Glass fiber reinforced polypropylene (GF-PP) composites have proven a great potential in the designation and manufacturing of control arms, leaf springs, barriers, beams and bridge decks. Two-pass injection molding of GF-PP was investigated in this study. Polypropylene (PP) was injected with different weight fractions (w%) of chopped glass fibers (GFs) with different fiber feedstock lengths (FFSLs). The composites were then crushed and re-injected once again. Some specimens were burned out to check the actual weight fractions and fiber lengths after the injection processes. The fiber lengths dramatically decreased due to damages during two-pass injection processes and crushing. The manufactured specimens were tested in tension, and the results indicated that the tensile strength increased slightly at w = 10% of GF. Further increase of GF weight fraction leads to a drop in the tensile strength below neat PP. The results of SEM micrographs showed an increase in air voids concentrations at high GF percentages which clarifies the reason behind the drop in the tensile strength. Also, in-plane shear tests were carried out using the Iosipescu fixture where a slight increase in the in-plane shear strength was noticed by increasing fiber weight fractions. In-plane shear moduli of all specimens were measured experimentally by strain gauges and calculated theoretically. The shear modulus was enhanced by glass fiber addition and a further increase was noticed by increasing the fiber weight fractions.
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