Aluminum Profile Bending
Bending and forming of aluminium extrusions
Aluminum can be extruded and bent to specified tolerances or to standard dimensional tolerances. While a product’s dimensions and bend angles can be methodically measured and re-measured, the end product will only be as precise as the bending equipment or method used.
Several factors should be considered when choosing which bending process is appropriate for a certain product. Aluminum extrusion engineers can provide crucial input on the bending, shaping, and forming of aluminum during the design phase of a project. Deformation of the inside or outside radii can be a design issue and can also determine which forming process to use. Before beginning the design process consider these parameters:
- What tolerances, or deviations, are expected on the inside radius, the outside dimension radius, and the overall length of the part?
- What surface areas are critical for appearance?
- What mechanical strength is required?
Bending compresses the inner side and stretches the outside. When bending a thin strip in the direction of it’s thickness, (not it’s width,) the difference in bend radius is small enough that the tensile and compressive forces aren’t great. As the work piece gets thicker though the challenges increase.
As an example of the difficulty, consider bending an “I” beam of the type commonly used in trailers and architectural applications. To start with, we’ll bend it in the direction of the top and bottom flanges. In this case the center web bends easily but as the inner and outer edges of flanges bend to different radii they experience more tension and compression. Tension thins the metal while compression can cause buckling.
Now if we bend the “I” beam in the direction of the web the problems multiply. Not only are top and bottom flanges forced to different radii, but so too is the top and bottom of the web.
The key to successful bending is controlling how each element of the extrusion moves. This is a lot easier when the extrusion is symmetrical, providing the axis of symmetry is aligned with the bend radius. Asymmetric profiles are extremely hard to bend without significant distortion.
Hollow section extrusion
Hollow section extrusion poses another problem: with nothing to support it there’s a tendency for metal under compression to buckle inwards. This can be overcome, to an extent, by filling the hollow interior with something that resists the buckling, but it makes bending a lot more complicated. Alternatively, thicken up the walls, incorporate generous internal radii and if possible, add internal stiffeners.
The product’s alloy, temper and cross-section also are important considerations. Once these factors are determined, aluminum extrusion manufacturers can begin the bending process using one of the following five common bending and forming methods.
Ram or Push Bending
Ram or push bending, as the name implies, uses a ram to force the extruded metal piece on a bending die. A die pushes the extrusion onto the pressure dies, forcing the extrusion into your desired bent form. With programmable bend angles, this form of bending allows close proximity to multiple plane bends, though only one radius can be bent at a time. Ram bending offers inexpensive tooling and good bend precision with low per-bend cost.
Ram or Push Bending is ideal for components such as boat gunnels, portable structure supports, wheelchair frames and medical beds.
Hydraulic Rotary Draw Bending
In the hydraulic rotary draw bending process, manufacturers place extruded aluminum onto a bender and hold it in place with a stationary or sliding pressure die and clamping block. The round bending die, powered by hydraulics, is rotated up to 90 degrees, bending the extrusion as it rotates. With this method, an extrusion can only be bent one radius at a time.
Incorporating a mandrel or other tool component to grip the rotary die can prevent creasing or misshaping of the product, though its use isn’t required. The single axis-controlled revolution can bend within one-tenth of a degree for extremely precise bend angles.
Hydraulic bending is often used when forming round tubes or pipes for applications such as handrails, and is ideal for extrusions with a large diameter, such as building signage.
Electric Rotary Draw Bending
Draw bending is typically used for tighter bends than roll forming down to 3:1 radius to section width ratio. Features of flow forming include:
Electric rotary draw bending uses the same process as the hydraulic method, but allows faster setup. The bends also are more accurate and easily repeated because angles and rotations can be automated in a machine’s programmable logic controller. Rotations of the extruded aluminum also can be mechanized for variable plane bends.
The electric rotary draw method is best for applications that require multiple bends per part in close proximity to each other, or different radii bends for each part.
- Enhanced equipment capability allowing forming of large cross sections up to 260mm in height and 250 in depth, such as bumpers and monorail lift parts.
- Forming hollow sections using floating mandrels or other fillers for irregular internal cross sections.
- Hydro’s recent investment in a multistack reversible head flow flowing machine allows multi radii and reverse bend components to be formed in one operation.
- Maximum bend arc up to 180 degrees.
- Tooling development enabling minimum bend radii to be reduced from 3:1 to 2:1, in some cases it is possible to achieve 1:1
CNC Three-Roll Bending
Three-roll bending pushes an extrusion around three different rolls placed in a triangular shape. The rolls are adjusted to form a precise angle, up to a 360-degree rotation, that can roll horizontally or vertically. As the extrusion is slowly moved across the power-driven rollers, it begins to curve and bend.
Extrusions are limited to a single bend per cycle, meaning a higher angle of bend would take longer to reach the desired angle. Though it may take longer, the maximum bend radius is unlimited. Symmetrical profiles are preferable for roll bending.
Roll forming is used for single and multi-plain bending, typically bend to section width ratio 10:1 in some cases it is possible to go down as low as 6:1. Features include:
- Minimal tooling costs with common tooling used for multiple radii, parabolic curves and edge forming on the same part. For simple profiles such as rectangles, and square section standard tooling is used,
- CNC equipment reduces set up costs and extends range of capability over the original 3 roll machines to include three dimensional curves.
- Reduced sensitivity to material batch variations through programme modifications rather than hard tool recuts.
- Allows for easy curve shape change, that allow rapidly change shape of end product.
- Ideal for low to medium volumes including prototype and development requirements.
Stretch Forming is the most accurate, repeatable and fastest of the forming processes. There are several features to the process:
- It can be applied to a wide range of sections making the process very flexible to a wide range of applications. In addition to bending in 2 planes we have developed the capability and equipment for 3D bending to create spiral type forms at lower cost than fabrication.
- For symmetrical parts left and right-hand forms can be produced in a single forming operation reducing costs.
- The control parameters can be set to incorporate compression bending to give tighter corner radii – overcoming the limitations of material yield strength.
During stretch forming, an extrusion is placed along a rounded, fixed bending die and clamped in place on each end. The machine begins to swing the clamped ends downward to angles up to 180 degrees, and the extrusion is bent around the die to reach the desired form.
The bend radius is unlimited with this method. A stretch forming machine can bend, twist and lift an extrusion simultaneously to create unique, specified shapes and angles for parts up to 25 feet long. This method also offers the most accurate and consistent bending through elongation control. Because of the way the rounded, fixed bending die pushes on the extrusion, stretch forming has the least amount of surface distortion and traffic marking on the extruded piece.
Stretch forming is commonly used for parts with a larger bend radius, as the minimum bend radius is generally two to three times greater than other forming/bending methods.
Each of these bending methods has various benefits. Designing for success and determining the best method ultimately comes down to an end product’s desired tolerance, appearance and strength. Input from aluminum extrusion manufacturing engineers during the design phase of a project can help OEM designers determine the best method for bending, shaping and forming an extruded product that will best bring designs to life.
CRITICAL FACTORS IN BENDING ALUMINUM EXTRUSIONS
Designing your aluminum-extruded components to get you to your final near-net shape is ideal. When this is not possible and your design requires a curved or bent feature, it is important to understand the critical factors during bending that can affect the quality of your component’s final shape.
Whether it is a 20-degree curve or a 180-degree u-shape, work with your extrusion supplier early in the product design stage to choose the appropriate bending equipment and technique for your shape’s needs. To form your product to its proper specifications, your extruder will address any pre-bending requirements during the design stage. Alloy choice and temper, for instance, need to be determined upfront. Your profile’s final geometry, tolerances, and surface finish requirements, also can influence the bending process used, as well as the consideration and management of spring back.
If an extruder makes changes to your design features and specifications, as they may often do, it is not to push their own ideas. They do it because they know how and where to avoid unexpected flaws in your extrusion caused by bending techniques that can affect your final product’s look, feel and functionality.
The extruder needs to maintain certain features to avoid problems with your part’s final fit and finish during the bending process, including:
- Inside and outside diameters
- Radii deformations or deviations
- Critical surface areas
- Mechanical strength
- Length of the part
Why Does Lag-Time Matter?
Keep in mind when using extruded aluminum components in your product designs, aluminum naturally ages and becomes stronger over time. The longer the lag-time between the extrusion and bending processes, the more difficult it is to bend or stretch-form your components to the design specifications. If you transfer extrusions from the extruder to the bending supplier, it will be harder to bend or form them accurately, consistently, and without surface defects or deformations.
Some manufacturers may also be transporting their components from the extruder to a bending supplier and then back to the extruder for artificial aging. This can stack-up lead-times, and add redundant handling and freight charges. To improve the reliability and quality of your formed extrusions, and save money on shipping charges, work with an extruder who also can bend your components, and provide other fabrication processes, such as machining and finishing.
Aluminum extruded components can be extruded and bent to specified tolerances or to standard dimensional tolerances. Keep in mind, the formed dimensions and bend angles of your product will only be as precise as the bending equipment or method used. Different types of bending and stretch forming methods deliver different results.
Ram or push bending
Ram or push bending works best for simple operations, shaping parts with a three-part tool where the bending die pushes the profile against the wipers. This type of bending is ideal for components such as fish house frames, trailer parts, boatlift beams, and ambulance beds.
Twin head compression bending is a fast method to make parts that have two bends, such as solar frames, chair handles and furniture frames.
Rotary draw bending
Rotary draw bending can create more complex, multiple bend parts, via CNC control, with or without a mandrel. This is best for applications that require multiple bends per part in close proximity to each other, or different radii bends for each. Ideal products for this type of bending, include swing set frames, and trailer side-rails.
Roller or three-roll bending pushes an extrusion around three different rollers placed in a triangle, and slowly moves the component across the power-driven rollers to create a curve This will bend extrusions in a large radius, and can roll a fully circular part. Compared to stretch forming, tolerances are looser and quality is not as great, but tooling cost is less and bends are programmable. Boat gunnels, trim components, bumpers, and signage trim are common product components made from using roller and three-roll bending equipment.
Stretch forming is for parts with a larger bend radius, as the minimum bend radius is generally two to three times greater than other forming/bending. Stretch forming is ideal for boat gunnels and trim, roof rails, medical imaging c-arms, and bumpers.