Lost foam casting technology offers high dimensional accuracy, excellent surface finish, minimal machining allowances, and low cost. Castings are free of flash and burrs, reducing cleaning costs. With these advantages, it has been widely adopted globally. In our experience, domestic lost foam grey cast iron processes and equipment are relatively mature. However, driven by lightweighting demands, replacing grey cast iron components with aluminum alloy castings has become a trend. Our company has dedicated nearly a decade to攻克 lost foam aluminum alloy process technology, achieving a yield rate exceeding 98%, which is leading domestically. Our existing production line comprises imported equipment like an American full-production line, a German melting furnace, an American aluminum purification system, a German robotic pouring system, and other auxiliary units. Domestically, there are no specialized design and manufacturing suppliers for such integrated systems. According to our company’s plan, we aim to transform an idle domestic lost foam grey cast iron casting line into an aluminum alloy line with minimal investment.
The original grey cast iron line consisted of two shared lines, each including two medium-frequency melting furnaces, one semi-automatic pouring machine, one molding and box-pushing line, two vacuum systems, one sand mixing and treatment system, one bag-type dust collector for the melting furnaces, three dust removal systems for the molding line and sand treatment, and auxiliary equipment like cooling towers, compressed air filters, and casting conveying chains. The transition to aluminum alloy production, while both use lost foam casting, requires significantly different equipment. Based on our experience with an imported American aluminum alloy lost foam line and the current equipment status, we summarize the key challenges and solutions below.
Melting Equipment
Medium-frequency induction furnaces generate eddy currents in metal charge via electromagnetic induction, creating a skin effect. Heat is produced within the charge itself, leading to fast melting rates and low metal loss, making them common in aluminum melting. However, they are primarily used for aluminum alloys with higher manganese, copper, or zinc content. During melting, the “hump” phenomenon in medium-frequency furnaces can cause aluminum melt to absorb hydrogen, leading to excessive porosity in castings. Additionally, due to limitations in the original tilting mechanism, production continuity was poor, requiring transfer to pouring stations, resulting in intermittent aluminum supply and significant temperature drops. Compared to natural gas furnaces, medium-frequency furnaces consume more energy and have higher costs. After over a month of trial operation with the original medium-frequency furnaces, casting quality and production efficiency did not fully meet our requirements for aluminum alloy production. Consequently, after discussion, we decided to abandon the original medium-frequency furnaces and procure new gas-fired furnaces. The energy consumption difference can be approximated by the formula for power consumption: $$E_{\text{mf}} = P_{\text{mf}} \cdot t_{\text{mf}}$$ where $E_{\text{mf}}$ is energy consumption of medium-frequency furnace, $P_{\text{mf}}$ is power rating, and $t_{\text{mf}}$ is melting time. For gas furnaces: $$E_{\text{gas}} = V_{\text{gas}} \cdot \Delta H_{\text{gas}}$$ where $V_{\text{gas}}$ is gas volume and $\Delta H_{\text{gas}}$ is heating value. The transition from grey cast iron to aluminum alloy melting thus involves a shift from high-temperature iron melting (above 1500°C) to lower-temperature aluminum melting (around 760°C), impacting overall thermal management.
| Parameter | Grey Cast Iron (Original) | Aluminum Alloy (Post-Transformation) |
|---|---|---|
| Melting Temperature | >1500°C | ~760°C |
| Furnace Type | Medium-Frequency Induction | Natural Gas Reverberatory |
| Energy Efficiency | Lower (High Electrical Consumption) | Higher (Gas Combustion) |
| Metal Loss | Low | Controlled via Cover Fluxes |
| Continuous Operation | Intermittent (Batch Transfer) | Continuous (Integrated Melting-Holding) |
Aluminum Melt Purification System
Melt purification is unique to aluminum alloy casting, requiring all-new equipment. Two common industrial approaches were evaluated. The first involves: melting → tapping into transfer ladle → degassing unit → holding furnace for settling → secondary degassing in holding furnace → pouring. The second流程 is: melting → flowing via launder into holding furnace for refining → settling in holding furnace → online degassing unit → filter box → pouring furnace → robotic pouring. Comparing these, the first method requires multiple forklift transfers, increasing hydrogen absorption risk, with temperature drops up to 100°C. Density index tends to be higher and unstable. Pouring efficiency is low, approximately 4 minutes per piece. Additionally, due to tilting mechanism limitations in pouring furnaces, dosing accuracy is poor, reducing overall aluminum utilization—每 2 tons of melt can vary by 120-180 kg. In contrast, robotic pouring offers flexibility, enables constant-flow pouring, and provides high precision and stability. The purification efficiency can be modeled using the equation for hydrogen removal: $$C_{H} = C_{H0} \cdot e^{-k t}$$ where $C_{H}$ is hydrogen concentration after time $t$, $C_{H0}$ is initial concentration, and $k$ is degassing rate constant. This highlights the importance of integrated systems for aluminum compared to simpler grey cast iron processing.
Molding and Box-Pushing Loop
The original loop used hydraulic cylinders to push boxes, with transfer via shuttle cars. Issues included: (1) Relative movement between sand box cars caused wear on bumpers. Assuming each bumper wears 0.5 mm on average, for 38 cars, total length reduction is 19 mm. In practice, combined with car deformation and iron slag adhesion, push lengths varied, preventing fixed-point pouring for dosing furnaces or robots. (2) Our existing aluminum line’s dual-drive hydraulic cylinder pushing caused severe shaking upon stopping, leading to air entrapment or deformation in freshly poured, un-solidified castings. Post-transformation, all cars are fixed via connecting pins, eliminating gaps and relative movement, thus no wear. After forming a closed loop, hydraulic positioning devices are installed at molding, fixed-point pouring, and knockout stations, ensuring push accuracy. The drive system is upgraded from hydraulic cylinders to servo ball screws, offering better synchronization, high control precision, and minimal shaking after stopping, improving product yield. The positional accuracy $\Delta x$ can be expressed as: $$\Delta x = \frac{L_{\text{total}} – \sum_{i=1}^{n} \delta_i}{n}$$ where $L_{\text{total}}$ is total loop length, $\delta_i$ is wear per bumper, and $n$ is number of cars. For grey cast iron lines, such precision was less critical due to different pouring dynamics.
| Aspect | Original Grey Cast Iron Line | Modified Aluminum Alloy Line |
|---|---|---|
| Drive Mechanism | Hydraulic Cylinders | Servo Ball Screws |
| Car Connection | Loose, with Gaps | Fixed via Pins, No Gaps |
| Positioning Accuracy | Low (Variable Wear) | High (Hydraulic Positioning) |
| Shaking upon Stop | Significant | Minimal |
| Impact on Casting Quality | Acceptable for Grey Cast Iron | Critical for Aluminum Alloy |
Sand Treatment System
Grey cast iron is poured above 1500°C, while aluminum alloy is poured at 760°C. This large temperature difference reduces sand temperature at knockout from about 300°C to below 200°C. The original sand treatment capacity was excessive, leading to high energy consumption and overly low sand temperature, which fell outside the required process window and caused defects. After recalculation, we optimized the system by removing two fluidized bed coolers. Lost foam aluminum alloy processing, unlike lost foam grey cast iron, does not require vacuum systems; lower pouring temperatures result in more EPS residues, primarily adsorbed by molding sand. Oily coatings on sand grains affect flowability, hindering vibration compaction and reducing interstitial spaces for adsorption, causing issues like back spray. We added an intermittent online sand calciner installed upstream of the sand cooler, ensuring hot sand enters directly, saving energy. The calciner uses natural gas direct heating; at 600°C, most surface contaminants are removed via exhaust, and sand color changes from black to white. The sand treatment capacity is 30 tons/hour. Based on trials, calcination rate is designed at 3 tons/hour, 10% of total throughput, sufficient for process needs. Excessive calcination increases energy consumption and cost. The heat balance for sand cooling can be described as: $$Q_{\text{sand}} = m_{\text{sand}} \cdot c_p \cdot \Delta T$$ where $Q_{\text{sand}}$ is heat removed, $m_{\text{sand}}$ is sand mass flow rate, $c_p$ is specific heat, and $\Delta T$ is temperature drop. For grey cast iron, higher $\Delta T$ necessitates more cooling, whereas for aluminum, reduced cooling capacity is adequate.
Other Equipment
After process modification, generated fumes contain substantial unburned EPS residues, which are viscous and unsuitable for bag-type dust collectors. We selected a three-stage wet scrubbing system for this transformation. In our imported line, carbon steel pipes in the wet scrubber corroded entirely within six months, and the scrubber shell corroded within a year, rendering the system inoperable. Hence, for this改造, pipes and scrubber are made of 304 stainless steel, and circulating pumps are corrosion-resistant nylon pumps. This contrasts with grey cast iron lines, where dust primarily consists of particulate matter manageable with bag filters. The corrosion resistance requirement can be quantified via material selection criteria based on environmental factors like pH and temperature.
Conclusion
After comparing multiple suppliers, we chose a melting-holding furnace with 2 tons/hour melting efficiency and 4 tons holding capacity from a domestic manufacturer, an aluminum purification system from an American provider, and a redesigned box-pushing loop and sand calciner from a domestic engineering company. The transformed line achieves an efficiency of 25 boxes per hour, operates with 7 personnel, boasts equipment uptime over 85%, and maintains a yield rate above 98% for heavy transmission housings. This successful transition from a grey cast iron lost foam line to aluminum alloy production demonstrates how tailored equipment modifications can address material-specific challenges, leveraging insights from both grey cast iron and aluminum processes to optimize performance and cost-effectiveness. The overall transformation underscores the importance of integrating advanced melting, purification, and automation systems to meet the stringent demands of aluminum alloy lost foam casting, while learning from the foundations laid by grey cast iron technology.