Troubleshooting HP Laser Printer Paper Paper Path Problems
Fully half of all customer complaints are about paper path problems. Moving paper is the most difficult, if not the most complicated, thing the average laser printer has to do. Compounding this problem is the mix of media available to the user envelopes.
Beyond the standard 8-1/2" x 11" paper, customers need to print on card stock, microfilm cards, transparencies-just about everything but napkins are specified out, and baseball caps. While many of the more exotic choices are specified out, printers are expected to perform acceptably for a wide variety of media.
In this article, we'll look at paper path problems true mechanical wear and failure problems (not bad sensors or error messages). While we will discuss the role sensors play in troubleshooting jams, most of what we will deal with here is simple mechanics. We're concerned here with real jams, not fake or phantom ones.
Moving paper through office machines is a task that manufacturers have been wrestling with for the last 40 years. Through that period, a lot has been learned about the mechanics of moving individual sheets of paper.
Surprisingly, the technology for doing so so remains largely unchanged from year to year. There are only so many ways to move paper. The problems remain the same, as well. There are essentially six problems involved in moving paper through a laser printer:
Separate and control a single sheet of paper.
Align the paper so that the image will be properly registered on the page.
Feed the paper through the machine at a constant speed, synchronized with the image system.
Prevent static charges from affecting the movement of the paper.
Use heat and pressure to melt the toner-cast image into the fibers of the paper.
Eject the paper from the machine in the desired fashion.
While all this is happening, the system must not deform the paper in any way and must protect it from vagrant debris in the paper path.
Laser Printer Paper Separation Characteristics
The first challenge is to acquire control of a single sheet of paper. The earliest machines accomplished this by having the operator insert single sheets of paper sequentially. Today's environment requires machines to feed paper at high speeds up to 30 sheets per minute. The difficulty this presents is that paper sheet tend to Cling together. Push on one piece, and you automatically put pressure on the paper beneath it. Getting one sheet of paper al a tme turns out to be a demanding technical problem. Separating the top sheet of paper from the pages beneath it is accomplished in one of three ways:
1. Pressure Between a D-Roller and a Separation Pad
The stacked paper is presented to the machine with no impediment on the leading edge. The paper is held to the top of the tray With firm pressure from below. When the D-shaped roller grasps the top sheet, sheets beneath are carried forward by a combined friction/static adhesion to the top sheet by the same pressure from the D-Roller. Lying in the path of the D-Roller's asymmetrical tum is a cork-covered Separation Pad.
The pressure between the Separation Pad and the leading edge of the D-Roller provides resistance against any paper being fed through, separating the top sheet from any sheets tagging along beneath it. The requirement is that the coefficient of friction must be greater between the Separation Pad and a sheet of paper, than exists between two sheets of paper. If the pad is worn smooth, or if a strong static bond exists between the individual sheets, multiple feeds are possible. If the D-Roller is worn, the pad can prevent any paper feed.
Separation Pad or Sep Pad. This serves to separate paper by having a higher coefficient of friction between the pad and a piece of paper than between any two pieces of paper. Eventually, it gets worn slick and the Coefficient of friction is reduced below the threshold for holding the underneath side of paper.
2. Pressure Between a D-Roller and an In-line Clip
The stacked paper is presented to the machine with an impeding corner clip. The impact of the D-Roller snaps one shect out of the clip and forces it into the feed.
3. Back Pressure From a Separation Roller
For higher-speed feeds, grouped rollers are used. Some, uses three Pickup/Separation Rollers to accomplish pickup, feed and separation. The first roller engages the paper, propelling it into the machine. The second roller, mounted below, back-feeds the paper under the object sheet, preventing multiple feeds, while the third roller continues the feed into the machine.
Printer Paper Separation Failures
Multiple feeds do sometimes occur, most commonly due to worn Separation Pads, for printers that have them. In the other two separation systems, multiple feeds are relatively uncommon.
Failure to feed at all is a more common problem. Jams at the Pickup Assembly çan usually be attributed to a worn Pickup Roller. In the Canon SX engine, the D-Roller acquires a crescent shape at its leading edge. The more pronounced the crescent shapc, the more likely the roller is to fail. Before a permanent failure, D-Rollers may fail intermittently, because they have accumulated dust and debris. On a service call for pickup jams, you would routinely change the Pickup Assembly, even though it might have considerable life left.
The duplex version of the many laser printers engine uses an identical roller on a similar axle for lower feed. However, this roller must work twice as hard in the printer. For every page fed, the Lower Roller must make two revolutions. Compounding this problem is that many firms use the Lower Tray for letterhead.
Since bond paper wears a roller down more than twice as quickly as short-fiber copy paper, it will cut the life span of the Lower Pickup to one-fourth that of the upper one, or about 25,000 prints. Worse, the cotton-rag bond papers sacrifice much more dust and debris during the process.
Some laser printers may have two other feed problems. The guide tab at the left forward top of the paper tray become bent down over time. When the tray is more than half filled with paper, this will introduce a skew to the right; this can result in a jam and, typically, interferes with proper registration.
The second problem is the Pickup Clutch. Many technicians replace Pickup Rollers without rebuilding the Clutch Assembly. Clutches can bind up with toner, allowing them to overcome the armature of their controlling solenoids. When this happens, the print is double-fed into the Registration, creating a jam at that point, occasionally with multiple feeds combined.
The most common failure with a printer Paper Feed Assembly is caused by poorly cleared jams from the Paper Cassette Tray, particularly labels. Some printers have the S-shaped paper path, labels fed from the Paper Cassette are highly likely to delaminate within the Paper Feed Assembly. The initial feed and turn from this cassette duplicates the action a user would take to delaminate labels manually.
The paper path from the Paper Cassette (and the optional Lower Cassette) is submerged within the Paper Feed Assembly, virtually inaccessible in the field, even after the assembly's removal. Only highly experienced technicians, operating under favorable working circumstances, should attempt to disassemble the Paper Feed Assembly, although partial disassembly of the upper guides will permit clearance of most simple jams.
Printer Paper Alignment and Registration
Once the paper is separated, it can be fed into the machine. Of critical importance is the position of the image on the paper. The machine's imaging system will beg to move toner toward the paper path as soon as it is told to by the DC Controller. It is important that the leading edge of the paper arrives above the Transfer Charge Mechanism before the toner-imaged OPC rotates into position above it. It is also important that the paper be fed in with the proper orientation so that lines of text or graphics are oriented properly to the edges of the paper. Printers use one of two methods to accomplish this:
1. Leading Edge Registration
The paper is fed into the machine and forced against a full-width Registration Assembly. The Registration Assembly has two rollers that will continue to feed the paper through the machine. The movement of the rollers is timed by the DC Controller and controlled with a Solenoid and a Clutch in the mechanism.
Prior to the release of the Clutch, the paper is forced against the junction of the two rollers and then some, forcing the paper to buckle and aligning the leading edge of the paper with the Registration Rollers. Once the paper is aligned, the Solenoid releases the Clutch, engaging the rollers and allowing the paper to continue its feed.
2. Side Edge Registration
Paper here is fed forward with an Oblique Roller that lines the paper up with the side edge of the printer feed path. Although the results are highly satisfactory, the Oblique Roller is subject to wear.
In some printer models, registration adjustments can be made to preposition the image on the paper. This is usually an electronic registration adjustment to space the first possible scanning line from the leading edge of the paper. Since this is a function of the imaging system, we mention it here in passing.
Laser Printer Registration Failures
Registration is a function of the Registration Assembly, mounted just forward of the Transfer Roller Assembly. The Clutch from the Registration Assembly ends with a ratchet gear to catch the armature of the Registration Solenoid. (The Registration and Pickup Solenoids are mounted on the Paper Control PCA, which, in turn, is mounted to the DC Power Supply.) Jams and failures in the Registration area of laser printers have several sources.
The most common problem is wear of the lower (rubber) roller. Over time, paper debris and OZone onspire to produce surface coating and oxidation that makes the roller slick and ineffective. Generally, a thorough cleaning with solvent will restore the assembly to complete usefulness.
Parts to a Laser Printer Registration Assembly.
Another printer problem is that the Registration Solenoid tends to stick. Their is a pad mounted on the Solenoid body that is impacted by the armature. This pad tends to disintegrate over time, resulting in this sticking condition. Replacing the pad corrects the problem and can be replaced just as easily.
The final problem is caused by the swivel arrangement of the top Forward Feed guide. Over time, these bushings rack and cause uneven separation between the two rollers. The result is uneven feed, poor registration and jams. The only effective repair is to replace the Registration Assembly.
When a gear fails, change the whole assembly, not just the offending gear. It just stands to reason that if one gear is worn out, the rest of them are worn and damaged. Gear assemblies are cheap, callbacks expensive.
Feeding Paper at a Constant Speed
For a new machine, constant feed speed is a simple matter of gearing. If you link all the rollers together with appropriate gearing, all the feeds will turn at the same speed. Unfortunately, as machines age, things can happen to interfere with feed speed, including:
Gear wear gets to the point where gears can slip or "bounce" and introduce small differences in speed.
Rollers that work together can separate over time, diminishing the pressure necessary for positive feed.
Rollers can become coated and/or oxidized, again causing pressure to lessen.
Newer machines can also experience #3 above. In addition, many of them employ more than one motor, the action of these motors must be synchronized because the gears they drive are separated and do not mesh. This is less a problem than it sounds. The DC Controller allocates power to the motors and, after they are powered, initiates their drives by activating Solenoids controlled by the Paper Control PCA. Because DC Controllers are extremely reliable, Solenoid timing is virtually always perfect.
Problems Maintaining Constant Feed
The vast majority of industry experience is with variable feed. Variable feed is a geriatrics problem, caused by excessive wear and deferred maintenance. Well maintained machines never suffer from the problem.
The most common problem is variant feed that occurs between the Fuser and Registration Assemblies---- we call this "push-me pull-you" failure. Generally the culprit is the Fuser. Toner dust buildup on the Pressure Roller causes it to slip, while the Registration is pushing from behind at a constant rate. The resulting failure produces no jam, but is diagnosed as an image problern.
Because the paper is moving through the Fuser at a slower rate than it emerges from the Registration, the excess paper in between buckles. This modest buckling causes the paper to actually brush the toner on the OPC as it transfers, causing it to smear and blur on a vertical axis. Characters are totally blurred, at worst, or modestly elongated up and, the down, at best.
Registration failure is less well understood. When the elongated text is very minor and appears at the bottom of the page, there is a good chance that the Registration is failing, and the paper is as to the causes here. I am uncertain of their validity, so I will simply comment that, in some instances, differential feed is a function of Registration Assembly failure, and that changing the Registration Assembly corrects the problem.
Inks used for textured or raised lettering" letterhead frequently are not thermally stable they often melt at temperatures below 420° F, becoming liquid inks once more. Since they cannot remain on the Heat Roller's Teflon surface, they transfer to the Pressure Roller, where they act like a colored glue, grabbing toner and paper debris. As the debris hardens on top of the inks, the Pressure Roller begins to slip over the paper.
One problem is that, over time, Delivery Assembly rollers harden and oxidize. The first symptom of this problem is a slight squealing or grinding sound that begins just as the paper starts to exit into the Face-down Tray. This persists and becomes progressively more pronounced, ultimately producing jams as the rollers have a more and more difficult time moving paper.
Static Charge Elimination
One of the interesting problems confronting every laser printer is static charge elimination. This is particularly troubling with regard to elimination of the positive static charge placed on the paper to allow it to attract toner. The problem this introduces is potentially catastrophic.
If the paper lacks sufficient rigidity, its positive charge could cause it to be attracted to any negatively charged mass in the paper path -- such as the OPC drum. The OPC is in close opposition to the paper as it is being charged. It is not inconceivable that the paper could then wrap itself around the Photodrum, effectively ending the process.
Problems With Static Charge Elimination
As mentioned earlier, there aren't many problems involving static charge elimination. Nevertheless, a potential jam is possible in this area. Paper that jams here usually is the result of a sticking Registration Solenoid. Ozone contamination dissolves the impact pad, making it sticky, causing the armature to stick to the solenoid.
Fusing the Toner to Paper
The Fusing Assembly represents the most vulnerable area in any electrostatic imaging machine, whether it is a copier, printer or fax machine. This assembly combines intense heat, movement and abrasive debris, sometimes mingled with lubricants a mix guaranteed to create problems.
Paper enters the Fuser, generally striking a guide plate that leads it to the juncture of two rollers. The roller usually mounted on top is a Heat Roller a Teflon-coated hollow tube, either aluminum or film sleeve.
Through the hollow shaft of the tube runs a high-intensity heat source. Older fusers used light bulbs or Lamps. Newer fusing technology utilizes a ceramic heating element located as part of the fixing film assembly. This heat source is is alternately turned off and on to maintain a temperature approaching 400 F. At this temperature, toner melts.
Melting the toner is only half the job. The melted toner must be permanently affixed to the pager This is accomplished by the second roller-the Pressure Roller. Its function is to press the molten toner into the fibers of the paper, annealing the two dissimilar materials to form a lasting image. The lower Pressure Roller historically was made of a spongy silicon rubber, while newer ones are sleeved with a Tellon like coating that eliminates the need for Fuser oil.
Laser Printer Fuser Temperature Control
Fusens have two electrical circuits an AC circuit for power, and a DC circuit for control The heat source inside the Heat Roller / FFA is lit from AC power. In Canon / HP engines, there's a safety feature, called a Thermoswitch or Thermoprotector, that is wired in series with the Lamp in the AC circuit.
If the temperature exceeds a certain point, a metal lead within the Thermoswitch expands, pushing a pin that opens the circuit. Older IBM and Lexmark engines, a small in-line Thermal Fuse replaces the Thermoswitches. Unlike Themoswitches, fuses cannot be reset.
The DC circuitry is wired separately and serves to communicate information to the DC controller. Generally, a Thermistor is mounted next to the Upper Roller in order to measure the roller's temperature and communicate this information to the controller.
The Themistor is a resistor, its resistance varies predictably and proportionately according to temperature changes. Beyond that, the DC circuitry generally includes a Photosensor for jam indications at the Fuser.
When the temperature readings get too high, the Controller signals the Triac in fie Power Supply to shut down power to the heat source. When the temperature drops to low the reverse happens and the heat source is powered on. Because the Fuser may the continuously supplied with cold imaged paper, the on-off cycle will be repeated sevel times. As printers have become faster, the film sleeve materials have changed dramatically.
Solving Laser Printer Fuser Problems
The Fuser is the biggest contributor to service problems, creating both image and paper-path errors. Feed problems at the Fuser generally result from gear failure. This gear mates with other gears like the large Drum Gear, the Exit Arm Gear and the gear that drives the Coupler Assembly at the Delivery Assembly.
Some of these gears are very small and are not truly inline with any of the other three gears that it meshes with, hence it receives differential torque from three sources. It i not uncommon for this gear to "freeze" to its axle, then split under pressure. We suggest you routinely replace gears whenever the Fuser is out of the machine for any reason.
When a Gear fails, paper may not feed through the Face-down Tray, though sometimes it will feed through the Face-up Tray for a period of time. The reason for this is that the gear wears differently, based upon the way the surrounding gears bear on it. The classic symptom of this failure is accordion folding of the paper as it exits the Fuser, or, in the worst case, while still contained within the Fuser's exit assembly. This failure is sometimes silent, seldom creating gear noises beyond a very light rattle.
The other common gear failure is the Forward Arm Gear. This is the part that engages with the Main Drive when the printer is closed and operational, and disengages when the printer is open. The engage/disengage feature allows a jam within the Fuser to be removed without engaging and turning the drive for the entire machine.
When the Forward Arm Gear fails, paper stops right at the entry to the Fuser, sometimes barely embedded between the two rollers following force from the Registration Assembly. Accompanying this failure is gear noise, as the Main Drive Lead Gear is in contact with the core of the Arm Gear because of the spring pressure that engages it.
While gears represent 70 percent of all Fuser jams in fuser assemblies, there are three more things that can cause them. A contaminated Lower Roller can create the "push-me, pull-you" condition that, if left untreated, can lead to jams. Typically the paper feeds through the Fuser and is beginning to exit the other side when the machine shuts down. Diagnosis of this failure is made difficult, because it resembles another degenerative problem caused by cable failure. The cable connecting the Fuser to the DC Controller is subject to failure at the female connector at the board end. Historically, techs may have been taught to "tweak" these connectors
Finally the some Fuser's contain Exit Roller that may cause jams. The Exit Roller is a shalt running the full length at the back of the Fuser. On that shaft are several small rubber rollers about 3/4" long and about 3/8" in diameter. At one end of each roller is a hollow end with nipples protruding from it. It is the task of these nipples to push the paper through the Face-up Tray at the rear of the machine. Over time, the nipples wear down, then off.
New laser printer engines that followed the 4250 have been designed with more durable Fuser Assemblies. New models have nearly eliminated the problem of Upper Roller abrasion by minimizing and relocating contact surfaces.
Whereas the Thermoprotector and Thermistor have been moved out of the paper path. They abrade the roller, it doesn't matter. Since it's out of the paper path, the abraded section of the roller doesn't have to melt toner
One common jamming problem with the newer engines' Fusers. Paper and transparencies (especially) can wrap themselves around the Upper Roller or film sleeve. Usually, the Fusers can be salvaged without damage if the wrapped media is carefully removed, but it is generally a good idea to replace the customer's Fuser with one of your own.
Is there any rule about what sort of paper should be used in which tray in an HP printers?
The short answer is "no." But there are some commonsense possibilities. The Lower Roller makes two rotations to do the job that the Upper Roller does same in one. Consequently, the Lower Roller should propel the paper least used. If bond paper is used over half as much as copy paper, the bond should be in the upper tray, so as not to prematurely age the Lower Roller.
Ejecting the Paper From the Laser Printer
Once the image has been fixed to the media, the last task is ejecting the media from the printer. Almost always, power for this step comes from the gear chain running through the Fuser. If there is a Fuser gear problem, it may demonstrate itself simply simple task, as a failure of the exit system. While this sounds like it would be a truth, jams in exit systems occur for a variety of reasons.
Laser Printer Paper Exit Problems
Drive continuity is accomplished by the series of gear assemblies. Over its life, there's a good chance that the gears will experience serious deterioration from ozone alone. You can see this in older machines, where the once shiny, black gears have been bleached nearly white by ozone. The contamination also causes them to dissolve and become brittle.
Of course, while the gears are being degraded, they are also wearing down. Unfortunately, when they fail, they create a symptom similar to a tooth Gear failure an accordion fold in the paper just outside the Fuser. Unlike a Tooth Gear failure, however, this Coupler Assembly failure can't cause paper to be trapped within the Fuser.
In addition, paper will always feed out the back into the Face-up Tray if Coupler Assembly failure is the proximal cause. There are two other problems that can result in delivery failure. If the Upper Rollers within the Delivery Assembly are hard and slick, they can get to the point where they will fail to feed paper.
The most obscure cause of jams in this area is Diverter problems. The Diverter behind the Fuser is movable, and actuated by opening or closing the rear door. On older machines, it is not uncommon for body parts in this area to sacrifice their gnarled bushings that hold attaching machine screws, causing the parts to slip loose. This can cause Diverter failure and jams. The Diverter itself is spring loaded and has been known to fail at the spring clip.
Duplexing or 2 Sided Printing in Laser Printers
While I didn't list it as one of the six steps, duplexing is an important part of the work cycle of some printers. Duplexing is the ability to automatically print on both sides of the paper, a feature critical for some applications. While duplexing is relatively simple, problems can occur.
Duplexing With the HP Printers
The first requirement of duplexing is a reliable paper path that will, in effect, flip the paper over without causing problems. In some HP printers the duplexers were built into these printers, rather than being add-on options, and are extremely reliable. The majority of duplex failures are caused by improper handling of the printer, and most duplex jams result from interrupting a job.
Paper feeds from either tray through the Registration and out the Fuser. Instead of exiting to the Face-down Tray or the Face-up Tray, the paper is carried back by the Switchback Assembly. It pulls the paper clear of the machine, then feeds i through the Duplex Rollers, image side up.
At the end of that path is a Diverter that rolls the paper straight up to the Registration Assembly again. The Registration completes the turn, leaving the previously imaged side down, feeding the paper toward the Fuser and the second imaging. Because the paper is turned end-to-end, the first image is printed upside down. At the completion of the second imaging, the paper is exited into the Face-down Tray. If the out-tray selection knob on the Switchback Assembly is turned to select the Face-up Tray, attempting to duplex will trigger an error message.
If a client calls saying, "I'm getting a jams, but there's no paper in the machine," nine times out of ten there will be paper stuck in the Duplexer. This is almost never a mechanical jam. It may be that a mechanical jam in some other area of the machine has interrupted printing; the customer may have overlooked it when clearing. Or, the customer prematurely stopped the print cycle.
Since the machine prints the back side (page two) first, the paper can stop in the Duplexer, confusing the machine. In either case, the problem can be solved by opening the side access door, removing the captured media. Other problems can occur with these systems, but they're so uncommon they aren't worth noting here.
Some routine maintenance is appropriate for the Switchback Assembly at the rear of the printer. When problems occur while duplexing, they are frequently caused by dirty rollers in the assembly, which has to nearly eject the paper, stop, then reverse the paper through the Duplexer path. As you perform preventative maintenance on these duplexing printers, flip the trays open from the top to expose and clean the rubber rollers below.
Because the HP printer's operate at 50 - 80 PPM page per minute speeds, small problems can have huge consequences. Use labels cautiously. A little stray adhesive can stop paper cold in the Duplexer.
The next common trouble area is at the Flapper," the assembly that controls whether paper travels up to the exit trays, or down to the Duplexer. The Solenoid the controlling the Flapper must engage with the first print of each duplexed page. Over time, this fatigues the Flapper, causing it to fail.
IN CLOSING Paper path problems may account for more HP printer service calls than all other causes combined. It is inevitable that every single machine ever made will at some point n its life trigger a paper jam. With that in mind, spend some time with your own LaserJet printers.
Listen to the tempo, the rhythm of the machine as established by the motors and the solenoids. Frequently, jams will advertise themselves with an extraordinary noise.
Take the printer covers off or do whatever else is necessary to get a good look at the paper as it moves through the machine. Time spent learning the mechanics of paper handling will prove to be a good investment in the future.
Other Helpful HP LaserJet Printer Support Resources
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