Influence of induction welding on PEKK crystallization

Abstract

Current studies about crystallization kinetics of polymers are aimed at Differential Scan- ning Calorimetry (DSC), characterized by constant conditions of temperature, cooling and heating rate. This is of limited use to industry, as non constant cooling/ heating rates occur during most manufacturing processes. This research is aimed at predict- ing the crystallization kinetics for non constant cooling rates of carbon fibre (CF) and Polyetherketoneketone (PEKK) during the induction welding process. An analytical model on the basis of the isoconversional method is developed and cou- pled to multi physics finite element simulations to investigate the effect of process pa- rameters on the degree of crystallinity at the weld interface. The study was validated by static and dynamic welding experiments. Careful setting of the process parameters has to be chosen to result in an increase of the degree of crystallinity at the weld interface. Using the proposed models welding times were adjusted to optimize the degree of crys- tallinity at the weld interface. The models are able to establish a relation between an arbitrary thermal program and the degree of crystallinity produced during this program. Linking the crystallization model to the simulations allowed investigation of the crystallinity throughout the work piece, which is not possible to do with experiments only. It was found that induction welding results in a localized heat point, which will result in fast cooling rates at the weld interface. In contrast to previous studies, our research is able to relate the crystallinity to non constant cooling programs, which allows direct application in the induction weld- ing process.