Based on the heat transfer theory

by simplifying the complicated and unstable question

the condition used to get metal castings without shrinkage porosity in the optimum state was presented.The optimum temperature gradient at the directional solidification direction and the solidification direction from side to center is in direct proportion to the difference between pouring temperature and solidus temperature

and is inversely proportional to the flow distance of liquid alloy at the solidification direction

meanwhile

temperature gradient at three direction should be kept pace.The way of getting the optimum is to make the thickness of coat at the mold cavity become thicker and thicker along the solidification direction

meanwhile

the gradient of coat thickness at the mold cavity should be less in order to produce castings without shrinkage defect efficiently under the optimum temperature gradient with decreasing in the thickness of mold.In addition

the heat conductive coefficient of coat is decreased

while the heat conductive coefficient of mold is increased as a result forced cooling at the beginning stage when the mold-wall of the permanent mold is decreased.