Developer and Toner

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Laser printer developers turn the electro-static charge on a photo-conductive belt or drum into an image. The developer does this by presenting controlled amounts of toner-powder to the photo-conductor, where there is a charge difference some toner is transferred from developer to drum.Laser Printer Developer

The image is first created on the photo-conductive surface by charging it with high voltage then discharging some areas with light from the scanner, creating a latent image in static electricity. Laser Imaging Process

Toner. Click for the Laser Printer Index & Overview Page

Toner is the "ink" that will colour the paper. The layer of toner placed on the paper is the only material that a laser printer inescapably must consume. Other parts will probably wear out.

Toner is usually supplied as a powdered solid with the following kinds of properties:
 
Evenly sized small particles that will move fairly easily in strong electrostatic fields
Triboelectric properties that allow particles to take up and retain a charge of their own
Maleability sufficient to stick to paper after pressure, melting or both
Robust adhesion to the media in use, typically paper but could be transparency
Strong colouration of the right kind - Cyan, Magenta, Yellow or Black

Ingredients. Click for the Laser Printer Index & Overview Page

The main ingredients of toner are plastic or wax and a colourant. A typical plastic component is styrene acrylate or polyester resin. Colourant for black toner is typically carbon black. Colourants for cyan, magenta and yellow are not usually declared

Sometimes a toner cartridge delivers a built in developer component as well. Developer components can also be separate - the iron filing developers often are. Kyocera "Ecosys" toners contain a ceramic powder to polish the amorphous silicon drum.

Many different materials could be used in toner, with different melting points and grain sizes. Some toners are plastics, others can be more of a resin or wax. Seemingly identical print engines used by different manufacturers can be designed for quite different types of toner, and use of the wrong grade can require a very expensive strip-down and cleaning of a machine. A printer's developer and fuser could be wrecked if it is fed the wrong grade of toner.

Recent toners may have a microcapsule structure with a wax core surrounded by a resin. The coat gives the right triboelectric properties for imaging, the core gives low temperature zero-silicone fusing to the paper.

Printer makers aren't exactly forthcoming about toner ingredients - after all it's one basis for their competitive advantage. Whilst this attitude is certainly understandable its a potential obstacle to cartridge recycling.


Grain Size. Click for the Laser Printer Index & Overview Page

Toner powder has a consistency similar to corn-flour - its a very fine powder. A merely fine-grained structure isn't good enough, grains ideally have a precise size. The dimensions and electrostatic properties of toner particles will obviously be important in determining image quality. Fine-grained toner (and developer) can give a finer-grained image. The toner used in a 1200 dot per inch machine is likely to be finer than that used in a 300 dot per inch model. The grains in 300 dpi toner are around 12 micrometers in diameter. Toners in recent 2400 dpi printers are more like 5 microns - about half the length of a pixel.

If the photoconductor is capable of some greyscale response then a finer grained toner will probably be used so that the quantity attracted is more nearly proportional to the electrostatic field.

Nanoengineered Powders. Click for the Laser Printer Index & Overview Page

The major components of toners haven't (yet) reached nanoengineering dimensions. Nanoengineered powders with dimensions below a thousandth of a millimetre are becoming quite common, even in things like household cleaners and cosmetics. Materials this fine aren't the major components in toner because:
 
the smallest details that can be resolved by the wavelength of light are ten times or more larger.
there are concerns that nanoengineered materials may sometimes have unexpected behaviours - very small particles are common in natural science, but deliberately engineered plastics are not. 

Toner granules themselves may be micrometer scale but they can be finely engineered. 

Photocopiers and laser printers use hot rollers called a fuser to adhere toner to a page - heat melts the toner and pressure makes it bond to the page. The toner would bond to the fuser rollers as well. The fuser is kept clean partly by using a teflon surface and in older and faster machines by maintaining a thin coat of silica oil on the fuser roller surface. Recent toners can do away with the silica oil by using a two component toner. Toner granules can be made microencaspulated, with a resin shell around a wax centre. In the fuser the wax melts and this provides just sufficient  lubrication to keep the fuser clean.

Toner grains should be approximately spherical in shape so that when they aggregate at a pixel they do so in a predictable way.  Taken together with microencapsulation ideas the toner powder is precision made, not just finely ground powder.

Although toner itself might not be nano-engineered some of it's components may be. Charge control and surface additives are nanometer sized particles that adhere to toner particles to control the triboelectric charge and it's adhesion to surfaces.

Powder quality is important, grains should all be the same size. Grains should not stick together because of pressure, heat or damp - if they do there will be erratic blotches in the image. 


Melting Point. Click for the Laser Printer Index & Overview Page

Low melting-point toners should allow the paper to move faster through a cooler fuser unit. Compared with the speed of the Raster Image Processor and polygon mirror the fuser is not generally regarded as a bottleneck in printer designs. The fuser does need to reach an operating temperature before it can be used, and the lower this is the faster the printer will be ready for action. Hot fusers ensure good adhesion with the page. Cool fusers can also deal with materials like overhead transparencies.

The components used in a toner primarily need the right tribolectric properties but a low melting point is desirable. Coating a particle with the right triboelectric material but having a soft core is one approach to getting an ideal material.


Colourant. Click for the Laser Printer Index & Overview Page

As the name "toner" implies the primary purpose is to impart colour to the page - the other components are only there to assist in delivering and adhering the colour.

Colourant might behave in one of two ways - binary or linear.

Binary colouring is what most printers do. A pixel is either coloured or not. If this is what the printer mechanism delivers then the colourant should ideally have the maximum effect in the minimum volume. The limiting point would be where, if a pixel is black, a layer one molecule thick should absorb all light.

Linear response is less common but the electrophotography process may be capable of it. Ideally grains of colourant would now have less effect so that a weak voltage would attract few grains and give a light grey, whilst a strong voltage would attract many grains and give black.
 


Developer. Click for the Laser Printer Index & Overview Page

The developer allows the latent electrostatic image on the photoconductor to take up sufficient toner to give a strongly coloured image.
 
Toner layering on the photoconductor should accurately reflect the strength and contours of the electrostatic image. 
Exchange of toner between the developer and the photoconductor should use the lowest possible voltages.

Toner powder might be
 
 sprinkled from a hopper 
 dusted with an air jet

Both have been tried, historically. Given that the OPC is a roller, application with another roller would be simplest.


Iron Velvet. Click for the Laser Printer Index & Overview Page

Until recently laser printer developer designs were almost always based on a technique that might be described as iron-filing velvet. The toner is mixed with iron filings, longer in one dimension than another (commonly 50 to 100 microns long). The filings are kept coated with toner by stirring in the developer hopper, where the triboelectrically charged toner clings to the metal particles. There is often an electrical feed into the developer unit to give the metal particles the right charge to attract toner. At the front of the hopper, adjacent to the photoconductor drum, the iron filings are picked up by a magnetised roller that rotates to carry material across the surface of the drum. The amount of material carried by the developer is controlled by a gate - usually a brass strip adjusted over the developer roller - this is sometimes called the "doctor blade" - a term derived from paper making and print inking. The developer roller continually rotates to carry fresh material. In some designs of developer the magnets inside the roller are static so that the iron filings are always at the same angle when they contact the drum. In other designs the magnets within the roller rotate more quickly so that waves of toner-bearing material pass across its surface.

The iron filings escape only spradically from the developer because of the magnet, so they are carried back in and re-mixed with toner.

Toner attached losely to the filings is attracted to any areas of the photoconductive drum that carry a different charge. Normally the developer roller and its cargo all have the same charge as unexposed areas of the drum- so in those areas there is no attraction and no transfer. Exposed parts of the drum pick up a layer of toner.

Developer designs are changing. Older designs arrange for the developer to brush very lightly against the drum. Some recent designs have impacted the developer quite hard onto the drum – it is a surprise that the image doesn’t smear. One reason for the close contact may be to reduce the transfer voltage.

Printer developers are consumable, their iron-filing content or resin will be used up or wear out.

In many older and larger machines the developer and toner are separate components. Typically the software in a printer will keep track of when a new consumable has been installed and will then count through its lifetime, either warning when it is exhausted or else stopping the machine.

Toner Dosing. Click for the Laser Printer Index & Overview Page

Another approach to keeping the right mixture of toner powder and iron-filings in the developer is to measure its quality. Electrical resistance, magnetic and sound properties will all change depending on the iron-filing toner mix. Most printers seem to use a magneto-resistive sensor. A motor or clutch turns a mixing paddle to give the developer a dose of toner when the mix is too weak.

Resin. Click for the Laser Printer Index & Overview Index & Overview Page

Most recent combination toner / developer cartridges use a resin coated roller or sheet instead of the iron-filing technique. The resin coat presents a much finer "flock" of toner to the OPC held on a plastic sheet or roller by electrostatic action. The toner particles gain a  triboelectric (static) charge as they pass through the developer - just as dust can be picked up with a statically charged comb.

The triboelectric technique seems to have been in use for some time. In older photocopiers the charge could apparently be created by using steel and quartz balls in the developer

The resin fuser removes the need for the iron filings - on the other hand the cartridge designer now has to select materials that will generate exactly the right static charge


Cartridges. Click for the Laser Printer Index & Overview Page

The toner, developer and waste bottle can all be integrated into one cartridge- this is the approach taken in the Canon engines starting with the LBP8 / HPII and in the Fuji/Xerox machines. One big advantage of cartridges is that no consumable ever falls far below its optimum performance, and it simplifies fault finding because there is only one module to change. The disadvantage is that parts of the cartridge will be perfectly usable when it is scrapped - so it may seem ecologically unsound.

The consumables in a laser printer tend to be used up in an orderly fashion.

  • Several toners are often used for each drum.
  • Several drums are used for each developer
  • Developers and fusers both last a long timer - perhaps a tenth to half the life of a printer.
In practice the amount of toner shipped can be raised a bit to match the life of the drum. Developer does not cost much to manufacture, so integrating toner, developer and waste bottle into a single cartridge and throwing them out with the drum may impose little cost.

Printer manufacturers commonly run recycling schemes to reduce the wastefulness of cartridges. Alternatively users can get their cartridges refurbished by a host of little local firms. Both methods raise some problems:

Recycling back to the manufacturer probably means a long and rather inefficient logistics chain - a lot of sorting depots and diesel.

Recycling locally should be more efficient. The problem is that a local ecycler dealing with any one of a hundred different types of cartridge isn't necessarily going to be very effective. Local recyclers also have the problem that the national concern, protective of its own methods, wont necessarily tell them what grades of material to use. More about cartridges


Problems. Click for the Laser Printer Index & Overview Page

Toner Failure.

Toner basically just runs out. When it's gone there isn't enough to create an image. A printer should normally warn of this and ask for more toner. The printer would know it needs more toner because of the number of page print cycles that have elapsed and/or because regardless of what the toner-mix detector says when the dosing motor can no longer bring the mix up to strength.

If there just doesn't seem to be enough toner this might be evident to an engineer. Touch the front of the developer lightly with a clean dry finger and it should become a rather black finger.

One way to check the image intensity is to set the printer away making an image, then stop it half way through by turning the power off. Take the drum or cartridge out and have a look at the image - which should look quite strong on the green material. If the image is faint suspect bad voltage connections first - but it is possible that the toner dosing mechanisms have failed.

Toner of the correct grade is unlikely to fail outright - at least if it has been stored in cool dry conditions and not kept for an unreasonable length of time. Toner will denature over time - the plastic and wax materials are likely to have some volatile components. Simple pressure and hot-cold cycles will also denature toner.  Boxes of toner or catridges that contain it need to be stored in cool, dry conditions - as does paper  

Refurbished cartridges could give difficulty. Given what has been said about toner particle size and melting points it is understandable that refills don't always work perfectly. Intermittently a refurbished cartridge causes annoyance.

Toner can leaks into the printer through a bad seal. This smudges the paper and collects on the charge elements wrecking their voltages.

Toner with the wrong electrostatic properties and too much is drawn onto the drum and dumped into the printer.

Toner has the wrong melting point and sticks to the fuser rollers.

Developer Failure. Click for the Laser Printer Index & Overview Page

Developers get replaced due to dramatic failure or just gradually wear out.

Wearing out an iron filing velvet seems to mean there is just too little material of the right grade left to do the job. What is most likely to happen is that the roller gets bald patches at one side or the other resulting in faint or blank print. A bit of shaking and pushing the rollers round by their cogs will show whether this is happening - if it's impossible to get an even cover then there isn't enough material. Don't do this in a clean white shirt, do it where a toner spill won't matter. Over time as the iron filings churn they grind one another down and escape through the doctor blade with the toner. In principle the developer hopper could be given another dose of iron filings - some toner does, others seem to rely on one charge and then replace the entire developer unit.

Roller damage gives a vertical mark on the page. Magnetic developers quite commonly suffer damage from extraneous objects, particularly when the printer has recently been worked on. The characteristic sign is a vertical wear mark down the page. The flock on the roller is uneven and there will be a mark on the roller where a bit of metal of some description has eroded into the aluminium roller. The normal repair is simply to replace the entire developer. It is possible to replace the roller but spares aren't common except from other scrap developers and it's a messy job.

Although the roller carries an electrical charge it is diffuse and a score mark is often very localised.  It may be possible to repair the roller by running a thin layer of filler into the score-mark but since dismantling a developer to the point where this can be done is very messy and time consuming this won't often be feasible

The doctor blade that controls toner flow sometimes moves. Characteristically the movement is uneven so the "velvet" coat is thicker on one side of the developer roller than on the other. It seems unlikely that this could happen by anything other than user interference - bashing the developer against something. It has happened often enough that an alternative theory might be that standing waves might occur in toner -developer mix and dislodge the blade.

Refurbished cartridge developers could have the wrong grade and move the wrong amount of toner. This will result in either a faint or dark page immediately after the developer is changed. First check the electrical contacts between a cartridge and the printer body because a bad contact can have the same effect.

Resin Developer components rely on the triboelectric properties of their materials. These are carefully engineered by use of various grades of plastic and will wear out

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Perhaps the surprise is that most refilled or refurbished cartridges actually work quite well.
 
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© Graham Huskinson 2010

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