I/O Ports and Devices

This chapter is from the book 

Printing Fundamentals

There are four types of printers you should be aware of:

• Laser
• Inkjet
• Thermal
• Impact

The following sections introduce you to the basic features of each printer type:

• How printers create a page (note in particular the steps used by a laser printer to create a page)
• Major components of each printer type covered (thermal printers as well as impact, inkjet, and laser/LED)
• Typical printer operation and output problems and their solutions
• How printers are interfaced to the computer

Laser Printers

Laser printers are similar in many ways to photocopiers:

• Both use an electrostatically charged drum to receive the image to be transferred to paper.
• Both use a fine-grained powdered toner that is heated to adhere to the paper.
• Both must feed the paper through elaborate paper paths for printing.

However, significant differences exist between the photocopier and its computer-savvy sibling:

• Laser printers produce images digitally, turning individual dots on and off; most copiers, however, are still analog devices.
• Laser printers work under the control of a computer; copiers have a dedicated scanner as an image source.
• Laser printers use much higher temperatures than copiers to bond printing to the paper; using copier labels or transparency media in a laser printer can result in damage to the printer due to melted label adhesive, labels coming off in the printer, or melted transparency media.

One type of laser printer is the LED printer. LED stands for light-emitting diode, which the LED printer uses as its light source. The essential difference between a laser and an LED printer is in the imaging device. The laser printer uses a laser to transfer the image to the drum, whereas an LED printer uses an LED array to perform the same task. Otherwise, these technologies are practically identical.

Although the technology of laser printers is interesting, we cannot forget what they use to print and what they print on. Laser printers use toner cartridges that can be replaced when they are empty. These printers print their information best on paper that is optimized for laser printers. Let's discuss toner cartridges and laser printing paper now.

Toner Cartridges

Most monochrome laser printers use toner cartridges that combine the imaging drum and the developer with black toner. This provides you with an efficient and easy way to replace the laser printer items with the greatest potential to wear out.

Depending on the model, a new toner cartridge might also require that you change a wiper used to remove excess toner during the fusing cycle. This is normally packaged with the toner cartridge.

When you install the toner cartridge, be sure to follow the directions for cleaning areas near the toner cartridge. Depending on the make and model of the laser printer, this can involve cleaning the mirror that reflects the laser beam, cleaning up stray toner, or cleaning the charging corona wire or conditioning rollers inside the printer. If you need to clean the charging corona wire (also called the primary corona wire on some models), the laser printer contains a special tool for this purpose. The printer instruction manual shows you how to clean the item.

Keep the cartridge closed; it is sensitive to light, and leaving it out of the printer in room light can damage the enclosed imaging drum's surface. Figure 3-25 shows a typical laser printer toner cartridge and mirror cleaning tool. The tool above the toner cartridge is used to clean the printer's mirror.

Color laser printers differ from monochrome laser printers in two important ways: They include four different colors of toner (cyan, magenta, yellow, and black), and the imaging drum is separate from the toner. Thus, instead of waste toner being reused as in a monochrome laser printer that has a toner cartridge with an integrated imaging drum, waste toner in a color printer is sent to a separate waste toner container.

Depending upon the toner transfer technology used, a color laser printer might require four passes to print a color page (one pass per color), or only one pass to print all four colors.

Laser Printer Paper and Media

For best results with laser printing, use these guidelines when selecting paper and media:

• Use paper made for laser or photocopier use. Extremely rough-surfaced specialty papers might not enable the toner to fuse correctly to the paper.
• Use envelopes made for laser printing, especially if the printer doesn't offer a straight-through paper path option. Standard envelopes can lose some of their flap adhesive or have the flap stick to the back of the envelope when used in a laser printer.
• Use only labels made for laser printers; these labels have no exposed backing, requiring you to separate the labels from the backing after printing.

  CAUTION

  Labels made for copiers have exposed backing, and the labels can come off inside the printer, leading to expensive repairs.

• Use only laser-compatible transparency stock; it can resist the high heat of the fuser rollers better than other types, which can melt and damage the printer.
• Avoid using paper with damaged edges or damp paper; this can cause paper jams and lead to poor-quality printing.
• Load paper carefully into the paper tray; fan the paper and make sure the edges are aligned before inserting it.

Inkjet Printers

Inkjet printers (also known as ink dispersion printers) represent the most popular type of printer in small-office/home-office (SOHO) use today and are also popular in large offices. Their print quality can rival laser printers, and virtually all inkjet printers in use today are able to print both color and black text and photographs.

From a tightly spaced group of nozzles, inkjet printers spray controlled dots of ink onto the paper to form characters and graphics. On a typical 5,760x1,440 dots per inch (dpi) printer, the number of nozzles can be as high as 180 for black ink and more than 50 per color (cyan, magenta, yellow). The tiny ink droplet size and high nozzle density enables inkjet printers to perform the seemingly impossible at resolutions as high as 1,200dpi or higher: fully formed characters from what is actually a high-resolution, non-impact, dot-matrix technology.

Inkjet printers are character/line printers. They print one line at a time of single characters or graphics up to the limit of the printhead matrix. Inkjet printers are functionally fully formed character printers because their inkjet matrix of small droplets forming the image is so carefully controlled that individual dots are not visible.

Larger characters are created by printing a portion of the characters across the page, advancing the page to enable the printhead to print another portion of the characters, and so on until the entire line of characters is printed. Thus, an inkjet printer is both a character and a line printer because it must connect lines of printing to build large characters. Some inkjet printers require realignment after each ink cartridge/printhead change to make sure that vertical lines formed by multiple printhead passes stay straight; with other models, alignment can be performed through a utility provided as part of the printer driver when print quality declines due to misalignment.

Ink Cartridges

Many inkjet printers, especially low-cost models, use a large tank of liquid ink for black and a separate tank with separate compartments for each color (typically cyan, magenta, and yellow; some models feature light versions of some of these colors for better photo-printing quality). However, the trend in most recent models has been to use a separate cartridge for each color. This improves print economy for the user because only one color at a time needs to be replaced. With a multicolor cartridge, the entire cartridge needs to be replaced, even when only one of the colors runs out.

Figure 3-26 shows some of the typical components of an inkjet printer.

Depending on the printer, the printhead might be incorporated into the ink tank; be a separate, user-replaceable item; or be built into the printer.

Some inkjet printers feature an extra-wide (more nozzles) printhead or a dual printhead for very speedy black printing. Some models enable the user to replace either the ink cartridge only or an assembly comprising the printhead and a replaceable ink cartridge.

An inkjet printer is only as good as its printhead and ink cartridges. Clogged or damaged printheads or ink cartridges render the printer useless. If an inkjet printer fails after its warranty expires, you should check service costs carefully before repairing the unit. Failed inkjet printers are often "throwaway" models and can be replaced, rather than repaired, even during the warranty period.

Always use the printer's own power switch, which enables the printer to protect the ink cartridges and perform other periodic tasks (such as self-cleaning) properly.

Inkjet Printer Paper and Media

Although papers made for copiers and laser printers provide adequate results, you must use inkjet-specific media of the following types to achieve best print quality:

• Glossy photos
• Transparencies
• Business cards
• Labels (especially clear labels)

It is also critical to use the correct print setting for the media type to avoid smudging, lines, and other print defects.

Thermal Printers

Thermal printers use heat transfer to create text and graphics on the paper. Thermal printers are used in point-of-sale and retail environments, as well as for some types of portable printing.

Thermal printers are available using three different technologies:

• Thermal transfer
• Direct thermal
• Dye sublimation

Thermal printers can use a dot-matrix print mechanism or a dye-sublimation technology to transfer images. Some thermal printers use heat-sensitive paper, and others use an ink ribbon to create the image. Let's start by discussing the thermal printer ribbon.

Thermal Printer Ribbons

Thermal transfer printers use wax or resin-based ribbons, which are often bundled with paper made especially for the printer. The most common type of thermal transfer printer uses dye-sublimation (dye-sub) technology to print 4x6 continuous-tone photographs. Examples of dye-sublimation printers include Kodak printer docks and Canon's Selphy CP series.

Figure 3-27 illustrates a typical dye-sublimation ribbon for a Canon Selphy CP printer.

Thermal transfer printers used in point-of-sale or retail environments typically use non-impact dot-matrix printheads.

Thermal Printer Paper

Direct thermal printers use heat-sensitized paper, and thermal transfer printers might use either standard copy paper or glossy photo paper, depending upon their intended use.

If the printer uses direct thermal printing, heat-sensitive paper with characteristics matching the printer's design specifications must be used. For portable printers using direct thermal printing such as the Pentax PocketJet series, the usual source for such paper is the printer vendor or its authorized resellers. If the direct thermal printer is used for bar codes or point-of-sale transactions, you can get suitable paper or label stock from bar code or POS equipment suppliers and resellers.

If the printer uses thermal transfer and is not designed for photo printing, most smooth paper and label stocks are satisfactory, including both natural and synthetic materials. However, dye-sublimation photo printers must use special media kits that include both a ribbon and suitable photo paper stocks.

Impact Printers

Impact printers are so named because they use a mechanical printhead that presses against an inked ribbon to print characters and graphics. Impact printers are the oldest printer technology, and are primarily used today in industrial and point-of-sale applications.

Dot-matrix printers, the most common form of impact printers, are so named because they create the appearance of fully formed characters from dots placed on the page.

Dot-matrix printers use a thermal or impact print head containing multiple pins that are used to form characters and graphics. Dot-matrix and other printers that print a line at a time are sometimes referred to as line printers. Typically, the term "page printers" refers to laser or LED array to change the electrostatic charge on a drum to attract toner, which is then transferred to paper to form the page.

The print mechanism of the dot-matrix printer is almost always an impact mechanism: A printhead containing various numbers of fine wires (called pins) arranged in one or more columns is used along with a fabric ribbon, similar to typewriter technology. The wires are moved by an electromagnet at high speed against the ribbon to form dot patterns that form words, special characters, or graphics. Figure 3-28 shows actual print samples from a typical 9-pin printer's draft mode, a typical 24-pin printer's draft mode, and the Near Letter Quality (NLQ) mode of the same 24-pin printer. The narrower pins of the 24-pin printhead produce a reasonably good NLQ printout but hard-to-read results in draft mode.

Figure 3-29 illustrates a typical impact dot-matrix printer.

Impact Printer Ribbons

Printer ribbons for impact printers use various types of cartridge designs. Some span the entire width of the paper, and others snap over the printhead. Figure 3-30 compares various types of ribbons for impact printers.

Impact Printer Paper and Media

Impact printers use plain uncoated paper or labels in various widths and sizes. Impact printers designed for point-of-sale receipt printing might use roll paper or larger sizes of paper. When larger sizes of paper are used, these printers typically use a tractor-feed mechanism to pull or push the paper past the printhead. Paper used with tractor-fed printers has fixed or removable sprocket holes on both sides of the paper.