double layers of sand falling opening are arranged below the sand storage bin, which respectively are a staggered sand falling opening on an upper layer and a sand-pressing-plate sand falling opening on a lower layer;

the staggered sand falling opening on the upper layer comprises a left sand falling plate and a right sand falling plate which are staggered, the two sand falling plates form a horizontal staggered opening at a bottom position, and a vertical spacing of the horizontal staggered opening is adjustable; a vibration device is arranged on one side of the left sand falling plate or the right sand falling plate, and in a working state, sand flows out of the staggered opening and enters the sand-pressing-plate sand falling opening on the lower layer; and

the sand-pressing-plate sand falling opening on the lower layer comprises left and right sand pressing plates, a vertical sand falling opening is arranged between the sand pressing plates on two sides, a width of the vertical sand falling opening is adjustable, and the sand pressing plates on the two sides are fixedly connected to the frame via supporting members arranged on two sides below the frame.