A fluid die is a precision tool that uses fluid pressure, or hydraulic pressure, to form metal. The process of using hydraulic pressure and a fluid die is called hydroforming. This process allows metal to be formed into complex shapes that are both strong and lightweight. Metal components formed using a fluid die and hydraulic pressure are often used in manufacturing automobiles, bicycles, and other products requiring complex structural components.
In metal forming, the die is customized for the final product being manufactured. Some may have inserts or additions that can be used to produce variations of the basic final product. Dies can be manufactured in various sizes, from small dies used in jewelry-making to car-sized dies used to create the frame for a vehicle. The die is usually held in a die shoe to increase precision in the manufacturing process. The assembly of dies and shoes is known as a die set.
Simple dies have been used in metal forming for thousands of years. In the earliest stages of metal crafting, sheets of soft metal were hammered into stone or wood forms, allowing the craftsman to create multiple near-identical pieces of precise sizes and shapes. These crude, simple dies were eventually replaced with metal dies that were stronger and more durable. In the mid-1800s, fabricators started using paired dies, also called male and female conforming dies, to more precisely shape metal. In this process, the two dies are forced together around sheet metal, also called a blank, using pressure.
Paired conforming dies can only form limited shapes, because the dies must be able to be fitted together and separated. Though paired dies can form intricate, detailed shapes such as coin faces, they cannot be used to form complex tubes or other closed shapes without multiple steps and complex dies. This is why manufacturers developed the technique of using a fluid die for hydroforming.
In the process of hydroforming, a metal blank is sealed between multiple fluid dies. The blank is internally pressurized with fluid, expanding the blank into the contours formed by the dies. The blank is sealed, so the fluid presses the blank out, also called bulging, and shapes it to fit the dies. A fluid die can be used to create a simple shape, such as a T-junction pipe, from a single tube, or it can be used to form the complex shapes required for a unibody automobile frame.