Study of Different Joints for Ultrasonic Welding of Semicrystalline Polymers

Abstract

U ltrasonic welding (USW) uses vibratory energy at high frequencies that are beyond the range of human hearing to produce low amplitude (20-30 mm) mechanical vibrations. The vibrations generate heat at the interface of the parts being welded, resulting in melting of the thermoplastic materials and weld formation after cooling. Figure 1 shows the schematic of the USW process.There are two basic types of joints practiced by the industry for USW of thermoplastics, the energy director (ED) joint and the shear joint. An ED is a mold in protrusion located at the joint interface that is used to concentrate the ultrasonic energy at the interface. Industry uses a triangular-shaped ED for simplicity of production. The other type of joint mostly used by the industry is the ''shear joint,'' which is an interference type of joint. Figure 2 shows the two types of joints practiced by the industry.The ultrasonic bond is usually accomplished in two ways, by contact sealing, also called near-field welding (distance from horn is less than 6 mm), and remote sealing, also called farfield welding (distance from the horn is greater than 6 mm), or a combination of both. Figure 3 shows the near-and far-field configurations. Practically all thermoplastics can be contact sealed and components having rigid shapes can be remote sealed. Amorphous thermoplastics like acrylonitrile butadiene styrene (ABS), polycarbonate, acrylic, polystyrene, and polysulfone are rigid and easily welded ultrasonically. As they do not greatly dampen the ultrasonic waves, they can be welded both in the near-field and in the far-field regions. Semicrystalline thermoplastics, on the other hand, are soft and are characterized by regions of ordered molecular structure, which acts like springs in the solid state and absorbs a large part of the ultrasonic vibrations instead of transmitting them to the joint interface. They include high-density polyethylene (HDPE), acetal, nylon, polyester, polyethylene, polypropylene, and polyphenylene sulphide. As they are difficult to weld, especially in the far-field region, attention has to be given to the design of the joint, length of the specimen, and fixturing of parts.A few standard joint designs practiced by the industry to weld semicrystalline polymer HDPE were studied in terms of the interface temperature, collapse, energy absorbed, and strength at fracture to arrive at the suitable joint configuration for welding semicrystalline polymers in the far-field region. The joints were grouped as ED joints and non-ED joints. Simulation of temperature distribution was carried out using ANSYS (ANSYS, Inc., Canonsburg, PA) for the different joint designs and compared with the experimental results. LITERATURE REVIEWBenatar and Cheng 1 studied the USW of thermoplastics in the far-field configuration. They found that both amorphous and semicrystalline polymers had stronger ultrasonic welded joints when the joint interface was at a displacement antinode. Grewell 2 studied the far-field welding of polypropylene and was successful in weldi...

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“…The semi-crystalline PP weld strength was three times higher than the amorphous PS weld strength (26 MPa versus 8 MPa, respectively), and required more energy to achieve optimum bond strength than the amorphous PS. The shape of the energy director is also a significant parameter and studies have been conducted to assess the effect of shape differences on bond quality [79,81,82]. Chuah et al assessed three energy director shapes in far-field ultrasonic welding of amorphous ABS and semi-crystalline polyester [81].…”

“…Additionally, the technique has limited applicability to semi-crystalline polymers. Two approaches are available for ultrasonic welding of thermoplastics; near-field (interface less than 6 mm from the ultrasonic horn) or far-field (interface greater than 6 mm from the horn) [77][78][79]. All thermoplastics can be welded in the near-field configuration with the correct choice of parameters.…”

Influence of Distribution Size of Micro Energy Director Array on Ultrasonic Precise Sealing of Polymer MEMS Device

“…For energy director in ultrasonic welding, Rani et al studied different joint forms, finding that surface with a rough structure has greater sealing strength than larger-sized energy director. Meanwhile, the absorbed energy is smaller, and the micro-device is less deformed [12] ; Qiu et al believed that for surface structure with a certain height and roughness, there is a suitable pressure to enable the optimal bonding effect [13] ; Buckley et al simplified the rough surface to an ideal rectangular model, simulated the wetting process based on approximation extrusion fluid theory. The research results indicate that polymer diffusion and wetting process is quite sensitive to surface morphology of the sealing surface, and surface with low roughness is easy to cause uneven diffusion on the sealing surface, thus increasing the occurrence probability of defects [14][15] .…”