Stretch forming refers to a method of metal forming in which a sheet of metal is stretched and bent simultaneously on a die in order for large contoured pieces. Stretch forming can be done on a stretch press. A piece of sheet metal is secured along its edges using gripping jaws. By navigating this site you can more information about the stretch forming process.
Image Source: Google
Each gripping jaw is attached to a carriage which is pulled by hydraulic or pneumatic force to stretch the sheet. This tooling is a stretch block, also known as a form block. It is a solid contoured piece against the sheet metal that will be pressed.
Most stretch presses are vertically oriented. The form die rests on a table that can be lifted into the sheet using a hydraulic ram. The forming die is driven into the sheet and is held tightly at its edges. As a result, the tensile force increases, and the sheet plastically form into a new shape.
Horizontal stretch presses place the form die on a stationary table while the gripping jaws pull horizontally the sheet around the form. Stretch forming can be used to create compound curves in sheet material, especially for large sheets panels.
Spring back is possible without any allowance. For any dimensional changes, it is important to make allowances. When you stretch, your length will increase while the width will decrease and your thickness will decrease. One die is sufficient, made from inexpensive material.