SIZE
Maximum and minimum sizes for P/M components vary greatly within the industry. The Wakefield Corporation produces parts ranging from a couple of ounces to approximately 10 pounds. The length-to-diameter ratio should be no greater than 2.5, and the length-to-wall thickness ratio should not exceed 10 times.

SHAPES
P/M can produce a variety of shapes, but those with uniform dimensions in the pressing direction are the easiest to form and eject from the press. Commonly produced P/M shapes include cams, bearings and gears.

Smooth sidewalls are needed for ejection. Reverse angles, under-cuts and threads cannot be formed in P/M tooling, but can be added after sintering by machining operations.
Multi-level shapes require complex, multi-motion tooling. Large, thin selections attached to long hubs can be tooled, but are difficult to eject from a multiple level die. To avoid cracks, stress and density gradients, large projections should be designed as thick as possible. Radii under shoulders and on horizontal edges aid in ejection and eliminate sharp edges on punches.

Designing away from abrupt changes in cross sections avoids density changes and die ejection problems. For example, a round component with integral pins may be redesigned with a uniform cross-section and corded holes. The increased wall section around the corded holes provides extra strength and reduces tooling stress. Taper pins are inserted afterwards. Wakefield will perform such an assembly.

WALL THICKNESS
Die fill is important to the integrity of the finished part. Walls and cored holes greater than 1/16" (1.575mm) promote proper material fill and extend tooling life.

CHAMFERS AND BEVELS
Chamfers, rather than radii, are necessary on part edges to reduce the effect of burrs and to break sharp corners. Large angle chamfers can be produced by bevels in the dies or core rods. A 60-degree chamfer is preferred and we recommend a minimum chamfer of 45 degrees. Chamfers less than 15 degrees must be machined. A minimum flat of 0.005" (.127mm) is added to the end of chamfers to prevent edge feathering on the punch.
Vertical radius on part edges causes featheredges on the punches. This is avoided through chamfer edges. The chamfer edges should be as far apart as possible to eliminate breakage.

DETAILS
Standard holes, such as round, square and D-shaped, as well as special shapes such as keyways and splines, can be added to the workplace in the pressing direction (vertically). Countersinks, flanges, and bosses can be designed into P/M components.
Other details may be "plunged" or pressed into the top of the work piece. The depression or detail should not exceed 25% of the total length of a part. Conversely, male projections should be limited to 25% of the total length.

ASSEMBLIES
A component not suitable as a single P/M structure may be designed as an assembly of two or more P/M parts. Dissimilar materials may also be joined together. For example, a bronze bearing can be inserted in a ferrous structure part. Joining can be accomplished through conventional methods including staking, press-fitting, brazing, welding or epoxying. Unique to P/M is the opportunity to join parts through sintering. This produces a metallurgical bond for strength and durability, as well as providing a cost-effective one-piece assembly.

TOOLING CONSIDERATIONS
The flow behavior of metal powders and the pressing action.

Two occurrences in powder compacting significantly influence part design: the flow behavior of metal powders and the pressing action. Although spherical, metal powders do not flow hydraulically due to friction between the particles and the dies. Therefore, the part design should assure adequate powder distribution within the die cavity. Secondly, metal powders have limited lateral flow, placing limitations on the contours that can be produced.