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Sheet Rolling vs Plate Roll Bending Process, Which One Do You Need?

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Sheet vs. Plate

To determine the designation of sheet vs. plate in general terms we can say that anything 1/8″ and thicker is a plate and anything less than 1/8″ is a sheet. The thickness of sheet metal is normally designated by a non-linear measure known as gauge. The larger the gauge number, the thinner the metal. Commonly used steel sheet metal ranges from 30 gauge to about 6 gauge.

There are multiple manufacturing processes that sheet metal can be formed by bending, curling, incremental sheet forming, laser cutting, perforating, press brake forming, punching, roll forming, rolling, spinning, stamping, water jet cutting.

What is Metal Rolling Process?

Metal rolling is the deformation process of a metal that is widely used in the metal forming process, in which Metal(s) in its semi-finished or finished form is passed between one or more pairs rolls to reduce the material, which reduces the metal’s thickness and make the thickness uniform. The rollers rolls around the metal as it squeezes in between them.

Two types of Sheet/Plate rolling

Rolling is classified according to the temperature of the metal rolled, it has two types:
Hot Rolling: It is a hot rolling if the temperature of the metal is above its recrystallization temperature, the recrystallization prevents the metal from work hardening.
Cold Rolling: Whilst it is known as cold rolling if the temperature of the metal is below the recrystallization temperature, this increases the strength to about 20% high through strain hardening and enhances the surface finish and also holds tighter tolerances.

Three types of rolling product

Plate: It is the Mill product, with thickness more than 6 mm.
Sheet: It is a Mill product, with thickness less than 6 mm, and width greater than 600 mm.
Strip: It is a Mill product with thickness less than 6 mm, and width less than 600 mm.

The four Most Common Terminology used In Rolling Process

Ingot: It is the Starting Metal that is Provided Input to the Rolling Process. The Ingot is a forging terminology, where metal is taken out from the cast with various defects.
Bloom: It is the first rolled product of Ingot, with a Cross-section area of more than 230 cm2.
Billet: The product is obtained by further rolling of Bloom, having an area of Cross-section greater than 1600 mm2.
Slab: It is a hot Rolled Ingot, with a Cross-section area greater than 100 cm2 and Width>=2*thickness.

Six Types of Rolling Machine (Mills)

The rolling process is done both hot and cold, which is accomplished in rolling mills. A rolling mill is a complex machine having two or more supporting rollers, working rollers, drive motor, roll stands, working rollers, coupling gear, flywheel, etc.

Six Types of Rolling Machine
Six Types of Rolling Machine

Rolling mills are classified on the basis of the number and arrangement of rolls in a stand.

Two-High Rolling Mill

It Consists of two High stands, and two rolls placed exactly one over the Other. In this type of Rolling Mill, the Rollers rotates in Opposite direction and their direction changes after each Metal pass. The Metal (Ingot) is passed continuously and approximately 25-30 passes are required to convert Ingot to Bloom.

Three-High Rolling Mill

It consists of three high stands and three Rollers present in the same vertical plane. The Top and bottom roller rotate in the same direction, and the middle roller rotates in the Opposite Direction. In this type of Rolling mill, the Direction of the drive is not changed after each pass. It is more Productive and easier with respect to the two-High Rolling Mill.

Four-High Rolling Mill

It Consists of two backup rollers and two Working rollers, arranged One over the Other in the same Vertical Plane. The Diameter of Backup rollers is always greater than the Working rollers. This type of rollers is mainly used in Sheet Rolling.

The Two Working Rollers of small diameters are used to reduce the power demands, but it increases the chance of bending of working rollers, and as a result, non-Uniform compression of sheets. This is the reason we use Backup rollers for reducing the bending of Working rollers.

Cluster Mill

It consists of Two Working rollers and two or more backup rollers. The number of Backup rollers depend upon the amount of support required for working. It is mostly used in Cold rolling Operations.

Universal Rolling Mill

It consists of two vertical rollers and two Horizontal rollers. The Vertical rollers are arranged between the bearing of horizontal rollers in the vertical plane. It is widely used to produce blooms from Ingot, and for rolling wide flange H-Section beams.

What Is Roll Bending Process

In roll bending, a cylindrical shaped product is produced from plate or steel metals. The rolls change shape during rolling because of the forces acting on them, which tends to bend the elasticity of the rolls during rolling. If the elastic modulus of the roll material is high, then the roll deflection would be smaller. Compared to its edges, the rolled strip tends to be thicker at the center. We can avoid this problem by grinding the rolls in such a way that their diameter at the center is slightly larger than at their edges.

Plate or sheet roll bending process

Plate or sheet roll bending is a fabricating process that is used to form various types of sheet metal, mainly stainless, aluminum, and steel into circular, conical, round or any other shapes.

Plate rolling machines (aka “plate rollers” or “roll bending machines”) give materials their shape by using force through their rollers that are powered by mechanical, hydraulic or
electrical systems. Read More: How To Choose The Right Plate Rolling Machine?

Work Principle

While bending, the work piece is positioned between the lower and upper rollers of the plate bending machine. After placing the sheet between the rollers, the upper roller is lowered to secure the work piece, and at the same time it has achieved sufficient pressure on the sheet, generating plastic bending. When rotating the roller there is a friction between the sheet metal and the roller, so the sheet moves along its longitudinal direction. At the same time, the upper roller adds pressure to the work piece.

When the roller crosses the lower limit of the upper roller (the roller deformation zone) in one turn and the pressure exceeds the bending limit, plastic deformation occurs and the sheet is folded to its desired dimensions by its entire length. By adjusting the relative position between the lower and upper rolls, the sheet can be folded to a diameter that is not less than the diameter of the upper roller.

Because of the friction between the sheet metal and the rollers, the work piece moves in advance. By adjusting the position of the upper roller, the desired bending angle can be achieved. If the work piece does not achieve the proper curvature after the first bending, it can be adjusted and started the process in the opposite direction until the desired shape is achieved.

Three methods to eliminate the un-bent flat end

After bending due to the position of the rollers on the work piece inevitably remains a flat part at the ends of the sheet(we called un-bent flat end). This flat part is approximately half the length of the gap between the centers of the lower rollers. In addition, there are three methods which we can use in order to easily eliminate this problem:

  1. Pre-bending: this solution involves bending the edges of the work piece on a press brake before the roll bending process.
  2. Adding a “base” for the work piece during the pre-bending: this method involves the installation of a “base” together with a workpiece designed for bending and performing the edge pre-bending process on the circular bending machine itself. The application of this method should take into account the possibilities of the circular bending machine and calculate the strength of the applied force on the “base” and the workpiece.
  3. Leaving allowance at the ends: if you add appropriate allowanceat plate ends before the bending, you can, after the bending, cut off those parts into a desired bending shape.

Applications

Roll bending is used in the production of many familiar components like: Wind towers, Pressure vessels and heat exchangers, Storage tanks and silos, Water, oil, and gas pipes.

Two Different Metal Roll Processes For Different Applications 

Hopefully, by now we’ve made it clear that sheet rolling and plate roll bending are two entirely different metal fabrication processes. That being said, we certainly understand how they are often confused with each other. So as a simple reminder:

  • The sheet rolling is used to produce long, linear components with uniform profiles
  • Roll bending is used to produce large, round shapes like cylinders, cones, and rings

Works Cited: What is Rolling Process: Definition, Working, Types, Defects and Application