The clamping force is a critical parameter to the stability of mold during injection molding process. An improper setting of the mold clamping force can adversely affect the service life of movable and stationary platens, tie bars, and molds. Thus, the clamping force is a key factor in product-quality consistency. This study bonded strain gauges to both sides of the movable platen to measure, in real time, strain changes in the platen under different clamping-force settings. The results were assessed using the corresponding cavity pressure and mold separation values to determine the molding clamping force's effect on product quality (with respect to thickness, weight, and appearance). The findings indicated the following. (1) The mold clamping force is significantly correlated with cavity pressure, mold separation, and platen deformation. A low clamping force can cause mold separation to increase, which in turn results in greater platen deformation. (2) For the prediction of mold separation, strain gauges that are mounted on the movable platen can sufficiently replace displacement transducers that are placed within the molds. In other words, the condition inside the molds can be predicted using sensing devices outside the molds. (3) The analytic results for platen deformation and product quality indicate that an insufficient mold clamping force potentially results in excessive mold separation and platen deformation, causing flashing and an increase in product weight and thickness.