Draft Angle in Die Casting - Rules, Exceptions & Tooling Cost

Draft angle in die casting is the taper applied to walls, ribs, bosses, holes, and textured surfaces so the solidified part can eject from the die without sticking, tearing, or distorting. Without adequate draft, ejection requires high force that marks surfaces, distorts geometry, or breaks fragile features �?shortening tool life and increasing scrap.

Draft is a small design detail with disproportionately large cost consequences. It determines tool life, ejection force, surface finish quality, parting line placement, and whether expensive slides or secondary machining operations are necessary.

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Recommended Draft Angles by Feature and Alloy

Feature	Aluminum	Zinc	Magnesium
External walls (cavity side)	1-2°	0.5-1°	1-2°
Internal walls and cores	2-3°	1-2°	2-3°
Deep ribs (depth >15 mm)	3-5°	2-3°	3-5°
Boss outside diameter	1-2°	0.5-1°	1-2°
Boss inside bore (core pin)	2-3°	1-2°	2-3°
Textured surfaces	Base angle + texture allowance	Base angle + texture allowance	Base angle + texture allowance
Polished cosmetic surfaces	1° minimum �?less if highly polished	0.5° minimum	1° minimum

Internal walls require more draft than external walls because aluminum and magnesium shrink onto the core as the casting cools, increasing the contact force the part exerts against the steel. This thermal shrinkage adds to the mechanical gripping effect that draft must overcome.

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Aluminum Shrinkage and Its Effect on Effective Draft

Aluminum shrinks approximately 0.5-0.7% volumetrically as it solidifies in the die. On a core 50 mm deep, this equates to 0.25-0.35 mm of radial shrinkage pressing the casting against the core surface. This shrinkage effectively reduces the functional draft that the geometry provides �?which is why internal walls consistently need more nominal draft than external walls pulling away from the cavity.

Magnesium shrinks at a similar rate (~0.5-0.6%). Zinc shrinks less (~0.3-0.4%), which is part of the reason zinc die casting can reliably hold tighter dimensional tolerances and use smaller draft angles.

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Textured Surfaces �?Draft Angle Calculation

Surface texture in die casting is specified using VDI 3400 (European standard), Mold-Tech, or custom texture specifications. Textured surfaces need additional draft beyond the smooth-wall minimum because the texture pattern mechanically keys into the casting and resists ejection.

The standard design rule for textured surfaces:

Add approximately 1° of draft for every 0.025 mm (0.001 inch) of texture depth beyond the base smooth-wall draft.

Texture Depth	Additional Draft Required
0.025 mm (VDI 12 approx.)	+1°
0.05 mm (VDI 18 approx.)	+2°
0.10 mm (VDI 24 approx.)	+4°
0.15 mm (VDI 27 approx.)	+6°
0.20 mm (VDI 30 approx.)	+8°

Example: an aluminum wall needing VDI 24 texture (0.10 mm depth) requires 1° base draft + 4° texture allowance = minimum 5° total. Insufficient draft on a textured wall causes scuffing, tearing, and cosmetic defects every cycle.

Buyers specifying texture depth on cosmetic surfaces must also specify the required draft �?or accept that the toolmaker will decide. Mismatched texture-to-draft ratios are a common cause of cosmetic rejection at first article.

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The Cost of Inadequate Draft

Design Choice	Production Consequence	Cost Impact
Normal draft on all pull-direction walls	Clean ejection, stable cycle, long die life	Lowest cost
0.5° less than minimum on a deep rib	Drag marks, tool polishing required, early wear	Tool repair every 20,000-50,000 shots
Zero draft on a cosmetic wall	Ejector marks visible, inconsistent finish, surface rework	Scrap or hand finishing cost per part
Textured surface without adequate draft	Tearing and surface scuffing �?may require tool texture removal	Tool rework and re-texture
Undercut requiring a slide	Slide adds $3,000-10,000 to tool cost; adds cycle time	Higher tooling and production cost per part
Zero draft requiring machining	Adds CNC operation after casting	$0.50-3.00 per part depending on operation

The tradeoff between draft and machining is often straightforward: if a functional surface requires zero draft and the machining allowance is small, machining after casting is cheaper than the tool complexity and maintenance of low-draft dies.

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Depth-to-Draft Guidelines for Ribs and Bosses

Draft requirements increase with depth because longer features have more contact area and more shrinkage force acting over a greater length.

Rib or Core Depth	Minimum Draft (Aluminum)
Up to 10 mm	1-2° typically sufficient
10-25 mm	2-3° recommended
25-50 mm	3-4° �?review with toolmaker
Over 50 mm	4-5° or more �?or consider slide or machining

A deep rib with 1° draft that looks fine in CAD will cause ejection problems in production as tool wear reduces the polished surface quality over thousands of shots. Draft should be designed for long-run stability, not just first-shot release.

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Exceptions and Trade-Offs

Some functional surfaces cannot accept standard draft. Common exceptions:

• Bearing seats and precision bores: Require close-tolerance cylindrical surfaces that machining provides after casting
• Gasket and sealing faces: Flatness and surface finish requirements are achieved by CNC face milling, not die casting
• Connector and press-fit interfaces: Controlled by post-cast machining
• Thread forms: Cut or rolled after casting in virtually all cases

For these features, the standard approach is:

1. Add machining allowance (typically 0.5-1.5 mm depending on surface) and machine to final dimension after casting
2. Reorient the feature in the die layout so it pulls in a direction where draft is achievable
3. Accept that a slide is required, with the associated cost and maintenance

A "zero draft" callout should appear only on surfaces that will be machined. Leaving it on a cast surface is a manufacturing conflict that will be resolved during tooling �?ideally before steel cutting, not after.

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Polishing Direction and Draft

Die cavities are polished in the draw direction (the direction the part ejects). Polishing reduces the coefficient of friction between the casting and the die surface, which helps compensate for marginal draft. However, polishing is not a substitute for adequate draft �?it is a complement that helps marginal geometry eject consistently and improves cosmetic surface quality.

Fine polishing (SPI A1-A2 finish) on cosmetic surfaces allows slightly lower draft angles to be used reliably. Rough or unpolished surfaces need more draft to compensate for friction. Buyers specifying high-gloss or optical surfaces on die castings should confirm the tool polishing standard at DFM review, since the polishing plan affects both draft requirements and tooling cost.

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Common Buyer Mistakes

Mistake	Result
Specifying texture without adding extra draft	Cosmetic scuffing and tearing at first article
Marking zero draft on all holes to "keep them cylindrical"	CNC required on all holes �?cost may not be justified on non-functional holes
Ignoring draft on deep ribs because "it looks fine in CAD"	Production instability after tool surfaces wear
Specifying heavy texture on a deep internal boss	Combination of depth and texture may require unreachable draft or tool redesign
Not reviewing draft before tooling release	Changes after steel cutting are expensive �?EDM or weld repair to the die

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RFQ Information to Send

Send the supplier:

• 3D model and 2D drawing, including any draft angle callouts
• Cosmetic surface map: which surfaces are visible and to what standard
• Texture specification with VDI or Mold-Tech reference and depth
• Functional surfaces requiring zero draft or machining
• Assembly fit requirements for bores, slots, and mating faces
• Annual volume (draft decisions interact with cycle time and die life)
• Target alloy

KastMfg reviews draft, parting line, slide requirement, texture-to-draft ratio, and machining plan during DFM before tooling release. Submit drawings through the RFQ page.

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FAQ

What is a normal draft angle for aluminum die casting?

External walls in aluminum die casting typically use 1-2° of draft. Internal walls and cores need 2-3° because aluminum shrinks onto the core during solidification, increasing ejection force. Zinc can use smaller draft angles due to less shrinkage and lower injection temperatures.

How much extra draft does a textured surface need?

The standard guideline is 1° of additional draft for every 0.025 mm of texture depth, added on top of the smooth-wall base draft. A moderate VDI 24 texture (approximately 0.10 mm depth) on an aluminum wall needs roughly 5° total draft �?1° base plus 4° for texture.

Can die cast parts have zero draft?

Yes, but only on surfaces that will be machined after casting. A zero-draft callout on a cast surface creates ejection problems and tool wear. Functional surfaces needing zero draft should have machining allowance added and be machined to final dimension after casting.

Why do internal walls need more draft than external walls?

As the casting solidifies and cools, aluminum and magnesium shrink onto the die core, gripping it. This thermal contraction adds to the mechanical force that must be overcome during ejection. Internal walls experience this gripping effect directly, while external walls pull away from the cavity naturally.

What happens if ribs have too little draft?

Ribs with inadequate draft tear on ejection, leaving drag marks on the rib flanks. In mild cases, this is cosmetic. In severe cases, ribs break during ejection or the die requires frequent polishing and repair, shortening tool life and increasing scrap rates.

How does draft affect die life?

Adequate draft reduces ejection force, which reduces die wear, surface erosion, and the frequency of tool polishing and repair. Low-draft dies wear faster, require more maintenance, and have shorter production life. The cost of inadequate draft is paid over the production life of the tool, not at tooling buyoff.

Does more draft affect part dimensions?

Draft adds taper to walls, which slightly reduces wall thickness at the top of a feature compared to the base. For most structural and functional features, this is within tolerance. For precision features requiring consistent wall thickness, the machining plan must account for the cast draft before final machining.