Aluminium Die Casting Tolerances — What to Specify and Why

Kinaguiden is a precision aluminium die casting company based in Stockholm, Sweden, specialising in HPDC, mould design, re-engineering, and global logistics.

Tolerance specification is one of the most consequential — and most commonly misunderstood — decisions in a die casting project. Over-specify and you add unnecessary cost through CNC machining on features that don't need it. Under-specify and parts may not assemble correctly or perform as intended. This guide explains what as-cast HPDC can realistically achieve, where CNC machining is needed, and how to write tolerances on your drawing that reflect what you actually need.

What Tolerances Can HPDC Achieve As-Cast?

High-pressure die casting is one of the most dimensionally consistent metal casting processes. Under normal production conditions, well-designed parts and well-maintained tooling can achieve:

• General dimensions: ±0.1 mm to ±0.2 mm for dimensions up to ~100 mm
• Larger features: ±0.3 mm to ±0.5 mm for dimensions in the 100–300 mm range
• Flatness: typically 0.2–0.5 mm across a 100 mm span, depending on part geometry and wall thickness
• Hole diameters (cored): ±0.1 mm is achievable for small cored holes; larger holes have proportionally wider tolerance

These figures represent what can reliably be held in series production without secondary operations. Individual parts in a trial run may perform better, but drawing tolerances must be written to what can be consistently maintained across thousands of parts.

Tolerance Standards for Die Casting

The most widely used international standard for die casting dimensional tolerances is ISO 8062 (Geometrical product specifications — dimensional and geometrical tolerances for moulded parts). Grades CT4 through CT7 are typically applicable to HPDC aluminium depending on feature type and part size.

In practice, many buyers simply write a general tolerance block on their drawing (e.g., "general tolerances ±0.2 mm unless otherwise stated") and apply tighter tolerances only to specific critical dimensions. This is a practical and recommended approach — it focuses manufacturing attention where it matters.

What Affects As-Cast Dimensional Accuracy?

Several factors influence how accurately a die cast part matches the nominal drawing:

Alloy Shrinkage

All aluminium alloys shrink as they solidify, typically between 0.5% and 0.7% for common HPDC alloys. The mould is designed with shrinkage compensation built in — but different alloys shrink at slightly different rates, and thermal variation during production can introduce small dimensional shifts. This is managed through careful mould design and process control, but it sets a fundamental limit on achievable tolerance.

Parting Line Flash

Where the two halves of the mould meet, a thin fin of metal (flash) can form. Even after trimming, there is typically a witness mark at the parting line that adds a small amount of material. Dimensions measured across the parting line are inherently less accurate than dimensions within a single mould half. For critical dimensions, design them to lie within one mould half wherever possible.

Ejector Pin Marks

Ejector pins leave small circular witness marks on the surface they contact. These marks are slightly recessed (typically 0.0–0.3 mm) or raised depending on mould wear. Any dimension that references a surface with an ejector pin mark will be affected.

Wall Thickness and Cooling

Non-uniform wall thickness causes uneven cooling and thermal stress, which leads to warping and dimensional variation. The more consistent the wall thickness of a part, the better its dimensional repeatability across production.

When Is CNC Machining Required?

For dimensions tighter than approximately ±0.05 mm, or for features where surface finish Ra better than 1.6 µm is required, CNC machining after casting is necessary. Common machined features on die cast parts include:

• Precise bore diameters for press-fit or bearing seats
• Flat sealing faces for O-ring grooves or gasket surfaces
• Threaded holes (usually drilled and tapped post-cast)
• Mating faces for assembled components requiring tight flatness
• Precision location features (dowel pin holes, register diameters)

CNC machining adds cost — typically in the range of 15–40% of total part cost for moderately machined components. It is worth reviewing every tight tolerance on a drawing to ask: does this feature functionally require this tolerance, or has it been carried over from a machined part drawing?

Practical Tolerance Table for HPDC Aluminium

Feature Type	As-Cast Achievable	With CNC Machining
Linear dimensions ≤50 mm	±0.10 mm	±0.01–0.02 mm
Linear dimensions 50–150 mm	±0.15–0.20 mm	±0.02–0.05 mm
Linear dimensions 150–300 mm	±0.25–0.40 mm	±0.05–0.10 mm
Cored hole diameter ≤20 mm	±0.10 mm	±0.01 mm (reamed)
Flatness per 100 mm	0.2–0.4 mm	0.02–0.05 mm
Surface roughness Ra	1.6–3.2 µm	0.4–0.8 µm
Across parting line	Add ±0.15–0.25 mm	Eliminates parting effect

How to Write Tolerances Correctly on Your Drawing

1. Use a general tolerance block — apply ISO 8062 CT6 or a specific general tolerance such as ±0.2 mm to all non-critical dimensions. This avoids the need to tolerance every dimension individually.
2. Identify and call out critical dimensions explicitly — apply tight tolerances only to dimensions that are functionally critical (assembly fits, sealing faces, bearing seats).
3. Distinguish as-cast from machined features — mark machined surfaces clearly. This tells the manufacturer which surfaces require secondary operations and which are accepted as-cast.
4. Review inherited tolerances — if your drawing was adapted from a machined-part drawing, tolerances may be tighter than needed everywhere. Review and relax non-critical tolerances to reduce cost.
5. Discuss with your supplier before finalising — a good die casting supplier will review your drawing for manufacturability and flag any dimensions that are difficult or unnecessarily tight. Kinaguiden does this as part of every quotation at no charge.