Method of forming box-like frame members

Abstract From the Canadian patent

A box-like frame member is formed by compressing an internallypressurized tubular blank within a die having a cavity conforming to the final box-like cross section desired for the product and, increasing the pressure to exceed the yield limit of the wall of the blank to expand the blank into conformity with the die cavity. The blank is selected so that the final product and the die cavity have a circumference preferably no more than about 5% larger than the circumference of the blank to avoid weakening or, cracking of the blank through excessive circumferential expansion. The internal pressure forces the blank evenly into the corners of the die on closing and allows the blank to be confined …