DIE CAVITY DEPTH
The process of forging is essentially one of “flowing” metal, heated to a temperature whereat it is soft or plastic, into the desired shape. This temperature is approximately 750° F. for aluminum alloys, and 2200 F. for steels.
This flowing process(http://www.myplasticmold.com/plastic-moulding-process.html) is accomplished by the hammer blows of the upper die half, and is easier to accomplish when the metal is either equally flowed into the two dies, or entirely into the lower die.
When approximately half the forging is formed in each die-half it is possible to keep the cavity in each at minimum depth, with resultant smaller dies, lower die cost, and reduced draft angles.
In the case of small, simple parts it is possible to greatly reduce die cost by placing the parting line at the top of the part, which locates the entire cavity in the lower half. This reduces die cost nearly 50%, as the upper die is then merely a flat block of steel.
Bearing in mind that forging is a process of hammering a semi-solid bar of metal into the required shape, it is obvious that highly irregular sections will be difficult to forge.
The ideal condition would be one wherein the cross-section area would be constant for the entire length of a forging, even though the section profile might vary. In such a case the metal bar used to form the forging would simply be deformed or shaped by the blows of the upper die, and little “flowing” of metal would be involved.
While this condition can rarely be attained, it represents the ideal forging Forging with extreme section variations are difficult and expensive to forge, as these frequently require progressive dies and several forging operations to complete the part.
This means that one or more “blocking dies” are used to form the part to a rough outline of the required forging, and that a “finishing die” is used to complete the forging. When this is done at least two sets of dies are required, with correspondingly greater die cost, and the forging time is greatly increased by the necessity of heating the forging billet for each of the progressive dies.