The threaded connection of the ropewire tensioner mandrel 5 is self-locking and the mandrel 5 can be placed in the position of the mold roller to change the length of the core. A handle 51 is welded to the drive end of the mandrel 5 to facilitate rotation of the mandrel 5. In the fixed mode I, the de-molding is made more stable by the horizontal connection of the limit bar 14 of the 2nd mode 21, which prevents the drift of the moving 2nd moving mode 21. The working principle of the screw tensioner: molten metal liquid enters the mold cavity through the injection hole 11 on the fixed mode I. After the molten metal liquid is solidified by pressure and the workpiece is formed, the second moving mode 21 moves in the direction of the screw 4 axis because the water wheel 41 drives the screw 4 and the screw 4 axis is restricted by the fixed mode I. The second moving mode 21 moves with the hook 3 under the restriction of the limit block 13.