Not all subjects are perfect, why do common-used injection molds have transparency defects? let's analyse the reasons. The transparency defects reasons: welds spots, silver marks, crack polystyrene and transparent products of PMMA material, through which the sparkling silver marks can be seen under light. The sliver marks is called cracks, the stress is caused by tensile stress, and makes polymer molecule flow at stress direction, and the difference in refractive index between stress direction and without stress direction, silver marks appear between the two interfaces under light.