The Die Process: Closed-Die Forging Vs. Open-Die Forging

When it comes to contorting and deforming metal, there are multiple techniques that involve die forging. Two of these techniques are open die forging and closed-die forging. While both utilize the same basic processes of heating and deforming metal with calculated force, they differ when it comes to the actual die plates themselves.

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Let’s explore the definitions, the forging process steps, differences and various pros and cons of each die forging process.

What Is  Open-Die Forging

Open-die forging, also known as free forging or smith forging, is the process of striking a hammer to deform a piece of metal, typically placed on a stationary anvil. Another approach is to use compression to press the metal between simple dies. These simple dies are typically flat, semi-round or V-shaped. Regardless of the die shape, the metal is never completely encased in the open-die forging process. The dies hammer or press the metal through a series of repetitions, altering the material until achieving the desired shape.

Open-die forging is not recommended for complex shapes, and often requires precision machining and forging equipment to complete the process since the dimensions produced by hammering during the open die forge process can be imprecise.

Pros of Open-Die Forging

• Little or no tooling cost
• Reduced lead time
• A variety of size options, from a few centimeters to nearly 100 feet

Cons of Open-Die Forging

• Not suitable for forming intricate, precision parts or other close-tolerance applications
• Machining is often required to complete open-die forgings
• Process doesn’t always produce consistent results or desired features

What Is Closed-Die Forging?

Closed-die forgings, or impression-die forgings, involves metal being placed between one or more custom-shaped dies. The metal is hammered or pressed, causing it to flow and fill the shaped-die cavities.

Closed-die forging is one of the most commonly used methods for forging steel parts. This closed die forging process works with the metal’s internal grain structure and can produce stronger, longer-lasting products.

Little to no machining is needed for closed-die forgings. The closed die forging process simply utilizes the force from a closed-die forging hammer or press, and heat, to deform the metal to fill the die impression or mold.

The Advantage of Flash in Closed-Die Forging

Where both die forgings can result in a thin layer of flash (excess material squeezed out during the die forging process) the closed-die process uses the flash to its advantage. As the flash promptly cools, it increases pressure on the impression, which encourages the metal flash to flow into details/crevices. The excess flash is then manually removed.

Pros of Closed-Die Forging

• Better surface finish and superior mechanical properties
• Reduced or no machining
• Cost-effective for large production runs
• Dimensions with tighter tolerances and various shapes can be achieved
• More precise, consistent impressions
• Ability to reproduce nearly any shape and/or size

Cons of Closed-Die Forging

• Not typically economical for short or small production runs due to the cost of die production
• Higher setup cost due to costly machines and furnaces

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Finding the right forging method for your metals requires knowing your options. Here is the complete guide on open die forging vs. closed die forging.

As of , the global metal market is worth around $3,949 billion. This makes sense considering the number of applications metal has in our daily lives. Metal is a staple of our existence, from movable bridges to aluminum space equipment to consumer electronics.

Open die vs. closed die forging are two metal forming options for those who want to have custom metal parts manufactured. Here, we will discuss these two metal forming processes and how they impact grain flow and utility. Read on to learn which approach best meets your needs.

What Is Open Die Forging?

Open die forging is a metal forming process that uses multiple dies. None of these dies completely enclose the material as the component is being shaped. Instead, the dies hammer the metal over and over until the forger creates the desired end shape.

Before the process begins, the metal is heated in a furnace. This makes it malleable so that dies can alter its shape.

Hammering and pressing the metal lets you achieve any shape. Assuming that the expert making the material understands metalworking, they will know exactly how to change the billet's dimensions without needing to fully enclose the metal. Open dies are simpler than closed dies because they do not require a specific shape to contain and shape the metal.

Open Die Grain Flow, Benefits and Downsides

The grain flow structure is one of the most significant open die forging benefits. As the metal is worked into the desired shape, grain flow is strengthened as it follows the contour of the piece. The grain flow movement is possible because force is applied to the metal during various forging operations, including upsetting, drawing out and/or rolling.

These methods of forging create a final product that is not porous. In fact, the final product will be stronger and more durable due to its even grain flow and improved microstructure.

Some other open die forging benefits include:

• Continuous grain flow and finer grain size

• Consolidation of voids in the material

• Greater fatigue resistance for long-term use

• Eliminates shrinkage and similar defects

• Less material waste (eco-friendly)

• Better production scheduling as different parts can run through the shop at the same time

There are also a few disadvantages:

• Usually requires some machining to create the correct dimensions

• The inability to create shapes with intricate inner passages

• Rough surface finish (unless refined later)

Open die forging is the most conventional metal forging process available. As a result, despite its shortcomings, it is ideal for R&D, prototypes, oversized components or small-run applications.

How Is Closed Die Forging Different?

Closed die forging is a slightly different process than open die alternatives. It is not a free forging process. Instead, it is often called "impression die forging" because it uses a die impression to give the metal its shape.

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Like open die forging, the metal for closed die is heated in a medium frequency induction heating furnace. However, it is then placed in an enclosed die molded in the desired shape of the final product. Then, the metal is compressed and formed into the end product through high pressure or multiple hammer impacts.

Molding dies are made with precision machines before a metal preform is placed between the top and bottom dies. Depending on the component type, machining might not be needed after the end product is removed from the die. This is because the closed die process creates intricate components depending on the detail of the dies.

Closed Die Advantages and Disadvantages

Like open die forging, internal grain flow is formed since material reduction is involved in creating the product.

There are some things closed die forging excels at:

• A cost-efficiencies are realized with this method for large production runs

• Little machining is required in most cases (for close tolerances)

• The ability to create tighter tolerances

Still, other areas are lacking compared to open die forging. These downsides include:

• High costs are associated with the manufacturing of the dies making this process too expensive for short production runs (since the die production costs are high)

• The time to produce the dies can push out your deadline

• Constrained capacity in closed die shops (Once a run is booked, they are locked into that one part until production is complete)

Ultimately, certain applications require closed die forging methods. However, it is less conventional than open die methods.

Which Metal Forging Process Should You Choose?

Open die forging is generally used for products manufactured in small quantities, require large sizes, and are critical for the end user or R&D. Generally, open die forging shapes are easy and used for more straightforward shaped products (rather than complex ones).

Some examples of products made from open die forging include:

• Small discs

• Forged and rolled rings

• Forged cylinders

• Domes

• Rollers

• Forged shafts

• Metal sleeves

These products are often used in shipbuilding, mining, defense and energy industries. However, they are also used in the machines that manufacture semiconductors, food/beverage equipment, industrial machinery, paper and more. Furthermore, with a skilled Forge Development team, like the one at Scot Forge, many complex shapes can be manufactured through the open die forging methods.

On the flip side, the closed-die forging process is generally reserved for creating smaller and more complex products and large production runs.

• Forged fittings

• Lifting hardware

• Rigging hardware

• Small/precise car parts

• Valves

Because of the precision level, closed die forging is essential in industries like commercial aircraft and automotive markets. In general, forgings keep users safe from part malfunction due to the contoured grain flow resulting in higher strength components.

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