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, falla de fatiga, 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: Mecanismo, Características, and Risks
To effectively address die casting coils, it is first necessary to clarify how they form and what harm they bring. Esta sección utiliza un 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 (a menudo 3-8 EM), 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 Características clave
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.
- inspección por rayos x: 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).
- Fallo funcional: Para piezas que soportan presión (P.EJ., válvulas hidráulicas), los poros pueden causar fugas, haciendo que el componente no pueda mantener una presión estable.
- Aumento de los costos de producción.: Las piezas de fundición defectuosas requieren reelaboración o desguace.. En producción en masa, incluso un 5% La tasa de defectos puede aumentar los costos generales al 15-20%.
2. Core Causes of Die Casting Coils: Four Key Factor Groups
Las bobinas de fundición a presión no son causadas por un solo error sino por la combinación de problemas hidrodinámicos., defectos de diseño del molde, problemas de calidad de materiales, y parámetros de proceso inadecuados. La siguiente tabla clasifica estas causas y sus mecanismos de formación de defectos para facilitar la resolución de problemas..
Grupo de factores | Causas específicas | Mecanismo de formación de defectos |
Hydrodynamic Factors | 1. Excessively high injection velocity2. Unreasonable gating system design (P.EJ., 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 (por metal solidificado prematuramente) en piezas de paredes delgadas con cavidades profundas | 1. Los métodos de escape tradicionales no pueden soportar el aumento instantáneo de presión del aire debido al llenado a alta velocidad., forzando el ingreso de aire al metal fundido.2. En cavidades profundas, El metal fundido se solidifica temprano., obstruyendo las vías de escape y atrapando el gas en el interior. |
Mala calidad de fusión | 1. Contenido excesivo de gas en metal fundido. (especialmente aleaciones de aluminio y magnesio)2. Uso de materias primas húmedas o agentes refinadores con agua cristalina. | 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. El corto tiempo de retención no compensa la contracción, expandiendo poros diminutos hasta convertirlos en defectos visibles.3. La baja temperatura del molde acelera la solidificación de la superficie, Bloquear el gas interno para que no flote y escape.. |
3. Targeted Improvement Measures for Die Casting Coils
Abordar las bobinas de fundición a presión requiere un enfoque multifacético, cubriendo la optimización de procesos, rediseño del molde, control de materiales, y adopción de tecnología avanzada. Las siguientes soluciones han demostrado ser eficaces en la práctica industrial..
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 “lento-rápido-lento” speed profile.
- Initial stage: Baja velocidad (1-2 EM) to avoid splashing when metal enters the cavity.
- Middle stage: Alta velocidad (4-6 EM) 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. Usar velocidad más baja (3-4 EM) for thin-walled areas (to prevent splashing) and slightly higher speed (5-6 EM) for thick parts—equipped with buffer devices to reduce impact.
- Extend pressurization and holding time: Después del llenado, apply high pressure (1.5 times the working pressure) and hold for 2-5 artículos de segunda clase. 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% de aire atrapado. 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 (P.EJ., 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 (P.EJ., 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 horas para eliminar la humedad.
- 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:
- Casting de vacío: After mold clamping, extract cavity air to a high vacuum (-90 to -95kPa) antes de la inyección. Esto es ideal para piezas de sistemas de propulsión de automóviles, ya que reduce la porosidad interna en más de 90%.
- Fundición a presión semisólida: Inyectar lodo parcialmente solidificado (con 30-50% fase sólida) en lugar de metal completamente líquido. La fase primaria esférica del lodo reduce la turbulencia, bloquear el arrastre de gas. Esta tecnología combina la densidad de la forja con la conformación casi neta de la fundición a presión..
4. Practical Case Studies: Verifying Improvement Effects
Las aplicaciones del mundo real demuestran que las medidas anteriores eliminan eficazmente las bobinas de fundición a presión.. He aquí dos casos típicos.:
Caso 1: Automotive Gearbox Housing
A major auto parts manufacturer faced severe die casting coils in its aluminum alloy gearbox housings, llevando a un 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 en 4.5 m/s and extending the holding time by 3 artículos de segunda clase.
Resultados: 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 minutos). The defect rate dropped to 0.8%.
Caso 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).
Resultados: Surface pinholes were completely eliminated, and the yield rate rose to 98%.
5. Yigu Technology’s Perspective on Die Casting Coils
En la tecnología yigu, 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 con monitoreo en tiempo real: Use CAE to predict gas entrainment risk areas before mold production, and install sensors to track injection speed, temperatura del molde, 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. Preguntas frecuentes: Common Questions About Die Casting Coils
Q1: Can die casting coils be detected without destructive testing?
Sí. Non-destructive testing methods like X-ray inspection and ultrasonic testing are effective. Las radiografías revelan la ubicación y el tamaño de los poros internos., mientras que las pruebas ultrasónicas detectan estructuras sueltas analizando los reflejos de las ondas sonoras.. Para la producción en masa, Las líneas automatizadas de escaneo de rayos X pueden detectar rápidamente bobinas de fundición a presión con una precisión de detección de más de 95%.
Q2: Will reducing injection speed definitely eliminate die casting coils?
No del todo. Si bien la velocidad excesivamente alta es la causa principal, una velocidad demasiado baja (abajo 2 EM) puede provocar un llenado incompleto o una solidificación prematura del metal fundido. La clave es hacer coincidir la velocidad con la geometría de la pieza fundida.: use una velocidad más baja para paredes delgadas y una velocidad moderada para partes gruesas, combinado con una curva de tres etapas para evitar turbulencias.
Q3: Is vacuum die casting suitable for all die casting parts?
No. La fundición a presión al vacío es más adecuada para piezas de alta confiabilidad. (P.EJ., Bloques de motor automotriz) que requieren defectos internos mínimos. Es menos rentable para productos de bajo valor., piezas simples (P.EJ., corchetes) debido a una mayor inversión en equipos y costos de producción. Para tales partes, optimizar las ranuras de escape y los procesos de desgasificación es una opción más práctica.