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"Is inkjet catching up with electrophotography? Not just yet!"

Jul 1, 2008 12:00 AM


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Lode Deprez, vice president, Toner & Developer Group, Punch Graphix Intl., shares the Punch Graphix / Xeikon stance on inkjet vs. electrophotographic production printing.

For years, the conventional wisdom has been that printers choose electrophotographic (EP) presses for high quality and inkjet for high speed and low cost. A closer examination reveals that the decision is no longer so clear. Advances in technology have enabled the latest generation of EP presses to print at high speeds, without sacrificing quality. While the up-front cost of an inkjet press still is likely to be lower than an EP machine, this is only part of the cost picture. Inkjet printheads need frequent replacing for maintain production print quality, causing significant direct (new parts) and indirect (loss of revenue due to downtime) costs to be incurred. In addition, environmental issues are an increasing concern, and dry toner-based EP has significant advantages to help the printing industry achieve a more ecofriendly position.

Technology considerations

Printhead life and cost of ownership | One of the most critical components of an inkjet machine is the printhead. On average, every nozzle is expected to produce 20-50 billion ink drops during its lifetime. This is considered to be the case for piezoelectric (piezo) heads; the lifetime for thermal printing heads is substantially lower. The frequency of ink drop production is directly linked to resolution and speed. For example, to print at a speed of 0.3 meter per second with a resolution of 1,000 dpi (the starting point for a graphic application), the head must fire with a frequency of 12,000 ink drops per second. Assuming a “best case”  average life expectancy of 50 billion total ink drops, the head then should be able to operate for approximately 1,150 total hours before needing to be replaced.

In a double-shift operation (4,000 hours) the heads need to be replaced 3 or 4 times per year. For a 4-color duplex inkjet machine printing on a 20-inch-wide web, you need at least 160 of these heads. The current cost per nozzle in a piezo head is approximately $2 (for thermal this could be a factor of 10 less, depending upon the technology used to produce it).  Final calculations reveal that 640 printing heads (each $2,000) will have to be replaced per year.  

There is no large difference between prices for ink versus toner in the professional markets, so the main cost difference is found in the usage of parts. When placing next to each other the above described “theoretical” inkjet digital press (1,000 dpi) with an actual commercially available EP digital press both printing duplex at 30 cm/s on a paper width of 20 inches, the cost of usage parts per A4 side is just below $1 (all included) for EP compared to $2.00 to 2.50 for inkjet (only heads), or a factor 2.5 higher.

Inkjet appears to be less expensive and complicated upfront, but the costs associated with using the technology at industrial speeds and resolution are much different than its traditional applications, such as the SOHO environment and large-format printing. Industrial inkjet is known to have the capability of reaching high speeds with lower resolution, but the above shows that increasing the resolution certainly doesn’t come for free.  There will continue to be new technical hurdles for inkjet to overcome as the technology looks to improve its capacity to produce faster, high-quality graphics in the same capacity EP is already accomplishing,

Substrate handling and movement | At higher speeds, inkjet printers capture a small amount of air on the surface, creating turbulence just above the substrate and making it difficult to predict where the ink drop is going to hit the surface. This is directly proportional to the speed and dropsize of the ink and the surface roughness of the substrate. The dimensional changes are due to the high amount of liquid that has to be absorbed to keep the pigments at the surface of the paper, requiring expensive special coatings or the use of a fixing agent, which can be applied imagewise prior to jetting the pigmented inks.  By doing the latter, the machine only puts a bonding agent on the places where it will put ink, but therefore it needs another row of nozzles to do so. Because EP is a contact process whereby the toner particles are transferred to the substrate directly, this problem does not occur.  
 
Ink vs. toner | The composition of the inkjet inks has to be designed in such a way that it is fully compatible with both the head and the susbtrate. The pigments have to be made small (<80 nm) to create a stable solution and not obstruct or abrade the nozzles of the head. To keep the pigment on top of the substrate, sometimes complex special designer molecules are added, making the performance of the inks increase exponentially with the rise in cost of both the ink production and ink materials. In addition, the inks frequently have to be adjusted to the type of substrate, to deal with the different surface energy and adhesion.  This is far less of an issue with toner materials. When polyester is used as a basic resin, compatibility with a large amount of substrates can be realized. Toner manufacturing also has advanced with respect to raw materials, manufacturing processes and energy consumption, resulting in more complex formulas without a substantial cost increase. 

Environmental impact

In addition to the technological differences between EP and inkjet, today’s printers must consider the environmental impact. With the global trend in ecofriendly technologies — and associated pressures not only from environmental groups but, increasingly, government mandates — environmental concerns are becoming a real priority for print manufacturers.

Volatile organic compounds | There is a global trend to reduce Volatile Organic Compounds (VOC) emissions to meet new environmental requirements and standards. Dry toner-based systems are far superior in this area, both with respect to VOC emissions and paper recyclability.  Solvent-based inks or liquid toners cannot show the same track record in VOC reduction to meet the high standard set forth by the graphic industry. Pigment-based inkjet inks and liquid toners also are not in the same league with electrophotography with respect to paper deinking performance.

Recyclability | Currently there is no good way to deink UV-curable inks or liquid toner inks when printed paper materials are recycled. During the recycling process, repulping causes these inks to become ink specks, too large to be removed by flotation and too elastic to be removed by screening. When looking at pigmented inkjet inks, the flotation process doesn’t work because the particles are too small and therefore cannot be removed. Recycling plants are concerned that these materials will end up at their mills, slowing down the production cycle and creating additional expenses or even losses.

Dry toners have been around for many years and, because of the specific brittle and hydrophobic nature of the particles, the toner images have the same behavior as offset or rotagravure inks during the deinking process.

Combining superior image quality and process stability without compromising printing speed, electrophotographic presses outperform other industrial printing methods on a wide range of substrates. Add to that the absence of solvents and the proven deinkability of dry toner printed material, and electrophotography has a significant advantage not only for the operator but also for the environment. Dry toner already has proven its strength and will gain importance as more demanding graphic applications emerge and stringent environmental legislation is implemented. It will take some years before inkjet comes to the EP level when dealing with paper printing, not only from the environmental point of view but also in performance, reliability and cost aspects.