Far field welding

Near field welding refers to joint V4 in. or less from area of horn contact far field welding to joint more than / from contact area. 

Figure 14.21 Near and far field welding, (a) Near field welding the contact surface of the horn is 6 mm (0.25 in.) or less away from the joint surface, (b) Far field welding the distance between the horn surface and the joint surface is greater than 6 mm (0.25 in.). Vibratory energy is transmitted through the top part to the joint interface.

Mattrial Gommem Trade Names Near Field Far Field Welding Anplitude 20 kHz Owraad... 

Side drilled holes are widely used as reference reflectors, especially when angle beam probes are used (e.g. for weld testing). However, the distance law of side drilled holes is different to that of a flat bottomed hole. In the literature [2] a conversion formula is given which allows to convert the diameter of a side drilled hole into the diameter of a flat bottomed hole and vice versa, valid in the far field only, and for diameters greater than 1.5 times the wave length. In practical application this formula can be used down to approximately one nearfield length, without making big mistakes. Fig. 2 shows curves recorded from real flat bottomed holes, and the uncorrected and corrected DGS curves. 

Early work lumped all of the heat input to the workpiece into a point source and used the Rosenthal solutions directly, demonstrating via instrumented welds that this was reasonably accurate for the far field (i.e., for distances greater than the shoulder radius) (Ref 6). Given the distributed nature of the heat source, it is also common to use the Rosenthal equations for FSW by integrating infinitesimal heat inputs distributed over the tool area, for example, assuming a constant power density or a power intensity that varies with the radius from the tool axis (Ref 7-9). If this level of detail is applied to the heat source, then other issues become significant, for example, the thermal boundary... 

More rigid materials can be welded with the joints farther away from the welding horn because the rigid polymer will transfer the vibrational energy moderately well. However, tough or elastomeric materials cannot be welded as far-field joints because they absorb the vibrational energy. 

At certain locations, generally near high-power broadcasting antennas or near high-power RF sealing or welding equipment, fields are Hkely to exceed the exposure guidelines. As NIER power radiates from a source, the spherical surface over which the wave spreads increases as the square of the distance from the source. This leads to the so-called inverse square law by which it is known that, in the far field of an antenna, the power density decreases rapidly. Calculation of RF exposure conditions rehes on this fact from basic physics, and the incorporation of certain conservative assumptions set forth by the FCC in its... 

Benatar, A., and Cheng, Z., Far-Field Ultrasonic Welding of Thermoplastics, yFVTEC Conf Proc., Soc. Plastics Eng., 1989. 

Two types of ultrasonic seal are possible, ie contact or near field such as with high frequency welding, where the electrode or applicator is applied directly to the material where the weld is to be made, and remote or far field which is suitable for joining rigid mouldings. With the latter, the tip of the work horn is applied to a point of the work which may be some inches away from the faces to be welded, and the vibrations are transmitted through the material. 

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