A mold structure has a first die, and a second die. A first buckling element is protruded downwardly from the first die, and defines a first contact surface and a bottom surface. The bottom surface has an end adjacent to the first contact surface protruded downwards to form a jamming portion, and the other end protruded downwards to form a blocking portion spaced away from and beyond the jamming portion. The jamming portion has a triangle cross-section, with a point facing downwardly and away from the blocking portion. A second buckling element is extended upwards from the second die, and has a gap at a top end thereof and passing through a side thereof. The gap defines a second contact surface, corresponding to the first contact surface, and an inclining surface, intersecting with the second contact surface with an acute angle formed therebetween for engaging with the jamming portion.