The direct long-fiber-reinforced thermoplastic (D-LFT) process is an efficient, one-stop manufacturing process starting from raw materials to a final product, and includes various types of equipment. Tandem twin-screw extruders are the main components of the D-LFT process, and their control dictates productivity and properties of products. This study investigates the effects of extruder temperature and screw speed on molecular weight and thermal properties of glass fiber-reinforced polyamide 6 (PA6) composites throughout the D-LFT process. Viscosity number measurements, thermogravimetric analyses (TGA), and differential scanning calorimetry (DSC) analyses were performed on samples taken from different locations along the D-LFT process. It was found that viscosity number, which is a measure of molecular weight of the PA6 base resin, decreased with increasing extruder temperature and decreasing screw speed. In contrast, TGA results showed that the low screw speed of the extruders increased apparent activation energy of the final product. Non-isothermal DSC crystallization analysis revealed no substantial changes to the material's degree of crystallinity with the variations in extruder temperature and screw speed; however, isothermal DSC crystallization analysis showed that the low screw speed of the extruders increased crystallization half-time of the final material. POLYM. ENG. SCI., 58:E114-E123, 2018. (c) 2017 Society of Plastics Engineers