The integration of Additive Manufacturing (AM) into traditional foundry practices has revolutionized rapid prototyping and low-volume production. Among various AM techniques, Fused Deposition Modeling (FDM) stands out for its relatively low cost, material versatility, and ease of operation, making it a promising candidate for generating sacrificial patterns for the lost wax investment casting process. This study delves into the optimization of a rapid investment casting workflow specifically for gas turbine blade production using FDM-fabricated patterns. The primary challenges addressed include shell cracking due to the thermal expansion and flow of melted pattern material, as well as ensuring superior surface finish and dimensional accuracy in the final metallic casting.
The core of the lost wax investment casting process involves creating a precise sacrificial pattern, building a ceramic shell around it, removing the pattern (dewaxing), and subsequently pouring molten metal into the resulting cavity. When employing FDM patterns, typically made from thermoplastics like Acrylonitrile Butadiene Styrene (ABS), the dewaxing stage is replaced by a thermal removal process. The intense heat applied to burn out the plastic pattern can cause significant gaseous pressure and volumetric expansion, posing a high risk of fracturing the delicate ceramic shell, thereby ruining the mold. This research systematically investigates and optimizes each step—from pattern design and shell building to pattern removal and metal pouring—to mitigate these issues and achieve high-integrity castings.
This investigation is structured around a comparative experiment. Two types of blade patterns were fabricated using an FDM printer: a solid pattern (Pattern A) and a hollow, sparse-fill pattern (Pattern B). Both patterns were subjected to the same optimized lost wax investment casting process chain. The resulting steel castings were then rigorously evaluated for surface quality, internal integrity, and most critically, dimensional fidelity to the original digital model. The findings demonstrate a clear pathway for reliable and precise manufacturing of complex components like turbine blades through this hybrid approach.
Material and Method: Pattern Fabrication and Process Optimization
The blade geometry was based on a representative gas turbine airfoil design. A Stratasys Uprint SE FDM system was utilized for pattern fabrication. For this study, ABS was selected as the pattern material due to its common use in FDM and its well-defined thermal properties relevant to burnout. Two distinct printing strategies were employed:
- Pattern A (Solid): Printed using the printer’s “solid” mode, resulting in a fully dense ABS model.
- Pattern B (Hollow/Sparse): Printed using the “sparse” mode. This mode creates an external shell with a defined wall thickness (set to 3.0 mm for this experiment) and fills the internal volume with a lightweight, honeycomb-like support structure made of the same ABS material, rather than leaving it completely empty. This approach conserves material and printing time while maintaining sufficient handling strength.
Both patterns were printed with a layer thickness of 0.254 mm. Key printing parameters are summarized below.
| Parameter | Pattern A (Solid) | Pattern B (Hollow) |
|---|---|---|
| Print Mode | Solid / High Density | Sparse / Low Density |
| Wall Thickness | Fully Solid | 3.0 mm |
| Internal Structure | Solid ABS | Honeycomb ABS Supports |
| Material Usage | High | Moderate |
| Primary Aim | Baseline for strength | Reduce mass & material |
Following printing and light post-processing (deburring sharp edges), the patterns were assembled onto a wax gating system. The design of this gating system is crucial for successful lost wax investment casting. An optimized stepped gating system was designed and fabricated in wax. The system featured a reinforced pouring cup, a central downsprue, and carefully positioned ingates and feeders at the blade’s thermal hotspots to ensure proper metal feeding and shrinkage compensation during solidification. Computational casting simulation was performed to validate the fluid flow and thermal behavior of the design prior to physical execution.
Ceramic Shell Building: A Multi-Layer Approach
A robust ceramic shell is the cornerstone of withstanding the thermal and mechanical stresses of the burnout and pouring stages. To counteract the potential pressures from decomposing ABS patterns, the conventional shell-building process was enhanced. The number of ceramic layers was increased, and the slurry/sand compositions were tailored for higher green and fired strength. The detailed shell build-up sequence is presented in the following table.
| Layer | Slurry Binder | Primary Refractory (Slurry) | Stucco Sand | Slurry Viscosity (s) | Drying Time (h) |
|---|---|---|---|---|---|
| 1st (Prime) | Silica Sol | Zircon Flour (320#) | Zircon Sand (70-140 mesh) | 32 ± 2 | 8 |
| 2nd | Silica Sol | Chamotte Flour (200#) | Chamotte Sand (30-60 mesh) | 24 – 30 | 6-8 |
| 3rd | Silica Sol | Chamotte Flour (200#) | Chamotte Sand (16-30 mesh) | 28 – 32 | 8-10 |
| 4th | Silica Sol | Chamotte Flour (200#) | Chamotte Sand (16-30 mesh) | 28 – 32 | 12 |
| 5th | Silica Sol | Chamotte Flour (200#) | Chamotte Sand (16-30 mesh) | 28 – 32 | 12 |
| 6th | Silica Sol | Chamotte Flour (200#) | Chamotte Sand (16-30 mesh) | 17 – 20 | 12 |
| Seal Coat | Silica Sol | Chamotte Flour (200#) | — | 17 – 20 | 13 |
This multi-layer protocol ensured a shell with excellent permeability for gas escape during burnout, coupled with high mechanical strength to resist cracking.
Pattern Removal: A Critical Two-Stage Thermal Process
The removal of the FDM-ABS pattern is the most critical phase where shell failure is likely. A two-stage thermal process was implemented to manage the phase change and decomposition of the ABS plastic safely.
Stage 1: Low-Temperature Wax Removal. The assembled cluster (wax gates + ABS patterns) was first subjected to autoclave dewaxing. The wax gating system was melted out using pressurized steam at approximately 168°C for 20 minutes. The autoclave’s pressure (capped at 0.8 MPa) helps counteract the internal pressure from melting wax, preventing shell damage at this initial stage. This step removes the majority of the sacrificial material (the wax gates) while leaving the more resilient ABS patterns intact within a now-partially hollow shell.
Stage 2: Controlled ABS Burnout. Subsequently, the shell containing only the ABS blade patterns was transferred to a furnace for high-temperature burnout. To prevent sudden gas pressure build-up from rapid ABS decomposition, a carefully controlled, slow heating ramp was employed. The heating rate was optimized to allow the ABS to soften, melt, vaporize, and ultimately combust at a controlled rate, with the gaseous products escaping freely through the permeable shell and the open ingate channels. The complete thermal cycle ensured minimal residual carbon ash within the cavity. The fundamental thermal behavior during this stage can be related to the polymer’s properties. The rate of gas generation $\( \dot{V}_{gas} \)$ is a function of the decomposition kinetics of ABS:
$$
\dot{V}_{gas} = A \cdot e^{-E_a/(R T)} \cdot m_{ABS}
$$
where $\( A \)$ is the pre-exponential factor, $\( E_a \)$ is the activation energy for decomposition, $\( R \)$ is the gas constant, $\( T \)$ is the absolute temperature, and $\( m_{ABS} \)$ is the mass of ABS. A slow ramp rate controls $\( dT/dt \)$ , thereby managing $\( \dot{V}_{gas} \)$ to stay below the permeability limit of the shell, avoiding fracture.
Following the burnout cycle, the ceramic shells were preheated to an appropriate temperature for casting. The molten metal, a heat-resistant cast steel (A297HK), was poured into the prepared shells. The chemical composition and key mechanical properties of the casting alloy are listed below.
| Element | Composition (wt. %) | |
|---|---|---|
| Carbon (C) | 0.20 – 0.60 | Mechanical Properties: Ultimate Tensile Strength: ≥ 450 MPa Yield Strength: ≥ 240 MPa Elongation: ~2.0 – 2.5% |
| Manganese (Mn) | ≤ 2.0 | |
| Silicon (Si) | ≤ 2.0 | |
| Phosphorus (P) | ≤ 0.04 | |
| Sulfur (S) | ≤ 0.04 | |
| Chromium (Cr) | 24.0 – 28.0 | |
| Nickel (Ni) | 18.0 – 22.0 | |
| Molybdenum (Mo) | ≤ 0.50 | |
Results and Analysis: Quality and Dimensional Assessment
After cooling, the ceramic shells were knocked off, and the castings were cut from the gating system and subjected to standard post-casting processes like shot blasting.
Surface and Internal Quality Inspection
Visual and non-destructive testing (NDT) was conducted on both Casting A (from solid pattern) and Casting B (from hollow pattern).
- Surface Quality (Magnetic Particle Inspection): Both castings showed no indications of surface cracks or major surface defects like cold shuts or massive slag inclusions. The shells had successfully survived the burnout and pouring processes without visible cracking.
- Internal Quality (Radiographic Inspection): X-ray inspection revealed the internal soundness of the castings. No gross porosity or shrinkage cavities were detected in either part, confirming the effectiveness of the optimized gating and feeding design for this geometry and alloy. However, both castings exhibited some finely scattered non-metallic inclusions, classified at a level comparable to B4 according to reference standards like ASTM E466. These are likely remnant oxides or very fine slag from the melting process or possibly minute residues from the ABS decomposition, though the latter’s contribution is difficult to isolate. This indicates an area for further process refinement in the metal handling and pouring aspect of the lost wax investment casting sequence.
Dimensional Accuracy: A Critical Comparison
The most significant finding of this study pertains to dimensional accuracy. The cast blades were digitized using a non-contact 3D optical scanning system to obtain dense point clouds of their surfaces. These point clouds were then aligned and compared to the original CAD geometry (the nominal model) to compute a comprehensive deviation map.
The results were striking:
| Dimensional Metric | Casting A (From Solid Pattern) | Casting B (From Hollow Pattern) |
|---|---|---|
| Maximum Positive Deviation (+) | ~ +4.7 mm (on feeding system) | ~ +3.0 mm (on feeding system) |
| Maximum Negative Deviation (-) | ~ -4.7 mm | ~ -3.0 mm |
| Standard Deviation of Errors | 0.846 mm | 0.750 mm |
| Mean Positive Error | +0.643 mm | +0.449 mm |
| Mean Negative Error | -0.730 mm | -0.521 mm |
| Aerofoil Profile Deviation | Within ±0.28 mm | Within ±0.15 mm |
Casting B, produced from the hollow FDM pattern, demonstrated superior dimensional accuracy across all metrics compared to Casting A from the solid pattern. The aerofoil contour of Casting B was particularly precise, falling within a ±0.15 mm band, which is a highly acceptable result for many investment casting applications and minimizes final machining stock.
Discussion: The Mechanics of Pattern Expansion
The root cause of the dimensional discrepancy lies in the thermo-mechanical behavior of the ABS pattern during the burnout stage of the lost wax investment casting process. When heated, the ABS undergoes a sequence of physical changes: thermal expansion, followed by glass transition, softening, melting, decomposition, and finally combustion.
The critical initial phase is thermal expansion. The volumetric thermal expansion $\( \Delta V \)$ of the pattern material before it softens can be estimated as:
$$
\Delta V = V_0 \cdot \beta \cdot \Delta T
$$
where $\( V_0 \)$ is the initial volume of ABS, $\( \beta \)$ is the volumetric coefficient of thermal expansion for ABS, and $\( \Delta T \)$ is the temperature increase from room temperature to the softening point.
For Pattern A (Solid), $\( V_0^{A} \)$ is very large because the pattern is fully dense. Even with a moderate $\( \beta \)$ , the resulting $\( \Delta V^{A} \)$ is substantial. This expanding volume exerts significant pressure $\( P \)$ on the constraining ceramic shell wall before the material can soften and flow out through the ingates. According to a simplified mechanical model, the induced stress $\( \sigma \)$ in the shell is proportional to this pressure. This stress can cause microscopic or even macroscopic yielding of the shell, permanently distorting the internal cavity dimensions (a phenomenon akin to core shift in sand casting). The final casting metal solidifies in this slightly enlarged cavity, leading to larger positive deviations from the nominal size.
For Pattern B (Hollow/Sparse), the effective $\( V_0^{B} \)$ of solid ABS material is much smaller due to the internal honeycomb structure. Consequently, $\( \Delta V^{B} \)$ is significantly reduced. More importantly, the internal voids within the honeycomb structure provide “free space” into which the expanding material can initially deform with minimal resistance. This drastically reduces the pressure $\( P \)$ exerted on the ceramic shell during the critical heating phase. Therefore, the shell cavity experiences far less distortion, preserving dimensions closer to the original pattern geometry. This fundamental advantage of the hollow pattern design is the key to achieving higher precision in the lost wax investment casting process when using FDM patterns.
The entire optimized rapid investment casting workflow, highlighting the critical comparison, is summarized in the following high-level process flow:
$$
\text{CAD Model} \xrightarrow[\text{Sparse/Hollow}]{\text{FDM Printing (Solid / })} \text{ABS Pattern} \xrightarrow{\text{Assembly to Wax Gating}} \text{Cluster}
$$
$$
\text{Cluster} \xrightarrow{\text{Multi-layer Ceramic Shelling}} \text{Green Mold} \xrightarrow{\text{Stage 1: Autoclave Dewaxing (Wax)}} \text{Mold with ABS}
$$
$$
\text{Mold with ABS} \xrightarrow{\text{Stage 2: Controlled Furnace Burnout (ABS)}} \text{Fired Ceramic Mold} \xrightarrow{\text{Metal Pouring \& Solidification}} \text{Raw Casting}
$$
$$
\text{Raw Casting} \xrightarrow{\text{Knock-off, Cut-off, Cleaning}} \text{Final Blade Casting} \xrightarrow{\text{3D Scanning \& Analysis}} \textbf{Result: Hollow Pattern → Superior Dimensional Accuracy}
$$
Conclusion and Future Perspectives
This study successfully developed and demonstrated an optimized rapid investment casting process chain for manufacturing gas turbine blades using FDM-generated ABS patterns. The principal challenge of shell cracking was mitigated through a multi-faceted approach: an enhanced ceramic shell build-up with more layers and tailored materials, and a critically controlled two-stage thermal pattern removal process. The comparative experiment between solid and hollow patterns revealed a decisive conclusion: utilizing hollow (sparse-fill) FDM patterns is vastly superior for dimensional accuracy. The internal structure of the hollow pattern accommodates the initial thermal expansion of the ABS, minimizing distorting forces on the ceramic mold and thereby yielding castings that are significantly closer to the intended design dimensions. This FDM-based rapid investment casting method provides a viable, efficient, and cost-effective pathway for prototyping and small-batch production of complex, high-value components like turbine blades.
Future work can focus on several promising avenues to push the boundaries of this hybrid manufacturing technique further:
- Advanced Pattern Design for Burnout: Incorporating deliberate, small venting channels or “pressure relief” features into the hollow pattern design, strategically sealed with easily removable wax plugs after shelling, could further facilitate the escape of decomposition gases, potentially allowing for faster burnout cycles without compromising accuracy or shell integrity in the lost wax investment casting process.
- Material Science Integration: Investigating alternative FDM filaments specifically engineered for investment casting. These could include:
- Low-Expansion Composites: Filaments filled with ceramic or other low-expansion particles to reduce $\( \beta \)$ .
- Combustible/Fugitive Catalysts: Materials designed to burn out more cleanly, completely, and at lower temperatures, leaving minimal residue. The quest for the ideal sacrificial material for rapid lost wax investment casting continues.
- Process-Compensated Modeling: Developing a predictive numerical model that accounts for the multi-physics phenomena of shell heating, pattern expansion/flow, and ceramic deformation. This model could be used to pre-distort the original CAD model (applying a negative offset based on predicted expansion) before printing the FDM pattern, thereby achieving net-shape or near-net-shape castings after all process-induced distortions. The compensation factor $\( C_f \)$ could be a function of pattern volume, wall thickness, and orientation:
$$
\text{CAD}_{\text{compensated}} = \text{CAD}_{\text{nominal}} – C_f(V, t, \theta)
$$This represents the ultimate goal for precision manufacturing via this rapid prototyping and lost wax investment casting synergy.