Laser printers make use of a process called Xerography which is defined as, “a dry copying process in which powder adheres to parts of a surface remaining electrically charged after being exposed to light from an image of the document to be copied” (Oxford English Dictionary). Laser printers and photocopiers both make use of a Xerographic printing process but achieve their results in different ways: see Laser Printer and Photocopier – What is the Difference? for further information. A document printed on a laser printer is produced as a result of a 7 stage process, although variations in the way different laser printers implement the stages do exist, e.g. Fuser assemblies using infra-red ovens rather than a Quartz Lamp.
The following is a basic overview of how laser printers make use of Xerography.
1. Preparing and Storing Data
The arriving vector-type document (e.g. Adobe Postscript) passes via the printer’s Communication Port (e.g. Parallel, USB) to the Printer Controller Unit. The laser printer’s Raster Image Processor (RIP) converts the data into a bitmap of raw binary dot patterns that are stored in the Raster Memory.
A characteristic of laser printers is that they store the data of every dot on the page in their memory before beginning the process of creating the image on paper. The laser printer’s mechanical design determines that printing must be carried out in a single and continuous process in order to eliminate the risk of visible gaps or dot misalignment. In contrast, inkjet/dot matrix printers receive a stream of data and print in bursts of activity with possible pauses as the printer awaits/processes further data.
Colour laser printers usually use 4 toner inks CYMK (Cyan, Yellow, Magenta and Black) which adds complexity to the print process and memory storage required. They typically use 4 times the memory of a monochrome printer to print the same sized document because each individual colour needs to be rasterised and stored in memory prior to printing. The price of memory is a major contributing factor to the cost of laser printers, so significant price reductions of this component in recent years has enabled laser printers to enter the budget range for most small businesses and home offices.
2. Charging the Photoreceptor/Photoconductor Drum
Note:
The example used throughout this article describes the following type system:
drum (‘background’) = positive charge
image = negative charge.
However, the system can work in the opposite way with the charges reversed i.e.
drum (‘background’) = negative charge
image = positive charge.
A photosensitive coated rotating drum or belt (Photoreceptor/Photoconductor) is typically given a uniform electrostatic positive charge/polarity via a Corona Wire (older printers) or Primary Charge Rollers (modern printers), and the rasterised data held in the memory is ready to be streamed to a fixed positioned laser.
3. Exposing the Image
The light photons emitted from the laser pass via a horizontally moving polygonal mirror through lenses and these photons discharge points on the photoconductive drum that correspond to the original document/graphic data. Every bitmap dot equates to a pulse and an empty space corresponds to no pulse from the laser.
The resulting image is an exact electrostatic copy (negatively charged) of the original data which becomes ‘etched’ onto the positively charged areas of the drum’s background: see Laser Printer and Photocopier – What is the Difference? for a diagram of this process. The Printer Engine Computer synchronises precisely the drum rotation and laser assembly activity. An image produced in this manner is referred to as ‘write-black’ or ‘Blackwriting’ and is the opposite to photocopiers that use ‘write-white or ‘Whitewriting’. Monochrome (Black and White) laser printers have 1 laser assembly whereas colour laser printers have 2+ units.
4. Developing the Image
Toner is a dry ink powder made from a blended mixture of pigment and plastic which is held in the Toner Hopper. Via the Developer Unit, the powder becomes positively charged and is attracted electrostatically to only those areas (i.e. the actual image) that are negatively charged on the drum’s surface.
5. Transferring Image to Paper
The Transfer Corona Wire charges the paper with a negative charge stronger than the image on the drum. In perfect synchronisation, the paper attracts/pulls the toner powder from the rotating drum to leave an exact image on the paper.
6. Fusing of Image to Paper
The Detac Corona Wire or Secondary Transfer Rollers discharge the paper and in addition, Teflon coated rollers help prevent the powder from sticking to the drum and the Fuser Assembly. Dry toner powder is bonded to the paper using heat (typically a Quartz Lamp elevating the temperature up to 220C/428F) and pressure (rollers). The paper then passes through at speed in order to prevent it from scorching or burning. Despite using cooling fans, a characteristic of laser printers is that the paper exits the printer still warm to the touch.
7. Cleaning of Photoreceptor
The final stage involves discharging any remaining charge on the drum by the Discharge Lamp and physically cleaning the drum of excess toner using a rubber blade. Waste toner is removed to the waste toner compartment or, in some printers, returned to the developer unit for re-use.
