Controlling Dot Gain on Garments

Dot gain is a challenge for garment screen printers, whether the job involves four-color process, simulated process, or index separations.

Dot gain is a challenge for garment screen printers, whether the job involves four-color process, simulated process, or index separations. Decorating textiles means working with surfaces that are 65-90% solid, which leaves a 10-35% chance of missing a printable surface.

Additionally, the substrate’s surface is absorbent and highly compressible, and the twisted yarn structure is highly irregular and not uniform. Forming a good gasket with the stencil in order to maintain dot integrity is almost impossible. We can’t do much about the nature of the garments, so our focus shifts to controllable variables on press.

Heads and platens
Dot gain increases as variation between heads and platens increases. In order to remedy the situation, you must have a way to determine platen-to-head distance. Numbering the heads and platens gives you a way to see what is happening between any given head or platen combination. How much distance qualifies as a step up or step down is up to us to determine based on our line count and tolerance for color variation within the run. A good starting point is to set the minimum off-contact to 0.06 in. An increase of 0.03 in. would constitute a step up to the next level. The highest level would be 0.12 in. or higher.
You also need to examine the ink. It must be suitable for use in printing halftones. A general-purpose mixing system may have too heavy a pigment load and create colors too dense for halftone printing.

Having the proper color strength measured before you even print the dot allows you to assess proper dot-gain profiles. Any change in total fabric height (thread diameter) or stencil profile (emulsion over mesh, EOM) will adversely affect the results. Use a relatively thin mesh, such as 280 or 305 threads/in. with 34-micron thread diameter in plain weave to control the physical growth of the dot that results from high ink volume. The thicker the mesh and stencil profile, the thicker the ink film. As the ink-film thickness increases, it provides physical volume that can smash and move around.

Likewise, thinner EOM and total stencil height will deliver a thinner ink film. This thinner film will appear not as dark—not as dense. The lower volume of ink means less physical dot gain.

Corrective measures
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.

View more from this Screen Printing issue