Deformation of Visco-Elastic Solids and Fluids

After compaction, the green body must be ejected from the die. During die ejection the green body is released from compression at high pressure to very low pressure and then pushed out of the mold. The primary force to be overcome during die ejection is that of wall friction. 

FIGURE 13J37 Pressing cycle for visco-elastic solids. 

Wall friction depends on the roug ess of the die wall and its lubrication. For walls with a scale of roughness larger than the diameter of the powder, the wall friction is essentially the same as the internal friction of the powder. This is because the rough wall traps small amounts of the powder in its reguosity and the sliding of the powder mass against this trapped powder produces the effective wall friction. If the wall has a scale of rou mess smaller than the diameter of the powder, the wall friction is controlled particle—wall friction. In both 

FIGURE 13.40 Maintaining the green body in compression during die ejection. 

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