The question was posed to me in final exam.
My query is from Gravity Die Casting topic in division Special Casting Techniques of Casting-Forming-Welding I
To explain I would say: Air cooling is not sufficient except for thin and small castings. Generally, air cooled dies are provided with fins on the outer surfaces to increase the heat extraction. And cooling water can be also circulated through the portions of the mould that are likely to have large heat concentration.