Green Strength

Green strength refers to the mechanical strength of a cold pressed powder compact. Adequate mechanical strength of a green part permits cold pressing of the powder, ejection of the green part from the die, and transfer of the green part to the sintering furnace without breakage. Strength of green compacts results mainly from mechanical interlocking of irregularities on the particle surfaces, which is promoted by plastic deformation during pressing. Figure 12 shows green strengths of various powders versus compaction pressure.

Fig. 12 Green strength versus compaction pressure for various types of powder. (a) Copper, iron and steel powders. (b) Iron powders

For most applications, green strength values of about 5.5 MPa (800 psi) or more permit pressing and safe handling of green parts. Lower green strength values may produce insufficient strength and horizontal lamination in the part. These laminations appear when the part is ejected from the die as a result of a small expansion of the part, due to the release of elastic stresses. This type of defect can be minimized or avoided by providing the die with a slight taper (draft angle) or a small radius at the upper rim. Parts with sharp contours or thin sections, or parts made from a blend of several powders in which some of the components detract from the overall strength of the part, require powders with higher green strength characteristics.

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