The Process Itself. A conceptually different step to prevent pinching and allow forming of more complex parts applies appropriate lower internal pressure while the die is closing and higher pressure after the die is closed.
The steps and benefits of pressure-sequence hydroforming are displayed here.
Varying low pressure during die closing makes the fluid-filled tube act as a semi-solid and forms the part by supporting the tube wall on the inside while the cavity surface defines the outside. The tube wall is forced into the corners as the die closes, because the cavity is designed to be nearly the same periphery as the start tube. The tube wall, which is not stretched or thinned, fills the die cavity.
This compressive force, combined with low pressure, makes it easier for metal to overcome friction and move over the cavity surface without lubricants, making very Polyurethane injection grouting unnecessary.
When the die is closed, higher pressure is required to complete forming the tube walls. Holes then are pierced by punches mounted in the forming die. Lubricants are not applied, nor are they advantageous in PSH.
PSH hydroforming fluid provides limited rust prevention. The highest pressure used to finish forming the part can be whatever is required (for example, 172+ MPA, or 25,000 PSI).
The PSH method can apply high pressure when it is needed, such as in expansion without end feeding. However, it requires bigger and more expensive equipment and somewhat longer cycle times. All possible ways to create a part feature should be explored to find the most economical solution. High pressure should be used only when it is the best alternative and justifies the extra cost.
Normal high pressure for PSH is 34 to 69 MPA (5,000 to 10,000 PSI). Using high-strength material, thicker walls, and sharp (3T) corners has minimal impact on required pressure.
That forming the tube causes the material to yield by bending from the starting round to a finished shape of flat areas and forms sharper corners along its whole length. This nearly eliminates springback of the cross section and dimensionally stabilizes the whole part.
Part features. In this author’s opinion, the range of features in PSH is potentially even wider than for HPH. All of the available elongation for a particular material can be used to form the part rather than using a portion of it to avoid pinching. If a part can be bent, it can be pressure-sequence hydroformed.
Elongation needed for hydroforming is relatively low. In production, joint-free parts with bends up to 120 degrees and bend radii down to 1.5D can be formed, while other methods may require joining two or more parts. Fewer parts and joints tend to reduce cost, weight, and dimensional variability.
Additional operations, if any, are minimized, and the problems caused by making the cavity bigger than the start tube are avoided.
Local severe deformation can be built into a part to fit with another part, to avoid interfering with it, or to form a feature that causes a die lock condition. An actuator forms the feature when the die is closed.
Pressure sequencing is less demanding on material formability. It is designed to allow use of materials specified by the customer for best part function and economy, with no need to tailor the material properties to suit the process. This allows the use of mild steel, stainless, HSLA, and galvanized steel, high-strength lower-elongation material, and aluminum.
Yield strength increases 5 to 7 percent with PSH for mild steel, similar to that achieved by HPH. The whole cross section yields, bending to form the finished shape from the starting round.
N-value, a feature critical to HPH success, is of little concern with PSH, which in normal steel- or tube-making is not watched closely.
Die Considerations. PSH dies are finished to normal die standards. Some have run up to two million parts, with no die surface refurbishment or wear inserts needed.
Hole changes can be easier and less expensive than those for stampings. Hole punching capability is well-developed in the PSH system. Hole sizes in production range from 3.1 millimeter (0.122 inches) diameter to a 40- by 45- millimeter (1.575- by 1.772-inch) rectangular slot.
Holes punched in the fashion illustrated here are used for numerous applications, like self-tapping screws, clips, and pins.
At one company, more than 100 million holes currently are punched every year, with as many as 57 holes in one die. These numbers reflect customer need, not limitations. Holes are used for self-tapping screws, clips, pins, drainage, access, and other applications.
Lower internal pressures increase the hole count in the hydroforming die because punch cylinders can be smaller and the die strength needed to resist pressure lower.
Equipment. Factors such as corner sharpness, material strength, and thickness that increase HPH equipment size requirements make little difference for PSH. With the PSH process, forming the same part requires about 20 to 30 percent of the internal pressure and equipment needed for HPH. Large end-feed cylinders are not required except for expanded sections.
Section Expansion Before and During Hydroforming. Figure 8 shows 15 percent tube expansion without end feeding during normal pressure sequencing using less than 48 MPA (7,000 PSI). Expansion with end feed modifies the process pressure curve to minimize wall thinning with 55 MPA (8,000 PSI).
Mechanically expanding the round tube after bending and before hydroforming thins the wall (7 percent for 41 percent expansion) less than does hydroexpansion in the die (14 percent for 41 percent expansion), because friction aids end feeding in the first situation rather than resisting it, as in Injection Packers.
Cycle Time. Production cycle time for the engine cradle is less than 22 seconds, including handling and forming. The 35 percent lower cycle time allows 55 percent higher production capacity from one production line.