A draft angle is the intentional taper on walls parallel to the ejection direction so a casting releases without scuffing, sticking, or bending cores. As metal solidifies it shrinks against the tool; without taper, ejection force turns into high frictional shear, causing drag marks, die wear, and distortion.
Required draft depends on process and amplifiers such as wall height, surface condition (polished vs. textured/coated), local stiffness (ribs/bosses/pockets), and production intent (cycle time/tool life). Practical starting points (per side) are sand 1.5–3.0°, permanent-mold 1.0–2.0°, HPDC 0.5–1.5°, investment 0.5–1.5°. Add ~0.25–0.5° per 10 mm of wall height above ~30 mm; add +0.5° for internal cores/deep cavities; add +0.25–0.75° if the surface is textured or will be powder-coated.
Zero-draft walls are credible only for very short features with mirror-polished cavities, robust lubrication/ejection, or when the surface will be post-machined. In most production, the goal is the lowest draft that ejects cleanly—then document direction, magnitude, and inspection zones on the drawing.
This guide translates that into action with cross-process ranges, adjustment rules, a face-type → draft selector, CAD/drawing practices that avoid GD&T conflicts, and inspection methods to control draft from model to mass production.
How much draft angle is needed for each casting process?
Use this table as your 10-second selector. Values are per side. Start from “Common” then adjust using the rules in Sections 2–4.
| Casting Process | Smooth Visible Walls | Internal Cores / Deep Pockets | Ribs & Bosses | Textured / Blasted / Etched Surfaces | Notes (when to tighten/relax) |
|---|---|---|---|---|---|
| Sand casting (green/chem-set) | 1.5–3.0° (Common) • Min 1.0° | 2.0–5.0° | 1.5–3.0° | +0.5–1.0° extra | Larger grains and collapsible sand need more draft; high walls add more. |
| Permanent-mold / Gravity die | 1.0–2.0° • Min 0.75° | 1.5–3.0° | 1.0–2.0° | +0.5–0.75° | Metal mold, slower cooling; draft depends on wall height & release coatings. |
| High-pressure die casting (HPDC) | 0.5–1.5° • Min 0.25° (polished) | 1.0–2.5° | 0.5–1.5° | +0.25–0.5° | Ejection speed & lube quality matter; mirror finish can reduce the minimums. |
| Investment (lost-wax) casting | 0.5–1.5° • Min 0.25° | 0.5–2.0° | 0.5–1.5° | +0.25–0.5° | Pattern & shell shrinkage usually allow smaller draft; exceptions exist. |
| Lost-foam casting | 0.5–1.0° | 1.0–2.0° | 0.5–1.0° | +0.25–0.5° | Foam vaporizes—draft can be small, but geometry & burnout path still matter. |
| Plaster molds / 3D-printed sand | 1.0–2.0° | 1.5–3.0° | 1.0–2.0° | +0.25–0.75° | Fine media gives better surfaces; still add draft for reliable ejection. |
Add-on rule of thumb for wall height:
For every 10 mm of vertical height above 30 mm, add +0.25–0.5° per side (process-dependent). For very short walls (≤10 mm), you may hold the low end of each range.
How do face type, wall height, and surface finish change draft angle?
Choose the row that matches your face, then adjust for height and surface.
| Face Type | Base Draft (Per Side) | Height Adjustment | Finish/Texture Adjustment | Extra Tips |
|---|---|---|---|---|
| Outside vertical wall | Process common (see table above) | +0.25° per +10 mm above 30 mm | +0.25–0.75° if bead-blast, etched, powder coat | Powder coat builds film; add draft to keep fit/clearances. |
| Inside cavity / core | +0.5° over outside wall | +0.25–0.5° per +10 mm | +0.25–0.5° | Internal surfaces scuff first—lube and polish help but don’t replace draft. |
| Ribs & bosses | 0.5–1.5° (HPDC) • 1–3° (others) | +0.25° if rib is >3× wall height | +0.25° for texture | Draft both sides of ribs to hold thickness ratio and flow. |
| Slots/Keyways | 1.0–2.0° (HPDC 0.5–1.5°) | +0.25–0.5° if depth >20 mm | +0.25° if textured | Consider ejector access—blind deep slots stick easily. |
| Cosmetic Class-A | Start at low end of process range | +0.25° per +10 mm | Plan texture-ready draft | Align draft with viewing direction; hide parting line. |
Can you use zero-draft walls in casting—and when is it realistic?
Sometimes, but only under strict conditions. Use this traffic-light matrix:
| Condition | HPDC | Permanent-mold | Investment | Sand |
|---|---|---|---|---|
| Short feature (≤5–8 mm), mirror-polished cavity, generous lube, strong ejection | 🟢 Feasible | 🟡 Risky | 🟢 Feasible | 🔴 No |
| Deep cavity (>25–30 mm) or internal core | 🔴 | 🔴 | 🟡 | 🔴 |
| Critical tolerance + post-machining planned (skim mill) | 🟢 | 🟡 | 🟢 | 🟡 |
| Heavy texture / powder coat | 🔴 | 🔴 | 🔴 | 🔴 |
Safer alternatives:
- Add 0.5–1.0° and hide it with parting line placement or gentle blends.
- Use inserts/secondary machining for the few faces that truly must be zero-draft.
- Re-orient features so the “must-be-straight” face is not the ejection direction.
How do you model, specify, and verify draft from CAD to production?
CAD—Adding Draft Correctly
- Set a single draft datum/direction (the ejection vector). In SolidWorks/Creo/NX, use Draft on faces, not entire bodies, to avoid unintended taper.
- Apply by face type: outside walls first → internal cores → ribs/bosses → small features.
- Relate draft to parting line: if parting shifts (DFM), update draft directions consistently.
- Pattern features after draft, not before—mirrors/arrays copy the correct taper.
Drawing—How to Call It Out (and not fight GD&T)
- State “Draft angle per side unless noted” near title block.
- On each affected face chain, add:
DRAFT 1.0° PER SIDE TOWARD DATUM -Z. - Avoid contradictory GD&T: parallelism/flatness should reference the drafted datum feature or be limited to local zones.
- For textured surfaces, add finish note (e.g., Ra 1.6 μm / VDI 28 — add 0.5° draft).
Process Check—Before Cutting Steel
- DFM + Moldflow review: confirm fill/vent/ejection and that draft is adequate for the predicted ejection force and lube plan.
- Trial coupon on first shots: gauge any scuff pull, then bump draft by +0.25° where needed.
Inspection—Measuring Draft
- CMM/optical: measure angle over a defined height window (e.g., 5–35 mm from datum).
- Gage wedge / sine bar works for quick checks on ribs/short features.
- In FAI, record location, height span, measured angle, direction for each critical face.
What mistakes cause sticking, drag marks, or core damage—and how do you prevent them?
| Mistake | Real-World Effect | Prevention |
|---|---|---|
| Draft too small on deep core | Sticking, scuff lines, bent core pins, die wear | Add +0.5–1.0°, polish, improve lube/ejection pattern |
| Draft forgotten on textured face | Orange-peel, print-through, rework | Add texture-based draft increments; note on drawing |
| Only outside walls drafted | Internal pockets still seize | Draft internal features more than externals |
| Over-draft on mating features | Assembly gaps, seal line shift | Localize draft, or machine critical interfaces |
| Draft directions inconsistent after parting-line change | Geometry clash, unexpected taper | Freeze a global draft direction & re-run DFM before release |
Step by Step to Choosing Draft for Your Design
Step 1: Identify process & finish
Pick the casting process (sand/HPDC/etc.), target finish/texture, and production volume. Higher volume → prefer metal molds with lower draft and better surfaces.
Step 2: Classify faces
Tag outside walls, internal cores/pockets, ribs/bosses, slots, cosmetic Class-A zones, and machined-after surfaces.
Step 3: Start from the cross-process table
Read the row for your process and take the “Smooth visible wall” value for outside faces.
Step 4: Adjust for face type, height, and texture
Use Section 2’s table to add increments for internal, tall, or textured faces. Note any must-be-zero faces and plan secondary machining.
Step 5: Model draft in CAD (by face)
Apply draft per face toward the ejection vector, keep feature tree clean, and pattern after draft.
Step 6: Add drawing notes & GD&T
Declare “per side,” set directions, limit GD&T to local zones where needed, and add finish/texture notes with extra draft.
Step 7: Validate with supplier
Run DFM with your foundry: confirm ejection force, lube, polish, and trial shot plan. Adjust +0.25–0.5° where trial data shows scuffing.
How does draft angle affect cost, yield, and tolerance stack-ups?
- Too little draft → more sticking → longer cycle, die repair, and scrap; top cost driver in early runs.
- Too much draft → machining stock increase or fit loss on seals/latches; adds downstream cost.
- Sweet spot: the low end of each process range for short, cosmetic faces; mid-high for deep/internal features to protect tooling and OEE.
How did draft changes improve an electronics enclosure?
- Original: HPDC aluminum, 60 mm tall walls at 0.5° outside / 0.5° inside; bead-blast + powder coat.
- Issue: Ejector scuffs on internal walls; powder coat raises interference at snap lugs.
- Fix: Outside kept 0.5°; internal walls → 1.5°; ribs → 1.0°; snap-lug faces machined 0-draft post-cast.
- Results: No scuff marks, +6% OEE, seal line held with minor machining, cosmetic grade maintained.
FAQs
Q1. What’s the maximum draft before I risk assembly fit or sealing?
For box-type enclosures, staying ≤2.0–2.5° per side on mating walls usually preserves gasket and PCB fits. If larger draft is needed, machine the fit surfaces after casting.
Q2. Do internal cores need more draft than outside walls? Why?
Yes. Internal faces contact hotter, expanding metal longer and see higher ejection friction. Plan +0.5° vs outside walls of the same height/finish.
Q3. How much extra draft should I add for textures or bead-blast?
Add +0.25–0.75° per side depending on roughness (light blast +0.25°, heavy etched/powder-topcoat +0.5–0.75°).
Q4. Can investment casting truly run with no draft?
On short, accessible faces with proper pattern design and shell control—sometimes yes. Deep pockets or complex cores still benefit from 0.5–1.0°.
Q5. How do I measure draft on a short feature (≤10 mm)?
Specify a local evaluation zone (e.g., 2–8 mm from datum) and check with optical CMM or a calibrated angle gage block/wedge; report span + angle.
Q6. What is “1.5 TPI in degrees”?
TPI depends on thread geometry; if you mean taper per inch (rise/run), use deg = arctan(rise/run). For example, 1/32 in per inch ≈ 1.79°.
Are you looking for a casting & die-casting supplier to validate your draft and process?
Yongzhu Casting supports sand, permanent-mold, investment, and aluminum high-pressure die casting with DFM-first engineering.
Send us:
- STEP/3D + drawing, target process (or let us recommend), annual volume
- Surfaces that must be cosmetic / machined / zero-draft
- Finish/texture (Ra/VDI or coating), critical mates (seal lines, latch fits), and allowable tolerance stack
You’ll receive:
- A cross-process feasibility map with recommended draft per face group
- Parting-line & ejection concept, HPDC press/tonnage estimate if relevant
- A concise starter process plan and risk notes so you can choose confidently
Need help now? Share files at yongzhucasting@gmail.com and we’ll respond within one business day.
