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Modern Group, Inc.
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What is a Closed Die Forging?
Forging is the only metal forming process that orients the grain of metal to the shape of the part. Closed die forgings are made by deforming a hot metal billet between a set of forge dies in which cavities have been precisely cut to conform to the desired finished forging. The forging is then trimmed of excess material, or flash, and staged for required secondary operations.
What are the Benefits of a Forging?
- Near net shape; reduced machine cost
- Exceptional strength
- High density; no internal porosity
- Lower weight final components
- Superior surface finish
- Enhanced mechanical properties
- Structural uniformity
- Statistically repeatable dimensions
Basic Forge Design Considerations
Draft
Draft angle of 1 ½º to 7º is normally added to all surfaces perpendicular to the forging plate, allowing for easy removal of the forged part from the die. But sometimes the part can be tilted in the die to produce "natural draft" without angling its surfaces.
Fillet Radii
When necessary, forging can produce radii as small as any other metalforming process. But smaller radii are more costly to machine in the die, and can cause checking of the die surface and reduced wear life. Fillet radii should be as large as practical to ease metal flow and avoid laps and cold shuts.
Corner radii
Like fillet radii, corner radii should be as large as possible to ease metal flow during forging and reduce die wear. But absolute minimums for corner radii are usually about half the minimums for fillet radii. Corner radii on bosses are also dependent on the height of the boss, because deeper die impressions are more difficult to fill.
Webs
How thin a given web can be made depends on its minimum longitudinal dimension and the alloy being used. It also depends on whether the web is "confined" or "unconfined." In an unconfined web, the metal is free to low in at least one direction during forging. In a confined web, metal flow is impeded by neighboring ribs, making forging more difficult.
Ribs
Limitations are similar to webs, except that the "confined" and "unconfined" distinction does not apply. The minimum thickness of a rib depends only on its height and the alloy being used. As a general rule, a rib confining a web should be no higher than the width of the confined web, and intermediate ribs should be no higher than the enclosing rib.
Holes and recesses
Recesses can be readily built into a part during forging, saving machining later. Holes can be made by forging recesses from both sides, then removing the residual web with a simple punching operation.
Finish Allowance
Though some forged parts are used directly without machining, an "envelope" of excess material is normally left around the part for finish machining. The minimum allowance varies with the alloy and the total area of the part. For carbon and common alloy forgings, typical minimum allowance for small parts is 0.06".
Seminars
Modern holds regular forging seminars to aid our customers in design and continuous improvement projects. If you would like to attend a seminar, or desire to have us hold a seminar at your facility, contact us at sales@modernforge.com.
