Hydraulic counter-pressure forming, alternately referred to as fluid forming or sheet hydroforming, was first utilized by Amino Corporation in the 1980's and has been applied to the automotive industry since 2005. Sheet hydroforming was initially developed to overcome the restrictions of deep drawing to form complex part geometries. Now, it is being utilized to reduce tooling costs for low volume panels for niche vehicles and to achieve more aggressive styling for special vehicles. Furthermore, the fact that there is no metal die surface contacting the outside show surface of the part means less scratching and damage to the part.
The Amino sheet hydroforming method uses a method called passive pressurization. In passive pressurization, rather than using a pump to pressurize the water, the water pressure is increased by the displacement created as the punch draws the sheet into the water chamber. The water pressure is controlled by a pressure relief valve, which can be programmed to maintain more or less water pressure depending on the displacement of the punch.
In a typical sheet hydroforming tool set, the upper punch is the male side and has the shape of the part machined into it. The lower die (water chamber) is typically much less detailed than a traditional stamping die, and thus a significant tooling cost reduction is achieved. The upper binder (blankholder) is used to provide controlled clamping force to the blank using hydraulic blankholder cylinders.
Initial hydroforming research conducted by Nakamura (Study For FFP of Sheet Metal, 1985) showed that a sheet hydroformed cup can achieve a much greater depth of draw than a conventionally drawn one. The greater depth of draw observed in hydroforming can be used to achieve a much higher draw ratio (2.92) when compared to the conventionally formed cup (2.08). The deeper drawability for both steel and aluminum sheets observed in hydroforming cup drawing experiments is also observed in automotive part production. Sheet hydroforming technology is best utilized to form deep drawn class A panels with detailed features. In sheet hydroforming, the water pressure holds the panel against the punch throughout the drawing process, which can increase the depth of draw by reducing localized thinning in critical areas. In addition to the increased depth of draw, sheet hydroforming is able to form sharp radii (on the order of 1.5x material thickness) and reduces wash out of character lines that can be observed in stamping when material is drawn over a feature.