Casting Draft Angle Guide: When Designers Need to Add Draft

If you buy metal casting from a foundry, you have likely seen “draft” noted on a drawing and wondered why it matters. Draft is the slight taper on vertical walls that lets a casting release from a mold without sticking, scraping, or tearing. It is a small geometric detail with a large effect on cost, lead time, and part quality. Draft is not optional. It is a requirement of the process, and the right time to handle it is before the tooling is cut, not after.

This guide explains when designers need to add draft, how much to add for each casting process, and what you as the buyer should send to the foundry so the quote you get matches the price you actually pay.

Draft Rules Differ by Casting Process

Waking runs three casting lines in the same factory: investment casting, gravity aluminum casting, and low-pressure aluminum casting. The draft requirements are different for each.

Investment casting (lost wax). The wax pattern is injected into a metal die, then coated with ceramic shell. The wax melts out, and the shell is broken away after the metal has solidified. Because the shell is destroyed to release the part, the metal never needs to be extracted from a rigid mold. Draft still exists, but only because the wax pattern must eject cleanly from the wax die. In practice, 0.5° to 1° per side is sufficient, and simple shapes can be made with almost no draft when the wax die has adequate ejector pins.

Gravity and low-pressure aluminum casting. These are permanent-mold processes. The mold is a steel die, and the casting is extracted from that die every cycle while it is still hot. Here draft is critical. Without it, the casting hangs up in the die, the operator has to pry it out, and the part can distort or tear. The die surface wears unevenly, cycle time climbs, and rejection rate goes up. Every rejected casting is cost in somebody’s quote.

The distinction matters when you compare processes. A zero-draft design may be workable in investment casting but difficult and expensive in gravity casting.

How Much Draft Should You Add?

Use these numbers as starting points, not guarantees. The final amount depends on depth, wall thickness, alloy, and the position of the parting line.

• External walls, permanent mold aluminum: 1° to 3° per side. Start at 1.5° per side and go higher for deeper walls.
• Internal walls, holes, and cores, permanent mold aluminum: 2° to 5° per side. The metal shrinks onto a core, so the core holds the part tighter than the outer cavity. More taper is better here.
• Investment casting wax tooling: 0.5° to 1° per side is usually sufficient.
• Draft is measured per side, not as a total included angle. A 2° per side taper means each face is angled 2°. A common quoting error is to specify total included angle, which halves the effective draft.

The deeper the feature, the more draft it needs. A deep pocket is far more sensitive to missing draft than a shallow rib. When in doubt, add more draft to internal features and let machining produce a straight final wall where the design requires one.

The Zero-Draft Traps That Push Costs Up

Certain features cause trouble in almost every review. Watch for them in your own parts.

Deep blind pockets. A housing with a deep pocket and no taper locks the casting onto the core. This is one of the most common problems in automotive housings. Give pocket side walls at least 2° per side.

Internal threads. You cannot draft a thread form. Cast a blank and machine the thread afterward. Mark threads as machined features in the drawing.

Ribs and bosses. Ribs running perpendicular to the parting line need draft like any other wall. Screw bosses also need a tapered base. Both show up constantly in industrial machinery parts.

Lettering and logos. Raised lettering on the part becomes recessed in the die, and the letter walls need taper. The taller the letters, the more taper they need to eject cleanly.

Sharp internal corners. They concentrate stress and make ejection harder. Radius them whenever possible.

Valve bodies, a product family Waking casts regularly in 304, 316, CF8, and CF8M stainless steel, combine an exterior with modest draft needs and internal flow passages formed by cores. The cores are where draft discipline matters most. The same logic applies to aluminum housings for automotive use.

If a feature truly cannot take draft, flag it and let the foundry propose a solution. The solution may be a secondary operation, a side core, or a different parting line. It will cost more than a drafted design, and you want that conversation before tooling is cut, not after the quote arrives.

What CNC Machining Can and Cannot Solve

Waking runs its machining in-house with 400-plus CNC machines. The interplay between cast draft and machined surfaces is therefore part of daily workflow here, not a handoff to a third party.

Machining solves one draft problem: a surface that gets machined does not need draft in the finished geometry. You can specify a zero-draft, straight wall because the cutter removes the taper. That is how designers get truly straight walls on a cast component.

Two things need to be said clearly.

1. Tell the foundry which surfaces will be machined. If the foundry does not know, it will add draft to a face you intended to machine flat, and the first article may not clean up.
2. Machining cannot rescue a core that refuses to eject. If an internal feature has no draft, the casting may not leave the die at all, and the tooling itself has to be reworked. Machining fixes geometry after the part exists. It does not fix a bad die design.

On any surface left un-machined, the draft remains in the finished part. That taper is functional, not a defect. As-cast faces are supposed to have it, and your approved drawing will show it.

What to Send at the Quoting Stage

Waking works a three-step process: send drawings and requirements, receive an engineering review and quote, then move to casting, machining, and delivery. The engineering review is where draft issues are caught before any metal is poured.

To get the most from that review, send the following together:

• Both 3D and 2D files. The 3D model shows true geometry. The 2D drawing states tolerances, finish, and machining notes.
• An indication of draft direction or parting line. If you do not specify one, the foundry will choose a draw direction that maximizes castability, and that choice may add cores or side actions that change the price. Make the call yourself if the direction matters to your design.
• A list of post-casting machined surfaces. This prevents the foundry from adding draft where you do not want it.
• Flags on zero-draft features. Let the engineer see them and respond with options instead of surprises.

A good foundry will not silently quote a geometry that cannot be produced. It will come back with drafting suggestions, a revised parting line, or a clear cost difference for the harder approach. At Waking, that review happens inside a real casting factory, not at a trading desk. The engineers who review your drawing work in the same plant where your parts will be poured, and the in-house CMM room with COORD3 and ZEISS machines checks the as-cast dimensions against the approved drawing before samples ship. Specify draft correctly up front, and the parts you receive match the geometry you approved.

Frequently Asked Questions

Why does investment casting need less draft than gravity or low-pressure aluminum casting?

Investment casting uses a wax pattern ejected from a metal die and a ceramic shell that is broken away after solidification, so the metal never needs to be extracted from a rigid mold. Gravity and low-pressure aluminum casting are permanent-mold processes where the hot casting is pulled from a steel die every cycle, making draft critical to avoid sticking, distortion, or die wear.

How much draft should I allow on internal holes and cores in permanent mold aluminum castings?

For internal walls, holes, and cores in permanent mold aluminum, allow 2° to 5° per side. The metal shrinks onto the core during cooling, so the core holds the part tighter than the outer cavity, and deeper features need more taper. When in doubt, add more draft and machine the final straight wall if required.

Can CNC machining remove the need for draft on all surfaces of a cast part?

No. Machining can remove draft only on surfaces that will be machined, producing a straight finished wall, but the foundry must know which surfaces will be machined beforehand. Machining cannot fix a core that refuses to eject because of missing internal draft, since the casting may never leave the die and the tooling itself must be reworked.

What should I include in my drawing at the quoting stage to avoid draft-related cost surprises?

Send drawings and requirements, clearly mark which surfaces will be machined, and mark internal threads as machined features rather than cast geometry. Also flag any feature that truly cannot take draft so the foundry can propose a secondary operation, side core, or different parting line before tooling is cut.

Key Parameters at a Glance