Die casting cold partitions (also known as cold shuts) are a prevalent and damaging surface defect that plagues metal forming processes. They occur when two or more streams of molten metal meet in the mold cavity but fail to fuse completely due to excessive cooling, leaving visible seams or even hidden cracks. This defect not only ruins the appearance of castings but also severely weakens their mechanical strength—for critical components like automotive brake calipers or hydraulic valves, cold partitions can lead to catastrophic failure, product recalls, and significant financial losses. This article systematically explores the nature of die casting cold partitions, their root causes, and a comprehensive solution framework to help manufacturers eliminate this issue and improve production quality.
1. Understanding Die Casting Cold Partitions: Definizione, Caratteristiche, and Risks
Before tackling the problem, it is essential to clearly define what die casting cold partitions are and recognize their potential impacts. Questa sezione utilizza a Struttura del punteggio totale to cover core concepts, con i termini chiave evidenziati per chiarezza.
1.1 Definizione fondamentale
Die casting cold partitions refer to a defect where molten metal, during the filling process, splits into multiple streams that cool down excessively before merging in the mold cavity. The cooled metal streams lose their fluidity and fail to form a homogeneous bond, resulting in a distinct separation line (seam) on the casting surface. Unlike minor surface scratches, cold partitions are not just cosmetic flaws—they often extend into the casting’s interior, creating weak planes that compromise structural integrity.
1.2 Caratteristiche chiave
You can identify die casting cold partitions through the following distinct traits, both visual and structural:
| Characteristic Category | Tratti specifici | Metodo di rilevamento |
| Caratteristiche della superficie | – Irregular, linear seams (often curved or zigzagged) with smooth, bordi arrotondati- Dull, matte appearance along the seam (no metallic luster)- Localized depressions or grooves adjacent to the seam | Ispezione ad occhio nudo (after surface cleaning) o lente di ingrandimento 10x; the seam is easily distinguishable from the surrounding metal |
| Structural Traits | – Incomplete fusion between metal streams (visible gap under microscopic examination)- Concentrated pores or shrinkage voids near the partition line- Reduced material density along the seam (compared to normal casting areas) | Analisi metallografica (sample sectioning and etching with 5% acido nitrico); rilevamento dei difetti ad ultrasuoni per identificare le estensioni interne della partizione |
1.3 Potential Risks
La presenza di partizioni fredde comporta rischi significativi sia per le prestazioni del getto che per le operazioni del produttore:
- Degrado delle prestazioni meccaniche: Le partizioni fredde fungono da punti di concentrazione dello stress. La resistenza alla trazione lungo la linea di partizione può diminuire del 25-40%, e la durata a fatica può essere ridotta di 50-70%. Per esempio, una staffa di sospensione automobilistica in lega di alluminio con una partizione fredda potrebbe rompersi sotto normali carichi di guida, comportando rischi per la sicurezza.
- Guasto funzionale: For pressure-bearing components (PER ESEMPIO., cilindri idraulici, iniettori di carburante), cold partitions can cause leakage. The incomplete fusion creates tiny channels that allow fluids or gases to escape, making the component unable to maintain the required pressure.
- Production Losses: Castings with cold partitions often require rework or scrapping. Nella produzione di massa, even a 5% defect rate can increase production costs by 15-20% due to material waste, labor rework, and delayed delivery.
- Reputational Damage: If cold partition defects reach the market, they can lead to product recalls. A single recall of 10,000 defective parts can cost a manufacturer millions of dollars in replacement costs, legal fees, and lost customer trust.
2. Root Causes of Die Casting Cold Partitions: A Comprehensive Analysis
Die casting cold partitions are not caused by a single factor but by a combination of failures in temperature control, design dello stampo, parametri di processo, and material selection. La tabella seguente utilizza a factor-cause-mechanism structure to identify the source of the problem, with real-world examples for practical reference.
| Causa categoria | Specific Failures | Defect Formation Mechanism | Esempio nel mondo reale |
| Temperature Control Issues | 1. Low molten metal pouring temperature (below the alloy’s liquidus temperature)2. Insufficient mold preheating (mold temperature below the recommended range)3. Excessive heat loss in the runner system (lungo, uninsulated runners) | 1. Low-temperature molten metal has high viscosity and loses fluidity quickly, failing to fuse when streams meet.2. A cold mold absorbs heat from the molten metal, causing rapid cooling of the metal stream surfaces.3. Lungo, uninsulated runners allow the molten metal to cool down before reaching the cavity, resulting in cold stream fronts. | An aluminum alloy ADC12 casting plant set the pouring temperature at 650°C (10°C below the liquidus temperature of ADC12). Cold partition defects increased from 2% A 15% within a week, and the defective castings failed tensile tests. |
| Muffa & Gating System Design Flaws | 1. Unreasonable runner layout (sharp bends, sudden cross-sectional changes)2. Improper inner gate placement (leading to chaotic metal flow and stream splitting)3. Inadequate exhaust (trapped gas prevents metal stream fusion) | 1. Sharp bends and sudden cross-sectional changes disrupt the molten metal flow, splitting it into multiple streams that cool independently.2. Poorly placed inner gates cause the metal to flow in conflicting directions, resulting in non-simultaneous filling and stream cooling.3. Trapped gas creates a barrier between metal streams, preventing them from merging even if they are still hot enough. | A zinc alloy toy manufacturer used a mold with a 90° sharp bend in the main runner. Cold partitions formed at the bend in 30% of castings. Redesigning the runner with a 15mm radius and adding an auxiliary gate reduced defects to 1.5%. |
| Process Parameter Mismatches | 1. Velocità di iniezione lenta (prolonging filling time and heat loss)2. Insufficient injection pressure (failing to push metal streams together for fusion)3. Excessive release agent application (creating a cooling barrier between metal streams) | 1. Slow injection extends the time the molten metal is in contact with the cold mold and runner walls, leading to excessive cooling of the stream fronts.2. Low injection pressure cannot overcome the resistance between cooled metal streams, preventing complete fusion.3. A thick release agent film acts as an insulator, reducing heat transfer between merging metal streams and inhibiting fusion. | An automotive parts supplier used an injection speed of 1.8 m/s for a 2mm-thick aluminum dashboard bracket. Cold partitions appeared in 22% of castings. Increasing the injection speed to 4.2 m/s and reducing release agent usage by 30% eliminated the defect. |
| Proprietà materiali & Management | 1. Poor alloy fluidity (PER ESEMPIO., low silicon content in aluminum alloys)2. Contaminated raw materials (mixed with oxides, impurità, or moisture)3. Improper return material ratio (high proportion of cold, oxidized return material) | 1. Alloys with low fluidity cool quickly and lose the ability to fuse, even if streams meet shortly after splitting.2. Oxides and impurities in the molten metal act as barriers between merging streams, preventing homogeneous bonding.3. A high proportion of cold return material lowers the overall temperature of the molten metal, increasing the risk of stream cooling before fusion. | A magnesium alloy casting plant mixed 40% unscreened return material (with oxide scales) into new ingots. Cold partition defects rose by 18%. Reducing the return material ratio to 20% and adding a 50μm ceramic filter cut defects to 3%. |
3. Systematic Prevention & Solution Strategies for Die Casting Cold Partitions
Eliminating die casting cold partitions requires a “full-process, multi-dimensional” approach that addresses temperature control, design dello stampo, ottimizzazione del processo, and material management. Questa sezione utilizza a step-by-step framework with actionable measures and measurable targets.
3.1 Fare un passo 1: Optimize Temperature Control Throughout the Production Process
Temperature is the primary factor influencing molten metal fluidity and fusion. Stabilizing temperatures at all stages is critical to preventing cold partitions:
- Molten Metal Temperature Management:
- Set the pouring temperature 10-20°C above the alloy’s liquidus temperature (PER ESEMPIO., 680-700°C for ADC12 aluminum alloy, 450-470°C for ZAMAK 5 lega di zinco).
- Usa un double-furnace system: The main furnace (higher temperature) ensures complete melting, and the holding furnace (precise temperature control) maintains the molten metal at the optimal pouring temperature. Install online infrared thermometers (accuracy ±2°C) to monitor temperature in real time and trigger alarms if deviations exceed 5°C.
- Preriscaldamento dello stampo & Temperature Maintenance:
- Preheat the mold to the recommended temperature range: 180-250°C for aluminum alloys, 120-180°C for zinc alloys, and 220-280°C for magnesium alloys. Use mold temperature controllers with zone-specific heating to ensure uniform temperature distribution (deviation ≤±10°C).
- For large molds or complex cavities, install additional heating elements in cold spots (PER ESEMPIO., cavità profonde, thin-walled sections) to prevent localized cooling of the molten metal.
- Corridore & Gate Temperature Insulation:
- Insulate the runner system with ceramic sleeves (thermal conductivity ≤0.5 W/m·K) to reduce heat loss. For long runners (length >300mm), add electric heating tapes to maintain the runner temperature at 50-80°C below the molten metal pouring temperature.
3.2 Fare un passo 2: Redesign Mold & Gating System for Smooth Metal Flow
A well-designed mold and gating system ensures that molten metal flows uniformly, avoiding stream splitting and cooling. Key improvements include:
- Runner System Optimization:
- Utilizzo streamlined runner designs with gradual cross-sectional transitions (taper angle 1-3°) and large-radius bends (radius ≥10mm) to prevent flow disruption. The cross-sectional area of the runner should decrease gradually from the main runner to the inner gate (Rapporto di riduzione 1:0.8) to maintain consistent flow velocity.
- For complex castings with multiple cavities, adopt a balanced runner layout to ensure that molten metal reaches each cavity simultaneously. Use CAE simulation software (PER ESEMPIO., MAGMA, AnyCasting) to verify flow uniformity and adjust the runner size accordingly.
- Inner Gate Design & Posizionamento:
- Position inner gates to ensure that molten metal fills the cavity in a single, continuous stream. Avoid placing gates opposite each other (which causes conflicting flows) or at the end of long, narrow sections (which increases flow resistance and cooling).
- Optimize the inner gate dimensions: The gate width should be 3-5 times the gate thickness, and the gate length should be as short as possible (≤5mm) to minimize heat loss. For thin-walled castings (<2mm), use fan-shaped inner gates to distribute the molten metal evenly.
- Exhaust System Enhancement:
- Aggiungere serpentine exhaust grooves (depth 0.1-0.15mm, width 5-8mm) at the last-filling positions of the cavity to remove trapped gas. The total cross-sectional area of the exhaust system should be at least 1/3 of the inner gate cross-sectional area to ensure effective gas evacuation.
- For deep-cavity or complex castings, utilizzo vacuum exhaust technology (vacuum degree >90kPa) to eliminate gas barriers between metal streams and promote fusion.
3.3 Fare un passo 3: Adjust Process Parameters to Promote Metal Fusion
Optimizing injection parameters ensures that molten metal streams merge before cooling excessively. Focus on the following adjustments:
| Parametro di processo | Optimization Measures | Target Value (for Aluminum Alloy ADC12) |
| Velocità di iniezione | Adopt a “two-stage speed profile”:1. Initial slow speed (1-2 SM) to fill the runner and avoid splashing.2. Fast speed (4-6 SM) to fill the cavity quickly and reduce heat loss. | Total filling time ≤2 seconds for castings with a maximum dimension of 200mm |
| Pressione di iniezione | Set the specific pressure to 80-120MPa to ensure that molten metal streams are pressed together for fusion. Increase pressure by 10-15% for complex castings with multiple flow paths. | Pressure should be maintained until the metal at the inner gate solidifies (tempo di trattenimento: 5-10 Secondi) |
| Applicazione dell'agente di rilascio | Utilizzare un basso volatile, distaccante resistente alle alte temperature (PER ESEMPIO., a base di grafite) e applicarlo in modo sottile, pellicola uniforme (spessore 5-10μm) utilizzando un sistema di spruzzatura automatico. Evitare una spruzzatura eccessiva. | L'agente distaccante dovrebbe coprire completamente la cavità dello stampo ma non formare goccioline visibili o strati spessi |
3.4 Fare un passo 4: Strict Material Control & Management
Il metallo fuso di alta qualità con una buona fluidità è essenziale per prevenire partizioni fredde. Implementare le seguenti misure di controllo dei materiali:
- Ottimizzazione della composizione delle leghe:
- Select alloys with good fluidity for die casting: Per leghe di alluminio, ensure silicon content is 11-13% (ADC12) O 7-9% (A380); for zinc alloys, use ZAMAK 5 (con 4% alluminio) per un flusso migliore. Add trace elements (PER ESEMPIO., 0.1-0.2% rare earth elements for aluminum) to improve fluidity by 15-20%.
- Conduct spectral analysis for each batch of raw materials to verify alloy composition. Reject batches with deviations exceeding ±0.5% from the standard.
- Raw Material & Return Material Management:
- Use clean, dry raw materials. Store ingots in a dry environment (relative humidity ≤60%) and preheat them to 120-150°C before melting to remove moisture.
- Materiale di ritorno del vaglio con un setaccio a maglie da 1 mm per rimuovere le scaglie di ossido, impurità, e frammenti di metallo freddo. Limitare la percentuale di materiale restituito a ≤30% (mescolato con 70% nuovi lingotti) per mantenere la qualità e la temperatura del metallo fuso.
- Raffinazione del metallo fuso:
- Affinare il metallo fuso utilizzando il degasaggio rotativo ad argon (15-20 minuti, portata di argon 2-3 l/min) per rimuovere idrogeno e ossidi. Utilizzare un filtro ceramico (50-80dimensione dei pori μm) per filtrare il metallo fuso prima della colata per eliminare le impurità solide.
4. On-Site Diagnosis & Emergency Treatment for Cold Partitions
Anche con misure preventive, Occasionalmente possono verificarsi partizioni fredde. This section provides quick-response steps to minimize production losses and restore normal operations.
4.1 Rapid Diagnosis
Follow this 3-step process to confirm the presence of cold partitions and identify the root cause:
- Ispezione visiva: Check the casting surface for linear seams with dull edges. Use a small hammer to tap the area around the seam— a dull, hollow sound indicates a cold partition (compared to a clear, resonant sound for normal metal).
- Microscopic Verification: Take a small sample from the suspected area, polish it, and etch it with 5% acido nitrico. Under a 100x microscope, a cold partition will appear as a distinct gap or incomplete fusion line between metal grains.
- Parametro & Process Review: Analyze recent production data to identify potential deviations:
- Did the molten metal temperature drop below the set range?
- Was the injection speed or pressure lower than normal?
- Did the mold temperature in the defect area fall below the target?
- Was there a change in the raw material batch or return material ratio?
4.2 Emergency Countermeasures
If cold partitions are detected, take the following immediate actions to resolve the issue:
- Temperature Adjustment: Increase the molten metal pouring temperature by 10-15°C (within the safe range) and raise the mold preheating temperature by 20-30°C. Test 10-20 samples to verify if the cold partitions are eliminated.
- Process Parameter Tweak: Increase the injection speed by 0.5-1 SM (up to the maximum safe speed for the mold) and raise the injection pressure by 10-15% to enhance metal stream fusion. Reduce the release agent application amount by 20-30% to avoid cooling barriers.
- Muffa & Runner Maintenance: Clean the runner system and mold cavity to remove residual oxide scales or cold metal fragments. For molds with sharp bends or poor exhaust, temporarily add auxiliary exhaust holes (0.5-1diametro mm) at the last-filling positions to improve gas evacuation.
- Material Adjustment: If the return material ratio is high, reduce it to 20% and add new ingots to improve molten metal fluidity. If the alloy composition is off-spec, adjust it by adding the necessary elements (PER ESEMPIO., silicon for aluminum alloys) to restore fluidity.
5. Yigu Technology’s Perspective on Die Casting Cold Partitions
Alla tecnologia Yigu, crediamo che le pareti divisorie per pressofusione non siano solo un difetto di produzione ma un riflesso di problemi sistemici nel processo di produzione. Molti produttori si concentrano esclusivamente sul trattamento dei sintomi (PER ESEMPIO., aumento della temperatura di colata) senza affrontare le cause profonde (PER ESEMPIO., progettazione dello stampo difettosa o qualità del materiale incoerente), portando a difetti ricorrenti e spreco di risorse.