Quick Answer
Die casting design — successful die casting parts require specific design rules: uniform wall thickness (2–4 mm), adequate draft angles (0.5–2°), generous fillet radii, proper gate placement, and understanding of the process constraints. Parts designed without die casting DFM rules cause tooling damage, high scrap rates, and costly redesigns. This guide covers the design rules every OEM buyer should know before ordering die cast parts.
Why Die Casting Design Rules Exist
Die casting is a high-pressure, high-temperature process that stresses the tooling and the metal simultaneously. Design errors that would be minor in sand casting become serious problems in die casting:
- Unequal wall thickness: Causes uneven cooling, warpage, sink marks, and internal stress
- Insufficient draft: Causes the casting to stick in the die, damaging the tooling and the part
- Sharp corners: Create stress concentrations and die wear; promote hot tears in the casting
- Excessive thin walls: Causes misruns — the metal solidifies before filling the cavity
- Deep pockets: Causes venting problems and incomplete fills
Designing for die casting is not about limiting complexity — it is about understanding the process constraints and working within them.
Wall Thickness Design
Recommended Wall Thickness Ranges
| Part Size | Minimum Wall (mm) | Recommended (mm) | Maximum Practical (mm) | Notes |
|---|---|---|---|---|
| Small (< 50 mm) | 0.8 mm | 1.5–2.5 mm | 6 mm | HPDC can fill 0.8 mm; 1.5+ is safer |
| Medium (50–150 mm) | 1.2 mm | 2.0–3.5 mm | 10 mm | Thicker for strength |
| Large (> 150 mm) | 2.0 mm | 3.0–5.0 mm | 15 mm | Thick sections risk shrinkage |
The Uniform Wall Thickness Rule
Rule: Wall thickness should be uniform throughout the part, with graduated transitions between sections.
Why: Unequal wall thickness causes differential cooling. Thick sections cool slowly, thin sections cool quickly. When the thin section solidifies, it may still need feed metal from the thick section — but the thick section is still liquid. This causes:
- Shrinkage porosity in the heavy section
- Sink marks on the surface above heavy sections
- Warping and distortion as the part cools unevenly
- Internal stress that causes cracking in service
Maximum Wall Thickness Transitions
| Transition | Risk Level | Recommendation |
|---|---|---|
| 1.5:1 (thin:thick) | Low | Acceptable, no special measures |
| 2:1 | Moderate | Use graduated transition if possible |
| 3:1 | High | Use ribs or cored holes to reduce mass |
| 4:1 or more | Very High | Redesign; shrinkage defects likely |
Solutions for Non-Uniform Walls
- Use graduated transitions: 3 mm → 5 mm → 8 mm instead of 3 mm → 12 mm
- Add cored holes: Reduces mass in heavy sections without changing external dimensions
- Use ribs: Adds stiffness without adding weight; avoids the heavy section problem
- Split the part: Consider two smaller die castings joined by fasteners or welding
Draft Angle Requirements
Draft Angle by Surface Type
| Surface Type | Minimum Draft | Recommended Draft | Notes |
|---|---|---|---|
| External flat surfaces | 0.5° | 1–1.5° | Standard requirement |
| External curved surfaces | 0.25° | 0.5–1° | Curved surfaces release more easily |
| Internal surfaces (cores) | 1° | 1.5–2° | Cores are more fragile |
| Deep sections (> 75 mm) | 1.5° | 2–3° | Deeper = more drag |
| Appearance surfaces | 1° | 1.5–2° | Extra draft for quality surface |
Draft Angle Impact on Part Design
Draft angle has two effects:
- Part dimension: Each degree of draft adds to the dimension perpendicular to the parting line
- Wall thickness: Draft adds material at the parting line end of the wall
Example: A 100 mm tall part with 1° draft on both sides has an actual dimension of ~103.5 mm (100 mm + 2 × 1.75 mm from the 1° taper on each side). The parting line must be designed to account for this taper.
Fillet Radii
Why Fillets Matter
Sharp internal corners cause two problems:
- Hot spots: Metal pools at sharp corners, cooling slowly and causing shrinkage defects
- Stress concentration: Sharp corners create stress concentrations that reduce fatigue life
- Die wear: Sharp corners in the die cavity wear faster and crack
Minimum Fillet Radii
| Corner Type | Minimum Radius | Recommended Radius | Why |
|---|---|---|---|
| Internal corners (wall junctions) | 1–2 mm | ≥ wall thickness | Eliminates hot spots and stress concentrations |
| Junction of 3+ walls | 3–5 mm | 5–10 mm | Highest stress concentration point |
| External corners | 0.5 mm | 1–2 mm | Sharp edges are possible but reduce die life | ≥ boss diameter × 0.3 | ≥ boss diameter × 0.5 | Prevents sink marks on boss face |
Gate Design and Location
What Is a Gate?
The gate is the opening where molten metal enters the cavity. Gate design affects:
- Filling pattern and speed
- Surface quality at the gate area
- Gate removal and finishing
- Weld line location
- Porosity risk
Gate Placement Principles
- Place gate away from appearance surfaces: Gate area has rougher surface (Ra 3.2–6.3 µm) and requires removal
- Gate at the thickest section: Thick sections need the most feed metal; gate there first
- Minimize flow distance: Gate so metal travels the shortest path to the furthest point
- Avoid weld lines on critical surfaces: Two metal streams meeting create a weld line (cold shut risk)
- Single gate preferred: Multiple gates create multiple weld lines and increase porosity risk
Gate Removal and Finishing
- Gate must be cut off after casting; specify gate location where removal is easy
- Gate area will have a rough surface — machine or finish if appearance matters
- Specify gate removal method: saw, bandsaw, or machining
Bosses and Ribs
Boss Design
Bosses (cylindrical protrusions for fasteners) require specific design to avoid sink marks:
- Boss diameter: Minimum 4–5 mm for M3–M4 fasteners; larger for heavier loads
- Boss height: Maximum 2× diameter; taller bosses risk collapse
- Boss wall thickness: 1.5–2× nominal wall thickness; thicker boss walls need gussets
- Boss fillet: Large radius at boss base (≥0.5× boss diameter)
Rib Design
Ribs add stiffness without adding weight — but improper rib design causes sink marks:
- Rib thickness: 0.5–0.7× nominal wall thickness (60% rule)
- Rib height: Maximum 3× rib thickness
- Rib-to-wall junction: Generous fillet radius (≥0.5× rib thickness)
- Rib spacing: Minimum 4× wall thickness between ribs
Undercuts and Core Pulls
When Undercuts Occur
An undercut is a feature that prevents the part from releasing from the die without a side action (core pull or slide):
- Bores perpendicular to the parting line
- Internal undercuts
- External undercuts
Solutions for Undercuts
- Core pull (side action): A mechanism in the die that pulls the core out before the die opens. Adds significant tooling cost ($2,000–$10,000 per core pull).
- Ceramic cores: For investment casting; cores are burned out after casting
- Redesign to eliminate: Often the best solution — redesign the part to avoid the undercut
- Machine after casting: Cast the part without the undercut, then machine the feature
Real Design Examples
Example 1: Bracket with Ribs
Problem: 8 mm thick flat plate with insufficient stiffness.
Solution: Reduce plate to 3 mm, add ribs at 25 mm spacing and 2 mm thick (0.67× wall). Result: same stiffness, 40% less material, no sink marks.
Example 2: Boss with Sink Mark
Problem: 6 mm boss on 3 mm wall causes sink mark visible on the outer face.
Solution: Reduce boss wall to 4.5 mm (1.5× nominal), add gussets from boss base to wall, increase fillet radius at junction. Result: no sink marks, boss remains functional.
Example 3: Deep Pocket
Problem: 80 mm deep pocket requires venting that die casting cannot provide.
Solution: Split the design into two die castings — top half and bottom half — joined by fasteners. Each half is shallow enough for proper venting. Alternatively, redesign to machine the pocket after casting.
Before finalizing the sourcing decision, many OEM buyers also compare Die Casting Process, Casting Tolerances, HPDC vs Gravity Die, and Surface Finish to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
Buyer Checklist
- Is wall thickness uniform (max 2:1 transition, graduated preferred)?
- Are draft angles specified (0.5–2° depending on surface type)?
- Are fillet radii specified on all internal corners?
- Are ribs and bosses designed per the 60% rule?
- Has gate location been considered and marked on the drawing?
- Are undercuts necessary, or can they be redesigned out?
- Has a die casting DFM review been completed with the foundry?
- Have parting line and draft been verified on the 3D model?
FAQ
Can die casting produce undercuts?
Yes — using core pulls (side actions) or slides in the die. However, each core pull adds $2,000–$10,000 to tooling cost and increases cycle time. Consider whether the part can be redesigned to eliminate the undercut, or whether machining after casting is more cost-effective.
What is the minimum hole size in die casting?
Through holes: minimum 2 mm diameter for small parts, 3–4 mm for larger parts. Blind holes: minimum 3–4 mm diameter. Smaller holes can be cast but require very tight process control. Machining holes after casting is often more reliable and cost-effective for small diameters.
How does draft angle affect the finished dimension?
Each degree of draft adds to the dimension perpendicular to the parting line. For a 50 mm tall part with 1° draft on both sides, the dimension at the parting line is 50 + 2 × (50 × tan(1°)) = 50 + 1.75 = 51.75 mm. Account for this in your dimensional requirements.