Abstract: Numerical simulation by finite element method and experimental investigation of the mechanism of void closing were carried out. It was found that the degree of closure λ, of a void is directly proportional to the maximum principal compressive strian ε1 arround the void. Contact friction between material and tool effects the distribution of stress and　strain states in the billet, but not the relationship ofλandε1. In the flat-die drawing　of a billet, the minimum ratio of die breadth to billet height which makes the top maximum compressive strain El to get at the billet center is 0.51. The optimum die breadth is0.7 of the billet height. For the FM method of drawing, the minimum die breadth is 0.4　of the billet height and the optimum is 0.6. The calculated results are. In good agreement　with experiment and now applied in industrial practice.

Key words: 孔洞闭合度, 压应变, 砧宽比