Printer's Guidebook, Part XII
Curing/Drying
48. Curing temperature
Textile inks must reach a certain specific temperature in order to be permanently
cured to the garment.
You must adequately heat the resin in an ink to achieve a cured ink film that will resist laundering and normal wear and tear. Solvent-based inks use heat to first evaporate their solvent, then to heat the resin which makes the ink stick to the shirt. Instead of an evaporative solvent, plastisol ink uses a plasticizer to make it flow. When heat is applied, the resins-made of polyvinyl chloride (PVC)-dissolve into the plasticizer. When it cools, the ink film is flexible, but solid and durable.
Five stages of cure For both plastisol and water-based inks, much of the curing challenge is the same. The systems do, though, diverge, as cure nears completion. Liquid: At room temperature, the resin particles and plasticizers-or solvents-are dispersed evenly and the ink can flow under the control of your squeegee blade through the screen. Pre-gelation: As the temperature is raised, the PVC resins in plastisol begin to absorb plasticizer; in the case of water- and oil-based inks, the solvents begin to evaporate. At this time, the ink is very unstable and would crumble if you touched it, since it has not yet fused. Gel: At 155°F, water-based inks stop raising in temperature until all the water is gone. At 180°F, the major difference between these two ink types is that in one, the solvent is gone and, in the other, the PVC resins have absorbed all the plasticizer. Although the resin particles begin to touch, this condition will vary up or down, depending on the resin type and plasticizer level. Partial fusion: At 280°F, the molecules of plasticizer begin to work their way between the polymer molecular chains, and fusion begins (plastisol only; see below). If the ink were cooled at this temperature, you would have a proper film, but with minimal physical strength. Fusion: Full fusion (full strength) of a plastisol deposit is accomplished when the plastic film is brought to approximately 325°F before cooling. The important step for water- and solvent-based inks is the initial rise in temperature-at the gel stage-that drives off the solvent (water or oil) so the resin can fuse. Often, simply raising the temperature won't remove sufficient solvent; thus, the air must be continually changed in the chamber, as well, for best drying results. Infrared (IR) panels by themselves are not very good at driving off water; convection-type dryers work best. Water in the shirt also slows down the total time it takes to achieve cure, because all the water must be removed first, to get that shirt above 155°F. Unfortunately, screen printing inks are very poor conductors of thermal energy. The result is that temperature differences can occur through the ink film that create a weakness in the film strength at lower levels where the ink actually attaches to the shirt (see illustration with Variable 50). The experience gained in the design of textile-based curing systems demonstrates that the fastest, most efficient technology for raising the temperature of an ink film is from within the ink film itself, through radiation from IR panels. Absorbed/reflected heat Your dryer generates heat to fuse the ink onto the shirts. Like a house furnace, it generates heat on demand which is controlled by a thermostat. Too much heat and your shirt will scorch. Pastel and white nylon will discolor at 315°F and scorch at 330°F. Most fabrics like cotton and poly/cotton blends start to scorch at 350°F. Dyed fabrics also start to sublimate their dye above 350°F, which causes discoloration of light ink, especially if printed on dark garments. These conditions create a narrow band of allowable heat: from 300 to 340°F. We use controllers of one sort or another to monitor the heat source, but since each job is different in terms of the shirt, its moisture content and the ink, we have to measure temperature right on the shirts. First, the chamber has to be controlled with a thermostat, because shirts steal heat from the chamber and don't return it. As you send shirts through the chamber, the heat source in the dryer has to make up for the heat the shirts take away. This make-up heat represents the actual condition of a production run-you should test ink fusion only under such conditions. Measuring temperature The best way to set belt speed and temperature is to adjust the belt to the slowest speed that will still keep up with your presses. Then use a donut-type or infrared sensor to set the temperature according to that belt speed. This will mean a savings in utility bills, because you can lower the heat and enjoy more appropriate micron wavelengths (see Variable 49: PANEL INTENSITY). Wasting too much heat on shirts raises your utility bills more than the cost of a temperature sensor. The slowest belt speed also ensures the most even temperature through the chamber, eliminating wild temperature differences. Thick deposits of white or metallic inks are extra trouble because they reflect infrared light, undermining the efficiency by which the thermal energy reaches the surface of the shirt. Different shirt characteristics-such as fabric weight, color and moisture content (see Variables 38-41)-alter processing parameters. Although extra trips through a 270°F dryer will never fuse an ink that must reach 320°, extra trips to remove moisture will have a cumulative effect. It is important that the entire ink film reach the critical temperature, especially where the ink actually touches the shirt. If, for example, T-shirt temperature only reaches 270°F, the point at which the ink touches the shirt cannot be much more than that-not enough for good film strength. You must check the shirt and ink temperature regularly to see if you are running your dryer speed and temperature settings correctly. When you test it with only one shirt, you are only testing a singular situation-not the condition of regular production. You must wait until you have the machine running for quite a few minutes and the cabinet has become accustomed to the load of numerous shirts. Thermometer basics Thermometers are used to measure temperature. The most critical measurement in any shop is the temperature change of shirts going through the dryer or oven. To measure the temperature of an object, you take a sample of that temperature, by bringing the thermometer into contact with the object you want to test. All thermometers respond to the temperature around them. They indicate the temperature that they themselves reach, which is supposed to tell you the temperature of that which you wish to measure. A thermometer will match the temperature of an object with which it is in contact because, when two different objects are placed in contact with each other, the hotter object will transfer heat to the colder until both are at the same temperature. When a nurse puts a thermometer under your tongue, your body is hotter than the thermometer (hopefully), and your mouth heats the thermometer until its temperature is equal to that of your mouth. When there isn't any more change, a state of equilibrium has been reached. If the thermometer is hotter than your mouth (perhaps you have dipped it in the soup your mother brought you?) the thermometer will try to heat up your mouth, but the result will be negligible, because your body is able to absorb all the heat the thermometer has, with very little change to its own temperature. Additionally, the nurse leaves the thermometer in your mouth for five minutes to make sure it completely duplicates your body temperature. Sealed liquid-in-glass thermometers have been used for this purpose for years because they're simple to read and inexpensive. Unfortunately, their five-minute response time is too slow for industrial use. (I use one only when I calibrate the temperature and response time of my other thermometers.) The best thermometers for industry respond quickly. They have to indicate changes between normal shop temperature and the 300-350°F peak temperature in the dryer chamber. Temperature tapes A popular, disposable indicator is the temperature "tape." Made from two layers of clear plastic, with a chemical sandwiched inside and an adhesive backing, they're easy to use, and provide a permanent record of the peak temperature in the dryer. The most popular is a tape that offers five temperature samples (identified on its surface). This allows you to see how high or low the temperature is, relative to your aim point. You stick the tape on a shirt and send it down the dryer belt. The chemical patch is calibrated to react within one second to a specific temperature-the reaction changes its color from white to black. People who say temperature tapes are not accurate can't support their claims. Such a chemical change is very reliable. Unfortunately, our use of the tapes is not. They are an excellent measure of the air temperature in the dryer, but unreliable as a measure of shirt temperature. They are designed to be used directly on a heat source (such as an automobile engine or electric motor) that itself generates heat, or an object (such as metal) that isn't effected by coolants that may be absorbed in the object. Moisture in the shirt, though, will affect the speed at which a shirt heats up, because the water acts as a coolant. The thermal energy is spent evaporating the moisture instead of heating up the shirt and ink. Both water-based and plastisol inks suffer the same problem, but in different ways. Plastisol ink must reach ±300°F to fuse the ink to the shirt. Water-based ink needs all the water evaporated first, before its resins can heat up and fuse to the shirt. The problem is determining whether the shirt, ink, cabinet air and temperature tape are all at the same temperature-and there's absolutely no reason to believe they'll all heat up at the same rate. Imagine what would happen if you put a temperature tape on an ice cube, and ran it through the dryer. This may be an extreme example, but what happens is that the tape reaches 300°F (as it will prove to you when it exits the dryer) before the ice melts. Try it. Naturally, you won't be printing plastisol on ice cubes; the thing to remember is that different objects will heat up at different rates. To be safe, we want shirts to stay in the oven for as long as possible to ensure that they're completely "well done"-"medium rare" won't cut it. Older versions of Flexible Products' Textile User's Manual, before 1990, have a diagram of how to determine if your shirt is truly "baked" through-and-through using three tapes: one on the shirt's surface next to the print, one on the inside of the shirt under the print to see how much heat got to the bottom of the ink film where it adheres to the shirt, and one on the back of the shirt, just to see how much heat actually got down there. My favorite tip is to cut your tapes in half down the center, giving you twice as many tapes. It doesn't affect the measurement and doubles your spending money. Transducers, thermocouples, RTDs For the constant monitoring of industrial devices such as dryers and flash-cure units, manufacturers control the heat temperature with transducers-the fancy name for devices that respond to changes in temperature with changes in electrical voltage. When they're wired to a volt meter that is calibrated for that device, they become an electric thermometer. Modern nurses use transducers when they take your temperature with a probe they put under your tongue, hooked up to a digital readout. These devices can give you an accurate temperature reading almost instantaneously. The most common industrial thermometer is the thermocouple. These are made from wires of two different metals (commonly iron and copper-nickel) joined together at one end and attached to a voltage-measuring device at the other. The voltage change is created by the metals' relative reaction to temperature, and the difference can accurately report that temperature change. Much more expensive, but much more accurate, are resistance thermometer detectors, or RTDs. The best are made of 99.995 percent pure platinum, so you only find them in the best equipment. Once calibrated, an RTD's change in electrical resistance is related to temperature. You'll find thermocouples in most drying units. It's not enough to know the temperature, though; knowing where these thermocouples are located in the unit is critical to understanding what the readout means. The dryer or flash-cure temperature controller is wired to a thermocouple which can be inserted in the IR panel, in the airflow of a convection dryer, or below the panels near where the shirts pass-each location will present a different temperature reading. Inside the panel will be the hottest, where temperatures of 900°F are common, but this doesn't tell you what temperature the shirt will be. Airflow-temperature measurement is very uniform and a probe below the panels will tell you the air temperature in the cabinet. But let me ask you: What is the temperature of a bottle of champagne you just moved from the closet into a 42°F refrigerator? Also consider the vast temperature and moisture differences between heavy black sweatshirts and lightweight white T-shirts. On-contact thermometers So, back to the shirts again. Every thermometer manufacturer sells a probe attached to a digital readout. These are better, but the problem is that we can't stick our arm into the 300° dryer and test the shirts, so we test them immediately as they exit the dryer. When the shirt comes out of the dryer at (we hope) 320°F, the shock of the shop's 75°F air cools the shirt far faster than any contact probe can go from that 75°F up to the shirt's exit temperature. So, as the shirt cools and the probe heats up, they meet somewhere in the middle-not a reliable measurement. Not long ago, a technical-instruments company collaborated with an ink company to develop a donut-shaped thermocouple which plugs into one of their standard hand-held digital readouts. The donut is a thick white Teflon ring with two tiny wire cross-hairs stretched across one of its faces. The tiny wires are very responsive and match the temperature of whatever they're in contact with almost instantly. If you place the probe with the wires directly on a shirt fabric or ink patch, the readout will tell you their temperature. If you place the donut with the wires up, the readout will tell you the temperature of the air nearby. The donut is connected to the digital readout with a 15-foot cable (optional cables of greater length are available). The manual supplied with the donut goes through every step of profiling dryer and shirt-curing requirements. The donut is sometimes clumsy to use because you have to deal with the 15 feet of wire and the hot ring every time you want to take a measurement. But because it stays in intimate contact with your product through its fusion cycle, it is reliable, quick and accurate. Traveling through the dryer as it does, there is no better way to profile your dryer's weak and strong points. The other thermometers have their advantages too, but each has a fundamental flaw in its accuracy. Thus, not using a donut means at least some guesswork or halfway measures. Non-contact thermometers The least expensive thermometer I have is a self-contained bi-metal style. It is rugged and reliable, but doesn't react very fast. The bi-metal strip is two different metals bonded together and then wound as a spring. When exposed to heat, one metal expands faster than the other, causing the spring to move. There is a pointer attached to the spring which moves across a scale to indicate temperature. You run the thermometer down the belt, but it will only tell you air temperature, because it never comes in contact with the shirt. The sexiest and most expensive are the non-contact infrared-radiation thermometers. These measure the infrared radiation coming off the shirt and convert it to a temperature measurement. They need constant calibration because each ink, shirt and combination of these emits a different level of radiation. If you don't calibrate for each combination, you won't get an accurate measurement. Once calibrated, though, these are the ideal device to measure whether both ink and shirt reached 320°F. You can aim the device at shirt after shirt immediately as they come out of the dryer, and get your answer in an instant-rather than sampling a single shirt as it travels through the cabinet, then dealing with the tangle of wires. Non-contact probes are perfect for high temperatures where you can't touch the product, but you also have to be able to point the probe at the shirt and ink to read the radiation it gives off. This only happens inside the dryer, where my arms don't reach (another vote for the donut). Thus, when I got my non-contact probe, I built a holder for it that sat right at the end of the dryer, precisely aimed at the shirts as they came out. It was a fabulous success to be able to lower the temperature of the dryer and not waste heat by adding an extra few degrees just to make certain each shirt was cured! |
49. Infrared-panel intensity
Must be assessed in terms of wavelength, temperature and watt density.
Infrared (IR) panels emit invisible electromagnetic waves that travel at the speed of light. They are similar to X-rays, visible light, and radio waves in that they can have different wavelengths within their range or category. The waves radiate from the IR panel depending on its construction and how much energy is put into it. (Such panels don't, incidentally, have to be electric; they can be gas-fired, but such are quite rare in this industry). IR panels radiate heat, which means that heat energy is transferred through space. Unlike conduction (heat through a solid) or convection (heat through a fluid-air, in this case, is considered a fluid because it flows), infrared radiation doesn't depend on a material to transfer it. Fluids (especially air) do not easily conduct heat because there is so much space between molecules to insulate them. This is why IR ovens are not designed to heat up air. IR radiation is best suited to heating up solids such as shirts, ink, temperature tapes, the conveyor belt or anything else that will absorb it. Water, such as the moisture in inks or shirts, is definitely not on that list. As a fluid, it doesn't absorb IR radiation very well. And, since IR doesn't evaporate moisture well, it shouldn't be used on water-based inks; it is, though, adequate for plastisol. How it works We heat up the panel, it emits IR radiation, the radiation heats up the molecules in the ink and shirt. Each different material absorbs IR radiation in a specific narrow wavelength just like a radio will receive the best reception of a given signal in only one dial position. Different inks and shirts absorb or receive the IR wavelengths differently, but most are receptive to 3.3-3.7 microns. As each different color responds best to a unique wavelength, we could theoretically cure plastisol in record times if we fine-tuned (as with a radio receiver) our IR panel to each specific color. It's not practical, though, to fine-tune our panels to the exact wavelength, because we routinely put different deposit thicknesses of different colors on each shirt. We also carelessly toss them on the belt and the distance from the panel varies slightly from shirt to shirt. Therefore, ink companies recommend a "color-blind" or broad-band wavelength of 3 to 6 microns. Hotter is not necessarily better. To complicate things, each different panel emits a different wavelength at each different temperature. Check with your panel manufacturer for their chart on emitter temperature and wavelength versus temperature. This chart will tell you exactly what panel-temperature range will emit 3 to 6 micron wavelengths. In the chart you can see that, as the watt density increases, so does the temperature, but the wavelength gets shorter. If you use a temperature controller that works like a thermostat and turns off the panel when it reaches a pre-set heat, then turns it on again when it cools, the wavelength so produced will fluctuate wildly. Try to use a controller that maintains a 5-10°F variance. Watt density The power of an infrared panel is measured (for comparison) in terms of watt density. Watt is the power measure and you can compare the output by comparing the number of watts per square inch of panel area. Find out from the manufacturer's label on the panel what the wattage is. Divide the wattage by the number of square inches for watt density. Dig out your old school books and look up the Ohm's law formula you thought you'd never use. Invite your electrician over and, while the dryer is turned off, have him show you how to measure the amperage, voltage and resistance-then calculate existing panel density. I like hooking up an ammeter to the panel circuit to see the electricity go on and off and see how many amps are being used. Panels can burn out just like light bulbs. If you monitor the increase in resistance (shown by an increase in amperage) you can predict the failure of a panel or replace a panel that still "works" because you recognize it is costing more than it should in utility bills. (If only they gave off visible light, everything would be easier.) |
50. Curing duration
Must balance against curing temperature in order to adequately cure ink
without damaging substrate.
Ink manufacturers tell us to combine the elements of time and temperature for proper ink cure. Since everybody has different equipment and prints differently, the ink makers issue instructions for the lowest common denominator, typically: three minutes at 320°F. This is very safe, but not very practical. A fantasy cure would be a very, very hot oven that heats up the shirt very, very fast and then snatches it out just in the nick of time as it gets to 320°F. This also is not very practical, though, as the shirt and the ink don't heat at the same speed, and the heat has to penetrate through the entire ink film for a proper cure. Our curing goal is a requirement of the resin for optimum strength and flexibility, but time is only relevant as a measure of how long it takes for the heat to penetrate the ink film, and heat the shirt underneath the ink. (Think of cooking a pancake from only one side: ink and pancakes need to be cured through and through, unlike a hamburger, where medium-rare is acceptable. If the bottom of the ink film is medium-rare, it will not adhere to the shirt.) The element of time is dependent on the thickness of the film and the power of the dryer. Since ink companies don't know if you are using a candle flame or a blow torch, they play it conservatively when telling you how to cure their ink.
To measure the proper cure, you must measure the time the shirt spends in the chamber when the chamber is at a known stable temperature. Typical chamber temperature is very unstable, because shirts steal heat out of it and this heat must be constantly made up. This is why there is a temperature controller on your dryer. You actually measure the shirt temperature and calibrate it with the eventual shirt temperature (see Variable 41: GARMENT MOISTURE CONTENT). The problem with any temperature-measuring device is determining whether the shirt, ink, cabinet air and, for example, temperature tape are all at the same temperature. There is no reason to believe that they will all heat up at the same rate so, to be safe, we want shirts to stay in the oven for as long as possible to assure that they are completely "well done." Belt speed Adjusting belt speed (rather than actual temperature) can give you exact, instantaneous curing adjustments on the printed product, because the chamber temperature is always fluctuating (unstable), while belt speed (once adjusted) is constant. To measure belt speed, label the side rail in one-foot increments. Place something on the belt and time how long it takes to travel one foot, then multiply that by the number of feet in your chamber. Since most dryer belts are controlled by a DC motor, the speed control is nothing more than a voltage controller that changes the amount of voltage that goes to the motor. You can attach a volt meter and calibrate voltage to time in the chamber. You can also make new labels for the volt meter or speed control that directly read in terms of chamber time. The best cure is made when panels are tuned to a temperature that helps radiate 3.3 to 3.7 micron waves and are about two inches from the substrate. With dryers that have convection, the slowest belt speed is best for the most thorough cure. Check the rise of shirt temperature as it goes through the chamber and don't let it go above 345°F. Ink modification with too much plasticizer can be dangerous because you increase the time needed for the ink to fuse, you change the bleed resistance, the ink may not cure at all, and you may get too much garment penetration, reducing opacity. Many of these quality changes can be tolerated, but fusion is not debatable. I ordered "well done," thank you. |
51. Airborne contamination
The combination of garment lint and adhesive overspray is a deadly one in
the screen-print shop.
In a garment-printing shop, dust and lint are necessary evils. Most of it is lint from spun-cotton shirts. This is the same lint we find in our home clothes dryers when we do the laundry. When we combine all that lint with spray adhesives we use to tack our platens, it's no wonder it causes problems. Lint will stick-with the aid of adhesive-to walls, floors and presses. Yes, it's unsightly, but is also thwarts maintenance and makes us stick to everything. It is also a fire hazard. If the flammable combination of lint and adhesive ignites, it will spread through the shop like wildfire. Good housekeeping Regular brooming or dust-mopping of the walls along with solvent cleaning of the presses will keep the mess down. Some printers actually oil the metal of the presses-as they would a rifle or other machined parts-so ink and dust won't stick to them. I have never had a problem with airborne lint clogging screens. With coarse screens, I've had lint get into the mesh directly from the shirts-electric-shaver style-where the (squeegee) blade literally shaves shirt hairs into the screen. This didn't cause a problem with clogging the screen openings, but I did have problems with streaks in the print. (I've also never had problems with dust contaminating open ink buckets. I've demonstrated this by taking broom sweepings and throwing them into a screen of white ink. Perhaps a chunk would cause a streak that was easily removed from the blade, but otherwise no problems.) Filtration Filters for air blown into and around the room will help clean that air. Common window fans equipped with furnace-style filters are a good start. Remember to regularly clean fans and filters in the heating, cooling and drying equipment you have. If the ventilation is blocked, motors will overheat and dryers won't operate efficiently. Many of the machines with disposable filters can be fitted with permanent, metal filters that are washable. In a busy shop, it is not uncommon to change or clean filters daily. Screen room The screen room is much more critical than the shop at large, in terms of keeping dust under control. Start by adopting the philosophy that if you spend more than one minute with a brush blocking out a screen millimeter by millimeter, you ought to take steps to prevent the dust that causes the holes in the first place. (In fact, avoid committing a sin common to all screen rooms: wasting time. Some screen makers look at pinhole-fixing as a chance to sit down, listen to the radio and look busy, rather than getting some real work done. If yours do, they're costing you money.) Dust control in the screen room can be a challenge, because emulsion is like flypaper. Direct emulsion is a water-soluble plastic-resin adhesive much like a carpenter's wood glue. Try this experiment: take some extra emulsion and spread it thinly on two pieces of smooth, flat wood. Press the wood together and let it dry. Next morning, the wood will be glued together. When you coat screens, you spread this emulsion/adhesive on a screen, then let it dry. If nobody's watching, you dry it by leaning it against a wall (with one end on the floor, so it's closer to the dirt) and aiming a fan at it because you're in a hurry. Down there on the floor, you now have a little dust storm aimed directly at your screen that's coated with glue! Remember, dust in the screen today means pinholes in the stencil tomorrow. (To all screen makers: Next time your bosses tell you they want to have the best printing in the world, tell them you need a screen room with filtered air blowing into the room; this creates positive air pressure-that blows out of the room whenever a door is opened, rather than sucking contamination in-and helps keep dust out of the cracks. The best printing depends on the best screens and the best screens don't have dirt in them.) Dust on positives Clean your positives whenever they need it with a rubber-cement solvent (such as Bestine). Your stencil can't tell the difference between a spot that's part of your image and a piece of dust. If a spot blocks the light of exposure, it will create a hole in your stencil. Dust on the vacuum-frame glass Old ink and emulsion, sticky tape residue and plain old dust all stick to the vacuum-frame glass. The glass on your camera copy board also likes to get dirty. Keep them clean with regular use of a razor blade (in a safe handle) for tape and other solid residue, and graphic-arts glass cleaner for the rest. It's worth it to buy graphic-arts glass cleaner because it doesn't contain dust-attracting ammonia. If you believe you are having to spend too much time cleaning the glass, discover the source of the contamination and take broader action. Capillary-film pinholes If you use capillary film (which is nothing more than emulsion coated, at the factory, on a polyester backing sheet), you can still get pinholes from poor housekeeping. Dirt caught on the surface of the film because you cut it on a dirty table can be removed by wiping it with a "tack rag" common to the auto-body industry for removing dust from car bodies just before spray-painting. The cloth is impregnated with a sticky chemical and the dust likes the rag more than your table or film. |
52. Ambient temperature
People and ink move more easily in a friendly climate.
Affect on personnel Temperature in the shop should always be examined in relation to humidity. Humidity in the shop is affected by changes in printing and screen making, as well as by the weather. The most important (though most ignored) affect temperature has is on the people who do your work. Office and shop workers are more productive when they are comfortable, a term many employers seem unfamiliar with. Comfortable heating in the winter and cooling in the summer are both very important. If your people are distracted by being too hot or too cold, they aren't working well. Affect on production One of the battles we fight constantly is the affect the dryers or ovens have on the temperature in our shops as the day progresses. At 7:00 a.m., we may start at 55°F, but the heat is up to 90°F by the end of the day. This gives us problems because of the affect it has on ink flow and evaporation. Temperature fluctuations in the shop will also affect the relative humidity (see Variable 53: AMBIENT HUMIDITY). Drying rates will change because of the temperature (and, therefore, humidity) and you will see changes in the time it takes for inks and stencils to dry. Affects of changing temperature on ink viscosity are very perplexing, especially since few printers have the equipment to measure ink-flow changes (see Variables 6 and 7). Cold temperature will reduce the ink flow, heat will increase it. If the room temperature changes during the day, so will the ink characteristics. You can control some of this by keeping your ink- and supplies-storage areas at a constant temperature. Epoxy catalysts and mesh adhesives will mix and perform much better if kept at 70-80°F. One thing you must not do is store ink on the floor on cold days. The floor is usually colder than the air and it will only take longer to get the ink moving in the morning. Stirring and keeping your ink in a temperature-controlled room will help, but when you take the ink out to the press, its characteristics will change as the climate in the room does. Thus, it makes sense not only to seek consistency from one department to the other, but from one hour to the next, as the production day progresses. Controlling this variable may not be very practical for some shops, but if you are having problems it will be worth the effort. |
53. Ambient humidity
In terms of drying both stencils and ink, humidity must be controlled.
Air can be compared to an absorbent sponge. A sponge will soak up water until it reaches its capacity, or saturation point. At this point, any additional water will be ignored as fast as it is supplied. Like the sponge, air will continue to absorb moisture only until its saturation point is reached. Air that has a high moisture content to begin with does not have the capacity to absorb moisture from your screens, inks or shirts. This humid condition is common during the hot summer months or near large bodies of water. Air that has little moisture content acts very much like blotting paper placed on a wet spot. This is an excellent condition for drying screens because we want to attract all the water out of the emulsion and into the atmosphere as quickly as possible, for a dry stencil. You can't do this in a humid room. The best condition for screens is dry as a desert (although your screen makers' skin, lips, nostrils and mucous membranes won't like it). To eliminate, stabilize or control humidity in a shop, dehumidify it when needed (or continuously) in the screen room where the humidity is constantly being raised by the drying of screens (see Variable 17: STENCIL MOISTURE CONTENT). Temperature and humidity are very interrelated because a change in temperature will change the relative humidity of the air. When you raise the air temperature, the water content stays the same but, because the air has expanded (as all gasses do when heated), the percentage of moisture-to-air (ie: relative humidity) is reduced. Conversely, if you lower the air temperature, the relative humidity will go up. It is helpful in the shop to keep the relative humidity between 45 and 60 percent. This is partly because wild fluctuations in any area create the worst conditions for printing. Additionally, workers work better in medium-to-dry rather than humid conditions. Their hands and skin won't be covered with perspiration and stick to things, they will think better and won't fatigue as quickly. In closed areas, dry conditions will also reduce condensation and rust on metal parts. Plastisols can absorb moisture which will change the way they print, but that is nothing compared to the effect humidity has on solvent-based or evaporative inks. Because humid air doesn't have the capacity to absorb excess moisture, it will take much longer for such an ink to dry. Printed garments themselves will also absorb moisture from the air which will cause them to take longer to heat up and dry. Paper for transfers will drastically change shape as it absorbs and loses moisture during curing. It will never expand and contract the same way twice (making the paper an inconsistent variable). Putting transfer paper in air-tight plastic bags may do some good in preventing it from taking on moisture, but if it does, the change during the drying cycle will frustrate your registration. Rx Buy a thermometer/hygrometer (such as the one Radio Shack has in its front counter) and keep track of the daily changes in humidity. You will begin to find a correlation between curing time on humid days vs dry days, winter vs summer and so on. If you have a digital hygrometer in your screen room, you can actually watch the humidity go up as water evaporates out of the stencil, then back down again as the dehumidifier condenses the water in the air. The conditioning of the air in the screen room is very important for consistent screens. Start by using an air-conditioner to cool, and remove moisture from the air. This is why people like to hang out in a well-controlled screen room during the summer. The added benefit is the filtration and positive pressure forced into the room that will help keep dust out. |
1997 National Business Media, Inc.
