Mechanism of Damage Caused by Elastic Aftereffect on Pressed Shape

When the mould is knockout, the elastic aftereffect is too large, and will cause the following hazards:
1. dimension deviation.
2. cracks or delamination occurred on the pressed shape.

The dimension deviation is easy to understand, but the crack caused by elastic aftereffect needs to be deeply analyzed

The size of elastic aftereffect mainly depends on two factors both inside and outside, the internal factors from the powder characteristics, including powders, types, component, state, particle shape and size, external factors include pressing, pressure, pressing mode, lubrication condition and structure and characteristics of mould etc.

Damage mechanism of elastic aftereffect

1. Cracks produced by radial expansion
This kind of crack often occurs in the blank which has greatly horizontal cross sectional area and complex shape. When the pressed shape knockout from the cavity block, have to remove part of elastic stress release, produce radial expansion, the part un-knockout have to endured the constrain of the pressed shape, the stress difference between these two parts, and then produce the radial shear, crack, extension in the horizontal direction.

Countermeasure:
1) the cavity block at the end of the exit have taper, and the taper shall not be too large for holding pressure. Otherwise, will cause, will cause the dimension deviation, and cause edge hair.
2) knockout speed should be properly controlled, stress fully realease when it too slower, resulting in cracks if it too fast.
3) use protective knockout to reduce the effect of elastic aftereffect by pressure and limitation of uprush.

2. Gas expansion
In compression, the compression of powder volume is largely the compression of the volume of gas in the powder. Compressed gas may discharged or retained in a pressed shape.
In the early days of pressing, the density of the compacts is low and there are many channels left for gas discharge. With the increase of pressure, the density of pressed shape increases continuously, and the exhaust channels of powder particles are blocked and the gases can not be expelled. This part of gas to be constantly compressed with the increasing pressure, seven hole pressure increasing. When the pressure is removed, the inner pressure of the pores causes the pores to expand elastically, and the extent degree of pores in direct proportion to the compressed of pores or internal elastic stress, may lead to cracking of the body, this crack usually occurs in the direction perpendicular to the compression, which is another reason for the elastic aftereffect and the form of expression.

Countermeasure:

1) the pressing speed should not be too fast, increase the holding time, and let the gas have the opportunity to exhaust.
2) the design of the die should be designed with chamfering, air guide and so on.

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