Flat receptacle, used in extruding the large aluminum wallboard, is under such work conditions of high pressure, temperature
and friction. It is recommended to apply combined structure to improve the flat receptacle strength and prolong its life span. However, due to the irregularity of inner hole, inconsistent resistance of shrinking fitting affects the inner hole within which nonuniform
deformation occurs. If the nonuniformity range exceeds the specified dimension accuracy, mould or (and) die repairing have to be done. In order to reduce nonuniform deformation, taken prestressed two layer flat container (670?mm×270?mm) applied in 80?MN extruder as research model, deformation regularity of inner hole for is obtained by FEM (finite element method), which shows that uneven deformation distribution of inner hole results from by stiffness difference because of different thickness of major and minor axes. Thus, concept of roundness is presented based on the above regularity, i.e. selecting outer layer with elliptic inner diameter and inner layer with elliptic outer diameter to change the uneven deformation. Then the prerequisites of selecting shrink range and verifying indexes are also presented to assure the prestress and improve die strength. The comparison on the numerical results under different roundness shows that the roundness applied in outer and inner layers can reduce quantitative performance index (
unevenness), save labor of die repair and expensive die material and improve dimensional precision. Moreover, we conclude that the unevenness can be lowed if K is in the span of 0.475 and 0.74, and that the unevenness reaches the minimum if roundness is 1.15 times larger than gap ratio. These results gives support to mold repair, design of shrink fitting and structural optimization.