Printed documents are created by attaching ink to paper. The process involved and the quality of the output varies widely depending on the type of printer used. Historically there have been many methods of printing and duplicating, but I will only talk about computer printers in this blog.
Printers are classified in one of three categories based on how they produce output. These categories are character printers, line printers and page printers. As you might guess the output is transferred to paper one character at a time, one line at a time or one page at a time respectively. The most popular early printers were dot matrix printers. These could be either character or line printers. Dot matrix line printers typically had a row of small wires lined up across the width of the paper with an ink saturated ribbon between. The ribbon is fed horizontally across the row of wires while the paper is moved vertically. For each vertical step of the paper a combination of wires would be pushed at the paper causing the ribbon to touch and leave an entire line of dots across the page. As the paper continues to move up the characters of the output are formed. These printers are very fast, but are usually limited to text or basic graphics.
Another type of dot matrix printer uses a moving print head with a small number of wires (typically 9 or 24) placed vertically. The head is moved horizontally across the paper while an ink ribbon is fed horizontally across the head. As the head moves across the wires are fired in a pattern to create each character of text one at a time. The head may fire as it travels in one direction or both depending on the printer and the quality of output. After a line of text has been completed the paper is fed vertically to position for the next line. Higher quality text could be created using a printwheel or ball printer. These printers used the same method as many typewriters spinning either a spoked wheel with embossed characters or a gimbled ball with embossed characters. In the case of a printwheel, a single hammer would fire to push the character on to the paper with the ribbon between leaving the character impression. Ball printers would lift the entire ball to the paper with the correct character positioned to push the ribbon against the paper. Neither of these types of printer could usually print graphics, but different fonts were available by changing the wheel or ball. These are also considered character printers and all of the technologies I’ve mentioned are able to print multi part forms because they use impact to transfer the image.
Newer technologies have replaced most dot matrix printers. Ink jet printers are character printers that are very similar to dot matrix character printers except that they force ink directly onto the paper through tiny nozzles on a moving head. The head is moved across the paper while small dots of ink are ejected using either bubbles created by heat or electrically induced constrictions of the nozzles. As with the dot matrix printers the dots form the characters as the head moves. Because there is no impact, multi-part forms are not able to be printed. The small size of the dots used to form the images allow for very high quality text or graphics to be printed. Most inkjet printers also use multiple print heads moving together to produce full colour output. Although there are a number of printers that use six or more heads, it is more common to have only four; black, cyan, magenta and yellow. The cyan, magenta and yellow can be placed on the paper in close groups of dots causing your eye to see the combined colour. This allows for a full spectrum of colour reproduction. Although these printers produce very high quality output, they are usually quite slow and expensive to operate due to the high cost of ink.
For better speed and lower cost of operation without too much sacrifice in image quality there are laser printers. Laser printers are considered page printers because the entire page or text or graphics are produced within the machine, then transferred to the paper. There are a number of laser technologies currently in use, but the most common uses a statically charged rotating drum. As the drum rotates a laser beam is fired at the surface. A spinning mirror directs the laser from side to side on the rotating drum as it is pulsed on and off. Where the laser hits the surface of the drum the static charge is removed. Farther along the rotation the surface of the drum is passed by a brush of charged ink particles called toner. The toner is attracted to the drum surface where there is no charge laying an image. Paper is then fed between the drum and another charge which attracts to toner to the paper. The paper is then fed between a hot roller and a pressure roller to fuse the image to the page. Colour images are made by rotating the drum four times past four different toner brushes, cyan, magenta, yellow and black, before feeding the paper past to receive the image. Laser printers are usually more expensive to purchase than inkjet printers, but cost less to operate over time.
Recent Comments