Closed-die forging (impression-die forging) yields a mirror image of the product being made. It has been called "20th Century Blacksmithing". Steel, aluminum, titanium and other alloys can be forged into a variety of 3-D shapes that range in weight from ounces to over 25 tons. Closed-die forgings are produced on hydraulic presses (50,000-ton capacity), mechanical presses (20-ton capacity) and hammers (50,000 lb capacity). Parts from 1 lb to 40 lbs range made by the closed-die forging method include railroad draft gears, truck differentials, steam traps, special wrenches, rail couplings, valve T-fittings and truck/tread links.

The multi-part dies used are made from tool steels. "Soft" dies are suited for cutting. Dies with high hardness tend to crack. Preheating the dies overcomes cracking in processing. Laps and seams on parts generated from test or prototype dies indicate improper metal flow. Die imperfections can be remedied with weld fill. Die life (cycles) is affected by a host of factors including the metal being forged.

As the name implies, the dies contain impressions of the part that are transferred to the stock being forged. The stock undergoes plastic deformation. Because the metal flow is restricted by die contours, the process can yield more complex shapes and closer tolerances than open-die forging.

Closed-die forgings deliver strength where needed. This is achieved by grain-flow optimization (directional strength). Forging parts to near-net shape yields greatly improved properties such as strength, increased fatigue resistance and overall performance improvements. Products forged to near-net shape require less machining.

The Forging Industry Association (FIA) offers the following information relative to tolerances on forgings made in hammers and presses:

• Length/width tolerance is +/-0.003 in./in. This tolerance includes allowances for shrinkage, die sinking and polishing variations.

In addition to the forging equipment, induction, electric and fossil-type heaters are used. Hot forging is accomplished in a range from 1750F to 2300F. Closed-die forging of ferrous parts from 1 lb to 40 lbs is done within a 2100F to 2250F temperature range.

When evaluating a closed-die forging source, the following criteria should be considered:

• Does the forger have experience in applications similar to the one being considered?
• Is design assistance offered?
• Does the forger have the equipment required to produce the part?
• Is the forger able to provide related services like heat treating, machining, testing etc.?
• Is the forger accustomed to producing the volume required? Does the company specialize in long runs, short runs or quick delivery?
• The forger should be asked to help evaluate the tradeoffs between reduced machining and increased die and processing costs as represented by the ability to reach near-net shape forging.
• Guidance on material selection, part configuration, and appropriate standards should be a part of the selection process.

Additional functions to be evaluated in determining how to buy forgings, particularly closed-die parts, are surface finishes, inspection and testing (which should include Statistical Process Control methods) and delivery.