How Does a Copier Work?

Making a Master Copy

To make photocopies of an original document, a copier must first make a master copy. Inside the machine is a cylinder made of a highly conductive metal such as aluminum coated with a photoconductor such as selenium, material that holds a charge in darkness but loses the charge when it is exposed to light. The surface of the cylinder is first electrically charged, then exposed to a bright lamp reflected through the original document. The area of the original that's blank will reflect light back onto the cylinder, discharging those areas, but where the document is dark, the photoconductor remains charged. The result is an electric map of the original image.

With the development of digital technology, copiers could scan the original and use the digital image to guide a laser over the cylinder instead.

Applying Toner

The electric map of the master copy can be transferred onto paper through an electrostatic medium. In copiers, this is the toner, powdered ink that is attracted to the electrically charged parts of the master copy. Toner is applied to the cylinder, and it sticks to the charged regions the way hair sticks to the surface of a balloon if the two are rubbed together. With the toner in place, an ink replica of the dark areas on the original document can be transferred onto a sheet of paper.

Paper Transfer

The toner must be melted for it to transfer onto paper, by heat and pressure. This must take place in the dark deep inside the machine, or the master copy on the photoconductive cylinder will be erased. A series of belts carry paper to the cylinder, where the toner is melted onto the paper. The paper is ejected and the process repeats.

When a new copy is made, the photoconductive cylinder is exposed to a set of lights that remove all charge. This causes any of the lingering toner to fall away, erasing the master copy and leaving the cylinder to be electrically charged again for a new image.

The use of photoconductive materials to make copies is much less expensive than older technologies such as carbon paper and the mimeograph. This explains why photocopiers came to replace these older methods so quickly after their introduction in the 1960s. The rise of electronic communication reduces some of the need for paper copies, but photocopiers and electrostatic toner remain the standard for physical reproduction of documents.

Resources

• How Does a Photocopier Work?