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Hydroforming is a process that uses fluid pressure to shape metal parts. Sheet hydroforming, one variation, can produce formed parts that are impossible to make using a stamping process. Also, sheet hydroforming shapes deep objects in a single operation, whereas stamping might require multiple operations.
Another advantage of hydroforming is that the tooling typically costs much less, as little as one-tenth the cost of that used in metal stamping. This can make hydroforming more attractive for low-volume applications.
Hydroforming basics
The process has been used by metal fabricators for more than half a century. Following initial interest during the aerospace boom of the ’50s and ’60s, however, development of new machines and technical innovation languished through the latter part of the 20th century. More recently, hydroforming has experienced a measurable resurgence, thanks to the introduction of precision motion controls.
Case in point is a new line of sheet-hydroforming machines called Triform presses, made by the Pryer Technology Group of Tulsa, Okla. In these presses, a bladder filled with hydraulic fluid surrounds the tooling form. A hydraulic-cylinder-driven punch presses the tooling and metal sheet into the bladder, causing it to conform to the required shape.
As fluid pressure in the bladder builds, it clamps the material and forces it against and around the form. Simultaneously, a hydraulic cylinder presses the form against the bladder, bending the metal sheet to conform to the tooling. It’s important to control how quickly the material bends. Otherwise, it will tear or wrinkle, according to Darrell Harrelson, technical director with the Beckwood Press Co. of Fenton, Mo., a design partner with Pryer on the new Triform hydroforming machine project.
Sheet hydroforming requires the management of several variables to ensure quality parts: position and speed of the punch; bladder fluid pressure; lubrication of the part, which is relatively easy to control; and material variances, which can be controlled by working with metal suppliers.
When making complex parts on traditional hydroforming presses, operators face differences in pressure from cycle to cycle, causing the position of the punch to vary, says Scott Pryer, president of Pryer Technology Group. Historically, highly skilled and experienced machine operators were needed to deal with this variability.
And often, even that wasn’t enough. Some older machines used two joysticks for control and dials to monitor pressure in the chamber. Operators couldn’t see the actual process, resulting in poor consistency among parts made by a single operator; and even less uniformity in parts produced by different operators.
“Metal fabricators demand machines that perform consistently, every time, so that operators can focus on improving other process factors such as the materials and lubrication,” says Pryer.
Position and pressure control
To solve the control problem, engineers designing the Triform presses turned to RMC electrohydraulic motion controllers from Delta Computer Systems Inc. of Battle Ground, Wash. They precisely control position and pressure throughout the process and reduce the number of forming steps in the manufacturing cycle.
On smaller Triform machines, the Delta RMC controls two motion axes: the punch cylinder that pushes tooling into the bladder, and the cylinder that varies bladder pressure. On larger presses, it also controls containment clamps and the upper chamber that closes around the bladder and workpiece. On the largest machine, a 32-in. model, eight axes are under closed-loop control.
Programs developed for the motion controller let operators select production “recipes” that contain up to 30 different steps in a press cycle. Each step corresponds to a target bladder pressure and punch position. Besides the target values, motion instructions specify how the machine will reach the next target.
For example, at the beginning of a machine cycle, a quick move instruction brings the punch cylinder to its start position. At the same time, the bladder is precharged to a designated pressure. Then, during the forming cycle, the punch axis receives a position command with an acceleration and deceleration value for each step in the process. Meanwhile, to control bladder pressure and coordinate its operation with punch-axis movements, the RMC is given a gearing command. This sets up the punch axis as master and forces bladder pressure to follow the punch as a slave. To smooth motions between steps, acceleration of the axes ramps up and down gradually using an S-curve motion command.
An intensifier cylinder pressurizes the bladder. It works like a syringe to inject or remove oil under both position and pressure control. A pressure transducer monitors bladder pressure and sends the data to the motion controller. The transducer can typically read between 1 and 10,000 psi. The RMC also commands a directional valve to control the position of the intensifier cylinder. A linear magnetostrictive-displacement transducer (LMDT) mounted inside the cylinder provides position feedback. A second LMDT monitors punch position. To ensure precise punch operation, outputs from the motion controller drive a servo-quality proportional valve that controls the punch hydraulics.
“The trick in the hydroforming process is to avoid wrinkling the sheet as it first bends around the corners of the tool,” notes Harrelson. To accomplish this, most of the programmable motion steps in each press cycle occur within the first inch of punch travel. Cycle time varies, depending on the profile of the part and the needed pressure. The machine’s operator interface locks in all of this information for each part by calling up a preprogrammed recipe from the RMC. There’s no guesswork.
In some larger machines, the motion controller is used to close the outer shells before the pressurization cycle begins. Closed-loop position control for this operation ensures smooth motion, avoiding any shocks that would create maintenance problems over time. The controller also monitors shell-position feedback to detect any faults or errors. For example, if a wrench left in the machine prevents the shell clamps from closing properly, a following error (a mismatch between target motion and actual motion) would signal a problem that needs to be fixed.
Similar controls activate the lid on a 32-in. deep-draw press. Four cylinders, one at each corner, each with its own proportional valve, raise the lid. The motion of these cylinders is linked using the multiaxis synchronization capability of the Delta controller to avoid wracking or jamming the lid frame.
“The biggest design challenge in developing the Triform was gearing the bladder pressure to the punch position,” says Harrelson. Getting them to synch requires calculating appropriate ramp times in the open-loop section. To test the design and tune it for optimal performance, Beckwood’s engineers used Delta Computer Systems’ RMCTools software. It includes a Plot Manager and pressure and position Tuning Wizards that analyze the system response to control outputs and produce the proper gains for the control-loop equation. “It simplifies tuning the system for best performance,” says Harrelson.
Results
The design goal of the Triform machines was to permit experienced operators to spend less time controlling the machine and more time improving the process, while also letting inexperienced operators use the recipes to make complex parts consistently, with little training.
Precise control improves quality and performance. “On moderately difficult forming projects in the past, 10% of the material would often be wasted due to lack of precision,” says Scott Pryer. And on complex forming operations, it could be as high as 25%. Now, with the repeatable process, the scrap rate is almost zero.