Die casting cold material is a common yet destructive defect that undermines the quality, prestazione, and appearance of castings. It forms when molten metal loses too much heat during filling, leading to reduced fluidity and incomplete mold cavity filling. This defect not only causes scrap rates to spike (fino a 20% in severe cases) but also poses safety risks for critical components like automotive engine parts or aerospace structural elements. To help manufacturers identify, prevent, and resolve this issue, this article systematically breaks down the nature of die casting cold material, its root causes, and a step-by-step improvement framework—backed by practical data and industry best practices.
1. Understanding Die Casting Cold Material: Definizione, Manifestations, and Risks
Prima di risolvere il problema, it’s essential to clearly define what die casting cold material is and how it impacts production. Questa sezione utilizza a 总分结构 to cover core concepts, con i termini chiave evidenziati per chiarezza.
1.1 Definizione fondamentale
Die casting cold material refers to a defect where molten metal experiences excessive cooling (either in the melting stage, transfer process, or mold filling) before fully filling the mold cavity. This cooling reduces the metal’s fluidity, causing it to solidify prematurely or form irregular structures that fail to bond with surrounding metal. Unlike surface blemishes (PER ESEMPIO., graffi), cold material is often a “hidden threat”—it may appear as minor surface marks but hide internal flaws like shrinkage or pore clusters.
1.2 Typical Manifestations
You can recognize cold material through four observable signs, both on the surface and inside the casting:
| Observation Dimension | Tratti specifici | Metodo di rilevamento |
| Caratteristiche della superficie | – Ruvido, dull patches (no metallic luster)- Obvious flow lines or layered “striations”- Localized material shortage (small unfilled gaps) | Ispezione ad occhio nudo (dopo la sabbiatura) or 5x magnification lens |
| Difetti interni | – Concentrated shrinkage voids (0.1-0.5diametro mm)- Aggregated pores (often near cold material areas)- Unmelted solid particles (residues from incomplete melting) | X-ray inspection or metallographic analysis (incisione del campione con 5% acido nitrico) |
1.3 Key Risk Areas
Cold material is not random—it tends to form in specific parts of the casting, where heat loss is most severe:
- Thin-walled areas far from the gate: These sections have a large surface area-to-volume ratio, accelerating heat dissipation. Per esempio, a 1mm-thick aluminum shell 100mm from the gate is 3x more likely to develop cold material than a 5mm-thick section near the gate.
- Deep cavity bottoms: Molten metal takes longer to reach these areas, and heat is trapped against the mold wall—cooling occurs before full filling.
- Low-temperature mold zones: Areas without proper preheating (PER ESEMPIO., mold corners, sections near cooling water channels) act as “dissipatori di calore,” rapidly cooling the metal.
2. Root Causes of Die Casting Cold Material: A Comprehensive Analysis
Cold material forms due to a combination of failures in temperature control, design dello stampo, parametri di processo, and material management. La tabella seguente utilizza a factor-cause-mechanism structure to identify the source of the problem, with specific examples for clarity.
| Causa categoria | Specific Failures | Defect Formation Mechanism | Esempio nel mondo reale |
| Molten Metal Temperature Control | 1. Incomplete melting (alloy not heated to process temperature)2. Holding furnace power shortage (temperature drops by 20-30°C)3. Unpreheated pressure chamber (room temperature vs. required 150-200°C) | 1. Unmelted particles remain in the metal, acting as “cold cores” that reduce overall fluidity.2. Cooled metal solidifies at the front of the flow, blocking subsequent filling.3. The cold chamber absorbs heat from the molten metal, causing front-end solidification in 2-3 Secondi. | An aluminum alloy ADC12 casting plant used a faulty holding furnace—metal temperature dropped from 700°C to 650°C. Cold material defects increased from 3% A 18% In 1 settimana. |
| Design dello stampo & Condition | 1. Poor runner design (sudden cross-sectional changes, sharp bends)2. Uneven mold temperature (cooling water too close to cavity: <5mm)3. Agente distaccante eccessivo (pellicola spessa >0.1mm) | 1. Runner irregularities increase flow resistance, prolonging filling time and heat loss.2. Cold mold areas cool the metal to below its liquidus temperature, stopping flow.3. A thick release agent film acts as an insulator, preventing heat transfer from the mold to the metal (worsening cooling). | A zinc alloy toy manufacturer used a mold with a 90° sharp bend in the runner. Cold material formed at the bend in 25% of castings—redesigning to a 15mm radius reduced defects to 2%. |
| Process Parameter Mismatch | 1. Velocità di iniezione lenta (<2 m/s for aluminum)2. Incorrect pressure holding timing (too early, compressing cold metal)3. Excessive pouring volume (residual cold material accumulates in the chamber) | 1. Slow flow extends the time the metal is in contact with the cold mold, accelerating cooling.2. Early pressure compresses partially solidified metal, creating layered cold material.3. Residual cold material from previous cycles mixes with new molten metal, reducing overall temperature. | An automotive parts plant used 1.5 m/s injection speed for a 2mm-thick aluminum bracket. 30% of parts had cold material—increasing speed to 4 m/s eliminated the defect. |
| Proprietà materiali & Management | 1. Alloy composition deviation (low silicon in aluminum: <9% contro. necessario 11-13%)2. Unscreened return material (mixed with oxide scales, impurità) | 1. Low silicon reduces aluminum’s fluidity (silicon acts as a “flow enhancer”), making it more prone to cooling-induced solidification.2. Impurities and oxides act as nucleation sites for solidification, triggering premature cooling. | A magnesium alloy plant mixed 50% unscreened return material with new ingots. Cold material defects rose by 12%—reducing return material to 30% and adding a 50μm filter cut defects to 4%. |
3. Systematic Improvement Plan: From Prevention to Resolution
Eliminating cold material requires a “full-process” approach—addressing temperature control, design dello stampo, parametri di processo, and material management. Questa sezione utilizza a step-by-step framework with actionable measures and measurable targets.
3.1 Fare un passo 1: Build a Precise Temperature Control System
Temperature is the root of cold material—stabilizing it across all stages is critical. Key measures include:
- Melting Stage:
- Adopt a double-furnace process: Use a main furnace (720-750° C.) for full melting and an auxiliary furnace (680-710° C.) for precise temperature adjustment to the upper limit of the process window.
- Install online infrared thermometers (accuracy ±2°C) to monitor metal temperature in real time—trigger an alarm if it drops below the lower limit (PER ESEMPIO., 670°C per ADC12).
- Transfer & Fase di iniezione:
- Utilizzo heated transfer ladles (equipped with 5kW electric heaters) to maintain metal temperature during transport—reduce heat loss to <5° C..
- Preheat the pressure chamber to 150-200° C. (alluminio) O 180-220° C. (magnesio) using electric heating jackets—never start injection with a room-temperature chamber.
- Mold Preheating Stage:
- Impostato gradient preheating temperatures based on alloy type: Aluminum molds → 200-250°C; Magnesium molds → 220-280°C.
- Utilizzo zone-specific heating (PER ESEMPIO., install additional heaters in cold spots like deep cavities) to ensure temperature uniformity (deviation ≤±10°C).
3.2 Fare un passo 2: Optimize Mold Design for Heat Retention & Flow
A well-designed mold minimizes heat loss and ensures smooth metal flow. Focus on these improvements:
- Runner Redesign:
- Replace sudden cross-sectional changes with transizioni graduali (taper angle 1-3°) to reduce flow resistance.
- Utilizzo smooth curved runners (radius ≥10mm) instead of sharp bends—cut filling time by 30% and heat loss by 25%.
- Aggiungere buffer grooves (volume 1.2x runner volume) at the entrance of thin-walled areas to stabilize flow and prevent front-end cooling.
- Thermal Balance Adjustment:
- Embed ceramic heat-insulating inserts (conducibilità termica 0.5 W/m · k) in thin-walled or deep cavity parts—slow heat dissipation by 50%.
- Adjust cooling water channel spacing: Keep channels ≥8mm from the cavity surface (contro. common 5mm) to avoid over-cooling.
- Mold Surface Maintenance:
- Polish the cavity surface to Ra ≤0.8 μm (using diamond grinding wheels) to reduce friction and heat loss from metal-mold contact.
- Control release agent application: Usa un mist sprayer to apply a thin, pellicola uniforme (spessore 5-10μm)—avoid excessive spraying that insulates the metal.
3.3 Fare un passo 3: Dynamically Adjust Process Parameters
Process parameters must match the mold and material to avoid cold material. Key optimizations:
| Parametro | Adjustment Measures | Target Value (Aluminum ADC12) |
| Velocità di iniezione | Adottare “fast-slow-fast” three-stage speed: 1. Initial fast (4-6 SM) to reach the cavity quickly.2. Middle slow (2-3 SM) for thick areas.3. Final fast (3-5 SM) for thin walls. | Filling time ≤2 seconds for parts <200mm di lunghezza. |
| Mantenimento della pressione | Start pressure holding 0.2-0.3 Secondi after cavity filling (not earlier)—use cavity pressure sensors to trigger timing. | Trattenere la pressione: 80-120MPA; tempo di trattenimento: 70-80% of total solidification time. |
| Pouring Volume | Calculate the exact single pouring volume using the formula: Volume = (Casting Volume + Runner Volume) × 1.05 (safety factor). | Avoid residual cold material in the chamber—clean the chamber after every 50 colpi. |
3.4 Fare un passo 4: Strict Material & Ingredient Management
Poor material quality exacerbates cold material—tighten control with these steps:
- Alloy Composition Control:
- Establish an alloy database and conduct spectral analysis for each batch of raw materials—ensure silicon content in ADC12 is 11-13%, magnesium in AZ91D is 0.7-1.0%.
- Aggiungere flow-enhancing elements se necessario: Per alluminio, aggiungere 0.1-0.2% rare earth elements (cerio, lanthanum) to improve fluidity by 15-20%.
- Return Material Management:
- Screen return material with a 1mm mesh sieve to remove oxide scales and impurities.
- Limit return material proportion to ≤30% (mix with 70% nuovi lingotti)—higher ratios reduce fluidity and increase cold material risk.
- Raffinazione & Degassante:
- Utilizzo argon rotary degassing (15 minuti, 2L/min argon flow) ridurre il contenuto di idrogeno a <0.15ml/100g Al—this also removes small oxide inclusions.
- Let the molten metal stand for ≥15 minutes after refining to allow slag to float—skim off slag before pouring.
4. On-Site Diagnosis & Emergency Treatment
Anche con misure preventive, cold material may occur. This section provides quick-response steps to minimize production loss.
4.1 Rapid Diagnosis
Follow this 3-step process to confirm cold material and identify the root cause:
- Ispezione visiva: Check for rough, dull patches or flow lines—tap the area with a small hammer: a dull sound indicates internal cold material (contro. a clear ring for normal metal).
- Microscopic Check: Take a small sample from the defective area and polish it—if unmelted particles or layered structures are visible under 100x magnification, it’s confirmed cold material.
- Parameter Review: Check recent data logs:
- Did molten metal temperature drop below the process range?
- Was injection speed slower than usual?
- Did mold temperature in the defect area fall below the target?
4.2 Emergency Countermeasures
If cold material is detected, take these immediate actions to restore production:
- Temperature Adjustment: Increase molten metal temperature by 10-15° C. (PER ESEMPIO., from 680°C to 695°C for ADC12)—test 10-20 samples to verify improvement.
- Chamber Cleaning: Stop production and clean the pressure chamber with a steel brush to remove residual cold material—preheat the chamber to 200°C before restarting.
- Parameter Tweak: Increase injection speed by 0.5-1 SM (within the safe range) to reduce filling time—avoid exceeding 8 SM (which causes turbulence).
- Mold Touch-Up: Apply a small amount of high-temperature release agent (a base di grafite) to cold mold zones—this temporarily reduces heat loss until full mold maintenance is possible.
5. Yigu Technology’s Perspective on Die Casting Cold Material
Alla tecnologia Yigu, we believe cold material is not just a “production defect” ma a “system warning”—it signals failures in temperature control, design dello stampo, or process management. Many manufacturers only treat the symptom (PER ESEMPIO., increasing metal temperature) without addressing the root cause (PER ESEMPIO., a faulty holding furnace), leading to recurring defects.
Raccomandiamo un data-driven prevention strategy: Install IoT sensors to monitor temperature, injection speed, and mold condition in real time—build a “cold material risk model” that predicts defects 1-2 hours in advance. Per esempio, our system alerts operators if mold temperature in a deep cavity drops by 15°C, allowing adjustment before defects form.
We also advocate DFM (Progettazione per la produzione) reviews: Our engineers work with clients to optimize casting design (PER ESEMPIO., thickening thin-walled areas near cold spots) and mold structure (PER ESEMPIO., adding buffer grooves) before production—this cuts cold material defects by 60-70% in the first batch. By combining real-time monitoring with proactive design, cold material can be controlled to a rate of <2%.
6. Domande frequenti: Common Questions About Die Casting Cold Material
Q1: Can cold material defects be repaired, or must the casting be scrapped?
Minor cold material (surface-only, no internal voids) can be repaired via argon arc welding (use matching alloy filler, current 80-100A) followed by grinding to restore surface smoothness. Tuttavia, castings with internal cold material (restringimento, pores) or cold material in load-bearing areas should be scrapped—repairs cannot restore structural integrity, and these parts may fail under stress.
Q2: How to distinguish cold material from other similar defects like cold shuts?
Cold material and cold shuts both involve premature cooling, but they differ in three key ways: 1. Posizione: Cold material forms in thin-walled/far-from-gate areas; cold shuts form at the meeting point of two metal flows. 2. Structure: Cold material has unmelted particles or layers; cold shuts have a clear “seam” with no particles. 3. Impatto: Cold material causes internal weakness; cold shuts mainly affect surface appearance (if no internal separation).
Q3: Does cold material affect the mechanical properties of the casting?
Yes—cold material significantly reduces mechanical performance. Per esempio, an aluminum ADC12 casting with cold material has: 1. Resistenza alla trazione reduced by 20-30% (from 310MPa to 220MPa). 2. Allungamento dropped by 50% (da 3% A 1.5%). 3. Vita a fatica shortened by 60-70% (fails after 50,000 cicli vs. 150,000 cycles for normal castings). This makes cold material parts unsuitable for critical applications like automotive engine components or aerospace brackets.