Die-casting sand holes—also called porosité ou trous d'épingle—are tiny voids in die-cast parts, allant de 0,1 mm à 2 mm de diamètre. Ils apparaissent comme des piqûres d'épingle en surface, cavités sous-cutanées, ou pores internes, et peut réduire la résistance à la traction d'une pièce de 15 à 30 % (données de l'industrie). Pour les pièces critiques telles que les supports de moteur automobile ou les composants de dispositifs médicaux, sand holes even lead to scrapping rates as high as 12%. But what causes these defects? How to distinguish different types of sand holes? And what systematic solutions can eliminate them long-term? This article answers these questions with actionable, data-backed strategies.
1. Types of Die-Casting Sand Holes: Morphology & Harm
Not all sand holes are the same—different types have unique characteristics and impacts. The table below classifies common sand holes and their key details:
| Sand Hole Type | Morphological Features | Typical Location | Harm Level (1–5, 5=Severe) | Detection Method |
| Surface Dispersion Pinholes | Tiny, scattered holes (0.1-0,3mm); visible to the naked eye | Part surfaces, near parting lines | 3 (ruins aesthetics; no structural risk for non-load parts) | Inspection visuelle + magnifying glass (10×) |
| Concentrated Atmospheric Pores | Larger holes (0.5–2mm); clustered in groups | Thick-walled areas, final filling zones | 5 (causes stress concentration; leads to cracking under load) | X-ray flaw detection + essai de densité |
| Subcutaneous Needle-Like Stomata | Mince, needle-shaped voids (0.1–0.5mm); hidden under the surface | Near gates, runner connections | 4 (exposed after machining; weakens local strength) | Ultrasonic testing (UT) + sectioning inspection |
| Heat Treatment Reaming Pores | Small holes that expand (to 0.5–1mm) après traitement thermique | Heat-treated parts (par ex., T6 aluminum alloys) | 5 (renders load-bearing parts unsafe; 100% taux de rebut) | Post-heat-treatment X-ray + essai de traction |
2. Core Causes of Die-Casting Sand Holes: A 3-Dimension Analysis
Sand holes arise from failures in material preparation, conception de moule, et contrôle des processus—three interrelated links. Below is a detailed breakdown with quantitative thresholds:
UN. Material-Related Causes (30–40% of Sand Holes)
Impure or improperly processed molten metal is a top trigger:
- Excess Gas Content: Hydrogen content >0.3cc/100g Al (for aluminum alloys) causes gas to expand during cooling, forming pinholes. This often happens when melting is not protected by inert gas.
- Inclusion Contamination: Oxide slag or foreign particles (>0.1mm) in the metal block gas flow, creating voids. Common sources: mixing different alloy grades, or using ingots with oil stains/corrosion.
- Poor Raw Material Management:
- Return material reused >3 times: Increases oxide content by 20–30%, leading to inclusion-based pores.
- No preheating: Ingots cold-charged directly into the furnace create temperature gradients (>100°C), causing uneven gas release.
B. Conception de moules & Maintenance Failures (25–35% of Sand Holes)
Mold issues trap gas or disrupt metal flow:
| Mold Problem | Technical Details | Impact on Sand Holes |
| Inadequate Exhaust | Exhaust groove depth <0.1mm; blocked by carbon buildup (>0.05mm d'épaisseur) | Gas in the cavity cannot escape; forced into the metal to form pores |
| Poor Gating Design | Gate angle >60° (not 45° oblique); no buffer nest/slag collection | Metal splashes and rolls in air; creates concentrated atmospheric pores |
| Worn Mold Surfaces | Cavity roughness Ra >1.6µm; wear pits (>0.2mm de profondeur) | Metal flow is hindered; air is trapped in pits to form pinholes |
| Excessive Paint Thickness | Mold paint >8µm d'épaisseur; uneven coating | Paint burns and releases gas during casting; gas is trapped as surface pinholes |
C. Process Parameter Mismatches (30–35% of Sand Holes)
Uncontrolled injection, température, or pressure settings exacerbate sand holes:
- Injection Speed Errors: Low-speed section >0.3m/s (for aluminum alloys) causes turbulent flow—metal splits and traps air. High-speed section with sudden acceleration (>5m/s²) leads to gas entrainment.
- Temperature Imbalance:
- Mold preheating gradient >40°C (par ex., 260°C on the current surface vs. 210°C at the far end) causes local overheating and gas expansion.
- Molten metal temperature <650°C (alliages d'aluminium) leads to premature solidification—gas cannot escape before the metal sets.
- Pressurization Timing Delay: Pressurization triggered >0.2s after filling completion allows gas to expand, forming subcutaneous stomata.
3. Systematic Solutions: From Material to Process
Resolving sand holes requires a holistic approach—fixing one link alone is ineffective. Below is a step-by-step solution framework:
UN. Material Control: Purify & Standardize
| Measure | Implementation Details | Expected Outcome |
| Inert Gas Protection | Use argon/nitrogen to blanket the melt throughout melting; débit: 5–10L/min | Reduces hydrogen absorption by 40–60%; gas content ≤0.2cc/100g Al |
| Deep Degassing | Use rotating degassing rods (vitesse: 400–600rpm) + compound refiners (rare earth-based); degassing time: 15–20min | Removes 80% of oxide slag; inclusion content <0.05% |
| Raw Material Management | – New material proportion ≥70%; return material reused ≤3 times.- Preheat ingots to 300–400°C before melting.- Forbid mixing different alloy grades or contaminated ingots | Reduces inclusion-based pores by 30–40%; stabilizes melt quality |
| Standing Precipitation | Let molten metal stand in the holding furnace for ≥15min; température: 680–700°C (alliages d'aluminium) | Oxides/inclusions settle to the bottom; melt purity ≥99.9% |
B. Mold Optimization: Enhance Exhaust & Flow
- Exhaust System Upgrade:
- Install serpentine exhaust ducts (profondeur: 0.1–0,2 mm) at final filling zones; add exhaust grooves at parting surface-movable block junctions.
- Verify exhaust patency with a smoke test during trial runs: Smoke should exit smoothly without backflow.
- Clean exhaust ducts weekly to remove carbon buildup (<0.03mm thick after cleaning).
- Gating System Reconstruction:
- Adjust gate angle to 45° oblique impact cavity (reduces metal splash by 50%).
- Add buffer nests (volume: 5–10% of cavity volume) and slag collectors at cavity ends to trap cold materials/inclusions.
- Design runners with proportional cross-sections: Main channel > diversion channel > inner gate (ensures laminar flow; Reynolds number <2000).
- Mold Maintenance Strengthening:
- Polish cavity surfaces monthly to Ra ≤0.8μm; repair wear pits/cracks with laser cladding.
- Add sealing rubber strips/O-rings to insert joint surfaces (clearance ≤0.03mm) to prevent metal leakage.
- Control mold paint thickness at 5–8μm; apply uniformly with an airbrush (avoids paint-induced gas).
C. Process Regulation: Contrôle de précision
| Process Stage | Key Parameter Settings | Monitoring Method |
| Vitesse d'injection | – Low-speed section: ≤0.3m/s (fills 80% of cavity).- High-speed section: Smooth acceleration curve (≤3m/s²); speed matches part thickness (0.5–1m/s for thin walls). | Real-time speed curve monitor; deviation ≤±0.1m/s |
| Temperature Field | – Mold preheating: 220–260°C (current surface), 180–200°C (far end); gradient ≤40°C.- Molten metal temperature: 680–720°C (alliages d'aluminium); fluctuation ≤±10°C. | Infrared thermal imager + thermocouples (10 points in cavity) |
| Pressurization | – Trigger timing: 0–0.1s after filling completion.- Pression de maintien: 40–60MPa (alliages d'aluminium); holding time: 5–8s.- Pressure building time: Synchronized with metal solidification time. | Pressure sensor + radiographie (verifies no pore expansion) |
D. Auxiliary Measures: Boost Defect Prevention
- Coulée sous vide: Apply to complex thin-walled parts; ultimate vacuum degree ≥90kPa. Use a three-stage exhaust system to reduce gas content to <0.1cc/100g Al—cuts sand holes by 50–60%.
- Filtration Integration: Install ceramic foam filters (CFM) at the cross sprue front end; porosité: 10–20 PPI. Keep filter-cavity distance ≥50mm to avoid blockage—traps 90% of inclusions.
- Vibration-Assisted Casting: Mount high-frequency vibrators (50–100Hz, amplitude 0.3–0.5mm) near inner gates. Vibration breaks metal surface tension, promoting gas escape—reduces subcutaneous stomata by 30%.
4. Standardized Monitoring & Continuous Improvement
To avoid sand hole recurrence, implement strict monitoring and optimization:
UN. Production Process Control
- First-Part Inspection: Check each shift’s first part for sand holes—focus on thick-walled transitions and distal dead corners. Use 10× magnifying glass for surface pinholes; UT for subcutaneous defects.
- Parameter Recording: Log injection speed, température, and pressure for each batch. Establish a defect traceability file (link sand holes to specific parameters).
- Equipment Maintenance:
- Clean pressure chamber/punch residual chips daily (prevents impurity inclusion).
- Calibrate pressure curves monthly; maintain die casting machine hydraulic system quarterly (eliminates pressure fluctuations >±2MPa).
- Replace worn punches/cores yearly (dimensional deviation ≤±0.05mm).
B. Effect Verification & Optimisation
- Testing Methods: Use X-ray flaw detection (porosity grade ≤2 per ASTM E446) and density testing (density ≥2.65g/cm³ for aluminum alloys) to verify improvement.
- Orthogonal Testing: Optimize parameter combinations (par ex., injection speed × mold temperature × holding time) via orthogonal tests. Par exemple, a 3-factor, 3-level test can identify the optimal process window.
5. Yigu Technology’s Perspective on Die-Casting Sand Holes
Chez Yigu Technologie, we see sand holes not just as defects, but as indicators of process instability. Pour les clients automobiles, our integrated solution—argon gas protection + moulage sous pression sous vide + AI parameter control—reduced sand hole rates from 11% à <1.5% dans 2 mois. For medical device manufacturers, our rare earth-based refiners and CFM filtration cut inclusion pores by 80%, meeting ISO 13485 normes.
We’re advancing two key innovations: 1) Real-time hydrogen sensors (response time <0.1s) that alert to excess gas before casting; 2) Digital twin simulation (MAGMA software) to optimize mold exhaust/gating upfront. Our goal is to help manufacturers stabilize their process window, turning sand hole prevention into a cost-saving advantage—cutting scrap rates by 60% and boosting production efficiency by 15%.
FAQ
- Can sand holes be repaired after casting, or must defective parts be scrapped?
Minor surface pinholes (≤0.3mm) can be repaired with aluminum alloy filler (for non-load parts). Cependant, concentrated atmospheric pores (>0.5mm) or heat treatment reaming pores must be scrapped—repairing masks structural risks. We recommend fixing root causes (par ex., improving exhaust) instead of relying on post-repair.
- How much does it cost to implement a sand hole prevention system, and what’s the ROI?
A basic system (inert gas protection + filter + mold upgrade) frais \(15,000–)30,000 for a mid-sized die caster. For a facility producing 10,000 parties/jour (scrap rate reduced from 10% à 1.5%), the ROI is ~6 months—savings from reduced scrap and rework far outweigh the investment.
- Do sand hole prevention measures work for all die cast alloys?
Oui, but adjustments are needed: For magnesium alloys (flammable), use nitrogen instead of argon for protection; for copper alloys (point de fusion élevé), increase mold preheating to 280–320°C. The core logic—gas control + inclusion removal + process stability—applies universally. We tailor solutions to each alloy’s unique properties.