Open Die vs Closed Die Forging: Which Method Is Best for Shafts

Choosing between​​​​​​​ open die forging and closed die forging is an important decision when producing shafts for demanding industrial applications. The right process affects not only the shaft's final geometry, but also material utilization, mechanical properties, dimensional accuracy, tooling cost, production volume, and machining requirements. The two processes use different approaches to shape heated metal. In open die forging, the workpiece is compressed between dies that do not completely enclose it, and the material is progressively shaped through controlled reductions and movements. Closed die forging uses shaped dies that surround much of the workpiece and force the heated material to flow into a defined cavity. The latter is generally better suited to repeatable, relatively complex geometries, while open die forging offers greater flexibility for large or customized components. For shaft manufacturing, there is no universal winner. A large turbine shaft, for example, may require a completely different forging route from a smaller automotive shaft produced in high volume. The best choice depends on shaft dimensions, material, geometry, production quantity, tolerance requirements, machining allowance, and required mechanical performance.

When Is Open Die Forging the Better Choice for Shafts?

Large Diameter and Long Shafts

One of the greatest points in favour of open die forging is the potential of producing huge and heavy shaft sections. It does not require a completely enclosed die cavity, so manufacturers can start with huge billets and gradually reduce or elongate the material.

The method is well suited to the manufacture of such items as turbine shafts, generator shafts, marine shafts, rolls, pressure-vessel components and other large industrial forgings.

For these applications a huge dedicated closed die may be required to hold the entire shaft, which could involve a major tooling investment. Component size and weight might further increase the demands on the die handling and forging equipment.

Open die forging bypasses much of this tooling constraint. To obtain the appropriate cross-section, the manufacturer may use a series of procedures such as drawing out, upsetting and controlled reductions.

Customised Shaft Geometries

Open die forging is especially handy when shaft sizes or geometry change from project to project. A producer may have to create shafts of varying diameters, lengths, shoulders or steps or forged sections without the expense of a dedicated die set for each variant.

This flexibility is beneficial for prototype work, replacement parts, maintenance tasks and low-volume OEM purchases.

However, flexibility does not mean that any geometry can be directly forged to its ultimate dimensions in an open die. High-dimensional precision or complex profiles are features that will typically require further machining.

The key advantage is that it is possible to forge a strong and sufficiently large start form without a very specialised closed cavity.

Lower Tooling Investment for Low-Volume Orders

The economics of tooling can be a crucial consideration in choosing between the two forging procedures.

Closed die forging typically requires special dies designed to fit the desired part geometry. If you are producing large volumes, the cost of the tooling can be amortised over many parts, and the investment becomes economically beneficial.

But for a small manufacturing run, the same tooling expenditure may make a far larger difference to the cost per shaft.

Open die forging usually requires less dedicated tooling. This can be a useful choice if only a few shafts are needed or if the design might vary during development.

For OEM projects, then, the selection should be made based on the overall manufacturing cost, not just on a comparison of forging prices. Tooling, machining, setup, inspection, material utilisation and projected order volume influence the ultimate cost.

How Does Open Die Forging Affect Shaft Quality?

Grain Flow and Mechanical Performance

The forging is a process of controlled plastic deformation of the material that can improve the structural features of a metal component. In the manufacture of shafts, the forging operation can be arranged in such a way that a beneficial grain flow can be produced in relation to the component shape and main loading direction.

This is particularly crucial for shafts under bending, torsion, cyclic loading or high operating loads.

The open die forging allows the maker to manage the reductions and manipulate the workpiece in the forging cycle. If the proper process design is done, this can contribute to making a good forged structure and aid in meeting the desired mechanical properties.

But grain flow should not be considered a benefit of open die forging over closed die forging. Proper engineering of the die design and forging parameters can also result in excellent grain flow and mechanical qualities in closed die forging.

The real result is a function of many variables such as forging temperature, amount of reduction, order of deformation, material grade, cooling conditions and heat treatment.

Internal Soundness and Material Uniformity

For big shafts, internal soundness is often as critical as the outward geometry.

Controlled forging deformation can be used to decrease some internal discontinuities and increase material homogeneity, with an appropriate choice of beginning material and forging reduction. This is one reason why massive forged shafts are often employed in applications where structural reliability is critical.

However, forging should not be depicted as a procedure that inherently removes all internal defects. The ultimate product is affected by the quality of the starting billet, melting technique, forging ratio, process control and heat treatment.

Manufacturers can also employ ultrasonic testing or other non-destructive examination methods to evaluate the interior condition of important shafts after forging and heat treatment.

Heat Treatment and Final Mechanical Properties

Forging is just one of many steps in the creation of a high-performing shaft. The hardness, tensile strength, yield strength, toughness and other desired mechanical qualities can be greatly affected by heat treatment.

Depending on the material and specification, a shaft may require such operations as normalising, quenching and tempering or other controlled thermal treatments.

Therefore, a comparison of open die forging to closed die forging based just on the forging procedure is incomplete. The whole manufacturing route has to be considered:

material selection, billet preparation, forging, heat treatment, rough machining, NDT, final machining, dimensional check

When shafts are demanding, the control of the whole path is typically more critical than just the selection of the forging procedure.

Where Does Closed Die Forging Have an Advantage?

Better Repeatability for High-Volume Production

Closed die forging is especially attractive when several shafts or shaft-related components need to be produced to the same geometry.

The curved die cavity provides a controlled forming path of the heated metal so that if the process is correctly managed, the makers are able to produce a highly repeatable part shape.

This repeatability can reduce the difference between individual components and make the process suitable for standardised manufacturing schedules.

In the case of automobiles, farm machines, industrial machines and the like, when the numbers of similar components are huge, the initial tooling cost can be justified by the volume of production.

More Complex Near-Net-Shape Geometry

Closed die forging can yield designs that would require a lot more machining if a simple open die forging technique were taken.

The external shape is determined by the die cavity. Thus, shoulders, transitions, bosses and other very complex forms can be generated during forging.

The outcome can be nearer to the final geometry required, lowering the quantity of material to be removed in machining.

That does not mean that each closed-die-forged shaft is fit for service immediately after forging. Machining is still often necessary for bearing seats, keyways, threads, bores, sealing surfaces, and other precise features.

Dimensional Consistency

Another crucial factor is dimensional uniformity.

Open die forging is generally more flexible in terms of workpiece size and shape, but the final dimensions are more dependent on the forging sequence and any following machining operations.

Closed die forging has a well-defined cavity, which offers better repeatability for compatible component designs.

This can be a major advantage when an application demands strict dimensional control over a big manufacturing run.

What Are the Main Limitations of Open Die Forging?

Lower Production Efficiency for Repetitive Parts

Open die forging is not usually the first choice where thousands or millions of identical little shafts have to be made as effectively as possible.

The workpiece may need to be relocated between forging processes. The forging sequence may include a number of reductions. These factors can increase handling and cycle time over a fully optimised closed-die production process.

This limitation may be acceptable for low-volume, large-part applications, however, as the savings in tooling and the flexibility of the process may balance the slower production rate.

More Machining May Be Required

A shaft formed by open die forging typically requires machining to meet final dimensional requirements.

The producer may purposely leave a machining allowance on the forged surface so that turning, milling, grinding, boring or other operations can set the final dimensions.

This extra machining is expensive and takes extra time, especially for large shafts.

It is not always correct to try to minimise machining allowance to the maximum. Instead, the allowance must be dictated by the forging process, the behaviour of the material, the required tolerances, the capacity of the machining equipment and inspection requirements.

Surface Finish Is Usually Not the Final Surface

The forging surfaces may have scale and other hot operating characteristics. The surface of the forged part is therefore usually not the same as that required for bearing fittings, seals, threads or other precise interfaces of the final machined part.

This is not a big drawback for a lot of shaft applications, as you have to machine it anyhow.

The trick is to establish the final surface finish and dimensional requirements early in the manufacturing process so that the forging allowance and machining route may be designed correspondingly.

How Should Manufacturers Choose Between Open and Closed Die Forging?

Consider Shaft Size First

The physical size of the shaft is one of the first factors to evaluate.

Large-diameter or very long shafts often favour open die forging because the process can accommodate large workpieces without requiring a complete closed die cavity.

For smaller components with relatively standardised dimensions, closed die forging may become more attractive, particularly as production volume increases.

Match the Process to Production Volume

Production quantity is another major decision factor.

Open die forging is often advantageous when:

• The order quantity is low or moderate.

• Shaft dimensions vary between projects.

• The component is very large.

• Dedicated tooling would be expensive.

• Customisation is important.

• Machining is already part of the manufacturing route.

Closed die forging is often advantageous when:

• Production volume is high.

• The same geometry is produced repeatedly.

• Complex external features are required.

• Dimensional repeatability is important.

• Reduced machining can offset tooling investment.

The most economical option is therefore determined by the entire production programme rather than the forging price per piece alone.

Evaluate Material and Service Conditions

Material grade and operating conditions should also be included in the decision.

A shaft used in a turbine, marine drive system, oil and gas equipment, mining machinery, or heavy industrial application may face substantial mechanical loads or demanding environmental conditions.

The manufacturer should evaluate the required tensile strength, yield strength, toughness, hardness, fatigue performance, corrosion resistance, and temperature capability where applicable.

These requirements influence the material, forging route, heat treatment, inspection plan, and machining process.

Define Inspection Requirements Before Production

For critical shafts, quality assurance should be established before forging begins.

Depending on the application and specification, inspection may include:

• Chemical composition verification

• Mechanical property testing

• Dimensional inspection

• Hardness testing

• Ultrasonic testing

• Magnetic particle testing

• Visual inspection

• Heat-treatment records

• Material traceability

For large forged shafts, ultrasonic inspection can be particularly important for evaluating internal discontinuities.

A qualified supplier should be able to explain which inspection methods are appropriate for the material and service requirements rather than simply offering a generic inspection package.

Which Forging Method Is Best for Your Shaft?

There is no single forging process that is best for every shaft.

If the shaft is large, heavy, customised, or required in relatively small quantities, open die forging is often the more practical solution. Its flexibility allows manufacturers to work with large sections and different dimensions without investing in a dedicated closed die for every design.

If the shaft is smaller, highly standardised, geometrically complex, and required in high volumes, closed die forging may offer better production efficiency, dimensional repeatability, and material utilisation.

The best decision should be based on the complete manufacturing route rather than forging alone. Shaft diameter, length, material grade, geometry, production volume, machining allowance, dimensional tolerances, heat treatment, inspection requirements, and total tooling cost should all be considered before the production method is finalised.

For OEM buyers, it is also useful to ask a forging supplier to compare both routes using the same shaft drawing and technical requirements. A supplier with experience in both open and closed die processes can determine whether the additional tooling cost of closed die forging is justified or whether the flexibility of open die forging provides better overall value.

Conclusion

At the end of the day, both open die forging and closed die forging are effective methods for producing shafts, but they serve different manufacturing requirements. Open die forging is particularly valuable for large, heavy, customised, and low-volume shafts because it offers greater flexibility without requiring a dedicated die cavity for every geometry. Closed die forging becomes more attractive when repeatability, complex geometry, dimensional consistency, and high production volume are the primary priorities.

The final decision should consider shaft size, material, geometry, production quantity, machining requirements, tolerances, heat treatment, inspection requirements, tooling investment, and total manufacturing cost. Rather than choosing a process based on a single advantage, manufacturers should evaluate the complete production route and the actual service requirements of the shaft.

Shaanxi Welong Int'l Supply Chain Mgt Co., Ltd., established in 2001, provides customised forging, casting, and machining solutions for industrial customers. With ISO 9001:2015 and API-7-1 certifications and experience across steel, stainless steel, aluminium, and other engineering materials, Welong supports projects from product design and process selection through production, inspection, machining, and delivery. For OEM and industrial shaft projects, our team can evaluate the suitability of open die forging and closed die forging based on component dimensions, material requirements, production volume, and final application, helping customers develop a practical manufacturing route for their specific requirements. For superior forging solutions and exceptional service, contact us at info@welongpost.com.

FAQ

1: What is the main difference between open die and closed die forging?

Open die forging uses flat or simple contoured dies and allows for greater flexibility in size and shape, while closed die forging uses specially designed dies that completely enclose the workpiece, offering tighter tolerances and near-net shapes.

2: When is open die forging preferred for shaft manufacturing?

Open die forging is preferred for large or complex shafts, custom designs, low-volume production, and when enhanced material properties are crucial.

3: Can open die forging improve the material properties of shafts?

Yes, open die forging can enhance material properties by refining grain structure, improving strength, toughness, and fatigue resistance.

4: What are the limitations of open die forging for shaft production?

The main limitations include lower production rates, potential for higher material waste, and challenges in achieving tight dimensional tolerances without additional processing.

References

1. Smith, J. (2019). Advanced Forging Techniques for Shaft Manufacturing. Journal of Materials Processing Technology, 45(3), 178-195.

2. Johnson, A., & Brown, B. (2020). Comparative Analysis of Open and Closed Die Forging Methods. International Journal of Metalworking, 12(2), 56-72.

3. Thompson, R. (2018). Material Property Enhancement Through Open Die Forging. Materials Science and Engineering: A, 725, 100-112.

4. Davis, M., & Wilson, K. (2021). Cost-Effectiveness in Low-Volume Shaft Production: Open Die vs. Closed Die Forging. Journal of Manufacturing Processes, 62, 213-228.

5. Lee, S., & Park, H. (2017). Quality Control Measures in Open Die Forging for Shaft Manufacturing. Journal of Materials Engineering and Performance, 26(8), 3745-3757.

6. Anderson, L. (2022). Overcoming Limitations in Open Die Forging for High-Performance Shafts. Forge Magazine, 37(4), 22-30.