Tailored irradiation by VCSEL for controlled thermal states in thermoplastic tape placement
In this paper, the use of Vertical-Cavity Surface-Emitting Lasers (VCSEL) for thermoplastic tape placement is presented. Thanks to their individually addressable emitters, high-power VCSEL modules enable a customized irradiation, whereby a (speed-adaptive) control of the materials' thermal state becomes viable. Especially of interest for us was the temperature gradient in through-thickness direction within the tape. The irradiation approach promises a new level of thermal controllability through not only controlling the surface temperature (state of the art), but also the thermal penetration and thereby the melt pool depth, melt dwell time and bond interface cooling rate. The novel control approach could improve the placement rate of (in-situ) thermoplastic tape placement beyond 1 m/s, without losses of the weld strength. To achieve a purposeful control of the materials' thermal state with the VCSEL, a prior inverse computation was required to predict the appropriate irradiation setting. The methods and results of inverse computations will be presented - numerical and analytical. A comparison of the computations with experiments was done by thermal imaging equipment. The experimental setup will be shown as well as the realization of the ""tailored"" irradiation by the VCSEL (the approximated ideal irradiation distribution). Finally, the advantages for the thermoplastic tape placement process will be discussed. These include an influence on the degree of intimate surface contact, autohesion and state of crystallization in the bond interface. The research work focuses on the thin tape, but is transferable to the generally thicker substrate.