【3D Printing】Industrial Investment Casting with SLA 3D Printed Patterns

What is Investment Casting?

Investment casting, also known as lost-wax casting, is a precision casting process used to produce complex metal parts. Its applications include lightweight automotive parts, golf clubs, jet turbines, industrial impellers, and art sculptures. It can create geometries that are difficult to achieve with traditional machining methods and offers excellent surface quality and part integrity.

Traditional wax patterns usually require CNC machining of metal molds, followed by wax injection. This process involves specialized equipment, multiple steps, skilled labor, and higher mold costs and lead times, especially disadvantageous for rapid prototyping, single-piece, and small-batch production.

How Does 3D Printed Investment Casting Shorten Lead Times and Reduce Costs?

Using SLA 3D printing to create expendable sacrificial patterns eliminates the need for metal molds and wax injection processes. Patterns can be produced directly from digital files, making them suitable for rapid turnaround, customized parts, design validation, and small-batch casting, while also freeing up CNC equipment and skilled operators.

SLA processes offer fine features, good dimensional accuracy, and smooth surfaces, allowing the pattern's surface and geometric features to be transferred to the final metal part. Compared to traditional wax patterns, 3D printing also makes it easier to create undercuts, curved channels, thin walls, and complex internal structures.

The Formlabs SLA ecosystem is easy to deploy, operate, and maintain, allowing printing, washing, and post-curing to be integrated into existing casting workflows.

Castable Resin Material Properties and Casting Tests

Castable Resin is an antimony-free, low-ash material with ash content below 0.02%. Formlabs collaborated with the Foundry 4.0 Center at the University of Northern Iowa (UNI) and industrial, service, and art foundries to validate its dimensional accuracy, surface quality, burnout effectiveness, and part costs.

Partner foundries have used this process to cast bronze, brass, A356 aluminum alloy, Ti-6Al-4V titanium alloy, 4140 steel, 8620 steel, 316 stainless steel, and 17-4 PH stainless steel. Test patterns were burned out in furnaces at 700°C to 900°C without an autoclave; most patterns were attached to standard wax sprues, and ceramic shells were then produced according to each foundry's existing processes.

A356 Aluminum Alloy Investment Casting Case Study

UNI Foundry 4.0 used Materialise's lattice module to create patterns as tetrahedral lattices with a 0.5 mm lattice wall thickness and 1 mm lattice diameter. These were then printed on a Form 3L with a 100 μm layer height and cleaned and post-processed according to Formlabs' standard procedures.

Patterns were attached to standard sprues using adhesive wax. The ceramic shell used a 100% silica system, with Remet RP-1 as the facecoat grit and RG-1 as the backcoat grit, typically consisting of two facecoats and three backcoats; grit was applied after the second facecoat and after every subsequent dip. An automated system maintained uniform coating, and the production time for a single pour tree was about 9 to 10 hours.

1. After the ceramic shell dried, it was flash-fired at 900°C (1650°F) for 2 hours.
2. After cooling, the ceramic shell was transferred to the casting area.
3. Before casting, the ceramic shell was preheated to 540°C.
4. Aluminum alloy was poured at 700°C to 750°C.
5. After the metal solidified, the primary shell was knocked off, and residual ceramic was removed by sandblasting.

"Our primary reason for purchasing the Form 3L was pattern cost. We previously used traditional wax injection molds to make impellers, which was difficult and time-consuming; after switching to PMMA printing, a single pattern cost over $300."

John Farr / Diversified Metal Fabricators

"The Formlabs system and Castable Resin allow us to produce high-precision patterns for art casting faster and more consistently, with labor costs a fraction of traditional hand-carved wax patterns; adding equipment also allows for rapid capacity expansion."

Julian Musi / Digital Atelier

Castable Resin Test Results and Cost Analysis

Feedback from partner foundries indicates that Castable Resin 3D printed patterns can produce investment cast parts with quality close to traditional wax patterns. Printed patterns may be more brittle than wax patterns and require careful handling and assembly; however, after burnout, no significant ash residue was found in the visible areas of the ceramic shell, and no abnormal defects appeared in the final metal parts.

Directly printing sacrificial patterns eliminates the need for metal molds, soluble wax cores, or other complex wax pattern forming techniques, allowing for the creation of undercuts, curved channels, and thin-wall features that are difficult to achieve with wax injection.

Overview of 3D Printed Investment Casting Workflow

The following methods are compiled from actual feedback from over 10 foundries, covering sacrificial pattern design, SLA printing, post-processing, assembly, ceramic shell production, pattern burnout, and metal pouring.

Step 1: Design Hollow Lattice Sacrificial Patterns

Sacrificial patterns must be strong enough to withstand ceramic slurry and handling pressures, while minimizing material to allow for smooth burnout and reduced ash residue. It is recommended to use thin outer walls with internal lattice structures instead of printing solid patterns.

Materialise Magics Investment Casting Tools or other software can be used to hollow CAD models and create internal lattices. Lattice settings can control shell thickness, internal support dimensions, and drain hole locations.

Setting Pattern Shell Thickness

Thinner outer walls reduce expansion during burnout and material residue, but can decrease print success rate and handling strength. Formlabs recommends starting with a 0.5 mm wall thickness and only increasing it if printing fails or dimensional accuracy is insufficient. Detail size can initially be kept at 0.5 mm.

Setting Internal Lattice Structure

Lattices support the pattern's outer walls, prevent warping, and support thin-wall printing. Increasing lattice strut thickness enhances pattern strength, but also increases material expansion and ceramic shell cracking risks. It is recommended to start with 0.75 mm strut thickness and 3 mm strut length for testing.

Adding Drain and Vent Holes

Drain and vent holes reduce suction effects during printing, allow uncured resin inside the pattern to drain, and improve airflow during pattern burnout. Before designing holes, confirm the print orientation.

1. Place holes at the highest and lowest points in the print direction.
2. Prioritize placing holes on sacrificial surfaces that can be cut or processed later, such as sprues.
3. Start with a minimum hole radius of 1 mm; it can be appropriately enlarged if it does not affect critical surfaces.
4. Use straight holes (r1 = r2) or slightly chamfered holes (r1 > r2).
5. Place vent holes near sprues to improve oxygen flow during pattern burnout.
6. If lattice pattern files are large, export them in 3MF format to reduce file size and speed up support generation.

Step 2: 3D Printing Patterns with Castable Resin

Step 3: Sealing Holes, Assembly, and Pattern Preparation

1. Fill drain and vent holes with standard casting wax or a UV resin pen.
2. If using a UV resin pen, cure according to material instructions.
3. Inject low-pressure compressed air into the last hole to check if other holes are sealed.
4. Arrange gates, branches, and pattern assembly according to existing procedures.
5. Use adhesive wax to secure the pattern to the wax tree and configure a T-bar suitable for dipping.
6. Standard wax sprues can be used, or sprues can be integrated into the 3D printed pattern.

For single-piece, extremely small-batch, or time-sensitive parts, additional vent channels can be added to the pattern to improve airflow during burnout and increase first-pass success rates. After burnout, the ceramic shell must be cooled, and vent holes repaired with embedding material.

Step 4: Producing Investment Casting Ceramic Shells

After pattern assembly, the ceramic shell can be built according to the foundry's existing slurry and stuccoing procedures. Some foundries use slurries with higher expansion tolerance or increase dipping times to reduce cracking risks; Formlabs recommends starting with existing standard procedures for testing.

UNI used an automated robotic unit to complete six dips:

1. One dip in facecoat slurry without stuccoing to maintain surface quality.
2. One dip in backcoat slurry with facecoat grit.
3. Three more dips with backcoat slurry and backcoat grit to increase ceramic shell thickness and strength.
4. Finally, one dip in facecoat slurry without stuccoing to form a sealing layer.

Step 5: High-Temperature Burnout of 3D Printed Sacrificial Patterns

Burn out patterns in a high-temperature furnace according to regular foundry procedures. Common flash burnout conditions are 705°C to 900°C (1300°F to 1650°F) for about 2 hours. The furnace must be able to maintain high temperatures and provide sufficient oxygen for complete pattern combustion.

After burnout, check the ceramic shell for cracks or burrs and remove internal ash. Castable Resin is antimony-free and has an ash content below 0.020%; combined with a hollow lattice design, residual ash is minimized.

Step 6: Metal Pouring and Casting Finishing

After pattern burnout and ceramic shell inspection, proceed with existing metal casting and finishing processes. The UNI case study followed these steps:

1. Preheat the ceramic shell in a kiln at 345°C (650°F) for 30 minutes.
2. Pour metal and wait for it to cool and solidify completely.
3. Remove the ceramic shell using high-pressure water blasting or sandblasting equipment.
4. If necessary, carefully use chisels to remove localized shell layers, avoiding damage to the casting.
5. Separate parts from the pouring tree using a band saw or other suitable tools.
6. Perform machining, grinding, sandblasting, or other finishing as required by the part.

Conclusion on Castable Resin and 3D Printed Investment Casting

Castable Resin allows foundries to produce sacrificial patterns directly from digital files, eliminating traditional metal molds and complex wax forming processes, making it suitable for rapid prototyping, quick turnaround, customized parts, and small-batch investment casting.

This workflow also enhances design freedom for undercuts, tortuous channels, thin walls, and complex geometries. Manufacturers can produce patterns on demand, defer investment in hard tooling, and validate designs with actual metal parts before committing to mass production molds.

Case results show that Formlabs SLA equipment and Castable Resin can significantly reduce pattern and mold costs and shorten lead times by weeks to months for specific projects, with only minor adjustments to existing processes. Actual benefits still need to be individually evaluated based on parts, production volume, materials, and casting conditions.