Permanent mold casting
Permanent mold casting is a casting manufacturing process that uses reusable metal molds, typically made of gray iron, steel or graphite, for the series production of parts with low-melting-point non-ferrous alloys. Gravity is the most common method for filling the cavity, although variants with gas pressure and vacuum exist. The molds are preheated to temperatures between 150 °C / 302 °F and 200 °C / 392 °F before the first cycle to facilitate metal flow and reduce thermal fatigue. The process offers typical dimensional tolerances of ±0.25 mm / ±0.010 in for small linear dimensions and a surface finish ranging between 2.5 µm / 100 µin and 7.5 µm / 250 µin average roughness.
Operating parameters and tolerances
Section titled “Operating parameters and tolerances”The pouring temperature is kept at the lowest practical value to minimize cracks and porosity, while the mold temperature is maintained continuously to ensure thermal uniformity between cycles. Mechanical ejector pins are incorporated when refractory coatings are insufficient to extract the part. Typical linear tolerances represent up to 2 % of the nominal dimension, and the draft angle applied is 1° to 3° on outer surfaces and 2° to 5° on inner surfaces. Cycle time and pouring temperature vary by alloy, as detailed in the material parameters table.
| Parameter | Typical value |
|---|---|
| Initial mold preheat temperature | 150–200 °C / 302–392 °F |
| Achievable surface roughness (Ra) | 2.5–7.5 µm / 100–250 µin |
| Linear tolerance (general) | 2 % of nominal dimension |
| Draft angle on outer surfaces | 1° to 3° |
| Draft angle on inner surfaces | 2° to 5° |
| Minimum wall thickness (length ≤ 75 mm / 3 in) | 3 mm / 0.12 in |
| Minimum inside radius (relative to nominal wall thickness) | 1 × nominal thickness |
| Minimum outside radius (relative to nominal wall thickness) | 3 × nominal thickness |
| Part weight range | 50 g to 70 kg / 1.5 oz to 150 lb |
| Typical process yield (percentage of metal utilized) | ≤ 60 % |
Parameters by material
Section titled “Parameters by material”| Cast material | Mold preheat temperature | Pouring temperature | Typical cycle time | Minimum wall thickness |
|---|---|---|---|---|
| Aluminum alloy | 200 °C / 392 °F | 700–750 °C / 1292–1382 °F | 1–3 min | 3 mm / 0.12 in |
| Magnesium alloy | 200 °C / 392 °F | 650–700 °C / 1202–1292 °F | 1–3 min | 3 mm / 0.12 in |
| Brass and bronze | 200 °C / 392 °F | 950–1050 °C / 1742–1922 °F | 2–5 min | 3.2 mm / 0.13 in |
| Zinc alloy | 150 °C / 302 °F | 370 °C / 698 °F | 0.5–1 min | 1.5 mm / 0.06 in |
| Gray iron (graphite mold) | 200 °C / 392 °F | 1370 °C / 2500 °F | 3–8 min | 4 mm / 0.16 in |
Applicable materials
Section titled “Applicable materials”Non-ferrous metals with low to medium melting points dominate the application of the process. Aluminum, magnesium and copper alloys (brass and bronze) constitute the primary selection. Zinc, tin and lead alloys are also successfully processed. Gray iron and steel are cast exclusively in graphite molds due to the high pouring temperature that would compromise the integrity of a conventional metal mold. Mold materials include gray iron — the most common option for its thermal fatigue resistance —, hardened steel, bronze and graphite, selected for their hardness and erosion resistance.
Advantages and limitations
Section titled “Advantages and limitations”The permanent mold casting process reduces the cost per part in production runs of one thousand units or more, with a break-even point that depends on geometric complexity. The accelerated solidification imposed by the metal mold produces fine grain and superior mechanical properties, along with a smoother surface finish and greater dimensional accuracy compared to sand casting. The main limitation lies in the high initial tooling cost, which is only justified with medium to high production volumes. The mold lacks collapsibility, requiring early opening to prevent hot cracking and restricting the geometric complexity of the parts. High-melting-point ferrous alloys are not viable in metal molds due to the accelerated thermal deterioration the tooling would suffer, and are limited to graphite molds.
Selection guide
Section titled “Selection guide”Permanent mold casting is preferred when tighter dimensional tolerances and a finer surface finish than those provided by sand casting are required, and when the projected production volume exceeds one thousand parts. It is suitable for small to medium-sized parts — between 50 g / 1.5 oz and 70 kg / 150 lb — made of aluminum, magnesium or zinc alloys. If the geometry includes complex internal cavities, combination with disposable sand cores should be considered. For low-volume ferrous parts or geometries with deep undercuts requiring collapsibility, sand casting or investment casting offer more suitable alternatives. The slush casting variant applies when the part requires a hollow ornamental shape with good surface detail but without strict wall thickness uniformity requirements.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What distinguishes permanent mold casting from sand casting?
Section titled “What distinguishes permanent mold casting from sand casting?”Permanent mold casting uses a reusable metal mold that provides superior cooling rates, generating a finer grain size and higher mechanical properties. Dimensional tolerances are reduced to approximately ±0.25 mm / ±0.010 in compared to ±1.5 mm / ±0.060 in typical of sand, and the surface finish improves to a range of 2.5 µm / 100 µin to 7.5 µm / 250 µin. The initial tooling cost is 5 to 10 times higher, therefore it is only justified for runs of 1000 parts or more.
What is the correct preheat temperature for a permanent mold?
Section titled “What is the correct preheat temperature for a permanent mold?”The mold is preheated to a range of 150 °C / 302 °F to 200 °C / 392 °F before the first casting cycle. During continuous production, the residual heat from previous parts maintains the operating temperature without the need for external reheating.
What dimensional tolerances can be expected in permanent mold aluminum parts?
Section titled “What dimensional tolerances can be expected in permanent mold aluminum parts?”Aluminum parts produced by permanent mold casting exhibit typical linear tolerances of 2 % of the nominal dimension, with a process capability that can reach ±0.25 mm / ±0.010 in for small cross-sectional dimensions in optimized production.
Is it possible to manufacture parts with wall thickness less than 3 mm / 0.12 in?
Section titled “Is it possible to manufacture parts with wall thickness less than 3 mm / 0.12 in?”Yes, for high-fluidity alloys such as zinc, minimum thicknesses of 1.5 mm / 0.06 in can be achieved. For aluminum and magnesium, the practical minimum thickness remains at 3 mm / 0.12 in for lengths up to 75 mm / 3 in, and increases proportionally in larger sections.
What is the main advantage of permanent mold casting over die casting?
Section titled “What is the main advantage of permanent mold casting over die casting?”Permanent mold casting operates with gravity filling, which significantly reduces porosity from gas entrapment compared to the high-speed injection of die casting. This allows subsequent heat treatments and welding on the parts, practices that are problematic in die-cast parts with air entrapment.
What materials cannot be processed in a permanent metal mold?
Section titled “What materials cannot be processed in a permanent metal mold?”Ferrous alloys with pouring temperatures above 1400 °C / 2550 °F, such as carbon steels and stainless steels, are not cast in conventional metal molds because the temperature compromises the thermal fatigue resistance of even the best cast iron. For these alloys, graphite molds are used, which tolerate thermal shock but have a limited service life.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/assembly-shrink-fits-d_951.html
- efunda.com: https://www.efunda.com/processes/metal_processing/permanent_mold.cfm
- manufacturingguide.com: https://www.manufacturingguide.com/en/kokillgjutning