Closed die forging also know as impression die forging is where metal is placed in a die resembling a mold attached to an anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the workpiece causing the metal to flow and fill the die cavities. The hammer makes contact repeatedly within milliseconds. Depending on the size and complexity of the part the hammer may drop multiple times in a series of quick successions. The excess metal that is squeezed out of the die cavities is referred to as flashing. The flash cools more rapidly than the rest of the material and the flash is usually stronger than the metal in the die so it helps prevent more flash from forming. The flash also forces the metal to completely fill the die cavity and after forging the flash is removed.
In commercial closed die forging the workpiece is usually moved through a series of cavities in a die to get from ingot form to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the material needed later in the cavities. This impression is called an edging or fullering or bending impression. The cavities after this are called blocking cavities in which the piece is worked into a shape that more closely looks like the final product. These different stages usually give the workpiece generous bends and large fillets. The final shape is forged in a finisher or final cavity. If there is only a short run of the parts then it may be more economical to forgo the final impression cavity and instead machine the part.
Closed die forging has been improved over recent years through automation and includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after the forging process.
A variation of closed die forging is called flashless forging or true closed die forging. In this type of forging the die cavities are completely closed which keeps the workpiece from forming flash. The advantage to this process is that less material is lost to flash. Flash can account from 20 up to 45% of the starting material. Of course there are disadvantages also which are the process includes additional cost due to a more complex die design and a need to better lubricate have workpiece placement.
There are other variations of part formation that are integrated in closed die forging. One incorporates casting a forging perform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is then trimmed then the part is quench hardened. Another variation goes somewhat along the same process as outlined above but the perform is produced by the spraying deposition of metal droplets into shaped collectors.
Closed die forging has a high initial cost for the die production and required design work to make the die cavities. However, it has a low production cost for each part, thus forgings become more economical with higher volume. This is one of the major reasons closed die forgings are often used in high volume production such as automotive or the tool industry. Another reason forgings are common is because forgings are generally higher in strength to weight ratio compared to cast or machined parts of the same material.
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