Welding ultrasonic horn

Ultrasonic welding of parts fabricated from ABS, acetals, nylon, PPO, polycarbonate, polysulfone, and thermoplastic polyesters should be considered as early in the design of the part as possible. Very often, minor modifications in part design will make ultrasonic welding more convenient. The plastic resin manufacturer or ultrasonic equipment suppher can recommend best joint design and ultrasonic horn design. 

Advanced thermal welding Advanced welding is similar to conventional thermal welding except that the heating is achieved by indirect contact of the fabric with a source such as ultrasonic horn, electromagnetic field, or laser. A separate heat-activated adhesive material can also be used. Fully or partially synthetic fabrics (usually woven) with thermoplastic components that are chemically and physically compatible when fused together. 

Ultrasonic horns can vary in size up to a maximum of about 24 cm by 4 cm if rectangular, or about 9 cm in diameter if circular. The actual shape of the hom relates to the particular welding operation to be achieved. One of the advantages of ultrasonic welding is that the same process can incorporate an ultrasonic cutting edge to enable cut and seam processing. 

Figure 14.19 Ultrasonic welding using an energy director. The energy director is molded onto the upper part being joined. During welding, the horn is in contact with the upper part, and force is applied perpendicular to the axis of the weld interface. Vertical vibratory motion applied to the part results in heat generation and melting of the energy director.

Ultrasonic head forming and welding is a fast assembly technique. It is a very rapid operation of about 2 seconds or less and lends itself to full automation. In this process high-frequency vibrations and pressure are applied to the products to be joined, heat is generated at the plastic causing it to flow, and, when the vibrations cease, the melt solidifies. The heart of the ultrasonic system is the horn, which is made of a metal that can be carefully tuned to the frequency of the system. The manufacture of the horn and its shape is normally developed by the manufacturer of the equipment. The results from this operation are not only economical, but also most satisfactory from a quality control standpoint. 

Ultrasonic welding the two basic techniques are plunge welding and continuous welding. In plunge welding, the parts are placed under a tool or horn the horn descends to the part under moderate pressure and the weld cycle is initiated. In the continuous welding process, the horn may scan the part or the material is passed over or under the horn on a continual basis. 

A noncrystalline polymeric material that has no definite order or crystallinity. A polymer in which the macromo-lecular chain has a random conformation in solid (glassy or rubbery) state. On the one hand, an amorphous polymer may show a short range order, while on the other, a crystalline polymer may be quenched to the amorphous state (viz., polyethylene terephthalate (PET)). The maximum value of a periodically varying function, e.g., used to describe the energy transmitted from the ultrasonic welding horn to the weld joint. 

Five components are essential in ultrasonic welding power supply, converter, horn, fixture, and a stand. The power supply converts a 60 Hz line frequency to 20 kHz. An electrostrictive converter mounted in the stand then converts this electrical energy to 20 kHz mechanical vibratory energy. This is accomplished by the use of a piezoelectric transducer. 

In ultrasonic welding, the vibrating horn contacts one of the mating parts under controlled pressure through a time cycle (usually less than 1 s). Ultrasonic vibrations pass from the horn to the interface or joint resulting in localized heat. The matrix melts, flows, and forms a weld. Both mating halves remain cool except at the joint where the energy quickly is dissipated. After the ultrasonics are turned off, the horn maintains a brief... 

With the exception of glass-reinforced fluorocarbons, such as polytetrafluoroethylene and fluo-rinated ethylene-propylene, most all materials can be ultrasonically welded. Table 5-5 compares the welded strength of several glass-reinforced materials. However, horn wear problems can arise from the abrasiveness of the fiberglass. This problem is usually minimal with less than 20% glass. Welds can be made with glass levels between 20 and 35% but some wear will result. Strong welds cannot be assured at levels above 35% due to insufficient fusable resin . 

Figure 8.8 Stages in the ultrasonic welding process. In Phase 1, the horn is placed in contact with the part, pressure is applied, and vibratory motion is started. Heat generation due to friction melts the energy director, and it flows into the joint interface. The weld displacement begins to increase as the distance between the parts decreases. In Phase 2, the melting rate increases, resulting in increased weld displacement, and the part surfaces meet. Steady-state melting occurs in Phase 3, as a constant melt layer thickness is maintained in the weld. In Phase 4, the holding phase, vibrations cease. Maximum di lacement is reached, and inter-molecular diffusion occurs as the weld cools and solidifies. ...

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