Hydroforming Case Study
New Hydroforming Application uses Motion Controller from Delta Computer Systems
Take a look at some of the uniquely shaped spouts on lavatory sinks the next time you're at the home supply store. If you see spouts on kitchen or bathroom sinks that look tubular, with continually changing cross sections, and which don't have plastic bottoms, those spouts may have been hydroformed by H&H Tube Corp. of Cheboygan, MI.
Hydroforming is a manufacturing process whereby a metal tube is formed into a special shape by filling the inside of the tube with high pressure water and literally inflating the tube. Hydroforming is used where the cross section of the part changes throughout the part. There's no conventional way to make spouts look this way except via casting; No cold-forming technique will do this. The spout looks like it was cast, but without the finishing costs and processing complexity of making a cast part. The reason for hydroforming is to shape the spouts aesthetically.
Hydroforming as a process worldwide has been around for decades, but has gained renewed popularity in the last few years. The precision of the process has only been possible with the advent of the new generation of hydraulic motion controllers.
The spout manufacturing process starts with a blank, a tube of uniform diameter, which is bent into roughly the shape of a spout before the hydroforming step. Because it is the easiest material to work with, brass is the tube material of choice. (Copper generally doesn't expand uniformly.) The pre-bent tube is placed in a mold. The mold halves are closed and two hydraulically-activated punches (rams) are extended, one into each end of the mold to form a water-tight seal at each end of the tube (see diagram). Next, water is fed in from one end through one of the punches and is pressurized inside the tube, causing the tube to inflate and conform to the shape of the mold cavity.
As the tube expands under water pressure, the punches push on the ends of the tube. The hydraulics are actively controlled to maintain pressure on the ends of the tube as the tube inflates. The continuous ram force has the effect of feeding more metal into the mold to help the tube expand to fill the mold cavity. The servo control system controls how fast the punches move and hence how fast they feed in the metal. How much metal to feed is based on experience and part geometry and the material being inflated. Some faucets require only a small amount of inflation and involve only small amounts of punch motion. Others, like some laboratory faucets, require the tube to expand up to 70% in diameter, and can require over 2 ½" of extra tube to be fed into the mold.
The secret to controlling the process lies in feeding metal in at a certain rate so that the wall of the tube won't crumple or ripple. Controlling this process to ensure that high-quality faucets are manufactured repeatedly and consistently requires a sophisticated servo control system.
Control System Elements
The diagram below shows the main elements in H&H Tube's hydraulic control system.
At the core of the application is an RMC100, a powerful hydraulic motion controller from Delta Computer Systems, Inc. of Vancouver, WA. The RMC100 controls the hydraulic rams through proportional servo-quality valves, using position feedback obtained from magnetostrictive displacement transducers (MDTs) mounted along the hydraulic piston axes. No additional interfacing hardware is needed as the RMC 100 has direct MDT and analog interfaces built-in. The RMC also has digital I/O interfaces, which it uses to communicate with a PLC and to activate a valve that controls the water pressure in the tube.
The PLC is the master system controller. It issues an I/O signal that causes a hydraulic clamp to close the mold and pressurize the press (the mold is held closed during the hydroforming process by more than 300 tons of pressure). Once the mold is clamped shut, the PLC issues a digital command to tell the RMC motion controller to initiate its motion sequence.
The motion controller runs through a programmed sequence of hydraulic commands to extend the punch rams to their desired positions. Next, the motion controller issues a digital command to tell the water to fill the tube. (The RMC 100 doesn't control the water pressure, which is brought up to between 10,000 and 20,000 psi depending on the amount of inflation required.) When the inflation phase is complete, the RMC turns the water off and retracts the punches.
Finally, the motion controller sends a digital message back to the PLC telling it to depressurize the mold holding clamp and open the mold.
The motion steps are programmed into the RMC using a PC with a Windows-based programming interface. The sequence of motion steps are tuned to be optimal for the manufacture of a particular faucet part and then the sequence file is stored in the PC for retrieval by the manufacturing engineer as he/she sets up the machine to manufacture that part.
Why use this particular motion controller?
One of the main reasons why the Delta Computer Systems RMC 100 motion controller was chosen for this application is its versatility and ease of use.
"A person doesn't have to know a lot about motion control to set up a program on the Delta controller," said Mark Robertson, H&H Tube design engineer. "I can enter measurements in inches. If we have a problem with how the tube material (e.g., brass) responds to a given motion profile, it's very easy to change a few parameters and get the system running again.
"The Delta controller's Windows-based programming interface is friendly and easy to use," said Robertson. Delta's RMCWin development software allows the user to set control parameters and program the motion sequence by loading values into a step table in the RMC 100 controller. Overall, H&H's application uses about 20 steps out of the 255 possible that the Delta controllers support.