 How Much Ink Do You Need?

(April 2000) posted on Wed Jun 14, 2000

Learn how to calculate the total amount of ink needed for any job based on mesh parameters and run length.

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Determining Image Area Knowing the height of the ink layer that a particular mesh will deposit only represents one piece of the estimating puzzle. Remember, you're trying to determine the volume of ink that you'll need for a job. The height of the ink deposit is a one-dimensional measurement--volume is three dimensional. So to get the value you're after, you need to multiply the ink height by the area the image will cover. Here's where the estimating process can get a little tricky. If you're printing a flood coat of ink through your mesh, determining the image area is no problem: Just multiply the length of the open mesh area by the width. Most jobs, however, don't involve flood coats--they involve a mixture of open and blocked mesh cells across the entire screen. As you might expect, halftones pose the greatest challenge. To help you determine the area covered by complex images, you have a few options. First, your design programs may have features for determining image area. Photoshop, for example, will provide coverage data, functioning somewhat like an automatic densitometer. But you can only measure specific areas of the image. For total coverage over the entire image, you have to take multiple measurements across the image and average the results. Another option is to invest in software and equipment designed specifically to measure coverage area and other halftone characteristics. One such system is the Dot*Spec product offered by Quality Imaging Products, Marietta, GA (http://qip.com/dotspec/dotspec.html). The system, which features a microscope, digital camera, dedicated PC, and special software, provides a wide assortment of data on halftones, including average dot size, percentage of coverage, and line spacing. You can also use an image area calculator. This is a clear film with a grid pattern that you can place over the image (generally on your film positive). Once the grid is positioned, you can estimate how much of each square is filled with image areas, then add up the results to get total coverage. Most of these solutions, however, are too complex, time consuming, or expensive for the average screen shop. So most printers attempt to "guesstimate" the percentage of this area that is actually covered by image elements. While visually estimating coverage is not the most precise method, it can provide reasonably accurate results if you take a systematic approach. For example, you may be looking at the magenta separation for a particular process-color job. The image for this screen falls inside an area that is approximately 50 x 50 cm. Image elements show up over roughly 80% of the screen, and most of the halftone dots fall in the midtone range. So you can estimate the coverage area per dot at about 50%. So now you have 80% coverage of the image area with 50% dots, which would be equivalent to covering 40% of the total image area with 100% dots. In other words, 40% of your 50 x 50-cm image area, or 1000 cm2, would actually contain image elements. Putting it all together Once you've determined how thick your ink deposit will be and how large an area it will cover, you can calculate the approximate ink volume (V) for a single print. Simply multiply the ink-deposit height by the coverage area (V = I x A). Note that your ink-deposit height will likely be in microns, while your coverage area will be in square centimeters. So before you complete the multiplication, you'll need to convert all the values to the same units. The easiest option is to divide the micron measurement by 10,000, which will give you the ink height in centimeters. You then multiply the image area and deposit height to get an ink volume per print in cubic centimeters. Next, you want to project this volume across the total run length to get the total amount of ink necessary for the job. Simply multiply the volume per print times the number of images you expect to print. You'll end up with a very large value in cubic centimeters. Obviously, you don't buy ink by the cubic centimeter. It's much more likely that you're buying it by the gallon. So you'll need to do that final metric-to-standard conversion I mentioned earlier. To simplify the task, just divide the volume in cubic centimeters by the factor 3785 (the number of cubic centimeters in a gallon). The result will be the total volume of ink you'll need. To cover test prints, setup, and slight overage, it's generally wise to inflate the final number by another 5%--just in case. The entire sequence of calculations is illustrated in Figure 2.

 Figure 2: Calculating total ink required Print quantity: 10,000 pieces Image size: 76 x 102 cm (approx. 30 x 40 in.) Coverage: 35% Mesh: M = 140 thread/cm (355 thread/in.), F = 58 microns Calculate wet ink-deposit height (I) Formula: I = 0.35 x F I = 0.35 x 58 I = 20.3 microns Convert ink-deposit height to centimeters Formula: I = height (microns) x 1 cm/10,000 microns I = 20.3/10,000 I = 0.00203 cm Calculate coverage area (A) Formula: A = width x length x coverage A = 76 cm x 102 cm x 35% A = 7752 cm2 x 0.35 A = 2713.2 cm2 Calculate ink volume per print (V) Formula: V = I x A V = 0.00203 cm x 2713.3 cm2 V = 5.51 cm3 Calculate ink volume for total job Formula: Total ink = V x # of pieces Total ink = 5.51 cm3 x 10,000 Total ink = 55,100 cm3 Convert volume to gallons Formula: Total ink = volume (cm3) x 1 gal/3785 cm3 Total ink = 55,100/3785 Total ink = 14.6 gal (15.3 gal with 5% extra)

Keep in mind that your results will be approximations of the ink you'll need. Nevertheless, they are approximations based on actual mesh characteristics, so they'll be much closer to your actual needs than the arbitrary guesses you may have relied on in the past.

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