What is Under base and why it is required for Screen Printing?

What is an Underbase?

An underbase is a layer of ink (generally white or other light color) that is printed as a "base" on a dark shirt for other colors to sit on. This gives the top colors more brilliance. Since the underbase is generally a high opacity ink, it is flash-cured before the top colors are printed over it. Not only does underbasing slow production, but it is often an extra color that the customer did not plan on - or one that sales did not get enough money for. Underbases are also called an underlay (very common) or a mask (not very common).

Contrary to popular belief, underbasing has NOT been around since the beginning of T-shirt printing. In fact in the original version of my Encyclopedia of Garment Printing, first published in 1986, we only touched on the topic. "Back then" we tried to use high opacity inks straight from the can through low mesh counts and hoped that the print would be acceptable. It wasn't that it couldn't be done, just that most small shops only had four-color presses and the larger shops were just looking at upgrading to an 8-color. What in the world would we do with all those colors and where would we fit in a flash-cure unit?

The other funny "history lesson" is that for the first ten years of my printing career no one had even invented a flash-cure unit. We all made our own version of a heater wired to a stand.

Now that 6-color MANUAL presses are the norm, and all of the large shops wouldn't even look an automatic smaller than 8-colors, underbasing has come of age.

Why use an underbase?

Although you could print each color as a high opacity ink and flash after each color, an underbase allows the print to have a bright look yet be soft to the touch because the underbase is printed through lower mesh counts and the top colors very high mesh counts.

The Rules

Underbasing isn't really hard if you just follow some simple rules. Break these rules and you will wonder why your prints just look awful and will stop a bullet.

  • Proper Art Prep

    The secret to creating artwork for an underbase is to make the underbase invisible (unless part of the underbase is going to be a color in the design). It would be easy to just duplicate the artwork or stack all of the overlays together and use this art to burn the white screen. The problem would be the "gain" of the thick ink when it is printed. When the top color goes over the underbase you will see the white edge peeking around it.
    The best underbase art is "choked" slightly or made skinnier. Choking is not the same as reducing the size. It does not alter the size proportionally, it just makes the outline a little thinner. If you have a very tight press and are using sturdy frames you can get away with a choke as little as 1 point. If you need a little more room to play with, use a 2 or 3 point choke.
    The secret that makes choking work is that when you print an all purpose top color on the white it looks bright, but when that same color falls on the dark shirt you will barely see it. You really have to look close to see the ink falling on the black background.

    figure1 (figure 1)

    The shirt in figure 1 has a white underbase through a 49 mesh (threads per cm).

    figure2 (figure 2)

    The shirt in figure 2 is the same print without the underbase through a 90 (cm) mesh for the red and the blue. You can see (or maybe you can't) how dark the ink is without the underbase.

    figure4 (figure 3)

    Figure 3 shows an exaggerated choke.
    A choke is traditionally done in the camera (there is an article out there somewhere telling how to do a camera choke) or it can be done in the computer. The choke of the underbase is the KEY to good underbasing! Don't neglect it. The best underbase is one you can't see!

  • Mesh

    Obviously you will be using a variety of meshes. In fact, the mesh will vary depending on whether you print manually or with an automatic. The chart in Figure 5 shows the recommended mesh counts. These can be varied depending on the amount of detail and the number of colors. If you are trying to stack five colors wet-on-wet on an underbase you will have to increase your mesh counts to help minimize ink pick-up on the bottom of the screens.

Image Type Manual Press Automatic Press
Simple block areas 34 49
Detailed images 49 55
Fine line halftones 55 - 78 63 - 78
Top Colors
Simple block areas 78 90
Detailed images 90 120
Fine line halftones 90 140

  • The Stencil
    Although I am a big fan of direct emulsion, you could also use direct film. It is helpful to have a thick stencil to lay down a thicker deposit of ink. If using direct emulsion, try coating/drying/coating to build up a thicker layer. A dual-cure emulsion will give a smoother stencil.

  • The Ink
    You will be using two types of ink. The underbase will need to be a high opacity plastisol. Generally, high opacity inks are too thick to print properly. Although some manufacturers claim you should not thin these inks because you will lose opacity, you will find that by making them a little smoother they actually print easier and you can use less squeegee pressure. The result? A brighter and smoother underbase. Use a small amount of curable reducer to make your high opacity ink smooth.
    Although you can use a low-bleed as the underbase, they seem to be a little more "bubbly" when cured. I prefer a normal high opacity - unless printing on a 50/50 shirt where you have to use a low-bleed.
    For added speed you can use the special flash-cure underbase whites (either standard or low-bleed). These were designed for automatic presses to help speed up the flash times AND reduce the cool down because these inks cure tack free! They are not as opaque as a standard high opacity so if this color is also going to be part of the design you may not get the brightness you want.
    I am often asked why not use a clear for the underbase. My response is - what's the point? If you are going to have to make a print it might as well be a color you can use like white!
    There are times that the underbase is not white though. If there is a lot of gray in a design you may be able to make the gray the underbase and part of the design. It requires a little thought when creating overlays and thinking out the print and flash sequence.
    The top color should be an all purpose plastisol. It too can be reduced slightly to help it flow better. Obviously, if using an automatic press you can reduce the ink viscosity less because the press can handle a thicker ink. You can also use a fluorescent or even a process color as the top color. The more transparent the top color the less you will see it when it falls off the choked underbase.

  • Printing Technique and Set-up

    Technique is VERY IMPORTANT. You must set the press off-contact so the thicker underbase will clear from the screen cleanly. The smoother the underbase the better the top color will look. Any peaks or stringing of the underbase will prevent good coverage of the top color. Uneven underbases are more common when printing on soft surfaced sweatshirts.
    To get a smooth print use a slow stroke to let the ink flow. You may even want to use a slightly dulled squeegee to help smooth the print. The first few prints may not be what you want but with a little practice you will get just the right angle and pressure.
    The ink MUST clean out of the screen. If you are in the habit of leaving ink in the image area after the print stroke, this will be an uneven area in the print.

  • Brighter Whites

    figure5 Figure 4

    It may be hard to get a really bright white with just one print. You will have to do at least two strokes to get a respectable white. For the BEST white, try printing the white - flash curing it - printing it again - and flash curing it again. Yes, this slows production - but what a white (figure 5)!
    For the top color you can use a little more speed. Good pressure is important here. You need to drive the ink around any peaks and valleys in the white. If the white is not smooth you will get "pitting" in the top color. You may need to do two strokes on the top to cover the white adequately.

  • Flash Curing

    There is no underbasing without a flash-cure unit. If you plan to do a lot of flashing make sure the unit is large enough. Some of the smaller units can barely cure a standard garment size image size.
    You should position the flash unit so that you can get a cure in a matter of seconds. Plastisol will cure to the touch at around 120 degrees C. If you fully cure it, the top colors may not adhere to them and can flake off when washing the shirt. More isn't better here.
    The ink may need to cool down after the flash. If using a manual press you can use the rotary feature and the ink will be cool when the print comes back around. If using an automatic press you may need a cool down station (or a typical arrangement of fans or blower between stations).
    The more colors you print on top of the white, the harder it is to get a good print without flash curing more than once. It is fairly easy to print a couple of colors, but by the third or fourth you start to lift the under colors off and give the colors peaks and uneven areas. For designs with lots of colors you will need to add a second or even third flash somewhere in the sequence. No two designs are the same and this is where experience and experimenting pays off.

Other Assorted Ideas

  • Underbase With Puff

    To make puff really jump off the shirt try putting an underbase below it. You will get a smooth and very high puff print. This is also helpful if the puff is bleeding on a 50/50 shirt. Use a low bleed underbase to block the dye migration.

  • Halftone Underbase

    A great trick to keep the print soft yet bright is to print a halftone white underbase under a solid top color. Use a high percentage (60% or 70%) halftone dot through a coarse mesh. The top color won't be quite as bright, but the print will have great softness.

In Summary

This article has been a basic primer on the fundamentals of good underbasing. Keep all of the above rules and ideas in mind and watch your prints jump off the shirt!
This is the starting point of doing GREAT simulated and real process color on dark shirts. In future articles we will look at creating halftone underbases and highlight white printers using Adobe Photoshop.