By Del Williams
Within the metal forming industry, a wide variety of heavy-duty, metal forming, hydraulic presses with capacities of 10,000 tons or more are required to bend, form, bond, or straighten aluminum, Titanium, and alloys into structures and components.
On the production floor, specialised machines such as stretch forming presses, hot stretch straighteners, contour rolls, diffusion bonding presses, and powdered metal compaction presses are often required to manufacture to custom metal forming specifications under process-specific applications of pressure and temperature.
Because these machines are customised and require precise controls and processes to reliably achieve the specified final product, partnering with a manufacturer with deep expertise in such metal forming is frequently essential for success.
"A custom-engineered machine and control solution is often required when precise control of position, axis synchronisation, force, and heat and force is essential for the forging\forming process," says Bill Goodwin, Vice President of Sales and Engineering at Erie Press Systems, a company that manufactures custom-engineered hydraulic presses for applications including metal forming, stretch forming, composite compression molding presses, cold extrusion, and forging. The company was acquired by Park Ohio and is now a part of the largest forging equipment supplier in North America: Ajax-CECO-Erie Press.
Drawing on Erie Press's decades of experience in aerospace applications, Goodwin surveys some of the essential equipment utilised in custom metal forming.
Stretch forming machines
When applicable, stretch wrap forming methods provide several unique advantages over pure bending and other types of metal forming. Stretch wrap forming machines operate by stretching the metal to its elastic limit, then wrapping the part around a forming die. This process increases the metal's yield strength and results in a stronger part. Since stretch forming machines also keep the metal under constant tension throughout the process, they minimise imperfections such as "cans" or "buckles."
Stretch forming machines also perform a task in one step that would otherwise require several machines and multiple steps, improving efficiency and saving time and money. For some parts, where it would otherwise be impossible, stretch forming machines allow for production runs of the part.
Sheet Stretch Forming: According to Goodwin, stretch forming machines for sheets are designed to meet all tonnage, length, and width specifications. Historically, these machines have been widely used in stretch forming aluminum fuselage, wing, and engine cowl panels. Still, the process has now evolved to forming exterior panels on a high-speed locomotive, recreational vehicles, and buses, as well as proprietary aerospace and commercial rocket applications. Additionally, a special adaptation of high tonnage stretch forming machines can manufacture the main structural support spar used on large commercial aircraft. Heavy cross-section beam material is gripped in specially designed jaws, stretched to its yield point, and bent over a die that follows the plane's curvature.
Extrusion Stretch Forming: Extrusion stretch forming machines were developed to bend and form complex aluminum, Titanium, and stainless steel structural components that have been challenging to work with using other manufacturing processes. Due to highly accurate and repeatable part production, extrusion stretch forming machines have also gained wide acceptance in structural applications within high-speed locomotive, rail, transit, and automotive industries.
"Typically, aluminum tends to wrinkle if bent. However, the stretch forming process can eliminate those defects, because the first step is stretching the part to yield, and then bending it so that you can bend extrusions without wrinkling," says Goodwin, whose company designs and builds stretch forming machines for extrusions, with all associated tooling.
High Production Rate Stretch Forming:Aluminum Stretch Forming (ASF) machines have been developed to service the automotive industry's high production requirements. CNC-controlled AC servo motor technology combines with high-resolution load cells and rigidly guided, low friction components to optimise the speed and accuracy for high production rate stretch forming of extrusions. ASF machines are ideal for stretch-forming extrusions with profiles up to 90⁰ total bend angles, such as bumper and luggage rack support rails and structural frame components used in the automotive industry.
ASF machines use two vertically mounted jaw carriage assemblies to apply full stretching and forming force on the part against a stationary die. Each jaw assembly includes systems that automatically center and clamp the piece, program tangency tracking, engage a support mandrel into hollow extrusions, and provides an inertia suppression system to protect the machine from damage if a part breaks during forming.
Hot Stretch Straightener: This forming machine press is utilised to resistively heat, stretch, and straighten long extruded tubes, or flat plates, because the cooling process after the heated extrusion cycle tends to introduce curvature. When using the hot stretch straightener, the pressure is gradually reduced as the long, extruded metal component cools. This maintains a tensile load on the part, maintaining part straightness during the cooling process.
Contour Roll Forming: Specially designed for stringers and frames, contour roll forming machines bend and twist on three planes and allow for theoretically unlimited lengths. These multi-axis bending machines are utilised to manufacture rounded, structural aircraft parts, such as those used around a fuselage. Generally, a straight piece is run through the machine, and it is bent and curved to the desired shape.
Forging superalloys and other materials with a low forgeability can create difficulties during a metal forging process. Also, the mechanical properties of some metals may vary greatly over small temperature ranges. Isothermal forging may help eliminate some of the problems associated with manufacturing with these types of materials, particularly concerning more complex parts. This process is also called hot die forging.
Isothermal forging is a hot working process that attempts to maintain the workpiece at its maximum elevated temperature throughout the entire operation. This is achieved by heating the die to or slightly below the starting work piece's temperature. As forces exerted by the die form the work, cooling the workpiece between the mold work interface is eliminated, and thus, the flow characteristics of the metal are greatly improved. Isothermal forging may or may not be performed in a vacuum. Isothermal forging is a hot working process that attempts to maintain the workpiece at its maximum elevated temperature throughout the entire forging operation.
"These are typically high tonnage machines that must have the capability to move at a wide speed range, including very slow velocities down to .5 mm per minute to generate a precise strain rate curve required to form these parts," he explains.
Powder compaction presses
Sometimes, manufacturers will take a special blend of metals to achieve certain material properties and turn them into a hybrid powder. After hydraulic presses compact the powder to a specific force, the powder is heated and forged to create a special hybrid piece.
"With powder compaction, manufacturers spin a powdered material at thousands of RPMs while heating it to specified temperatures. The process produces two different metals: a lower-cost metal at its base and a very high-cost metal at its skin. The process then compacts the two layers of powder together and uses this as a raw material to forge into a final multilayered piece," explains Goodwin, whose company offers a variety of designs to accommodate compaction processes for powder metal, ceramics, carbon anodes, and more.
The future of metal forming
With all the demands placed on manufacturers for lighter, stronger materials and greater efficiency, innovation must be a priority within metal forming.
Teaming up with a partner that can provide a wide range of specialised, high-tonnage machinery, along with the ability to customise, can be critical to ongoing success. Because metal forming providers like Ajax-CECO-Erie Press continue to invest in innovation, such a partnership can also provide an edge in future applications.
"In the next 5 to 10 years, instead of hydraulic drives, we expect electric presses driven by geared servo motors and larger roller screws to become increasingly important to manufacturers for any application that is sensitive to oil leaks or where precise control is required. So, we are developing the technology today," concludes Goodwin.
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