Printer Faults - Odd Specks on a Page

The odd random visible toner dot is normal in laser printing, the developing and transfer processes are not completely perfect.

Individual toner specks are not very noticeable amongst slight blemishes in the paper.

If toner scatter increases to the point where it is noticeable or the background to printing is grey then it needs attention. Try reducing the print density settings.

Very dry conditions change the static fields and give speckling worsening to grey. The human beings in an office will probably be complaining and it might be worth trying a humidifier.

Heavy media can also cause speckling. Toner scatter around the character can be media resisting the toner - a sign that the paper (or whatever) is unsuited to laser printing.

Toner

Toner is a fine powder usually based on styrene acrylate. Toner can be made by milling a solid and historically that is how it was done. Older toners have particles around 8 microns in size suited to printing at 300 or 600 dpi.

Photographs require better techniques. This means printing at 1200 dpi and trying to extract some grey-scale behaviour from the printer. Toner particles need to be finer - sizes around 5 microns perform better, especially if they tend to be rounded and more evenly sized. Modern toners can be air milled or they can be grown in solution. Growing in solution gives rounded grains with an even size distribution.

With any current production technique toner grains are not all completely even sizes and shapes. There is an assortment of sizes - research suggests a normal distibution.

Toner powder is first given a triboelectric charge by stirring in the developer hopper. Most of the particles pick up a negative charge but apparently smaller ones can pick up a positive charge. Most of the particles electrically repel on another but some attract. The toner mix is attracted to the developer roller by a magnetic and / or electric field and given a further charge by the negative DC bias on its surface.

Developer.

Toner is carried around on the developer through a doctor blade which only lets a fine layer through. In some designs rubbing against the blade contributes to the triboelectric charge on the particles. As the developer turns the magnetic attraction diminishes

Toner on the developer surface is then presented to the photoconductor which carries the latent image written by the laser in electrical fields on its surface. In most designs areas intended to be white also carry a negative charge so they repel the negatively charged toner. Areas intended to be black are neutral and attract the charged toner.

The process looks highly controlled but underlying it is a set of probabilities. Toner grain sizes, shapes and charges differ. Some toner particles are coupled to others with an opposite charge. As the toner particles travel on the developer they are attached to a surface by a magentic field and take on its charge.

At the nip point between developer and photoconductor the two usually do not touch. Toner may be brought into contact with the photoconductor by being brushed near to it on the developer

Transfer Roller.

Much the same process is repeated at the transfer roller. The transfer roller and hence the paper are given a charge which should attract the toner - so if the toner particles are negatively charged a positive charge will attract most of them. Yet again the nature of the process is stochastic, depending on the process about 90 - 95% of the toner transfers, a small amount goes on and has to be removed by the scraper blade and waste handling parts of the cartridge.

Laser printer designs achieve a very high concentation of toner in areas intended to be black and very little elsewhere. The odd bit of toner will take on a different charge and do something unexpected.