The primary purpose of the roll forming process is to shape steel metal sheets to achieve a certain cross-section profile. To build a high quality product at the right price, an engineer or manufacturer should have the right material for the job. Choosing the ideal material for a job can be a daunting task; therefore, the designer ought to consider several factors.
This section aims at helping roll formers pick the right metal by informing them about specific and general metal characteristics.
Engineers always emphasize that it is a requirement for any product to withstand a certain amount of load without getting permanent deformation. This means that the product must have enough resistance against a particular load. Resistance is achieved by ensuring the cross section, as well as the strength of the material, is perfect. The most vital mechanical properties to focus on are elongation, yield strength, and tensile strength. To determine these properties, tests must be conducted. An engineer will have to apply tension on the materials so that he or she examines their mechanical properties.
Experts claim that products that have a good design rarely get permanent deformation. This can be explained by the fact that their actual stresses do not get to the yield stress. Such products get permanently deformed only when subjected to an accidental overload. A good example is when a car crashes, it is distorted beyond repair.
It is always important to roll form metals with stresses that are above the yield stress. When materials are formed with a stress that falls below yield, they will be able to retain their original shape. On the other hand, when the forming stress is more than the maximum tensile stress, the product will definitely tear and crack during the process of metal roll forming.
According to the various tests conducted, it is extremely difficult to form metals that have high tensile and yield stresses and have an elongation close to zero.
Cold Working and Hot Working
When an engineer or roll former wishes to increase the tensile and yield strength, he or she can do it by alloying, cold working, heat treating, or a combination of both. A good example is when a wire is bent several times, it hardens and breaks eventually. The wire breaks because of the repeated bending, which is referred to as cold working.
This phenomenon also happens during the process of metal roll forming. To achieve a certain metal thickness, the former forces the metal strip through rolls that have decreasing gaps between them. This metal rolling process causes the material to become hard that rolling it further will break or crack the strip. Most cold-worked metals used by the industrial companies must be annealed or heated before they are shipped to clients.
Hot working does not require much force; however, it helps achieve great thickness reduction during the metal roll forming process.
Most sheet metal rollers do not use pure iron for industrial purposes. Commercial steels always have a certain percentage of carbon and impurities that are absorbed during steel manufacturing. Even insignificant amounts of impurities such as phosphorous can influence some properties of steel.
Mild steels have a carbon content of around 0.20 percent; this carbon content makes welding and metal rolling easier. Unfortunately, their strength and hardness cannot be increased through heat treatment.
High Carbon steels have a content of around 0.4 to 1.0 percent; this makes them have higher strength and yield as well as lower elongation. They cannot be welded normally, but they can be subjected to heat treatment.
There exist various metallic coatings that prevent carbon steels from rusting. The common metals used to coat are tin, aluminium, zinc, and chromium. Coatings are achieved through the electrolytic process, dipping the steel into molten metal, hot spray, vapor deposit or lamination with a chemical bond.
The most preferred and used coating is zinc. To come up with galvanized steel, industries have to cover steel in zinc or zinc alloys. It is possible to roll form both galvanized and uncoated steel, but damage to the coating ought to be stopped and zinc pickup prevented at all costs. Zinc build-up can be prevented by using good lubrication together with good tools of the right set up.
What You Need to Know About Forming Machines
Steel roll formers are designed to bend long pieces of materials into various sizes and shapes. The steel roll formers are grouped into various types so that each can fulfil a particular purpose. The great thing about roll forming is that the industry has been flourishing nowadays regardless of the kind of construction; they all require steel rolls of varying shapes and sizes.
The Four Types of Basic Machines
Roll forming machines can be categorized into four basic application types. Any other models that goes beyond the four machine types is simply a theme variation, or a compilation of some simple applications combined into one. On the other hand, there exist some specialized machine applications that differ completely with the four basic types. In this section, we will focus on the basic types. Machine applications are mostly categorized by the servo system. This system controls press tool or material positioning. In addition, roll forming machines can also be classified on the basis of whether the press operations happen during the job or whether material stops. Below are the four application types:
- Closed Loop Stopping
- Open Loop Flying Die
- Closed Loop Flying Die
- Open Loop flying Die
Before looking at the applications, it is imperative to comprehend the basics of the categories as well as the reasons why these categories actually exist.
There exists three key factors that determine the type of machine that will be used in the production of a specific roll formed product: accuracy, throughput, and cost. The system might cost more when high throughput and accuracy are considered important.
Open Loop Stopping
Also known as Start/stop machines, they are known for their low cost roll forming solutions that are characterized by having low throughput rates. When doing the job with these types of machines, the material has to stop for press operation. This helps a manufacturer to achieve high accuracy when making the product. The ability of the material in question to stop consistently determines length repeatability.
It is important to know that the Tolerance present in open loop stopping functions is always ± 0.060”, 1.5 mm, or more. As long as the length control system is able to make minor corrections before cycling the press, engineers or manufacturers are assured of controlled length accuracy.
Most experts will agree that this machine type comes in handy when manufacturing metal building components as well as light gauge steel framing. The open loop stopping machine model has always been praised for its ability to offer high quality components and relatively low cost equipment for DIY builders and minor operations.
Some of the key features of the feed-to-stop model included:
- Press variations do not have impact on length
- Moderate accuracy
- Low cost
- Sensitive to tracking variations/timings
- Lower average line speeds
A roll former machine that falls under the open loop stopping systems offers a relatively low cost and enough accuracy solution for all applications. You’ll find that it provides low throughput when there are tolerance requirements. Amazingly, reliable stopping action is achieved from avoiding mechanical backlash and hitting uniform speeds.
Open Loop Flying Die
A forming machine in this category can be described as a low cost solution that provides a high throughout. The Accuracy level of this machine model is similar to the Open Loop model. This means that these precision roll formers are able to achieve tight accuracies. These machines also require high maintenance and precision during setup; this ensures that the roll former machine functions optimally.
One of the most popular uses of this application/ function is during steel stud production. There are two main variations or models of the open flying die application, which included the boosted and unboosted dies. A boost is a pneumatic or hydraulic cylinder that is used to pull or push the die to match material speed. Some systems are normally fitted with a proportional valve that helps to match the die with the material speed in a more accurate manner.
- Low cost
- Sensitive to tracking/ timing variations
- Moderate accuracy
- High cycle rates
- Higher accuracies that need regular maintenance
Other than providing affordable, high speed solutions for roll formers, these high precision roll formers come in handy in terms of accuracy. The open loop flying die model utilizes the press reaction parameters in order to achieve great part lengths through speed changes. For those machines that have boosted dies, their Xl200 controller allows the customer to adjust the system to suit their equipment needs.
Closed Loop Flying Die
Also referred to as the die accelerator, the press tolling is accelerated up to material speed and kept in line with the target position on-the-fly. These metal forming machines allow high quality press operation to be performed without interrupting with the throughput. This type of application is mostly used in high-speed track lines and studs.
One of the unique features is that the cut off of this machine is mostly a belt-driven servo shear. On the other hand, its punches are mainly open loop since the overall length is more critical than the tolerances.
- Requires high levels of expertise during startup and troubleshooting
- High accuracy
- High cost
- Not sensitive to the timing variations of equipment
- High speed
- Extremely sensitive to encoder tracking disparities
It is evident that this metal roll former is capable of producing accurate parts even at high speeds. They are also able to produce excellent results regardless of the user’s experience or skill level. They require more expertise during setup and are more expensive.
Closed Loop Stopping
This sheet metal roller type has similar features or work in the same manner as the open loop stopping systems. However, they provide a higher level of performance in terms of accuracy and throughput. Another major difference is that it offers impressive results regardless of outside factors. On the other hand, the results are determined by consistent conditions and the equipment’s repeatability. This metal roll former has an accuracy that ranges at around 0.08mm or better.
These systems depend on the servo systems to achieve tolerances that are too tight. Closed loop stopping systems feature a low throughput due to the fact that the material has to halt for press operations. The good news is that proper servo sizing can assist in reducing the impact of stops. This type of forming machine system is appropriate where hole-to-hole tolerance is too tight or when flying punches would look impractical.
- Impressive accuracy
- Roll-feeder slip might need the inclusion of a second encoder
- Higher cost
- Production rate depends on dwell times & spacing of targets
Manufacturers prefer going with the closed loop stopping systems since they offer high accuracy levels during punching jobs. Although the equipment is expensive, the system is not affected by external conditions and does not require any tweaking by the user.
Roll Forming Techniques
There exist two major methods that are suitable when shaped components are roll formed. They include:
- The post-cut roll forming
- Cut-to-length roll forming
Post-Cut Roll Forming
It’s described as the least problematic, consistent and most efficient process. It has been widely used especially for manufacturing corrugated metals. This technique is normally supplemented by various auxiliary and secondary operations such as:
- Die forming
Cut-To-Length Roll Forming
Also known as the pre-cut roll forming method, it entails cutting the material to length prior to feeding it into the roll forming machine. This technique includes a stacking and feeding system that drives the material into the sheet metal roller and finally to the after production conveying and stacking structure.
This technique works well for low volume parts as well as that instance when notching seems difficult in a post-cut line.
To manufacture a high quality product at a cheap price requires dealers to have the right knowledge about metal forming machines. There are several advantages that are available by selecting a roll forming machine. This is why clients are advised to think about this process from the design stage.