Backflow prevention device for in-line screw injection molding machine

A backflow prevention device for an in-line screw injection molding machine, the device being used for preventing backflow of resin from a front of a screw to surroundings of the screw, the device comprising:

• a screw tip provided on a distal end of the screw with a shaft interposed therebetween, the shaft extending from a rear end surface of the screw tip, and the screw tip having a plurality of grooves in a peripheral portion of the rear end surface;
• a check ring fitted around the shaft, the check ring being configured such that a rear end surface of the check ring is separated from a front end portion of the screw and thereby a resin path, in which resin is sent to a front of the screw tip from the surroundings of the screw through a space between the check ring and the shaft, is connected when the screw is rotated in a forward direction to send the resin forward, and such that the rear end surface of the check ring is brought into contact with the front end portion of the screw and thereby the resin path is closed when the screw is advanced, the check ring being provided with a plurality of teeth with which the respective grooves are engaged,
• wherein
• each of the grooves has one side surface which is pressed against the corresponding engaged tooth when the screw is rotated in the forward direction and another side surface located on a reverse side of said one side surface, said one side surface is formed vertical to the rear end surface of the screw tip, said another side surface is inclined such that a width of each of the grooves is gradually narrowed toward a bottom thereof,
• each of the teeth has one side surface against which the corresponding engaged groove is pressed when the screw is rotated in the forward direction and another side surface located on a reverse side of said one side surface, said one side surface is formed vertical to a front end surface of the check ring, and said another side surface is inclined at the same angle as that of the grooves such that a width of each of the teeth is gradually narrowed toward a distal end thereof, wherein
• a stepped portion is provided in the middle of said inclined another side surface of each of the grooves of the screw tip, and the stepped portion is configured such that the rear end surface of the check ring contacts the front end portion of the screw when the stepped portion contacts a distal end of the tooth.