Like any other part fabrication methods, PM has it's own set of design guidelines for producing well engineered, economical products. Designing for powdered metallurgy requires close cooperation between the part user, or the buyer and the producer, especially in the initial design stages. An improved, lower cost PM part often can be achieved through small changes in an assembly. Frequently early designer-manufacturer interaction results in an expansion of the PM concept that can simplify the design and reduce costs.
At Falcon, our engineers are always eager to assist a powdered metal part design for mutual benefit.
The two major factors in the compacting operation influence or control part design are the flow behavior of metal powders and the pressing action. Metal powders do not flow hydraulically because of friction between the particles and dies. The design therefore should ensure adequate powder distribution within the die cavity to allow satisfactory compaction.
Part Size
- Since compaction occurs in the vertical direction only top and bottom motions, part lengths in the pressing direction are limited Compression ratio, the ratio of the height of the loose powder filling the die to that of the compacted part, also tends to limit vertical part lengths
Shapes
- Shapes with uniform dimensions in the pressing direction are easiest to produce and eject from the press
- Cams, gears and sprockets are readily made
- Thin walls and projections may require fragile tools which need careful consideration
- Face forms on the upper or lower punches can provide bosses, pads, lettering, countersinks etc.
- More complex, multi-motion tooling is required is required to maintain consistent density throughout parts with more than one level
- Both mechanical and hydraulic presses are available for making parts with five or more levels.
- Parts weighting a few grams to a few pounds are possible
Holes
- Holes in the pressing can have any shape (like round, spline, D-shaped, through profile etc). Holes are created by the tool members called core rods.
- The core rods must remain straight and should not buckle while compacting. Diameter to length of the core rod required can require changes on the product design.
- Blind holes, blind steps in the holes and tapered holes are readily produced
- Side holes are produced after sintering usually through machining
Wall Thickness/Flange thickness
- Die fill is all important. Normally wall thickness below 1.50 mm (0.06 inch) are extremely difficult to fill.
- Long and thin walls also result in fragile tooling and parts with tendencies toward variations in density.
- Where ratio of length-to-wall thickness is as high as 8 to 1 or more, special precautions must be taken to achieve uniform fill. In such situations, density differences are unavoidable.
Flatness
- 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
- Projections and 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
Fillets & Radii
- Generous fillet radii are most desirable
- Tooling with generous fillets are normally long lasting and stronger
- Parts made with generous fillets are made more easily and more quickly o Parts with generous fillets have greater structural integrity
Chamfers & Bevels
- Chamfers rather than radii, are necessary on the part edges to prevent burring
- Chamfers between 30~45 deg with a flat edge of 0.005~0.015 inch (0.13~0.38 mm) are common to avoid sharp edge and feather edges on the punches
- Large angle chamfers can be produced by bevels in dies or core rods. Production rates would be slowed with such tooling because of the need for caution in preventing die-fill and between-tool powder wedging problems
Flanges
- A comparatively small flange, step or overhang can often be produced by machining a shelf or step or step in the die. Too large a flange causes ejection problems and normally separate punch is required to generate large flanges. A large juncture radius will help.
Bosses
- 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 deg per side to avoid sticking of the compacted
Hubs
- Hubs which are complementary parts sections to gears, sprockets, or cams can be easily produced by the PM process.
- Remember to include a generous radius between hub and flange section, and to maximize space between hub outer diameter and gear or sprocket root radius
Studs
- While studs with drafted sides can be made like bosses, sometimes no draft is allowed or stud height-to-diameter ratio is relatively large. In such case, tooling with additional punches is required. Always consider fragility of the green part prior to sintering
Slots and Grooves
- Grooves can be pressed into either end of a part from projections on the punch face. However some limitations apply
- Curved or semicircular grooves to a maximum depth of 20% of overall part length
- Rectangular groves to a maximum depth of 15% of overall part length, provided that surfaces parallel to the pressing direction have up to 12 deg. Draft and all corners have radiuses.
- Deep narrow slots and grooves require fragile tool members and should be avoided
Undercuts
- Undercuts on the horizontal plane (perpendicular to pressing direction) can not be made since they prevent part ejection from the die.
- Annular grooves may be machined as secondary operation
Tolerances
The tolerances which can be held in the PM process compared favorably with other parts fabricating processes.
*Reprinted with permission from Powdered Metallurgy Design Solutions , 1999, Metal Powder Industries Federation 105 College Road, East Princetown, NJ USA, 2001
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