Office copiers and laser printers both use an electrostatic process to produce their image. Although the term photocopy is often used in relation to office copies it is actually inaccurate and refers to an older obsolete process.
This advice explains the technologies and their suitability for creating records for long-term retention.
Electrostatic (photocopying) processes
Xerography, the most common photocopying process, uses the basic principles of static electricity to create an image on a sensitised surface and then transfer it to paper, as described below.
- A strong electrical charge is applied to the surface of a photosensitive drum.
- The image is projected onto the sensitised surface resulting in a surface charge called the latent electrostatic image.
- Toner powder is electrostatically attracted to the latent image, resulting in a visible image of toner particles.
- A sheet of paper is brought into contact with the drum surface and a charge applied to the back side of the paper attracts the toner image to it. The paper leaves the plate with the toner clinging to it electrostatically.
- The toner is fused to the page by heat and pressure.
The laser printing process is very similar to the xerographic process. The major difference is that the exposure step for laser printers employs a laser beam to record the image onto the sensitised surface to form the latent electrostatic image.
Toners and their effect on image permanence
A typical toner for both xerography and laser printing is composed of a heat-sensitive polymer, such as acrylic or styrene - or both, and a pigment such as carbon black.
Some toners may have an additive known as a 'carrier' added. A carrier is typically an iron powder, which may be coated with resin and is added to improve the ability of the toner to attach to the charged area of the paper. The presence of carriers in the image on paper may adversely affect the longevity of documents.
Full-colour photocopying uses the same principles as single-colour photocopying except that the process is repeated for each colour – yellow, magenta and cyan, and then black. That is, each colour is separately laid onto the paper.
Adhesion of toner to paper
The physical durability of a photocopy depends primarily on how well the toner adheres to the paper. Most copy machines ensure good adhesion but a poorly adjusted or malfunctioning machine may produce poorly adhered images which flake off or smudge.
Full-colour images are thicker than black toner images because they are composed of four distinct layers of toner. Consequently, adhesion is not as good, and the thicker layer is more prone to cracking and flaking during flexing or folding.
The durability of photocopied and laser-printed documents depends largely on the quality of the paper used, as well as the degree to which the paper and toner adhere after processing.
- Toners composed of stable resin materials and a stable pigment such as carbon black are capable of strong bonding to the paper surface. Copies using these toners and printed onto permanent or archival quality paper can be considered permanent and suitable for long-term storage.
- Do not stack or weight dry toner images at high temperatures. Do not place them in direct contact with PVC as this makes the toner sticky and capable of transferring to adjacent surfaces (polyester or polypropylene film is safe).
- Copy machines should be maintained regularly by qualified technicians to ensure that they are operating at the correct temperature to fuse the toner.
- There is evidence that colour electrostatic images do not last as long as black-and-white copies. Most international archival institutions do not recommend the permanent or long-term storage of colour electrostatic copies.
- Maximise the life of colour copies by minimising exposure to light and avoid flexing or folding them as the print layer may crack.
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