The Joining Techniques for Thermoplastics Materials in Automotive Industries: A Comprehensive Literature Review

Nowadays the use of thermoplastic materials has been increasing steadily, especially in automotive industries because of its positive effects on vehicle weight which is directly related to fuel consumption. These materials also provide a cost reduction for companies comparing with the steel or other similar materials. The other benefits of the thermoplastic materials are their high stiffness, excellent crashworthiness due to their energy-absorption characteristics, strength-to-weight ratios, fatigue and optimum design. Through their structure occurred by the polymer resins, thermoplastic materials can physically become a homogenized liquid when heated and hard when cooled. The thermoplastic materials are able to reheat, remolded and have good thermal and chemical stability. Also, these materials can be easily recycled which provides a lower environmental impact on the automotive industry. Due to the advantages of the thermoplastic materials, automotive industries have been using these technology in vehicle parts such as door panels, seat backs, load floor, engine cover, front end module, airbag housing, crash boxes, bumpers, instrument panel, air intake manifold, air duck, cross car beam, pedal brackets, gas tank carrier, etc. In order to produce the thermoplastic materials, a number of different methods (i.e. mechanical fastenings, ultrasonic assembly, metal inserts, snap fits, electromagnetic and heat welding, solvent/adhesive bonding) are proposed in the literature and most of them are successfully carried out in industrial applications. However, the identifying the joining technique according to the application area is an important issue to obtain appropriate material. Therefore, this paper presents a literature review of joining methods for thermoplastic materials and classifies the methods according to the structure of the joining technique. Within this context, more than 50 studies about joining techniques for thermoplastic materials are considered the methods are grouped into three main categories: chemical joining techniques, mechanical joining techniques, and thermal joining techniques. Chemical joining methods melt the surfaces of the materials by using a chemical solvent. By using the solvent, one plastic material is joined to itself or the material is joined to another type plastic that dissolves in the same solvent. In mechanical joining techniques, the materials are bonded by using some physical methods such as clipping, clamping, screwing, riveting, etc. Similarly, in thermal joining techniques the surface of the materials to be joined are heated and a pressure is applied until the thermoplastic material is formed. As a result of the review, the differences and efficiency of the joining methods are pointed out in the study with paired comparisons. Moreover, the real life applications of joining methods for thermoplastic materials in the automotive industry are presented. In this paper, effects of the joining techniques on pedestrian and occupant safety are also reviewed by taking into account the high-stress concentration factor, the inconvenient manufacturing process and, the reaction force peaks. Finally, the future challenges of the three categorized are summarized.