Copiers and Printers

The basic principles and mechanics behind photocopiers

Static electricity is at the heart of a photocopier’s functionality. Inside a photocopier, you will find a type of drum or belt which can be charged with static electricity in order to attract the extremely fine black powder known as toner.

Toner is made up of two ingredients, plastic, and pigment. The pigment provides the colour, and the plastic helps the pigment stick to the paper once it is heated.

To create text or an image on paper, the drum can be charged selectively so that only certain parts attract the toner. This is achieved by making the drum out of photoconductive material.

With the document sitting on the glass surface of the copier, a very intense beam of light is projected up through the glass. Through a series of mirrors, the image is reflected and sent to either the lens, or CCD, and in the case of a digital machine, the image is sent to the laser assembly via a polygon mirror.

Older analogue copiers machines use a positively charged drum and paper, with a negatively charged toner.

In older analogue machines, wherever light hits, electrons are released from the photoconductive atoms in the drum, and neutralize the positive charges above. Areas that are dark do not reflect light onto the drum, leaving areas of positive charges on the drum’s surface. The negatively charged, dry toner is then spread over the surface of the drum, and the particles stick to the positive charges.

On the other hand, modern digital copiers utilise a negatively charged drum and paper, and a positively charged toner. The laser light only strikes the drum to reflect the dark areas, and not the light areas producing a ‘reversed’ image.

To attract the toner that is inside the drum onto paper, a sheet of paper of opposing charge is passed over the surface of the drum. Becuase toner is heat sensitive, the particles are fused onto the surface of the paper using heat and pressure rollers and the image is formed.