Stretch Forming Process -Advantages and Disadvantages

What is Stretch Forming?

Stretch Forming is a forming process that involves metal wherein a piece of metal is stretched and bent. This process is usually done on a stretch press wherein the metal is gripped at the edges so that it cannot break loose. The metal is held together by gripping jaws that are attached to a carriage. The carriage is held together by either a pneumatic force or hydraulic force which also helps stretches the metal sheet. When the metal sheet is stretched while tightly secured by the gripping jaws, it is usually stretched vertically wherein the form rests on a press table and is raised upwards with the help of a hydraulic ram. As the sheet is raised upwards, it stretches. Once the metal sheet has stretched to its limit, it begins to deform and takes another shape. This helps customize the metal sheet into anything that aircraft companies want it to be. The shapes that the stretch forming process can make vary from one shape to another. It can be formed to a simple curved shape or to a more complex form with cross sections. All of these can be done through expert use of the method of stretch forming. Stretch forming is a very complicated process that involves high accuracy.

stretch Forming
stretch Forming

The four methods of stretch forming are:

  1. Stretch draw forming 
  2. Stretch wrapping, also called rotary stretch forming 
  3. Compression forming 
  4. Radial draw forming .

Application Of Stretch Forming 

  • Almost any shape that can be produced by other sheet-forming methods can be produced by stretch forming. Drawn shapes that involve metal flow, particularly straight cylindrical shells, and details that result from such compression operations as coining and embossing cannot be made. However, some embossing is done by the mating-die method of stretch draw forming
  • Stretch forming is used to form aerospace parts from steel, nickel, aluminum, and titanium alloys and other heat-resistant and refractory metals. Some of these parts are difficult or impossible to form by other methods, for example, the titanium alloy gas-turbine ring
  • Stretch forming is also used to shape automotive body panels, both inner and outer, and frame members that could be formed by other
    processes but at higher cost.

Advantages Of Stretch Forming :

Stretch forming has the following advantages over conventional press forming methods:

  • Approximately 70% less force is needed than that required for conventional press forming.
  • Stretch forming can reduce material costs by as much as 15%. Although allowance must be made on the stock for gripping, it is gripped on two ends only. The allowance for trimming is usually less than that in conventional press forming.
  • Because stretch forming is done on the entire area of the workpiece, there is little likelihood of buckles and wrinkles. Tensile strength is increased uniformly by approximately 10%.
  • Hardness is increased by approximately 2%.
  • Springback is greatly reduced. There is some springback, but it is easily controlled by over forming.
  • Residual stresses are low in stretch-formed parts.
  • Form blocks are made of inexpensive materials, such as wood, plastic, cast iron, or low carbon steel, and are approximately one-third the cost of conventional forming dies. If the workpiece is formed hot, the dies must be able to withstand the forming temperature. However, most stretch forming is done at room temperature.

Limitations.

Stretch forming is subject to the following limitations:

  • It is seldom suited to progressive or transfer operations.
  • It is limited in its ability to form sharp contours and reentrant angles. It is at its best in forming shallow or nearly flat contours.
  • If the piece is not pinched between mating dies, there is no opportunity to coin out or iron out slight irregularities in the surface of the
    metal.
  • In some applications, especially in stretch wrapping, the process is slower than competitive processes, and it is not suited to high volume production. However, stretch draw forming with mating dies can be done as rapidly and automatically as conventional
    press operations. In fact, punch presses are used with dies incorporating draw beads or other means of gripping the blank in order to
    perform some stretch-forming operations.
  • Metals with yield strength and tensile strength very nearly the same, such as titanium, necessitate the use of automatic equipment for
    determining the amount of strain for uniform results,
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