Theory and description of various ceramic ink and inkjet printing technologies for ceramic tile, the issues technicians and factories face, inket printer product overview.
This is an outline of the state of the art in inkjet decoration of tiles technology (from the convention held in Modena, Italy, May, 27th 2009 and latest news).
From year 2000, when the first machines for inkjet decoration of tiles were shown at Cevisama in Spain, there has been a steady evolution of mechanical and electronics, inks, pigments and chemicals. Of course, strong competition among different companies has been a catalyst for this.
Currently several companies are competing to develop new machines able to decorate tiles 24 hours-a-day at 30-50 m/min (1000-1500 m2/hour output depending on the resolution). Inkjet decoration machines perfectly fit into glazing lines and improve efficiency. They can have up to 1000 nozzles and work with 6 different inks up to 1000 dpi resolution (for most of tiles 200-300 dpi is sufficient). However, the most important feature is the amazing versatility of these devices. Different machines for digital decoration are available with a variety of characteristics but for all we can enumerate these advantages:
There are more advantages but we must also admit that digital printing cannot always substitute for traditional methods of printing, especially when we need higher thicknesses of paste or when we need strongly colored surface areas. Work is in progress so we can expect improvements and further cost reductions because inkjet technology is still in evolution (after overcoming initial problems). Inkjet technology is currently expanding its reach to different kinds of tile and, of course, the technology aspect itself is arousing the interest of more developers.
In digital printing there is no contact with the tile surface. The most popular technology adopted by most producers of machines is called Drop-On-Demand (DOD). It is based on a piezoelectric device that deforms under voltage enabling droplet expulsion. The most popular color system is the well known CMYK standard. This is an acronym for the colors Cyan, Magenta, Yellow and Black (for black K is used instead of B to avoid confusion with Blue). In the CMYK system the black separation contains the details, thus it is called the "Key plate". Cyan, magenta and yellow overlap in different percentages to produce the colors in the following figure. They overlap at 100% of each to produce a very dark brown, for this reason the additional black ink is employed.
Depending on whether there are 3, 4 or 6 inks in the machine a specific colorimetric procedure is employed to separate colors in the original image to obtain a plate for each. In the figure we can see an example.
The final image is reproduced by overlaying these ink colours.
To enable high output (more than 1000 m2/hr) the print head assembly containing the nozzles spans the entire width of the tiles. When the color assemblies are arranged in succession we have following configuration (by Durst).
The mechanism of this method is as follows: The ink, made from a suspension of extra fine pigments, at the right temperature and with the right viscosity, passes from a main reservoir to another that is smaller which has a "firing nozzle". The ink is shot from a connected nozzle (having an internal diameterof about 0.04 mm) to the target (whose distance can vary from a few millimeters to 1 cm). The piezoelectric element can be in contact with the ink or outside the channel where ink flows (depending on whether it is composed of a liquid having polarity (like water) or not). Since the piezoelectric element is deformed to an extent that varies according to a voltage input a computer controlling the waveform to the print head can control the size, speed and frequency of the drops. Available machines from different suppliers use different frequencies depending on the resolution needed.
This technology uses a spray instead of drops. The main reservoir is connected to a vibrating capillary tube that shoots the spray of ink. By controlling the frequency of the of the vibrations and the time we obtain a printing cone wherein the intensity of the color can be controlled. This technology is thus also called "Flatjet". In the variable spray the drops leave the capillary tube with a frequency of 1000 per second. Nozzles have an internal diameter of about 0.5 mm so there is a greater flow of ink (about 40 mm3/sec/nozzle), this is about 25 times more than the drop-on-demand system. In addition, this machine can utilize inks having particles up to 10 microns, that is 10 times the maximum size allowed by other machines. An associated dvantage here is that pigments show a better stability on firing because they can be less finely milled.
One company has developed a technology that produces drops of different sizes from the same nozzle. This produces different chromatic intensities depending on the size of the drops (for instance grays with different intensities). While almost all producers of digital decorating machines are using the same-drop-size technology, this technology shows key advantages. Drops having different sizes are obtained by changing the frequency and length of pulses stressing the piezoelectric element. For example a standard nozzle could aim one drop having 6 pl volume or concentrate a sequence of 7 drops having 42 pl volume whereas this technology can do the same in two drops. Higher color definition is possible while at the same time the system still supports lower color intensity with less definition. By overlapping drops with different colors different chromatic effects can be obtained (creating an impressive range of tones and colors). Moreover, since changes in drop volume correspond to a dilution of ink only a few inks are needed (making the machine very flexible).
Inks must have consistent properties. Variation in any parameter can affect color, intensity and definition (and therefore quality). A key aspect to maintaining the density, viscosity and surface tension parameters of inks is keeping them at a constant temperature. This is necessary in all cases, including particle suspensions and organometallic compounds both for organic solvents and water. In some machines the entire ink reservoirs are kept at a constant temperature whereas in others just the small nozzle chambers or only the nozzles themselves. Inks are continuously stirred and flow in a cycle from the main reservoir to the small chambers (and nozzles in some cases) and back to the main reservoir again. There is always a washing procedure and steps are scheduled, all without any break in the printing process.
Three-firing producers have used inkjet equipment since its inception and we can foresee even greater adoption of this technology in the future. Producers can create tiles and panels of any size having photographic quality using 3, 4 or 6 basic colors made from normal and very fine ceramic pigments stable for these temperatures. Only by inkjet is it possible to lay down decoration on top of frit grains because it is done without contact. Wonderful effects can be obtained with semi-transparent colors and diffusion. For third-firing, inkjet represents a huge opportunity for savings for on-demand production (reducing warehouse stock and the number of pigments needed).
For single fired wall tiles we have developed new effects that define new quality in the high rank market. For vitrified tiles soluble inks have already been used for many years. Soluble inks, made from soluble salts, can penetrate into the green body or into the green glaze so polishing can be done without loss of color or definition. While better definition is normal for ceramic pigments the layer is very thin and so cannot withstand polishing or heavy traffic without a protective over glaze. Thus we can combine the inkjet process using inks made from soluble salts and those made using traditional ceramic pigments with standard roller roller printing machines to obtain valuable aesthetic effects.
The first inks used for inkjet decoration were organometallic compounds because all the applications required an optimized system with low viscosity, moderate surface tension, a high color covering capacity and non-toxic solvents.
Inkjet decorating machines work with 3, 4 or 6 basic colors and by using these all other colors must be created. However this goal has not been fully realized. While in recent years we have studied and developed more environmentally friendly glazes and bodies that deliver color stability, some specific colors have been elusive. In ceramics we have to face the lack of a true magenta or a true black, often we must resort to mixtures of other colors. However, new organometallic colors hold promise, there are a wide range of pigments with particles sizes of less than 1 micron. Moreover we also need further research because interactions bwtween pigments having different crystal structures can produce different colors depending on firing conditions and fineness of particles.
We know that pigments can lose their staining power when particles are too fine because the increased surface area exposes the crystal structure to the chemical attack of glazes and bodies. Up until now results are encouraging and a wide range of pigments are available that perform without settling, clogging of nozzles and with good coloring properties (solvents must not dry and clog the nozzles during breaks but, at the same time, they must evaporate within just a few seconds after application). These problems have been solved by adopting soluble heavy metal complexes with traditional liquid ink technology (the required colors are developed during tile firing, i.e. when the corresponding oxides are formed). This has proven to be a way of producing very stable inks without solid particles.
Solvents are important for the formation of drops. For instance some companies are proposing mixtures of water and derivates of paraffin or glycol (whereas soluble organometallic compounds contain cobalt, chromium, ruthenium, gold, etc). This approach offers the following advantages:
One problem is that the migration of colored elements reduces color intensity and definition. To overcome this problem inks of mixed ceramic pigments with solvents are available. In this case we have a dispersion of granules with a density 3-4 times higher than that of the solvent. Producers are looking for the maximum stability, without settling (not only during storage but also in all parts of the machine where mechanical stirring is not performed). Pigment particles are typically less than 1 micron in size and suspensions are made using organic polar solvents (ethers and glycol esters) with additives to improve their wetting properties. This approach offers the following advantages:
The problem of accurate rendition of the original tones is a constant issue for those who have the task of reproducing an image in the most exact and repeatable way. For ceramic tiles, the challenge is even more complex because the decoration is fired in a kiln between 900-1200°C, and thus involves chemical phenomenon or solid state interactions not easy to foresee or manage.
However work is in progress and every day we have improved colors that overcome problems with intensity, shade and reactions with glazes and bodies. In addition there are companies dealing with reactions between nanoparticles and metallic ions in order to obtain very small colored particles without milling.
Likely the most important aspect of using digital inkjet decoration is not in the mechanics of it on the glazing line itself. Rather it is the rationalization that must occur to set up the printing process so that it faithfully reproduces the original digital picture despite all of the complex machine setup, ink, pigment, glaze and firing issues described above. Thus it is no surprise that in a manufacturing facility a comprehensive mix of specific expertises are needed: software, hardware, graphics, ceramic production and ceramic engineering specialties.
Following are some of the latest equipment innovations as shown during the convention in Modena, Italy (also other sources).
KERAjet is an inkjet printing system developed by Ferro in the past few years. In this a 100% digital system for ceramic decoration to create, modify, transmit and apply a design directly to the tile. This process is called the 'variable image' system and it uses a four-color-process decoration (Cyan, Magenta, Yellow, BlacK) to create the finished design. The KERAjet system, already used by many of the leading tile manufacturing companies in the world, represents a decoration system alternative to other traditional methods and opens new opportunities of creativity in tile ceramics. The four ceramic inks are fed automatically ('drop on demand system') through industrial electric printing-heads consisting of 500 injectors per head and transmitting 4000 drops/second each at an application speed of 6 ml/sec.
Durst also utilizes the classic CMYK inkjet technology without contact. Gamma printers enable decoration up to 1200 m2/h with maximum resolution of 924dpi (with appropriate software). Inks for Gamma inkjet printers can work with the same glazes used in traditional decoration equipment and are not sensitive to environmental conditions.
The decoration technology selected by Sacmi is the "Flatjet". It can be considered a method of "spray on demand" because a cone of droplets are sprayed on the surface of the tiles. In this case the open side of the capillary tube is free and it can vibrate when excited by a piezoelectric disc while the opposite side is connected to a reservoir. The color intensity of the printed area can be controlled by changing the vibration time of the piezoelectric disc. Thus it is possible to print, if needed, using larger amounts of color than the drop on demand method. Moreover Sacmi digital printer nozzles have diameters of 500 microns while other printers using drop-on-demand technology have 40 micron diameters. Thus we can use normally milled pigments in water and decorate with larger amounts of color per time unit because the flow of pigment is about 25 times greater than typical inkjet printers. The printer has independent printing units, one for each color, and the number of units can vary from 3 to 6, depending on the need. There is a panel unit for control and calibration. In each printing unit the distance between nozzles in a row is 2.54 mm, thus 5 rows of staggered nozzles are needed in order to have 50 nozzles per inch (50dpi).
In the CretaPrinter inkjet decoration system the ink is in constant movement, even in the interior of the shooting chamber (within the walls of the piezoelectric material). Using this technology it is possible work with inks of up to 100 cps of viscosity at 40ºC (compared to the 20 cps of current systems). This fact makes CretaPrinter the first existing solution to enable the use of inks having a higher content of oxide coloring particles and consequently with higher color intensity inks. CretaPrinter can work with drops of different volume (grey scale mode), while in the rest of systems the generated drop is always the same size (binary mode). As a result, pixelized (separated dots) images are avoided, and uniform color gradation is obtained. From between 3 to 8 impression bars can be selected for each machine with an impression width in multiples of 70mm to a maximum of 1,120 mm.
Rotodigit is a piezoelectric drop-on-demand printer that can be assembled and can work in synchronization with Rotocolor units (see picture). Silicon rollers can transfer 800 picoliters per nozzle while inkjets can transfer about 65 picoliters per nozzle, thus both can be used depending on the amount of color and definition needed. This approach minimizes costs because of course inkjet heads are expensive. In addition, by employing both methods better decoration quality can be acheived by compensating the limits of one technology with the advantages of the other. Specifically, inkjets can decorate close to edges and surfaces with heavy structures where rollers experience problems. On the other hand inkjets cannot transfer large amounts of color while rollers can. This system can also interplay with Rotoplotter for the development of graphic research. This software is dedicated to this purpose, it can consider the color of body and available pigments and can print a graphic equivalent to Rotodigit for color tone and color rendering. With this evaluation ability adjustments can be made before industrial tests.
Jet Digital Printer 08 is an inkjet printer for third firing and special pieces and it can decorate flat or eneven surfaces, for double or single firing. It is fully digital and it works like the four-color process but can use up to eight colors maximum.
KeraLab is a laboratory service dedicated to color management for digital printing. As noted, the problem of accurate and repeatable reproduction of colors on decorated tiles is a constant challenge for people operating both inside factories as well as in support services. The KeraLab system provides a video preview or a printout of a proposed image, on which it is possible to make numerical measurements to evaluate the likelihood of a production match with the original. Decisions can then be made, on the basis of prefixed thresholds, whether to proceed with reproduction in the factory with the inkjet printing device. The process thus starts with the availability of the materials and ends with the electronic color separation files to reproduce the image on the target printer. The complete process enables better understanding of what is or is not possible to obtain and users have various objective data and numerical indications about the quality of the achievable results as well as the corrective actions to make.
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By Nilo Tozzi