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Electrophotography

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Electrophotography (also known as xerography) is a complex process commonly used in copiers and faxes, as well as in digital printers. It is an imaging technology that takes a digital file and utilizes a photoreceptor, light source, electrostatic principles, and toner to produce the printed output. Before Electrophotography was used for digital printing, it was extensively used in analog copiers where a lamp illuminated the page being copied, and then a series of mirrors reflected the page directly onto the surface of a drum. Digital copiers replaced the direct light path with a sensor that converts the analog image into digital information, then a laser or an LED array writes the image onto the drum. Many digital printers today are based on the same platform as digital copiers. The technology has seen many improvements over the years, but the electrophotographic process at its core remains relatively unchanged. The photoreceptor is commonly referred to as a drum. It is a cylinder coated with a material that becomes conductive when exposed to light. Areas that are not exposed have a high resistance, which allows these areas to hold the electrostatic charge necessary for the process. Light sources used in digital printing include LED arrays or, more commonly, lasers. VCSEL (vertical cavity surface emitting laser) is an advanced type of laser used in the most current digital presses in the market. A VCSEL array can position its beam with high accuracy (addressability) for optimal clarity, resolution, and image positioning. This makes it ideally suited for a digital press. To understand electrophotography, we must first understand some basic electrostatic principles. When certain materials come in contact then separate from each other, these materials can become electrically charged. Rubbing these materials together can increase this effect. This is called the triboelectric effect. Static electricity buildup on your clothes in a dryer or from rubbing a balloon on your hair are examples of the triboelectric effect. Charges can have either a positive or negative polarity (see image). Like charges repel each other while opposite charges are attracted, in much the same way as the polarities in magnets. These properties are at the core of the technology and are utilized in almost every stage of the digital imaging process. Toner is a very fine, dry powder medium used in the electrophotographic or xerographic process. It is composed primarily of a resin and includes pigment, wax, and process-enhancing additives. The term xerography, in fact, is derived from the Greek words xeros, ‘dry’ and graphia, ‘writing,’ reflecting how toner rather than ink is used in the imaging process. Toner particles become electrically charged when stirred or agitated through a triboelectric effect. The composition of the toner not only contributes to its imaging characteristics but to its ability to maintain and control its charge properties. The shape of the toner also is a factor in its charging capability. This electrical charge is what allows the toner to be precisely manipulated throughout the process. There are two basic types of toner production, pulverized and chemical. Pulverized toner was commonly used in earlier digital printers and is manufactured by successive compound mixing and grinding steps, until the desired consistency and size is achieved. The resulting toner particles are irregular in size and shape and typically average around 6.2 to 10.2 microns in size. Pulverized toner produces good results, up to 600 dpi resolution; however, a consistent size and shape along with a smaller particle size is required to produce better clarity and detail at higher resolutions. Chemical toners were introduced later to overcome those limitations and are in common use today. Each manufacturer has its own process for creating this type of toner and unique names as well. Xerox’s EA toner, Ricoh’s PxP toner, and Konica Minolta’s Simitri toner are all examples of chemical toners. As the name suggests, chemical toners are created through a process of building or ‘growing’ the particle chemically. This process allows for the precise control of the shape and size of the toner particle (under 5 microns in some cases), resulting in higher definition and resolution capabilities. Resolutions of 1,200 dpi and 2,400 dpi are possible largely due to the use of this type of toner. Other benefits include much lower energy consumption, both in the manufacturing process and printing process, as well as narrower particle size and charge distributions. A video of how chemical toner is made, is available to view here: https://youtu.be/852TWDP61T4. Dry toner comes in two forms: mono component and dual component. Both rely on magnetic iron or iron oxide particles to ‘hold’ the charged toner on a magnetic roller. Mono component toners incorporate the magnetic material in the composition of the toner particle itself where dual component toners have the magnetic material mixed together with the toner but as separate components. This mixture is called developer. ElectroInk is a unique form of toner used in HP Indigo digital presses. The toner comes in the form of a paste and is mixed internally in the press with imaging oil, a lightweight petroleum distillate. This type of toner is considered a liquid toner as the particles are suspended in the liquid imaging oil, but still uses an electrophotographic process for imaging. One of the important advantages of this type of toner is its particle size. ElectroInk toner particles are 1 to 2 microns, significantly smaller than the smallest dry toner particle. At this size, a dry toner would become airborne and would be very difficult to control. The toner and oil suspension achieves higher resolutions, uniform gloss, sharp image edges, and very thin image layers. A thin image layer allows the toner to conform to the surface of the substrate, producing a consistent look between imaged and non- imaged areas. A drawback of this toner, however, is that substrates may need to be pre-treated in order for the toner to adhere properly. There are substrates available for use specifically on HP Indigo digital presses, but typically these are more expensive or may not be compatible with other printing methods. Some Indigo presses are equipped with a pre-treating station that expands substrate compatibility extensively and even surpasses that of other forms of digital printing. Nanography is a very new and exciting print technology currently in development by the creator of the Indigo digital press, Benny Landa. It borrows some of the same concepts used in the Indigo but with a different approach to the implementation of these. The technology centres around NanoInk, a breakthrough ink with pigment sizes in the tens of nanometers. In comparison, pigments found in good-quality offset inks are in the 500 nanometre range. Colorants intensify and ink density increases at this microscopic level, thereby expanding the ink’s colour gamut considerably. The ink uses water as a carrier instead of imaging oil making it more cost effective and eco-friendly. Billions of ink droplets are jetted onto a heated blanket, not directly onto the substrate as in inkjet printing. The ink spreads uniformly on the blanket and the water quickly evaporates leaving only an ultra-thin (approximately 500 nanometres), dry polymeric film. This film transfers completely onto the substrate on contact and produces a tough, abrasion-resistant image. This print technology can be used with almost any substrate without pre-treatment and, due to its minuscule film thickness, does not interfere with the finish. Whether high gloss or matte, the ink finish matches that of the substrate. Although the technology is poised to revolutionize the print industry, the first press to use it is currently in beta testing. You can find the latest news and more information on nanography on this webpage: http://www.landanano.com/nanography. The electrophotographic process consists of seven stages. For the purpose of this topic, we will be describing the process using a negatively charged dry toner. The process is the same for a positive toner except the polarity would be reversed in each stage. In the first stage, a high negative voltage of approximately -900 volts is provided to a charge roller (see image). The voltage used varies by manufacturer and model. The charge roller applies a uniform layer of negative charge to the surface of the drum. The resistivity of the unexposed photosensitive drum coating allows the charge to remain on the surface. A laser is used to write the image onto the charged surface (see image). Because the photosensitive coating on the drum becomes conductive when exposed to light, the charges on the surface of the drum exposed to the laser conduct to the base layer, which is connected to a ground. The result is a near zero volt image and a negative background. This is known as the latent image. Many digital printers and presses use a dual component development system (see image). The developer is a mixture of non-magnetic toner and a magnetic carrier. As the developer is stirred and the particles rub up against each other, a triboelectric charge is generated between them. The toner becomes negatively charged while the carrier becomes positive. The opposite charges cause the toner to be attracted to the carrier. A magnetic development roller holds the mostly iron carrier in alignment with magnetic lines of force forming a magnetic brush. This magnetic brush in turn ‘carries’ the attracted toner to the surface of the drum. A high negative bias is applied to the development roller repelling the toner onto the drum. The toner is attracted to the areas of the drum exposed by the laser, which, being close to zero volts, is much more positive than the negatively charged toner. In this way, the latent image is developed. As the carrier remains on the development roller, it continues to attract toner from the hopper to maintain the optimal concentration on the magnetic brush. A sheet of paper or substrate passes between the drum and a transfer charge roller that has a high positive voltage applied to it (see image). The negatively charged toner of the developed latent image on the drum is attracted to the more positive transfer roller and adheres to the sheet in-between. The charge applied to the back of the sheet causes the paper to cling to the drum. A high negative voltage is applied to a discharge plate immediately after the transfer charge roller, to aid in the separation of the sheet from the drum. The curvature of the drum along with the weight and rigidity of the sheet also aid in the separation. A more advanced method of transfer utilizes an intermediate transfer belt system. This is most common on colour digital presses where four or more colours are transferred onto the belt before transferring the complete image onto the sheet. Charge rollers beneath the belt, under each drum, pull off the developed latent images of each separation directly onto the belt. In the transfer stage, a transfer charge roller beneath the belt applies a negative charge to push the toner onto the sheet. A second roller, directly beneath the first on the other side of the belt, applies pressure keeping the paper in contact with the belt and aiding in transfer for more textured stocks. The lower roller may have a small positive charge applied to it or may be grounded. Some systems can also alternate the charge applied to the transfer charge roller, to further aid toner application onto textured substrates. After this stage, the sheet moves on to fusing where the toner permanently adheres to the substrate. The next two stages described below are post-imaging steps that are necessary to prepare the drum surface for the next print cycle. After the transfer stage, some toner may be left behind on the surface of the drum. If left there, the background of each successive print would slowly become darker and dirtier. To prevent this, a cleaning blade removes any residual toner from the drum’s surface (see image). Some systems will recycle this toner back to the developing unit, but mostly the waste toner is collected in a container for disposal. In the Erasing stage, an LED array exposes the length of the drum, bringing this area of the drum to near zero volts. This prepares the drum surface for the charging stage of the next print cycle. Fusing is the final stage in the electophotographic process. The fusing mechanism, or fuser, consists of a heat roller, a pressure roller, and cleaning mechanism (see image). Toner is composed mostly of resin. When the toner is heated by the heat roller and pressure applied by the complement pressure roller, it melts and is pressed into the fibres of the sheet. The toner is never absorbed by the paper or substrate but rather is bonded to the surface. A negative charge is applied to the heat roller or belt to prevent the toner from being attracted to it and the cleaning section removes any toner or other contaminates that may have remained on the heat roller. Heat may also be applied to the pressure roller (at a much lower temperature) to prevent the sheet from curling. Along with the transfer stage, fusing can be greatly affected by the paper or substrate used. The thicker and heavier the sheet, the more heat it absorbs. Because of this, these sheets require higher temperatures so there is sufficient heat remaining to melt the toner. Insufficient heat can cause the toner to scratch off easily or not bond at all. Too much heat can cause moisture in the substrate to evaporate quickly and get trapped beneath the toner causing tiny bubbles that prevent the toner from sticking wherever they occur. This issue is seen more on thinner stocks that do not absorb as much heat. Too much heat can also cause toner residue to stick to the heater roller and deposit it on subsequent sheets. The heat roller can heat up quite quickly but may take much longer to cool down. This can cause delays in producing work that switches between different paper weights. To combat this, some devices use a thin belt that can be both heated and cooled quickly in place of the heater roller. In some cases, a cooling mechanism is also employed further mitigating the cooling lag.