Printer's Guidebook, Part VIII
Ink, continued
27. Adhesive properties
Those characteristics of an ink that enable it to adhere-mechanically or
chemically-to a substrate.
Most textile inks adhere mechanically rather than chemically. This means the ink is deposited onto and into a highly irregular surface-the garment fabric-that has considerable surface area and texture to which it can adhere. When the ink is properly cured or fused, it can't be pulled off because it is physically, mechanically hanging onto the individual fibers. Still, it never becomes an integral part of those fibers. In the case of nylon inks, cross-linking catalysts are added in sufficient quantities to chemically tie the ink to the surface of the nylon, at the molecular level. This is necessary not only because nylons are woven rather than knit, but because their individual fibers are smooth-little or no texture. You could probably adhere mechanically to fabric that was knitted from nylon threads, or woven fabrics of natural fibers such as cotton. The combination, though, of the synthetic and the weave presents too great a challenge for mechanical adhesion, and a chemical bond is required. The bulk of mechanical adhesion develops when the ink surpasses 110°F. It is at its lowest viscosity point at this temperature. Whether you push it with a blade, cure it through the dryer, hit with a flash unit or simply do your printing in a Phoenix garage with no air conditioning in July, at some point the ink surpasses 110°F and, at that low viscosity point, mechanical adhesion is achieved. An ink's adhesive properties come into particular play in this industry when we're printing on top of a flash-cured underbase. If, for example, you take your underbase to the point of total fusion underneath the flash, then let it cool and try to print over the top of it, you'll will have to duplicate the amount of fusion in your over-printed color in order for the two inks to adhere to each other. Successful "intercoat adhesion," therefore, relies on not compromising your underbase's adhesive characteristics by over-flashing. |
28. Dry/cure rates
Typically expressed as a function of time and temperature, those conditions
necessary to dry/cure textile inks.
At one-thousand degrees Fahrenheit, you could fuse an ink in less than a second. Conversely, at 200°F, you could get most plastisols to remelt, given adequate duration. Over time, the industry has developed a standard, of sorts, which balances the curing characteristics of plastisol with the demands of production and the equipment available. The result is the frequently referred to "300-350°F" range at which plastisol is said to cure. There are three temperature levels in a plastisol's curing cycle that are important to know: gel, fusion and remelt. All three are expressed as given temperatures at given times. This information may be obtained from the manufacturers. It may also be very practical for, as you begin to appreciate the manufacturers' "lab-oven" conditions and the curing parameters they yield, you can use the three numbers to draw correlations to what you can use in your plant. The three curing stages are rarely linear. Thus, you can have inks with a high gel and a low fusion temperature, or those with a low gel and a high fusion temperature. In the case of heat-transfer printing, for example, you want an awful lot of distance between the gel and the fusion points. But when direct-printing, you only need some distance there-so you can take an ink to gel, in the case of an underbase, without risking a full cure; it is unnecessary that it be as wide. Additionally, when printing an underbase, you want it to gel at a very low temperature, very rapidly; at that point, you want the resin to have consumed a great deal of plasticizer which will minimize after-flash tack. Despite the fact that curing specifications are typically offered in terms of time and temperature, the ink manufacturer should technically only provide a temperature specification. This is because, for a plastisol to fuse, it must reach a given temperature. Specifically how long it takes to reach that temperature, though, may be affected by many factors such as the type and efficiency of the drying equipment, the load placed on that equipment, and other ambient conditions that prevail in the shop. Assume, then, that the ink manufacturer is not an expert on curing equipment and that the equipment manufacturer knows little about ink. This being the case, the ink manufacturer should provide a temperature spec only (it's your job to achieve that temperature). Conversely, the equipment (dryer) manufacturer should offer: "Given a certain mass (weight) of material to heat, it's going to take such-and-such retention time to reach so-and-so temperature." For example, the real fusion temperature may be 260°F, but it would be totally impractical to run at that level, and wait for cure to be achieved. It would take 10 minutes and you'd need an 80-foot dryer. Thus, the manufacturing community determines what's reasonable, what printers can assimilate and have confidence in, and they express their specifications accordingly. |
29. Blade durometer
The measure of a squeegee blade's resistance to penetration or compressibility.
The durometer of a squeegee blade is a value that reflects the physical hardness, strength or rigidity of the blade material and is based on resistance to penetration (or compressibility).
Blade durometer values range from 50 to 95, and it is generally agreed that 60 refers to a soft blade, 70 to a medium and 80 and higher to hard blades. As long as all blades are the same size and shape there is some validity to durometer as a comparison, but blade height and spring rate are much more important to the screen printer. Spring rate and blade height The extent to which the blade material extends beyond the blade holder has a significant bearing on how much it will bend; that bending affects blade pressure and ink deposit. A durometer reading only gives you a measure of the strength of the material that the blade is made of. If we take the durometer reading on a standard 2" X 3/8" polyurethane blade and a block of polyurethane 2 inches by 1 inch thick, we may get the same durometer reading, but we know that a 1"-thick blade will behave very differently from one 3/8" thick. The 1" blade will be stiffer (will have a lower spring rate) and offer greater resistance to other forces such as ink and screen tension. The squeegee blade is a cantilever beam sticking out of a handle. A cantilever beam is supported on one end only and projects out into space. A diving board is an excellent example of a cantilever beam. It is securely fastened on one end and a load (the diver) can be supported on the free end. We know from experience what happens when we apply a load to the end of a cantilever: it bends. Engineers refer to the bending of a beam as deflection. (A tension meter measures the deflection of the "beam" of mesh between opposite frame members). The effect of removing (as if with a blade sharpener) 10 percent of the free blade in effect doubles the resistance, which affects blade pressure and deposit. Thus, if all the blades in your shop are different heights, their relative durometers have no meaning. Color-coded It is very hard to tell one durometer blade from another without a durometer gauge, especially when they are covered with ink. So most manufacturers color-code their synthetic blades by durometer. Put a coded chart on the wall and anyone in the hop should be able to memorize the hardness of each color. While you're printing, if you are using an orange blade and you want less deposit or a sharper image, switch to a blue (harder) blade. You can also tell from across the room what durometer blade (plus or minus five percent) someone is using. When you see the new guy trying to print white ink with a soft (orange) blade, you can identify it from a distance without getting your fingers dirty, and correct the situation. Blade material The least expensive blades available are those constructed of natural rubber. While used extensively in the screen-printing industry due to their low cost, rubber blades tend to suffer from poor abrasion resistance and susceptibility to strong solvents. Neoprene, a synthetic rubber compound, is also a popular blade material. While neoprene is slightly more expensive than natural rubber, it offers better chemical and abrasion resistance. Urethane, a synthetic plastic material, is often used to make blades designed for extended use, and for automatic and semi-automatic equipment. While urethane is more expensive than rubber or neoprene, it offers a much better resistance to both physical and chemical abrasion. Deposit and ink resistance Typically, the durometer will be directly determined by the substrate and the screen mesh. The blade durometer directly affects the way the ink is deposited. By changing blade durometer, one can increase or decrease ink deposited on the substrate. A soft blade will deposit more ink than a harder blade. A softer blade will bend substantially during the print stroke. This bending causes more downward (rather than lateral) force on the ink, resulting in a greater deposit on the substrate. A harder blade produces results opposite those of a soft one. As force is exerted on the blade, the material bends very little, so less force is exerted on the ink, resulting in a lesser ink deposit. The goal of many blade manufacturers has been to combine various of these characteristics into a single squeegee blade. Combination durometers One of the characteristics common to all blades is the constant value of the durometer. In other words, a 70 durometer blade has an equal hardness throughout its length and width. While this is suitable for most applications, certain jobs require blades with special printing features. The first effort to combine the positive characteristics of two different durometers was performed by graphics printers and was a soft polyurethane blade tip glued to a rigid fiberglass board. The fiberglass kept the blade from bending, producing excellent ink shear, while the soft polyurethane edge laid down a thick layer of ink. Unfortunately, these blades are much more expensive than conventional urethane blades and can't be sharpened much. For years many printers have also used a thin piece of metal to support the softer blade like a brace. Another method is laminating two pieces of polyurethane together to form a single blade. One side of the blade is a hard piece of polyurethane, the other a softer piece. This combination, called a dual-durometer blade, produces results similar to those of the fiberglass backed blade, but at a lower cost. Since the blade is made entirely of polyurethane, it can be cut and sharpened like a single-durometer blade. Another dual-durometer blade is known as a composite blade. It has a hard blade material with softer material at the tip. Then there are the popular triple-ply, double-durometer blades, simply two softer layers sandwiched around a harder layer. Again, the purpose here is to provide a bend-resistant blade without sacrificing low-durometer characteristics. Resistance to wear The printing equipment itself can also affect blade choice. Hard-durometer blades are normally recommended for use on high-speed automatic presses due to the high degree of abrasion that occurs during a production run. Softer durometer blades are typically preferred for low-pressure, low-speed manual and semi-automatic runs. |
30. Blade shape
The cross-sectional profile of a squeegee blade has considerable affect
on ink deposit and image quality.
Changing the profile of the squeegee blade is a brute-force method of changing your ink deposit, as the shape of the blade affects both the amount of ink deposited and the crispness or clarity of the image.
A variety of blade profiles are available with the most common being A and B.
The square edge is the most accurate and the most common, with the standard size being 2" X 3/8". Sharp, square-edged blades deposit the smallest amount of ink with the greatest image clarity. With a small area of contact between blade and screen, this edge scrapes the screen clean of ink, like a knife scraping butter off toast. (Use the back of a spoon to spread butter, and you'll get a much, thicker deposit.) A sharp blade not only wipes the screen, but pushes excess ink along in front of itself (rather than through the stencil) which produces the sharpest results-very important with fine lines, halftones and process work. Beveled blades are typically used for printing rounded surfaces where fine definition is required. While double-sided beveled blades are more efficient on high-speed automatic machines, single-sided beveled blades produce excellent results when printing heavy solids. Along with round blades, they are typically used for puffs and other novelty inks where excessive deposit is needed and there aren't fine lines or halftones in the design. Rounded-edge blades improve coverage in open areas by allowing a small but significant amount of ink to roll under the rounded edge, rather than before it. The extra angle of attack provided by these edges drives extra ink through the mesh but with a loss of fine-line control. This blade still cleans excess ink from the screen and pushes most of the excess ahead, but does so less efficiently. In addition, the rounded edge of the blade makes contact with a greater area of the screen for a slightly longer period of time, tending to increase build-up. If the other blade variables of speed, angle and pressure stay the same, the round edge will deposit more ink onto the surface than the sharp-edged blade. The blade edge should come in contact with (bridge) at least two threads of mesh at all times. If a blade's edge is too sharp for the mesh it will bounce up and down into and out of the valleys of the mesh and sound like a needle dragged across a phonograph record with a high-pitched scrape. With the proper blade, you should hear little or nothing. If the blade is too round, it will surf up on top of the ink and won't push the ink through the mesh. It can sometimes help to dull the edge until it better suits the mesh on which you are printing. Molded vs poured Most blade material is poured or extruded, then cut into strips. Others blades are molded, one by one. Being from a mold, they have a sealed edge that resists abrasion and solvent penetration. Because of this molded edge, excellent printing results and long life may be expected. No wear means no sharpening, and no sharpening means every blade height is the same, making set-ups much easier. Sharpening also opens up the pores of the material, making it susceptible to solvents. Maintenance and storage When printing with inks that contain aggressive solvents, each side of the blade should be used for no more than two hours. Excessive swelling or softening can result from printing with the same blade edge for too long. If ink residue begins to build up on the inside of the screen, the blade should be replaced with a new one. If a blade has absorbed solvents, it will usually recover after 24 hours and can be returned to the printing press. Check to see if the swelling has damaged the blade edge and resharpen if necessary. Polyurethane blades should be stored in a dry and relatively cool area (60-70°F), lying flat. Occasionally, polyurethane blades stored at temperatures below 60 degrees will increase in hardness. This will not, however, affect blade performance or life expectancy. A dull or nicked blade will not allow the ink to transfer evenly through the screen. If the inside of the screen is wet with ink residue, the blade is not performing as well as it should. Wear & tear A number of factors affect blade performance. Abrasion against the screen slowly dulls the blade, reducing its ability to shear the ink from the screen. Harsh solvents will also degrade the blade. Solvent-based inks and UV-curing inks can drastically limit the life expectancy of a blade, often causing it to swell or soften on the press. All these factors can ultimately affect print quality. Sharpening Resharpening can be performed with a belt sander, a high-speed grinding wheel or, ideally, a squeegee-blade sharpener specifically designed for the purpose. If too much pressure is applied to the grinding wheel, however, the blade edge will begin to melt due to excess friction. When sharpening, remove as little material as possible over several passes. When using a belt grinder, the belt should be approximately 200 to 300 grit, with a belt speed of 40 to 60 feet per second. To gain a smooth edge, the blade should be moved perpendicular to the belt with a slow constant motion. The blade should never be held stationary under the grinding wheel. |
31. Blade angle
A change in this variable affects the amount of ink deposited.
Blade angle is more complicated than just the setting you make on your automatic press or with your wrist if printing manually. Blade angle determines the angle of attack at which the squeegee blade forces the ink across and down into the mesh and stencil, filling them with ink. It is an interdependence of many variables that finally effect the true angle of attack: the shape of the blade at the tip where it meets the screen, and the only angle that really matters. The true angle of attack is difficult to measure and hard to see because it is always obscured by the roll of ink in front of the blade. It is critical though, and must be accounted for.
The enemy of angle of attack is too much pressure. The novice printer sees a result from adding or removing pressure (see Variable 33: BLADE PRESSURE) but doesn't see the other affects. Increasing or decreasing the pressure does not, in fact, affect ink deposit. Ink deposit is affected by a change in angle at the blade tip. In the worst scenario, the blade tip actually bends into a banana shape, because it touches the platen or press bed. To avoid this, set the angle first, then re-adjust the blade pressure so the blade tip gently brings the stencil in momentary contact with the substrate. Pressure is very difficult to duplicate, while blade angle is easy to measure and repeat. Final angle of attack is also determined by blade height, profile and durometer, blade pressure and mesh tension. Usually invisible to your eye are the changes the blade undergoes from too much bending and overexposure to solvents which can change the spring rate of blade materials. Input settings Many presses allow the press operator to set the blade angle. This adjustment changes the amount of downward force applied to mesh and ink. When the blade angle is increased from 20° to 35° (measured from the vertical) this will increase the ink deposit due to the additional downward force applied to the ink. This additional downward force is a function of the ink contact area as the ink tumbles in front of the blade. The smaller the angle, the smaller the contact area; the greater the angle, the greater the contact area. Experience has taught us that the ideal angle of attack is 60° to 70°, which is measured as an angle by itself, rather than compared to a vertical axis like the angle we set on the press. For example, when printing white ink on a black substrate, the ink may appear gray in color if insufficient ink is deposited. If the press operator increases blade angle slightly, and re-adjusts the blade pressure accordingly, the ink deposit may be increased sufficiently to appear white on the black substrate. (The disadvantage here is the need to adjust blade pressure-very easy to do with instant results even while the press is printing-after changing the blade angle-usually requiring tools on an automatic press-which is not as easy to do.) |
32. Stroke speed
Balanced against ink and screen variables, stroke speed affects the efficiency
of ink transfer.
Obviously, stroke speed has a lot to do with how fast we can print. The speed at which the stroke can be maintained, though, is largely dependent on the ink and screen selected. A slower stroke speed gives ink more time to flow through image-area openings in the stencil. When the stroke speed is fast, it allows less time for ink to flow through stencil openings and, therefore, less ink is deposited. This is significantly affected by the viscosity or flow factor of the ink. For example, with a high-viscosity (thick) ink, the speed will have less affect on deposit than with a low-viscosity (thin) ink. Understand that some new inks, formulated for high-tension mesh or special applications, may respond differently. (Union's Tru-Tone process colors, for example, deposit more ink when you stroke fast and less when you stroke slow. They are very sensitive to stroke speed which can make life frustrating for air driven heads that don't have repeatable instruments or controls for speed.) All inks resist flow. If the ink hasn't time enough to flow onto the substrate, it won't print. If the blade moves too fast, we reach the limits of the ink's rheological range and it won't print. On the other end of this range, a blade pass that's too slow creates other problems related to the ink's thixotropic characteristics. When it is not under shear, the ink will change viscosity, making it harder for the ink to separate from the mesh. We must, therefore, determine what stroke speed gives us the desired print. Stroke speed is measured in inches per second, yet few presses have gauges to provide feedback for repeatability. To monitor your stroke speed under such circumstances, first measure the length of the stroke on your press. Set the stroke speed to 10 (which generally represents 100 percent of the voltage-usually 90 volts on a 100-volt motor because more than 90 volts burns out the brushes), then measure with a stopwatch how long it takes for the head to cycle, or for the blade to land on the shirt and stroke across. Now turn the dial to the lowest setting and time that stroke speed. Check as many points as you like along the dial and record your observations. Now decide if you want to use a "stroke time" which is practical on a fixed-stroke-length press, or calculate the inches per second by dividing the inches traveled by the number of seconds it takes to stroke. For example, a stroke of three seconds that spans 22 inches indicates a stroke speed of seven inches per second. You can also attach a volt meter to the circuit and note the relationship between the voltage and seven inches per second. |
1997 National Business Media, Inc.
This image (courtesy Fimor/Euroscreen)
shows two different durometers of the popular dual-durometer, triple-ply
blade configuration.
