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![]() The Variables of Calculating Ink Consumption
By Jeff Burton, Digital Printing Analyst, SGIA
Hopefully the answer is a lot, as that would translate to high volumes of billable printing, with the majority of the ink going on to media, and directly billed to a client.
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Tracking ink usage has become a little easier these days thanks to drop counters and job accounting software. But why is tracking ink use so difficult? Ink use numbers have a tendency to evade collection for a number of reasons. When you purchase ink, is the shipping cost distributed or added to the cost per milliliter for each ink container shipped? When performing print testing for color or design for a client, is the ink use tracked? How about ink use for media and ICC profiling, cleaning cycles, head purges and ink used during non-use overnight or machine idle time? Let's look at each of these in a little more detail.
Old School This method also assumes that the ink from the container goes directly to printing, which may or may not be the case. Some equipment may simply load ink from the cartridge/pouch/box into an onboard secondary ink reservoir for future use. In systems that have this secondary ink storage, keeping ink use statistics becomes really frustrating. You can still count ink containers and total square feet printed, though it may take longer to get to some usage numbers.
Print, Clean, Repeat
Counting Drops I ran a comparison between three commonly used RIP software packages to gain a better understanding of ink consumption numbers generated. For all of the RIP software, the first order of business is to enter the picoliter drop size or sizes (variable drops) that your printer is using. In some circumstances, this number changes whether you are running the printer in variable droplet or in a fixed droplet mode. In other cases, the RIP software has prepopulated this number. The drop size usually is supplied to RIP software manufacturers in a software developers' kit (SDK) from the inkjet printer manufacturer. After entering all of the drop size numbers, you will then enter the cost of the ink, usually in dollars per milliliter ($/ml), or in weight measurement. To figure this, just divide the cost of your ink container (plus shipping) by your total ink volume, very simple. In-RIP drop counting has a few distinct advantages.
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Ink use for the print can be calculated without actually printing the job, as the RIP calculates the cost either as it RIPs the job or in a subsequent pass. Paper costs are usually added to this calculation, with actual square feet printed, or print area plus waste generated at the same time. The cost of paper can also be introduced, so that you can have a projected/actual cost log for media plus ink costs per image printed. Back to my comparison… I printed to an HP Z6100 PS printer using two different standalone RIP packages and printed direct using the HP print driver from Photoshop on a Mac. With the HP, cost accounting software is built into the device driver utility, with all processed jobs listed and results that can be exported to Excel. The print size for this test was only 15 by 20 inches, but for the print made directly via the Mac HP print driver, the ink usage was the highest, at 2.82 milliliters. This was due to the fact that there are no ink restriction modifiers in a print driver aside from the choice of paper, which is the ink limit setting in an OS print driver (no RIP used). Print drivers also have no GCR/UCR settings with which to optimize ink use, so direct print drivers would be expected to have the largest ink use versus software RIP packages. The second print was made the same size and printed in the same mode - 1200 x 1200, eight-pass bi-directional - as the HP print. I used a popular RIP software package profiled for that paper and those same particular printer settings. This software stated that the ink used for the same print was down to 1.7 milliliters, a 39 percent ink savings for popular RIP versus no RIP. The percent savings get smaller when you compare RIP to RIP because the majority of RIPs have profiling features and varying manufacturer methodologies for ink restriction, GCR/UCR and light/dark ink start points. The last print to test ink consumption was with another popular RIP software that was profiled to use the same printer resolution, media and passes as the other two tests. This software reported a very close 1.66 milliliters of ink used for the test image. For this RIP, the savings was in the neighborhood of two percent, although differences between RIP ink usages for the same print could range from 10 to 30 percent. This was just a cursory test, as ink limits and other RIP variables are difficult to set and assess when trying to create an "equivalent print" across multiple software packages without aim points (using visual assessment only). Some RIPs now have an export or cost accounting function or module that allows job-costing functionality. I think this is the right direction to move for RIP manufacturers, as digital printers need this information quickly and seamlessly. But what affects ink consumption in the RIP?
Controlling Ink Use in the RIP At the stage of media profiling, right before actually building the profile, there is an area in the software that will let the operator change how the profile treats grey component replacement (GCR) and/or under color removal (UCR). GCR and UCR, when used correctly, will enable users to get the best print using optimized ink levels for greatest ink savings. But dialing in these particular controls takes a little patience and testing, depending on the types of images you print. GCR and UCR are often misunderstood and confused with each other, because they originate in the offset print domain. GCR uses the simple principle that black should be used to replace neutral (CMY gray component) areas in the original image. The two controls are often used in combination with each other, GCR defining how much black to use, and UCR to control how much CMY to remove. Settings for GCR and UCR at the RIP level are global and affect all files processed by the RIP. One could make various profiles using different degrees of GCR/UCR and save those profiles as separate printing environments, but this is too time consuming and cumbersome.
Ink Optimization Software
Conclusion Jeff Burton is SGIA's Digital Printing Analyst, and provides solutions to digital printing production, color management and application issues, as well as digital equipment and vendor recommendations. In his more than 20 years in the industry, he has worked as production manager, member consultant, trainer, author of numerous technical articles, and presented at a variety of industry events. His extensive background in digital inkjet printing, digital photography, graphics applications, and production methods, serves members by supplying customized solutions to their daily imaging problems. Jeff currently instructs the Color Management Boot Camp, at SGIA's headquarters in Fairfax, Virginia. This article appeared in the SGIA Journal, July / August 2013 Issue and is reprinted with permission. Copyright 2013 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.
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