In the power generation industry, coal crusher screens are critical wear components subjected to severe impact, abrasion, and shear stresses during the primary crushing of bulk coal. The traditional method for manufacturing these screens involved sodium silicate-CO2 sand casting. However, this method consistently yielded products plagued with defects such as sand inclusions, blowholes, and slag inclusions, leading to high scrap rates and compromised performance in subsequent machining operations. To overcome these challenges and significantly improve production yield, we implemented the lost foam casting process as a superior alternative. This article details the comprehensive development, optimization, and results of applying the lost foam casting process to fabricate high-quality coal crusher screens.
The traditional sand casting process yielded a screen with a scrap rate of approximately 20%, primarily due to subsurface defects revealed after initial machining. In contrast, the developed lost foam casting process achieved a 100% yield. Furthermore, the production efficiency increased by 1.5 times, as more castings could be produced per mold box. The following table summarizes the key comparative metrics between the two processes.
| Process Parameter | Traditional Sand Casting | Lost Foam Casting |
|---|---|---|
| Castings per Mold | 4 | 6 |
| Production Yield | ~80% | ~100% |
| Relative Efficiency Factor | 1.0 (Baseline) | 1.5 |
| Major Defects | Blowholes, Sand Inclusions, Slag | Effectively Eliminated |
The fundamental principle of the lost foam casting process involves replacing a disposable foam pattern with molten metal. The pattern, an exact replica of the desired part, is assembled from foam sections, coated with a refractory slurry, and embedded in unbonded dry sand within a vented flask. When molten metal is poured, the foam thermally decomposes and is displaced by the advancing metal front. The process’s success hinges on carefully balancing the rate of foam degradation, gas evacuation through the coating and sand, and metal filling speed. The decomposition of expanded polystyrene (EPS) foam can be modeled kinetically. The mass loss rate is often described by an Arrhenius-type equation:
$$ -\frac{dm}{dt} = A \cdot e^{-E_a/(R T)} \cdot m^n $$
where $m$ is the mass of the foam, $A$ is the pre-exponential factor, $E_a$ is the activation energy, $R$ is the universal gas constant, $T$ is the temperature at the metal-foam interface, and $n$ is the reaction order. Controlling this decomposition through vacuum assistance and proper gating is crucial to prevent defects.
Pattern Making and Assembly
The initial and most critical step in the lost foam casting process is creating a high-fidelity foam pattern. We selected expanded polystyrene (EPS) with a density range of 0.016–0.018 g/cm³. Lower density foam lacks sufficient strength for handling and can lead to mold wall collapse or pattern distortion during sand filling and vibration. Higher density foam increases the volume of gaseous decomposition products, raising the risk of carbon pickup in the steel and the formation of gas-related defects like porosity. Patterns were meticulously cut from foam blocks using a hot-wire cutter. A shrinkage allowance of 1.02% was applied to the pattern dimensions to compensate for solidification contraction of the metal. Complex features or the gating system were assembled using a specialized white glue (PVA-based), ensuring strong, seamless joints. After assembly, patterns were dried for 24-48 hours to eliminate moisture, which could cause violent steam generation and “reverse eruption” or blowholes during pouring.
Refractory Coating Application
The coating serves multiple vital functions: it provides a barrier to prevent sand penetration (burn-on), strengthens the fragile foam pattern, allows degassing products to permeate through it, and facilitates easy shakeout. We formulated a water-based coating with the following composition, mixed in a ball mill for 4-6 hours to achieve optimal homogeneity and suspension.
| Component | Function | Weight % |
|---|---|---|
| Quartz Flour (SiO₂) | Refractory Filler | ~60-65% |
| Water | Carrier | ~30-35% |
| Cellulose | Suspension Agent | ~1-2% |
| PVA (White Glue) | Binder | ~3-5% |
The coated patterns were dried in an infrared oven at a controlled temperature of 52°C ± 3°C to achieve a uniform, crack-free dry coating thickness of approximately 0.8-1.2 mm. This thickness is critical; too thin a coating risks metal penetration, while too thick a coating can hinder gas escape and cause fold defects.
Gating System Design and Mold Compaction
This phase was paramount to solving the primary defect encountered. Our initial design employed a vertical gating system where the ingates were attached to the screen’s teeth. While this seemed straightforward, it resulted in severe “burn-on” or sand adhesion in the central mounting hole. Analysis revealed that in the vertical orientation, the dry sand in the complex, recessed area of the central hole could not be adequately compacted, leaving low-density zones that allowed molten metal to infiltrate the sand grains.
We redesigned the system to a horizontal pouring arrangement. The key modifications were:
- Horizontal Orientation: The screen pattern was laid flat, ensuring all surfaces, especially the central hole, were accessible for uniform sand compaction.
- Riser Placement: A feeding riser was strategically placed above the central hole. This served a dual purpose: providing thermal mass for directional solidification towards the riser to eliminate shrinkage porosity, and creating a metallostatic pressure head that helped keep the sand grains compacted in the critical area.
- Open Gating System: The system was designed with progressively increasing cross-sectional areas from the sprue base to the ingates to maintain a non-turbulent fill. Ingate dimensions were 45 mm x 45 mm, and the runner was 50 mm x 50 mm.
The mold filling process in the lost foam casting process must balance filling velocity $v$ with foam degradation. A simplified model for the required metal head pressure $P_m$ to overcome gas back-pressure and push degradation products through the coating is:
$$ P_m = \rho g h \geq P_{gas} + \frac{\mu L v}{\kappa} $$
where $\rho$ is metal density, $g$ is gravity, $h$ is metallostatic head, $P_{gas}$ is pressure from decomposed foam gases, $\mu$ is gas viscosity, $L$ is coating thickness, and $\kappa$ is coating permeability. The horizontal design with a riser provided a sufficient and stable $h$.
Mold compaction followed a strict procedure using a bottom-vacuum flask. After placing a base layer of dry silica sand, the cluster of six coated patterns was positioned. Sand was added in stages with intermittent vibrations. Special attention was paid to manually sturdifying the sand beneath and around the central hole of each screen using rods to eliminate any potential “bridging” or voids. The final sand fill height ensured a mold wall thickness (sand margin) of 100-200 mm around the cluster.
Melting, Pouring, and Solidification
The steel for the screens was melted in a 250 kg medium-frequency induction furnace. Charge calculation was based on the required wear-resistant steel composition (typically low-alloy Cr-Mo steel). A key practice was thorough deoxidation to minimize the source of oxide inclusions. The pouring temperature was tightly controlled between 1590°C and 1610°C. After tapping, the ladle was held for 3-5 minutes to allow slag separation and temperature homogenization.
Pouring is the most dynamic stage of the lost foam casting process. The vacuum pump was activated before pouring, maintaining a negative pressure of -0.06 to -0.04 MPa during the pour. This vacuum serves to:
- Draw decomposition gases through the coating and sand bed.
- Increase the strength of the unbonded sand mold, preventing wall movement.
- Assist in pulling the metal into the mold cavity.
The pouring speed was critical. It must match the foam degradation rate. Too slow, and the collapsing foam cavity can cause mold wall collapse. Too fast, and turbulent flow can trap liquid degradation products, leading to slag inclusions or “fold” defects. We adopted a “fill-the-sprue-quickly-then-pour-steadily” technique. After pouring, the vacuum was reduced to -0.04 to -0.03 MPa and held for 15-20 minutes to ensure complete solidification under pressure. The entire mold was then cooled in the flask for 6 hours before shakeout.
Heat Treatment for Required Properties
As-forged or as-cast microstructures are seldom optimal for wear resistance. To achieve the desired combination of hardness and toughness, the screens underwent a quenching and tempering heat treatment. The specific thermal cycle was developed based on the steel’s continuous cooling transformation (CCT) diagram and tempering response. The process can be summarized by the following schematic cycle and resulting hardness.
Heat Treatment Cycle:
1. Austenitization: Heat to $A_{c3}$ + 30-50°C (approx. 880-920°C). Hold for sufficient time based on section thickness (approx. 2 hours).
2. Quenching: Rapidly quench in agitated oil to achieve a martensitic microstructure.
3. Tempering: Reheat to a sub-critical temperature (380-420°C). Hold (approx. 3 hours), then air cool to relieve stresses and impart toughness.
The final hardness was measured at multiple locations on the screen using a Brinell hardness tester. The results consistently fell within the range of HBS 243-269, fully meeting the service requirements for impact-abrasion resistance.
Defect Analysis and Resolution
The systematic approach of the lost foam casting process allowed us to identify and eliminate the defects common in sand casting. The quantitative comparison of defects per casting is revealing.
| Casting Process | Blowholes (avg. count) | Sand Inclusions (avg. count) | Slag Inclusions (avg. count) | Burn-on / Penetration |
|---|---|---|---|---|
| Traditional Sand Casting | 3.2 | 2.8 | 3.1 | None |
| Lost Foam (Initial Vertical Pour) | 1.3 | 1.2 | 1.0 | Severe in Central Hole |
| Lost Foam (Optimized Horizontal Pour) | 0.3 | 0.2 | 0.1 | None |
Root Cause and Solution Summary:
- Burn-on in Central Hole: Cause: Inadequate sand compaction in deep recesses of vertical pattern. Solution: Switch to horizontal pouring with meticulous manual sand sturdification and the addition of a pressure-feeding riser.
- Blowholes/Gas Porosity: Cause: Entrapment of foam pyrolysis gases. Solution: Optimization of coating permeability, application of controlled vacuum (-0.06 to -0.04 MPa), and synchronization of pour speed with degradation rate.
- Slag Inclusions/Folds: Cause: Turbulent metal flow trapping liquid residue from decomposed foam. Solution: Design of an open, pressurized gating system for laminar filling and proper melt deoxidation.
Conclusion and Advantages
The implementation of the lost foam casting process for manufacturing coal crusher screens proved to be a transformative success. The process demonstrated clear, quantifiable advantages over the conventional sand casting method:
- Superior Product Quality and Yield: The optimized lost foam casting process eliminated the major casting defects, achieving a near-perfect product yield of 100%, a 25% absolute improvement over sand casting.
- Enhanced Production Efficiency: The ability to cluster six patterns per mold, compared to four in sand casting, increased throughput by a factor of 1.5. The elimination of core-making and mold assembly further streamlined production.
- Excellent Dimensional Accuracy and Surface Finish: The use of a precise foam pattern and the absence of parting lines resulted in castings with excellent dimensional consistency and a superior surface finish, reducing cleanup and machining time and cost.
- Process Simplicity and Cost-Effectiveness: The process eliminates the need for bonded sand systems and complex cores. While pattern production requires attention, the overall flask preparation is simpler and more automatable, leading to lower long-term production costs despite the initial cost of the foam pattern.
In summary, the lost foam casting process is not merely an alternative but a technologically and economically superior method for producing complex, high-integrity castings like coal crusher screens. The key to success lies in a systematic approach encompassing precise pattern engineering, optimized coating and gating design, controlled pouring parameters with vacuum assistance, and disciplined foundry practices. This case study validates the lost foam casting process as a robust solution for enhancing the manufacturing of critical industrial wear components.