Printer Faults - Marks at Vertical Intervals

Vertical repetitive defects are almost always caused by a problem with one of the print process rollers. Likely culprits are the OPC drum, transfer roller and fuser because they wear out quite quickly and are exposed to accidents.

A mono printer can have 7 rollers and a colour machine more than 20, so identifying the most likely culprit might be difficult. Luckily roller faults have a characteristic pattern - the rate at which the fault repeats is given by the circumference of the roller.

Measure the pitch between marks and try to match it with the rollers in the printer. Service manuals generally have a "defects ruler" but it is possible to gauge which roller is likely to be to blame by eye.

Likely causes of faults are contamination or damage to:

• Print cartridge drum, also known as the organic photoconductor (OPC). The OPC is both delicate and the key to how the printer works, so it is a likely source of trouble. One of the reasons for making print cartridges is to make swapping the OPC easy.
• A transfer roller pulls the image from the print cartridge onto the page. In recent printers it is almost always a transfer roller because they give very little trouble compared to an older technique still found in copiers called a corotron. Rollers do get dirty and damaged; they are usually quite easy to clean or change. Colour printers have a transfer belt which is an electrostatic belt covering four or five transfer rollers; they can be more trouble
• The fuser in a laser printer binds the image in toner powder created by the OPC onto the page. The fuser uses a combination of heat and pressure to do this. Fusers wear out because the heater might fail and the constant pressure between the rollers causes fatigue fractures. The point of interest here is that the non-stick surface that heats the toner can lose some of its qualities and makes marks. The non-stick surface will tend to lose its properties with age.

Any of the print-process rollers could be to blame for regular marks. The visible rollers are most likely to be to blame because they are exposed. Lexmark printers have the pre-charge roller as a separate item on a hinged arm that lowers when the cartridge is inserted. On many printers the developer roller is part of the print cartridge and can't be seen. Lexmark have a toner feed roller in the cartridge to load toner onto the developer. Rollers that are inside the cartridge give a fault that will disappear when it is changed

Stop Test.

Quite often the OPC and fuser hot roller are quite difficult to see,they are pretty much the same size and there is no defects ruler to hand. The answer is to do a "stop test". Set a bit of printing like a configuration page away and wait a second or so until it must be in between the print station and the fuser. Opening the cartridge door and the safety interlock will stop the printer. Take the cartridge out so the page with loose toner can be seen. Now examine the page. Is the fault on the page before it gets to the fuser - if so the fuser can't be to blame.

A stop test also helps eliminate the transfer roller. Turn the print cartridge over and open the OPC shutter. About one quarter of the drum will have solid print on it so there is a 25% chance of catching the fault with one stop test. If the fault can be seen on the OPC then clearly it's happening inside the cartridge - not being caused by the transfer station. It might take several attempts to catch the fault in the quadrant that contains solid print.

Print Cartridge.

Take the print cartridge out of the machine and open the shutter. Look at (but do not yet touch) the photoconductive drum - they can usually be rotated by thumb pressure on the drive cog in the direction it would travel when printing. Don't touch the OPC surface itself as that will cause a mark and will degrade the material. Look for marks that correspond with that on the page.

The OPC material is usually green, blue or some hue in between. The material is not very robust. Scoring and abrasions will tend to give dark marks on the printed page because the drum surface can't carry the charge needed to repel toner.

The drum is exposed to the print media and whatever it might bring into the machine. There is a strong electrostatic attraction between the print media and the drum but normally material like paper is too stiff to bend up and touch the drum. Something like a sticky label coming part unstuck can leave an adhesive smear on the drum which will then become a persistent mark.

Transfer Roller.

In a mono printer the transfer roller is just that - a plain black or grey roller in the bottom of the printer just below the drum. The page passes over the roller, making contact with its top surface, The rollers job is to transfer a charge to the paper so as to pull as much as possible of the toner from the OPC drum onto the page.

Transfer Belt.

Colour printers have a more complicated transfer system because they use a succession of voltages. Transfer belts have a roller behind the belt for each colour. The belt protects the individual colour rollers - but at the cost that it is exposed itself. The quoted life is typically around 100,000 pages. When they are new transfer belts are nice and shiny and they should remain like that although they tend to get some light surface abrasion.

Transfer belts work a couple of different ways. In one the paper travels with the belt across the toner cartridges. The transfer roller picks up the paper using an attachment roller. Another technique prints the succession of colours on the belt then transfers the toner onto the page with a secondary transfer roller. One way the image is only exposed to the belt, the other it is exposed to both the belt and the roller

Roller or belt, either way the rule holds that the circumference of the roller (or belt) will be reflected in the problems with the image. With a belt the circumference could be about 2.5 to 3 times the page length so the problem with image might be on every second page or so.

Fuser.

Fusers normally have a top and a bottom roller. The pair of rollers nip the print media and put some pressure on it as it passes. The upper roller is heated to somewhere between 120°C and 240°C so that the toner becomes sticky and adheres to the page.

The hot roller softens the toner by contact. It is vital that the roller surface is a material that won't itself stick to the toner otherwise the image would partly stick to the roller, go round once then part-transfer to the page. The ghost printing caused by this is called offsetting and usually indicates that the fuser is too hot (or that the toner is the wrong type).

Fusers can suffer from heater failure and degeneration of the hot roller. Marks on the page are the faults of interest here. Once again an instant diagnostic is the defects ruler but a stop test is nearly as good. If the page goes into the fuser looking perfect but emerges with a series of marks, that is conclusive proof of where the fault occurs if not why.

There are two types of hot roller.

The traditional kind is a metal tube sufficiently strong to resist the loading springs on the lower roller. Down the center of the tube runs a halogen heater. The outer surface of the roller is coated in non-stick material. The problem with this kind of fuser is that the roller wall needs to be quite thick to resist bending under pressure from the lower roller. Thickness of the metal means it takes a minute to warm through. Either:

• The printer keeps the fuser hot so that impatient users get their pages quickly - this wastes electricity.
• or the fuser saves power but has a tendency to keep users waiting; in business this might waste time.

Foil-type fusers use a top roller that is just a layer of non-stick material made as a flexible tube. The tube wraps around a steel crosspiece on the base of which is a ceramic heater. The foil and heater element have very little thermal capacity so they come to temperature in a couple of seconds - power doesn't have to be kept running and the user experiences no noticeable delay.

The designs are very different but the failures are quite similar.

EdgeWear.

Pressure changes when a page goes through the fuser. Although 80gsm paper is only about 0.1mm thick there is extra pressure at the edges and this distorts the upper roller a bit. The paper edge is also sharp. As it handles many pages the constant abrading and pressure from paper wears out the edges of the hot roller causing grey margins called "edgewear".

There is a sensor for temperature and a cutout for overheating - these cause similar marks to edgewear but nearer the middle of the page.

Dirty Roller.

Fusers run hot, so things like sticky labels are rather inclined to come away from their backing sheets. A fuser surface contaminated by glue will pick up toner and bake the result leaving marks on the page.

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Removing Marks.

Sticky marks on the drum or fuser rollers can sometimes be removed by gentle rubbing with a cotton bud moistened with water, saliva or isopropyl alcohol. Do not rub hard on foil fusers - it will burst the foil. Don't use cleaning fluids on the OPC - any trace of ammonia will ruin it.

There are, of course, special fluids for cleaning OPCs. In practice they come in litre bottles and only people who refurbish cartridges for a living are ever likely to need a whole bottle. Experience suggests isopropyl alcohol is best and meths can probably be substituted. As every mother knows, warm spit on a tissue often works.