Rotary draw bending of small diameter copper tubes: predicting the quality of the cross-section

The purpose of the present work is to study the correlation between the main geometrical tube parameters of the rotary draw bending process and the resulting quality of the cross-section of small hydraulic pipings. The method of choice, named free rotary draw bending, is a rotary draw bending process where no internal support from an internal mandrel or plug is possible, because hydraulic tubes cannot be lubricated on their internal diameter. Furthermore, no axial boosting of the tube or of the pressure die is applied. Free bending is mainly used when small-diameter tubes are involved, with high length and relatively thin wall. A unique quality estimator QCS of the cross-section geometry of bent tubes has been developed, which is able to synthetically represent the main functional and aesthetical properties of tubes: the maximum ovalization, the minimum hydraulic diameter, and so on. In the case of copper tubes for hydraulic applications a strongly deformed cross-section after bending means a poor quality of the product, since it delivers an increase of head losses and of pressure drops. The deformation occurring in the cross-section of the tube has been studied through a set of FEM simulations, run with an explicit code with shell elements, and the model has been validated by comparing the experimental and numerical results of specific cases, in terms of geometry of the cross-section in a 90 bend. The plan of simulations has been designed using the typical copper tube dimensions for hydro-thermo-sanitary applications and a few runs have been added with modified parameters with the aim of completing the plan for evaluating the sensitivity to a change either in geometrical variables (thickness t, outer diameter OD, mean bending radius RM) or in the tube material. An indicator (bending factor) is proposed, able to reliably estimate the difficulty of a bending process with respect to tendency to ovalization and collapse of the crosssection. Some design guidelines of the bending tools are provided.