This image represents the typic tube hydroforming technology.

Here there are quite a lot of factors worth being noted, such as geometrical, material features of formed tubes and the quality of the finished parts related to the density allocation or dimension accuracy.

These features are crucial for the successful process, and each of the mentioned components needs processing in various phases. Thus, the major factors being considered in the tube hydroforming system are as the follows:

  1. Tube and deformability
  2. Frictional lubrication
  3. Pre-form and pre-bend operations
  4. Equipment
  5. Cycling duration and productiveness.

The following article relates to tubes, their deformability, pre-form, pre-bend operations as well as equipment.

Pipe and Deformability

Feed pipe quality determines the success of the hydroforming technology. Stuff characteristics (material contexture, welding types, yielding force, the maximum extensile strength, extension and flow properties) and the tube dimensional features (diameters and thicknesses) should be specified in accordance with the demands of the finished parts and should be retained and monitored carefully while manufacturing. To gain realistic data on the tube material characteristics a testing operation similar to the hydroforming technology can be realized. Here is a draft extension test sample conducted at the State University of Ohio.

Fig.. 1

In the test the tube tips are locked and the tube becomes free to expand through the hydraulic inner pressure.

In Fig. 2 some types of 304 stainless steels are bulged at various levels of pressure. (Figure 2) 

In the given sample a piece of SAE 1008 steel is exposed to formation under the pressure of 2.654 PSI (18.3 MPa). 

The levels of pressures and the utmost bulging diameters are calculated under various grades of pressure in case of a certain pipe and thickness. The following information along with the suitable software foresees the tube material flowing strength as an equation and a function, which is possible to use in the finite element method (FEM) process to imitate a pipe hydroforming operation.

Pre-form and Pre-bend Operations  

The hydroformed start pipe shape may be either direct, or pre-bent and pre-shaped. It depends on how complex the finished part should be. (Fig.3) In certain cases some parts of the piece get pressed while the mold closes and standardized to the mold surface complete dimension.

Fig. 3

To gain accomplished plastic deformity all along the piece and meanwhile reduce the back spring, the pipe perimeter has to be a bit less compared to the mold geometries in every charter. 

The tension in the curved pipe has an effect on the pipe deformity and dilution in the hydroform process, as well as defines the allocation of thickness and force along the finished piece (this should be noted while designing to ensure the required features for the formed parts prior to assembling).

Manufacturers should refrain from exceeding wrinkles in the pressed part of curve pipes not to achieve hydroformed parts with no wrinkles 

Manufacturers should as well consider the pipe dilution, occurring when it gets extended, to discover whether the pipe is of proper thickness to meet the needs of hydroformed parts.  

One more thing, orienting the welded part of the tube is essential to make it appear in a neutral area and prevent it from being affected by pressure during deforming procedure. 

Equipment

The hydraulic press key function lays in opening and closing the mold to provide clamp loading in the formation process, eliminating elastic deformity as well as mold disconnection. Axle strength cylinders and pressure amplifiers also take an immediate part in the following process.

Nowadays a hydraulic press is used to ensure larger clamp force in this process. Yet, the hydraulic press commonly requires larger capital investments. Certain research work is currently being carried out to come up with equipment of lower costs, which will open and close molds through several stages, meanwhile providing sufficient clamp loads.

One design bears a top mold ram driving half upwards and downwards through a little cylinder. When the ram comes to the base central location and the mold closes, the both distance blocks get pushed amongst the ram and pressing frame through the pneumatic or hydraulic cylinder. Next, some short-stroke cylinders push the gaskets and the bottom half part of the mold eliminating the clearance between mold halves. Such design makes it possible to gate fast, however, demands less force for closing, clamping or opening the molds.

Conclusion

An innovative process has been developed over the last 5 years by researching hydraulic formation of tube fittings in industries, which is suitable for mass production. Still, in comparison with the traditional stamp operation, pipe hydroformation appears to be newer and there is some lack of ample knowledge on tooling as well as designing the process. The full comprehension of computer software will support engineers in developing dependable controlling models for axle feed, inner pressures and timing to contribute deformability of hydroforming stuff.

As tube hydroformation involves considerably higher pressures, testing with soft molds is impossible. Molds have to be out of steel possessing some hardness as well as coat specifications, and modification requires high costs. The less the trial and error process, the more is the pipe hydroformation potential. It is accessible through computer modelling, which supports to gain more detailed and clear insight of interactions among each process parameter.

Any prediction (thickness allocation, inner pressure, clamp forces, frictions, rebounding) offered by computer modeling helps to identify any possible form defect or crack in the design process and enables designers to better or correct mold designs prior to mold hardening.