Cold forging is an impact forming process that plastically deforms a piece of raw material, under high compressive force, between a punch and a die within suitable equipment such as a machine press.
Some basic techniques include extrusion (forward, backward, forward and backward), coining, upsetting, and swaging. These techniques may take place in the same punch stroke or in separate operations, depending on the specific application requirements.
In essence, cold forging is a displacement process that forms existing material into the desired shape; contrast this with conventional machining, in which material is removed to create the desired shape. As seen in the following sections, this distinction offers several significant advantages. The final section provides some of the key factors that should be kept in mind when considering cold forging as a manufacturing process.
There are generally 2 types of dies used in cold forging:
- Open Forging: Material is allowed to escape after the cavity is filled.
- Advantage: Lower stress and load
- Disadvantage: Some post-machining may be required, depending on application requirements
- Closed Forging: Die cavity volume is exactly the same as material volume in order to achieve outputs of net shape or near net shape.
- Advantage: Eliminates need for post-machining
- Disadvantage: Higher stress and load; die could be seriously damaged if the material is in excess
Higher Productivity for High Volumes
A primary reason for many companies moving to use cold forging is their need to achieve higher throughput from the production line. In many cases, conventional processes (such as machining, welding or other fabrication methods) involve multiple-pass operations to remove material and finish the part (e.g. vertical, horizontal, bulk removal, detail touch-up, etc.). In contrast, cold forging is typically a single-pass forming process that deforms the existing material into the desired shape.
Depending on part-specific parameters, the time savings per piece can deliver major productivity improvements. For example, some parts that take 3 to 5 minutes per piece to machine can achieve a throughput of over 50 parts per minute when cold forging is used instead.
The opportunity to achieve productivity improvements of over 100 to 200 times offers a fast return on investment in cold forging die and tooling. Thus, many companies have opted to use other methods only for prototyping or during the early production phases, with a transition to cold forging planned for the lead up to higher volume production ramp-up.
Material Savings and Cost Reduction
Another key advantage of cold forging is the elimination of wasted material. Instead of removing a significant amount of raw material, the cold forging process makes use of raw material in all its entirety.
Inputs to its process are in the form of billets of material, which are cut from raw stock bulk material (coils, beams, sheets, etc.). Each billet is the exact amount of material needed for the final part, so there is no wastage or loss of material. This waste-free process can offer significant benefits in high-volume production where waste-per-part is a key cost consideration, and/or in situations where raw material is costly, such as when specialized alloys or scarce metals are used.
Improved Part Integrity and Strength
A very important factor that companies consider when deciding to use cold forging is its ability to significantly improve the strength and integrity of the final part. Forging yields much stronger parts than its counterparts manufactured by casting, welding, powder metal processes, or the machining of raw bar/plate metal.
The high compression forces in cold forging actually displace and rearrange the grain of the base material to minimize any inherent weaknesses. This is particularly important for part designs where the required shape experiences weak points along the existing grain of the base material— for example, long protrusions that cut across the grain or narrow points that could be prone to breakage under stress. The cold forging process overcomes these problems by reducing engineers’ worries about issues pertaining to the underlying grain of the raw material.
Enhanced Appearance and Surface Finishing
Cold forging can also offer distinct advantages over machining, progressive stamping, casting, welding and other fabrication processes in that the output does not typically require post-processing steps to achieve presentable appearance and/or required surface smoothness.
Depending on the specific end-application requirements, some parts could require cleaning up to remove burrs, grooves, striations, or other artefacts from the machining process. This is not a problem with finished parts created via the cold forging compression process.
While cold forging may not be appropriate for every application, it can offer very significant advantages in the appropriate situations. Given that it requires specialized equipment as well as tooling and die investments, the use of cold forging should be balanced against overall production volumes, material costs, part strength requirements and Return on Investment (ROI) projections.
In some instances, where strength, shape and surface smoothness are of critical importance, cold forging is the only process that can efficiently produce parts that meet the required specifications. Consequently, some of these parts— such as complex pinion gears— are designed specifically to suit the cold forging process because they cannot be manufactured via machining or other processes.
Outsourcing production to an experienced cold forging partner can offset a company’s capital equipment investments so that Non-Recurring Engineering (NRE) costs can be focused on tooling and die creation. Costs aside, one should look for a cold forging partner with extensive experience in a wide range of end applications, and who has the know-how to address key process optimization issues such as:
The billet size must be controlled precisely if closed forging is employed. Excess material has nowhere to escape in the closed die cavity when being compressed; this can cause excessively high stress within the die, risking serious damage to the tool. On the other hand, if open forging is employed, extra material will generally not cause similar damage as mentioned above, as material escape routes are usually designed into the process.
Bonderizing is a dipping process that coats billet surfaces with phosphate and soap to ease material flow over the punches or dies during the forging process. This helps to reduce friction, force and stress and also improves surface quality.
Annealing is a process that softens the material and lowers the flow stress for easier material flow. Intermediate annealing, applied in between forging stages, is necessary when cold forging induces work hardening to the extent that no further cold working of the given material is practical or possible.
In cold forging, the use of high-viscosity oil is critical to reducing bare metal-to-metal contact. However, in order to also dissipate heat generated, adding the right amount of thin oil is usually also necessary.
Understanding cold forging trade-offs and selecting a partner with deep experience in cold forging applications, including vertical integration with other processes, can offer designers and production engineers a valuable alternative to conventional machining or casting processes.
The key to success is to begin evaluating early in the design process and to consider overall production volume and ramp-up requirements so that cold forging can be leveraged for optimal ROI and quality results.
Compared to other competing technologies such as machining, die casting, plastic injection molding, welding and metal injection molding, cold forging creates products with higher impact strength, improved structural integrity, and better accuracy while using less material. The process is also highly productive, and optimal for surface finishing.