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Embedded Heating Element

Fig. 7.60. Resistance Heating Methods for Joining Thermoplastic Composites1

Embedded Heating Element

Fig. 7.60. Resistance Heating Methods for Joining Thermoplastic Composites1

Ultrasonic Welding. Ultrasonic welding is used extensively in commercial processes to join lower temperature unreinforced thermoplastics and can also be used for advanced thermoplastic composites. As shown in Fig. 7.61, an ultrasonic horn, also known as a sonotrode, is used to produce ultrasonic energy at the composite interfaces. Electrical energy is converted into mechanical energy. The sonotrode is placed in contact with one of the pieces to be joined. The second piece is held stationary while the vibrating piece creates frictional heating at the interface. Ultrasonic frequencies of 20-40 kHz are normally used. The process works best if one of the surfaces has small asperities that act as energy directors or intensifiers. The asperities have a high energy per unit volume and melt before the surrounding material. The quality of the bond is increased with increasing time, pressure, and amplitude of the signal.38 Again, it is common practice to incorporate a thin layer of neat resin film to provide gap filling. Typical weld parameters are less than 10 s at 70-200 psi pressure.44 This process is somewhat similar to spot welding of metals.

Induction Welding. Similar to resistance welding, induction welding techniques have been developed in which a metallic susceptor may, or may not, be placed in the bondline. It is generally accepted that the use of a metallic susceptor produces superior joint strengths. A typical induction set-up, shown in Fig. 7.62, uses an induction coil to generate an electromagnetic field that results

Welding Force

Welding Force

Fig. 7.61. Ultrasonic Joining Method for Thermoplastic Composites44

in eddy current heating in the conductive susceptor and/or by hysteresis losses in the susceptor. Susceptor materials evaluated include iron, nickel, carbon fibers, and copper meshes. As with resistance heating, it is normal practice to place a layer of polymer film on each side of the metallic susceptor. Typical welding parameters are 5-30 min at 50-200 psi pressure.44

A comparison of single lap shear strengths produced in thermoplastic composites, using the various techniques described above, is given in Fig. 7.63. Note that adhesive bonding yields lower joint strengths than the fusion bonding techniques and is very dependent on the surface preparation method used. Autoclave co-consolidated (melt fusion) joint strengths approach virgin autoclave molded strengths. Typically, resistance and induction welding strengths exhibit similar properties, both of which are superior to those of ultrasonic welding.

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