Dot gain is a challenge for garment screen printers, whether the job involves four-color process, simulated process, or index separations.
Skips and voids with bad penetration into the garment result when off-contact distance is greatest (high/low, low/high combinations). To fix the problem, start by numbering all heads and platens sequentially. This establishes a relationship between individual heads and individual platens that you can measure and document. Next, measure the distance between the head and platen for each combination on the press.
Level the press head-to-head, and level the platens to the same value. The distance between all heads and platens will then be the same, and each platen and each head will also be in the same plane. The platens must be level and parallel to each and every head. The acceptable tolerance of variation depends on the mechanics of each press, as well as how the squeegee and floodbar pressures are controlled. A good basic starting point for off-contact is 0.06 in. Even if you cannot make each combination of head and platen perfectly level or parallel, you can still use the variances in dot gain from your initial test prints to determine how much gain to expect on that press.
Print enough samples to reach an equilibrium point. Regardless of the dot gain you experience, it should be consistent between heads and platens within 1-2%. When the machine is not calibrated, the differences can be greater than 40% between each head/platen combination. Also consider the line count you’re printing, because the number of dots per square inch increases very rapidly. You’ll have more trouble controlling gain as the density of dots per square inch increases, and you’ll need to account for proportionally greater optical dot gain in addition to the physical gain.
Here are three effective ways to reduce the physical and optical gain at any given line count:
• Always use a wet white highlight or detail white screen at the end of the process. It’ll mix with the wet color and create a pastel color, thereby reducing or canceling the optical and physical gain in the highlight to quartertone range.
• Add some high-density base (up to 3% by weight) to the ink to dramatically reduce physical dot gain without appreciably thickening the ink.
• Mix line counts within the separation. For example, lowering the line count of the black from 65 lines/in. to 55 lines/in. decreases the dot density from 4225 to 3025/sq in. The dots are farther apart, thereby decreasing our dot gain for the same given area.
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