a water - cooled mold for continuous casting includes two water - cooled wide copper plates arranged opposite to each other in a front and back direction and two water - cooled narrow copper plates arranged opposite to each other in a left and right direction . the upper portion of the cavity of the mold is a sprue area and the lower portion of the cavity is a mold cavity area . the sprue area is gradually narrowed in the casting direction and smoothly transited into the mold cavity , corresponding to the shape of a slab to be cast . the inside surfaces of the water - cooled narrow copper plates are smooth planar surfaces . a portion of the inside surface of the water - cooled wide copper plates in the sprue area is a curved surface and another portion in the mold cavity area is a planar surface . the curved surface and the planar surface portions form a continuous smooth surface .