Ink wash-off has been one of the problems that has confounded the overall success of the in-mold decoration (IMD) process for injection molded parts. In this report, an experimental matrix design based on the Taguchi method was conducted to examine the influence of processing parameters on the ink washoff of molded parts. The resins used in the study were 20% glass fiber filled poly(ethylene terephthalate) (PET). A commercially available PET film pre-printed with grids was employed to prepare the parts. Specimens of three different thicknesses were molded using a pin injection gate on an injection molding machine with a reciprocating screw system. After molding, the severity of ink washoff was determined by an image analysis program. The experimental results suggested that molded composite parts of smaller thicknesses exhibited more severe ink washoff. For the parameters selected in the experiments, the melt temperature and mold temperature were found to be the principal factors affecting the ink failure of IMD injection molded composites. Furthermore, numerical simulations using a commercially available code were completed to better understand the mechanism of the ink washoff problem. The calculated results suggested that the ink was mainly washed off by the high shear stress of hot polymer melts. It is shown that the formation mechanisms of ink washoff can be explained, and, this being done, steps can be taken to ensure that the washoff is minimized. This provides significant advantages in terms of improved product quality.