Design Considerations with Powder Metallurgy
Two major factors in the compacting operation influence design considerations with powder metallurgy: the flow behavior of metal powders and the pressing action.
Metal powders do not flow hydraulically because of the friction between particles and the dies. The design, therefore, should ensure adequate powder distribution within the die cavity to allow satisfactory compaction. Because metal powders also limited lateral flow, there are some limitations on the contours that can be produced.
Successful design for powder metallurgy rests on an understanding of how the unique aspects of the technology affect the countless details that make up the design of any structural part, which is why you should keep your chosen parts manufacturer involved in every step of the design.
Learn how the following topics apply to powder metallurgy:
- Holes in the pressing direction can be round, D-shaped, include keyways, splines, and so on. Tooling members which create holes are called core rods.
- The core rods must remain straight and not buckle during compaction, or ejection and dimensional problems may result.
- Lightening holes are frequently added to large parts to reduce projected pressing area, thus making parts easier to press and lighter.
- Blind holes, blind steps in holes, and tapered holes are readily produced.
- Side holes have to be produced after a sintering operation, usually by machining.
- Die fill is all-important; as a general rule, do not make walls thinner than 1.52 mm (0.060 in.).
- Avoid designing long, thin walls; they require tooling that is fragile and the parts themselves have a tendency toward density variation.
- Where the ratio of length-to-wall thickness is as high as 8:1 or more, special precautions must be taken to achieve uniform fill, and variations in density are unavoidable.
- Total measured flatness depends on part thickness and surface area.
- Thin parts tend to distort more than thick parts during sintering or heat treatment.
- Re-pressing improves flatness.
- Projection bosses are easier to bring to flat than entire face areas.
Tapers and Drafts
Draft is generally not required or desired on sides of parts. While draft on outer sections for ejection is sometimes helpful, it demands careful timing of the tools and slower production rates.
Fillet and Radii
Generous fillet radii are most desirable:
- Tooling with such fillets is more economical, longer lasting.
- Parts made with generous fillets are made more easily and more quickly.
- Parts made with fillets have greater structural integrity (see figures that follow).
Chamfers and Bevels
Chamfers, rather than radii, are necessary on part edges to prevent burring. For example, on a bushing (Figure 1), a 30°–45° chamfer and a 0.13–0.38 mm (0.005–0.015 in.) flat to eliminate feather edges are the preferred practice.
Large angle chamfers may be produced by bevels in dies or core rods (Figure 2). Production rates would be slowed with such tooling because of the need for caution in preventing problems in die-fill and between-tool powder wedging.
A countersink is a chamfer around a hole for a screw or bolt head. When the countersink is formed by a punch, a 0.25 mm (0.010 in.) nominal flat is essential for avoiding sharp, fragile edges on the punch.
A comparatively small flange, step, or overhang can often be produced by machining a shelf or step in the die. Too large a flange causes ejection difficulties. Bottom flange edge and juncture radii should be generous.
Boss-forming cavities may be located in punch tools. Cavities may not be too deep in relation to part height (15% less) and draft angles should be at least 12° per side to avoid sticking of the compacted boss to the punch.
Hubs, which are complementary part sections to gears, sprockets, or cams, can be readily produced by the PM process. It is important to include a generous radius between the hub and flange section and to maximize space between the hub O.D. and the root diameter of the gear or sprocket.
While studs with drafted sides can be made like bosses, sometimes no draft is allowed or the stud-to-diameter ratio is relatively large. In that case, tooling with additional punches is required. Always consider the fragility of the green compact prior to sintering.
Slots & Grooves
Grooves can be pressed into either end of a part from projections on the punch face, with the following general caveats (see figure):
- Curved or semicircular grooves are limited to a maximum depth of 20% of the overall part length.
- Rectangular grooves are limited to a maximum depth of 15% of the overall part length, provided that surfaces parallel to the pressing direction have up to 12° draft and all corners are radiused.
Deep, narrow slots and grooves require fragile tool members and should thus be avoided.
- Undercuts on the horizontal plane (perpendicular to the pressing direction) cannot be made since they prevent part ejection from the die.
- Annular grooves may be machined as a secondary operation (Figure 1).
- For a part such as that shown in Figure 2, where a juncture undercut is needed to allow fit-up to a dead corner, an alternative approach is shown in Figure 3.
The tolerances which can be held in the PM process compare favorably with those of other parts-fabricating processes. For reasons of economics, tolerances no closer than necessary should be specified. The following table illustrates tolerance characteristics (die dimensions, unless stated otherwise) of PM and competitive near-net-shape forming methods:
The following are some fabricator concerns related to holding tolerances via the PM process:
- What is the relationship between sintering conditions and the metal powder used?
- What is the largest dimension which must be held?
- What tool wear will this metal powder cause when compacting to the chosen green density?
- Assuming a symmetrical part shape around an I.D., what run-out tolerance is required?
- If dealing with a ferrous part, will it be heat treated?
- Are tolerances demanding enough to require sizing?
Three prototyping options are commonly used:
- Machining from a PM blank—PM blanks, as pressed and sintered or infiltrated, are available from PM parts fabricators. Machining and finishing steps in fabricating the prototype part from the blank should maintain the surface finish and porosity typical of the PM part. The blank will be uniform in density and may not accurately reproduce the density variation found multi-level parts. Evaluating several density levels is suggested.
- Pressing from a set of tools that contain certain defined features (such as an I.D. spline, sintering, and then machining the remaining features (such as gear teeth and a hub).
- Pressing the entire part from a set of tools and sintering. This option is viable when the part configuration is relatively simple or where a large number of prototypes is required. This option is the most costly and time consuming, but it is the most reliable.