How does an old, aged, damaged, dirty, contaminated or bad PCR affect your print/copy quality and machine. January 2020 Week 4

Probably one of the areas in a digital copier or laser printer that is neglected the most in relation to the importance of the device  is the overall condition and operation  of the Primary Charge Roller, so lets look at one does the PCR in the machine when it is not well looked after, or what are the signs and causes of a PCR wearing out.

Lets first in a nutshell look at the role or functioning of the PCR. The job of the PCR is to provide and even electrical charge to the outer coating of the OPC (Organic Photo Conductor) Drum, and this electrical charge is then discharged when it comes into contact with the laser beam that shines on it, and produces and electrical or latent image in the shape/s of the image one is printing, the toner is then repelled by the charged areas of the OPC Drum, and this is the white areas of print on the page, and the toner adheres to the discharged areas, forming a visible image, be it on a Black & White/Monochrome print or a colour print. The PCR also discharges the OPC Drum with an AC (Alternating Current), just before it recharges the drum, and this discharge ensures that there is no residual electrical charge on the image areas of the drum that were just printed.

Firstly as most PCRs have a rubber coating as the main component that transfers the electrical charge to the OPC drum, one needs to be sure that the PCR one is purchasing and installing in your machines you service is manufactured using high grade rubber. At CET we use only the highest grade of rubber raw materials from Japan, and the PCRs are manufactured under the strictest environmental and Quality Controls, as small things such as the curing temperatures and there must be zero humidity are critical to ensuring there is even curing of the rubber when the PCRs are molded. The accuracy of the molding with the metals PCR shaft to be exactly in the centre so as to ensure the PCR runs 100% smoothly against the OPC Drum. If not, then there will be an uneven charge delivered to the drum and one will get an uneven print on the page. The PCR will also more than likely damage the drum well before the actual life of the OPC drum too, if not well manufactured.

As there is an electrical charge delivered through the PCR to the OPC Drum, the rubber used to manufacture the PCR is impregnated with carbon, allowing it to conduct electricity from the centre shaft, through the rubber to the OPC Drum surface. Every model of digital copier/laser printer is designed with a specific voltage that is applied to the PCR in order to achieve the desired charge on the OPC Drum surface, so the electrical resistance of the PCR rubber is critical, as if the resistance of the PCR is too high the charge delivered to the OPC Drum will be too low, and one will get background on the prints, and if the resistance is too low, too much voltage will be applied to the OPC Drum and the prints will be light, and one may even get a short circuit, where sparks are produced between the PCR shaft and the OPC Drum, severely damaging the OPC Drum surface.

What actually happens to the PCR rubber as it gets older? As the rubber PCRs as used in most makes of copier and laser printer are the rubber PCT that runs on the surface of the drum, there is spring pressure pressing the PCR against the OPC Drum so the rubber on the PCR compacts or compresses slightly all the time, so with time on every rotation the PCR compacts slightly and with time will reduce in diameter. Then, as there is a relatively high electrical charge of between 500VDC and 1500VDC depending on the machine, this electrical charge causes the rubber to perish, decay, degrade, or degenerate, whatever one may refer to the degradation of the rubber, so the overall strength and conductivity of the PCR will reduce over time.

The PCR has a far smaller diameter than the OPC Drum, so rotates many more times per rotation of the OPC Drum too. As the PCR rubber degenerates, it tends to conduct less electricity, causing the prints to start producing background print, and technicians often look for other causes such as blaming the toner, or developer if there is developer in the machine. As the PCR runs against the OPC drum, and the only drive that causes the PCR to rotate is the friction against the OPC drum, so on long print runs, the PCR will also heat up from the friction between the OPC drum and the PCR.

The background on the prints can be rectified by cleaning or replacing the PCR, but worse still if the PCR rubber has severely decayed, gaps or splits in the rubber of the PCR, then arcing may occur between the PCR and the OPC drum, this is when sparks form from short circuit between the PCR shaft and the OPC drum due to there being no insulation or resistance between the PCR shaft and the OPC drum, and this will go through the OPC layer on the drum, damaging the drum permanently.

Lets then look at a few points here that one should take into consideration regarding the PCR in the digital copier/laser printer, and that is that the PCR should always be kept as clean possible to ensure optimum performance. Once the life of the PCR is over, one should replace the PCR with only a quality PCR, such as those manufactured by CET. This will maintain the print quality plus will extend the OPC drum life in most cases too.

Importance of using a good PCR and maintaining PCR well for good print quality

January 2020 - Week 3

A- Is the outer carbon impregnated polyurethane and rubber roller that makes up the main part of the PCR. The outer coating, the hardness/softness and texture of the roller, then the electrical resistance to be as close to 6Ohms possible, are all factors that will affect the output contact voltage that the PCR delivers to the OPC Drum. If the PCR roller is too hard it will damage the OPC  Drum, and if too soft the contact area becomes too large and roller gets contaminated. As the outer of the PCR makes contact with the OPC Drum, it will always get residual toner and other compounds from the toner that will build up on the outer PCR, so keeping the outside of the PCR clean is critical. The (A) rubber roller fo the PCR also forms a shield between the metal roller that carries the main electrical charge and the OPC Drum, so if the outer rubber layer of the PCR becomes damaged at any time there is a good chance that there will be an electric arc or spark from the metal roller to the OPC drum, damaging the drum, and if the drum makes use of an Amorphous Silicon Drum then this can be very costly when it could have been prevented. So choosing a good PCR is always a wise choice.

B- Is the metal inner shaft or the PCR, and it is not only the inner shaft, but it also carries the main electrical charge from machine, that then is carried to the drum through the outer rubber roller. This electrical charge is generally delivered to the shaft via a small copper strip, and it is important that the contact area of the copper strip is clean where it contacts the metal inner shaft, so the maximum voltage is always transferred to the PCR, as if the copper strip contact point is dirty, or if it is damaged, it will not be able to provide the correct voltage to the PCR, resulting in bad copies. One can also apply a conductive grease to the contact point of the copper strip where it contacts the PCR. One must also make sure the PCR shaft is clean in the bushes it runs in so it rotates freely, otherwise one gets an irregular rotation, and jitter marks on the images.

C-  The contact between the PCR and the drum needs to be equal across the PCR and drum, and also the PCR must run straight along the length of the drum, otherwise one will get uneven copies due to irregular application of electrical charge from the PCR to the drum.

D-                       The copper metal strip the carries the electrical charge to the PCR is often neglected and bent or broken during installation of the PCR. The constant pressure, and clean contact points on this strip are critical to good crisp copies at all times.

E-  Applying of equal and constant pressure of the PCR to the drum is also crucial to having the PCR not only function properly, but if too little pressure, the PCR will not deliver enough electrical charge, and if too much pressure there is a good chance the PCR and/or the drum become damaged. Preferably do not change the pressure on the springs that apply this pressure.

F-   Drum, be it an OPC Drum or an Amorphous Silicon Drum, the condition of the drum is critical to making good prints and should be clean at all times, and outer coating should not be damaged in anyway.

G-                      Application of a Bias voltage and/or Earthing/Grounding of the drum is also critical to maintaining good print quality. Many drums have a copper strip inside the drum that makes contact with the drum shaft, and it is also important that the contact strip stays clean and in good condition to allow it to let the drum maintain an electrical voltage and then release the voltage in the areas that the laser bean strikes the drum, and this voltage then needs to be released to through the Ground/Earth of the drum. If the drum is not earthed one will get irregular print quality.

Colour copiers and printers have different ways of regulating the toner density and ratio in the developer. Some use an ATR, Automatic Toner Replenishment sensor, to measure the toner in the carrier/developer

On other colour copiers/laser printers, there is a colour patch processed on the drum and then onto the transfer belt and this patch is then read by the machine to determine the correct ratio of toner in the developer in the imaging unit, and with anything wrong in the PCR and charge area of the copier/printer, a machine may be over toning or under toning and one will look for the problem been caused by the toner or the imaging unit, when it can be caused by a simple dirty contact, dirty PCR, or any of the above.

What different kinds of PCR do we get.

Week 2 January 2020

All digital copiers are designed differently, they have different OPC drums, different toners, different laser units and print at different speeds, so the charging of the OPC drum in each machine is different and therefore different brands of machine use different types of PCR (Primary Charge Roller). Also then comes down to the cost of the units, and low cost toner cartridges that have the OPC drum and PCR in the toner cartridge such as in many monochrome machines, the PCR will be of the lowest cost, so simplest and easiest manufacturing methods.

One finds 3 basic types of PCR,

1. Dense Carbon Impregnated Rubber - most commonly used

As you can see from the picture, this is a hard rubber, that has a certain percentage of carbon in it, allowing it to have certain conductive properties, and most commonly used in smaller laser printer engines, or laser printer cartridges. It is the easiest and cheapest version of PCR to manufacture. This PCR runs on the surface of the OPC drum, at slow speed, and low pressure.

2.  Solid PCR with Nylon Bushes at each end of the PCR

The PCR with Nylon Bushes at each end is difficult to produce for aftermarket manufacturers to the same level or performance of the OEM due to the extremely accurate tolerances in production. The Nylon Bushes run on the edge of the OPC drum where there is no OPC coating, and are generally about 0.1mm larger than the charge portion of the roller, and imagine, just 0.1mm off the surface of the OPC drum, so manufacture needs to be extremely accurate. These PCRs also have a cleaning system to keep the roller surface from becoming contaminated with toner or developer. Most commonly used in Ricoh machines.

3. Foam Core with Solid Rubber Sleeve PCR 

As with the Nylon Bush PCR, this PCR requires a few steps in production to manufacture these PCRs. One needs to mold the foam core first, and this core needs to be a certain sponge property to it, and is able to compress a bit. Then only once the core is cured, can one mold the outer solid rubber sleeve. This PCR runs on the OPC Drum, and being softer than the solid rubber core PCR, this PCR is used more in the larger Canon copiers.

So important factors in designing and manufacturing PCRs needs to be the accuracy of the machining of the  PCR shaft. Then the resistance or conductivity of the rubber needs to be good and consistent, and maintain the same conductivity through the life of the PCR. The rubber also needs to be of a high quality in larger machines as a PCR is subjected to high voltages causing a deterioration in the rubber, and once the PCR rubber wears down or cracks, arcing of the high charge occurs between the PCR shaft and the OPC drum, severely damaging the OPC drum. Then the accuracy and hardness of the rubber portion of the PCR is critical, as they run either on the OPC drum or 0.1mm off the OPC drum, so if the PCR does not run smoothly or even along the surface of the OPC drum, it will cause irregular or bad printing.

Next week - How and why to maintain the PCR well, and the benefits.