Printers are commonly used output devices that produce a hard copy of document stored in electronic form, i.e they put information from computer on to paper.

There are various kinds of printers available today like Impact printers, Bubble-jet printers, Laser printers, Thermal printers etc.

1.2 Types of printers

1.2.1 Impact printers

Impact printers are among the old printing technologies, which make use of inked ribbon to make an imprint on the paper. Impact printers are considered noisy when compared to other printers.

The most commonly known impact printers are

Daisy-Wheel Printers
Dot-Matrix Printers
How Daisy-Wheel Printers work

A Daisy-Wheel Printer works on the same principle as ball-head typewriter. The daisy wheel printer consists of a disk made of plastic or metal on which characters stand out along the outer edge. The printer rotates the disk to print a character until the desired letter is facing the paper, after which a hammer called solenoid strikes forcing the character to hit an ink ribbon making a mark of the character on the paper. Their speed is rated by cps (number of characters per second).
	Fig. 1:Daisy-wheel,

	Fig b.: A section in detail

Advantages and Disadvantages

The main disadvantage of this printer is that they make noise when printing and these kind of printers cannot print graphics.

The advantage is that they are not expensive and can produce letter-quality text.

ii. Dot-Matrix Printers:

A typical dot-matrix printer is shown in the figure below. It consists of a print head, sheet guide assembly, platen knob, and covers.

Fig.: Dot-matrix printer

Fig.: Continuous feed paper

The name Dot-Matrix refers to the mechanism the printer uses to print characters on paper i.e, dots.

In this type of printer, it consists of a column of pins on the printhead that form letters and numbers as the printhead moves across the paper. The most recent dot matrix printers are equipped with 24 pins

Figure: 8-pin dot-matrix print head with examples of printed letters.

The pins, contained in the printhead, are about one inch long and are driven by several hammers, which force each pin into contact with the ink ribbon (and paper) at a certain time. The force on these hammers comes from the magnetic pull of small wire coils (solenoids), which are energized at a particular time, depending on the character to be printed.

Print Width

Most printers can print 80 columns, and 132 columns per line.

Advantages of Dot-matrix printers include the following:

They can print on multi-part stationary or carbon copies
Lower printing costs compared with Inkjet or Laser, the reason being that dot-matrix printers use a ribbon and rugged printing process.
These can withstand unclean or dusty environment whereas Inkject or laser jet printers require clean environment
Suitable for traction fed paper or continuous paper feed. Sometimes you may need to print an activity using continuous paper feed. Inkjet and Laser printers use discrete sheets of paper, and normally do not use continuous paper feed.
Using these printers require negligible operator training.
These printers usually cost less (initial purchase cost may be more, but running cost is negligible)

The main disadvantages of Dot-matrix printers are:

Dot-matrix printers are noisy but not as much as Daisy-wheel printers
The quality of print is not as good as laser and inkjet printers.
These printers cannot be used for producing good quality images or for photo printing.
These printers are slow compared with laser printers.

1.2.2 Solid-Ink Printers

Solid-Ink printers use ink in a waxy solid form than liquid form which avoids problems like spillage. And these kinds of printers print one line at a time and these printers are best suitable for graphic companies that need true color at a price lower than a color laser printer.

Advantages and Disadvantages

Advantages are good print quality, ease of use and generate less waste and the disadvantages include more power consumption and odour of wax

Solid ink printers are less sensitive to paper thickness and paper fibers. This allows printing on a number of different surfaces including recycled and handmade paper.

1.2.3 Thermal Printers

Thermal printers are of two kinds.

Direct Thermal printer and
Thermal wax-transfer printer

i. Direct Thermal printer

Direct Thermal printer use a heated printhead to burn dots into the surface of special heat-sensitive paper. It is similar to older fax machines. The disadvantage in this is that the paper that is used gets darken early, thus making difficult to read.

ii. Thermal wax-transfer printer

Thermal wax-transfer printers use a heat-sensitive-ribbon instead of heat-sensitive paper. Thermal printhead melts wax-based ink from the ribbon on to the paper. The disadvantages with thermal transfer printers are that the heating and cooling of the print head determines their speed which means that since the printhead is extensively used, it has to be replaced often.

1.2.4 Inkjet Printers

Inkjet printers are those that place extremely small droplets of ink onto paper to create an image. They use a reservoir of aqueous ink, a pump and an ink nozzle to accomplish this. These dots are extremely small and can have different colors combined together to create photo-quality images. They essentially work by shooting ink onto paper. Both inkjet and laser printers are non-impact printers in the sense that they do not have mechanisms that physically touch paper in order to create images. However, unlike laser printers, inkjet printers use aqueous ink that spontaneously colors the paper (unlike toner from laser printers that has to be fused into the paper with a fuser).

Parts of a typical ink jet printer are shown in the figure below:

Edge guide - Helps load the paper straight. Adjust the left edge guide so that it fits snugly to the width of your paper.
Sheet feeder - Feeds a stack of paper automatically.
Paper support - Supports the paper loaded in the sheet feeder.
Feeder guard - Prevents objects placed on the document cover from falling inside the printer when opening the document cover.
Document cover - Open and close when you place a photo or document.
Output tray - Receives ejected paper.
Output tray extension - Supports the ejected paper.
Scanner unit - Open and close when you replace an ink cartridge.
On button - Turns the printer on and off.

i. Print head assembly

Print head :. The core component of the inkjet printer is the print head that contains a series of nozzles that spray dots of ink onto paper. There are two main mechanisms that generate the spraying of ink from the nozzles. The first mechanism relies on a thermal bubble. In these systems, current flows through certain nozzles and heats up resistors near those nozzles. This heat vaporizes some the ink and generates a bubble that expands. As the bubble expands, ink is sprayed out of the nozzle. Then, the current decreases, and the bubble pops, sucking more ink from the cartridge to fill the empty space.

The other mechanism that inkjet printers rely on to spray ink from their nozzles involves piezoelectric materials. These materials change shape based on the electric field around them. A transducer is placed at the base (top) of the nozzle. An electrical stimulus excites the transducer so that it changes shape, causing ink to spray out of the nozzle. When the stimulus stops, ink from the reservoir flows back into the cartridge to fill the void.

Ink Catridges : These come in various combinations such as separate black and color catridges or even a catridge for each ink color.

Print head stepper motor : A stepper motor moves the print head assembly (print head and ink cartridges) back and forth across the paper

Belt : This is used to attach the print head assembly to the stepper motor

Stabilizer bar : The print head assembly uses a stabilizer bar to ensure that movement is precise and controlled.

ii. Paper feed assembly

Paper tray/feeder : This is actually a tray that you load the paper into.

Rollers : These rollers pull the paper in from the tray or feeder and advance the paper when the print head assembly is ready for another pass.

Paper feed stepper motor : This powers the rollers to move the paper in the exact increment needed to ensure a continuous image is printed

Advantages and Disadvantages

The low cost and relatively high quality of prints that are offered by the inkjet printer is suitable for most day-to-day tasks and thus used in home and offices.

Inkjets have the advantage of practically no warm up time and lower cost per page, no noise and the disadvantages are ink cartridges are expensive. Inkjet printers can print on a variety of papers.

1.2.5 Laser printers

Laser printers are the fastest and most popular printers on the market today. They produce extremely high quality images – some near photo quality.

Main Principle of Laser Printer

The main principle in the working of laser printer is static electricity i.e, they use electrophotography, or an electrophotostatic process, to form images on paper. The basis of the principles involved here is the science of atoms – oppositely-charged atoms are attracted to each other, so opposite static electricity fields cling together.

Parts of a laser printer are

The basic parts that a laser printer consists of are toner catridge, photosensitive drum, erase lamp, primary corona, transfer corona, fuser assembly. Each of these parts have a very important role to play in the printing process.

How it works?

The drum is the main component in a laser printer and is oftentimes located near the center. It is usually made of a highly photoconductive material that can be charged or discharged by light. The drum interfaces directly with the paper and places the toner at the correct locations to produce the image.

The way that the drum works is that it is given an initial charge to begin with. As the drum rotates in circles, the laser shines upon certain areas of the drum. The parts of the drum that get exposed to the laser experience a change in charge. For example, in certain laser printers, the drum is initially given strong negative electro-static charge and the laser causes exposed areas to change from a negative to a positive electro-static charge. In this way, the laser generates an electrostatic image on the drum.

Then, the printer exposes the rotating drum to negatively charged toner particles. The toner particles are attracted to the positive areas of the drum that were exposed by the laser. As a result, an electrostatic image is developed on the drum surface that will get transferred to the paper at a later state..

Now, the paper is given a strong positive charge (much stronger than that of the drum) and is slid beneath the drum. Since the paper has a stronger positive charge than the drum, it takes the toner off of the drum so that the pattern from the drum is translated to the paper. Then, the paper goes through the fuser and the toner particles are fused into the paper.

In brief, the steps involved in the working of a laser printer are given below:

1. Paper feeding

The printer moves a sheet of paper from the proper tray onto a series of rollers, through the imaging and fixing areas, and to the output hopper.

2. Drum Cleaning and Charging

Any residual toner from past jobs is scraped from the printer's photosensitive drum. A fine wire (the primary corona) produces a negative electrical charge across the entire face of the drum. The image is set in raster lines as a series of fine dots on the drum.

3. Imaging the Drum

The information from the raster-image processor is read from memory and sent to the print engine, one line at a time. The laser sets a positive charge in the areas of the image to be filled with toner.

4. Transferring Toner to the Drum

A film of fine plastic power is placed on the toner transfer roller, which is turning close to the photosensitive drum. This toner is then attracted to the positively charged areas of the drum.

5. Transferring Toner to the Paper

The corona wire places a positive electrical charge on the paper as it moves close to the drum. The toner is attracted to the page, forming an image.

6. Fusing the Toner

The page passes through a pair of rollers. The roller on the side toward the toner that has been placed on the page is heated just enough to melt the plastic toner particles onto the page without smearing. The roller on the other side supplies the needed pressure.

Advantages and Disadvantages

Laser printers have a number of advantages over the rival inkjet technology. They produce much better quality black text documents than inkjets, and they turn out more pages per minute (100 to 200 pages per minute are typical) at a lower cost per page than inkjets.

Laser printers are well known for their speed and they can handle large volumes and another advantage is that they are not messy as inkjet that is, there is no ink spillage as the ink is created from powder and they can print on any type of paper and the disadvantage is that laser printers are expensive.

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