Abstract
Pressure loss through a bend in the runner of an injection mold was studied with full 3D flow simulation. Two types of runners were used for this study. One was a straight runner and the other was a bent runner, where these runners only differed in the presence of the bend for the bent runner. The tested polymer was polycarbonate. It is well known that a bend causes an additional pressure loss when the fluid is water. For the polymer melt, however, the simulation showed that the pressure loss in the bent runner was smaller than that in the straight runner. This phenomenon, negative pressure difference, was also observed by experiment. By investigating the calculated results in detail, we found three reasons for the negative pressure difference. The first is the decrease in wall resistance because part of the inner side wall vanishes at the bend. The second cause is in the flow path at the bend. Since the main flow path is diagonal in the bend, the flow length in the bent runner is short in comparison with that in the straight one, and the pressure loss of the bent runner is consequently reduced. The third reason is because the viscosity near the side walls in the region beyond the bend is lower than that in the straight runner, and this difference in the viscosity is caused by a change in the temperature profile in the width direction of the runner.
