Co-authors​ :

 Amal ALLIYANKAL VIJAYAKUMAR (ITALY), Muhammad ZAHID , Stefano G. CORVAGLIA , Francesca LIONETTO , Alfonso MAFFEZZOLI (ITALY) 

The use of thermoplastic sandwich structures is associated with their lightweight, high-specific bending strength and stiffness, thermal insulation, recyclability, and shock-absorbing capabilities. In the manufacturing of sandwich structures, the bonding of skins to the core is a crucial and critical process. Conventionally used sandwich manufacturing techniques, such as compression moulding, vacuum bagging, double belt lamination, and in-situ foaming, possess drawbacks such as a limited variety of geometries, high energy consumption, and weak interface bonding. In some cases, it also requires high-cost and labour-intensive operations in the presence of complex working conditions. In this study, a novel joining technique derived from welding thermoplastic matrix composites has been adopted to join the same thermoplastic skin and foam core. The proposed approach aims to develop thermoplastic sandwich structures using an ultrasonic or induction welding process characterised by fast processing and great flexibility of core thickness and geometries. For instance, polypropylene (PP) and polyethylene terephthalate (PET) have been studied as skin and foam core matrices, whereas, E-glass and carbon fibres were used as skin reinforcements for PP and PET, respectively. The influence of the fundamental process parameters during welding has been optimised as a function of the interfacial adhesion. Similarly, the effect of thermoplastic film thickness and other types of energy directors at the interface on welding quality and interfacial properties was investigated. Morphological analysis was performed to assess weld quality, fractured surfaces, and the collapse of foam cells. Single cantilever beam (SCB) fracture toughness tests (Mode I) were used to analyse interfacial mechanical properties. The sizing of the SCB sandwich specimens for measuring the debonding was configured according to the protocol proposed by Ratcliffe. Compared to traditional methods, the joining of thermoplastic skin and core through welding presents several advantages, such as high performance, ease of use, cost-effectiveness, versatility of applications, and reproducibility. Based on the proposed technologies, the performance of sandwich structures for aerospace, marine, automotive, rail, or sporting applications can be tailored. Thus, the overall objective is to develop a deep knowledge of the complex interplay between the processing conditions and the skin/core bond obtained through ultrasonic and induction welding processes.