Roll forming is one of the oldest but still misunderstood metal forming processes. And its capabilities continue to grow. Getty Images
The following article is partly based on the “Rolling Form Rationalization” prepared by Formtek Inc. Senior Application Engineer Brian Rodgers for the FABTECH meeting.
The basic concept behind roll forming can be traced back to Vinci. The first continuous roll forming line dates back to the 1910s. However, even after so long, many people still don’t know what the real roll forming is. In some fields, such as the automotive, aerospace and metal construction industries, it is essential, but for many people, the centuries-old metal forming technology is still brand new.
Knowing what can and cannot be done with rolling opens the door to many metal forming possibilities. In most cases, this starts with a continuous part profile with high production requirements.
Even those who know roll forming are still surprised by the possible methods. Nowadays, some manufacturers have developed proprietary methods to push the limits of roll forming, pressing metal into a mold to produce part contours that appear to be extruded, rather than being made from sheet metal rolls. Others have found ways to form shapes with discontinuous cross-sections, such as C-shaped channels “squeezed in” like a bow tie.
But most conventional roll forming applications still require a continuous cross section or profile. It can be a simple C or U channel, or a very complex irregular shape. Parts can be straight, curved, or even twisted into a spiral. But in all these geometric changes, the part profile is continuous.
To understand why, you need a basic understanding of roll forming methods (see Figures 1 and 2). The material comes out of the coil or blank through a precision straightening machine (if needed), and then enters the first frame of the roll forming line. The upper and lower roll tools on each stand perform a certain number of forming, which together form a so-called roll flower. This figure shows how the part is rolled to its final contour.
When the metal belt passes through, the upper and lower roller tools on each frame will form a specified amount of contour according to the metal thickness, grade, material yield and tensile strength, part geometry, feed speed and other variables. Like stamping and bending forming, roll forming must deal with springback. The metal is formed when it is squeezed through the first set of rolls, then slightly relaxes (due to springback), and then enters the next set of rolls or forming passes. The next stop formed the next “petal” segment of the flower pattern, illustrating the previous rebound of the previous traverse. Therefore, the process continues until it reaches the straightening station or the sweeping block. The process can eliminate or cause the camber (see Figure 3), and then complete or cut to a certain length at the end of the production line. The resulting parts can be straight, curved, and even spiral.
Roll forming production lines certainly include various rolls, but they also have at least one mold, but usually several molds. Each continuous production line has a cutting die, which cuts the final roll forming part to a certain length (see Figure 4). Some blank-fed roll forming lines are manually fed, while other blank pre-punching dies cut the strip to a certain length before feeding it into the roll forming line.
Other molds on the line will punch holes and other cuts. When it occurs before the metal is formed, it is called pre-scoring or pre-perforation. When the stamping operation takes place after or between the forming stations, this is called a mid-cut-this is sometimes a necessary step if, for example, punching a hole before forming causes undesirable stretching.
Slotting and blanking molds can remain stationary, fixed on rams and pillows, or travel along guide rails installed on the base, where the entire hydraulic or pneumatic press assembly (sometimes just the mold) moves linearly (see Figure 5). These flying molds increase the flexibility of the roll forming line.
Some production lines use special molds that form embossing, grooves, tongues, shutters and other value-added functions. In applications requiring high speeds, rotating molds can be used. As the mold rotates, cuts and forms will be created at a speed of more than 100 feet per minute.
If you see a roll-formed part that is spot welded along the seam and there is no second spot welding station on the floor, it is very likely that they will be placed in a process called rotary resistance spot welding, which will The material is clamped between two copper electrode wheels and allows welding while the material is moving. This process is used in the automotive industry for bumper beams and door frames. In fact, rotary spot welding has been the first choice for bumper beams for many years. Today, many roll production lines use high-frequency welders to produce customized welded pipes (see Figure 6).
Like any forming process, roll forming creates stress in the part, which may cause some kind of deformation. This includes so-called tail bombs. Without compensation, all roll-formed parts will have toes at the front edge of the part and flaring at the rear edge. This phenomenon usually occurs within the first 6 inches of the part length. The root cause is related to the elasticity of metal, similar to springback in stamping and bending forming.
To compensate for this, the anti-flaring device of the roll forming line consists of a series of blocks, rolls and mandrels. These elements bend parts of the part, causing it to spring back to the desired shape.
One of the biggest advantages of roll forming is its ability to form a variety of materials, including high-strength/low-alloy (HSLA) steel. Engineers can control the roll pressure by adjusting the inside and outside of the rolling mill stand to adapt to the bending stress. They can also reduce the degree of bending of each mill stand. For example, instead of bending the part by 25 degrees on one stand, an engineer can decide to shape it into 5 degree increments in 5 degree increments between five rolling mill stands.
Additional rolling passes can help solve the “material memory” problem, but these passes need to be strategically placed. The geometry of the parts sometimes requires certain rolling mill stands to handle materials better than others. Adding brackets can be said to be a wise choice, but they must be placed where they are most useful, and this may vary from application to application.
In any case, adjusting the number of brackets (and the tools on each bracket) will provide the engineer with another “turning knob” to fine-tune and complete the process. All these “adjustable knobs” are one of the reasons that roll forming can make a variety of materials into a variety of shapes.
Sometimes, roll forming prevents spirals and other shapes from being formed in any other way. However, when deciding the rolling form is based only on the part geometry, it is usually related to the size of the part.
In theory, roll forming has no limit on the length of the part. The only real restriction is practical: that is, what to do with the part when it comes out of the production line. Very small, short parts are sometimes processed with chutes. The bullet evacuation system under the mold can actually become a part evacuation system. In other words, the is part of it.
On the other hand, the part may be very long. The punch press has a bed size limit, and the press brake bed can only be so long. However, even the most compact roll forming production line has no theoretical limit on the length of its parts. The only limiting factor is to have enough space and ability to manipulate the molded part.
Regarding width, the only limiting factor is usually the width of the coil. As we all know, some roll forming lines can process workpieces of various widths (see Figure 7). These include duplex lines formed only near the edges of the material, which can be moved in and out to accommodate different widths without changing tools.
Figure 1 is formed by multiple sets of rolling tools to form a rolled profile profile. The frame was removed to illustrate the transition of the workpiece.
Duplex lines that automatically adjust direction illustrate the practical flexibility of some rolling lines, especially when processing part families that share certain attributes. Some roll forming lines in the bleach and seating industries can produce different parts one after another. The parts share the same outline, but have different lengths and hole patterns. All are packed and shipped in the exact order, and the installer needs to install it on the job site.
The rapid transition in rolling can occur in many ways. For example, a dedicated line can be established to roll several different parts, each part using a specific rolling mill stand. This only applies to certain types of part geometries and certain tolerance requirements (essentially, the roll forming line is tailored to the specific, usually mixed needs of high-volume parts), but in some cases it is one Kind of choice.
The new programmable controller can change the punching mode, part length and other attributes. For the different modes of each part, the conversion may not be immediate or instant, but it may happen within a few minutes.
Another quick change option is to use the drift line (see Figure 8), where the upper tools of the entire group are preset in the raft and can be lifted into place. The small roll forming line can be completely composed of a raft, which can be replaced as needed. This technology introduces rapid conversion to a wider product portfolio. Fast conversions are not immediate, but they are faster than traditional manual conversions, in which operators can spend hours manually swapping and aligning tools.
The labor cost in rolling usually accounts for 3% to 15% of the total work cost. Why is it wide? This is largely due to the degree of automation available and the product mix and complexity of the rolling mill production line.
Roller production lines that require manual conversion have a higher labor content, depending on the number of conversions required by the production line. On the other hand, some companies may have only one employee to manage three different departments. The controller manages most of the conversion, and the parts are automatically unloaded into the Kanban box. An employee may spend most of his time monitoring the process and preparing a new coil for the next job. In some high-volume environments, the roll forming machine may not have an operator at all.
Automatic calibration further reduces the artificial content, helps reduce some of the black magic of the roll paper settings, and in many cases, they can actually be adjusted in real time. Just as the bending machine provides real-time angle measurement, the roll forming line now also provides real-time adjustment to take into account the variability of the material and keep the geometry of the final part within tolerance.
After setting, roll forming always provides high consistency between parts. The standard contour tolerance is ±0.030 inches, and the angle is ±2 degrees. The torsion tolerance can be less than 0.120 inches above 40 inches; the camber tolerance can be within the range of 0.040 inches and exceed 40 inches. The bow tolerance and bow tolerance can be within 0.040 inches and above 40 inches. Nevertheless, these numbers vary from application to application. Depending on the geometry of the part, all these tolerances can be even stricter. In any case, automatic calibration will only bring consistency to a new level.
In summary, the artificial content of roll forming is just one of the mysteries. Even a roll production line with a high labor content can still help reduce overall costs, especially if the production line integrates a second process such as welding. For example, a large roll forming production line may require an operator to monitor the roll forming process and another operator to run and monitor the welding operation. The labor content of the roll forming machine may be increased to 15%, but due to the cancellation of multiple secondary operations, the overall cost has plummeted.
In fact, the elimination of secondary processing is a major reason why many previously extruded parts become roll-formed. Extrusion has the advantage of a simple, inexpensive tool that can produce extremely complex geometries. And because you don’t start with sheet metal, you don’t have to worry about end torches and other metal forming properties.
The solid model in Figure 2 (with the upper roller removed for illustration purposes) shows how the metal strip is formed as it passes through the different stations in the rolling mill.
Because we live in a global market, reducing labor costs is crucial. The quantity can vary according to the application, the geometry of the part and the market demand, but in typical cases, when the material content accounts for between 60% and 90% of the total work cost, roll forming is competitive.
The life cycle of many parts begins with a bending machine, but is ultimately produced by a roll forming machine. In fact, once the part volume exceeds 250,000 linear feet per year (note that this is in feet, not the number of parts), roll forming usually becomes the most cost-effective option.
However, this number varies with the complexity of the part. Parts with only some simple bends may require up to 500,000 linear feet per year to be roll formed. Again, you may only have 10,000 linear feet per year, but because the parts are too complex, roll forming is still the cheapest option. If the roll mill helps eliminate secondary welding or other operations, the number can be even lower.
Know that these numbers are just generalizations. Consider a part that starts with a manual press brake and then transfers to a mechanical press brake unit. The robot cell can meet customer needs, but the part itself may not take advantage of the bending cell. Offline simulation, smart material grading, flexible fixtures and automatic tool changes together create a system that can produce a wide variety of parts and is not limited to parts with continuous contours.
The decision to roll (or use any other process for this) comes down to making full use of metal forming capabilities. Sometimes this requires questioning the status quo. Considering the state of the world, all the uncertainties in the future, and the continuous progress of manufacturing technology, questioning the status quo has become more important than ever.
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Post time: Oct-27-2020