Die casting product sink—also called dent O shrinkage depression—is a prevalent surface defect characterized by smooth, sunken areas (0.1–2mm deep) that form in thick-walled sections of cast parts. It not only ruins product aesthetics (rejecting 5–8% of parts in mass production) but also weakens structural integrity: sinks in pressure-bearing components (PER ESEMPIO., valvole idrauliche) can cause leaks or even catastrophic failure. Unlike other defects (PER ESEMPIO., segni di flusso), sinks stem from systemic issues in design, processo, or equipment—requiring targeted fixes rather than quick fixes. But what exactly triggers these sunken areas? How to diagnose their root causes accurately? And what long-term solutions prevent recurrence? This article answers these questions with data-driven insights and actionable strategies.
1. Types of Die Casting Product Sink: Identify Before Fixing
Not all sinks are the same—their location and severity reveal clues about their root cause. The table below classifies common sink types and their key traits:
| Sink Type | Morphological Features | Typical Occurrence Areas | Severity (1–5, 5=Critical) |
| Localized Thick-Wall Sink | Piccolo, circular depressions (diameter 2–10mm); bordi lisci | Thick-walled cores (PER ESEMPIO., engine block ribs), wall thickness transitions (10mm → 3mm) | 4 (weakens local strength; visible on functional surfaces) |
| Hot-Joint Sink | Irregular, elongated sunken areas; often connected to internal shrinkage | Intersections of multiple ribs (PER ESEMPIO., EV battery frame junctions) | 5 (indicates internal voids; unsafe for load-bearing parts) |
| Surface Layer Sink | Poco profondo, widespread depressions (profondità <0.5mm); no internal defects | Grandi superfici piatte (PER ESEMPIO., automotive cover panels) | 2 (only affects aesthetics; no structural risk) |
| Post-Cooling Sink | Appears hours/days after demolding; caused by delayed solidification | Thick-walled parts (PER ESEMPIO., heavy-duty equipment brackets) | 3 (unpredictable; requires rework) |
2. Core Causes of Die Casting Product Sink: A 4-Dimension Analysis
Sink formation follows a clear causal chain: uneven solidification → volume shrinkage → lack of metal replenishment → surface depression. Below is a breakdown of the four key triggers, with quantitative thresholds:
UN. Design Deficiencies (30–40% of Sinks)
Poor casting or mold design creates conditions for uneven cooling and shrinkage.
| Design Issue | Dettagli tecnici | Quantitative Impact |
| Severe Wall Thickness Difference | Thickness ratio >3:1 (PER ESEMPIO., 9mm vs. 3mm) creates “hot spots”—thick areas solidify 2–3× slower than thin areas. | Shrinkage volume increases by 15–20% in thick sections; 80% of these cases develop sinks. |
| Unoptimized Hot Joints | Rib intersections without heat-dissipating structures (PER ESEMPIO., 3 ribs crossing at 90°) TRAP COLLE. | Local temperature remains 50–80°C higher than surrounding areas; solidification delayed by 10–15 seconds. |
| Ineffective Sprue Systems | Inner gate located >50mm from hot joints; cross-sectional area <2× the part’s wall thickness. | Metal can’t reach shrinking areas in time—replenishment rate drops by 40–60%. |
B. Process Parameter Mismatches (25–35% of Sinks)
Incorrect injection, temperatura, or timing settings fail to compensate for shrinkage.
| Parameter Issue | Key Problem | Data Threshold |
| Low Injection Specific Pressure | Pressure too low to push molten metal into shrinking gaps. | <50MPA (leghe di alluminio); <30MPA (leghe di zinco) → 70% sink rate in thick parts. |
| Insufficient Holding Time | Mold opens before thick sections fully solidify; no time for metal replenishment. | Holding time <0.8× solidification time (PER ESEMPIO., 5s for a 10mm-thick part) → 60% post-demolding sinks. |
| Excessive Pouring Temperature | High temperature increases total shrinkage volume; gas content rises, exacerbating voids. | >720° C. (leghe di alluminio); >430° C. (leghe di zinco) → shrinkage volume increases by 12–18%. |
C. Cooling System Failures (20–25% of Sinks)
Uneven mold cooling amplifies solidification differences.
| Cooling Issue | Dettagli tecnici | Impact on Sinks |
| Unreasonable Channel Layout | Cooling channels >20mm from thick sections; no targeted cooling for hot joints. | Temperature difference between thick/thin areas >40° C.; solidification asynchronized. |
| Blocked Cooling Channels | Scale/rust buildup (spessore >1mm) reduces heat transfer efficiency by 30–40%. | Local cooling rate drops from 15°C/s to <8° C/s; thick sections develop sinks. |
| Inconsistent Cooling Water Flow | Flow rate <2L/min for critical channels; pressure fluctuations >±0.2MPa. | Cooling unevenness increases by 25%; sinks appear in low-flow areas. |
D. Operational Errors (5–10% of Sinks)
Human factors disrupt process stability.
- Premature Mold Opening: Mold opened 2–3 seconds before solidification completion (detected via thermocouples). Surface layers soften and collapse under internal shrinkage.
- Over-Spraying Release Agent: Thick agent layers (>10μm) insulate the mold surface, slowing heat dissipation in local areas.
- Incorrect Alloy Composition: High copper content (>4% in aluminum alloys) increases shrinkage rate by 10–15%; magnesium deficiency (<0.3%) reduces fluidity, hindering metal replenishment.
3. Systematic Solutions: From Design to Maintenance
Resolving sinks requires a holistic approach—fixing one link alone is ineffective. Below is a step-by-step solution framework:
UN. Ottimizzazione del design: Eliminate Sink Risks Upfront
| Optimization Measure | Implementation Details | Risultato atteso |
| Balance Wall Thickness | Limit thickness ratio to ≤2:1; use gradual transitions (pendenza 1:5) between thick/thin areas. | Hot spot formation reduced by 70%; shrinkage volume stabilized. |
| Improve Hot Joints | – Add “heat-dissipating holes” (diameter 3–5mm) at rib intersections.- Use hollow ribs (wall thickness 2–3mm) instead of solid ribs. | Local cooling speed increased by 40%; hot-joint sinks cut by 80%. |
| Redesign Sprue Systems | – Locate inner gates within 30mm of hot joints.- Increase gate cross-sectional area to 2.5× the part’s wall thickness.- Add auxiliary feeders (volume 5–10% of the hot joint) per gran parte. | Metal replenishment rate improved by 50%; sink rate drops to <5%. |
B. Process Parameter Fine-Tuning
The table below lists optimized parameters for common alloys, tailored to prevent sinks:
| Parametro | Leghe di alluminio (Camera fredda) | Leghe di zinco (Camera calda) | Monitoring Method |
| Injection Specific Pressure | 60–80MPa | 30–50MPa | Real-time pressure curve (deviation ≤±5MPa) |
| Holding Time | 1.2× solidification time (PER ESEMPIO., 12s for 10mm-thick parts) | 1.0× solidification time (PER ESEMPIO., 8s for 8mm-thick parts) | Timer linked to mold temperature sensor |
| Pouring Temperature | 680–700 ° C. | 380–400 ° C. | Digital thermocouple (±2°C accuracy) |
| Temperatura della muffa | 200–220 ° C. (sezioni spesse); 180–200 ° C. (thin sections) | 150–170 ° C. (uniform across mold) | Infrared thermal imager (temperature difference ≤±5°C) |
C. Cooling System Upgrade
- Targeted Cooling: Install profiled cooling channels (shape matches part geometry) per sezioni spesse. Per esempio, use spiral channels around 10mm-thick ribs to boost heat transfer by 35%.
- High-Pressure Cooling: Apply 0.8–1.2MPa high-pressure water to hot joints; this thickens the quench layer by 0.5–1mm, accelerating solidification.
- Regular Maintenance: Clean cooling channels every 500 cycles with descaling agents; replace corroded pipes (flow rate restored to ≥2L/min).
D. Advanced Technologies for High-Risk Parts
For critical components (PER ESEMPIO., parentesi aerospaziali), use these cutting-edge solutions:
- Local Extrusion Technology: Integrate hydraulic extrusion pins (diameter 5–10mm) nello stampo. Apply 80–120MPa pressure during the semi-solid stage (solid fraction 60–70%) to push metal into shrinkage gaps—eliminates hot-joint sinks by 95%.
- Solidification Simulation: Use MAGMA or Flow-3D software to predict shrinkage areas. Per esempio, a simulation of an EV battery frame identified a hot joint sink risk, prompting a design tweak that cut defects by 70%.
- Profiling Weight Reduction: Hollow out thick sections (PER ESEMPIO., 10mm → 5mm with internal ribs) to reduce heat accumulation. This lowers shrinkage volume by 25% pur mantenendo la forza.
4. Defect Remediation: Fix Existing Sinks
For parts with minor sinks (not critical for safety), use these repair methods:
| Sink Severity | Repair Method | Implementation Details |
| Minor (profondità <0.5mm) | Lucidatura meccanica | Use 800–1200-grit sandpaper to smooth the surface; follow with buffing (Ra <1.6µm). |
| Moderare (depth 0.5–1mm) | Filler Repair | Apply aluminum/zinc alloy putty (matching the part’s composition); cure at 80–100°C for 30 minuti. |
| Severe (profondità >1mm) | Saldatura + Lavorazione | Use TIG welding to fill the sink; machine to restore dimensions (tolleranza ± 0,1 mm). Only for non-load-bearing parts. |
5. Yigu Technology’s Perspective on Die Casting Product Sink
Alla tecnologia Yigu, we see sinks as a “design-process mismatch”—not just a surface defect. For automotive clients producing engine blocks, our integrated solution (profiled cooling channels + local extrusion) reduced hot-joint sinks from 12% A <1.2%. For EV battery frame manufacturers, our solidification simulation tool identified sink risks upfront, cutting mold rework costs by 40%.
Stiamo portando avanti due innovazioni chiave: 1) AI-driven cooling control (adjusts water flow in real time based on mold temperature data); 2) Self-heating auxiliary feeders (maintain molten metal temperature to replenish shrinkage). Our goal is to help manufacturers shift from “defect repair” to “defect prevention”—turning sink elimination into a cost-saving advantage that boosts yield rates by 15%.
Domande frequenti
- Can sinks be detected before demolding to avoid wasting materials?
Yes—use real-time monitoring tools: 1) Mold temperature sensors (alert if thick sections stay >300°C after set holding time); 2) Pressure sensors (detect pressure drops in hot joints, indicating insufficient replenishment); 3) Ultrasonic testing during solidification (identifies internal shrinkage that will become surface sinks). These tools reduce wasted parts by 60%.
- Do sinks only affect aluminum/zinc alloys, or other die casting materials too?
All die casting materials are at risk, but severity varies: Magnesium alloys (tasso di restringimento 4.5%) are more prone to sinks than zinc alloys (tasso di restringimento 2.5%). Leghe di rame (Punto di fusione elevato) require stricter cooling control—sinks often form in thick sections if mold temperature exceeds 250°C. The solutions (design balance, pressure control) apply universally, but parameters must be tailored to each alloy.
- Is it cheaper to fix sinks during design or after production?
Fixing during design is 5–10× cheaper. A design tweak (PER ESEMPIO., adjusting rib thickness) costi \(500- )1,000 but prevents \(5,000- )10,000 in post-production rework/scrap for a 10,000-part batch. We recommend investing in solidification simulation upfront—this identifies 90% of sink risks before mold manufacturing.