Laser and LED Printers

Laser and LED printers combine the low noise of an inkjet printer with the waterproof printing of a dot-matrix printer and have a very low cost per page.

Laser/LED 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/LED printers produce images digitally, turning individual dots on and off; most copiers, however, are still analog devices.

• Laser/LED printers work under the control of a computer; copiers have a dedicated scanner as an image source.

• Laser/LED printers are optimized for both text and graphics, including continuous-tone photos; copiers must choose text or photo modes, with mediocre results at best on anything other than scanned text.

• Laser/LED printers use much higher temperatures than copiers to bond printing to the paper; using copier label or transparency media in a laser printer can result in damage to the printer due to melted label adhesive , labels coming off in the copier, or melted transparency media.

In larger offices, you might find that a networked copier/printer, such as the Xerox DocuCentre, has replaced traditional photocopiers and printers. The printing method used by these devices is similar to a laser printer, but these machines often use separate toner bottles and a long-life drum, rather than the toner/drum combination used in most desktop and smaller network laser printers.

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. The laser printing process described in the following section also applies to LED printers.

The Laser Printing Process

To master this section, make sure you

• Memorize the six steps involved in laser printer imaging.

• Master the details of each step and their sequence.

• Be prepared to answer troubleshooting questions based on these steps.

tip

Understanding the sequence of events in the laser printing process is essential to successfully passing the A+ Core Hardware Exam. About 70% of A+ printer questions concern laser printers, and 9% of the entire exam is printer related .

The laser printing process often is referred to as the electrophotographic (EP) process .

Before the six-step laser printing process can take place, the following events must first take place:

• Laser printers are page based; they must receive the entire page before they can start printing.

• After the page has been received, the printer pulls a sheet of paper into the printer with its feed rollers.

After the paper has been fed into the print mechanism, a series of six steps takes place, which results in a printed page:

1. The excess toner is cleaned from the drum and the electrical charge discharged to prepare for the next page.

2. The image drum is conditioned.

3. The page is written to the drum.

4. The image is developed on the drum with the toner.

5. The toner image of the page is transferred to the paper.

6. The toner image of the page is fused permanently to the paper.

See Figure 10.9 to learn where each of these steps takes place in the printer.

Figure 10.9. A typical laser printer's components . The heavy line indicates the paper path ; paper enters the printer at the right and is pulled through the printer to either the left output tray or the top output tray.

Step 1: Cleaning

To prepare the drum for a new page, the image of the preceding page placed on the drum by the laser (see step 3) is removed by a discharge lamp.

Toner that is not adhering to the surface of the drum is scraped from the drum's surface for reuse.

Step 1 prepares the drum for the conditioning step (step 2).

Step 2: Conditioning

The cylinder-shaped imaging drum receives an electrostatic charge of -600Vdc (DC voltage) from a primary corona wire or conditioning roller . The smooth surface of the drum retains this charge uniformly over its entire surface. The drum is photosensitive and will retain this charge only while kept in darkness .

Step 3: Writing

A moving mirror moves the laser beam across the surface of the drum. As it moves, the laser beam temporarily records the image of the page to be printed on the surface of the drum by reducing the voltage of the charge applied by the charger corona to -100Vdc. Instead of using a laser beam, an LED printer activates its LED array to record the image on the page.

Step 4: Developing

The drum has toner applied to it from the developer; because the toner is electrostatic and is also at -600Vdc, the toner stays on only the portions of the drum that have been reduced in voltage to create the image. It is not attracted to the rest of the drum because both the toner and the drum are at the same voltage, and like charges repel each other. This "like charges repel" phenomenon is the same reason two magnets repel each other.

Step 5: Transferring

While the sheet is being fed into the printer, it receives an electrostatic charge of +600Vdc from a corona wire or roller; this enables it to attract toner from the drum, which is negatively charged (see step 3). As the drum's surface moves close to the charged paper, the toner adhering to the drum is attracted to the electrostatically charged paper to create the printed page.

As the paper continues to move through the printer, its charge is canceled by a static eliminator strip, so the paper itself isn't attracted to the drum.

Step 6: Fusing

The printed sheet of paper is pulled through fuser rollers, using high temperatures (about 350F degrees) to heat the toner and press it into the paper. The printed image is slightly raised above the surface of the paper.

The paper is ejected into the paper tray, and the drum must be prepared for another page.

Color and Monochrome Laser Printers

Most laser printers for home and small office use print with black toner only. However, an increasing number of laser and LED printers also support color printing using the CMYK model.

Conventional color laser printers require four passes to print a color page (one pass per color), but color LED printers can print all four colors in one pass so they print color as fast as black and white.

Memory and Laser Printers

Because a laser printer is a page printer and the graphics, text, and fonts on the page all use memory, the amount of memory in the laser printer determines the types of pages it can print successfullyand on some models, how quickly the pages are printed.

All laser printers are shipped with enough memory to print with built-in typefaces, and most printers sold since the mid-1990s have enough memory for documents containing several scalable TrueType typefaces used by Windows. However, graphics, especially photographs, require a great deal more printer memory.

If a page is sent to a laser printer that requires more memory than the laser printer contains, the laser printer tries to print the page but stops after the printer's memory is full. The printer then displays an error message or blinks error status lights, at which point you must manually eject the page. Only a portion of the page is printed.

If the page requires an amount of memory close to the maximum in the laser printer, most recent laser printers have techniques for compressing the data going to the printer. Although this technique means that more pages can be printed successfully, compressing the data can slow down the print process.

If you reduce the graphics resolution, text resolution stays the same, so a document that is not designed for reproduction or mass distribution will still have acceptable quality.

However, graphics resolutions of 300dpi or less produce poor-quality photo output.

Three options can be used if the pages you need to print require too much memory:

• Reduce the resolution of the print job. Most laser printers today have a standard resolution of 600dpi or 1,200dpi. Reducing the graphics resolution to the next lower figure (from 1,200 to 600dpi or from 600 to 300dpi) will reduce the memory requirement for printing the page by a factor of four. The laser printer's Graphics or Advanced Printing Defaults Paper/Quality Properties sheet (see Figure 10.10) enables this factor to be adjusted as needed.

  Figure 10.10. The Graphics Properties sheet in Windows 98 (left) or the Advanced Printing Defaults Paper/Quality Properties sheet in Windows XP (right) for a typical laser printer enable you to adjust the graphics resolution from the default of 600dpi to 300dpi or less; text quality is not affected by this option.

  

• Eliminate or reduce the size of graphics on the page.

• Convert color photos to black-and-white photos before placing in a desktop publishing document or printing them directly from the file. This can actually enhance the output quality from a monochrome laser printer as well as reduce the memory requirement for pages with photos.

These options are temporary workarounds that are unsatisfactory for permanent use. The best solution to "out-of-memory" problems with a printer, as with the computer, is to add more RAM.

Printers generally use memory modules that are somewhat different from computer memory modules, although they might look similar. Recent laser printers often use a 100-pin SDRAM SODIMM (SDRAM SODIMMs used in portable computers have 144 pins), whereas older models often use 72-pin fast-page or EDO SIMMs. Other types of printer memory expansion include credit card sized proprietary modules or, on very old printers, slide-in boards that are populated with individual memory chips.

If the printer uses SIMM memory, see Chapter 7, "RAM," for installation tips. If the printer uses SODIMM memory, see Chapter 12, "Portables," for installation tips. However, if the printer uses proprietary modules or slide-in boards, see the printer's own documentation for details.

After the memory is installed in the printer, the printer is ready to use it immediately. However, some Windows printer drivers might not automatically detect the additional RAM. Because many Windows printer drivers try to compress data to fit into the amount of printer RAM known, an inaccurate RAM value will cause slower printing; Windows will ignore printer RAM it doesn't know about. After installing the RAM in the laser printer, open the printer Properties sheet in Windows and reset the total value for printer RAM to the correct value; a typical location for this option is Device Options (Windows 98) or Device Settings (Windows XP).

tip

To avoid memory problems, make sure at least 4 megabytes (MB) or more RAM is installed in 600dpi laser printers using PCL and at least 16MB or more RAM is installed in 600dpi PostScript laser printers; add more RAM for higher resolutions.

Toner Cartridges

Most laser printers use toner cartridges , which combine the imaging drum and the developer with 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.

Recycled toner cartridges are controversial in some circles, but I've used a mixture of new and rebuilt toner cartridges for several years without a problem. Major manufacturers, such as Apple, HP, and Canon, encourage you to recycle your toner cartridges by enclosing a postage -paid return label in the box.

Reputable toner cartridge rebuilders can save you as much as 30% off the price of a new toner cartridge.

When you install the toner cartridge, be sure to follow the directions for cleaning near the toner cartridge. Depending on the model of laser printer, this can involve cleaning the mirror that reflects the laser beam, cleaning up stray toner, or cleaning the charging corona wire on the toner cartridge itself (replaced by conditioning rollers on most recent models). If you need to clean the charging corona wire (also called the primary corona wire on some models, the laser printer will contain a special tool for this purpose. The printer instruction manual will show 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 10.11 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.

Figure 10.11. A typical laser printer toner cartridge. The inset shows the mirror cleaning tool in use after the old toner cartridge has been removed and before the new cartridge is put into position.

Paper, Labels, and Transparency Stock

Because the laser printer bonds the print to the paper during the fusing process, it can handle a wider range of paper stocks than inkjet printers. However, the same heat that produces an embossed feel to printing can also cause problems if the wrong types of labels or transparencies are used. For best results, follow these guidelines:

• 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.

• 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.

caution

When you change a toner cartridge, take care to avoid getting toner on your face, hands, or clothing. It can leave a messy residue that's hard to clean.

caution

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

Periodic Maintenance for Laser Printers

Because laser printers use fine-grain powdered toner, keeping the inside of a laser printer clean is important in periodic maintenance. Turn off the laser printer before using a damp cloth to clean up any toner spills.

To keep the paper path and rollers clean, use cleaning sheets made for laser printers. To use the sheets

1. Insert the sheet into the manual feed tray on the laser printer.

2. Create a short document with Notepad, WordPad, or some other text editor and print it on the sheet.

caution

Never use transparency media not designed for laser printers in a laser printer. Copier or inkjet media isn't designed to handle the high heat present in a laser printer and can melt or warp and possibly damage the printer.

As the sheet passes through the printer, it cleans the rollers. If a specialized cleaning sheet is not available, you can also use transparency film designed for laser printers.

Change the toner cartridge as needed, and the ozone filter on models that use one as required.