Color laser printers use the same basic technology as standard black-only laser printers, but they also overcome several additional challenges in order to produce high-quality color documents.
How All Laser Printers Work
You send a document to your laser printer and your printer turns it into a bitmap image—an image where the printer knows exactly what color to put in each dot of the image. If this is a black-only printer, each pixel is either empty (white) or full (black). For color images, the printer figures out the right combination of its different colored toner to create the color you want.
Then the printer creates a static electric charge on a special roller, called the imaging drum. The static charge will let toner stick to the roller the same way your clothes stick together when they have a static charge after being taken out of the dryer. But before the toner is applied, the printer uses a laser to imprint the bitmap image on the roller.
The special imaging drum roller (which can be a belt in modern laser printers) is made out of a material which conducts electricity when light hits it, so the laser eliminates the static charge on the parts of the roller it imprints. The rest of the roller remains charged—and sticky.
The roller then rolls over the toner reservoir in your printer. The charged (sticky) parts of the roller pick up toner. When the roller finally rolls over a paper sheet, the toner transfers to the sheet.
For a black-only printer, only one application is necessary. For a color printer, the roller must apply each color toner separately. Since most color laser printers have four color toners (cyan, magneta, yellow, and black), multiple rollers are often used one after another to speed up the process. (Typically two rollers in home/office printers with two passes each and four rollers in high-volume commercial printers with a single high-speed pass.)
Most modern toner is made of plastic (but in some applications, wax is used). The plastic won’t stay stuck to the sheet of paper on its own, so the paper runs through a second roller heated to about 200 degrees Fahrenheit. (Temperatures vary by printer.) This melts the plastic toner into the page, blending the colors and creating your final document.
The Special Challenges Of Color Laser Printers
Black-only printers have it easy—they apply toner in one pass and then melt the toner into the page. But color printers typically make four different passes and blend the ink colors together at the end. This can become a huge problem if the different rollers are even slightly out of alignment.
If your printer wants to create a particular color by blending together a specific ratio of its three different color toners, but the roller which applies one of those colors is out of alignment, the formula won’t work and you’ll get (for example) blue instead of green.
How precise does the alignment need to be? It’s easy to figure out for your printer—look at your printer’s maximum specified Dots Per Inch (DPI). It’s usually 600 or 1,200 DPI. Each dot (pixel) must be in alignment, so a printer with a 1,200 DPI rating can’t be more than 1,200th of a inch out of alignment (that’s 0.00083 of an inch or 21 micrometers—or roughly equal to the minimum width of a human hair, 17 micrometers).
This high degree of required precision is one reason color laser printers cost more than black-only laser printers. All of the technology for laser printing is basically the same, but color laser printers must be built to higher engineering standards in order to ensure the rollers and their lasers are built in perfect alignment and stay in perfect alignment for tens or hundreds of thousands of pages.
Considering that challenge, it’s incredible that color laser printers are as affordable as they are today.