There is no universal forging method. Forging can be best categorized by the temperature at which it’s performed (e.g., hot forging and cold forging), the equipment used (press forging, hammer forging, etc.), and the type of tooling used in the process (open dies and impression dies).
When comparing these different processes, it’s not a question of “Which one is inherently better?” but a question of “What are the best applications for each process?”
In this article, we’ll discuss open die forging and impression die forging processes, including benefits, limitations, and best use cases for each.
Open Die Forging
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The open die forging process involves using tooling (typically flat, parallel plates) to compress metal. In this method, the dies do not completely surround the preform, allowing excess material to flow out of the tooling.
When the dies open, the forging operator may reorient the billet for the next stroke. Multiple strokes or compactions are used to form the desired shape. As the dies are flat or gently radiused, open die forging can only flatten parallel surfaces, limiting the shapes that can be formed to circular, square, flat, or hexagonal.
Benefits
- Fast
- Forms high-strength metal parts
- Requires little tooling
- Possible to forge large shapes that weigh hundreds or thousands of pounds
Drawbacks
- Repeatability is poor, as there’s little control over the deformation process
- Cannot produce complex forms or fine detail
- Produced forgings may require significant machining
Best Use Cases
- Small quantity production, where tooling costs would be prohibitive
- Situations where there’s little time for tooling development
- Large parts that do not require complex geometries or have tight tolerances
- Used to produce shafts, blocks, bars, discs, and plates for use in sectors like ship building, civil and municipal engineering, mining, and railways
Impression Die Forging
In this forging method, the dies are not open. Instead, each half, upper and lower, contains an impression of the part to be formed. As they close around the preform, they force it into this shape. Unlike open die forging, impression die forging maintains complete control over how and where the metal flows.
The process consists of three steps:
- The preheated billet is placed in a die cavity. The upper die closes and starts the process of shaping the part. At this stage, the goal is to distribute metal to where it will be needed.
- The preform is then moved to a second cavity, and again squeezed to shape as the dies close. Flash emerges from between the two dies, but its movement is controlled to build pressure and force the billet to fill the furthest corners of the cavity.
- The metal part is shaped to its final dimensions. Corner radii are sharpened, draft angles are reduced, and fine detail like letters and numbers is added.
Benefits
- Produces smaller grains than open die forging, resulting in stronger components
- Better consistency as the tooling maintains tight control over dimensions
- Excellent surface finish
- Can hold tight tolerances
- Produces fine detail, such as lettering and thin ribs
- Can produce near-net parts/little machining required
Drawbacks
- High initial tooling costs
- Long lead time while tooling is designed and manufactured
- Need for periodic tool maintenance (after forging a large quantity of parts)
- Not suited to forging very large parts
Best Use Cases
- High-strength parts needed in large quantities, such as automotive driveline and suspension components
- Complex geometries such as those needed in aerospace parts
- Defense-related applications that use hard-to-machine alloys
- Valve bodies for industries like chemical processing and oil and gas
- Components for agricultural machinery where durability is paramount
- Brackets, plates, and mounts for architectural and civil engineering projects
See If Impression Die Forging Is Right for Your Project
Founded in 1967, Trenton Forging specializes in producing asymmetrical and symmetrical components using the impression die process. We produce millions of parts each year, offer industry-leading turnaround, and provide turnkey services, including machining, part design, and tooling maintenance.
If you’re not sure whether impression die forging is the right method for your project, contact us today to request a project analysis or first article sample.
