Injection molding method, screw for injection molding machine, and injection molding machine

An injection molding method, comprising:

• a plasticization step of supplying a solid resin raw material and reinforcing fibers to a cylinder including a screw, and rotating the screw in a normal direction to generate a mixture of the reinforcing fibers and a molten resin, the screw being rotatable around a rotation axis and being movable forward and rearward along the rotation axis;
• and
• an injection step of injecting the mixture into a cavity of a mold, wherein in the plasticization step,
• compression force that is higher than compression force on an upstream side of a constricting region is applied to the mixture in the constricting region, the constricting region being provided in at least a portion of the screw in the rotation axis direction,
• a pressure applied to the mixture passed through the constructing region is reduced in a reduced-pressure region on a downstream side of the constricting region, and
• the mixture is kneaded through the rotation of the screw after the pressure applied to the mixture is reduced, wherein the screw includes,
• a constricting section, a reduced-pressure section, and a kneading section, the constricting section being provided in at least a partial region through which the generated mixture passes and having an outer diameter D2 that is larger than a shaft diameter D1 of the screw on the upstream side of the partial region, the constricting section being formed in a ring shape having the outer diameter D2 larger than the shaft diameter D1 of the screw over an entire circumference, the reduced-pressure section being continuous with the constricting section on the downstream side and having a shaft diameter D3 that is smaller than the outer diameter D2 of the constricting section, and the kneading section being continuous with a downstream end of the reduced-pressure section and kneading the mixture,
• the constricting region is provided around the constricting section inside the cylinder,
• the reduced-pressure region is provided around the reduced-pressure section inside the cylinder, and
• the kneading section is provided around the reduced-pressure region inside the cylinder.