Open Die Forging Versus Closed Die Forging

Time was when forging was just forging, beating heated metals into shape with the aid of a hammer. The principle is still the same, the methods infinitely more sophisticated. Today we have methods that have evolved to the point where forging can be controlled to produce complex shapes, large forgings, and seamless rolled rings. But the two major forging methods are called closed die and open die. The basic differences between these two methods control the sizes and shapes that they produce.

Typically, closed die forging is used when hundreds or thousands of identical parts are required with lower mechanical properties.  Closed die forging has size limitations because the parts are produced within a mold.  Open die forging is used when part volumes are just one to several hundred, and when significant structural integrity is required.

Forging techniques, although evolved, are still somewhat of an art, and benefit from an expert operator. This applies particularly to open die forging, where there is no complete enclosure of the metal, and repeated hammering and pressing can lead to the production of any desired shape. Open die forging involves two dies, an upper and a lower, that come into contact with the workpiece. As mentioned, the metal is free to flow where it may, and is effectively under the command of the operator. There are really no limits to the dimensions that can be produced by this process.

Grain flow in open die forging follows the shape of the part being forged. Working of the material leads to a final product that is consolidated, in that it has no porosity, has a uniform grain size, and good mechanical properties. 

The open die technique is best used for relatively small quantities, and is good for discs, forged and rolled rings, cylinders, and shafts, although more complex shapes can be achieved through forging expertis. It is also good for the production of oversize parts.

Closed die forging is not a free forging process, rather it is restricted to the shape of the dies. In fact, the process is often referred to as impression die forging, whereby the metal is forced into the die to give the final part its shape. There are repeated hammer blows and the dies, made normally from hot-work tool steels, will serve to produce the required components.

By the very nature of the closed die process, and the significant costs involved in manufacturing the part dies, the process is best used for long production runs.  The work that is put into die manufacture results in an overall ability to create tighter tolerances, and complex parts, hence less machining required. 

Both methods have inherent advantages. Open die forging can produce very large parts that would not be possible from the closed die process. Theoretically the closed die process can produce parts whose dimensional tolerances will surpass those from the open die process. Both processes produce a grain flow and a lack of internal porosity that result in required mechanical properties. 

As a manufacturer of open die forgings and seamless rolled rings, All Metals & Forge Group can produce a wide range of shapes and part sizes in various metals and heat-treated conditions.  Many industries rely on both open die forged parts and closed die forgings for construction of machinery, turbines, drive shafts, fluid ends, xmas tree flanges, rollers, and hundreds of other finish machined components.