To investigate the conditions necessary for glass fiber bundle dispersion in the polymer compounding process, we developed a method for experimentally quantifying dispersion failure in this study. We employed a co-rotating twin-screw extruder in our experiment and investigated the dependence of frequency of occurrence of dispersion failure on screw configurations (a forward kneading disk (FKD) and a backward mixing screw (BMS) with different screw configurations), throughput (Q), and screw speed (Ns). From our experiments, we found that increasing the production rate promotes the occurrence of dispersion failure when a FKD is used. Dispersion failure is markedly suppressed when a BMS is used. We thus found that the production rate could be increased by using a BMS with a large length-to-diameter (L/D) ratio.