In the world of fluid dynamics, the impeller is the heart of the machine. Whether it is pumping cryogenic fuel into a rocket engine, circulating cooling water in a nuclear reactor, or driving a high-pressure turbocharger, the efficiency of the entire system rests on the geometry of the impeller.
Design engineers have always known that Closed Shroud Impellers (where the vanes are sandwiched between a top and bottom disc) offer superior efficiency and structural strength compared to open impellers. However, for decades, they faced a manufacturing brick wall. Designing the perfect fluid path is easy in CAD; casting it in metal is a nightmare. The complex, twisted internal channels create “undercuts” that make it impossible to withdraw a traditional metal tool.
This led to compromises. Designs were simplified to make them manufacturable, albeit at the expense of performance.
Today, that compromise is dead. By combining 3D-printed wax patterns with precision block moulding, Castech is producing complex closed shroud impellers with fine internal details that were previously impossible to manufacture.
This guide explores how additive manufacturing has liberated the foundry process, allowing for the creation of high-performance impellers without expensive tooling.
The Quick Answer
A Closed Shroud Impeller features internal vanes hidden between two solid discs. Traditional moulding cannot produce these because you cannot pull a solid tool out of the twisted internal spaces. The solution at castech.co.uk utilises 3D-printed pure casting wax patterns and a block moulding version of the Lost Wax Method. Instead of injecting wax into a metal die, we 3D print the pattern directly. These patterns are fixed to a runner system to form a wax tree, which is placed into a foundry box. A fine plaster investment is applied under a vacuum, which easily penetrates all external features and fills internal voids. The wax is melted out, the mould is fired, and molten metal is poured with vacuum assist. Because the pattern is printed layer-by-layer and the plaster mould is completely removed using a water jet, it can contain incredibly complex, curved, and thin internal vanes without needing a tool to be withdrawn. This eliminates tooling costs and unlocks unlimited geometric freedom.
The Engineering Challenge: The “Impossible” Geometry
To understand the breakthrough, you must first understand the problem with traditional manufacturing.
1. The Tooling Trap
In standard investment casting, you need a metal die (a negative mould) to inject the wax pattern. To get the wax out of the die, the die must pull apart.
Open Impeller: Easy. The die pulls up and down.
Closed Shroud Impeller: Impossible. The top shroud blocks the tool from pulling up. The twisted vanes block the tool from sliding sideways.
2. The Ceramic Core Solution (The Old Way)
Historically, foundries used “soluble cores.”. They would inject a ceramic core to form the internal vanes, place it into the die, inject wax around it, and then dissolve the core later.
Drawback: This is incredibly expensive, slow, and prone to “core shift,” where the internal veins move slightly, ruining the balance of the impeller.
3. Machining from Solid (The Wasteful Way)
You can use 5-axis CNC machining to cut an impeller from a solid block.
Drawback: The tool has to reach inside the shroud. It creates “blind spots” where the cutter cannot reach, limiting the curve of the vanes. It is also astronomically expensive and wastes 80% of the material.
The Solution: 3D Printed Wax Patterns
At Castech, we bypass these limitations entirely by removing the metal tool from the equation. We use Rapid Investment Casting.
Step 1: The Digital Twin
We take your CAD file (STEP or STL). We do not need to design “draft angles” (tapered walls to help the tool slide out) because there is no tool. We can print the exact fluid geometry you calculated in your CFD (Computational Fluid Dynamics) simulations.
Step 2: 3D Printing the Pattern
Using high-resolution industrial printers (such as ProJet), we print the pattern layer by layer.
Material: We use pure Casting Wax.
Precision: The printer lays down layers as thin as 25 microns. This allows for razor-sharp trailing edges on the vanes, which is critical for reducing turbulence in the fluid.
Step 3: The Shell and Pour
This printed pattern is treated using a block moulding version of the Lost Wax Method.
We fix the wax patterns to a runner system to form a wax tree.
A fine plaster investment is applied under a vacuum, penetrating all external features and faithfully filling internal voids.
We place it in a furnace where the wax is completely melted out, and the mould is fired.
Molten metal is poured into the mould with vacuum assist.
Finally, the mould material is removed flawlessly using a high-pressure water jet.
The Advantages of High-Performance Impellers
Why is this method superior for closed shroud designs?
1. Zero Draft Angles
In traditional casting, vertical walls must be tapered (drafted) so the tool doesn’t get stuck.
These tapers change the fluid flow, often reducing pump efficiency.
With 3D printed waxes, walls can be perfectly vertical or even reverse tapered.
This allows engineers to maintain the precise cross-sectional area of the fluid channel from inlet to outlet.
2. Thin Wall Sections
Traditional wax injection requires high pressure.
If a vane is too thin, the pressure can break the plaster mould core.
3D printing is a low stress process.
We can cast vanes with trailing edges as thin as 0.5mm.
Thinner vanes mean less blockage in the fluid path and higher flow rates.
3. Balanced Fluid Dynamics
A 5-axis machined impeller often has “tool marks” or geometric compromises where the cutter couldn’t reach.
A printed pattern has smooth, continuous curves throughout the entire internal passage.
This reduces cavitation risk and noise.
Rapid Prototyping to Production
This technology completely changes the development cycle for pump and turbine manufacturers.
The Old Cycle:
Design Impeller.
Order Tooling (£20,000, 12 weeks).
Test Castings. Find out the flow is 5% too low.
Modify Tooling (£5,000, 4 weeks).
Re-test.
The Castech Cycle:
Design Impeller.
Print Wax Pattern (2 days).
Cast and Test (2 weeks). Find out flow is 5% too low.
Update CAD file.
Print New Wax Pattern (2 days).
Re-test.
You can iterate your design five times in the time it takes to cut one metal tool. Once the design is frozen, we can continue to use 3D printing for low-volume production (e.g., 50 units a year), or we can transition to hard tooling if volumes soar to 10,000 units.
Material Versatility
Because the printed pattern melts out just like standard wax, we can cast these impellers in almost any non-ferrous alloy available in our foundry.
Aluminium (LM25, A356): For lightweight aerospace fuel pumps and turbochargers.
Bronze / Aluminium Bronze: For seawater applications where bio-fouling and corrosion resistance are paramount.
Case Study: The “Blind” Vane
We recently worked with a client designing a high-efficiency water pump. The design featured a “splitter vane”, a small secondary vane hidden deep inside the closed shroud to prevent recirculation.
Traditional Casting: Impossible. The core would be too fragile.
Machining: Impossible. The cutter could not reach behind the main vane to cut the splitter.
Castech Solution: We 3D printed the pattern in Wax. The splitter vane was printed integrally with the shroud. The resulting casting was a single, solid piece of metal with perfect internal geometry. The pump achieved a 12% efficiency gain over the previous open impeller model.
Conclusion
The “manufacturability” constraint that has held back impeller design for decades has been removed. Engineers no longer need to ask “Can this be cast?” but simply “Is this the best shape for the fluid?”.
Complex closed shroud impellers with fine internal details, splitter vanes, and zero draft angles are now standard capability at Castech. By leveraging the speed and geometric freedom of Rapid Prototyping, we deliver components that perform exactly as the simulation predicted.
If you are developing rotating machinery and want to push the limits of efficiency, contact our technical team to discuss how 3D-printed waxes can bring your fluid dynamics to life.