Metal fabricators are more apt to use extrusion, press brake, or stamping equipment, but roll forming has its place in modern manufacturing
September 13, 2010
Four processes are primarily used to form metal in industry today: extruding, press brake bending, roll forming, and stamping. Roll forming is likely the least used of all of these processes, but given the right applications, it can prove to be the most cost-effective alternative.
Four processes are used primarily to form metal in industry today: extruding, press brake bending, roll forming, and stamping. These four processes have helped to create the buildings we work in, the cars we drive, and plenty of other metal objects we use on an everyday basis.
Roll forming is just one of these processes, but given the right applications, it can prove to be the most cost-effective alternative. Before exploring roll forming further, however, it is important to understand the other metal forming processes.
Stamping presses, press brakes, and extrusion equipment have been around a long time, and these metal forming technologies still are widely used. Figure 1 shows the breakdown of process usage in U.S. metal forming.
Stamping involves the placement of coiled or sheet metal between two dies that press together to form a desired shape. The process has quick cycle times, can create complex parts, and can run unattended or with unskilled operators. These benefits have helped stamping become the process of choice for many years.
One drawback of the stamping process is its inability to produce long parts, which customers are looking for these days. For example, the emerging solar market has created a demand for parts in excess of 20 feet long. Some of the largest stamping presses can produce parts only about 84 in. long. Additionally, the cost of building dies that support 84-in. parts is astronomical when compared to other metal forming processes.
A stamping press could form shorter parts, which then could be joined together to create a 20-ft. part, but the secondary assembly activities add cost to the process. This increases the piece price for the customer.
The emergence of high-strength steel also has posed a problem for the stamping process. These high-strength steels, lighter and stronger than carbon steel, have allowed automotive designers to reduce the weight of vehicles while simultaneously meeting safety standards. But high-strength steels can prove to be difficult to stamp because of springback, which means the material is trying to return to its original shape after the forming process. Controlling springback requires additional tooling cost, and the presence of springback likely will interfere with a stamper’s ability to deliver high-tolerance parts.1
The makeup of the high-strength materials also can result in galling or scratching of the material surface during the forming process. Customers can find such marking to be unacceptable. Special tool coatings and lubrication may be needed to prevent this condition, or a tooling rebuild may be required. High-speed movement of material through the stamping press can exacerbate this condition.
Extrusions compete very closely with the roll forming process, especially with aluminum. The advantage of extruding is that by using a multiple cavity mold tool, more than one cross section can be created at a time. Another advantage is that wall thickness of the section also can be changed. The molds are set up so material is pushed through them, creating a continuous section or sections. Tight tolerances are possible because the material completely fits precisely manufactured molds.
Today, with the advancement of technology, longer sections of more than 20 ft. are achievable.
In certain industries roll forming and extruding are the two main choices for metal forming. These industries include office furniture, automotive, aerospace, and construction. The extrusion process normally has lower tooling costs than other metal forming processes and is attractive to those manufacturers looking for lightweight parts made of aluminum.
The extrusion process does have some disadvantages. No value-added activities, such as hole-making, can be added to this process. Extrusions also require a secondary artificial aging process, which results in additional cost for each feature. After the aluminum material is extruded through the molds, the material is very soft and can be bent or deformed very easily. Before any secondary operations take place to add value to the part, the part needs to go through an aging oven. The specified strength affects how much time is required in the oven. In some instances, the aging time can exceed 24 hours. With pass-through ovens, this process can be shortened but not eliminated.
The gauge thickness of extrusions is typically more than 0.045 in. Thinner material can be extruded, but the sections are prone to tangling, creating defects in the parts.
Another drawback is the extrusion process cannot extrude steel. This process requires a billet or block of material to be heated so it becomes a malleable solid that is soft enough to push through the steel tools. Steel cannot easily achieve the same level of malleability as aluminum. As a result, industries that require steel for strong parts can’t rely on extrusions.
The press brake is a competitive means of metal forming when compared to the roll forming process. The equipment can form steel and other materials with low tooling cost,2 and tooling changes can be achieved in a small amount of time.3
Press brakes are very common in metal fabricating because they are efficient tools that produce parts at low cost, but only in low volumes.
Roll forming and press brake operations are complementary in many applications. Many roll formed sections begin as parts that were bent on a press brake to create a section. At lower volumes, it makes economic sense to use a press brake to bend parts, and at times it is cheaper to use a press brake when a program is at the end of its life cycle.
The issue with using a press brake is that it’s limited in the length of the part it can form. For this reason, secondary operations are needed to join shorter sections that have been formed on a press brake.
Several other shortcomings of the press brake are:
Roll forming takes elements of the three previously mentioned processes and combines them to produce high-toleranced parts in high-volume batches with the ability to introduce other value-added processes during the forming process (see Figure 2).
Roll forming can produce parts of any length, really being limited only by the length of the coil material being fed into the line. The materials used in roll forming are those that can withstand the radius required. Boron, aluminum, and steel are all materials that can be roll formed.4 The process creates cross sections by slowly bending material to a desired radius in a continuous operation. It produces parts with tight tolerances and creates complex sweeps in a high-volume application.
One of the most attractive features of roll forming is that it offers the ability to consolidate secondary operations into the forming operation. Hole punching, embossing, and trimming to desired length can easily be added to the roll form process.
Welding also is commonly done in the roll forming process, something that is unusual in the other metal forming processes. Look at the tube forming industry, where inline welding speed can exceed 400 feet per minute. Welding in a roll forming process is generally slower, but still achieves a rate of at least 75 FPM.
Gas tungsten arc welding, laser, and rotary spot welding are the most common welding operations in a roll form line. These welding technologies allow manufacturers to create custom tube sections during roll forming.
Adding a stamping operation along with welding in a roll forming line can deliver a finished product in one process. Figure 3 shows a basic roll former used to create simple cross sections.
Sweeping of a section allows roll forming to bend the finished product at the end of the roll form line. This process often is used in the automotive industry to bend parts that follow the bend lines of the vehicle.
Roll forming is the only process capable of bending a curve in three directions without the assistance of an additional operation. This is possible with an extrusion process in which the material travels through a block that sets the desired radius.
Roll forming also can handle high-strength materials easily because of its slow forming speed made possible with specially designed roll form tooling. In stamping and press brake operations, the material is hit quickly to form the radius. In roll forming the material slowly travels through the tooling, which allows the roll forming engineer to create opportunities where tooling can work areas of the material to reduce springback.5
Roll forming costs do have to be considered when comparing it to other metal forming processes. Roll tooling is less expensive than stamping tooling for most parts, but more expensive than press brake and extrusion tooling. If a slow forming process is called for, roll form lines can become very long, and the need for extra tooling can increase the price.
The roll form line also needs to be run by experienced operators and engineers who have developed a successful process. These educated and experienced resources are not as plentiful as operators familiar with stamping, extruding, and press brakes.6 The obstacle in the roll forming community is education. Many of the experts in the field are retiring, and colleges do not offer classes in this field. Today a small group of professionals are trying to promote roll forming as a suitable metal forming alternative, and even with the current economy, the effort has proven successful. (For more information, see “Getting a Roll Forming Education.”)
Before ending the conversation about roll forming, one last disadvantage needs to be discussed: flare. During all metal forming processes, stress builds up in the metal during the forming process; in roll forming, this stress causes the parts to flare—the expansion of the cross section in the trailing edge of the part.
Roll form engineers and technicians have been fighting flare for years. Today with computer simulations and experience, roll forming engineers have been able to add features or add passes to roll forming lines to control flare.
The roll forming process can be very beneficial for the right application. For example, a part that normally is extruded and requires several holes or a part run on a press brake exceeds 5,000 ft. per run would be perfect candidates for roll forming.
The cost of tooling may be greater, but if the volume is high enough, the cost of tooling becomes less of an issue because the piece cost becomes less than with other processes. In addition, companies with high-volume applications benefit from the ability to produce parts in one operation in continuous mode.Actually, for high-volume runs, roll forming has the lowest tooling maintenance costs. Many sets of roll tooling have produced millions of feet of material with no maintenance required.
1. X. He Siguang, et al, “Springback prediction, compensation and correlation for automotive stamping,” in AIP Conference Proceedings, 2005, 778(1), pp. 345-350.
2. W.F. Wolf, “Selecting a press brake,” The FABRICATOR, August 2000, pp. 36-37.
3. T. Hays, “Primary considerations for press brake operations,” The FABRICATOR, December 1992, pp. 10-20.
4. Custom Roll Form Institute, Handbook of Custom Roll Forming, 2008, pp. 1-10.
5. J. Sheu, et al, “Simulation and optimization of the cold roll-forming process, in AIP Conference Proceedings,” 2004, 712(1), pp. 452-457.
6. L.A. Kren, “Keys to a successful tooling launch,” Metalforming, November 2005, pp. 56-60.
Interested in roll forming? The Fabricators & Manufacturers Association‘s (FMA) training seminars, on-site educational events, and online programs assist those who want to begin or further their roll forming education.
The FMA offers classes to train and certify roll forming operators. Knowledge of basic statistical process control, blueprint reading, and industrial safety, as well as practical work experience on a roll forming line, is all you need to get started toward earning that Level 1 certificate. Through classroom instruction and passing exams, you can climb the certification ladder, from Operator Level 1, Operator Level 2, Operator Level 3, and ending with Technician Level 4..
FMA’s Roll Forming Council offers training at its World-Class Roll Forming Workshop (April 19-20, 2011) and seminars at events such as METALCON (Oct. 20-22 in Las Vegas) and FABTECH (Nov. 2-4 in Atlanta). METALCON attendees can hear Brian “Tiny” Rodgers’ presentation, “Basic to Complex Roll Form Systems,” at 8:30 a.m. on Oct. 20.
For more information on FMA’s roll forming educational offerings and its Roll Forming Council, visit www.fmanet.org/technologies/roll-forming.cfm.