Screen mesh seems like such a simple part of the screen-printing process but, as we all well know, things are rarely as simple as they seem. Take, for example, its commercial and industrial uses as a filtration cloth that involve a myriad of numbers on the elements of screen mesh. The end-users need the numbers so chemical engineers can compute, and then model, fluid flow patterns, pressure drops and volumetric input/output. If all this sounds high-tech compared to simple screen-printing, don’t believe it! Mesh is truly a textile engineering feat, but its selection is often as misunderstood as its sophistication.
It’s important, then, to setup the criteria for mesh selection based on the data provided. This ordering will allow us to model input/output to serve as navigational tools when inks call for thick-film printing.
Thick-film fluid transfer (the extrusion method of screen printing) happens when ink is put under shearing energy—approximately the combination of fluid pressure and speed (a.k.a. how “hard and fast” the ink is moved with a squeegee). The fluid pressure is caused by the size and shape of the funnel (the shape that is made by moving a quantity of ink between the blade and the mesh) but also by the speed of the blade.
Here’s the rub: the ink wants to move out of the way while we are pushing it and, with sag-resistant stencils, they want to move out of the way even more. Therefore we need a mesh that will allow thick ink to flow with minimal restriction, at a volume that we can maintain without interruption, and at a speed that will efficiently fill the cells of the mesh.
If you want a tall column of ink, a.k.a. a thick-film, you will want to start with a mesh that supports it. Refer to your mesh supplier’s specs or use that provided here (see Figure 1). The thickest ink film will be obtained with the thickest fabric, number one (30/250) mesh. Also consider the resistance of the mesh to flow, the distance the ink has to travel, the volumetric requirements and the speed you can attain for both productivity and quality. In practice, it may not be possible to effectively use the fabric that will put down the tallest columns. Consider “thicker” means lots more film/coating, drying, flashing, cooling and raw material cost.
Ink transfer distance
Ink transfer distance, noted in the graphs for this article as ITD, is the distance the ink must flow in order to provide sufficient ink to create a seamless finish with minimum surface texture (see Figure 2). The best case scenario for ITD is number 15 mesh (110/80) and the number one mesh (30/250) is worst case.
Thick-film inks are notoriously resistant to flow. They don’t exhibit much fluid momentum—soon as we quit pushin’ they stop flowin’. If opting for a mesh with a long ITD, use a slightly thinner ink that will continue to thin under shearing energy. Such an ink will respond to shear-rate (speed) and will flow for a distance on its own. Be warned: the same ink will sag if it is too thixotropic (gets thinner as it is agitated).
When the blade passes over the mesh, the ink is divided between yarns and openings. The resistance is based on the size, topography and concentrations of the yarns. Think of resistance as the nozzle on a garden hose; turn it to the right and the pressure goes up, the volume goes down and the water shoots farther. Turn it to the left and the opposite happens.
The number one mesh (30/250) will allow something as thick as roofing tar through—it’s hassle-free but this screen fabric will require a high level of shear-stress on press; high tension, maximum off-contact, small footprint (contact area between blade and mesh) and large funnel. These will be mandatory in order to print at the highest speed possible to go the distance with such a robust mesh.
Theoretical ink volume
TIV is the theoretical ink volume—how much stuff you need to put through the chute. Here’s the bad news: if printing on an automatic press, you’ll be shoveling and carding fresh ink every minute or two with a mesh that has a high TIV (theoretical ink volume).
Inches per second
Higher print speed may generate a lot more profits, but as we learned a few paragraphs ago, transfer speed is the best thing you can do for the quality of the print. This is true providing the squeegee blade will permit it and that you can shovel enough ink in its path.
The footprint of the blade must cover one mesh period 0.033" with the 30 mesh but only 0.009" with the 110 mesh. The printing veteran will reach for a softer blade to form a tight seal over the coarser mesh, but there is one caveat with soft blades. The film thickness will be limited by the gap between the blade and the mesh at any point prior to transfer, so don’t go too soft.
On the other hand, if the blade is too hard, you will be forced to cantilever the blade to create an adequate footprint. As a result, you will be hard pressed to form a tight seal. Using either extreme hard or soft blades compromises print speed.
Square feet per gallon
Be very judicious in this case when attempting to print thick film for profit. At 0.001" (or one mil thickness), about a 305 mesh, you can get 1,604 square feet per gallon. With an 80 mesh, the thickness is a nominal 5 mils (0.005") so the yield is one-fifth or about 320 square feet. You can use your computer software to compute the image area with high precision. Whether you use wet and dry film thickness gauges or a press run and a gram scale be certain you know your costs per mesh count.
Choosing the Right Mesh
Pick a mesh that will support your process while remembering three facts—thicker ink films equal tougher printing, the stencil can be used to exaggerate fine details and there is not a lot of percentage in running very large solid areas in thick film unless you like bulletproof prints.
The optimal screen-mesh choices for thick-film are:
Mesh number three for extreme thick ink printing with an ultra-thick stencil; well-suited for manual printing.
Number five is well-suited for a thick print with the fewest (but long dwell) flashes and cooling, and a surface-enhancing high print speed.
Number seven works well as a base coat or for a multitude of layers (although you may want to go thinner as you add layers).
Mesh number 14 is great for detail-enhancement with thick stencils. Higher counts than our number 14 may be usable, but pretest for suitability and be sure the ink will go the distance at a reasonable speed under such high restriction.
As you progress with thick-film printing, Figure 3 will help to fine-tune your selection to fit your particular needs.