Printer Faults - Fading and Light Print Issues

Faded print is a fairly common problem with laser printers.

By its nature the print mechanism transfers just a little highly coloured toner.

It's generally preferable for printers to use too little toner; this will get noticed as fading and can be adjusted upwards. Using too much toner is wasteful and might not be noticed and can be adjusted down.

Assuming the print process is working at all (and to get feint print it is), then the laser print process relies on two sets of things:

• Cartridge components - toner, developer and drum together with a transfer roller or belt in colour machines.
• Voltages to transfer toner powder from the developer, onto the drum and then the paper.

Cartridges.

Fading often gets noticed when a cartridge is changed. The last thing changed is the top suspect but might not be the answer. Changing a print cartridge disturbs the machine, blows a bit of dust and toner around and inevitably slightly alters the spacing of parts of the printer.

Print cartridges are fairly complicated

The main components of a cartridge are:

• Toner - essentially just a hopper of styrene acrylate and colourant turned into a fine powder. Toners typically last 5,000 to 10,000 pages and often come in cartridges, although a straight-forward toner can be supplied in bags and bottles. For the print process to work at all there must be enough toner. When toner runs out print tends to fade over a hundred pages or so.
• Developer - usually a metal roller that rotates around a magnetic field. Some toners have their developer as a magnetic component mixed into the toner itself. Alternatively, the developer contains iron filings. If the developer material is distinct from the toner it tends to have a life of about 150,000 pages
• Photoconductive Drum - the drum is also known as an "OPC" or organic photoconductor. It is essentially two thin layers of conductive polymer coated on aluminium. The outer layer carries a charge. The inner layer becomes conductive on exposure to light (Materials such as polyvinylcarbazole have the right properties). Over time the drum becomes worn and scratched. Even if this did not happen exposure to light eventually breaks the transfer layers properties down. Drum life tends to depend on their size - somewhere between 20,000 and 50,000 pages seems typical. When a drum is exhausted it tends to give grey print lacking contrast and tre marks down one side or another of the page.
• Waste Bottle- it is difficult to get all the toner to transfer away from the drum, a fraction remains on the drum surface. This is cleaned off by exposure to light and a scraper blade. Re-using toner is difficult because it is partly de-natured, the waste toner goes into a bottle.

One Cartridge or Two?

Some larger printers and older models have the four components as separate items. Many of the big photocopiers placed on lease work this way as well. With expert users or a sales and engineering force to look after the machines this is practical.

Canon originated the idea of a single cartridge by wrapping all four functions in one unit. The original aim was to make personal photocopiers practical. It wouldn't be feasible to have engineers making regular service calls on low cost machines. Things were simplified so that the user could deal with most problems by changing the cartridge. A single piece cartridge wraps up all the normally consumable items so all the faults and issues they might cause are dealt with at one stroke.

Some manufacturers (and users) still prefer printers with a separate toner cartridge and photoconductor, drum or imaging unit. It does seem wasteful to be throwing away part used drums, even if they do go for recycling. In some ways having separate components is handy - a technician can test each component to see where a fault lies. However separating the components can make fault diagnosis more difficult for ordinary users.

Image Formation

An image is first formed in the printer's memory from data communicated in a print language of some kind. If this part of the process were to go wrong the printer would either not print anything or it will print gibberish - a mixture of heiroglyphs and blank pages. Just to be sure the computer is not to blame for faint pages print a configuration page straight from the printer front panel. If a configuration page is faded the fault does lie in the printer.

When the image in memory is complete the printer spins its polygon mirror and reads the image in memory as a "raster" - a line of dots at a time. (Dots may or may not correspond to pixels in memory, the laser might print several dots to handle one grey-scale pixel). It is possible for the laser to be faint, over thousands of hours use they lose brightness just as LEDs do. Another possible cause of low laser energy is dirty optics in the scanner - they usually aren't sealed assemblies.

Electrostatic Process

Meanwhile the drum has begun to turn past its pre-charge or conditioning station. The drum gets a charge of several hundred volts across its whole surface, The laser then discharges areas that are to print leaving a "latent image" of varying levels of static charge.

The developer is a cylinder placed close to the drum. The developer is usually an aluminium tube that rotates around a magnet. Toner particles and developer powder are pushed towards the developer by stirrer blades and attracted to it by both its magnetic and electrostatic fields. Toner itself has triboelectric properties, the action of the stirrers gives it a static charge. Developers and toners differ:

• HP and Canon have tended to use magnetic toner - there is some ferrous material such as magnetite mixed into the toner powder itself as mentioned above. The benefit of this is that the developer component is controlled during toner manufacture
• Other designs often use a separate developer powder or carrier which is essentially sharp iron filing powder. The roughness of the filings is important because it tends to trap toner powder. The developer hopper usually has a magnetic densitometer on it and when the toner level falls too low the printer senses it and runs the stirrers or a dosing motor to provide more toner.

The mix of toner and developer is coated onto the developer roller possibly with the assistance a of a feed roller. The toner/developer is held in the magnetic and electric fields on the developer roller and passes through a doctor blade. The doctor blade controls how much mix passes onto the drum facing surface of the developer. If the doctor blade is incorrectly set the print could be light or dark accordingly. Miss-seating of the blade would be unusual but it isn't unknown.

The developer should be carrying sufficient toner to give dense print if it transfers to the drum. If the toner isn't needed it goes back into the developer unit.

Development is a combination of factors. In a write-black system the toner will transfer off the developer roller to the OPC drum where the drum voltage has been discharged. The voltage difference has to be great enough for toner particles to make the leap to or stick to exposed parts of the OPC in preference to the drum.

The image is now "devloped" on the OPC and if the printer is stopped and the shutter opened it can be seen. (Thats one of the things a stop test helps with).

the image has to be transfered to paper. As the drum turns the paper will be driven forward by the registration rollers to meet the image comming round on the drum. At this point the High Voltage Power Supply applies several hundred volts positive to the transfer roller. The relatively negative charged toner on the drum is attracted to the positive charge under the paper.

The paper travels over a static charge eliminator which removes most of the residual charge leaving the image fixed in lose toner. If a stop test is done the image can be seen.

The final stage in printing is to pass the page through a fuser which uses a combination of heat and pressure to stick the toner to the page. Some recent fusers do carry charges in order to stop toner moving as it comes close to the rollers.

The drum tends to be about 3 to 4 inches in circumference - a large drum is expensive and would make bulky cartridges. The drum therefore turns about 3 times in printing a page. Most of the toner transfers off the drum onto the page but some remains. A scraper blade removes the surplus toner. At one time the scraper blade was invariably accompanied by a set of erase lamps but now an erase volatge might be used at the end of the print process instead.

Voltages.

A mono laser printer generates about five high voltages and these control the "electrostatic" printing. There are three DC voltages: pre-charge, developer and transfer. Control of print density is usually via the developer DC bias voltage; turning up this voltage increases print density. In a modern printer the stages tend to use around 600 volts negative but in machines with corona wires the voltages might be much higher.

There are also a couple of AC high voltages.

The developer AC bias is connected to the developer roller together with the DC bias. The AC component reduces the attraction of toner to both the developer roller and the unexposed parts of the drum latent image - it increases the density and contrast of the page.

The erase voltage replaces the erase lamps found in older printers. At the end of printing the drum makes one more revolution with AC applied to the primary charge roller and this removes any residual charges ready for the next print cycle.

We aren't being precise with voltages here for a few reasons:

• they specific to technologies, models and to a print engine series. Service manuals give the details.
• they aren't easy to measure, the printer shuts them down when any lid is open because they would certainly hurt and could be fatal. Electrical fields are more dangerous than the laser.
• even technicians used to laser printers may find it difficult to measure the voltages - they can go up to several thousand volts, way beyond the range of most meters. An HT probe is needed.
• experience suggests that the voltages in a printer with clean contacts tend to be correct. They are produced by little HT transformers that rarely go wrong. If an HT fault is suspected take a close look at the High Voltage Power supply. A fault often shows as scorching.

Bad Images

Shaking the Cartridge

The short answer is try it, cautiously, gently, keeping it horizontal and not over clothing or carpets.

The long answer is:

Some manufacturers say their cartridges are "non shake" designs, meaning the user doesn't have to periodically take the cartridge out and give it a shaking - which happened with the older printers. Older toners had a tendency to "cake" if they were left static for a time. Recent toners shouldn't be so inclined to do that. Reasons why shaking shouldn't be necessary any more include:

• Manufacturers incorporate several sets of stirrers in the cartridges shaped like a series of cylinders.
• Cartridges have chips with an expiry date so toners don't get to sit on a shelf for a long period and the problem of toner caking is reduced.
• Modern toners are so fine that a bit of shaking will still leave lumps - so it is liklely to be innefiective

Reasons to be cautious are:

• Toners for recent printers are a very fine powder with particles a few microns across. The toner behaves rather like a liquid, so if it is shaken it is inclined to leak out.

When a cartridge is sold it has had much the same life as any other bit of goods. Factory, warehouse, container, ship, re-stacked in a warehouse, put into a picking trolley, wrapped and finally chucked between couriers vans several times. Which end of the cartridge the toner is at is unknown.

First visually check the cartridge as it is taken out of the machine; is the machine generally quite clean or are there traces of toner and paper dust.

Take the cartridge somewhere a spill won't matter too much - kitchen or bathroom lino or tiles, or maybe outside

If printing seems faded at one side exercise some caution in handling it. Hold the cartridge horizontally and gently shake it back and forth, that should even out the toner in the hopper.

Modern toners are often designed to avoid the need for shaking to overcome caking and they have multiple stirrer blades to keep an even flow of toner to the developer - but still a bit of shaking sometimes helps.

Otherwise keep cartridges level once the shipment seals have been removed. Avoid handling cartridges unnecessarily. Keep cartridge surfaces clean and free from toner dust by wiping them lightly with a damp tissue.

Print Density and Media Settings

Most printers have two sets of controls that change the appearance of the print:

• First check the medium you are trying to print on. If there seems to be a problem use ordinary office grade 80gsm printer paper fresh from a packet. The paper should not curl as it passes through the printer - that is a sign of damp.
• Also check the media settings. The printer varies the transfer voltage to suit the medium and having a setting for card and printing on office paper will cause problems.
• Change the print density or "toner density" a notch or so. Not all cartridges are made the same and the reason for having a density control is to give a measure of adjustment.

It should be possible to get black print on a white background with

• the white background not turning grey or having excessive amounts of speckle.
• a black box being solid and not fading in the middle. (But people don't usually print black boxes, it wastes toner)

Cartridge manufacturers do aim for consistent products but there is no guarantee that you won't have to make ordinary user adjustments from time to time.

Refurbished, recycled and compatible cartridges are particularly likely to be a bit more inconsistent in their behaviour.

With some kinds of Brother cartridges note that you can't swap the cartridge between printers and get consistent results - the printer monitors and adjusts the toner over a period.

Settings Problems

Faded print is most likely to to be caused by the print density or media settings. There are two sets of print and media settings:

• settings applied by the user in the computer driver
• settings held in the printer. When the printer is turned on it will load default settings.

Settings in the computer are usually issued to the printer using a print language. The setting issued by the computer normally override both the default settings and any issued by a user at the control panel.

If the wrong drivers are in use on some computers on a network the printer might work in terms of interpreting page layouts but media selection and print density might not work properly.

A printer test page is produced by the printer itself rather than by the computers so it should be uninfluenced by their settings. To truly isolate any possibility of computer settings disconnect the printer from computers and the network and print a self test page.

Another possibility, to isolate any influence of external settings is the use the engine test button if the printer has one. The engine test page is often rather dull but not likely to be influenced by computer settings.

Cartridge Seating.

Cartridges and the body of the printer are both made of plastic. Particularly with mono printers the cartridges go into the printer at an odd angle - it makes it fairly easy to load the cartridge from the front and have the cartridge drum appear in the middle of the machine. It can be difficult to feel if the cartridge is fully in position and it can be possible to close the lid and have the printer work with the cartridge misaligned by a millimetre so that toner doesn't transfer correctly at one side.

Cartridge Connectors.

Cartridges use several high voltages in operation: There is usually a precharge voltage for the drum, a developer charge and a drum charge as well as a ground connector. These voltages typically aren't carried by special connectors but by axles and little tin plates on the side of the cartridge mating with springs and pins on the inside of the printer. Make sure the metalwork of the connectors is clean - use a damp tissue or cotton bud as appropriate. Obviously avoid bending the connectors and examine those you can see for damage. The printer may not work at all and can produce anything from a white to a black page if one of the contacts is not making properly.

Transfer Roller.

Another cause of light print is the transfer roller. The transfer roller carries a voltage that strips the image off the photoconductive drum and onto the media. HP, Canon and most other manufacturers design printers using a transfer roller under the photoconductive drum of the print cartridge. Paper passes between the drum and the transfer roller. On a mono printer this is visible when you take the cartridge out, its a page-width black roller - or at least it's black when its new. Colour printers tend to use a transfer belt instead.

Over time the transfer roller tends to pick up dust from the paper and to oxidise a bit. It becomes a rather lighter grey - the left and right ends might show something like the original colour.

The simplest answer is to replace the transfer roller which is usually quite easy. Rollers can be included in maintenance kits although that drives up the price a bit and replacing good rollers is a bit wasteful. HP and Canon printers tend to have a little hook to help pick up the replace the roller without touching it. If you are throwing an old roller out touching it doesn't matter, but it will matter with the replacement. Avoid touching the working area of a new roller as finger grease will de-nature it and this may lead to a repetitive fault in printing.

Transfers rollers can be cleaned with a brush. The paper dust can be removed by fairly hard brushing with a paintbrush or light brushing with a toothbrush. Gauge the brush and the technique to get rid of the dust and don't damage the roller. Perhaps it's just British parsimony that makes us suggest cleaning transfer rollers. If a printer has to work reliably without attention after a field service call its simplest to just replace the roller - the extra cost is trivial compared with the cost of the call.

Transfer rollers do wear out after a while. Its actually a surprise that they tend to build up dust because the leading edge of the paper is constantly chipping at them so after a few hundred thousand pages the dimensions and performance are likely to have moved out of tolerance.

Transfer Belts

Colour printers normally use transfer belts rather than individual rollers. There are two ways of using the belts:

• electrostatic transfer belts use charge first to grip the paper, then a succession of transfer rollers underneath pulling toner directly across onto the page.
• intermediate transfer belts pulling the image directly onto the belt using a succession of transfer rollers, then the belt passes across a secondary transfer roller where the image is pulled onto the page.

There usually isn't a choice about changing either kind of transfer belt. The printer control software measures its use and is likely to require another when it believes the belt to be exhausted. Printers often calibrate by printing onto the belt and measuring the effect with a colorimeter, so the printer can detect exhaustion.

Precharge Rollers and Corona Wires.

The photoconductor drum in a laser printer goes through five stages: precharge, illumination, developing, transfer and cleaning. At each stage the drum changes voltage. The precharge and transfer voltages are applied to the drum by a roller or a corona wire.

In HP and Canon printers the precharge roller is part of the cartridge. Lexmark tend to make precharge part of the printer - the roller should stay clean Many Brother cartridges use a corona wire for precharge.

At one time both the precharge and transfer were invariably done by corona wires. Coronas are thin wires that bleed charge off into their environment. There are a couple of problems.

• Corona charges are often sufficiently high to break air down to ozone. Regardless of what your granny told you ozone is bad for health.
• corona wires attract dust from the environment onto the wire, the dust them interferes with the wire's function causing lighter or darker areas on the page. With older photocopiers keeping the machine going meant taking the corona assemblies out and cleaning them every few hundred pages.

High Voltage PSU Fault

Laser printers have a high voltage power supply which provides the precharge, developer and transfer voltages. Recent printers may have voltages for the fuser rollers as well. Colour printers have a set of voltages for each of the four colours so the high voltage power supply gets quite impressive.

High voltage power supplies don't fail a great deal. Look at printer cleanliness before changing the HVPSU.