When it comes to choosing the right mesh to screen print halftones the answer is pretty simple, really. Mesh is measured in threads per inch, 110 mesh = 110 threads crossing in a square inch. Halftones are measured in lines per inch. 55 LPI, means that there are 55 lines of halftones in that in the area.
Your screen mesh has to be a tight enough weave in order to hold onto the halftones you are trying to expose to it. To calculate this, there is a very simple equation based around a 5 multiple:
Your LPI x 5 = Minimum mesh size
Your Screen Mesh / 5 = Max LPI
So, for example...
200 mesh count / 5 = 40 LPI, meaning you would want to put a max 40 line halftone on a 200 mesh screen.
AccuRIP's default LPI is 45, so that would mean you would want to put that on (45x5) a 225 mesh screen or higher. This is why we typically recommend 230 mesh or higher for spot process work.
Best Practices for Best Halftones
Of course, there are a lot of other variables in getting your halftones to properly expose onto a screen. Your screen mesh should be clean and haze free. Your screen mesh should be dyed for higher meshes 156 or 200+, the dyed mesh blocks light transmission that can happen during the exposure process and over expose small halftone detail. Most screen vendors will dye their 156+ mesh screens by default.
Your emulsion coat should be clean, crisp and work its way into the mesh by coating 1/1 or 2/1 or 2/2 by turning the screen 180 degrees between each coat. Your emulsion should be completely dry, with a minimum dry time of 4 hours, up to the optimal of overnight or 24 hours. Keep your dark room and screen storage area to 35-40% humidity.
Your films should be printed with a dark, opaque ink, like our BlackMax All Black Ink System. The positive contact between your screen mesh and your exposure glass should be tight, with at least 40-60 lbs of pressure. If you do not have an exposure unit with a compression lid, this can be achieved by setting something of the appropriate weight carefully atop your exposure glass.
Your light source should be fresh, not old, and optimized for screen exposure. Make sure to check and replace your bulbs often. Even before they go out completely, they can make exposure more difficult by being weaker.
Make sure your exposure is good. Underexpose and your emulsion will be soft and could separate from the mesh or wash away when you rinse your image out. Overexposed screens can be very difficult to wash out, and small details can be lost if it does wash out.
When you are washing the image area out, be careful with how much pressure you use. It is perfectly fine to use a pressure washer, or something similar, but you only need a wide, fairly gentle spray. Spray both sides down to soften the emulsion, then carefully wash down the screen until your image area washes out. Too much pressure can blow out parts of your image and ruin the screen.
The main exposure variable in halftone exposure and retention is the light source. if you have a multi point light source (fluorescent tubes) you will not be able to expose as fine of detail as a single point light source like the FX Exposure Unit. The 5x multiple is developed for multi point light sources whereas if you have a single point of light, realistically, you could go closer to the the 3.5x range.
Below are some other tips for choosing mesh, based on the ink type you are using and the garment you are printing on. Also, note the photo of our halftone LPI screen. If you zoom into the photo, you can see that around the 40-50 LPI dot sizes, this 200 mesh screen really started to lose the detail.
Perform the right steps, stick to the 5x or 3.5x5x and you do the proper steps above and you should be fine!