Powder Fill

The important consideration in P/M part production is the fill ratio required to produce parts to a density that is compatible with end use requirements. The fill ratios must remain constant for a given part to maintain dimensional reproducibility. Parts can be of single-level or multilevel design.

Single-level parts, designated as class I by the Metal Powder Industries Federation (MPIF), present the least difficulty to the tool designer, regardless of the size or part configuration. The main consideration is designing a die that is long enough guidance for the lower punch (usually 25 mm, or 1 in.) and providing adequate fill depth for compacting the powder to the required density. This challenge, coupled with the primary mechanical consideration of locating the center of mass in the press center, provides the best potential for producing a uniform quality part. Figure 4(a) shows basic geometries of MPIF class I parts.

Multilevel parts, with industry classifications II through IV, present two additional complications to the tool designer: powder fill and part ejection. Because metal powders tend to compact in vertical columns and generate little hydraulic flow, the tool designer must create fill levels in the tools that compensate for the thickness variations present in the final part configuration. Uniform density, neutral axis of compaction, and part ejection should be considered to determine the need to vary fill levels and the manner in which these variations are achieved. Excessive density variations contribute to green cracks and sintered distortion.

A common method of varying fill levels is by using multiple lower punches, which are timed to react to one another either through the use of springs or air, or by mounting on separate press platens. Other methods are less effective, because punches are not adjustable and are fixed on one of the tool members, such as the die or core rod.

Fixed levels are commonly referred to as die chokes, core rod steps, or splash pockets (Fig. 5). Fixed fills are sensitive to the apparent density of the material being compacted. In operations that control compacting pressure, such as in hydraulic pressing, fixed fills cause dimensional variations in part thickness. Because mechanical presses are set to operate to a fixed position relative to the die, the variation created by the apparent density of the powder causes overdensification or underdensification, resulting in a corresponding oversize or undersize peripheral area on the part. Green expansion occurs as a part is stripped from the die. Ideally, the part returns to die size through shrinkage during sintering.

Fig. 5 Methods of achieving fixed fill levels. (a) Fixed fill on an upper level using a step die. (b) Fixed fill using a splash pocket to permit a projection feature on an upper punch. (c) Stepped core rod forming an internal shoulder

When a part has more than one level in the compacting direction, the step height should be limited to one-quarter of the overall height for a single punch (Fig. 6a). If a larger step is required, multiple punches should be considered (Fig. 6b).

Fig. 6 Two-level compaction, (a) Single lower punch when h l^H/4. (b) Double lower punches when h > H/4.

Fill Height. The fill height is the depth of the loose powder required to give the required part thickness after compaction. The value is determined by the compressibility of the loose powder at the required density. The fill height, hf, is obtained by multiplying the finished part height by the compression ratio of the powder:

In this equation, t is the part thickness, kr is the compression ratio, and kr = q/qa, where q is the part required compaction density and qa is the apparent density of the loose powder. If the fill height is greater than the maximum fill height that can be accommodated in the press selected on the basis of the compacting load required, a larger capacity machine should be selected, which has the required fill height capacity.

Powder Metallurgy Presses and Tooling

Revised by John Porter, Cincinnati Incorporated

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