Understanding the Variables of Plastisol Printing
How ink characteristics and production variables influence one another through the printing process
On the surface (pun intended), garment screen printing seems like a simple process. We slide a squeegee across an ink-covered screen and the ink transfers to the substrate. It would be nice if that was all there was to printing, but unfortunately, the process is a bit more complex. Besides, if screen printing was too easy, everybody would just print their own shirts, and we'd all be out of business!
A number of variables complicate screen printing, regardless of the application. But several of these variables become even more challenging when we're working with plastisol inks on garments.
On the following pages, we'll take a closer look at each of these variables and identify just how they can influence our level of success with plastisols. We'll also consider the ways in which these variables influence each another.
All printing jobs begin with artwork. The nature of our artwork is the basis for every decision we make at each step of the reproduction process. The design may be as simple as large block type or as complex as a process-color or simulated-process halftone image. It may involve a simple single-color graphic on a white T-shirt or a more challenging multicolor plastisol print on a dark garment.
In all these situations, art characteristics have a direct bearing on screen selection. With simpler, lower-resolution designs (e.g., a single-color block-letter logo to be printed on the left front chest of white T-shirts), we have a lot of latitude. We could select anything from a 160- to 305-thread/in. mesh and achieve successful results without giving much thought to mesh-thread diameter or screen tension. But as our images become more complex, it becomes increasingly important to match the performance characteristics of our inks with the right mesh for the job.
Additionally, artwork for garment printing should be designed with the production process in mind. Among other things, the way images will be separated for screenmaking should be given careful attention. With multicolor jobs, we traditionally begin printing with smaller image areas and work our way up to larger images, or we work from the lightest colors to the darkest. These general guidelines are still useful for many jobs, but the wide range of specialty inks used today (high-density inks, gels, glitters, caviar-bead adhesives, etc.) can throw off the sequence.
Specialty inks are typically printed late in the sequence and require flashing of the previous colors over which they will be printed. An artist who understands screen-printing production can design images that take ink placement into consideration so that the need for flashing is minimized and other poten-tial problems are avoided.
Ink rheology is the science of how ink flows and reacts under the stresses of the printing process. Rheology has two fundamental components: viscosity, which refers to the ink's resistance to flow, and elasticity, which applies to its structure or "stickiness." We are most concerned with plastisol's viscosity.
In screen printing with plastisols, the ideal situation is for the ink to have a high initial viscosity that drops quickly under the shear force of the squeegee during the print stroke. Inks that flow more readily when shear force is applied are considered psuedoplastic. The ink should recover a higher viscosity immediately after transferring to prevent flowout on the substrate and maintain image integrity.
In a perfect world, all ink would react this way during printing. But garment screen printers don't live in a perfect world, and inks can perform very differently. Plastisol viscosity can run the gamut, from the consistency of light gravy to the thickness of cold tar. The actual viscosity level will influence other variables discussed later in this piece, such as the amount of screen tension and squeegee force required, and even which squeegee durometer (hardness) should be used.
As a general rule, the greater the opacity of the ink, the higher its viscosity. We can modify the viscosity of our inks by adding reducer; however, only small amounts of reducer should be used or opacity may suffer. Much of the research and product-development work conducted by ink manufacturers is aimed at achieving inks with ideal printing characteristics so that users can avoid modifying the inks.
Ink opacity/substrate color
Textile decorators screen print on every color of fabric imaginable. This single variable makes necessary a full range of ink opacities, which are determined largely by the concentration of pigments within the plastisol formulation. An ink that is suitable for a black shirt is wasted on a white one. Process-color ink, which has the lowest pigment load and opacity, doesn't even show up if we try to print it on a dark shirt. As printers, we need to have a full range of ink opacities available to satisfy the demands of every garment color we print.
Closely related to the opacity issue is the way a printed garment feels. If we are printing on a white T-shirt, today's plastisol inks will allow us to produce a print with the look and the soft hand of a water-based ink. With darker shirts, however, the hand will always be heavier, either because of the need for an underbase or because the opaque inks required for dark-garment printing are just naturally more dense and thick. But the hand on darks has improved a lot, thanks to developments like simulated-process-color and index-color printing, both of which result in less ink buildup than was possible a decade ago.
The composition of the fabric we are printing on has a very big impact on the quality of the print (Figure 1). Ring-spun yarn creates a nap that can result in fibrillation of short yarn fibers into the print. A very densely knit fabric, such as cotton sheeting, accepts a print differently than does a jersey T-shirt. Terrycloth is difficult to print with plastisol unless the tops of the loops have been sheared--and even then it's still not easy.
One of the best ways to "train" customers about what is possible with different materials is to take a good design, print samples of it on a variety of different substrates, then use these samples as a showcase of our capabilities and limitations. It is much better to show a customer what fibrillation looks like and explain why it occurs ahead of time than to have that customer find out unexpectedly after the garments are washed for the first time.
Durometer is a measure of hardness. In textile printing, the common durometer range for squeegees is 60-80 durometer (the lower the value, the softer the blade). Blades at the lower end of this range are commonly used when a heavier ink deposit is required, such as when printing an underbase or other large solid areas. However, if blades are too soft or used with too much pressure, they can deform or bow, which effectively changes the squeegee angle and leads to excessive ink deposit. Additionally, with high-viscosity plastisol inks, soft blades are more likely to ride over the ink on the screen rather than shear it for effective transfer to the substrate.
Harder blades resist the bowing problems associated with soft blades, and they are commonly used to print halftones and other fine details. But blades that are too hard can lead to shearing problems, insufficient ink transfer, and the need for excessive squeegee pressure.
To get the best of both worlds, many screen printers use multidurometer blades, which feature a hard center layer (usually around 90 durometer) and softer (60-70 durometer) outer layers. The rigid center section resists deflection when printing high-viscosity plastisols, while the soft edges provide a more substantial ink deposit than would be possible with a single-durometer hard blade.
A decade or two ago, it was quite common for textile printers to use an arsenal of different squeegee-edge profiles to control the amount of ink deposited. The common profiles included square edge for general use, round edge for heavier deposits (such as puff inks), and "V" edge for very heavy ink deposits (such as those required when printing towels, terry cloth, or similar fabrics). Today, it is hard to find anything but square- edge blades in most shops. The main reason for this is because of improved inks and screen and stencil materials, which provide more predictable control of deposit thickness than altering the squeegee's edge profile.
The angle of the squeegee as it contacts the screen greatly influences the amount of plastisol ink deposited during printing (Figure 2). In general, the greater the angle from vertical, the more ink that will be deposited. However, this effect diminishes once the angle exceeds approximately 45°. At 45° and more, the angle begins to negate the influence of durometer, and the squeegee just ends up riding up and over medium- and high-viscosity inks rather than shearing and transferring them to the substrate.
The speed of the squeegee has a marginal influence on the amount of plastisol ink deposited in garment-printing applications. In general, to increase ink deposit, squeegee speed is slowed down, while to decrease the deposit, the squeegee is sped up. The reason I say that squeegee speed has a marginal influence on ink deposit is that beyond a certain speed, friction between mesh and squeegee will alter the ink's rheology enough so that it can't be sheared cleanly and won't transfer correctly.
For many printers, adjusting squeegee pressure is the primary modification used to overcome the effects of other variables in the process. In most cases, however, adjusting the pressure means increasing it, which can lead to an ink deposit that's too heavy, poor image definition, and a host of other problems.
The best approach is always to keep squeegee pressure as low as possible. The advantages of minimal squeegee pressure are huge: Low pressure makes it easier to hold registration, helps prolong screen life, and prevents us from laying down too much ink. Squeegee pressure should be set at the lowest level that still allows the ink to transfer from the screen completely.
The profile of the floodbar is a moderately influential variable with regards to ink-deposit thickness. Floodbar profile includes not only the edge of the floodbar, which is typically slightly rounded, but also the face of the floodbar, which can range from flat to a slightly scoop shaped. Floodbar profile is best described by the type of ink movement it creates.
Assuming that the floodbar height is set to the proper level (where it just grazes the surface of the stencil), we see two kinds of ink movement, which I call smear and roll. Smear is the most common type and is generally associated with flat floodbars. The floodbar simply spreads the ink out as it moves across the screen. Roll is a more complicated flood effect and is usually brought about by scoop-shaped floodbars, which cause the ink to roll as it's being spread across the screen. This rolling effect tends to be more effective in prefilling the mesh, particularly with thicker stencils.Mike Ukena
Floodbar angle has a similar effect to that of squeegee angle (Figure 2). The greater the angle, the more ink that will prefill the mesh opening and the greater the final ink deposit. However, floodbar angle alone does not change the ink-deposit thickness nearly as much as the squeegee angle will.
Floodbar speed also can affect ink-deposit thickness. Faster flooding speeds result in less prefilling of the mesh and lower printed ink deposits while slower speeds lead to more prefill and a thicker printed deposit. The overall influence of floodbar speed on ink-deposit thickness is much less than that of squeegee speed.
Off-contact distance is a complicated variable in plastisol printing as in all other screen-printing applications. The main reason it is included in this list is because of its relationship to screen tension and other variables, including squeegee pressure, squeegee speed, squeegee angle, and even squeegee durometer.
Screen-tension level generally dictates what the minimum acceptable off-contact distance will be. The lower the tension, the more off-contact that is required in order to ensure that the mesh will release from the substrate immediately after the squeegee passes. But with greater off-contact comes a need for greater squeegee pressure to bring the mesh into contact with the substrate during the squeegee stroke. The greater this pressure, the shorter the life of the screen and stencil. Additionally, higher off-contact distances make it increasingly difficult to hold registration from color to color in multicolor and process-color work.
We turn to screen-mesh tension at this point because of its close relationship with off-contact height. Clearly, the only way to ensure a minimal off-contact height is to use high-tension screens. High-tension screens are particularly beneficial for working with plastisols because such screens promote better shearing and transfer of the high-viscosity inks.
Mesh thread count
The mesh count of the screen, measured in threads/in. (or threads/cm) is one of the best understood variables in terms of its influence on ink deposit--at least most printers understand that a lower count usually means larger mesh openings, which lead to a heavier ink deposit. (I say "usually" because of the influence of thread diameter, which is discussed in the next section.) But many shops still use mesh counts that are lower than needed for the job at hand.
Low mesh count does result in a heavier ink deposit, which might be desirable when printing on dark fabrics or when using specialty inks that require a thick deposit. But for the vast majority of applications, the high ink deposits only create greater ink costs for the printer and lead to heavy-handed prints that are less desirable to the consumer.
Mesh thread diameter
In general, the influence of mesh thread diameter on plastisol printing is less understood by printers than the mesh count itself. Thread diameter is most important when we want to select between two different meshes of the same thread counts.
For example, imagine we have two 305-thread/in. screens, one with 40- micron threads, the other with 34-micron threads. After converting these values to the same measurement units, we can use the formula % Open Area = [1 - (thread count x thread diameter)]2 x 100% to calculate that the mesh with 40-micron threads has an open area of approximately 27%, while the mesh with 34-micron threads has an open area of about 35%. Clearly, the mesh with the 34-micron threads will result in a greater ink deposit during printing.
The quality of the stencil is often overlooked as a culprit when a print just won't come out right. I am not talking about obvious problems that would impact print quality, such as premature stencil breakdown. What I'm referring to are those difficult-to-identify overex-posure and underexposure problems that occur from time to time.
On a highly detailed job, a slight underexposure could result in line and dot sizes that are bigger than they should be, and possibly even the wrong shape. A stencil than is slightly overexposed could hold lines and dots that are smaller than they should be. Anyone who has tried to register a job in which lines are choked knows what a problem this can be. Properly produced stencils not only yield better prints, they also speed up both job setup and print production.
This variable is an easy one. A good automatic press will provide more consistent squeegee and flooding characteristics during printing than a manual press. Automatics do not get tired, so print quality should remain unchanged throughout any run.
Automatics also tend to hold registration better than manual presses, although there is really no good reason why a well-built manual press shouldn't be able to do the same. For some reason, however, the screens and printheads on automatics don't seem to get bumped and mistreated as frequently as the various components of a manual press.
Additionally, most manuals use a single rear screen clamp for each printhead (Figure 3), which means it's much easier to knock their screens out of register. Automatics, on the other hand, are generally better supported and protected, and they are large and heavy enough to stay put. Automatics also tend to maintain off-contact better than manuals. On most automatics, the screens are stationary and the platens move. Even on those automatics where the heads move up and down, the screens tend to be fixed in a firm horizontal lock.
This variable concerns automatic presses and whether or not the print stroke is applied from the top of an image to the bottom or from the bottom to the top. What makes this a variable is the fact that most presses allow a certain amount of deflection in the platen when squeegee pressure is applied during printing. This deflection is most pronounced toward the top portion of the image (which we'll call the outboard end since it's the portion that is farthest away from the center on a carousel press).
In order to compensate for platen deflection, most printers set their machines to apply more squeegee pressure during the print stroke. This extra pressure is enough to ensure good contact between screen and substrate toward the outboard side of the stroke but is excessive for the inboard portion, which usually gets a heavier-than-needed ink deposit as a result.
We have two primary ways in which we can handle this situation. First, we can set our equipment so that the print stroke always moves from the outside toward the center of the press. I found this an effective way to reduce the amount of pressure compensation required for platen deflection. An alternate and more effective approach, however, is to address some of the other variables discussed previously, namely screen tension and off-contact. By using high-tension screens, we require less off-contact, which means less overall squeegee pressure will be required to transfer the image from screen to substrate and less platen deflection will occur.
The material (or materials) from which a press platen is made has a moderate effect on print quality. A softer platen surface is beneficial when a heavier ink deposit is desired, but the same soft surface can hurt the quality of fine detail and halftone prints. The overall rigidity of the platen is also important. If a platen can be deflected or has an uneven or warped surface (Figure 4), it will be almost useless for producing high-quality, high-resolution prints with plastisols.
Most printers simply turn on their flash-curing unit to a particular temperature and leave the machine at that temperature throughout the print run. If any adjustments are made during the run, they usually involve adjusting the printing speed higher after platens begin to get warm. However, if platens continue to build heat after press speed is increased, it's a sure sign that the flash unit is too hot.
Platen temperature is a serious issue for most textile printers using automatic presses, and any printer using manual equipment who is having a problem with heat buildup. When heat builds up in platens, it can cause ink to gel while it's still on the printing screen. And once plastisol begins to gel, it will no longer print properly. Additionally, excessive platen heat coupled with flashing can overcure printed colors that we only wanted to gel. The result can be adhesion problems with subsequent ink layers.
To avoid these problems, the best advice is to minimize the amount of flash curing required on any given job. On jobs involving underbases or special-effect inks, flashing may be unavoidable. But by giving proper attention to all of the variables discussed in this article, we can limit the negative effects of flashing. By using higher mesh counts, high screen tensions, and low off-contact, for example, we can print thinner deposits that require fewer flashes and lower flash-curing intensity.
If we can't avoid flashing, we must make sure to monitor platen and flash temperatures regularly in order to maintain the right equilibrium. I suggest aiming for a peak platen temperature of approximately 120°F (49°C) at which the platen will feel warm rather than hot.
A key point to take from this discussion is that changing any one variable in the printing process can have a major impact on one or more other variables. We cannot change screen tension without impacting squeegee-pressure requirements. We cannot reduce ink viscosity without adjusting squeegee and flood-bar parameters as well. When we switch from T-shirts to fleece in the middle of a print run, we have to make adjustments to variables such as off-contact, squeegee pressure, and flash temperature (among others) in order to maximize the print quality.
When any element of the process needs to adjusted, we must ascertain if the first one that comes to mind is the right one to change. In general, the best variable to alter is the one that has the least impact on all of the other variables and will require less overall tinkering and adjustment to the entire process.
Many articles have been written on each of the variables covered in this article. The goal here was to summarize each of them in a way that makes them easy to understand and control as part of any print shop's daily routine. The only way to improve the quality of the screen-printing process is to identify and eliminate the variables that bar the way.