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.
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.
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