The ancient process of forging is an indispensable part of modern metalworking. The basic idea is simple: you’re shaping metal by heating it and hammering it into a new form. Forging today is quite complex, with many different approaches and techniques. One of the most important distinctions is open die forging and closed die forging. Choosing between these techniques has a significant impact on your final product and can mean the difference between the success or failure of a project. This significance means considering the open die vs closed die forging question is crucial. In this guide, we will introduce the two approaches and consider how to decide between them.
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Let’s start with open die forging. This technique is the process of deforming and reshaping metal between dies that do not totally enclose the piece, resulting in free flow metal condition. The open die forging process stamps metal pieces between multiple dies until reaching the final shape. Though you’ve reached the desired shape, secondary machining is often required to ensure the metal pieces have the correct final dimensional characteristics. The open die process typically produces metal pieces like rings and shafts. However, the process can also make a variety of custom shapes. Open die forging increases the strength of metal, as the dies hammer impact distorts/refines the piece’s grain structure, making it internally more uniform. This feature creates final products with improved fatigue resistance and high strength. The level of increased strength and fatigue resistance is dependent on the level of deformation imparted to the original workpiece.
The other side of the open die vs closed die forging question is closed die forging, also known as impression die forging. This technique uses high pressure to compress metal into dies that enclose the piece. Where the open die forging process uses flat dies, closed forging moulds metal into the shape of custom dies. First, the metalworker presses or hammers the material into the bottom die or die cavity. Secondly, the top die compresses the material into the desired shape. Unlike open die forging, this process typically results in a more precise final geometric shape. Any subsequent machining is very minimal, thus resulting in reduced waste of metal and, generally, a final product of a greater geometric complexity. Like the other technique, closed die forging strengthens the metal piece by refining its grain structure.
These two forging techniques have distinct advantages and disadvantages. Though both processes increase the strength of the final shape, open die forging lends itself more readily to simpler and larger shapes. Also, open die forging can be used as a pre-forming operation in order to displace volumes of material, where required, then to be finished using closed die forging. The process’s simplicity means it is often cheaper to employ, especially when only needing a short production run. However, this simplicity means open die forging often cannot achieve high precision. Imprecision necessitates further work to create a wide range of complex shapes. Open die forging involves more human involvement than closed, which makes it a good choice for free-flow forging and custom pieces.b
Closed die forging is typically more expensive than open die forging. The moulding dies you need must be machined, which adds costs, and the process creates material waste, or ‘flash’. This disadvantage is much less noticeable when you’re producing large numbers of products, and the proportion of flash waste is minimal, compared to the mass of the final shape. The primary advantage of closed die forging is precision. The high pressure of impression die forging creates custom pieces with a clean surface finish, without sacrificing strength. These attributes make the closed die process ideal for producing smaller, complex shapes, and higher production run rates.
The two forging techniques do not directly compete. The decision in the open die vs closed die forging question depends on the producer’s needs. Open die forging is well suited to creating large metal parts such as long shafts and rollers. This feature means it is typically used in the railway and aircraft industries. By contrast, closed die forging is best used to create small, complex products. Many industries rely on closed die forging to create smaller parts and hardware, including the automotive, mining and agricultural sectors.
Deciding between open and closed die forging processes will come down to your particular needs. How many products are you aiming to use? Do you need a simple or a complex metal piece? How large are the parts going to be? Asking these questions as you consider your production needs will help you make the right decision. Ideally, you’ll have access to both processes. Outsourcing your work to forging specialists is a great way to achieve this.
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.
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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.
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