Sheet metal fabrication: leveling, roll-forming, and secondary operations 

Sheet metal made from steel, aluminum, copper, and other metals plays an essential role in everyday living. Objects made of sheet metal are, for example, the body of vehicles, trains, and other transport media, the facade cladding of buildings, rain gutters, garage doors, cable guides, and beverage cans. Sheet metal is characterized by a large surface combined with a comparatively small and constant thickness. Sheet metal pieces thicker than 6 mm are called plates, those thinner than 6 mm are sheets, and pieces under 0.060 mm (60 µm) are foils. 

Sheet metal shaping

In most cases, such as steel, sheet metal starts as a slab produced by ingot or continuous casting, then processed at high temperatures in a rolling mill. In the mill, the slab is flattened by roller pairs into a strip known as hot strip due to the elevated temperatures. Often, a subsequent cold-rolling step at room temperature refines the strip, producing a cold-rolled metal strip with precise dimensions. The cold-rolled sheet metal strip wound up in a coil and delivered in this way before undergoing further processing. Sheet metal is then uncoiled and leveled. There are two ways to process the leveled sheet metal:

1 – The leveled sheet metal is cut into pieces that can be individually shaped on press brakes, folding and bending machines, or by deep-drawing, punching, or notching.

2 – The leveled sheet metal is continuously formed by roll forming into the desired final shape and finally cut into the required lengths.

Roll forming

Roll forming, also known as cold rolling or roll profiling, was introduced to Europe in the 1950s from the United States and has been widely used since the early 1960s. It is an ideal forming process for sheet metal profile parts, including closed ones with long lengths and in large quantities. Roll-formed sheet metal profiles are typically characterized by uniform wall thickness.

The forming process is performed at room temperature and consists of a series of consecutive bends: the sheet metal is fed longitudinally through a sequence of roller pairs, each contributing to the plastic deformation of the sheet metal until the desired profile cross-section is achieved. The shape of the rollers determines the profile, while the number of the roller pairs and their arrangement depends on the sheet metal grade and the complexity of the final product. There are virtually unlimited possibilities for profile design, profile dimensions, and maximum profile lengths.

Secondary processes

Further manufacturing processes such as punching, embossing, perforating, notching, and slotting can be integrated into a roll-forming line. Closed tube-like profiles can also be produced by roll forming. In this case, an additional joining process is required, usually seam welding. Longitudinally welded tubes are an example of typical products made using this method.

Also, in sheet metal fabrication, cutting is essential. Shearing is commonly used for straight cuts on thinner sheets, while laser cutting is ideal for complex shapes and fine details. Waterjet cutting, with high-pressure water mixed with abrasives, cuts thick sheets without heat-induced distortions or changes in material properties.

Bending and forming are important techniques for shaping sheet metal into the desired form. Press brake forming, for example, is used to create precise bends by clamping metal between a punch and die: in this way, a wide range of bend angles can be achieved. Roll bending produces curved surfaces and cylindrical shapes by passing the sheet metal through rollers that gradually bend it to the desired curvature.

Stamping involves placing a flat sheet metal into a stamping press, covering processes like punching, embossing, bending, flanging, and coining, each tailored to produce specific shapes and designs. Deep drawing forms hollow shapes like pots, vehicle bodies, and sinks by drawing a sheet metal blank into a die with a mechanical punch, creating seamless parts. This technique enables the production of complex, high-strength components with minimal material waste.

Several joining methods are used to assemble sheet metal into complex structures. Each processing technique has its advantages and limitations, with the choice depending on material type, thickness, desired precision, and cost.

Sheet metal finishing processes

Finishing techniques serve both aesthetic and functional purposes. Painting is a common method that enhances the appearance of the metal and provides a protective layer against corrosion and wear. Powder coating electrostatically applies a dry powder to the metal, then cures it under heat for a uniform, high-quality finish with excellent resistance to chipping, fading, and scratching. In galvanic plating, a sheet metal is coated with another layer, like chrome or zinc to improve corrosion resistance, conductivity, or solderability, depending on the application.

CAD technologies

The planning, simulation, implementation, and monitoring of sheet metal roll-forming processes are now facilitated by computer-aided (CA) tools. Specific software tools can be used to measure the productivity and the Overall Equipment Effectiveness (OEE) of roll-forming lines.

Roll forming lines

The starting point of a roll forming line is a decoiler onto which the sheet metal coil is loaded and uncoiled. The starting end is tapered for smooth feeding into the forming roller pairs. The sheet metal is fed through a straightening device which levels it. Various secondary processes can be performed before or after the roll-forming process. For example, punching, piercing, notching, shearing, and nibbling can be performed before the forming process takes place. It is also possible to insert holes with or without thread, embossing, corrugations, or nuts into the sheet metal. During roll forming, the sheet metal is progressively shaped by passing through the roller pairs of the roller die stands, with the application of lubricants to reduce friction. For other closed profile cross-sections like tubes, the edges are welded longitudinally using laser or HF welding technologies. At the end of the line, a cutting device cuts the formed parts to length before unloading them for further processing or shipment.

Applications of profiled sheet metal

Roll-formed sheet metal profiles are essential components in automobiles, trains, logistics, photovoltaic (PV) plants, power distribution systems, buildings, warehouses, supermarkets, and other sectors. In vehicles, formed sheet metal parts act as decorative and reinforcing elements in body panels. In railway locomotives and passenger carriages, sheet metal parts are integrated into side walls, roofs, and floors. Building applications include walls, wall claddings, roofs, ceilings, floors, cable trays, and HVAC equipment. Profiled sheet metal components are also necessary for shelving posts, supports, cross-members, and pallet rails in warehouses. Furthermore, roll-formed sheet metal components are used in the commercial food storage sector, for example in the refrigerators, freezers, and shelves of grocery stores and supermarkets. Lastly, in PV plants, they are essential for mounting structures and channels that secure solar panels at specific angles.

Sheet metals: choosing the right materials

Steel is most commonly used for its strength and durability, especially galvanized one, which is essential in the automotive industry for body panels, frames, engine parts, and household appliances. In the construction sector, steel sheet metals are used for roofing, cladding, structural support, and HVAC systems.

Stainless steel is ideal for medical, healthcare, food processing, and architectural applications due to its corrosion resistance.

Aluminum, known for its lightweight and strong mechanical properties, is widely used in aerospace and automotive industries. In automotive, it reduces body panel weight; in the aerospace sector, aluminum and titanium are used for aircraft skins, structural parts, and hydraulic systems. Aluminum's excellent conductivity also makes it valuable in electrical and heat-exchange systems.

Copper is valued for its corrosion resistance, electrical, and thermal conductivity, making it suitable for electrical components, plumbing, roofing, and decorative uses.

Titanium is prized for its high strength-to-weight ratio, corrosion resistance, and biocompatibility, making it suitable for aircraft, spacecraft components, chemical processing devices, and automotive exhaust systems.

Zinc sheet metal offers excellent corrosion resistance, making it perfect for outdoor uses such as roofing, cladding, and flashing. In addition, zinc is also a very strong and durable material, allowing it to withstand extreme temperatures without cracking or warping.

Literature

Jaswinder Bhatti: A better understanding of roll forming and its tooling. Breaking down the roll forming process by studying each place the metal is changed. In: the fabricator, June 25, 2021.
https://www.thefabricator.com/thefabricator/article/rollforming/a-better-understanding-of-roll-forming-and-its-tooling

Omolayo M. Ikumapayi, Esther Akinlabi, Peter Onu, Abolusoro Olatunji: Rolling operation in metal forming: Process and principles – A brief study. In: Materials Today: Proceedings 26 (2020) pages 1644–1649.
https://www.researchgate.net/publication/339597007_Rolling_operation_in_metal_forming_Process_and_principles_-_A_brief_study

Ilyas Kacar, Fahrettin Ozturk: Roll forming applications for automotive industry. OTEKON 2014. 7. Automotive Technology Congress, 26 – 27 May 2014, Bursa.
https://www.researchgate.net/publication/282567993_ROLL_FORMING_APPLICATIONS_FOR_AUTOMOTIVE_INDUSTRY

ByeongDon Joo, HyunJong Lee, DongKyu Kim, YoungHoon Moo:
A Study on Forming Characteristics of Roll Forming Process with High Strength Steel. August 2011.
https://www.researchgate.net/publication/253533864_A_Study_on_Forming_Characteristics_of_Roll_Forming_Process_with_High_Strength_Steel

George T. Halmos: Roll Forming Handbook. CRC Press, 2005.
ISBN 9780824795634
https://www.roll-kraft.com/roll-kraft-resources/roll-forming-handbook-by-george-halmos

Kevin Sweeney, Ulrich Grunewald: The application of roll forming for automotive structural parts. In: Journal of Materials Processing Technology. Volume 132, Issues 1–3, 10 January 2003, Pages 9-15.
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The information has been compiled by Dipl.-Ing. Konrad Dengler, technical journalist and translator specialized in industrial topics.