Process Of Impression - Die Forging

Impression dies generally contains preliminary shaping steps to permit the change from the original forging stock to the finished forging without mechanical defects. Simple Symmetrical parts may be forged directly in the finished impression (finishing die cavity) without preliminary shaping. The more difficult or complex shapes may require several different steps to produce a finished forging. The most-used preliminary forging step is the edger, which serves to proportion the cross-sectional area along the length of the flowing metal from a section being reduced to a section being enlarged. The fullering step or fuller, reduces the cross-sectional area between the ends of the forging stock without appreciable change to the end sections. The bending step or bender forms the length of the forging stock to a shape for a finishing impression. The preform may be further shaped to bring it closer to the final configuration in a so called blocker die which assures proper distribution of material but not the final shape. Excess material is allowed to run out between the flat die surfaces and this flash is sometimes removed or trimmed prior to forging in the finishing die. The excess material is again allowed to escape into a flash, which must now be thin to assure die filling and close tolerances. The flash is reduced to its minimum thickness over only a small width called flash land, and the rest is allowed to flow freely into a flash gutter.

The product design engineer must exercise care in determining the amount of material to be inserted in the die. If there is not enough material, the part will not fill up the die. If there is two much material, the flash will be excessive , resulting in wasted material and greater die wear. In general around 10 percent excess material over calculation is allowed. 

Dies must, however be heated before the  first forging is made and often gas or electric heaters is used to keep the dies hot so that the forgings will not cook fast, shrink, and be difficult to eject. Many mechanical forging presses have lower die ejectors, and upper die strippers can be furnished. 

After the forging operation the part must be trimmed to remove the flash. The flash is removed hot or cold. If the carbon content is low and the forging small, it is usually removed cold. Most medium-sized and large forgings are trimmed hot. Subsequent operations to remove the scale or oxide include shot blasting, or pickling.