Branched and linear polyacetals prepared by cationic bulk polymerization were molded under high-injection rate and pressure, and the resultant 1-mm-thick specimens were investigated regarding the crystalline morphology, mechanical properties, and transparency. The branched polyacetal exhibited shear-induced transformation of crystalline morphology, namely, the spherulites, the elongated spherulites, and shish-kebab morphology parallel to the flow direction, with increasing shear viscosity. The degree of orientation of the branched polyacetal, calculated from the intensity distribution on the Debye ring of the (100) diffraction by WAXS, linearly and significantly increased with the increase of the logarithm of the shear viscosity. The difference of the crystalline morphology greatly influenced the mechanical properties and transparency of the branched and linear polyacetals. The branched polyacetal with the shish-kebab morphology had approximately 20%, higher tensile strength and modulus as compared with those with the spherulites morphology, and showed translucent with a higher light transmittance over a wide range of wavelength of incident light. The results indicate that a large number of fibrous crystals in the shish-kebab morphology result in the self-reinforcement of specimens parallel to the flow direction and diminishment of the scattering of incident light. (c) 2006 Wiley Periodicals, Inc.