The short answer is the dots in a halftone print grow as they are printed. So what do we do? Let’s take a look!
In any type of screen printing, paper or textile, the halftone dots grow or get larger when printed through a screen onto a sub-strait. This is more prevalent in t-shirt printing than flat stock printing because the inks are thicker and require more pressure. The increased pressure along with the porous sub-strait increase the amount or percentage of dot gain. The good news is dot gain is consistent and controllable.
The first image is what the dots look like on screen. The second image is what the dots look like after printing. That’s the effect of dot gain.
There are many tools in the process that need to be calibrated to succeed in the dot gain battle. First we will talk about Photoshop. Photoshop has the ability to simulate dot gain the the settings for channels. When separating the art into channels you can get an accurate representation of what the finished print will look like. Here is where you will adjust the settings in Photoshop for expected dot gain.
Edit/Color Settings/Spot Custom Dot Gain. Start with 90 at the 50% rang. Every monitor will display differently so some testing and adjusting will need to be done.
Now I know we talked about excessive gain in t-shirt printing, right? Well, it’s typical to see as much as 40% gain in the 50% dot size. That’s where the 90 comes from. 50% meaning the gradient should look half black and half white. In graphic printing 20% dot gain is more realistic, but remember t-shirt printing gains a lot more.
If your dot gain settings are not correct in Photoshop the only chance you have of printing somewhat accurate is with minimal pressure. What happens then is under saturated shadows and solid areas. Then the print looks washed out and flat.
Weather you are using Film or Computer to Screen you need to calibrate the output of the device. This is a lot trickier than the Photoshop adjustment but their is hope. Adjustments are usually available in the RIP software. The RIP software takes a percentage of a color and prints that color in dot patterns.
The output device needs to be calibrated as an exact amount. Tools are available to measure dot value but can get costly. Old fashion trial and error can be used too with the help of a reference tool. The key here is 10% needs to be 10%, 40% needs to be 40%, and so on… That way what you print is what you get.
Screens need to be consistent in tension and mesh count. Tension is how taught the screen is. Minimal deflection prints and releases the ink better with minimal pressure. Screens should be measured with a screen tension meter and be within 10% of each other.
Newton Meters is the standard measuring unit used in screen tension. If one screen measures 20 Nm and another measures 30 Nm you will have inconsistent registration and dot gain.
Squeegees are equally (if not more) important. Squeegees must be kept sharp and in like new condition. Dull squeegees will increase the amount of dot gain dramatically. Different durometer (stiffness) squeegees will create different amounts of dot gain. Pressure on the squeegee will also greatly effect the amount of dot gain generated.
Inks need to have consistent viscosity so the dots grow in equal amounts. Inks that shear or cut and release with similar pressure will print more consistently. Bases and reducers are often used to adjust the inks for halftone printing.
Unfortunately this one we can’t control. The better the shirt quality and the smoother the surface, the better the print quality. Recommendations can be made to guide customers to certain brands and styles but sometimes we have to print on less than desirable fabrics. Different squeegees and squeegee angle is usually the best fix in these situations.
This one is a lot trickier. In Illustrator or other types of vector programs you don’t have a dot gain simulator. So dot gain must be adjusted through the rip program. Here is the best approach for Illustrator Dot Gain adjustments.
After your output device is calibrated, test screens need to be made with 10% increments of halftones. Sample shirts then need to be printed and evaluated for dot gain. Adjustments are then made in the Rip Software for Illustrator output. The tests are repeated until consistent results are achieved. This takes a lot of work but the final product is predictable results.
Similar to the illustrator adjustments, the dot gain curve in Photoshop needs to be adjusted when comparing the final print product to the on screen image. This is a bit easier and usually faster than the Illustrator process. Seeing the image live on the screen and making adjustment to the curve speeds up the calibration process.
Just like dots growing when printed, line art grows equally when printed. If a 50% dot can grow to 1.5 times it’s size, so can a 1 pt line. Thin lines and small openings in letters can close in or disappear completely when printing. Unfortunately there is no adjustment for Line Art Dot Gain. Line art is either black or white so it prints exactly the size it sees in the art.
Consideration must be taken when designing art for t-shirt printing. Here is a great article which talks about Bold Is Best when creating your t-shirt designs. T-shirts are usually viewed from much farther away than a paper or a magazine. Negative space is critical in ease of reading. And what good is a t-shirt print if it is unrecognizable.
We hope that understanding the effects of dot gain guide you in the right direction when creating and separating your t shirt art. Happy Printing!
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