An injection molding device for thermosetting molding materials includes a male mold portion, and a female mold portion. The male mold portion includes an injection aperture and a sprue communicated with the injection aperture. The female mold portion is mated with the male mold portion to define a parting face therebetween and includes a cold slug well and a main runner. The cold slug well is disposed at an opposite end of the sprue relative to the injection aperture and has at least two inner sidewalls perpendicular to each other. The main runner is defined in the parting face and commutating with the sprue. The refractive index of the products molded using the injection molding device may be obtained via directly measuring that of the cold slug after molding. Therefore, the time spent to measure the refractive index of the products is less than the conventional method.