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18
Mar

Printer Guidebook---Screen Mesh 1

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Mesh-fiber composition

The molecular and mechanical characteristics of screen-printing mesh.

At present, the only suitable, modern mesh fiber is high-quality monofilament polyester: round and smooth. It has superior elongation and strength characteristics, has no loss of strength when wet and, in fact, absorbs less than 0.8 percent moisture. Absorption of moisture is an important consideration, as this is such a wet process.

Nylon, for example (or, by its generic name, polyamide, which can absorb up to 5 percent moisture), loses 10 to 20 percent of its strength when wet and elongates 26 to 40 percent before breaking-compared to polyester, which has elasticity of only 19 to 23 percent. Nylon mesh, therefore, should only be used when printing irregular-surfaced or three-dimensional objects (such as bottles) that might require such properties.

Identifying nylon

Why identify nylon? Because many such bolts of fabric are "out there," waiting for the unsuspecting printer to stretch them over a frame. Don't. Look on the edge of your roll of mesh for the brand name and model of the mesh you're using. Memorize the names different weavers give their monofilament polyester mesh.

I often come across the odd "bargain" bolt of mesh that has "nylo" or "mide" (from the generic term for nylon) in the name. Get rid of it! If you have a bolt that isn't labeled, try a burn test. Both polyester and nylon fuse and shrink away from a flame, but polyester will put out a black smoke that nylon won't. Also notice (if you happen to get it into your nose) that the odor of burned polyester is "sweeter" than that of nylon.

Molecular alignment

When used as screen-printing mesh, polyester's most important trait is its ability to orient its molecules parallel to each other when the squeegee-blade drags across the mesh fibers during printing. This is called "work hardening," and is not a mystery, but a documented physical principle of long-chain molecules. They can change from low-orientation-molecules crisscrossing each other and not parallel-to high-orientation where the molecules are very parallel. High orientation is associated with good fiber strength and low elongation; low orientation offers nothing of value to screen printers.

In polyester, molecules are joined to parallel molecules by associative forces known as "hydrogen bonding" (due to the effect of hydrogen atoms in parallel molecules bonding with other smaller electro-negative atoms). This bonding aids dimensional stability and lends mesh the ability to elongate-when acted on by off-contact and blade pressure-but then to return reliably to its original unstressed position. When molecules don't contain the polar groups necessary for hydrogen bonding, weaker associative forces can be established between molecules, provided they are very close to each other. The weaker forces depend on proximity and atomic charges-and, incidentally, are usually referred to as "van de Waal's forces." (I mention this mainly to illustrate that these principles are not just folk stories told by one generation to another, but actually have a scientific bearing on mesh performance.)

The work-hardening phenomenon doesn't happen when you stretch mesh on a frame but, rather, as a result of the directional stroking of the blade on the mesh during printing. It is important, therefore, to document the direction in which the blade is used and never reverse the direction; this causes the molecules to re-orient themselves in the opposite direction, leading to chaos in the fibers and odd, unpredictable changes in tension and image shape-even unexpected stencil breakdown.

Rapid tensioning

During the development of high-tension printing in the 1980s, the best results were achieved by stretching mesh, then letting it relax (a mechanical stretcher works best for this, as pneumatic stretching doesn't allow the mesh to relax), then re-tensioning it until a higher than normal tension was achieved. These steps were used to allow the mesh to acclimate to the force on it gradually and prevent mesh failure-typically ripping.

To make a critical screen dimensionally stable, some printers stretch and coat it, then cycle the squeegee blade across it in a single print head using a clear base as a lubricant-100 to 200 strokes, or until the screen stops losing tension-to workharden the mesh without an image. They then clean the base out, reclaim the emulsion, retension the mesh and re-coat it for a stencil.

 

The introduction of low-elongation (LE) polyester fibers has again changed the approach to stretching. The molecules of this mesh are oriented during manufacture to help make it more dimensionally stable. This wasn't demanded of mesh manufacturers until the development of retensionable frames that could easily take meshes to their single-stretch tension limit and beyond. Until then, mesh had been stretched well below its potential. The techniques taught for retensionable frames of softening the corners to relieve stress facilitate high tensions.

Tests by the SPTF (Screen Printing Technical Foundation) have demonstrated that, under controlled circumstances, a printer can proceed directly to the desired ultimate high tension in one stretch without significant loss of tension, compared to progressively elevating tension. Contact the SPTF and your mesh manufacturer for current instructions for using LE rather than traditional mesh.

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