Die casting coils, a critical internal defect in the die casting process, severely undermines the mechanical performance and reliability of castings. Unlike surface flaws like scratches or burrs, this defect hides inside castings, often only detectable through non-destructive testing (such as X-rays). Its core is the entrainment of air or gas into the molten metal during high-speed filling, which solidifies into pores or loose structures. For industries requiring high-precision parts—such as automotive powertrains or hydraulic components—die casting coils can lead to leakage, fatigue failure, or even safety accidents. This article systematically breaks down the causes of die casting coils and provides actionable solutions to help manufacturers resolve this issue.
1. Understanding Die Casting Coils: Mechanism, Characteristics, and Risks
To effectively address die casting coils, it is first necessary to clarify how they form and what harm they bring. This section uses a mechanism + characteristics + risks structure for clear comprehension.
1.1 Formation Mechanism
Die casting coils originate from the hydrodynamic behavior of molten metal during high-speed injection. When the injection punch pushes the molten metal into the mold cavity at high velocity (often 3-8 m/s), the metal’s inertia causes violent turbulence and splashing. This chaotic flow creates localized low-pressure zones in the cavity, which rapidly suck in surrounding air. As the molten metal cools and solidifies, the trapped gas cannot escape and becomes encapsulated inside the casting—forming either tiny, scattered pores (like pinholes) or larger, concentrated bubbles.
1.2 Key Characteristics
You can identify die casting coils through the following three typical signs:
- Surface clues: Dense pinhole-like bulges may appear on the casting surface, especially in areas with thick walls or complex structures.
- Section observation: When cut open, the casting shows a honeycomb-like loose structure instead of a dense metal texture.
- X-ray inspection: Irregular, cloud-like shadows are visible inside the casting, indicating the distribution and size of trapped gas.
1.3 Potential Risks
The impact of die casting coils extends far beyond material waste:
- Reduced mechanical properties: Pores weaken the casting’s compactness, lowering tensile strength by 10-30% and fatigue life by up to 50% (depending on the defect severity).
- Functional failure: For pressure-bearing parts (e.g., hydraulic valves), pores can cause leakage, making the component unable to maintain stable pressure.
- Increased production costs: Defective castings require rework or scrapping. In mass production, even a 5% defect rate can raise overall costs by 15-20%.
2. Core Causes of Die Casting Coils: Four Key Factor Groups
Die casting coils are not caused by a single error but by the combination of hydrodynamic issues, mold design flaws, material quality problems, and improper process parameters. The table below sorts out these causes and their defect-forming mechanisms for easy troubleshooting.
| Factor Group | Specific Causes | Mechanism of Defect Formation |
| Hydrodynamic Factors | 1. Excessively high injection velocity2. Unreasonable gating system design (e.g., sudden cross-sectional changes in sprue, sharp turns in runner) | 1. High velocity causes molten metal to splash against the cavity wall, forming vortex rings that trap air.2. Abrupt sprue/runner changes disrupt flow, creating turbulence and increasing air entrainment. |
| Mold Exhaust Limitations | 1. Overreliance on parting surface gaps or simple exhaust grooves2. Blocked exhaust channels (by prematurely solidified metal) in deep-cavity thin-walled parts | 1. Traditional exhaust methods cannot handle the instantaneous air pressure surge from high-speed filling, forcing air into the molten metal.2. In deep cavities, molten metal solidifies early, clogging exhaust paths and trapping gas inside. |
| Poor Melt Quality | 1. Excessive gas content in molten metal (especially aluminum-magnesium alloys)2. Use of damp raw materials or refining agents with crystalline water | 1. Alloys like aluminum-magnesium easily absorb hydrogen during melting. Undegassed melt releases hydrogen during injection, combining with entrained air to form a “double gas source.”2. Damp materials decompose into gas when heated, increasing the melt’s gas content. |
| Improper Process Parameters | 1. Wrong timing for switching from fast to slow injection (too early or too late)2. Insufficient holding time3. Too low mold temperature | 1. Early switching causes incomplete filling; late switching intensifies turbulence.2. Short holding time fails to compensate for shrinkage, expanding tiny pores into visible defects.3. Low mold temperature accelerates surface solidification, blocking internal gas from floating and escaping. |
3. Targeted Improvement Measures for Die Casting Coils
Addressing die casting coils requires a multi-faceted approach, covering process optimization, mold redesign, material control, and advanced technology adoption. The following solutions are proven effective in industrial practice.
3.1 Fine-Tune Injection Process Parameters
The injection process directly controls molten metal flow—optimizing parameters is the first line of defense against die casting coils.
- Three-stage injection curve: Adopt a “slow-fast-slow” speed profile.
- Initial stage: Low speed (1-2 m/s) to avoid splashing when metal enters the cavity.
- Middle stage: High speed (4-6 m/s) for efficient filling of the main cavity.
- Final stage: Decelerate to 1-3 m/s to transition smoothly to pressurization, suppressing turbulence.
- Segmented speed thresholds: Adjust speed based on casting geometry. Use lower speed (3-4 m/s) for thin-walled areas (to prevent splashing) and slightly higher speed (5-6 m/s) for thick parts—equipped with buffer devices to reduce impact.
- Extend pressurization and holding time: After filling, apply high pressure (1.5 times the working pressure) and hold for 2-5 seconds. This compresses existing bubbles and pushes molten metal into shrinkage gaps, reducing pore formation.
3.2 Optimize Mold Structure for Better Exhaust
A well-designed mold exhaust system can remove up to 80% of trapped air. Key improvements include:
- Efficient exhaust network: Add serpentine exhaust grooves (depth ≥ 0.1mm) at the last-filling positions of the cavity. Combine these with embedded exhaust blocks to form a graded exhaust channel, guiding gas out step by step.
- Vacuum exhaust for deep cavities: For complex thin-walled parts (e.g., mobile phone middle frames), install a forced vacuum system. Extract air from the cavity to -90kPa before injection, reducing initial gas content by over 90%.
- Improve gating system: Use inclined sprues or tangential inlets to leverage centrifugal force, separating gas from molten metal. Add buffer grooves or deflectors to guide smooth flow and avoid turbulence.
- Eliminate dead zones: Polish cavity transitions into rounded corners (radius ≥ 1mm) to prevent vortex formation in dead zones. Add overflow grooves at gas-prone areas to act as “gas collectors.”
3.3 Strictly Control Melt Quality
High-purity melt with low gas content is the foundation for avoiding die casting coils.
- Enhanced degassing: Use online rotary degassing (e.g., argon gas curtain purification) to remove hydrogen. Control the melt’s gas content below 0.15ml/100g Al—test regularly with a gas analyzer.
- Standardize raw material management:
- Dry furnace charge (especially return scrap) at 120-200°C for 4-6 hours to remove moisture.
- Select low-gas alloy ingots as the base material, avoiding ingots with surface oxidation or oil contamination.
- Clean the furnace regularly: Remove oxide residues and dross from the furnace every 8-12 hours to prevent secondary gas entrainment during melting.
3.4 Adopt Advanced Die Casting Technologies
For high-reliability parts, advanced technologies can fundamentally eliminate die casting coils:
- Vacuum die casting: After mold clamping, extract cavity air to a high vacuum (-90 to -95kPa) before injection. This is ideal for automotive powertrain parts—reducing internal porosity by over 90%.
- Semi-solid die casting: Inject partially solidified slurry (with 30-50% solid phase) instead of fully liquid metal. The slurry’s spherical primary phase reduces turbulence, blocking gas entrainment. This technology combines the density of forging with the near-net shaping of die casting.
4. Practical Case Studies: Verifying Improvement Effects
Real-world applications prove that the above measures effectively eliminate die casting coils. Here are two typical cases:
Case 1: Automotive Gearbox Housing
A major auto parts manufacturer faced severe die casting coils in its aluminum alloy gearbox housings, leading to a 12% defect rate and frequent pressure leakage failures. The solution included:
- Changing the single straight sprue to a spiral buffer sprue to reduce turbulence.
- Adding three-stage serpentine exhaust grooves and a vacuum assistance system.
- Lowering the injection speed from 6 m/s to 4.5 m/s and extending the holding time by 3 seconds.
Results: Internal porosity decreased by 82%, tensile strength increased by 15%, and the housings passed the ISO 16012 pressure seal test (no leakage at 1.2MPa for 5 minutes). The defect rate dropped to 0.8%.
Case 2: Mobile Phone Middle Frame
A consumer electronics factory struggled with surface pinholes (caused by die casting coils) in its magnesium alloy phone middle frames, with a yield rate of only 85%. The fix involved:
- Using local pressurized pin technology to compress pores in thin-walled areas.
- Adopting argon-protected die casting to reduce air contact with the melt.
- Optimizing the mold’s cooling system to slow surface solidification (allowing gas to escape).
Results: Surface pinholes were completely eliminated, and the yield rate rose to 98%.
5. Yigu Technology’s Perspective on Die Casting Coils
At Yigu Technology, we believe solving die casting coils requires a “prevention-first, data-driven” strategy—not just post-repair. Many manufacturers focus on reworking defective parts but ignore root causes like mold exhaust dead zones or unstable melt degassing. In reality, die casting coils are a “symptom” of systemic issues: they may signal outdated mold design, uncalibrated injection parameters, or inadequate raw material inspection.
We recommend manufacturers combine CAE simulation with real-time monitoring: Use CAE to predict gas entrainment risk areas before mold production, and install sensors to track injection speed, mold temperature, and cavity pressure during production. By dynamically adjusting parameters based on data, defects can be prevented early. For high-end parts, integrating vacuum die casting with semi-solid technology is the future—this combination balances efficiency and quality, helping achieve near-zero internal defects.
6. FAQ: Common Questions About Die Casting Coils
Q1: Can die casting coils be detected without destructive testing?
Yes. Non-destructive testing methods like X-ray inspection and ultrasonic testing are effective. X-rays reveal the location and size of internal pores, while ultrasonic testing detects loose structures by analyzing sound wave reflections. For mass production, automated X-ray scanning lines can quickly screen for die casting coils with a detection accuracy of over 95%.
Q2: Will reducing injection speed definitely eliminate die casting coils?
Not entirely. While excessively high speed is a main cause, too low a speed (below 2 m/s) can lead to incomplete filling or premature solidification of the molten metal. The key is to match the speed to the casting’s geometry: use lower speed for thin walls and moderate speed for thick parts, combined with a three-stage curve to avoid turbulence.
Q3: Is vacuum die casting suitable for all die casting parts?
No. Vacuum die casting is most suitable for high-reliability parts (e.g., automotive engine blocks) that require minimal internal defects. It is less cost-effective for low-value, simple parts (e.g., decorative brackets) due to higher equipment investment and production costs. For such parts, optimizing exhaust grooves and degassing processes is a more practical choice.