as mobile phones or tablet computers. In some cases, printers are built directly into a digital device.

Categories

Printers can be categorized broadly in two groups: paper and 3D.

Paper

Most printers in existence are designed to place text and/or images onto paper. Some printers, such as modern laser printers, handle single pages, whereas older printers used continuous rolls or fan-fold paper. High-speed printers used tractor feeds that fed continuous paper through the printing mechanism, keeping position by the use of projections that fit through evenly spaces holes on the left and right margins of the paper. Most printers used either 80-column-wide (9.5") or 132-column-wide paper. These printers can also print onto mailing labels, barcode labels, checks, photo paper (for pictures), CDs or DVDs, etc.

Plotter

For a more detailed treatment, see Plotter printer.
Plotter printers are used to produce images, usually on large sheets of paper. As plotter technology evolves, theses printers become faster, but they still generally take some time to complete their tasks. Since they are slower than most other modern printers, they are generally not used for regular-sized printing tasks.

3D

For a more detailed treatment, see Three-dimensional printer.
3D printers are used to create 3-dimensional objects. The object is built one small layer at a time from bottom to top. Most consumer 3D printers can create plastic objects up to about 10" in any dimension. To use this type of printer a user creates a 3D model on a computer, then sends the job to the printer. The printer then constructs the object on a mat, by moving the nozzle (or print head) on a dual-axis assembly and releasing small amounts of the material where it is needed. In general, these printers are very slow in comparison to 2D printers, but the technology is still evolving and improving significantly. These are used in private, educational, manufacturing, medical, and other venues.

Usage

Personal

Personal printers are usually small and inexpensive, and most commonly print using the inkjet method. They receive instructions directly from a primary device (computer) through a serial or parallel port (older methods) or USB or Network connection (such as wireless connections via Bluetooth or WiFi). They generally operate solely as a output peripheral, though sometimes they include fax, image scanning, and/or photocopy functionality.

Network

Network printers are not directly connected to a controller (usually a computer) but rather a print server, which exists as an accessible entity on the local area network. In some cases, the print server is an entirely separate server computer, through which all print jobs are managed. However, sometimes (especially in smaller installations) the printer uses an internal print server. Either way, when something is to be printed, the requesting device connects to the print server through the intranet (or sometimes internet) and spools the print job to it. The print server then has the printer complete the task.
Network printers are commonly used for frequent and high-volume printing, so they often use Laser printers, which are often somewhat larger than most other printers. They are often towers which stand three to four feet tall.

Virtual

Virtual printers have earned their name quite simply because they are not really printers at all. These printers have no physical module, but are rather computer programs. The operating system and other software recognizes these as printers, but when something is "printed" to one, the task is spooled into a file, rather than an actual printer. Perhaps the most common virtual printer is produced by Adobe, which "prints" to PDF.

History

Early printers came in two forms: with keyboards and without keyboards. In fact, some early printer lines used the exact same printing mechanism, with some models having additional features such as keyboards and/or paper tape punches/readers. The ASR33 teletype was one of these printer lines. With an associated keyboard, the printers could be used as terminals. This practice continued for many years for cheaper printers, including the Digital Equipment Corporation LA line. The DECWriter II (model LA36) was one of the most popular printing terminals to supplant the teletype.

Teletypes operated at 110 baud (about 11 characters per second), whereas the newer dot-matrix printers achieved 1200 baud (about 120 characters per second). Later printer speeds where measured in lines per minute or pages per minute.

Even as video terminals (and then PCs) replaced printing terminals, printers were still needed to create hard-copy output. Technological advancement was then focused on increasing print speed, increasing print quality, adding color, and reducing cost.

The 2010's saw the introduction of consumer-priced 3D printers.

Printing Mechanisms

Printer mechanisms have varied over time, with some falling out of favor as technology advances and costs come down. Typically, two or three different printing technologies vied for prominence at any given time with the older technologies providing a cheaper alternative and the newer technologies providing advantages in speed and/or capability.

Solid print heads

The original printers used a metal cylinder with the character glyphs embossed on the surface. The cylinder was rotated and raised/lowered so that the appropriate glyph faced the paper. The head was then forced forward against an inked ribbon and then the paper, forming the image on the paper in ink. The head was moved left to right with the process repeated for each column. When the head reached the right side, it returned to the left side to begin the next line.

Line printers also used metal cylinders (called "drums"), except that the cylinders were oriented horizontally with up to 132 stripes that circumscribed the drum - one for each column. Each stripe had all printable glyphs, so that if you looked at the drum while rotating it, you would see a line of 132 "A"s across the width, then a line of 132 "B"s, and so forth. The printer spun the drum at high speed and when the appropriate letter for a given column has spun into position, a hammer behind the paper would quickly push the paper into contact with the embossed glyph on the drum, with an inked ribbon between them, thus placing the character onto the paper. Since more than one instance of a given character would often occur on the same line, multiple hammers would strike the paper at the same time (for each column that used that character on that line), thus an entire line of text could be transferred to the paper during a single rotation of the drum.

Band printers used a similar approach except that the glyphs were embossed on a belt that spun in a horizontal direction. The belt had only a single instance of each character, but as the belt traveled across the paper, hammers would strike when the appropriate character was in the appropriate position - often with many hammers striking simultaneously when the appropriate glyphs lined up with the appropriate column.

With carbon paper, multiple copies could be made simultaneously. Because of their mechanical nature, these types of printers tended to be extremely noisy.

Dot-matrix

The next advance in printing terminals used a different type of print-head called a dot-matrix printer. The advantage is that these print heads could print faster than entirely mechanical print heads. Dot-matrix printer technology consists of narrow metal pins organized in rows and columns (typically 7x9). For a given glyph, a pattern of the pins were pushed out, through an ink ribbon, creating a representation of the glyph on the paper. The sound of the metal pins hitting the paper made the printers moderately noisy, but far less than solid print head printers. They could also use carbon paper. Although the use of finely controlled paper movement allowed arbitrary resolution, considerations of print speed, ink absorption by the paper, and pin size, the quality of the output was typically about 50 dpi, as opposed to the current 1,200 dpi of laser printers.

When printing:

1. The top edge of a continuous sheet of pin fed paper is aligned just above the print head
2. The print head moves from side to side, rapidly extending and retracting pins where marks should appear on the paper.
3. A ribbon runs between the paper and the print head, so the pins press this ribbon against the paper, leaving ink behind
4. The ribbon loop continuously feeds from the ink cartage to the head and back again to maintain the availability of ink.
5. When the print job is complete, extra paper is fed out, so that the product may be torn or cut off

Thermal

Thermal print heads use special heat-sensitive paper (which turns black in response to heat) and a print head that heats up a matrix in an appropriate configuration to transfer the image to the paper, much resembling the dot-matrix printer output, but with almost no noise. These printers are limited to black printing only, and the final product can be destroyed sunlight, hot surfaces, or friction. However, they are inexpensive and convenient for a number of purposes, including shopping receipt printing and label production, but they never caught on in the general consumer market. Thermal printers do not use any ink or toner.

The most common form of thermal paper contains bisphenol A, which raises health concerns because the substance transfers to the skin of the person holding the paper. BPA is then absorbed by the skin to varying degrees. Some thermal media also contains bisphenol S, which although believed to be slightly less harmful than BPA, is absorbed more easily thus increasing the exposure quantity.

Care and maintenance

Since no ink is used, thermal printers tend to stay fairly clean. However, paper fragments and dust can still build up, so these should be cleaned from time to time. Compressed air and/or a damp cloth can be used to clean it manually. Some printers can also be cleaned by feeding through a cleaning card.

Inkjet

Ink-jet printers superseded dot-matrix printers. They worked by spraying droplets of ink through extremely fine nozzle​s, which would then create the character on the paper, usually in a dot-matrix pattern. However, because the angle of the ink drops could be controlled, the resolution of characters could exceed 100 dots per inch (dpi), which was far superior to the quality of dot-matrix technology. Because the only mechanical movement was moving the nozzle from side to side, and the feeding of paper, these printers were extremely quiet. The printers often came with several colored ink cartridges which allowed color printing of graphics. The printing mechanism was simple and so the cost of the printers was often less than $100. However, the ink tended to be extremely expensive, quickly used up, tended to fade with age, and on infrequently-used printers, it would often congeal in the nozzles, causing printing issues.

This process (called drop-on-demand) can be performed several different ways.

bubblejet

The most common method of inkjet printing uses the bubblejet technique. Resistors in the print head are used to produce heat which vaporizes some of the ink, creating a bubble. This bubble of gas propels a droplet of liquid ink out of the head and onto the medium.

Piezoelectric (piezo)

Piezoelectric printers use, as the name implies, piezoelectric crystals to generate the required ink jets. Small quantities of one or more of these selected materials are placed in the print head. When power is applied to the material, it changes shape, pushing a small droplet of ink out onto the medium.

Care and maintenance

The main problem these printers experience is clogging of the print heads. They can usually be cleaned fairly easily using isopropyl alcohol. However, the printer must be opened up and the print heads removed to do this, and once done, the printer usually must print one or more alignment sheets (which consumes expensive ink). Many Inkjet printers have the print heads on the actual ink cartridges, which means that each time the cartage is replaced, so is the head.

Electrostatic

Electrostatic printers use the same technology as copy machines. A laser is used to create patterns on a charged metal drum as the drum turns, thus creating an electrically-charged version of the text or image to print. The drum rotates through a supply of toner, which sticks to the charged parts of the drum, but not the non-charged parts. The drum is then pressed against paper which transfers the toner to the paper. Finally, pressure and heat is used to fix the toner onto the paper, which is then discharged from the printer with the completed page on it. Black printers go through this process once, but color printer must do so four times. There are four photoreceptive drums, one is used for each color—yellow, magenta, cyan, and black. Each drum prints its color onto a belt, then a transfer roller moves the toner to the paper. and because the toner particles are so fine, the quality of text is as much as 1,200 dpi even on cheaper consumer modules, which is close to the 1,440 dpi quality of typeset.

The printing process is as follows:

1. Processing: Construct the page in memory
2. Charging: Put an negative electrostatic charge on the imaging drum
3. Exposing: Write the image on the drum using laser
4. Developing: Add negatively charged toner to the drum (it will be repelled by all areas except those written on by laser)
5. Transferring: Place the toner from the drum onto the paper (If printing color, transfer to a belt first, then the paper)
6. Fusing: Apply heat and/or pressure to the page
7. Cleaning: Remove excess toner

Care and maintenance

Since these printers use powder toner, they tend to get quite dirty over time, and should be cleaned periodically. However, compressed air should not be used in most cases, since the toner may become lodged in more vital areas, and this will generally make a mess of the surrounding area. Cold water on a cloth is usually the best option for cleaning most surfaces, and isopropyl alcohol cleans the rollers well.