The use of rubber for hydroforming has demonstrated widespread use and an enduring popularity.

Hydroforming, for those who are not familiar with the term, is a process by which hydraulic pressure is exerted upon the surface of a workpiece to form a desired shape. The mediums for transmitting this pressure are hydraulic fluid, cast elastomer shapes and rubber. Rubber, when it is confined in a die or press, transmits pressure in much the same way as hydraulic fluid. In this stead, rubber has had many decades of use. Although the utilization of direct action of pressurized fluids has recently risen in popularity, the use of rubber for hydraulic forming is still the preferred method for many companies.

The use of rubber in hydroforming takes advantage of its ability to flow. Rubber placed in a cylinder and pressured by a press slide results in equal pressure being placed on all sides of a workpiece. This is the same principal that is utilized in tubular hydroforming which uses oil or water as the working fluid. Rubber, incidentally, also possesses the property of cohesion, which is not a property of fluids. Cohesive ability plays an important part in the working of materials, however it has its limits. Caution must be exercised in the design of a tool so as not to expect too much from the material. Overstressed rubber will not provide a long service life.

Rubber processes generally require only the male die half, which results in a lower tool cost. The Guerin process, for example, utilizes a rubber pad contained in the half known as the press ram. This process is commonly used to form short runs of light metal parts with dies placed on the lower press platen. As the press lowers the rubber flows around the form block forming the blank to the block’s shape. Although it is one of the oldest hydroforming methods the Guerin process still remains in wide use. It is however limited to forming shallow parts in light materials. The rubber pads in this method may be constructed either solidly or laminated. The laminated pad is comprised of sheets of rubber placed over one another. The advantage they have is that the working surface can be restored by merely turning the top layer over or replacing it.

One of the first mediums used for hydroforming was rubber and its use is still current and widespread today.

Source: Hydroforming: A Historical Perspective
Dave Smith
Fabricating Equipment News
http://www.vulcanpub.com/fen