This investigation presents a methodology for the characterisation of thermoplastic composite materials under rapid processing conditions, as is needed for understanding the flow mechanisms in Unidirectional (UD) tapes. A bespoke compaction rig was designed and constructed to carry out a series of high temperature ramp-dwell loading experiments on CF/PPS (Carbon Fibre/Polyphenylene Sulphide) and CF/PEEK (Carbon Fibre/Polyetheretherketone) specimens. Significant deformations were observed above the melting points, where further increases in temperature led to gradually smaller through-thickness displacements and larger in-plane deformations. A phenomenological material model, initially developed for toughened thermosets, was applied, to assess its suitability for predicting the through-thickness behaviour of thermoplastic composites across a range of temperatures and pressures. The modelling approach was then deployed in Finite Element (FE) simulations of the compacted specimens, making use of the extracted parameters from the experimental tests.