What is Stretch Forming Process?
The stretch-forming process is a method of forming metal parts such as steel shapes, aluminum extrusions, and sheet metal, into permanently curved shapes of predetermined contour, the curved shapes are formed from sheet metal pressed, stretched, and bent over a die.
This is done simultaneously to form larger, contoured metal parts as needed. Different techniques will be utilized depending on the overall application ranging from hydroforming to cold forming.
Sheet Stretch Forming process
Sheet Stretch Forming is commonly used by aircraft builders to manufacture fuselage skin sections from special aerospace aluminum alloy sheets.
Extrusion stretch forming process
Extrusion stretch forming press designs for contouring aluminum aerospace alloys in the form of extrusion roll-formed profiles, or press braked shapes. These profile stretch presses are sometimes called stretch wrap or swing-arm presses. The basic press design has two arms or carriage beams that hold multiple-positioning gripping jaws. The jaws are attached to hydraulic tension cylinders that provide the stretch of the workpiece. The arms swing by rotating on large machined pins with bearings, thus allowing the workpiece to wrap around and against the forming die.
A simple explanation of the process
The stretch forming 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, then stretched into a state of tension. The carriage is held together by either a pneumatic force or hydraulic force which also helps stretches the metal sheet. As the piece remains at tension it’s wrapped around a contoured form block, or die, that has been created to match the desired shape of the finished product.
When the metal parts are 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 metal parts are raised upwards, it stretches. Once the metal parts have stretched to their limit, it begins to deform and takes another shape.
This helps customize the metal parts into anything that customers want them to be. The shapes that the stretch forming process can make vary from one shape to another. It can be formed into a simple curved shape or to a more complex form with cross-sections.
The features of the stretch forming process
Stretch forming is a very complicated process that involves high accuracy.
If a part were to be curved without first being stretched, two natural forces would occur during the process. First, one-half of the part would naturally be stretched. Secondly, the other half would go into compression, and the neutral axis would be the division between the two. Stretching the material compensates for the compression that would normally take place on the inner surface of the part, which, in turn, eliminates the wrinkling or twist.
It could be said that a stretch-forming machine bends parts into curved shapes, but does so while pulling the wrinkles out of the part before they can occur.
In this combination, of bending and stretching, the outer fibers of the part being formed experience an elongation stretch sufficient enough to put this fiber into yield i.e. it has been stretched beyond its ability to return to its original length after the stretching force has been relaxed. Therefore, the parts remain close to the shape of the form block after it’s been removed from the stretch-forming machine.
The stretching action taking place in the stretch-forming process not only eliminates wrinkling but also helps to eliminate the “spring-back”, of the part from the desired finished contour.
Although it is a cold form of bending, heat can be introduced in the stretch forming process to allow for the bending of exotic alloys.
Advantages of the stretch forming process
There are several advantages of the stretch forming process. From cost savings to metal variety, the advantages of the stretch forming process are numerous for metal parts stretch forming!
The main advantage of stretch forming is its ability to form complex shapes accurately while maintaining a parts dimensional cross-section shape and quality finish.
Aerospace stretch forming companies, like Jones Metal Products, are able to utilize the metal stretch forming process to create complex shapes and angles. Metal parts that are created through the metal stretch forming process come with minimal “thin-out” and they maintain extremely precise tolerances. This maintaining of precise tolerances allows for irregular pieces to be formed easily. This is perfect for companies that are in need of aerospace stretch forming in the form of smaller batches of intricate metal parts.
Approximately 70% less force is needed than that required for conventional press forming.
Springback is greatly reduced. There is some spring back, but it is easily controlled by over forming.
Residual stresses are low in stretch-formed parts.
As a cost-effective process, stretch forming uses less force than any other press forming method and can reduce material costs. Metal parts are created with a quality finish free of scuffs and or marking. The stretch forming process induces work hardening, which increases material strength while at the same time reducing residual stress to minimize spring back.
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.
Aerospace stretch forming, as it falls under the hydroforming umbrella, completely eliminates the need for dies, cast plastics, hard epoxy, and the hardened steel tools that would be used in stamping and traditional metal forming. And paired with the quality finish that is achieved through the metal stretch forming process, additional finishing or polishing is also unneeded.
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.
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%.
Since the metal stretch forming process does not make use of a punch or die, the need for finishing or polishing, later on, is essentially extinguished. Metal parts are created with a nearly perfect quality finish that is free of scuffs and other marks.
Disadvantages of the stretch forming process
- The stretch forming process is seldom suited to progressive or transfer operations. It is limited in its ability to form sharp contours and reentrant angles. The stretch forming process 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 stretch forming 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.
Stretch forming applications
Typical stretch-formed parts are large curved panels such as door panels in cars or wing panels on aircraft. The variety of shapes and cross-sections that can be stretched and formed is almost unlimited. Window systems, skylights, storefronts, signs, flashings, curtain walls, walkway enclosures, and hand railings can be accurately and precisely formed to the desired profiles.. Other stretch-formed parts can be found in window frames and enclosures. Close and consistent tolerances, no surface marring, no distortion or ripples, and no surface misalignment of complex profiles are important benefits inherent in stretch forming. A smooth and even surface results from the stretch forming process. This process is ideally suited for the manufacture of large parts made from aluminum but does just as well with stainless steel and commercially pure titanium. It is quick, efficient, and has a high degree of repeatability.
Almost any shape that can be produced by other sheet-forming methods can be produced by stretch 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 a higher cost.
Which metals can be used for stretch forming process
The stretch forming process is commonly used in the curving of aluminum sheets, extruded profiles, stainless sheets, bent or rolled profiles, structural shapes, various steel alloys, brass, bronze, copped, titanium, and even Inconel. It allows the curving of true radius contours, ellipses, compound bends, and helical bends and can include straight legs.
- Any ductile metal can be shaped by this method.
- The process is mainly applied to aluminum alloys for aircraft skins.
- Magnesium alloys are stretch-formed while hot.
- Stainless steel and titanium are stretch-formed on a commercial scale.
- Thinning and strain hardening are inherent in the processes. It is important to know the elongation values for the metal being used. Large elongations are best. Thickness reduction should not exceed 5% of the original thickness.