Fundamentals for ink storage, dispensing, measuring and mixing
A ghastly figure appears in your office, dripping large red and green spots onto the carpet. A thick yellow substance oozes from its hair onto its shoulders. "Hello, Stan," you say to the shape in the doorway. "More trouble in the inkroom?"
If this scene is typical in your shop, and if you're pitching printed garments because of poor ink quality on press, it's time to take a look at some simple ways to make ink handling easier, cleaner, faster, and more accurate. To this end, we'll focus our attention on the fundamental equipment any inkroom should incorporate for ink storage,, measuring, and mixing.
Step one: Know your ink
Before investing in any mixing or handling tools, make certain they're appropriate for the ink type you plan to use. Plastisols and water-based inks, whether purchased ready for use (RFU) or as pigment and base (also known as base and toner) systems, present different challenges to inkroom personnel. Storage and handling instructions for these ink types vary and consequently influence the tools you should select.
Step two: Store and handle your ink
When ordering ink, you probably purchase 1-, 5-, 30-, or 55-gal containers. It's a good idea to keep as few ink containers in the inkroom as possible--buying one, large container can be more economical than buying several smaller containers. Be aware, however, that you'll need to buy a pump to get ink out of large drums. We'll look at pumps more closely in the next section.
Many ink manufacturers offer totes, or "flow bins," that hold 250 gal of ink. These containers are pressurized and equipped with a tap in the bottom that allows ink to be dispensed into smaller, manageable containers. You may find a tote useful if you use bases, whites, or particular ink colors in large quantities.
Totes allow printers to get a price break by buying ink in a large volume. However, Jane Willey, ink management specialist for Flexible Products, Kennesaw, GA, points out that the decision to buy a tote should be made carefully since totes are very expensive. She adds that you also should check with your ink manufacturer to find out the recommended shelf life of the ink: Many recommend that you don't buy more than you can use in 14 days--after that time, the ink quality may suffer.
Next, look around your ink room and survey the types of tools there. Before you even open an ink pail, you should have the room stocked with appropriate tools for basic ink handling: plastic or metal cups, buckets, spatulas, scoops, and stir sticks.
For high-quality, stainless steel spatulas and scoops, visit your local industrial kitchen supply store. "You'll pay more for your spatulas and scoops there than at a chain hardware store, but they'll last a lot longer," says Marvin Page, sales manager for Jay Products, Cincinnati, OH. Stock up on these kinds of tools; you can never have too many.
Don't use wooden stir sticks and spatulas because wood can shed splinters into the ink, splinters that will end up on the screen and lead to voids and other image defects. The same goes for wax-coated mixing cups--the wax can shed off into your ink and eventually clog your screens.
Whether you use a water-based or plastisol ink, and whether you buy it in pint or 55-gal containers, you have to get the ink out before you mix or print it. If you buy ink in 5-gal pails or small containers, you may not need a pump. But a pump is a must if you're buying ink in drums.
The advantage of any pump is that it allows you to quickly and cleanly get ink out of its container. But before you buy a pump, talk with your ink distributor to find out the viscosity and pigment-particle size of the ink you're using. These factors will help determine what type of pump you should buy.
Pumps driven by pressurized air from an electric compressor are available for 5- to 55-gal containers. Single- and dual-post pumps (supported on a metal base by either one or two posts) are offered for both medium- and high-viscosity inks. They work by replacing the ink container lid with a large metal disk called a follower plate. When the pump is activated, the follower plate pushes down on the ink and closely runs along the container sides, minimizing ink waste by scraping the sides clean. A piston pump that sits in the middle of the follower plate draws ink from beneath the plate and feeds it through the pump's dispensing nozzle. The pneumatic mechanism that drives the pump and follower plate is called a ram assembly, and it keeps the pump primed and ready-for-use at all times.
A dual-post pump for a 55-gal container costs approximately $5000. According to Mike Sikora, sales and marketing manager for Ingersol-Rand's ARO Fluid Products Division, Bryan, OH, most printers make up the cost of the pump in several months based on the extra ink they're able to get from the container.
Another pump option is a stick pump. These pumps differ from post pumps in that they are not free-standing. Rather, these narrow, 5-ft-long pumps fit into the bung (a small opening) at the top of a 55-gal drum. Like the post pumps, they require pressurized air to operate, but they do not have a follower plate.
Other types of pumps include those designed for dispensing ink from totes. Small, portable electric air-driven pumps, fitted with a hose and a dispensing nozzle like that on a gasoline pump, are also available for 5-gal containers. And for smaller ink quantities, you can use manually driven pumps housed in lids that fit over standard 1-gal ink containers, or that come with their own containers. But since these manual models don't feature follower plates, they won't be able to remove all the ink from a container, which could lead to unnecessary waste. Note that all the pumps mentioned here are piston, not diaphragm, models. Diaphragm pumps are designed primarily to handle low-viscosity materials--not textile screen-printing inks.
Before you invest in a pump, you need to discuss with the manufacturer what pump pressure ratio you require. Stick pumps are 1:1 ratio pumps, meaning that they deliver as much fluid pressure as the air pressure you put into it. In other words, if your air input is 100 psi, the pump will develop a fluid pressure of 100 psi. Depending on the viscosity of the ink you're using, a 1:1 ratio pump may be sufficient. However, some plastisols may require higher fluid pressure to move the ink into the pump and out of the container.
Piston pumps on the single- and dual-post devices are equipped with electric air motors that allow them to deliver varying ratios of fluid pressure. The dual-post pump pictured in Figure 1, for example, is a 44:1 ratio pump capable of 4400 psi fluid pressure--more than enough to effectively move plastisol ink. A rule of thumb is that if the pump has a follower plate, it probably delivers a greater than 1:1 pressure ratio.
The ratio necessary to effectively get your ink from the drum varies not only with the ink viscosity but also the distance you want to move the ink. If you plan to pump the ink 600 ft from the drum to a mixing station, you'll probably need a 44:1 pump. If you're pumping it 300 ft, a 13:1 pump may be sufficient.
What complicates the issue further is the fact that plastisol inks are thixotropic, meaning they flow more easily (have a lower viscosity) as shear or force is exerted on them. Therefore, some amount of guesswork goes into getting a pump that will effectively move your ink. Bob Riethman, ARO sales representative, offers this advice: "If you can't pour [the ink] out of a cup, you need a pump with a follower plate."
While pumps are effective at removing ink from containers, they should not be mistaken for precision dispensing systems. Today's battery of dispensing systems are designed to remove ink from a container, but in specific amounts to guarantee accurate color matching.
If you're printing RFU ink colors right out of the bucket, you can bypass this section and skip to step five. However, if you do any color matching or pigment and base mixing, you may be interested in the dispensing systems now available.
To measure the right amount of ink or pigment for accurate color matching, you have two options: Do it by hand, or use an automated dispensing system. While techniques for ink dispensing and measuring vary, ink manufacturers agree on one thing: A high-quality digital scale is critical for accurate color matching. "Everything else is a nicety," says Steve Horton, president of Plast-O-Meric SP, Inc., Waukesha, WI.
For measuring quart-sized amounts of ink precisely, you'll need a scale that displays measurements from about 0.1-1500 g. For gallon size measurements, a scale with a range of roughly 1-6000 g is appropriate. And for measuring extremely large ink batches, a 10g-20 kg scale or similar is what you'll need. Most complete dispensing systems include scales as part of the package. But if you're measuring and dispensing your inks by hand or have a custom mixing system, your ink distributor can recommend a scale for you (Figure 2).
Once you've determined the formula you need for mixing a color (hopefully you have records, either on a computer or written down somewhere), you need to precisely measure those amounts of ink into a container for mixing. Starting on the low-tech end, many printers use ink scoops and spatulas to get ink into the mixing container. With practice using a good digital scale, you probably can measure relatively accurate amounts of each ink component you need.
Drawbacks of the scoop-and-measure system are obvious: It can be messy, and it's subject to human error. Also, the less ink you're measuring, the more precise you must be.
Precise measurement is particularly important when you're measuring pigment concentrates. Interviewed for a previous article ("Color-Matching Inks," by Tom O. Frecska, Screen Printing magazine, Nov. '95, page 58), Steve Presutto, technical service manager with Coates Screen, East Rutherford, NJ, explained, "Because there's so much more pigment in the concentrate than in the RFU ink, you have to be more critical when weighing. If you're 10% off on the concentrate compared to 10% off on a finished ink, obviously being 10% off on the concentrate is going to affect your color much more."
Many companies have developed automated precision dispensing systems to minimize the margin of error in measuring both RFU inks and pigment concentrates for color matching. While Wilflex is marketing a system designed specifically for plastisol inks, other manufacturers are retrofitting for plastisol use dispensers designed for other applications.
In addition to dispensing ink, many of these computer-driven dispensing systems also include software that stores ink recipes, manages ink inventories, and calculates ink requirements based on ink usage per print. (For more detailed information on these systems, see "Dispensing Systems for Color Matching" below.) Ink experts say that automated dispensing systems aren't economical for most printers. However, the systems are good time-saving equipment for companies mixing large quantities and a broad spectrum of ink colors.
For textile printers looking for a cleaner, more convenient way to measure inks, but who aren't in the market for a large automated precision-dispensing system, Rutland Plastic Technologies, Inc., Pineville, NC, offers the Southswell System for plastisols. The system consists of manually operated 5-gal pumps equipped with a custom control valve and a follower plate (Figure 3).
With the Southswell system, printers can purchase pumps for each of the inks in a color-matching system and set them up on 5-gal buckets arranged on a long shelf. A scale is placed on a flat wheeled cart located on a lower shelf. Printers simply place a bucket on the scale, zero the scale, roll the entire arrangement under the desired ink pumps, and dispense the amount of each ink component they need into the bucket. The operator opens and adjusts a control valve to dispense each ink at a desired rate when the handle is pumped. The user stops pumping when the scale indicates that the correct amount has been added.
This is not as precise as dispensing with a software-driven automated system, and it requires practice to accurately pump out small amounts of ink. However, it certainly is cleaner and less wasteful than using a scoop.
Mixing practices in the inkroom
As mentioned earlier, you may find yourself mixing ink for one of three reasons: to get a color match using a prescribed formula to blend pigments with a base to prepare inks for the press
Whatever your goal, it's important to have the right mixer for the job. Mixer types vary greatly, so it pays to have an idea as to what you need before you make a call to a distributor.
If you're using RFU water-based inks, you may be putting the ink directly from the bucket to the press. While this isn't the worst thing you can do, it's also not the best. "You should always premix your inks," says Frank Blanco, Jr., vice president of sales for Nazdar, Shawnee, KS.
For RFU water-based inks, this can be with a spatula or stir stick. For large quantities, you can purchase a mixer that essentially consists of a motorized propeller blade that fits into an ink container. Indco and U.S. Best, New Albany, IN, manufacture a variety of such mixers. Make certain you properly affix any such mixer to the side of the ink container to prevent the blades from cutting though the container sides.
If you mix your own water-based ink using pigments and bases, you must have a specialized, high-speed mixer called a homogenizer. Homogenizers thoroughly blend pigments into bases by pulling the ink up through the mixing blade and then pushing it back out into the container. If you are using a pigment concentrate water-based ink system, ask your ink distributor to refer you to a homogenizer manufacturer.
Plastisol inks require a different type of mixer. Because they cure at a lower temperature than water-based inks, they must be mixed at a very slow speed to prevent the ink from overheating and gelling on the mixing blades. For this reason, you should avoid using propeller-type mixers, which generally operate at much higher RPMs than plastisols can withstand. Even if you don't actually gel the ink, the heat generated by propeller-type mixers can damage the inks' rheological characteristics.
Some mixers are specifically designed for low-RPM mixing of plastisols, such as the M&R Turnabout and A.W.T. World Trade Tornado (Figure 4). Both are slow-turn mixers--turning at less than 50 rpm--that effectively blend colors with both RFU and pigment-concentrate ink systems. You can mix plastisols by hand, but because of the high viscosity, you really should have a plastisol mixer for mixing quantities of more than 1 gal. The Tornado accommodates container sizes from 1 qt-5 gal, and the Turnabout holds 1- to 5-gal containers.
If you want the first print to look as good as the 20th print, you must shear or prepare plastisols for the press. Plastisols change viscosity under pressure, so it's better to initiate that viscosity change before you put the inks on press. If you're pumping RFU plastisols directly onto the press, you may be able to skip the mixing step, since the pressure of moving the ink through a small orifice shears the ink. However, you'll find that many plastisols, whites in particular, must be mixed in order to get them press-ready; whites have a "warm-up" time of 30-45 min before they will print smoothly.
Many tools exist to make inkroom processes quicker, cleaner, and easier. If you're wasting ink by leaving it on the sides of drums and in your technician's hair, it may pay to investigate ink pumps. Whether it is economical to spend money on a dispensing system depends on the types of jobs you print and the quantity of ink you use. The real Golden Rules, however, are few and simple: Stock up on high-quality, stainless steel scoops, stirrers and spatulas Maintain records and samples of all your color mixes Do all your ink color matches with a scale, whether dispensing ink manually or with a dispensing machine Don't mix plastisols with a high RPM mixer Install good lighting Keep your shop clean and organized
With planning and the implementation of some of the above tips, you can choose the right tools to eliminate waste and mistakes, and keep your inks flowing smoothly to the press.
If you've been looking for a system to automatically dispense precise amounts of ink components, you're in luck. Such systems now exist both for RFU and pigment concentrate plastisol inks.
Wilflex's PCMaster is an automated, software-driven dispensing unit designed to minimize the margin of error in measuring pigment concentrates. The operator types a Pantone color and a quantity into a key pad, and the machine assembles a color match using 14 pigments (including black and white) and selected bases. The machine dispenses pigment in small squirts into a bucket until the scale below registers that the appropriate quantity has been dispensed. The PCMaster, which lists for $85,000, assembles ColorMaster ink components within 0.01 g accuracy.
Wilflex's DispenseMaster accomplishes a similar purpose with either RFU plastisols or pigment concentrates and lists for $150,000-180,000 depending on the options selected. It measures ink from 1 qt to 5 gal with 0.1 g accuracy. The DispenseMaster uses Wilflex's ColorMaster pigment concentrate series or MatchMaker RFU color-matching inks and can be programmed to dispense all the ink you'll need for an entire day's printing jobs.
This leads to another factor to consider when purchasing an ink dispensing system for plastisols: For mixing accuracy, most electronic dispensing systems are equipped with software that calculates formulas based on a certain brand of ink. In such cases, the user must use only that brand of ink for the formulas to work correctly.
The above dispensing systems were specifically designed for plastisol inks, which are of a much higher viscosity than inks used for applications other than screen printing. Several companies offer precision dispensing equipment that operates on the same basic principles, but most were designed for dispensing other types of materials.
The On-Color ink metering and dispensing system offered by Willamette Valley Company, Eugene, OR, for example, was not made to dispense plastisols, but General Manager Eldon Owen says the system has been tested and approved for use with plastisol inks. Likewise, Precision Dispensing, West Chester, PA, features the Paste Ink Dispensing System, which is designed to effectively dispense highly viscous materials, like plastisols. If you are considering the purchase of a precision dispensing system, you need to ask the manufacturer if the system has been approved for plastisol use, specifically to handle the ink's high viscosity and pigment particle size.
No manufacturer has yet designed an automatic precision dispensing unit for water-based inks, but dispensing equipment manufacturers may be able to modify an existing system to work with these formulations. Any dispensing system you'd use for water-based inks must have stainless steel components, or you'll have rusted parts to deal with.
You may discover a company that offers a dispensing system that works using flow meters. These are "volumetric" systems, and according to Michael Dempsey, sales manager for Precision Dispensing, you don't want this kind. Volumetric systems measure dispensed ink based on how much flows through the dispensing nozzle. Because textile inks vary greatly in viscosity, such a system requires a tremendous about of calibration and maintenance. Make certain that any dispensing unit you consider is a gravimetric system-one that relies on weight to ensure accurate dispensing.
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