Casting, Forging and Powder Metallurgy
Solidification time, risers, shrinkage, forging force and when to cast, forge or machine from bar.
Cast, forge, or machine from bar
| Route | Strengths | Weaknesses | Good for |
|---|---|---|---|
| Casting | complex shapes, hollow forms, almost any size, low cost per part at volume | porosity, lower strength and fatigue properties, pattern or mold cost, rough surface | housings, pump and valve bodies, engine blocks |
| Forging | grain flow follows the part, highest strength and toughness, no porosity | die cost, simple-to-medium shapes, machining still needed | crankshafts, connecting rods, hand tools, flanges |
| Machining from bar or plate | no tooling, any quantity from one, tight tolerances, good material | wasted material and time on complex parts | prototypes, small batches, precision parts |
| Powder metallurgy | near-net shape with little waste at high volume, controlled porosity | size limit, lower strength, tooling cost | small gears, bushings, filters, magnets |
| Fabrication (weld) | one-offs, large and structural parts | distortion and labor | frames, tanks, vessels (Welding Processes and Weld Design) |
Casting processes
| Process | Mold | Typical alloys | Tolerance and finish |
|---|---|---|---|
| Sand casting | sand packed round a pattern, used once | iron, steel, aluminum, bronze | ±1 to 3 mm per 100 mm; rough, Ra 12 to 25 µm |
| Investment (lost wax) | ceramic shell around a wax pattern | steel, stainless, superalloys, titanium | ±0.1 to 0.3 mm; fine, Ra 1 to 3 µm |
| Die casting (high pressure) | steel die, metal injected at 100 to 1 000 bar | aluminum, zinc, magnesium | ±0.1 mm; smooth, thin walls (1 to 3 mm) |
| Permanent mold / gravity die | metal mold, gravity or low pressure fill | aluminum, copper alloys | ±0.3 to 0.5 mm; better than sand |
| Centrifugal | a spinning mold throws metal outwards | pipes, rings, liners, bushings | dense, clean outer layer |
| 3D-printed sand molds and patterns | additive manufacturing of the mold | any sand-cast alloy | complex cores, no pattern needed (see 3D Printing) |
Solidification and Chvorinov’s rule
Cast metal freezes from the mold wall inwards. The time to solidify depends on the ratio of the volume to the cooling surface area, the modulus M = V ÷ A, through Chvorinov’s rule t = B M², where B is a mold constant (about 2.5 min/cm² for iron or aluminum in dry sand). Because of the square, a thick section freezes much slower than a thin one. Metal shrinks as it solidifies (3 to 6 percent for most alloys), so the last metal to freeze must be supplied with liquid from a riser that freezes after the casting. The casting section must be fed from thin to thick, and a good design has the thick sections near the risers.
Worked example: the cylinder block
Worked example: a cast iron block 200 × 200 × 60 mm and its riser
- Volume V = 20 × 20 × 6 = 2 400 cm³; surface area A = 2 (LW + LH + WH) = 1 280 cm². Modulus Mc = 1.88 cm; solidification time = 2.5 × 1.88² = 8.8 min.
- The riser should freeze 25 percent later: tr ≥ 11.0 min, i.e. a modulus 1.1 times the casting’s (√1.25), Mr ≥ 2.06 cm.
- A cylindrical riser with height 1.5 × its diameter has V = 0.589 D³ and area 3.93 D², so M = 0.15 D. An 8 cm riser has Mr = 1.50 cm (0.80 times the casting’s modulus) and freezes in only 5.6 min: too small (it would freeze before the casting and leave a shrinkage cavity in it). The smallest riser that meets the 1.25 rule is about D = 11.2 cm (M = 2.10 cm), height 17 cm.
- Pouring: from a 300 mm head the metal leaves the sprue at v = √(2 g h) = 2.43 m/s; through a choke of 4 cm² (discharge coefficient 0.8) the flow is 776 cm³/s, so a 12 kg shot (1 667 cm³) fills in about 2 s: fast enough to avoid cold shuts, slow enough to avoid erosion of the mold.
Casting design rules and allowances
- Uniform wall thickness, changing gradually (a taper of 1 in 4); avoid hot spots at junctions (the intersection of ribs has a larger modulus: use radii and cores).
- Draft of 1° to 3° on all faces parallel to the draw direction so that the pattern leaves the sand.
- Fillets and radii at least ½ the wall thickness; sharp corners crack and make hot spots.
- Machining allowance of 2 to 5 mm on sand castings (more on large ones), 0.5 to 1 mm on die castings, and datum surfaces chosen for machining.
- Minimum wall (sand): about 3 to 6 mm for iron and aluminum; die casting 1 to 3 mm.
- Pattern shrinkage allowance: the pattern is made larger than the part. For a 300 mm feature:
| Alloy | Linear shrinkage | Pattern dimension for 300 mm |
|---|---|---|
| Gray cast iron | 1.0 % | 303.0 mm |
| Ductile iron | 1.0 % | 303.0 mm |
| Cast steel | 2.0 % | 306.0 mm |
| Aluminum alloys | 1.3 % | 303.9 mm |
| Brass and bronze | 1.5 % | 304.5 mm |
| Zinc alloys | 1.1 % | 303.3 mm |
Forging
Forging shapes hot (or cold) metal by compressive force in dies. The metal flows to fill the die and its grain structure follows the shape, giving better strength, toughness and fatigue resistance than a casting or a machined bar. Types: open-die (large simple parts, shafts), closed-die (impression) (the usual production forging, with flash), upsetting (bolt heads), rolled rings and cold forging (accurate small parts). The press force needed for upsetting a cylinder of diameter d to a height h is about F = σf A (1 + μ d ÷ (3 h)), with σf the flow stress, A the contact area and μ the friction.
Worked example: upsetting a steel blank
- A hot steel blank (flow stress about 100 MPa at 1 100 °C), diameter 60 mm, now 30 mm high, μ = 0.3: A = 2 827 mm², F = 100 × 2 827 × (1 + 0.3 × 60 ÷ 90) = 339 kN (35 tonnes-force).
- The same upset cold (flow stress about 600 MPa for work-hardened steel) takes 2 036 kN: six times the force, which is why large parts are forged hot.
- Hammers and presses deliver energy in different ways: a hydraulic press (like the one in Hydraulics and Pneumatics, which delivers 80 kN) is slow and controllable; a mechanical press is fast; a hammer is cheap but noisy and hard on the dies.
- A forged part usually costs more in tooling but saves material and machining: machining a 120 mm length with a finished volume of 35 percent of the 100 mm bar wastes 65 percent of the stock as chips.
Powder metallurgy
Metal powder is pressed in a die (400 to 800 MPa) and sintered at about 80 percent of its melting point, so the particles bond without melting. It gives near-net-shape parts with almost no waste, controlled porosity (a self-lubricating bearing holds oil in its pores) and unusual materials (tungsten carbide, magnets, filters). A typical sintered iron part has a density of about 7.0 g/cm³ against 7.85 for solid steel: 11 percent porosity and a strength (and fatigue strength) well below wrought steel. Sizing, infiltration, forging and heat treatment raise the properties. Metal injection molding (MIM) extends the process to small complex shapes; metal 3D printing is a related powder route.
Cost break-even: a cast part costing 18 each after machining, with a pattern of 3 000, against 55 for machining from bar, breaks even at 3 000 ÷ (55 − 18) = 81 parts. Below that, machine it (see Prototyping Techniques for the quantity thresholds).
Related
- Previous: Corrosion and Protection. Next: Sheet Metal Forming.
- CNC Machining for finish machining.
- Materials and Processes: the whole guide.
FAQ
Why does a casting have a riser?
Metal shrinks as it freezes. The riser is a reservoir of liquid that freezes last and feeds the casting, so the shrinkage ends up in the riser and not inside the part.
Why are castings weaker than forgings?
Castings have a coarse dendritic grain structure and may contain porosity and inclusions. Forging closes the voids and refines the grain, and the flow lines follow the shape.
What is the cheapest process for a one-off?
Almost always machining from bar or plate (or fabrication), or 3D printing: no tooling cost.
What defects should I look for in a casting?
Porosity, shrinkage cavities, cold shuts, misruns, hot tears and sand inclusions: dye penetrant, radiography and sectioning of samples find them.
References
- Groover, M. P., Fundamentals of Modern Manufacturing, Wiley; Campbell, J., Complete Casting Handbook, Butterworth-Heinemann; ASM Handbook, Volumes 14 (Forming and Forging) and 15 (Casting).
- Kalpakjian, S., Schmid, S. R., Manufacturing Engineering and Technology, Pearson; German, R. M., Powder Metallurgy and Particulate Materials Processing, MPIF.
- ISO 8062 (casting tolerances) and ASTM A48, A536 (cast irons), A27 (steel castings).
Last reviewed: October 2026. Mold constants, flow stresses and allowances are typical.