Tooling Design for Warm Compaction

The tooling design for warm compaction is essentially the same as for regular compaction with typical radial tooling clearance of 0.01 to 0.02 mm (0.0004 to 0.0008 in.). The choice of carbide inserts or tool steel inserts is not critical. Carbide inserts have proven to be successful; however, the designer is cautioned that additional interference fits are required to compensate for the differential thermal expansion of the carbide insert compared to the steel stress ring.

One word of caution in the design of tooling is the stress involved during the compaction to near-pore-free densities. As the density increases, the tooling loads increase rapidly. This increase in tooling pressure necessitates that thicker stress rings be used and the allowances made for the greater tool deflections. Shown in Fig. 8 is the green expansion as a function of the green density of powder compacts compacted using both conventional room-temperature compaction in addition to warm-compaction conditions. Note that the green expansion at equivalent density is lower for the warm-compacted material. The rationale for the lower green expansion for the warm compacted material is explained by the fact that lower compacting pressure was required to achieve this same density; thus the tooling load was decreased. However, as the green density increases to near-pore-free density, the green expansion increases dramatically. With this increased density, the tooling loads increase, resulting in greater expansion of the part.

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Fig. 8 Green expansion and sintered dimensional change of warm-compacted material relative to conventional compaction techniques

This increased green expansion can cause microlaminations in the compacted part. These microlaminations are serious problems because they reduce the structural integrity of the sintered component. In multilevel parts, these microlaminations usually occur at the transition from one level to another. Incorporating top-punch hold-down during the ejection cycle often prevents these cracks from occurring. However, even top-punch hold-down is not sufficient to prevent microcracking if the density of the part exceeds 98% of the theoretical density.

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