Dot gain is a phenomenon that causes the screen-printed fabric to look darker than intended. This happens because the diameter of halftone dots increases during the pre-press and printing process causing tone value increases. Dot gain is an important factor in the screen-printing process. It is merely halftone dots enlarging or expanding. Total dot gain is the difference between the dot size on the original file and the final dot size on the printed result.
Further complicating predictable results, dot gain is not identical for all colors used in screen printing, mainly because of the wet-on-wet process, white printer or base-plate, and other factors. Not to mention dot loss is another phenomenon to compensate for in the process. As a result, the halftone color simulation is no longer accurate making colors look darker and sometimes lighter in areas. Due to halftone dot gain the image color is no longer accurate when compared to the original. Color change occurs because dot gain changes the densities in the color gradients.
There are several types of dot gain. The film output or CTS (computer-to-screen) systems are not always linearized. Under-exposure can contribute to increased dot size during development. Mechanical dot gain occurs on the press as ink transfers from the screen to the garment. Each time the dots get stepped on by subsequent screens the physical diameter of the printed dot gets squished and increases in size. The squeegee plays a tremendous role here. Sharpness, shore, pressure, angle, speed, and off-contact distance affect dot gain. When the ink is absorbed by the garment, both vertically and horizontally, the dot diameter increases again.
Pre-press and press operators try to minimize some types of dot gain but cannot avoid it. It is also the responsibility of the separator to be aware of it and try and compensate for it. Dot gain controls can be adjusted in Photoshop, as a numerical value. To predict and offset the variable dot loss and gain each part of the process must be controlled. The output should be linearized; consistent screens are most critical; and proper selection, tension, and stencil thickness, as well as accurate exposures and developing, are essential. Finally, the press must be dialed in. We call it the planar relationships. Being level and true are the starting points. All platens, screens, off-contact distances, and squeegees and floods must be parallel to each other.
Source: Lon Winters