Stretch Forming Process Modeling: The Role of Modern Stretch Forming Machinery Design and Performance

Abstract

Abstract Extrusion stretch forming is used extensively in the aerospace and architectural industries to add contour to extrusions and roll formed sections. Frame members, stringers, wing spars, curtain tracks and many other important aircraft parts are formed with this process. Forming is achieved by pulling an initially straight part in the tensile direction above the material’s yield point and then wrapping the section around a die to add contour. Local buckling and wrinkling that might appear in a pure bending operation can be avoided. There is current interest in improving the process for greater repeatability and less part rework to reduce cost while achieving tighter tolerances (e.g. [1,2]). The stretch forming die plays a significant role in the process. To this end researchers are interested in quicker die development techniques using non-linear beam theory and non-linear finite element modeling of the forming process. For a complete analytical picture of the process, a close look at the stretch forming machine’s performance must be included in the process model. Two major areas of machine performance are important; machine deflections and hydraulic control system performance. This paper provides a brief overview of the extrusion stretch forming process and then focuses on the structural and control system design of the modern stretch forming machine. Analytical models of the machine deflection as well as its hydraulic control system are developed. A short discussion concerning the difference between traditional “pressure forming” and modern CNC position forming is also included. Insight into the limitations of traditional PID control for the stretch forming machine can be seen from the analysis. It is evident that these machine models must be used to complete the process model to effectively create die designs for close tolerance and highly repetitive part production.