Casting Tolerances by Different Casting Processes

A casting tolerance specifies the allowable deviation between a feature’s nominal and actual size.

For instance, a ±0.5 mm tolerance on a 100 mm dimension means the finished part may measure anywhere between 99.5 mm and 100.5 mm.

Such precision influences component fit, mechanical performance, and assembly reliability.

At the same time, every tenth of a millimeter shaved off the tolerance budget can increase mold cost by 10–20%, raise scrap rates by up to 15%, and add two to four weeks of tooling lead time.

This article surveys a range of casting processes—from green‐sand to die casting—and quantifies their typical tolerance capabilities.

We’ll also review ISO 8062 and other industry standards, outline necessary pattern and machining allowances,

and recommend inspection and statistical‐process‐control methods that help you strike the optimal balance between cost and precision.

1. Understanding Tolerances in Casting

Before selecting a process, clarify these foundational concepts:

• Tolerance is the total allowable variation in a dimension.
• Allowance is the deliberate oversize or undersize built in for casting shrinkage, draft, or subsequent machining.
• Fit describes how two mating parts interact, ranging from clearance fits (loose) to interference fits (tight).

Moreover, casting tolerances may be linear (e.g., ±0.5 mm) or geometric (e.g., circularity, perpendicularity), defined using GD&T symbols.

Remember: each class of tolerance you specify can translate into tangible cost and schedule impacts.

Consequently, careful upfront planning—aligned with your manufacturing partner’s capabilities—pays dividends in quality and total cost of ownership.

2. Standards and Nomenclature

Before specifying tolerances, you need a common language. International and regional standards define both dimensional and geometric casting tolerances, so designers and foundries can speak with precision.

ISO 8062 Casting Tolerance (CT) and Geometric Casting Tolerance (GCT)

ISO 8062-3 defines Dimensional Casting Tolerance (DCT) grades from CT1 through CT16, where lower CT numbers correspond to tighter as-cast tolerances. In practice:

• CT1–CT4 (±0.05–0.3 % of dimension) suit high-precision die-casting and permanent-mold parts.
• CT5–CT9 (±0.1–0.8 %) apply to investment and shell-mold castings.
• CT10–CT14 (±0.4–2.0 %) cover various sand-casting methods.
• CT15–CT16 (±2.5–3.5 %) serve very large or non-critical castings.

For example, on a 200 mm feature:

• A CT4 part might hold ±0.6 mm,
• While a CT12 sand casting might allow ±4 mm.

Complementing CT grades, ISO 8062-2 defines Geometrical Casting Tolerances (GCT)—covering form (flatness, circularity), orientation (perpendicularity, parallelism), and position (true position).

Each GCT grade (G1–G8) layers geometric control onto the nominal CT dimensional envelope.

Regional & Industry Specifications

While ISO provides a global framework, many industries reference tailored standards:

NADCA (North American Die Casting Association):

• Normal tolerance: ±0.25 mm per 100 mm (approx. ISO CT3–CT4).
• Precision tolerance: ±0.10 mm per 100 mm (approx. ISO CT1–CT2).
• NADCA also defines separate classes for height, hole, and flatness tolerances specific to die-cast materials such as zinc, aluminum, and magnesium.

SFSA 2000 (Steel Founders’ Society of America):

• Provides sand-casting tolerances ranging ±0.4–1.6 mm per 100 mm, depending on mold type (green-sand vs. resin-bonded).
• Its tables correspond roughly to ISO CT11–CT13.

BS 6615 (British Standard for Foundry)

• Covers sand, shell, and investment processes.
• Typical allowances:

• • Sand casting ±0.5–2.0 mm/100 mm (CT11–CT14)
  • Shell casting ±0.2–0.8 mm/100 mm (CT8–CT12)
  • Investment casting ±0.1–0.5 mm/100 mm (CT5–CT9)

3. Casting tolerance table (unit: mm)

The following table lists the maximum total tolerance values ​​for different CT grades (Casting Tolerance Grade CT1–CT16) within different basic size ranges.

4. Overview of Major Casting Processes

Casting processes fall into three broad categories—expendable‐mold, permanent‐mold/pressure‐driven, and specialty techniques—each offering distinct tolerance capabilities, surface finishes, and cost structures.

Expendable-Mold Methods

Green-Sand Casting

Green-sand casting remains the most economical and flexible method for large or simple parts.

Foundries mix silica sand, clay, and moisture to form molds that yield typical ISO CT11–CT14 tolerances—about ±0.5–2.0% of any given dimension (i.e., ±0.5–2.0 mm on 100 mm).

Surface finish generally ranges Ra 6–12 μm, and tooling cost stays low (often <$500 per pattern).

Chemically-Bonded & No-Bake Sand

Upgrading to resin-bonded or no-bake sand molds tightens tolerances to CT9–CT12 (±0.3–1.2%), improves mold strength, and reduces wash-out.

Surface roughness drops to Ra 3–6 μm, making these methods well suited for medium-complexity parts where green-sand precision proves marginal.

Investment (Lost-Wax) Casting

Investment casting, also known as lost-wax, produces intricate shapes and thin walls with CT5–CT9 tolerances—approximately ±0.1–0.5% (±0.1–0.5 mm per 100 mm).

Its excellent surface finish (Ra 0.8–2.0 μm) and ability to maintain fine detail justify higher tooling costs (often $2,000–$10,000 per pattern) in aerospace, medical, and high-end industrial applications.

Lost-Foam Casting

Lost-foam casting combines expendable patterns with unbonded sand, offering CT10–CT13 capabilities (±0.4–1.5%).

While surface finish (Ra 4–8 μm) and dimensional control fall between green-sand and investment casting, this method excels at producing complex, single-piece assemblies without cores.

Permanent-Mold & Pressure-Driven Methods

Die Casting (Hot & Cold Chamber)

Die casting yields the tightest as-cast tolerances—CT1–CT4, or ±0.05–0.3% of dimension (±0.05–0.3 mm per 100 mm).

Typical surface finish ranges Ra 0.5–1.5 μm. High upfront tooling costs (often $10,000–$200,000 per die) pay off in cycle times as fast as 15–60 seconds and excellent repeatability for aluminum, zinc, and magnesium parts.

Gravity Die & Low-Pressure Die Casting

Gravity and low-pressure die casting, using reusable metal molds, achieve CT2–CT6 tolerances (±0.1–0.5%) with Ra 1–4 μm finishes.

Because they operate without high injection speeds, these methods reduce porosity and strengthen components—particularly in automotive wheel and pump applications.

Specialty Techniques

Centrifugal Casting

By spinning molds at 200–2,000 RPM, centrifugal casting forces molten metal outward, producing dense pipe walls and rings. Radial tolerance falls in CT3–CT8 (±0.1–0.5%).

Surface finish typically sits at Ra 3–8 μm, and directional cooling enhances mechanical properties in heavy-duty bearings and piping.

Plaster & Ceramic Mold Casting

Plaster and ceramic molds—largely used for art, jewelry, and small-batch aerospace parts—provide CT6–CT9 tolerances (±0.2–0.8%) and Ra 2–5 μm finishes.

Although slower and more expensive than sand, these processes accommodate fine detail and special alloys.

5. Tolerance Capabilities by Casting Process

In this section, we present a consolidated view of each process’s typical ISO 8062 CT grade,

its corresponding linear tolerance (as a percentage of dimension and in millimeters on 100 mm), and a representative surface finish.

6. Factors Affecting Casting Tolerances

Casting tolerances are not fixed properties of a process—they result from a complex interplay between material behavior, tooling design, process parameters, and part geometry.

Material Properties

The type of metal or alloy directly affects shrinkage, flowability, and dimensional stability.

• Thermal Contraction Rates: Metals shrink upon cooling. For example:

• • Gray iron: ~1.0%
  • Aluminum alloys: ~1.3%
  • Zinc alloys: ~0.7%
  • Steel: ~2.0% (varies with carbon content)

Higher shrinkage results in more dimensional deviation unless compensated by tooling design.

• Fluidity and Solidification Behavior:

• • Metals with higher fluidity (e.g., aluminum, bronze) fill molds more precisely.
  • Rapid solidification in thin sections or low-fluidity metals can cause voids and uneven shrinkage.

• Alloying Effects:

• • Silicon in cast iron improves fluidity but also increases expansion.
  • Nickel and chromium enhance dimensional stability in steels.

Mold and Tooling Variables

The mold system is often the single largest contributor to as-cast dimensional variation.

• Pattern Accuracy:

• • CNC-machined patterns achieve far better tolerance than hand-crafted ones.
  • Wear over time degrades precision—especially in high-volume sand casting.

• Draft Angles:

• • Required to release the casting from the mold, typical angles are:

• • • 1°–3° for external surfaces
    • 5°–8° for internal cavities

• • Excessive draft adds dimensional variation and must be accounted for.

• Mold Rigidity and Expansion:

• • Sand molds are compressible and expand under heat, which affects tolerances.
  • Metal dies (in die casting) are more dimensionally stable, supporting tighter tolerances.

• Thermal Conductivity:

• • Fast cooling (e.g., metal molds) minimizes distortion.
  • Slow cooling (e.g., ceramic or plaster molds) allows more time for material contraction and deformation.

Process Parameters

How the metal is poured, solidified, and cooled significantly alters final dimensions.

• Pouring Temperature:

• • Overheating increases mold erosion and exaggerates shrinkage.
  • Underheating leads to poor mold filling and cold shuts.

• Gating and Risering Design:

• • Poor gating can cause turbulence and air entrapment, leading to porosity and distortion.
  • Insufficient risers result in shrink cavities that reduce geometric integrity.

• Cooling Rate and Solidification Control:

• • Techniques such as chills, venting, and controlled cooling zones help refine dimensional accuracy.
  • In thicker sections, uneven solidification may cause differential shrinkage and warping.

• Section Thickness and Complexity:

• • Thin sections cool faster, resulting in smaller grain size and better dimensional control.
  • Complex geometries with varying wall thicknesses are prone to hot spots and internal stresses, affecting the final shape.

Part Size and Geometry

Larger parts accumulate more thermal and mechanical stresses, leading to increased distortion:

• A 1000 mm steel casting may vary ±3–5 mm, while a 100 mm aluminum part can maintain ±0.1 mm with investment casting.
• Asymmetrical parts often warp due to unbalanced cooling and uneven metal flow.
• Incorporating uniform wall thickness, ribs, and rounded transitions enhances dimensional predictability.

Summary Table – Key Factors & Typical Impacts

Factor	Typical Impact on Tolerance
Thermal Shrinkage of Material	+0.7% to +2.5% deviation from mold dimension
Pattern Accuracy (manual vs CNC)	±0.5 mm to ±0.05 mm variance
Draft Angle Requirement	Adds 0.1–1 mm per 100 mm of depth
Pouring Temp Deviation (±50°C)	Up to ±0.2 mm dimensional shift
Wall Thickness Variation	Can cause ±0.3–0.6 mm distortion
Mold Expansion (sand vs metal)	±0.1 mm to ±1.0 mm depending on mold type

7. Allowances in Pattern and Mold Design

To achieve final tolerances, designers build in specific allowances:

• Shrinkage Allowance: Add 1.0–1.3 mm per 100 mm for aluminum, 1.0 mm/100 mm for iron.
• Draft Allowance: 1°–3° taper per vertical face.
• Machining Allowance: 1–3 mm (depending on process and feature criticality).
• Distortion & Shake: Extra 0.5–1.0 mm in thin walls to counter pattern shake and distortion.

By meticulously applying these values, engineers ensure that the as-cast oversize positions critical dimensions into the desired tolerance window.

8. Design for Tolerance Control

Effective design minimizes the gap between as-cast and finished dimensions:

• Near-Net Shape: Aim to deliver features within ±10% of final size, reducing machining by 70%.
• GD&T Focus: Apply tight controls only to critical interfaces; allow CT-grade tolerances on non‐critical surfaces.
• Geometry Guidelines: Use generous fillets (>1 mm radius), uniform wall thickness (≤10 mm variation), and strategically placed ribs to limit distortion.

Such intentional feature design helps castings emerge closer to their target geometry, preserving both cost and quality.

9. Inspection and Quality Assurance

CMMs, laser scanners, and CT systems enable rapid, high‐density measurement:

• Vernier & Micrometer: Quick “spot checks” for first-pass verification.
• CMM/Optical Scanning: Full-field mapping against CAD models; typical uncertainty: ±0.005 mm.
• CT Scanning: Validates internal geometries, pore distribution, and wall‐thickness uniformity.

Quality plans should include First Article Inspection (FAI), PPAP for automotive, or AQL sampling (e.g., AQL 1.0) for high‐volume runs.

Root-cause analysis targets tolerance excursions—whether due to mold shift, thermal distortion, or pattern wear.

10. Statistical Process Capability

To quantify your casting operation’s ability to meet tolerance:

• Calculate Cp (process potential) and Cpk (process performance) values; aim for Cp ≥1.33 and Cpk ≥1.0 for robust tolerance control.
• Use SPC charts to monitor critical casting parameters: mold hardness, pouring temperature, and dimension trends.
• Implement DOE (Design of Experiments) to identify key factors and optimize gating, mold compaction, and cooling rates.

11. Conclusion

Casting tolerances represent a critical nexus of design intent, process capability, and economic reality.

By grounding decisions in ISO 8062 CT grades, aligning with NADCA or SFSA requirements, and incorporating proper pattern allowances, engineers and foundries can deliver parts that meet both performance and budget goals.

Moreover, rigorous inspection, statistical control, and emerging digital technologies—from 3D‐printed sand molds to real-time simulation—are tightening as-cast tolerances and reducing expensive downstream machining.

Ultimately, the right tolerance strategy ensures that your cast component transitions smoothly from pattern shop to assembly line, on time, on budget, and within specification.