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Roll forming
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Roll forming, also spelled rollforming, is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained.
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Encyclopedia
Roll forming, also spelled rollforming, is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities.
Overview
A variety of cross-section profiles can be produced, but each profile requires a carefully crafted set of roll tools. Design of the rolls starts with a flower pattern, which is the sequence of profile cross-sections, one for each stand of rolls. The roll contours are then derived from the profile contours. Because of the high cost of the roll sets, simulation is often used to validate the designed rolls and optimize the forming process to minimize the number of stands and material stresses in the final product.
Roll formed sections have an advantage over extrusions of a similar shapes. Roll formed parts are generally much lighter and stronger, having been work hardened in a cold state. Another advantage is that the part can be made having a finish or already painted. Labor is greatly reduced since volume is a major consideration for choosing the roll forming process.
Roll forming machines are now being produced so that for similar products such as stud and track profiles, a new set of profile rolls is not required. This is achieved by the mill being split along its center line and the web, flange and ear sizes are set using a control panel which moves the mill rafts centrally to increase or decrease the aforementioned features.
Roll forming lines can be set up with multiple configurations to punch and cut off parts in a continuous operation. For cutting a part to length, the lines can be set up to use a pre-cut die where a single blank runs through the roll mill, or a post-cut die where the profile is cutoff after the roll forming process. Features may be added in a hole, notch, embossment, or shear form by punching in a roll forming line. These part features can be done in a pre-punch application (before roll forming starts), in a mid-line punching application (in the middle of a roll forming line/process) or a post punching application (after roll forming is done). Some roll forming lines incorporate only one of the above punch or cutoff applications, others incorporate some or all of the applications in one line.
Process
The process of roll forming is one of the more simple manufacturing processes. It begins with a large spool of metal strips, usually between 1 in. and 20in. in width, and 0.004 in. and 0.125 in. thick. This is held by a device called a dispenser. The metal strip is then unrolled and fed into a machine starting with the stock feeder which is connected to the cutoff attachment. After the cutoff attachment, the metal strip is fed into the forming rolls. These mating die-set rolls are constructed to form the desired shape in stages sequentially by means of various shaped rolls. The layout of these rolls can be flower shaped as mentioned previously, progressive upper/lower rolls, side rolls, or as overhung spindle rolls. These different roll configurations are used according to the job that needs to be done.
Geometric Possibilities
The geometric possibilities can be very broad and even include enclosed shapes so long as it is the same cross-section throughout. Typical sheeting thicknesses range from 0.004in. to 0.125in., but they can exceed that. Length is almost unaffected by the rolling process. The part widths typically aren't smaller than 1in. however they can exceed 20in.
- Tolerances can typically be held within ±0.015in. for the width of the cross-sectional form, and ±0.060in. for its depth.
Production Rates
The production rate depends greatly on the material thickness and the bend radius, it is also affected by the number of stations or steps required. For bend radii of 50 times the material thickness of a low carbon steel .07 in thick can range from 85 feet per minute through eight stations to 55 feet per minute through 12 stations or 50 feet per minute through 22 stations.
The time taken for one product to take shape can be represented by a simple function. The function is as follows: where V is the velocity of strip through rolls (fpm), L is the length of the piece being rolled (ft), d is the distance between forming stands (ft), and n is the number of forming stands.
Other Considerations
While dealing with manufacturing, there are always things to be considered such as lubrication, the effect of the process on material properties, cost, and of course safety.
Lubrication provides an essential barrier between the roll dies and the workpiece surface. They help to reduce the tool wear, and allow things to move along faster. This table shows the different kinds of lubricants, their application, and the ideal metals to use them on.
| Work Material | Roll Lubricants | Application |
|---|
| Nonferrous | Chlorinated oils or waxes, mineral oils | Spray, wiping roller | | Ferrous | Water-soluble oils | Wiping, drip, spray | | Stainless steels | Chlorinated oils or waxes | Wiping roller | | Polished surfaces | Plastic film | Calendaring, covering, spraying | | Precoated materials | Film/forced air | |
The effects of the process on the material's properties for this process are very minimal. The physical and chemical properties virtually have no change. But the process may cause workhardening, microcracks, or thinning at bends when discussing the mechanical properties of the material.
The cost of roll forming is relatively low. Some things to be considered when calculating the cost of the process are setup time, equipment and tool costs, load/unload time, direct labor rate, overhead rate, and the amortization of equipment and tooling.
Safety is also a bit of an issue with this process. The main hazards that need to be taken into consideration are dealing with moving workpieces (up to 800 fpm), high pressure rolls, or sharp, sheared metal edges.
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