Printer Faults - Offsetting and Ghosting

Repeated but fading images often called "offsetting" or "ghosting" are seen when the the fuser is too hot and melting the toner onto the foil or hot-roller. In the print industry it's often called "offsetting" because offset transfer by rollers is a common print technique and that is essentially what is happening.

Regular marks might be caused by any of the rollers involved in the print process (Precharge, OPC, Developer, Transfer, or the fuser).

Offsetting where a previously printed image recurs just about has to be the OPC or the fuser because they are the only two pagewidth rollers that deal with a complete image. The fuser is the most likely culprit.

Offsetting caused by the fuser being too hot is the opposite fault from smearing caused by the fuser being too cool. That might be worth mentioning here because the next step is for the user to turn the fuser temperature down and get smearing. The ideal temperature for the fuser is to make the toner tacky but not actually to melt it and that might happen in quite a narrow range of temperatures.

Fuser Offsetting

Fusers can cause offsetting if they actually melt the toner on a page.

The fuser adheres the toner to the page using heat and pressure. The fuser has a heated roller coated in a layer of non-stick material and a pressure roller made of a rubber compound and loaded up against the heated roller by springs. The idea is that as the page passes through the fuser contact with the heated roller heats the toner and the page. The toner becomes plastic and melts into the flock and texture of the paper. Toner isn't intended to actually melt.

If toner does melt in the fuser it starts to adhere to the normally non-stick top roller coat; part-transfers to the page and part goes round to print again. If the effect is marginal, then about two thirds of the image will transfer to the paper and a third will re-appear one fuser-roller circumference down the page. The fuser roller will get dirty and toner drops will form so the page is likely to be covered in random specs as well.

Sometimes the print on the page actually looks molten - rather than having a shiny surface it has a crinkle as though it had melted too fluidly.

Check the pitch of the offsetting against the defects ruler for the printer.

A stop test will help identify offsetting in the fuser if there is any doubt. The image going into the fuser will look fine but what comes out has the problem.

If the fuser is easy to get at (through the rear flap for instance), have a look at it. If the rollers are covered in specks that suggests either they are worn out or they are retaining too much toner.

There are several likely causes of offsetting in the fuser:

The print cartridge has just been changed and the toner is inappropriate.

There are different toner formulations with quite different melting points. This is unusual but it does happen, particularly where one print engine and cartridge type is used by several different manufacturers.

The printer has been used for heavy media and the fuser temperature has been turned up.

Not every printer has adjustable fuser temperatures but some do. Set the media type and fuser temperature correctly. Normally the computer settings override the printer settings. With a mixture of computers with different drivers on a network it is possible (if unusual) for a printer to set a high temperature and another not to re-set it later.

Media may be inappropriate.

If the media won't take up the toner very well then some portion of it might go round on the fuser roller instead. This might happen with plastic or metallised materials.

Thermistor Problem.

Fuser temperature is controlled by a feedback loop. The engine controller turns the fuser power on. Fuser power is often on the low voltage supply. The fuser heats up and this is detected by the thermistor. The thermistor in turn feeds a value back to the engine control. If the thermistor is not making good contact the fuser temperature may rise a bit too high.

Fuser Control Problem.

Fuser power and therefore temperature often comes from the LV-PSU via the engine controller. The LV-PSU contains the triac that actually turns the fuser on and off and it is the most likely thing to be at fault. The engine controller is essentially a microprocessor and if it is doing everything else right then it's a question why it can't control the fuser - perhaps it has a damaged output transistor. Doubt makes fault tracing difficult.

Foil Fuser Lubricant.

HP and Canon have a patent on a very thin fuser top roller that is little more than a PTFE sleeve powerfully heated by a ceramic rod. Heating of the paper relies on distribution of a layer of lubricant. If the lubricant isn't right or hasn't had time to spread, temperatures will be uneven and there might be some offsetting.

Foil Fusers.

Canon/HP foil fusers (those with a sleeve rather than a roller) sometimes cause offsetting problems when they are first installed. The fuser relies on a lubricant layer to distribute the heat but when a fuser is first installed that lubricant may be uneven.

Foil fusers could need about 100 pages of test print before they settle down with an even temperature distribution.

OPC

The organic photoconductor (OPC) will cause offsetting if its cleaning process breaks down. To rehearse what the OPC does:

It is made from a material which acts as an insulator in the dark and a conductor in light. This capability is at the heart of the laser printer process, Normally the OPC is formed into a drum which turns about 3 or 4 times in printing one page. At the start of a cycle the drum is given an electric charge, which it holds because it is in the dark confines of a cartridge. A laser then scans across the drum and where the laser touches the electric charge drains away. Next the drum travels past a developer which presents toner powder to the drum, the toner sticks in areas where it is not repelled by the charge. The drum now turns over the paper and the toner is pulled away from its surface onto the paper by a transfer voltage. In use the OPC transfers between 90 and 95% of the image onto the paper. Finally the drum goes to a cleaning process which makes the surface ready to be used again.

The OPC will give offsetting if the cleaning process breaks down. The offset image will be in faded grey (because it only has 5 to 10% of the toner) and an OPC roller circumference from the original print (The service manual gives a defect ruler but it can be gauged by eye.

After a bit of use the pattern of repetition will still remain but the print background is likely to have become very grey because of the buildup of old patterns.

OPC cleaning has several elements.

The heart of the process is usually a scraper blade, this fits tight up against the drum. If the blade fails it usually degrades a bit over part of the print to one side or another. This tends to give "tyre marks" in one margin or another. A little nick of damage in the cleaning blade tends to leave a grey streak.

In some printers this is supplemented or replaced by a flock or foam roller carrying a voltage.

In older printers there is usually a page-width series of red LEDs. This strip of erase LEDS illuminate the drum so that the whole thing is discharged. Usually it will be one LED or another that fails, leaving strips about an inch wide not properly erased.

In recent printers the erase LEDS have been replaced by a roller carrying an AC signal. The AC disrupts and discharges the OPC. The erase signal can be placed on the precharge roller for one turn. Failure of the erase voltage will leave the drum with a grey background but the repeated pattern suggests the problem is the erase process and not an issue with the developer bias.

The easiest guide to whether the fault is coming from the drum is that it recurs at an interval corresponding to the drum circumference. If this doesn't give certainty do a stop test and look at the emerging print - is the pattern discernible on the drum itself?

If the erase process breaks down on modern printers then a change of cartridge should solve the problem. If that doesn't fix the problem (or if you want to salvage the cartridge) then first look for a voltage not properly connecting due to dirty contacts.

Other Causes

Printing large quantities of unusually narrow media can cause this problem; the fuser is not being cooled by the media as rapidly as would be expected so heat builds up. Heat detection is usually by a single thermistor and it tends to be in the centre of the print path so it might not see the rising temperature towards the edges.

Offsetting also happens with preprinted material, particularly letterhead pages and forms that have been printed on another laser printer. The temperatures used in laser printing vary and a pre-printed form might be printed with a cool-melting toner that re-melts in another machine.

Try reducing the fuser temperature then printing a few more pages to see if the problem clears itself. Try printing a cleaning page if the printer has this option. Examine the fuser to see if there is visible material stuck to its surface.

Offsetting can be a nuisance but things can get worse. The next step up in toner melting too far is for the toner to melt and coat the fuser rollers and then get a paper jam. At this point the printer will shut the fuser down so it will cool with a horrible mess inside it. When the printer is turned off and back on it will be unable to turn the fuser. With luck the result will just go straight to error and the printer won't make more than an initial attempt to turn. There is a danger of stripping a cog.