A roll forming machine may appear to be an extremely complex machine, with a wide range of stations and high speeds. It's worth understanding how it works, as it can perform unique functions and consistently create the same end product to the strictest specifications.
To better understand what a roll forming machine does, it's best to break it down to study each place where the metal is changed in some way-whether that's bending, stamping, cutting, or folding.
Rolling mold
Whenever sheet metal is changed on the roll forming line, the roll forming machine's tools gradually change the shape of the metal. The original coil goes through many changes before the finished product is packaged to the other end.
Forming is accomplished by rolling tool steel rollers over the workpiece metal. Each roller continuously changes the shape of the metal sheet as it advances along the production line. Eventually, as the progress of the required bending or folding increases, more wheels or molds are needed to achieve it.
Increasing the bending schedule too quickly can damage the material and make it difficult to produce the finished product within tolerances. Increasing the bending progress too slowly will result in the need for additional molds or forming points, which is more costly. Machines with more forming points also take up more shop space.
Metal Roll Forming Stamping and Cutting
Roll forming lines can form, stamp and cut a variety of metals in a variety of specifications and are suitable for many industries, including construction. It is important to determine whether the metal should be stamped before or after forming. Stamping it after forming may not always be possible or may be more costly.
Sheet metal can be cut at specific points on the roll forming line. Determining the best place to cut is important because the final product will vary depending on where you cut.
Either way, manufacturers must consider flare and determine how forming affects it. When a roll-formed part is cut, some of the residual stress created in the process is released. This can cause the ends of the part to open or deform. This is called terminal flare. If the part is formed in a pre-cut roll forming process, the stresses may be greater than in a part formed in a back-cut roll forming process.
Weighing the differences between pre-shear and post-shear operations can help determine the best approach.
Pre-cut advantages
Eliminates expensive cutting dies and their maintenance.
No burrs at the end.
Simple to use, low maintenance pre-shear.
Allows manual feeding of strips or sheets for small batch production.
Prevent cutoff distortion.
Because the thickness, yield strength, and elongation of the workpiece material vary during the part forming process, it is important to match the radius and other dimensions of the cutting blade to the part contour to obtain a good cutting result. Conversely, cutting without matching dimensions may result in deformation. When the part is formed in the pre-cut roll forming process, it is cut flat so there is no need to worry about matching the profile to the cutting blade.
Disadvantages of pre-cutting
Increased end flares occur, especially in deeper parts, because there is no support at the leading and trailing edges of the rollformer.
High strength steel (HSS) requires more overbending because HSS has higher springback than other types of steel and does not have any support at the leading and trailing edges.
More forming stations are required, making roll forming machines and tooling more expensive.
Generally, more floor space is required.
Controlling the quality of multi-bent parts becomes difficult because additional bends add more flare.
Presents challenges in forming short parts. For a part to go through a roll forming machine, it needs to be in at least two forming stations at all times. The shorter the part, the more difficult it is to join the parts together, which can lead to part quality issues.
An intermediate guide may be required, especially for short pieces. Adjusting these guides can be problematic for multi-width guides.
Advantages of post-editing
Productivity is increased since the uncut web passes continuously through the forming station. In contrast, during pre-cut roll forming, the cut material hits each forming station, so the line speed must be slowed to prevent the material from bending and lodging in the machine.
Promotes better part control, resulting in improved quality. Because material flows continuously through the machine, parts are formed simultaneously while the material is retained in each forming station.
Because it can be controlled, end flare is greatly reduced.
Better results are achieved with high-speed steels such as duplex and martensitic steels, as both the leading and trailing edges of the part are fixed in each forming station.
Produced in lengths as short as 2 to 3 inches.
The use of end straighteners is allowed and is most effective at cutting back, controlling camber, camber and twist.
Eliminates leading edge deformation of parts.
Extended roll life on large gauge and high-strength materials because there is no leading edge hitting the part when precut.
More friendly to material specifications and hardness deviations as the process and lack of a leading edge does not stress the machine or tooling. As opposed to the pre-cut process, parts tend to pass through the machine smoothly.
Punching/grooving can be provided during the parting operation, potentially eliminating one or more secondary operations.
Disadvantages of post-editing
The initial cost may be higher as cutoff machines and molds can be expensive.
Increased maintenance costs, especially for cutting dies.
Causes end burrs and rough ends.
Causes product end deformation.
When a new coil is introduced, it may be necessary to manually feed in the front end of the first part.
In many cases, all surfaces need to be supported to avoid end deformation. This is not possible if solid mold parts cannot be built into the mold or if openings must be left in the enclosed space.
Metal roll forming bends, cambers and twists
Metal defects are common and can create problems during the roll forming process. They can even occur during the roll forming process.
Using tension straightened steel or aluminum coils can help. Tension straightening is the process of pulling metal above its yield point to permanently change its shape and make it flat.
Any roll forming operation can result in defects, so it's important to ensure that your machine is operating properly and that every change the machine makes to the metal is within acceptable tolerances from start to finish. For example, misaligned input guides can cause defects.
Fortunately, bows, arches, and twists can actually be straightened out during the roll forming process. Hair straighteners can be part of a production line. It should be as close to the final molding process as possible. Even straightening can put tension on the metal, so it's best not to use straighteners if you don't need to.