Page 167 - Industrial Cutting of Textile Materials
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154                                         Industrial Cutting of Textile Materials

         Cutting nozzle
         A cutting nozzle guides the stream of compressed air into the cutting gap and protects
         optical lenses from flying sparks, vapours, and particles generated on the cut material
         surface. The use of the correct nozzle for the process is very important. If the nozzle is
         too small, the cut edge is not cleanly cut, and slag clings to the lower edge of the kerf.
         In the extreme case, the material is not even parted. If the nozzle is too large, the gas
         pressure is lower, and cutting gas consumption increases, but the cutting quality is not
         significantly affected.
           The distance between the nozzle and the cut material surface has also a great effect
         on the cutting quality. It determines the placement of the focal point on the cut material
         surface.

         10.2.1.3   Cutting gas jet (shield gas, assist gas, process gas)

         The compressed gas jet is used to not only keep focusing lens clean but also improve
         cutting quality. During the work process, the laser beam heats, melts, and partially
         or completely vaporizes the cut material. The stream of compressed gas (air or ni-
         trogen) is led to the material through a cutting nozzle to drive out the debris and
         molten material from the cut kerf (the groove made while cutting). Together with
         the debris, it is vacuumed off downwards through the cutting gap and surface of
         the vacuum table (see Section 10.3.2). The gas also cools the heat-affected material
         zone and thus reduces cutting width, ensures uniform cut edges, and minimizes
         material oxidization marks on cut edges. Clean air is mostly used as the cutting gas
         for processing textiles.
           The cutting quality is very much dependent on the pressure of the cutting gas. If
         the pressure is too low, the fluid slag can remain adhered to the cut material, forming a
         permanent burr or closing the kerf again. If the pressure is too high, the lower edges of
         the cut can be burnt out and often make the cut unusable. Cutting gas pressure has to be
         increased increasing the material thickness. Fine jets of air ensure neat-cut edges and
         material surfaces. Weak but wide air jets are beneficial to perform material engraving.


         10.2.2   Systems to guide the laser cutting tool
         There are two basic systems to guide the laser cutting tool, gantry system and galva-
         nometer (galvo) system.

         10.2.2.1   The gantry laser cutting system
         The gantry (linear, flat bed) laser cutting system works similar to blade cutters. A laser
         cutting device is fixed on the gantry moving along a set of tracks (see Fig. 10.3). The
         cutting device is powered by several servomotors that move the laser cutting head in X
         and Y directions. The Z-axis movement can be used to raise and lower the cutting head
         changing focal length. The beam is focused through the lens down onto the workpiece.
         The graphic design (CAD drawing) of the processed component is loaded into the
         laser system's control software. The control software then directs the cutting device in
         the work area in a linear motion in vector or raster mode (see Section 10.2.2.3).
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