Tooling Clearances and Design

As in many other manufacturing operations, process variables (e.g., the type of materials being processed, the density of the part being produced, the amount and type of powder or die lubrication, and production rate) dictate operating conditions. Density and production rate greatly affect tool clearances during a continuous production run in which tooling temperature increases as compacted density and/or production rate increases. Temperature variations and corresponding dimensional changes within the various tooling members must be considered.

Standard tooling clearance is 0.016 mm/25 mm (0.0006 in./1 in.) on the diameter total. Minimum clearance should be used initially, because materials can always be removed from the punch or die to provide additional clearance as needed. In addition, the clearances must be smaller than the size of the powder particles to prevent their entrapment. Smaller clearances will also help reduce possible variations in the dimensions of the parts. The density of the compact produced and the production rate have a great influence on the determination of clearances. Compacting loads will be higher for increased densities. These, as well as higher production rates, increase the tooling temperature. As the temperature increases, dimensional changes occur in the tooling members. For this reason, the clearances must be sufficiently large to prevent seizure of the tools.

A representative value for the clearances between die walls and punches is 0.005 to 0.008 mm for precision parts and an upper limit of 0.013 mm for other parts. Minimum possible clearance should be used initially, because it can be increased as needed by removing material from the punch or the die.

The expansion ("pop out" or "springback") of the compact upon ejection makes it essential that the top edge of the die cavity be properly rounded or flared to allow the compact to make a smooth transition during ejection. Provision of a shallow chamfer is a more practical solution in the case of gears.

Die cavities and punch faces should be lapped and polished to a very high degree of surface finish, preferably <0.25 /' m.

Shapes and Features. A shape or feature can be die compacted provided that it can be ejected from the tooling and the tools that form the feature have sufficient strength to withstand the repeated compaction loads. Due to the vertical closure of the tooling and the lack of tool motions perpendicular to the pressing direction, part removal from the tools controls many features. Examples of features that cannot be accommodated in die compaction, and therefore require secondary machining operations, include undercuts, reverse taper (larger on bottom than on top), annular grooves, and threads. The following guidelines provide assistance with many possible features in die compaction. Further details are provided in Ref 3.

Wall Thickness. Minimum wall thickness is governed by overall part size and shape. For parts of any appreciable length, walls should be not <1.5 mm (0.060 in.) thick. A maximum length-to-wall thickness ratio of 8 to 1 should be followed to ensure reasonable density uniformity and adequate tool life. Separate tool members (punches) should be used to provide density uniformity and proper ejection.

Steps. Simple steps or level changes not exceeding 15% of the overall part height (H) can be formed by face contours in the punches. A draft of 5° or more is needed to release this contour from the punch face during ejection. Features such as countersinks and counterbores can be similarly formed. This tooling method, as compared to multiple punches, will result in slight density variations from level to level. However, this approach offers the simplest tooling, lower-cost tooling, and closer axial tolerances than multiple punches.

Spherical Shapes. Complete spheres cannot normally be made because the punches would have to feather to zero width (Fig. 17). Spherical parts require a flat area around a major diameter to allow the punch to terminate in a flat section (Fig. 17). Parts that must fit into ball sockets are repressed after sintering to remove the flats.

Avoid Preferred

Fig. 17 Proper design of spherical shapes in P/M parts. Source: Ref 3

Taper and Draft. Draft is not generally required or desired on straight-through parts. While tapered side walls can be produced where required, the tools may demand a short straight surface (A in Fig. 18) at the end of the taper to prevent the punch from running into the taper in the die wall or on the core rod.

Fig. 18 Tapered hole design for P/M parts. Source: Ref 3

Holes. Through holes in the pressing direction are produced with core rods extending through the punches. Round holes require the least expensive tooling, but many other shapes, such as splines, keys, keyways, D-shapes, squares, and so forth can readily be produced. Blind holes, blind steps in holes, and tapered holes, are also readily pressed. For very large parts, lightening holes are added to reduce weight and the area of compacted surface.

Flanges. A small flange, step, or overhang can be produced by a shelf or step in the die. Separate lower punches are used if the amount of overhang becomes too great to permit ejection without breaking the flange.

Alphanumeric Characters. Numbers, lettering, logos, and other characters can be pressed into surfaces oriented perpendicular to the pressing direction. Recessed lettering is preferred because raised letters are fragile, easily damaged in the green compact, and prevent stacking of parts for sintering.

Chamfers, Radii, and Bevels. Chamfers are preferred rather than radii on part edges to prevent burring. It is common practice to add a 0.25 mm (0.010 in.) flat at a 45° chamfer; lower chamfer angles may not require the tooling flat.

Hubs and Bosses. Hubs or bosses that provide for drive or alignment rigidity in gears, sprockets, and cams can be readily produced. However, the design should ensure the maximum permissible material between the outside diameter of the hub and the root diameter gear or sprocket features.

Other Features. Additional information on these and other features (slots, grooves, knurls, studs, fillets, countersinks, etc.) can be found in Ref 3. Because shape complexity is a recognized limitation of die compaction, multipiece assembly is a useful alternative, especially where extensive machining would be required. Pulleys, spools, and sprockets have been produced using sinter bonding, brazing, and welding techniques.

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