Stretch forming equipment produces complex shapes by stretching a metal sheet, plate or extrusion over a form die. Stretch formed parts provide better shape control and surface quality than rolled or drawn parts. The stretch forming process subjects a metal blank to both plastic and elastic deformation. First, the blank is positioned on a die table and locked into place with hydraulic holding jaws. A ram-driven punch then stretches the material over the die to produce the desired shape. When the workpiece is unloaded, the plastic component of deformation is retained. Some stretch forming equipment is used to produce aluminum parts for the automobile industry or titanium parts for aerospace applications. Stretch formed parts are also used in household appliances and various sheet metal applications.
Types of Stretch Forming Equipment
There are two basic types of stretch forming equipment: longitudinal and transverse. Longitudinal equipment stretches the workpiece along its length. Transverse equipment stretches the workpiece along its width. For both types of stretch formers, specifications describe the jaws, die table, and hydraulic system. Jaw specifications include: number of jaws, jaw width, distance between jaws, stroke per jaw, tonnage per jaw, jaw swing, and gripping pressure. Die table specifications include: size and weight, minimum and maximum tonnage, workpiece length and width, tilt angle, forming speed, and stroke. The hydraulic system for stretch forming equipment has two main parts: the power unit and the ram. Motor speed, pressure, flow, and fluid type are important power unit parameters. Force, stroke, open/close rate and pressing rate describe the ram. Hydraulic system specifications also include minimum and maximum tension cylinder pull. With some stretch forming equipment, each arm is operated by its own cylinder.
Stretch forming equipment can be interfaced to a host computer and may include features such as computer numeric control (CNC) and an integral front panel or console. With CNC machines, an operator can program the radius of the jaw instead of manually adjusting the locking screws on the curving arm for each jaw assembly. Software for stretch forming equipment is also available.