Establishing Rules for Underbase Printing
A poorly printed underbase will produce washed-out prints or garments too heavy to wear. Find out how to establish procedures that will lead to a quality product, every time.
Deciding whether or not to use an underbase for a color can be complicated, involving a lot of factors that will affect the final print. Many printers throw in the towel and just underbase everything in the design. Of course, this makes the shirt feel as though a big plate of ink is sitting on top of it, and it also tends to make all of the top colors that are not flashed immediately begin to bleed outside of their image area. Then, when customers complain about these “bulletproof” prints, some printers overcompensate and underbase very little in an image, perhaps only specific colors such as bright red, yellow, or blue. This will cause the design to look dull and washed out on a dark shirt and can create unpredictable color shifts if you’ll be printing several shirt colors with the same image.
Neither of these quick fixes solves the problem, and both can lead to a lot of spoilage and even orders that your customer refuses to accept. So, what is the right way to produce a good underbase and get consistent results time after time?
The best practice is to develop some simple rules about when colors should or shouldn’t be underbased that are specific to your shop. It requires analyzing your process, developing underbase guidelines, and then doing quality checks of prints coming off the press to be sure the standards are working. The first steps are often the hardest because they require that everyone in your company remain objective, something that can push a lot of people out of their comfort zones.
Analyze Your Process
In a lot of screen printing shops, one person always makes the call as to what should and shouldn’t be underbased in an image. The judgment is often arbitrary based on the experience and background of the person making the decision. What happens when that person is on vacation or leaves? It’s better for everyone to understand the process of making a good underbase and the primary factors that dictate what the best choice will be.
This is usually the primary reason to use an underbase. Screen printing inks typically are not completely opaque when printed in the thin layers that printers strive for to control the hand and avoid producing garments that will be uncomfortable to wear. Therefore, any shirt color other than white will affect the color of the ink to a certain degree. In some cases, it’s okay for the shirt color to penetrate the image, such as in a vintage design with a washed-out appearance (see Figure 1). An underbase wasn’t used in parts of this design intentionally to create the vintage look. But for most prints on any garment color except white, the design should include an underbase for any brighter colors in the image.
Two issues that can potentially influence an underbase are the fabric composition of the shirt and any seams that fall close to the print area. A variety of issues with the fabric type can affect the underbase – the amount of absorption that the fabric has, for example (and whether it will soak up or repel the ink) or how fuzzy the surface of the material is. An ironic thing about ultra-soft, treated shirts is that they tend to have finer fibers that stick up and feel softer, but also cause an underbase print to not release well or peel back up into the screen (commonly called “orange peel”).
If you have any concerns about the printability of the fabric, always test it with a sample print prior to the production run. If you have to modify the inks, you’ll have time to do so before you’re on press and things get down to a rush.
Having samples of the garments ahead of time will also help you avoid any problems with the seams. Get the smallest and largest sizes of the garment that will likely be ordered. This will allow you to adapt the artwork to fit within the printable area for each size, or know you have an issue that might cause a reject with that specific style or size of shirt. For example, when you print different types of baseball jersey T-shirts, the sleeves can run at an angle that may hit a wide or vertical design near the shoulder seam. This can cause serious issues with an underbase print, problems that would also show through in the top colors and cause the shirts to be scrapped.
The more complex a design, the smaller the individual elements become, which can make creating a successful underbase more difficult. Designs with many different elements (shapes, spaces, lines, etc.) and/or a lot of color transitions usually require careful consideration. Although a simpler underbase can occasionally work with a complex design, usually it’s a sign of a poor-quality separation, where the underbase is too solid to properly support the detail in the top colors. It doesn’t have to be a nighmare, but it will be usually take considerably longer to prepare than the other colors in the design.
Screen tension is of particular concern when a design has very small image elements that the under- base needs to support. Thicker, tackier inks might pull the mesh slightly during the print if the screen isn’t tight enough. This can cause the white underbase to show up in unwelcome places and the top colors not to register properly. Even a small amount of distortion from screen stretch can be visible due to the lower margin for error with small, tightly registered image elements (see Figure 2).
The placement of the design on the garment can become a concern when seams or the edge of the garment cause the squeegee or screen to become unlevel during the print stroke. This is a common problem when printing across the zipper of a hoodie or over the seam of a garment’s shoulder sleeve. These “bumps” push the screen away from the surface of the garment, causing distortion in the underbase that you’ll typically see as bleeding, puckering, or image drop-offs in the final print.
You may be able to solve problems like this through equipment modifications. Platens with soft, spongy surfaces can help by pushing these areas of the garment down and leveling out the printing surface. Some squeegees are designed with a gap or slightly lifted area in the middle to compensate for center seams. At one of my old companies, we would cut a notch in the squeegee blade on higher-volume jobs to get around vertical seams, a job-to-job solution that costs a squeegee blade but prevents you from tearing the screen. Other common adaptations include using different types of ink that absorb into the garment and don’t pool or pucker as easily, such as water-based inks or reduced-opacity plastisols.
In the art department, you can compensate for these issues by making the design less hard-edged and more forgiving to slight printing distortions. One way to do this is to distress the artwork, adding an overlay layer in the design software that intentionally makes it appear to be worn out or damaged, allowing any printing errors to blend in easily (see Figure 3). Another solution is to break up the design into smaller pieces and use separate screens over each side of the bump in the garment, though this is obviously a less friendly option on volume and cost.
Screen and Press Variables
Sometimes, your underbase decisions will be limited by the number of colors you can print on the machine, depending on the design and the method you’ll be using to separate it. You might prefer to add a gray screen or another off-white color to extend the subtlety of the printed image and help the brighter colors to pop, but you won’t always have an open station to allow for that. If you are short on available stations for a job, you can try to create a gray value on a darker shirt using the underbase, a technique that may also save costs by allowing you to use fewer screens (see Figure 4). An experienced separator will account for the screen and press limitations in choosing the best approach to get the right amount of detail into the design that production can duplicate.
Of course, the subtle work that a separator does to vary the coverage of the underbase to get the best possible results without putting down too much ink and compromising the hand of the garment will be lost if your printing process isn’t under control. This means choosing the right mesh count, tensioning screens properly, having consistent stencil-making procedures, choosing the right squeegees, setting up the press properly, and so on.
With an underbase, the most important variable is the stencil thickness and the EOM (emulsion over mesh ratio) – the thickness of the emulsion that rises above the surface of the mesh, which will form the “well” that holds the ink after the flood stroke. The open area and thickness of the mesh are obviously important as well, but even a high mesh count can produce a thick print if the EOM is high enough. This is why thick capillary films are used in high-density printing, because the emulsion rising above the surface of the mesh will hold and print the thickest possible deposit.
Pay attention to your coating process and the resulting EOM that you achieve, because they will have a big effect on the quality of your underbase printing. At my former company, I once tested more than 25 white inks trying to find the brightest possible one for underbase printing. I printed them all under the exact same conditions and saw only small variations in brightness from one ink to the next. During the test, I accidentally double coated a previously coated screen. (Yes, I was really tired when I did that.) But the resulting print was more than twice as bright as any of the other prints from the test. This helped me realize the importance of controlling the EOM to get the brightest underbase.
When I work with clients, some of the ways I help them increase the EOM include putting the emulsion in the refrigerator, switching to an emulsion with a higher solids content, and adding extra coats using a more rounded scoop coater with less pressure. A coating machine will allow you to get consistent results from job to job. EOM gauges are also available that allow you to measure the exact stencil thickness as a quality-control measure.
The downside of using a screen with a high EOM for an underbase is that the thicker ink deposit can make the final print feel heavier and uncomfortable to wear. This is why the positive for the underbase screen shouldn’t just be complete coverage for every color in the design.
All of these variables – the number of screens available for a job, type of film positive, ink opacity, ink viscosity, press setup, press registration capabilities, skill of the press operator, and more – can be mapped out using some simple tests that will help you see where your production team is able to hold and replicate details in an underbase print (see Figure 5). Several styles of test prints are available that will help you document the detail and quality capabilities that your shop currently possesses. Use the tests to find the ideal settings at each step of the process and make sure you record each variable when you achieve the best print you can with your current processes and equipment.