Underfills are another geometrical defect commonly caused by inadequate press force, energy and/or power. These may be symptomatic of the use of undersized equipment on jobs that are too big for it.
In this type of underfill defect, we see that part produced by the normal forging technique is left with certain amount of billet part left under filled as the work piece volume is insufficient to fill the complete cavity space provided in between the two die halves. This type of defect is generally caused due to the improper die design. Also note the roughened surface near the region of such deficiency.
The part produced is left with major problem of dimensional inaccuracy, which later could not be corrected by subsequent machining operations and hence the part is supposed to be rejected. Thus it is recommended to check up for the die design or to take sufficiently large size of the billet at before hand.