Investment casting delivers cast iron parts at roughly ±0.1–0.3 mm, compared with about ±1–3 mm for sand casting, making the process choice a direct tolerance decision [S3].
Sand casting supports parts from a few ounces to more than 200 tons, while investment casting is most practical below roughly 200 lb, with a practical ceiling near 1,000 lb [S3].
CT Grades Define the Dimensional Gap
ISO 8062 uses CT1 through CT16 for cast dimensional tolerances, with CT1 as the most precise and CT16 as the least precise. The selected grade depends on the casting method, metal, part size, geometry, and production volume [S1].
Sand castings commonly occupy CT10–CT13, corresponding to approximately ±1–3 mm or about 1–1.5% of the selected dimension. Investment castings occupy CT4–CT7, corresponding to approximately ±0.1–0.3 mm on comparable features [S3].
One published process comparison lists green sand at CT10–CT13, shell molding and furan-resin sand at CT8–CT12, silica-sol investment at CT4–CT6, and water-glass investment at CT5–CT9. These are foundry capability ranges, not guaranteed tolerances for every geometry [S2].
For sand castings, ISO 8062-3 tables place hand-molded steel and grey cast iron within DCTG 11–14. A separate iron-casting assessment recommends production sand castings at DCTG 8–12, demonstrating how part geometry and the assessment method affect grade selection [S6][S9].
Compare Cost, Size, Finish, and Lead Time
Sand casting offers lower entry cost, with patterns estimated at roughly $500–$3,000 and 1–2 weeks of tooling. Investment casting requires wax injection dies estimated at $6,000–$30,000 and 4–12 weeks of tooling, but reduces machining requirements by producing a closer-to-net shape [S3].
Investment casting becomes less economical for very large castings because it works best below roughly 200 lb, whereas sand casting scales from small parts to castings above 10,000 kg. Minimum wall thickness is about 1.5 mm for investment casting, compared with approximately 3 mm for light-alloy sand castings and 5–6 mm for ferrous sand castings [S3].
Surface finish also affects total cost. Investment castings typically arrive at Ra 1.6–6.3 µm, while sand castings commonly arrive at Ra 12.5–50 µm. A sealing face, cosmetic surface, or mating feature may therefore remove enough machining from an investment casting to justify its higher tooling cost [S3][S10].
For cast iron housings, covers, and brackets, the decision is straightforward: specify sand casting when size, low volume, or low initial tooling cost dominates; specify investment casting when mating dimensions, thin walls, or as-cast surface finish control the functional cost.
Choose by Part Size and Production Quantity
Sand casting is the appropriate choice for large grey or ductile iron components, one-off prototypes, repair parts, and low production volumes. Its mold tooling is inexpensive, and castings weighing 200-plus tons or exceeding 10,000 kg are established applications [S3].
Investment casting is better suited to smaller cast iron components requiring controlled holes, slots, thin sections, undercuts, or close assembly fits. Its ceramic shell reproduces the wax pattern with limited dimensional loss, while the as-cast finish can approach that of light machining [S3][S10].
For medium production, the cost crossover depends on machining value. The choice between investment casting and sand casting depends on production quantity and the trade-off between tooling costs and machining, with neither process objectively better for every part.
Sand casting is not the right choice when the drawing depends on CT7 or tighter as-cast dimensions without machining allowance. Investment casting is not the right choice for a multi-ton component, an extremely low-volume item justified by inexpensive sand tooling, or a design that cannot meet the process minimum wall thickness.
Geometry and Shrinkage Must Be Included
Investment-casting tolerances are affected by wax or plastic temperature, injection pressure, die temperature, shell composition, firing temperature, cooling rate, position on the casting tree, and heat treatment. The same nominal tolerance can therefore vary between foundries, alloys, and part shapes [S4].
Linear inspection commonly covers length, concentricity, fillet radii, holes, flatness, straightness, corner radii, and curved holes. Geometric inspection covers profiles, true position, parallelism, contours, radii, roundness, perpendicularity, tapered holes, and cam profiles [S4].
Flatness is governed substantially by volumetric shrinkage, with dish concentrated toward the center of the casting mass. A rough straightness guide is an axial bow of 0.005 in per inch, while a difficult yoke geometry can produce parallelism loss of about 0.010 in per inch of length [S4].
Specify machining allowance where a feature controls fit, concentricity, flatness, or sealing. A raw CT grade does not guarantee that a machined bore will meet concentricity after drilling or reaming, and ribs and gussets can complicate straightness even when local dimensions meet the grade [S4].
Inspection and Sourcing Controls
Purchase the cast iron against the applicable ISO 8062-3 DCTG rather than an unsupported linear-tolerance statement. ISO 8062-2 separately defines GCT grades G1–G8 for form, orientation, and position, so linear and geometric requirements must not be merged into one drawing callout [S5].
A practical inspection plan should record the part number, material grade, casting process, nominal dimensions, required CT grade, GCT requirements, surface finish, and any machining allowance. Geometry-dependent effects such as dish, axial bow, and local distortion should be identified separately from general casting tolerance [S4].
Ask the foundry to confirm its actual process route. Silica-sol investment casting is listed at CT4–CT6, while water-glass investment casting is listed at CT5–CT9; the binder route therefore changes the purchaser's available precision [S2].
Material selection remains a separate decision from process selection. Grey cast iron, ductile iron, and malleable iron differ in mechanical behavior, and the process table identifies broad compatibility rather than guaranteeing identical properties or tolerances for every iron grade [S1].
Use Case Recommendations for Cast Iron Parts
For a large grey iron housing above 200 lb, sand casting is the practical baseline. Specify CT10–CT13, include machining allowance on bearing or sealing surfaces, and use ISO 8062-3 DCTG 11–14 as a compatibility check for hand-molded grey iron [S2][S6].
For a smaller ductile or grey iron bracket requiring a close-fit feature, investment casting can justify CT4–CT7 capability. Use it when the wall can remain at approximately 1.5 mm or more, the surface can meet Ra 1.6–6.3 µm, and the production quantity can support $6,000–$30,000 of wax tooling [S3].
For a low-volume prototype, compare the complete cost rather than the casting price alone. A $500–$3,000 sand pattern with 1–2 weeks of tooling may be rational, but add the machining needed to move from approximately ±1–3 mm as-cast accuracy to the drawing requirement [S3].
Track two signals before releasing production tooling: the foundry's confirmed DCTG capability for the exact casting size and geometry, and the inspection report covering the critical features after machining. For mechanical acceptance criteria, cast iron hardness by grade should be specified separately from dimensional tolerance so a tight CT grade cannot conceal an unsuitable material property.