BACKGROUND:

Before the advent of laser printers that were compact enough to sit on a desk, most printing was done on either a Dot Matrix or Daisywheel Impact Printer. The operation of these types of printers were relatively straight forward to understand, because they were modelled on the mechanism of a typewriter. Basically something hard struck an inked ribbon and that left a mark on the page. Bubble/Inkjet printers, although a little more more complicated, have essentially only substituted ink being forced out of a nozzle onto the paper, in place of an inked ribbon.

Laser printing involves a multistep process, and what’s happening is not obvious from watching the printer operate. It is a mysterious activity. Paper goes in blank... paper comes out printed. But how does it happen? You may recall Mimeograph machines. In principle, the operation of a laser printer mimics that of a mimeograph machine. The entire page on a mimeograph is printed on a master sheet wrapped around a rotating cylinder. As the handle is turned, the cylinder rotates past ink and paper, transferring the image to the document. Laser printers work much the same way, except of course, that the image on the laser printer’s cylinder is formed with lasers and electrostatic charges.

THE EVOLUTION OF PERSONAL PRINTING:

STEPS INVOLVED IN LASER PRINTING:

Click On Image To View Larger Diagram...

Step 1: Incoming Data

• First, prior to sending a print job to the printer, the computer queries the output port to make sure that a printer is attached, is online, and is ready to print. The printer sends a return status signal on the strobe line (pin 1) for parallel or DTR (line 20) for a serial interface, indicating whether or not it is ready to receive data.
• Next, if the print job requires any fonts that the printer does not have in its fonts library, such as TrueType Fonts, they are sent to the printer and stored in its memory, and finally, the print job itself is sent and stored in the printer's remaining available memory.

Note 1: If the job is larger than the printer’s memory can accommodate, the operating system (OS) either waits for memory to become available... common with with single-tasking OS's such as DOS, or uses its own Print Spooler to send the remainder of the print job to the printer as memory becomes available. This not only speeds up printing, but also allows the OS to continue with other tasks.. common with multi-tasking OS's such as Microsoft Windows. Lasers printers may also come with their own custom print spooler software, that takes over from the one in the OS. This often speeds up printing further, which is ideal in a heavy traffic environment.

Note 2: Laser printers fall under the category of Page Printers because they compose the entire page in their memory and then transfer it to paper. A full page with graphics occupies on average 512KB of memory. A laser printer should have at least 1 MB of memory to store at least one page plus any associated fonts. Laser printers with more memory can store multiple pages of a print job, which offloads the print job more quickly from the OS. 

Step 2: Drum Preparation (See Fig. 3)

• First, in preparation for printing, the drum must be cleaned to remove all traces of previous pages. First, a rubber blade wipes the excess toner from the drum, and then erase lamps (in older models) or a charged roller (in newer models) electrostatically clean the drum by neutralizing any residual electrical charges.
• Next, the printer conditions the drum to receive the next image by applying a uniform negative charge of 600 Volts (-600v) to its surface. This function is performed by either the Primary Corona, or by a secondary charged drum. 

Note 1: The Print Drum is an aluminum cylinder coated with photosensitive material, and is usually blue or green in colour. The drum may be part of the toner cartridge, or installed as a separate unit. The print drum's coating is photosensitive, and it can be damaged by exposure to light, so erase lamps are red-filtered. On some models of laser printers, red plastic is placed over the lamps to accomplish this. Allways store print drums out of the light, and preferably inside the black plastic bag that it came with originally.

Note 2: The Primary Corona is a thin wire located inside the toner cartridge. There are several corona wires involved in the print process. The primary corona must emit a charge of NEGATIVE 6,000 volts (-6,000v) in order to apply a NEGATIVE 600 volts (-600v) charge to the drum. 

Note 3: The negative charge applies evenly across the photosensitive drum because a Varistor (Variable Resistor) grid filters it. The Varistor does not conduct electricity until a specific required voltage level is achieved. 

Click On Image To View Larger Diagram...

Step 3: Drum Writing (See Fig. 3)

• The printer's Laser Scanner writes the data held in the printer’s memory to the print drum by directing a very precise laser beam to certain points on its photosensitive surface, altering the electrical charge at those points. 

Note 1: As the print drum rotates past the laser scanner, its beam sweeps across the surface of the cylinder, alternatively turning on and off to neutralize certain areas to about NEGATIVE 100 Volts (-100v). These neutralized areas will be where toner will adhere to later in the printing process and then be transfered to the paper.

Note 2: In early model laser printers, the on/off status of the laser scanner could be changed 300 times per inch (2.54cm), and the drum rotated 1/300 of an inch (1/762 of a cm), with each horizontal sweep of the laser beam, resulting in a 300 dots per inch (DPI) maximum resolution. Later model laser printers are capable of achieving 600 DPI and/or 1200 DPI in either some and/or all print modes. 

Step 4: Paper Feed (See Fig. 2)

• Paper is drawn into the printer from the paper tray by a series of Paper Pickup/Feed (Separation) Rollers. Rollers in the Registration (Input) Assembley hold the paper momentarily until it’s time for it to be released, making sure that the top of the paper feeds in exactly at the moment when the laser image of the page that’s on the drum rotates past it.
• Laser printers have built-in light sensors at critical points along the paper path, that check whether the paper is in the right place for that step to occur. The printer knows how long it should take for the paper to move from one sensor to the next, and if it is is delayed, the printer stops and indicates a Paper Jam message. 

Note 1: Some laser printer have a single paper pickup/feed (separation) roller, whilst others have both paper pickup and feed (separation) roller(s). On certain models... HP 5Si, 8000, 8100 & 9000 series laser printers, there's 1 x pickup roller and 2 feed/separation rollers per tray (excluding the Multi-Purpose (MP)/Fold-Down/By-Pass Tray).

Note 2: Paper trays also have a Separation Pad, which prevents multiple sheets of paper from being picked up and fed into the printer at once, causing a single page of print to be spread across more than one page, and/or resulting in potential paper jams. 

Step 5: Toner Pickup

• Toner cartridges contain a rotating magnetic metal-developing cylinder, a toner reservoir, and a height control mechanism that limits the amount of toner that the cylinder can pick up at any one time. Toner powder consists of plastic resin particles... the particles that are melted to produce the image on the paper, and iron oxide... the particles that are affected by magnetic attraction and electrical charges. The toner’s metal particles adhere to the magnetic cylinder. The magnetic cylinder presents the toner to the print drum as it passes by. The developing cylinder is charged to NEGATIVE 600 Volts (-600v), like the blank portions of the photosensitive print drum. The toner powder adhering to the cylinder also takes on that same charge.
• As the print drum passes by the magnetic cylinder, the toner powder ignores all the areas charged to NEGATIVE 600 Volts (-600v) on the print drum, because it is charged to the same voltage level, but is attracted by the areas charged to NEGATIVE 100 Volts (-100v). It jumps off the magnetic cylinder and clings to the areas with the lesser charge on the print drum. 

Note 1: Step Four: Paper Feed and Step Five: Toner Pickup occur virtually simultaneously... as the paper is drawn into the printer, toner is applied to the print drum.

Note 2: At this point, the image exists on the drum, complete with toner. If you could look inside the printer as it operates... you can’t, by the way because of the safety features in place, and stop the drum from rotating for a moment, you could see the page on the drum, just as it is to be printed.

Tip 1: If you accidentally spill toner powder onto fabric, you can remove it with a magnet, since toner powder is approximately 50 percent iron oxide... all Ferrous (containing iron) materials are affected by magnetic fields. 

Step 6: Toner Transfer To The Paper

• As the paper is fed into the printer, the Transfer Corona or Transfer Roller, applies a POSITIVE 600 Volts (+600v) charge to the paper. When the paper passes by the print drum, the NEGATIVE 100 Volt (-100v) charged toner powder jumps off the print drum and onto the positively charged paper.
• The paper runs past a Static Charge Eliminator, which is a row of teeth with a negative charge that reduces the paper’s highly positive charge.  

Step 7: Fusing The Toner To The Paper (See Fig. 4)

• The image is now on the paper, but it’s not permanent. It’s just loose toner held in place by gravity and a weak electrostatic charge... it's still possible to smudge the image at this point. For permanent application, it must be Fused... melting of the toner's plastice particles into the fibres of the paper. This is done by the printer's Fuser Unit.

 Note 1: A laser printer Fuser Unit has two major components... an Upper Fuser Heat Roller, and a Lower Fuser Pressure Roller.

• The Upper Fuser Heat Roller is either a nonstick teflon coated solid aluminium cylinder, or teflon coated plastic sleeve that rotates over an inner Fuser Core. Fusers Units with a solid cylinder have 1-3 high-powered lamps inside. Alternatively, Fuser Units with plastic sleeves have a metal heat element strip held in place on the under outerside of the Fuser Core. The heat lamp(s)/element heat the paper to around 330 to 355 degrees Fahrenheit (165 - 180 degrees Celcius). As the paper passes beneath the Heat Roller, the toner's plastic particles are melted into the fibers of the paper, bonding toner and paper permanently.
• On some models.. eg. Lexmark S & T Series laser printers, a fabric or felt-cleaning pad is in constant contact with the Fuser Heat Roller, helping to keep it clean. This pad needs to be changed either whenever you change the toner cartridge, or the Fuser Unit itself, depending on the printer model.
• Most laser printers have a required service Maintenance Interval... a preset number of pages that can be reliably printed, before certain internal components require replacement. For some HP laser printers for example, the default interval is either 150,000 or 200,000 pages, depending on the model. The Fuser Unit is one component that may require replacement at this time, depending on its physical and/or electrical condition. Other components that may require replacement at this time are, Separation Pad(s), Pickup/Feed Roller(s), and the Transfer Roller.
• The Lower Fuser Pressure Roller is a rubber roller that presses firmly against the Fuser Heat Roller, by the force exerted by left and right tension springs. The paper feeds between the Pressure and Fuser rollers on its way through the printer. The fuser roller can leave an indent... a noticeable flat spot on the softer compound of the Pressure Roller, because of the heat it generates, so the printer’s internal software will rotate the assembly periodically to keep this from happening.

Note 1: Flat Spots are also caused when the printer is switched off.

• Without the periodic rotation of the Fuser Rollers, due to normal printing, and the printer's own pressure compensating idle rotations, the tension generated by the left and right tension springs is sufficient to cause serious flat spot damage to the Pressure Roller, that cannot be repaired... the roller must be replaced. This is a very common problem on Lexmark S & T Series Printers, but is a problem for all laser printers in general, especially those with solid metal heat rollers.
• You can identify quite readily, whether or not you have a fuser flat spot. Simply switch on the printer, and you will hear a repetitive 'thumping' sound emenating from the fuser, timed with the flat spot passing underneath the heat roller. Alternatively you may remove the fuser, and rotate the rollers manually. The rollers will rotate smoothly past one another, until the flat spot passes beneath the heat roller, at which point the rotation will stall. You will be able to feel the point at which this happens.
• To avoid this potentially very costly problem, simply release the tension springs when the printer is not in use. All fuser units have a release mechanism, designed to allow access to the unit for removal of paper jams. This mechanism can also be employed to release tension on the rollers when the printer is being stored away, and not in use.

Note 2: Make certain that all media put though a laser printer is of laser grade to avoid potentially costly damage to the Fuser Unit.

• Do not use Inkjet Printer type transparencies in a laser printer, as they will melt and bond to the Heat Roller, destoying the teflon coating. Furthermore heat will not be able to dissipate, in which case the heat lamp(s) or element will burn out, resulting not only in the need for a complete Fuser Unit replacement, but is also a potential fire hazard. In any event, inkjet media is not designed to have toner powder bonded to it, so why waste your time or incur potentially costly problems.
• Do not use Inkjet Printer type labels in a laser printer, as they have the tendency to lift or peel off entirely, leaving glue residue throughout the paper path, and gumming up the works. Repetitive paper jams, and potentially costly fuser damage will be all that you can look forward to. And once again, they are not designed for toner powder in any event.

Tip: When using laser grade media such as labels, envelopes or transparencies, always choose the straighest possible paper path that you printer allows. If your printer has a Fold-Down tray... also referred to as Tray 1, Multi-Purpose (MP) or By-Pass tray, then print your labels, envelopes and transparencies from this tray. Also some printers will permit the paper to exit out the back of the printer.. eg. HP 2100, 2200, 2300, 4000, 4050, 4100, 4200, 4250, 4300 & 4350 Series, instead of having it pass out into the standard output bin. Having the paper exit the back is the ideal for these kinds of media, because it eliminates the necessity for the paper to 'curl'. When the paper curls, especially if it is after it has exited the fuser, then the potential for labels to lift or peel off, envelopes or transparencies to jam, is greater.