Thermal joining of foamed thermoplastic injection-moulded parts using laser radiation

Abstract

The foam injection moulding is an established process to produce plastic components with reduced density. Due to the fragile foam structure, contactless welding methods such as laser transmission welding are suitable for foamed components. In a research project, the correlation between the injection moulding parameters and the resulting foam structure is determined in preliminary tests. Specimens with four different foam structures are produced, which differ in terms of their surface layer thickness and cell density. Measurements of the transmittance, as well as the intensity profile after irradiation of the transparent specimen, show that the transmission of the foamed test specimens is lower than compared to compact specimens. In particular, foam structures with low cell density and low surface layer thickness show a low transmittance. The weld strength is influenced only by the foam structure of the transparent joining partner. Additionally, a possible post-expansion of the enclosed gas during the welding process has to be taken into account. The temperature within the laser-absorbing joining partner is locally increased above the melting point of the plastic. Once the surrounding material is melted, the gas can expand due to the temperature increase and defects during the solidification of the melt are formed.