Method of manufacturing hollow body
A hollow body formed of a thermoplastic resin and having uniform inner diameter and thickness and an excellent inner surface smoothness, and, in particular, a pipe-shaped hollow body having a bent portion is manufactured by injection molding. A method of manufacturing a hollow body includes injecting a molten resin 8 into a mold cavity 2 of a mold 1 , the mold cavity 2 having on its one end a pressure port 4 provided with a floating core 6 and on its other end an outlet 5 , pressure-injecting a pressurized fluid through the pressure port 4 after the injection of the molten resin 8 , and moving the floating core 6 toward the outlet 5 , and, at the same time, extruding the molten resin 8 from the outlet 5. In the method, the floating core 6 is constituted of a columnar portion and a top portion, which is continuously connected to one surface of the columnar portion and has a shape that a cross-sectional area perpendicular to a central axis of the columnar portion gradually decreases from one surface side of the columnar portion. When a diameter of the columnar portion is represented by A, the height of the columnar portion is 0.1 A to 1 A, and the height of the top portion is 0.3 A to 1.6 A.
1. A method of manufacturing a pipe-shaped hollow body having a bent portion, which comprises injecting a molten resin into a mold cavity of a mold, the mold cavity having on its one end a pressure port provided with a floating core and on its other end an outlet, pressure-injecting a pressurized fluid through the pressure port after the injection of the molten resin, and moving the floating core to the outlet side, and, at the same time, extruding the molten resin from the outlet, the method comprising:
the floating core is constituted of a columnar portion and a top portion, which is continuously connected to one surface of the columnar portion and has a shape that a cross-sectional area perpendicular to a central axis of the columnar portion gradually decreases from one surface side of the columnar portion, and when a diameter of the columnar portion is represented by A, the height of the columnar portion is 0.1 A to 1 A, and the height of the top portion is 0.3 A to 1.6 A, wherein
the floating core is fitted in the pressure port on the other surface side of the columnar portion, and
a peripheral edge of the other surface of the columnar portion of the floating core is subjected to a corner rounding processing with a curvature radius of A/1000 to A/3.
2. The method of manufacturing a hollow body according to claim 1 , wherein the floating core is fitted with a protrusion of the pressure port by a recess provided in the other surface of the columnar portion.
3. The method of manufacturing a hollow body according to claim 1 , wherein an area of one surface of the columnar portion is equal to an area of a surface on which the top portion is in contact with the columnar portion.
4. The method of manufacturing a hollow body according to claim 1 , wherein the central axis of the columnar portion is equal to the central axis of the top portion.
5. The method of manufacturing a hollow body according to claim 1 , wherein a shape of the top portion is a circular cone or a half-turned ellipsoidal body.
6. The method of manufacturing a hollow body according to claim 5 , wherein the half-turned ellipsoidal body is a hemisphere.