The importance of numerical investigations in joining processes increases with the development of advanced joining technologies, process designs and tool concepts. To predict the load bearing capabilities for fibre-reinforced thermoplastic (TPC) structural components, the knowledge of the resultant material structure is essential. In this paper, a forming process of a carbon fibre/polyamide 6-preform into a structured mould is investigated. An adapted integral bladder-assisted moulding process is used and numerically investigated by combination of an Arbitrary-Lagrangian-Eulerian method and a multi-filament approach. The objectives are to analyse the forming phenomena of bridging, compression and rearrangements of the fibres while forming. Due to the application of the Arbitrary-Lagrangian-Eulerian method, flow process of the matrix can be investigated. The proposed multi-filament modelling approach is presented in detail and the main phenomena of fibre rearrangement and flow processes during the forming process are discussed on the basis of micrographic and surface analyses of the experimental investigations. It is shown that the chosen numerical approach is suitable to model and analyse the phenomena of fibre bridging and the flow process to determine matrix rich zones. The quantitative comparison of the experimental and numerical results reveals, that the compaction of the fibre bundles is overestimated in the simulations. Furthermore, numerical parameters regarding contact behaviour and leakage significantly impact the modelling performance.