Stainless steel, plain weave mesh is used predominantly in hybrid circuit printing. This consists of every warp wire crossing alternately above and below every weft wire and vice versa. Warp wires of the mesh run lengthwise to the cloth as woven. They are bent at right angles (crimped) where they pass over and under the straight wires. These straighter wires are known as the weft or shute wires and run crosswise to the woven cloth.
Mesh count (M) refers to the number of wires per linear inch. For example, a 200 mesh count would be 200 x 200, or 40,000 openings per square inch in a plain weave.
Mesh Open Area
The open area (O) is expressed as a percent which indicates the portion of total screen area which is open space. It is dependent upon the mesh count (M) and wire diameter (D). Generally, a higher percent open area for a given mesh count is associated with more uniform printing results across the pattern area. In addition to standard meshes, MicroScreen can provide mesh with ultra-fine wire that offers a higher percent open area.
The theoretical overall mesh thickness™ of twice the wire diameter is hardly reached in practice. In extreme cases, the overall thickness can be three times the wire diameter depending on the crimping heights of warp and weft wires.
Differences in the mesh thickness within the same roll due to weft wire changes are limited to the absolute minimum in the stainless steel cloths used in MicroScreen's screens.
Ink, when deposited through the screen, appears as cubes or columns, whose size is governed by the wire diameter, mesh opening, and emulsion build up. The cubes then flow together to form the actual pattern on the substrates. Should either the wire diameter or weave thickness deviate to the high or low side of the specifications, the thickness of the deposit can be affected.
The mesh used by MicroScreen features highly controlled uniform wire cloth thickness. This uniformity is the result of specially drawn wire and the most advanced weaving on electronically controlled looms which minimize roll variations.
Mesh can be mounted 45º, 22.5º, 30º, or 90º orientations of mesh weave to the frame. Better printing results are often obtained with 22.5º and 45º orientations, particularly for fine lines or irregularly shaped images. The orientation of weft and warp wires to the frame is very important on those screens with orthogonally mounted mesh. Since screens fatigue more rapidly in the direction of the squeegee motion it is best to position the more flexible wire in the cross print direction. This more flexible wire is the warp wire. The straight wire, or weft, is the more stable of the two, and, therefore, less sensitive to the squeegee motion. For best printing results, the mesh should be mounted so that the weft wire runs parallel to the squeegee stroke. This will increase the life of the screen and the squeegee.
Mesh Tensile Strength
The mesh used in our screen making operation is a special Type 304 wire with precise dimensions and surface characteristics. Wire tensile strength is exceptionally high, producing a wire with a yield point up to 20% higher than
conventional types. From an application standpoint, this gives greater elastic reserve and longer screen life. We offer the higher print quality associated with higher tension, yet still retain the 0.5% ±0.1% elastic elongation reserve for the squeegee stroke.
Stretched mesh tension is very important and often an area that is overlooked. Tension can be indicated in newtons per centimeter. However, for measurement convenience, it is usually expressed in mils of deflection in the center of the screen, resulting from one pound of applied force. In this case, deflection is also related to the size of the frame.
Consistency and repeatability of measurement technique is most important.
Screens with excessive deflection (too low tension) can result in poor print definition, elongation of image, mesh marks in the printed pattern, and slower allowable squeegee speed. On the other hand, screens which have been overstretched beyond their elongation reserve are then permanently deformed, and develop the same problems associated with understretching.
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