Design powdered metal parts with walls of at least 0.060 in (1.5 mm), a length-to-diameter ratio of about 3:1 or less in the pressing direction, three or fewer pressing levels, and every feature parallel to the press axis. Use 45° chamfers instead of radii on edges. Expect about ±0.001 in per inch radially as sintered and ±0.003–0.005 in (±0.08–0.13 mm) in height.
SSG MFG · design guideDG-05 · 11 rules
11 rules, in the order you meet them on a model. Every value is typical; the principal quoting your part confirms its own limits.
| # | Rule | |
|---|---|---|
| 01 | Specify an MPIF Standard 35 gradeFor example FC-0208-50, where 50 is the minimum yield strength in ksi | For example FC-0208-50, where 50 is the minimum yield strength in ksi |
| 02 | Check projected area against press sizeCompaction takes about 30–60 tons per square inch (415–830 MPa) of projected area | Compaction takes about 30–60 tons per square inch (415–830 MPa) of projected area |
| 03 | Limit height to diameterLength-to-diameter ratio of about 3:1 or less; 2:1 or less for density-critical parts | Length-to-diameter ratio of about 3:1 or less; 2:1 or less for density-critical parts |
| 04 | Keep features on the pressing axisNo side undercuts, cross holes or threads as pressed | No side undercuts, cross holes or threads as pressed |
| 05 | Limit pressing levelsThree or fewer levels where possible | Three or fewer levels where possible |
| 06 | Hold the minimum wall0.060 in (1.5 mm) minimum, including hole to outside edge | 0.060 in (1.5 mm) minimum, including hole to outside edge |
| 07 | Keep core rods sturdyHoles at least 0.060 in (1.5 mm) in diameter; 0.100 in (2.5 mm) or more preferred | Holes at least 0.060 in (1.5 mm) in diameter; 0.100 in (2.5 mm) or more preferred |
| 08 | Radius the profile cornersInside and outside corners of the outline at least about 0.010 in (0.25 mm) | Inside and outside corners of the outline at least about 0.010 in (0.25 mm) |
| 09 | Chamfer rather than radius edges45° chamfers with a flat of about 0.010 in (0.25 mm) on the punch | 45° chamfers with a flat of about 0.010 in (0.25 mm) on the punch |
| 10 | Skip draft on straight walls0° on walls parallel to the pressing direction | 0° on walls parallel to the pressing direction |
| 11 | Tolerance height looser than diameterRadial about ±0.001 in per in (±0.1%) as sintered; height ±0.003–0.005 in (±0.08–0.13 mm) | Radial about ±0.001 in per in (±0.1%) as sintered; height ±0.003–0.005 in (±0.08–0.13 mm) |
These build on the short list on the powdered metal process page, with the failure each rule prevents and a drawing for each.
Change the height-to-diameter ratio of a pressed compact to see how density falls off toward the middle of the part, and why tall parts pressed from both ends still end up weakest at the center.
L/D 1.50 · DOUBLE-ACTION PRESSING
Works: even density
Short columns press to near-uniform density, so strength and sintered size are predictable.
Shading is illustrative, not a measured density profile. Typical limits: L/D up to about 3:1, and about 2:1 where density matters.
Each rule with its typical value, the reason behind it and what happens on the floor when it's ignored. Switch any drawing between Do and Avoid; point at a callout to pick it out.
Rule 01
For example FC-0208-50, where 50 is the minimum yield strength in ksi
Standard: MPIF Standard 35
Rule 02
Compaction takes about 30–60 tons per square inch (415–830 MPa) of projected area
Rule 03
Length-to-diameter ratio of about 3:1 or less; 2:1 or less for density-critical parts
Fig. 1Height to diameter
Do. Keep the height to about 3× the diameter or less so pressure reaches the whole column and density stays even.
Avoid. In a tall column, die-wall friction eats the pressure: the middle presses to lower density and becomes the weak zone.
Rule 04
No side undercuts, cross holes or threads as pressed
Fig. 2Undercuts and side actions
Do. Where an undercut is worth it, a slide pulls sideways before ejection. Better still, move the feature into the line of draw or use a pass-through core.
Avoid. A side hole with no side action locks the part in the steel; every slide or lifter added to fix it adds tool cost and cycle time.
Rule 05
Three or fewer levels where possible
Fig. 3Pressing levels
Do. Two or three levels, each with its own punch, keep tooling simple and density even across the part.
Avoid. Every added level adds a punch, a tool motion and a density difference between steps; past three, cost and cracking risk climb.
Rule 06
0.060 in (1.5 mm) minimum, including hole to outside edge
Fig. 4Uniform wall
Do. Every wall within about 15% of nominal T, so the whole part cools and shrinks at one rate.
Avoid. A section twice as thick cools last: it pulls a sink mark into the face above it and can leave a void inside.
Rule 07
Holes at least 0.060 in (1.5 mm) in diameter; 0.100 in (2.5 mm) or more preferred
Fig. 5Cored hole depth
Do. A drafted core pin no deeper than about 4× its diameter stays straight under injection pressure and releases cleanly.
Avoid. A long, thin pin deflects under metal pressure and heat checks; holes run off-position and pins break in production.
Rule 08
Inside and outside corners of the outline at least about 0.010 in (0.25 mm)
Fig. 6Inside and outside radii
Do. Inside radius at least half the wall and outside radius equal to inside plus T: the corner stays at thickness T and flow turns smoothly.
Avoid. A sharp inside corner concentrates stress about 3× and the corner runs thick: cracks start there, and it cools last.
Rule 09
45° chamfers with a flat of about 0.010 in (0.25 mm) on the punch
Fig. 7Chamfer, not radius
Do. A 45° chamfer ending in a flat of about 0.010 in (0.25 mm) lets the punch keep a strong, square edge.
Avoid. A full radius or a chamfer run out to a point needs a knife-thin punch edge that chips and breaks early.
Rule 10
0° on walls parallel to the pressing direction
Fig. 8Draft angle
Do. Walls taper by the draft angle in the line of draw; each side moves in by H × tan θ, and the part slides off the core.
Avoid. With zero draft the part grips the core as it shrinks: drag marks, scuffed texture, and ejector pins that dent or punch through.
Rule 11
Radial about ±0.001 in per in (±0.1%) as sintered; height ±0.003–0.005 in (±0.08–0.13 mm)
Standard: MPIF Standard 35
Fig. 9A complete drawing package
Do. Every requirement is on the drawing: general tolerance, material by specification, finish by specification, critical dimensions and revision.
Avoid. Whatever the drawing leaves blank, the supplier decides, usually with the cheapest local equivalent.
Radial dimensions are held by the die and move with sintering shrinkage or growth. Heights depend on fill and punch travel. Sizing, coining or machining tightens either, one operation at a time.
| Feature | Typical | Tight | Cost of tight |
|---|---|---|---|
| Radial dimensionsSizing adds a repress | ±0.001 in/in (±0.1%) as sintered | ±0.0005 in (±0.013 mm) sized | Moderate |
| Height in pressing directionTight usually needs machining or grinding | ±0.003–0.005 in (±0.08–0.13 mm) | ±0.001 in (±0.025 mm) | High |
| Bushing ID after sizing | ±0.0005 in (±0.013 mm) | ±0.0003 in (±0.008 mm) | Moderate |
| Concentricity, OD to ID | 0.002–0.003 in (0.05–0.08 mm) TIR | 0.001 in (0.025 mm) TIR | Moderate |
| Flatness | 0.002 in/in | 0.0005 in (0.013 mm) ground | High |
| Density | ±0.1 g/cm³ | ±0.05 g/cm³ | Moderate |
| Radial after heat treatQuenching opens tolerances | About ±0.002 in/in (±0.2%) | ±0.001 in/in sized first | Moderate |
Basis: MPIF Standard 35 grades; typical press-and-sinter practice. Put a general tolerance note on the drawing and tighten only the features that need it; every tight callout is priced. See the tolerance guide.
The same powdered metal rules shift with the alloy or compound.
Iron-copper-carbon, the most common structural PM grade. It presses easily at 6.4–7.0 g/cm³ and can be quench hardened, but copper growth during sintering shifts sizes, so sizing is common on tight fits.
Nickel steel with better toughness and impact strength than FC grades after heat treatment. Choose it for gears and parts that see shock loads.
Sintered 316L resists corrosion but is weaker and more porous than wrought 316. Sintering atmosphere controls corrosion resistance, so specify the MPIF grade and density.
Most often made by metal injection molding for small, complex parts under about 0.25 lb (100 g). MIM relaxes the pressing-axis and level rules but carries about ±0.3–0.5% dimensional tolerance.
Run these against the model before it goes out for quote. The builder's DFM check catches most of them on an uploaded part.
0 of 12 checked
About 3:1 in the pressing direction is the practical limit for parts pressed from both ends, and 2:1 or less is better when density matters. Friction on the die wall reduces pressing force with depth, so density falls toward the middle of tall parts. Longer parts are usually made in two pieces or by machining.
Powder does not flow like a liquid. Friction against the die wall and between particles uses up pressing force, so density is highest next to the punches and lowest far from them. Double-action pressing from top and bottom moves the low-density band to the middle. Tall parts and thin steps make the gradient worse.
FC-0208-50 is an MPIF Standard 35 material code. FC means iron-copper, 02 is about 2% copper, 08 is about 0.8% combined carbon, and 50 is the minimum yield strength of 50 ksi (345 MPa) as sintered. Calling out the full code fixes composition and minimum strength for every supplier quoting the part.
Not as pressed. The part ejects along the press axis, so side undercuts, cross holes, threads and knurls perpendicular to the stroke are machined after sintering. Grooves and undercuts that run parallel to the axis, such as keyways and splines, press directly. If many side features are needed, screw machining may cost less.
Plan on 0.060 in (1.5 mm) minimum, including the wall between a hole and the outside edge. Thinner walls fill unevenly, come out low in density and need fragile punch sections. Long thin walls in the pressing direction are the hardest case, so thicken them or shorten the part.
Iron-copper-carbon grades such as FC-0208 often grow slightly, about 0.1–0.4%, because copper swells during sintering, while other grades shrink by a similar amount. The toolmaker builds the die to the grade's known size change. That is why the material grade has to be fixed before tooling is cut.
Check your part against these rules.
Drop a STEP file in the builder for an instant DFM check and a budgetary estimate, then send it to principals who run powdered metal every day.
