Digital Technology Review

Transcription

1 Digital Technology Review Prepared By Digital Printing Systems Digital Printing Systems 300 Blaisdell Road Suite One Orangeburg, NY Tel: (845) Fax: (845)

2 2 History In 1977 a Canon researcher inadvertently touched an ink-filled syringe with a hot soldering iron. Ink sprayed out of the syringe after the heat caused an air bubble to form. This was the beginning of today s inkjet technology so the story goes. It wasn t until the early 1990s that inkjet technology reached the consumer on any substantial scale. Canon s accidental discovery led to a flurry of research, which culminated in the Bubble-Jet technology and eventually the Bubble-Jet Printers. Canon still uses that term for its printers, while all other manufacturers use the term ink jet. In 1992 Canon introduced its first laser-like inkjet printer. The price was almost $1,000 for a black & white printer with print resolution of 360 dots per inch (dpi). Piezoelectric inkjet printers soon followed, with Epson rolling out its first monochrome inkjet in 1993 (360 dpi) and the first color inkjet in 1994 (720 dpi). Over the past 10 years, several forward thinking companies began to adopt core head technologies (thermal & piezo) from the desktop market and integrate them into wide-format (greater than 36 inches) printers. These printers focused on the signage, billboard, and fleet graphics industries. These machines, which can vary from a narrow 36 inches to the super wide 16 feet Vutek printers, carry up to 50 different trade names. Most experts recognize that inkjet technology will have a profound affect on all printing industries. The demand for personalization, zero inventory, and print-ondemand capabilities is driving the technology forward at a rapid rate. The next few pages will review the most prevalent types of digital technology.

3 3 Thermal Inkjet Description Thermal inkjet printers use heat to produce a drop. After the controller sends a signal to the print head, a heating element in the print head activates and quickly warms the ink to a boil. As the ink continues to heat, a bubble forms in the ink chamber and grows until it begins to push ink out of the nozzle. After the ink drop is ejected, the bubble pops and the nozzle is already preparing for the next cycle. This heating and cooling cycle happens up to 10,000 times per second (10 KHz), depending on the print head technology. Heating the ink chamber has several negative impacts on thermal technology. Thermal heads are not well suited for high-speed applications since the ink chamber must cool down before it can fire again. Even though it takes only a fraction of a second, each delay adds up. The constant heating of the ink chamber also causes kogation; defined as the crystallization of ink on the nozzle orifice. Kogation eventually leads to nozzle clogging and head failure. This is not a problem with most applications, because each new thermal ink cartridge replaces the print head. Hewlett-Packard is the largest supplier of thermal heads in the world, followed by Canon and Lexmark. Ink Chamber Operating Schematic Nozzle Signal Heat Generating Resistor Ink Substrate

4 4 Piezoelectric Inkjet Description Piezoelectric inkjet printers use piezo crystals to control the ejection of ink from the nozzle. Piezo crystals change shape when an electrical current passes through them. In a piezo print head, the crystal is often placed inside or immediately adjacent to the ink chamber. As the crystal changes shape, it squeezes ink out of the nozzle. Today s piezo head can eject individual droplets of ink up to a rate of 25,000 times per second (25 KHz). There are several reasons why piezo technology is becoming the technology of choice for industrial applications. First, a piezo head can eject drops at a much greater frequency than a thermal head - enabling greater printing speeds. Second, there is no heat related stress, which equates to a much greater head life span. In the inkjet industry, lengthy head life is known as being robust. Another key factor is their ability to use a wide-variety of ink chemistry including solvent, water, and UV formulations. Epson is by far the largest producer of piezo print heads. However, smaller companies such as Spectra, Xaar, Trident, and Aprion Digital (DPS 65) have developed their own variations of the piezo technology and are more focused on the industrial sector than Epson. Operating Schematic Signal In Piezoelectric Transducer Nozzle Substrate Ink

5 5 Binary Continuous Inkjet Description Continuous inkjet (CIJ), like piezo, uses the deflection of a crystal to help eject ink for the nozzles. However, this is where the similarity ends. Piezo heads eject droplets of ink only when requested by the controller. Conversely, continuous inkjet heads eject a steady stream of ink that breaks up into individual droplets as it travels towards the substrate. The control of individual droplets is accomplished via a complex charging and deflection process. The ink stream first passes through an electrode that places an electrical charge onto each ink droplet as it breaks away from the stream. The droplets then pass through a deflection plate that has the ability to place an opposite charge onto each one. The computer controls which droplets it does not want to print by deflecting them into the gutter with an opposite charge. Droplets that do not get deflected remain on their original trajectory and fall to the substrate. Deflected droplets are re-circulated through the system and back to the print head. Continuous inkjet is the technology of choice for industrial marking purposes (Bar coding). CIJ is a good choice for high-speed applications, because CIJ print heads can produce drops at extremely high frequencies. A typical CIJ head produces up to 100,000 droplets per second (100 KHz). Operational Schematic Nozzle Charging System Deflector Gutter Substrate Ink

6 6 Multi-deflection Continuous Inkjet Description Like binary continuous inkjet, multi-deflection uses a complex charging and deflection process to control individual droplets of ink. However, a multi deflection print head can exhibit much greater control than a binary system. The deflection plate in multi CIJ heads has the ability to vary the deflection charge that it places on the individual ink droplets. This gives the system the ability to vary the trajectory of each droplet and thus its final placement on the substrate. There are several advantages in using multi-deflection CIJ. Multi CIJ allows the print engine to place more droplets of ink on the substrate using fewer ink jet nozzles. The fewer the number of nozzles used in the engine, the greater the reliability of the machine. In addition to increased reliability, multi-deflection CIJ heads provide a much higher drop velocity. Increased drop velocity leads to greater substrate penetration, a vital key in digital textile printing. In the case of the Jemtex multi-deflection CIJ print head, the heads are extremely robust and are designed with replaceable nozzles. The DPS 75T & DPS 85T use Jemtex multi-deflection continuous inkjet heads. Operating Schematic Print Head Charge Electrode Deflection Plate Re-circulation to Print Head Fabric

7 7 Electrostatic Technology Description Electrostatic technology has been in use since the 1960 s. It was originally developed as a means of imaging using electrostatic attraction of particles over wide widths. Paper specially treated (di-electrically coated) to be surface conductive is first passed over an electrically charged head (NAB) to create the image area, then over a developing station to attract oppositely charged liquid toner particles. The particles are fused to the paper by physiochemical means in a non-contact manner at the time of image development. Neither heat nor pressure is necessary. Electrostatic technology is inherently complex in chemistry, image generation and machine maintenance. The resulting print is expensive and most experts feel that electrostatic printers will be replaced by ink jet technology. In addition to high costs, there are only four manufacturers of electrostatic technology: 3M (NS-Calcomp), Xerox, Raster Graphics, and Phoenix. Equally as important, Seal and Rexam are the only two suppliers of the special electrostatic paper. Operational Schematic Unwind Rewind Electrostatic Writing Head Y Toner Applicator C Toner Removal M K

8 8 Electrophotographic Technology Description Electrophotography is a digital printing technology that does not fit into the inkjet category. Electrophotographic printing is accomplished via direct contact with a drum. Electrophotography is more closely related to other contact print methods; however, the process by which the ink is transferred to the print drum is what classifies this as a digital technology. The process begins with a corona charger that puts a uniform electrical charge on to the photoreceptor drum. The second station uses light (laser or LED) to create an image on the surface of the drum. As the drum passes the dry toner, oppositely charged toner particles are attracted to the imaged areas of the drum. The image is then transferred to the substrate in the contact area, which is charged from underneath by a second corona charger. The second charger acts as a magnet, attracting the toner particles to the surface of the substrate. Following the transfer of the image, the charge on the drum is neutralized by the eraser and cleaned to remove any miscellaneous toner particles. Once all four colors have been transferred, the toner is fixed to the substrate with high temperature and pressure. Operating Schematic Corona Charger LED Array Light Scanning Unit Cleaner Photoreceptor Eraser Developing Unit Pressure Roller Transfer Corona

9 9 Ink Technology General Each variation of digital printing technology requires its own highly specific ink chemistry. Below is a brief review of the various ink types currently used in digital printing. Phase Change Phase change ink is solid and behaves like a wax candle. The ink is melted prior to introduction into the ink chamber. Once the ink drop reaches the substrate, it instantaneously. Phase change inks have very good opacity due to their wax-like appearance but have very poor scratch resistance properties. Phase change inks are used mostly in the SOHO market, typically via Tektronics printers. Thermal Transfer Ribbon Each ink color is pre-coated onto a thin, clear carrier sheet. The ink is transferred from the ribbon onto the substrate using contact by a needle-like mechanism. The ribbon cannot be reused, and a lot of ink is wasted. The ink waste prevents transfer ribbon technology from moving into wide-format and other industrial type applications. Toner Very much like an office copier, dry toner used for electrophotography is supplied in powder form. It must be fused to the substrate through heat and pressure. Conversely, electrostatic printing uses liquid toners. The liquid is a fast evaporating solvent carrier and does not require any additional energy to fuse to the substrate. In order to achieve print durability, both types of electro printing require a combination of special substrates and ink chemistry. Solvent-Based Although solvent technology is well known in the world of analog printing, solvent inks must be specifically formulated for inkjet print heads. Environmental impacts are still an important concern, causing most OEM s to pursue water-based solutions. Both pigments and dyes are available in a solvent carrier. Water-Based Water based inks are the preferred choice for inkjet printing. There are fewer environmental concerns with water-based inks; however, it is more difficult to meet the physical specifications of certain industrial applications. The recent incorporation of binder into water-based formulations has shown progress and will eventually eliminate the need for specially coated substrates and/or expensive lamination. UV Curable The most recent advancement in ink technology is the introduction of UV curable ink. UV curable ink is typically substrate independent and can be fixed almost instantaneously. Speedy fixation and excellent durability provided

10 10 by UV ink is well suited for high-speed printing and industrial applications. Unfortunately, UV technology has not yet been integrated into a commercial ink jet printer. Frequently Asked Questions What is inkjet printing? A: Inkjet printing is a non-impact printing process in which text and images are formed by the precise placement of small (picoliter-sized) droplets of ink fired at high speeds from the nozzles of computer driven printheads. Droplets of cyan, magenta, yellow, and black (better known as CMYK) are combined to form precisely placed dots of the four colors, which when viewed from a distance, compose an image. What is a picoliter? A: A picoliter is one millionth of a millionth of a liter. Depending on the resolution of the printer, inkjet drop sizes range from five to more than 1500 picoliters. What is a Kilohertz? A: Usually represented by the letters KHz, a Kilohertz is defined as 1000 occurrences per second. In ink jet printing, a kilohertz is the unit of measure used to identify the number of ink droplets that are ejected from an inkjet nozzle in one second. This number is reported as the print head s frequency. For Example: A frequency of 25 KHz indicates that 25, 000 droplets of ink are ejected from an inkjet nozzle per second. What is a Piezo inkjet printer? A: A piezo inkjet print head uses an electrically stimulated crystal to change the shape of the ink chamber, creating pressure on the ink and forcing a droplet through the nozzle. Each nozzle is controlled by a computer and ejects a droplet of ink only when it is needed: creating the term drop-on-demand. A piezo ink jet printer is a print engine that uses piezo print heads. What is a Thermal inkjet printer? A: Thermal ink jet print heads are another type of drop of demand device. As the name implies, thermal inkjet printers use heat to control the ejection of ink droplets. In a thermal head, the ink is rapidly heated which creates a bubble

11 11 inside the ink chamber. The bubble forces a drop from the nozzles, and the ink is then cooled. Unlike piezo ink jet heads that can be engineered for long life span, thermal print heads are throw-a-ways. A thermal ink jet printer is a print engine that uses thermal print heads. What are some of the differences between piezo and thermal inkjet printers? A: Piezo and thermal print heads differ primarily in how droplets of ink are produced from the nozzles. This difference, the use of heat vs. the use of mechanical pressure, causes major differences in the head life spans. Thermal printheads are typically replaced after 500 ml of ink, and a piezo print head may last up to 5000 hours of use. This difference can be attributed to the repetitive heating cycles (thousands per second) needed for thermal inkjet printing. The heating cycles quickly degrade the printhead s electronics and cause kogation, a phenomenon defined as the build up of ink on the surface of the nozzle plate. The short life span of thermal heads prevents their use in industrial applications; however, piezo printheads are typically a good choice for long-term printing. There is also one other difference between the heads: Thermal ink jet heads must use water-based inks whereas piezo heads can use solvent, water-based, and/or UV curable chemistries. What is an Electrostatic Printer? A: As the name implies, electrostatic printers use electrostatic attraction of particles to create an image. Paper specially-treated to be surface conductive is passed first over an electrically-charged head to charge the image area, then over a developing station to attract oppositely charged liquid toner particles. The particles are then fused to the paper by physiochemical means. Raster Graphics, 3M, Xerox, and Phoenix manufacture this technology. What are some of the differences between inkjet and electrostatic printers? A: Electrostatic printers with few exceptions are faster than inkjet technologies and capable of variable information on every print. It offers the best transfer of chemistry to the media because toner consists of particles of plastic that become physically fused to the substrate surface. Electrostatic printing prevents the absorption, density loss, and colorant related stability problems found in inkjet. Inkjet technologies now exist that can compete with the output capabilities of electrostatic printers. The increased speed and reliability of these ink jet systems will eventually replace electrostatic printers for the following reasons: Electrostatic is an expensive technology to operate and maintain due to its inherent complexity. Reliability problems do not necessarily mean breakdowns, but can mean variable images and maintenance hungry chemistry processes. There is variability in substrate quality, sensitivity within machines to humidity, and a constantly varying chemistry mix.

12 12 What is an Electrophotographic printer? A: Also known as laser technology, electrophotographic printers employ both an electrical charge and a light source (laser or LED writing head) to transfer an image to a photoreceptor drum. The drum is then exposed to oppositely charged dry toner particles, which transfers the color onto the drum. Substrate is oppositely charged from underneath to attract the toner particles to its surface as it comes in direct contact with the drum. Once all four colors are printed, the substrate passes through a fusing station where the toner is exposed to high temperature and pressure. There are many mechanical elements such as the corona charger, eraser and cleaner that are involved in this technology which make it quite complex. Xeikon and Indigo are examples of this technology, which is primarily focused towards publication printing. Like electrostatic technology, electrophotographic technology is complex, has reliability issues and is expensive to operate. Is a printhead the same as a print engine? A: No. A printhead is the component from which the ink drop is produced and is just one element of the print engine, which also includes the mechanics and firmware to control the movement and operation of the printhead across the media. Can I use any ink in my printer? A: No. Ink is an integral part of an inkjet printing system because the ink chemistry must have the viscosity, particle size, surface tension, and stability necessary to flow easily and reliably through the printhead nozzles. Ink jet inks must also have the necessary drying characteristics to dry quickly on the substrate while conversely staying wet in the orifice of the nozzle to prevent clogging. The ink must also have the necessary color density and physical properties to meet the specifications of machine s market. Generally speaking, ink for each inkjet head manufacturer is developed simultaneously with the inkjet head, which ensures a properly matched system. For those inkjet head technologies that have been in the market for several years, aftermarket inks may be available. However, the use of these inks generally voids the warranties of the print head manufacturers. What are the different types of inks? A: There are 5 main ink types for digital printing: phase-change, solvent-based, water-based, toner and UV curable. The different in types are discussed in detail in the previous section.

13 13 What are the differences between dye and pigment inks? A: Dye-based inks are generally known for their exceptional color gamut and vibrancy whereas pigments typically produce less vivid colors. In textile printing, dye-based inks are substrate specific but produce high color prints with excellent resistance to wash, crock, and fade. Conversely, pigment inks typically have superior lightfastness to dye-based inks on paper. Pigments are not substrate specific, but digital pigment inks may be designed for a certain level of durability. For example, exterior signage pigment inks are guaranteed for two plus years, while dye inks require an over-laminate with a UV inhibitor to protect against fading. What are the differences between water-based and solvent-based inks? A: Solvent-based inks are generally known for their ability to print directly onto standard substrates such as paper and vinyl without the need for special treatments. However, the use of solvents raises some environmental and health issues that many printers prefer to avoid. Although environmentally friendly, the original water-based inks required specially treated substrates in order to achieve durable prints. However, new water-based technology can print directly onto standard substrates. The addition of binder in conjunction with integrated drying systems allows ink to form durable films when properly dried. What is the difference between dpi and ppi? A: Dpi is a measurement of printed dots per inch. Example: 600 dpi is equal to 600 dots per inch in any lateral or longitudinal direction. The size of each drop is measured in picoliters, previously defined. Ppi refers to the number of picture elements (pixels) gathered by a scanner or viewable on a screen. There is no clear correlation between the resolution of digital data (600 ppi) and the resolution of a printed image (600 dpi). What is the difference between true resolution, addressable resolution, and apparent resolution? A: True resolution is measured in dots-per-inch (dpi). A 600-dpi printer means that each dot is 1/600 th of an inch in diameter, placed in a 600-space/inch x 600- space/inch grid. A 300-dpi printer means that each dot is 1/300 th of an inch in diameter, placed in a 300 spaces / inch x 300-spaces / inch grid. Addressable resolution of 600 dpi means that dots larger than 1/600 th of an inch (e.g. 1/300 th of an inch) are placed in a 600 dots-per-inch grid. Apparent resolution isn t a mathematical measurement, but rather describes the quality of an image perceived by the human eye. In theory, smaller dot sizes will produce finer details, sharper text, and smoother curves.

14 14 How is output affected by resolution? A: Generally speaking, lower resolution (larger drop size) equals faster speed. Many printers on the market today offer a choice of print modes in which you can sacrifice higher resolution for faster speed. Creating high-resolution images (e.g x 1440) can be a painstakingly slow process because the printhead must traverse back and forth across the same swath of media several times, with the media advancing in small steps. The various print modes may involve one-pass, two-pass, or four-pass operation, with different nozzles firing during each pass to prevent the appearance of streaks (known as banding) due to misfiring jets. When printer manufacturers claim their printers can print 600-dpi images and have speeds up to 90 sq. ft/hr, be aware that the top speed is typically for a lower resolution image. What are the advantages and drawbacks of printers with more than four colors of ink? A: Printers with more than four colors of ink give you the ability to achieve smoother blends and graduations between colors (CMYKLcLm) or the ability to hit a wider color gamut (CMYKOG). While it is true that six-color printers have more printheads to maintain and more ink chambers to keep filled, six-color inkjet printers typically don t use significantly more ink than four-color models. A typical image produced by a six-color printer will actually use less of the base colors (CMYK) to achieve equivalent image quality. CMYKLcLm (Cyan, Magenta, Yellow, Black, Light Cyan, and Light Magenta) configurations add lighter densities of cyan and magenta ink to the traditional CMYK. Because light cyan and magenta can be used instead of printing the more conspicuous dark inks (cyan & magenta) in highlight areas, you get higher apparent resolution in highlights and midtones. The end result is fill areas that are less grain as well as improved pastels and flesh tones. CMYKOG (Cyan, Magenta, Yellow, Black, Orange, Green), commonly known as hexachrome, drastically increases the available color gamut. Using orange and green allows the software to print millions of colors that were unobtainable with simple CMYK. Generally speaking, the only downside to 4+color printing is the data involved in processing the images. What are the benefits of printing with Hexachrome (CMYKOG) vs. four colors? A: Hexachrome is a six-color process invented by Pantone. CMYKOG (Cyan, Magenta, Yellow, Black, Orange, Green) inksets add orange and green to the CMYK mix, expanding the range of colors that can be reproduced on the printer. Why does K in CMYK represent black? A: K stands for key. It was traditionally the reference color used to register the other process colors in printing. Black is used to reproduce text and line art,

15 15 neutralize the contamination of CMY inks, add density, and reduce total ink consumption in wide-format inkjet printing. What is a RIP and what does it do? A. A RIP (raster image processor) takes an ordinary data file and formats the image for the printer resolution. The file is adjusted for color and appropriate output size, and then sent to the printer. The RIP tells the printer exactly where to place the droplets of ink on the substrate. The RIP software and print controller determine the main functionality of an inkjet printer. Why are ink and media profiles so important? A. The inks and substrates produced for digital printing can vary significantly by supplier. The final printed appearance depends on ink color, as well as certain brightness, absorption, and reflectance characteristics of the substrate and the receptivity of its coating. In order to produce high-quality images and match the appropriate colors, the RIP software needs data about the color properties of the inks and media. Color profiles (ICC and others) provide this information. Why is lamination so popular for wide-format printing? A: Many inkjet prints (up to 75%) are laminated. In addition to adding rigidity and protection, different lamination films change the look of the print. Appropriate films can intensify colors and possibly add a textured or high-gloss finish. Generally speaking, lamination is a simple method of modifying the appearance of a print without altering the inks or media.