Light emanating from white threads also causes the stencil to close in on the open space, a phenomenon called “thread growth.” Thread growth is simply tubular sleeves of exposed emulsion extending into the open areas of the stencil (see Figure 3).

While dyed mesh resists light refraction and dilution, it also will take longer to expose; often, dyed mesh will take double the exposure time as white mesh of the same thread count and diameter.

An often overlooked benefit of dyed mesh, however, is the gain in exposure time flexibility. Acceptable stencil exposure time is referred to as the latitude of the emulsion; in simple terms, this is the time before and after the optimum exposure where the stencil produced is still of high quality - it could also be referred to as the available “screw-up latitude.”

Capillary Film as a Partial Exception

The nature of capillary film is to place the majority of its thickness on the surface of the substrate-side of the mesh, making it less susceptible to some of the negative effects of white mesh.

Capillary film will show some signs of thread growth - specifically, where it joins the mesh - but, as most of the stencil is past the knuckle high-point of the mesh weave, little of its bridging, resolution, or detail will be affected. Fine detail can be closed in, though, even when using capillary films on white mesh.

The reality is that few plants would stock white mesh higher than 200-tpi, and what little thread growth could close in on small detail would be overshadowed anyway, by the impact of the thicker threads of the mesh crossing and blocking the open areas of the stencil (see Figure 5).

The older compromise of white mesh on low thread counts and dyed on high thread counts first started years ago when dyed mesh was considerably higher in cost than white mesh. Dyed mesh was purchased only when the mesh thread count was high, and where required detail and resolution demanded a dyed mesh.

The reasons were obvious. Shops allowed price to dictate where compromise in quality was not as large a factor. The common practice was to choose white meshes in thread counts lower than 200-tpi and dyed mesh for higher counts.

The practice of using white mesh in low thread counts is so imbedded in the industry that it is not uncommon, even today, to see suppliers and manufacturers stock a majority of white mesh in the lower thread counts. Many plants both large and small continue this practice without actually understanding why the choices have become standard.

The current prices of dyed meshes now hover in ranges less than 5% higher than the same thread counts in white, making the old issue of price as a factor now a historical phantasm.

The price difference is now so small that suppliers and manufacturers of mesh products don’t even bother to keep dual pricing charts. And typically, there’s no additional charge for yellow mesh, regardless of count, for most suppliers’ products.

Thickness and Exposure

We all know lower mesh counts can take longer to expose than higher mesh counts, the reason being simple thickness of mesh profile combined with emulsion thickness. As the light has to move into and past emulsion and mesh threads, it is progressively filtered to lower intensity as it travels deeper into the emulsion and threads. White or dyed mesh will feature this effect, but the added light refraction of white mesh will cause an acceleration of the cone effect illustrated in Figure 2.

Thicker stencils and thicker mesh profiles will also increase the cone effect. With the greater distance traveled into the emulsion and mesh thickness, the more the light closes in on the open stencil areas. Simply put, distance adds to the cone effect similarly to the widening beam from a flashlight.

Individual thread thickness and the total mesh profile should be taken into consideration when judging exposure. The most obvious effect of this is that each and every change in thread count, individual thread thickness, and color will have effects on the optimum exposure time.

Knowledge to Factor Into Challenges

All screen printers progress in their experience and add to their wealth of knowledge. As shops take on more challenging jobs, smaller and often overlooked factors will become issues to be tackled. Smaller, technical nuances can often be converted from troublesome variables into useful tools with which to gain desired results.