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In fluid machinery such as turbo‑compressors, pumps, turbochargers, and hydraulic turbines, the volute casing is the core component that converts the kinetic energy of high‑velocity fluid into pressure energy. Internally, it features a spiral flow passage with varying cross‑sections; externally, it bears pressure and provides connection interfaces. This thin‑walled shell structure, with its complex internal cavity and irregular external shape, imposes extremely high demands on the casting process.
Sand casting, as the oldest and most resilient casting process, has become the mainstream method for producing large, complex, small‑batch volute castings, thanks to its high process adaptability, low tooling cost, and wide material‑selection range. It can be said that sand casting is the critical bridge that transforms a volute from a design blueprint into a functional physical component.
1. What Is a Sand‑Cast Volute Casing?
A sand‑cast volute casing refers to a volute‑type casting produced by the sand‑casting process (using silica sand as the primary moulding material) . Its key characteristics include:
- Formation of complex internal cavities: Sand cores are used to create the spiral flow passage with varying cross‑sections inside the volute.
- Excellent pressure‑bearing capacity: The casting has a dense structure, capable of withstanding high‑pressure, high‑velocity gas impacts.
- Broad material adaptability: Suitable for cast iron, cast steel, stainless steel, copper alloys, aluminium alloys, and many other materials.
- Large‑scale capability: Particularly suitable for producing large volutes weighing from several tens of kilograms to several tens of tonnes.
2. Process Characteristics of Sand‑Cast Volute Casings
Compared with investment casting, die casting, and other processes, sand‑cast volute casings offer the following notable advantages:
1. Highly Flexible Process Adaptability
Sand casting is not constrained by the size or complexity of the volute. Whether it is a turbo‑compressor volute with overall dimensions of 3100 mm × 1900 mm × 600 mm and a weight of 4.26 tonnes, or a blower volute with dimensions of 3900 mm × 3650 mm × 1500 mm and a rough weight of 20 tonnes, sand casting can handle it.
2. Resin‑Sand Moulding for Improved Quality
Modern sand‑cast volutes have widely adopted resin‑sand moulding processes, such as alkali‑phenolic resin sand and furan resin sand. Compared with traditional green sand, resin sand offers:
- Higher dimensional accuracy and better surface quality.
- Flow‑channel surface roughness that fully meets design requirements.
- Strong resistance to intergranular corrosion and pitting corrosion.
- Good permeability, significantly reducing gas‑related defects.
3. Deep Integration with Advanced Technologies
Traditional sand casting is increasingly integrating with modern digital technologies. Casting simulation software (such as ProCAST and AnyCasting) is widely used to optimise gating and riser systems and to predict shrinkage porosity defects. 3D‑printed sand moulds/cores solve the problems of complex core‑making processes and high costs, enabling rapid casting of volutes.
3. Material Selection for Volute Casings
Sand‑cast volutes offer a wide range of material choices, which should be determined based on the working medium, temperature, pressure, and corrosive environment.
| Material Category | Typical Grades | Key Characteristics | Typical Applications |
|---|---|---|---|
| Nodular cast iron | EN‑GJS‑400‑18‑LT | Good overall mechanical properties, relatively low cost | Turbo‑compressor volutes |
| Cast steel / carbon steel | ZG230‑450, WCB | High strength, good weldability | Large chemical casings, general‑purpose volutes |
| Austenitic stainless steel | ZG07Cr19Ni9, GX40CrNiSi25‑12 | Corrosion‑resistant, high‑temperature resistant, smooth flow passages | Chemical, pharmaceutical, food‑grade volutes |
| Heat‑resistant steel | Austenitic heat‑resistant steels | Good high‑temperature strength, oxidation resistance | Turbocharger turbine housings |
| Aluminium alloy | ZL101, ZL201 | Lightweight, good thermal conductivity | Automotive lightweight volutes, aerospace thin‑wall parts |
| Copper alloy | C90500 | Corrosion‑resistant, wear‑resistant | Special‑medium pump volutes |
| Martensitic stainless steel | Relevant grades | High strength, high hardness | Wear‑resistant, high‑pressure applications |
Selection Guidelines
- Chemical / corrosive environments: Prioritise austenitic stainless steels (e.g., ZG07Cr19Ni9), which offer both intergranular and pitting corrosion resistance.
- High‑temperature conditions: Prioritise heat‑resistant steels or heat‑resistant stainless steels to ensure microstructural stability and oxidation resistance at elevated temperatures.
- Large pressure‑bearing components: Prioritise nodular cast iron or cast steel for good overall performance and controllable cost.
4. Core Application Areas of Sand‑Cast Volute Casings
Sand‑cast volute casings are widely used across multiple industrial sectors:
| Application Area | Typical Equipment | Key Requirements | Common Materials |
|---|---|---|---|
| Turbo‑compressors | Multi‑stage turbo‑compressors | High airtightness, high flow‑channel accuracy | Nodular cast iron |
| Chemical equipment | Nitric‑acid production equipment | Corrosion resistance, pressure resistance | Austenitic stainless steel |
| Pumps / FGD pumps | Spray pumps, FGD pump volutes | Wear resistance, no shrinkage porosity | Stainless steel, high‑chromium cast iron |
| Turbochargers | Engine turbochargers | High‑temperature resistance, dimensional precision | Heat‑resistant stainless steel |
| Hydraulic turbines | Large turbine volutes | Large size, defect‑free | Cast steel |
| Blowers | Large blower volutes | Large dimensions, high pressure‑bearing capacity | Stainless steel |
5. Key Process Design Considerations for Sand‑Cast Volute Casings
1. Pouring Direction and Parting Line
The pouring direction is the primary decision in volute casting process design. For a turbo‑compressor volute, for example, to facilitate core fixing and smooth outgassing, the opening face is typically placed downward and secured to the lower mould box in advance.
2. Gating System Design
The internal flow passage of the volute demands high surface quality, so a well‑designed gating system is critical. A common approach is a bottom‑fed, initially‑closed‑then‑open gating system:
- A buffer sump is added at the bottom of the sprue to reduce turbulence.
- A filter screen seat is placed in the lower runner to prevent slag from entering the mould cavity.
- Ingates are distributed evenly with multiple entry points, feeding from the thicker sections.
3. Riser and Feeding System
The top of the volute is prone to feeding‑related defects. Risers are typically placed on the outside of the casting, using exothermic risers combined with chills in a zoned feeding arrangement.
4. Core Fixing
The spiral internal cavity is formed by a helical sand core. The positioning and fixing of this core are process challenges. A combination of dedicated high‑strength core irons and multi‑point handling ensures that the core does not shift or break during pouring.
6. Common Casting Defects and Prevention
| Defect Type | Cause | Preventive Measures |
|---|---|---|
| Shrinkage porosity / cavities | Insufficient feeding | Optimise riser parameters, add chills, use exothermic risers |
| Sand inclusion | Mould sand erosion, loose sand not cleaned | Improve mould strength, clean loose sand from cavity |
| Core shift / breakage | Insufficient core strength, excessive molten‑metal buoyancy | Use anti‑shift/anti‑bending core irons, dedicated high‑strength core irons |
| Slag inclusion | Slag entering the cavity | Design filter screen seats, use closed‑then‑open gating systems |
7. Selection Guide for Sand‑Cast Volute Casings
Step 1: Identify the application scenario and operating conditions
- Compressors / blowers → Focus on airtightness and flow‑channel accuracy.
- Pumps / chemical pumps → Focus on corrosion resistance and wear resistance.
- Turbochargers → Focus on high‑temperature resistance and dimensional precision.
Step 2: Select the appropriate material
- Determine the material based on operating temperature, corrosive media, and pressure rating.
- Refer to the material comparison table above for matching.
Step 3: Confirm the casting process plan
- Conventional large volutes → Resin‑sand hand moulding.
- Complex / rapid delivery → 3D‑printed sand moulds/cores.
- High precision requirements → Casting simulation software for process optimisation.
Step 4: Choose an experienced casting foundry
- Confirm that the foundry has resin‑sand moulding capability.
- Confirm that it possesses casting simulation and non‑destructive testing capabilities.
- Review the foundry’s track record in producing similar volute castings.