The performance of thermoplastic composites depends considerably on the local thermal history that the material undergoes during processing. The thermal history not only affects the polymer matrix morphology but also defines the magnitude of residual stresses generated due to volumetric shrinkage, both of which significantly affect the final composite properties. In semi-crystalline polymers, morphology and shrinkage depend largely on the crystallization kinetics, which, for a thermoplastic composite matrix, can be vastly different mainly due to the presence of fibers, but also due to the nucleating agents and modifiers that are usually added to the polymer during prepreg manufacturing. In this work, the crystallization kinetics and the resulting morphology of polyamide 6 (PA6) are systematically studied for three materials: 1) pure PA6, 2) nucleated PA6 and 3) glass fiber reinforced PA6 prepreg, representing the formulations of the polymer matrix during composite production. By employing multiple differential scanning calorimetry (DSC) methods it was observed that, for a broad temperature range, nucleated PA6 crystallizes one order of magnitude faster than the pure PA6, while the addition of fibers slows down the crystallization due to hindrance. A considerable increase in crystallinity and lamellar thickness was measured using X-ray for nucleated and composite PA6, which translates to higher measured yield strength. Finally, it is shown that a coupled crystallization- and thermal model is capable of predicting accurately the local thermal history of the various materials during compression molding. This toolset is a vital input for a future residual stress prediction model for thermoplastic composites.