Open Die Forging vs. Closed Die Forging - Steel Forging

Open Die Forging

Open die forging is the process of deforming a piece of metal between multiple dies that do not completely enclose the material. The metal is altered as the dies “hammer” or “stamp” the material through a series of movements until the desired shape is achieved. Products formed through open forging often need secondary machining and refining to achieve the tolerances required for the finished specifications. Open die forging is widely used for the products in small quantity that are simple, rather than complex, such as discs, rings, sleeves, cylinders and shafts. Custom shapes can also be produced with open die forging. The strength of the grain structure will be increased during the deformation process due to the repeated working of the steel billet. Another benefit of open die forging is that the fatigue resistance and strength of products will be improved. Besides, voids could be effectively reduced after open die forging.

If you want to learn more, please visit our website RUNCHI.

Closed Die Forging

Closed die forging (also known as as impression die forging) is a metal forming process that compress a piece of metal under high pressure to fill an enclosed die impression. For some special shapes, second forging operation is required to reach final shapes and dimensions. The type of material, tightness of tolerances, and need for heat treatment can determine the cost of a closed die forged part.

Differences between Open Die Forging and Closed Die Forging

Definition

Open die forging is also called free forging. In open die forging, the billet is placed between multiple dies that do not enclose the metal entirely. The dimensions will be changed by hammering and stamping the metal through a series of movement until the final dimensions are achieved.

Closed die forging process, is also referred to as impression die forging. It utilizes high pressure to compress the metal piece to fill an enclosed die impression for required shapes. A medium frequency induction heating furnace is used to heat the metal for pressing into dies.

Process

The technique of both open die forging and closed die forging is different. In general, for closed die forging, the molding dies are first precision machined and cost is high. The metal bars are then placed on between upper and lower dies and forged to any shapes needed. After closed die forging very less or no machining will be operated due to its high accuracy throughout the process. So closed die forging is also a net shape or near net shape process.

For open die forging, enormous compressive forces are imposed by the continuous strike of a forging hammer to deform the metal billets. Unlike closed die forging, dies of open die forging is very simple. And secondly machining operation is always carried out.

Advantages & Disadvantages

Open Die Forging Closed Die Forging Advantages Better fatigue resistance and improved microstructure The internal grain structure formation increases the tightness and strength of the products. Continuous grain flow and finer grain size Economic for large production runs. Increased strength and longer part life No material limitation Less material waste Better surface finish Reduced chance of voids Less or no machining required for its close tolerances Valuable cost savings Dimensions of tighter tolerances and net shapes can be achieved Disadvantages It is not capable of forming close tolerance and higher precision parts It is not very economical for short runs due to the high cost of die production Machining is often required to achieve desired dimensions Closed die forging presents a dangerous working environment

Application

Closed die forging and open die casting are the most common used forging methods for steel parts. Forging is popular because of its improved internal grain structure and can produce stronger and tougher products.

If you want to learn more, please visit our website open die forging service.

In Open die forging, a rough or precision machining is normally further required after the open die forging process as imprecise dimensions are achieved by hammering. Thus, open die forging is more suitable for large parts in a number of tons. Generally, the products in forging method include forged long shafts, forged rollers, and forged cylinders, which are mostly used for the application of railway and aircraft industry.

However, closed die forging is preferred for small critical parts with safety consideration because of its high precision. It is highly used to produce small products like forged fittings, forged lifting & rigging hardware, forged automotive parts, etc. Oilfield, automotive, forestry & agriculture, and mining are some of the industries that rely on closed die forging technique.

The question, “Open Die Forging or Closed Die Forging: Which is better?” can be answered simply. It depends; both processes have their advantages and disadvantages.

The question, “Open Die Forging or Closed Die Forging: Which is better?” can be answered simply. It depends; both processes have their advantages and disadvantages.

Closed-Die Forging

Closed die forgings, also known as impression-die forgings, are considered suitable for complex parts because they force metal into a predetermined shape, which can be something with intricate curves or multiple surface heights. Additionally, closed-die forgings are advantageous for parts that must be mass-produced. The closed-die forging process starts with heating metal to a malleable state, without liquefying it. The metal is then placed between two preformed dies, which resemble a mold. These dies are either pressed or hammered towards each other to guide the metal, making it flow to fill the desired shape. This process will result in a buildup of excess metal along the outer edge of the forging; this is known as flash, which must be cut away.

 The actual process of closed-die forging is where you can find the disadvantages of this type of forging. Because closed-die forging forces metal into shape, there is a possibility that the metal won’t fill all of the crevasses, creating an incomplete part wasting time and resources. To anticipate the likelihood of this happening, Forging Engineers use software to predict the outcomes of the closed-die process. But, even then, because you cannot see the part being forged, there is a possibility that the forging won’t meet specifications. Next, once the forge engineers have designed dies for the part, the dies need to be created, which can a holdup the process because it takes time and money to create them. Finally, when forging with a closed-die, there is the problem of the workpiece cooling at different rates. For example, the flashing cools much faster than the inside of the part, which affects the toughness and strength of the metal, possibly compromising the integrity of the part.

Open-Die Forging

Open-die forgings, or hammer forgings, are advantageous for shapes like discs, hubs, blocks, shafts (including step shafts or with flanges), sleeves, cylinders, flats, hexes, rounds, plate and some custom shapes, which require artistic finesse or custom work. Open-die forging begins with an ingot, a piece of cast metal, which is put into a furnace until it is ductile. Once in this state, the ingot is moved to the anvil, or the press depending on the size of the operation, where the blacksmith hammers the pliable ingot into shape. Since open-die forging doesn’t require custom-created dies, it can be a faster, more cost-effective technique. To get the desired shape with open-die forging, the blacksmith moves and orients the workpiece between hammer drops to force the metal to flow a certain way. Generally, the hammer has flat dies, but on occasion, a simple-shaped die is used for specialized pieces.

The disadvantage to the process of open-die forging is that, because this process is not as precise, the forgings tend to need machining after the piece is forged. But, the disadvantages of open-die forging tend to be mitigated when using a tenured company that can specialize in near-net shaped forgings. Advanced forge shops now offer shapes that were never before thought capable of being produced by the open-die forging process.

In conclusion, every project is different from the turnaround time to requirements; and knowing what process is best suited for your project may depend on the capabilities of the forging partner you are working with. Not all forging companies are equal, so the key to getting a successful forging is finding a company who is a true partner that will work to save you time and money with capabilities that are time tested. Scot Forge has more than 126 years of experience in making custom forgings. If you think you need a forging, let our forging experts take a look and help you find a solution that is right for you, contact us today.

Contact us to discuss your requirements of Open die forging solution. Our experienced sales team can help you identify the options that best suit your needs.