In the pressing process, due to various reasons, often have defects of powder metallurgy pressing (compact), Common cosmetic defects are uneven density of compact, delamination crack,  losing angle, 

oversize rug, surface scratching, the coaxial shaft tolerance, etc. These defects can be divided into: 1.the density is not up to standard. 2.the size does not meet. 3.tolerance and size. 4.appearance defects. Among them, appearance defects are the most common. Here are the discussion about several common cosmetic defects.

 

1. Delamination

Definition: edge present in lock block, and appear a certain angle (about 45 degrees) with compression, extended to the interior, form a neat interface.

Reason: internal elastic stress and shear stress in a lock block. If a mixture of cobalt carbide with low content of cobalt, high carbide, powder or material is fine, forming agent is too small or the distribution is not uniform, the mixture too wet or dry, pressure is too large, single block is too large, complex shape, mold smoothness is poor, uneven surface, may cause delamination.

Improvement measures: charging up uniformly, non overpressure, increase strength of the lock block, reduce the internal stress and elastic aftereffect of the lock block.

Inspection method: the lock blank can be inspected by cross section, the alloy can be tested with ultrasonic.

 

2. Crack

Definition: the phenomenon of irregular local fracture in a lock block is called a crack.

Reason: similar to delamination, it is also the result of elastic aftereffect. The difference is that the block blank will layered if the stress concentration area is on the compression surface, otherwise it will crack.

Measures: similar to delamination, and extend the dwell time or repeated pressure, reduced pressure,  substance, improve the mould design and increase the mould thickness, accelerate the knockout rate, increase the forming agent, improve the material bulk density can reduce the crack.

 

3. Not pressed well

Definition: the size of the hole between the particles in the compacts is too large to be completely disappeared during sintering, and there are still many voids in the alloy.

Reason: the pellet is too hard or coarse, the material is too loose; the loose grain is not uniformly distributed in the mould cavity, the single weight is low and the pressing pressure is too small.