Finite Element Analysis in Roll Forming Applications

by James Ede

Executive Summary

Finite Element Analysis (FEA) is a computer-driven modeling method used to evaluate the quality and performance of products and processes prior to actual production. When applied to the roll forming process, FEA qualifies the design of the roll tooling prior to its manufacturing. This white paper gives a brief overview of the roll forming process and then describes how FEA can positively affect roll-forming manufacturing applications by reducing development and testing time, which will enhance productivity and profitability for manufacturers and their clients alike.

About Roll Forming

Roll forming is a manufacturing process in which a continuous strip of metal (typically coiled) passes through consecutive sets of rolls, or "forming stands". Each forming stand gradually forms the strip into the desired cross sectional shape. Unlike other common metal forming methods, roll forming's flexibility allows secondary processes to be integrated into a single production process. Holes, slots, or other features are easily added by combining prenotching, midnotching, or postnotching to the roll forming process. Roll forming excels in applications that require high-volume production runs of metal parts with tight tolerances and consistent, complex cross sections. The process can accommodate material gauges typically ranging from .010" to .375" thick and is an effective alternative to extrusion, press brake and stamping processes.

Finite Element Analysis

Finite Element Analysis (FEA) is a mathematical modeling method. A wide range of studies can be performed by applying FEA - heat transfer, fluid dynamics, and structural analysis to name a few. It is also used for product design and structural analysis under both static and dynamic loading. RFC uses FEA for these traditional means, but what separates us is the use of FEA for analyzing the roll forming process. Therefore, this white paper is only discussing the topic of how FEA is applied to roll forming.

In the early years, FEA programs were based off the "linear" or elastic region of the stress-strain curve. Failure would occur once the structure exceeded the yield point of the curve. Today, because of advancements in software and computer technologies, FEA programs use both the elastic or "linear" and plastic or "non-linear" regions of the stress-strain curve. These advancements have provided the engineer with a tool that will allow the analysis of "deformation" or how a body will deform under various loading conditions.

This ability to study deformations is what makes FEA a great tool for analyzing the roll forming process. The material strip deforms as it contacts the forming passes. If modeled correctly one can see the effects on the forming of the rolled shape. These effects show up as residual strains and deformations in the part as it exists in its "free state" or unloaded position.

The FEA model is broken down into a mesh of small elements. Results needed from the study dictate the required density of the mesh. Typically, a higher mesh density in the area of study is used. The intersection points of the mesh represent nodes. The FEA program solves the calculations necessary to establish equilibrium at each node. Inputs to the model include the mechanical properties of the material. One can study the effects of different materials on the model by changing these properties. The results from the FEA study can be depicted in the nodes with each node having its own displacement and state of strain or stress. These values are graphically displayed on the computer screen and can be quantified for the engineer to analyze.

Today, FEA is a computer-driven tool used by engineers to conduct virtual research and development evaluations to test new product designs or to refine existing product specifications before prototype production—allowing engineers to more cost-effectively manage product design, development, and fabrication.

FEA in Roll Forming

When applying FEA to the roll forming process the material strip is considered a deformable body. The roll tooling is defined as a "rigid body" or fixed in position. One is studying the effects of the roll tooling geometry on the material strip. The strip is broken down into elements and the mechanical properties are defined for the strip. As the strip feeds through the roll forming mill it makes contact with the surfaces of the roll tooling (see Figure 1). This contact creates the loading on the strip. The strip will deform (the nodes will move) as it passes through the surfaces of the rolls. One can visualize and quantify the results in the nodes as it passes through the rolls
(see Figure 2)

An analogy can be made by applying the circle grid technology to roll forming. If one were to scribe a grid onto the surface of the strip and feed the strip through the forming rolls, one can see how the intersection of the grid displaces and moves around as it forms. FEA will allow the engineer to see how the strip progresses through the forming mill, how the material feeds from the forming stand to forming stand, and how the edge of the material strip strains as it progresses through the mill. FEA also allows the engineer to see the stretch and thinning of radii and analyze hole features (see Figure 3). Typically, for this case, the mesh density would be increased around the feature and any distortion of the feature can be viewed and measured as it progresses through the tooling.

The software will also allow one to toggle and view the roll tooling in an "on" or "off" state. This allows one to virtually see what is happening to the strip as it leads into a forming pass or view the "forming footprint" (what is happening underneath the forming pass). In the past, this would happen at the mill. One would need to take the time to disassemble the roll pass in order to cut out the required samples. This is time consuming, inefficient, and non-value added.

When used by an experienced designer, FEA will allow one to develop a ideal roll design. Roll problems are discovered and solved before the tooling is made. When combining FEA with design experience several iterations of tool tryout can be removed from the development process.

Benefits for Manufacturers, Clients & End Users

When it comes to producing quality roll-formed parts, there is no substitute for experience. In experienced hands, FEA is a powerful tool that increases efficiency, quality, uptime, and profitability for manufacturers, clients, and end users alike. To discover development problems and make necessary design changes after production has started can add weeks to a project and considerable expense for both the manufacturer and the client. The manufacturer has to pull the job, reschedule it, and redesign the rolls. As a result, the client's project is delayed, valuable floor capacity is wasted, and profitability for both parties is diminished. FEA helps to avoid this scenario with roll and product models that give manufacturers the ability to optimize uptime and productivity with designs that have been pre-tested and verified for quality before they ever hit the production floor. For end users, value is enhanced as these savings are passed along and the quality of parts is improved.

FEA is a helpful training aid and when reviewing the results, experience can be shared among engineers. This allows the opportunity to sharpen skills without the danger or expense of disassembling actual production lines to gain firsthand knowledge. In addition, FEA provides an unbiased, fact-based view of design performance and effectiveness so manufacturers and clients can easily document needed engineering changes.

Other Considerations

Because FEA modeling assumes "ideal" manufacturing conditions and homogenous materials in its calculations, the results it generates are only as good as the data entered. This is why it is most effective in the hands of experienced roll-form engineers who have the practical and technical knowledge necessary to fully utilize FEA's strengths as well as the ability to apply real world expertise to the interpretation of its findings.

Conclusions

Finite Element Analysis is a valuable and effective tool for experienced roll-form engineers to verify process and product quality from roll development all the way to part finishing. While FEA modeling cannot completely eliminate the need for traditional research and development with physical prototypes, it can greatly enhance the process and make it much more cost-effective. The ability to perform FEA is a great asset for any roll-form manufacturer and provides enhanced value for clients and end users that benefit from the cost savings, quality improvements, and shorter lead times made possible by this powerful modeling method.

About the Author

James Ede is a Senior Design Engineer with Roll Forming Corporation. He has been with the company for 30 years and has been involved with the design and development of over 1000 roll formed shapes. Jim holds a Masters Degree in Mechanical Engineering from the University of Louisville. He is also a licensed Professional Engineer in the state of Kentucky.

About Roll Forming Corporation

Roll Forming Corporation is North America's foremost provider of roll forming technology and business support services. Headquartered in Shelbyville, Kentucky, since 1947, Roll Forming Corporation was acquired by international metals processor voestalpine AG in 2000; expanding its client services capabilities with a global support network. From its three U.S. facilities, Roll Forming Corporation provides customized roll-form manufacturing solutions with integrated secondary processing operations as well as in-house engineering and tooling services. To offer clients a truly comprehensive partnership, Roll Forming Corporation provides complete supply chain management support that extends from material sourcing to stocking programs and logistics.

Publication date:
February/2011

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