Investment Casting (also known as lost-wax casting) is a precision forming process used to produce near-net-shape, highly complex metal parts. This process uses a sacrificial wax model to create a ceramic shell, achieving casting tolerances of ±0.1 mm and a surface finish of 1.6-3.2 micrometers. It allows for the integral molding of complex features, such as thin walls and internal cavities, without requiring draft angles. This enables the integral molding of complex structures from a single casting, significantly reducing manufacturing costs and development cycles for complex components in the aerospace, medical, and automotive fields.
This article will delve into the complete process chain from wax model to finished product, focusing on key design principles and cost optimization solutions. We will reveal how to maximize cost-effectiveness while ensuring part performance through rational structural design and material selection, providing practical guidance for the success of your investment casting project.
Investment Casting Process
The following picture shows the complete investment casting production process:
Investment Casting Process
Wax Pattern Making
Production Methods
Depending on production needs, part complexity, and quantity, the following three pattern-making methods are mainly used:
Injection Molding with Metal Mold
As the preferred solution for mass production, this method uses CNC-machined aluminum or steel molds to create wax patterns through high-pressure injection molding.
Its advantages include ensuring high-dimensional consistency and excellent surface quality of the wax patterns, making it particularly suitable for mass production orders.
3D Printing Rapid Prototyping
For product development and small-batch production, this technology directly creates wax or resin patterns from digital prototypes.
This method eliminates the traditional mold-opening process, significantly shortening the production cycle, and is particularly suitable for the initial prototyping of parts with complex internal structures.
Hand Carving
Primarily used in artistic creation and personalized customization, this method involves experienced artisans directly carving wax.
This traditional process achieves artistic effects that are difficult for machines to replicate, fully meeting the requirements for creating single artworks and special jewelry.
The following figure shows three methods of wax pattern making:
3 Methods of Wax Pattern Making
Design Considerations
Wax pattern design is not a simple replication; material shrinkage must be taken into account.
Both the wax pattern itself and the subsequent solidification of the metal will shrink; therefore, a shrinkage allowance of 1% to 2.5% must be allowed.
To form the internal cavities of the casting, a core made of water-soluble wax or ceramic must be pre-placed in the wax pattern.
Wax Pattern Assembly (Tree Assembly)
To improve production efficiency, multiple wax patterns are assembled onto a central wax gating system (including the pouring cup, runner, and ingate) using adhesive bonding, forming a tree-like structure.
This process is called “tree assembly.” This method enables mass production of parts and effectively distributes unit costs.
The tree assembly process can be completed manually or automated using robots to ensure higher consistency and alignment accuracy.
The following figure shows two methods of wax pattern assembly:
2 Wax Pattern Assembly Methods
Ceramic Shell Making
Shell Making Process
The assembled wax tree is repeatedly immersed in a ceramic slurry prepared from colloidal silica sol and refractory materials (such as zircon powder and fused silica).
After that, it is then immediately stuccoed with coarser refractory sand (such as fused alumina and aluminosilicate).
This “slurry-sand-drying” cycle is typically repeated 6 to 9 times to build a ceramic shell of sufficient thickness (5-10 mm).
Curing Treatment
After each coating layer, thorough drying is required under strictly controlled temperature and humidity to prevent cracking or deformation of the shell.
This layer-by-layer accumulation process ensures the overall strength and stability of the shell, laying a solid foundation for subsequent processes.
Purpose of Coatings
In addition to the basic structural layer, special functional coatings are sometimes applied.
For example, refractory coatings provide a better thermal barrier; anti-metal penetration coatings improve the surface quality of castings; and isolating coatings help control the cooling rate and optimize the microstructure of castings.
The following figure shows the manufacturing process of the ceramic shell:
Ceramic Shell Making Process
Dewaxing
After the mold shell has completely hardened, the internal wax model needs to be removed.
Typically, the mold shell is heated in a steam dewaxing kettle or flash furnace, causing the wax to melt and flow out of the shell–hence the name “lost-wax casting.”
It’s worth emphasizing that most of the recovered wax can be reused after processing, aligning with the principles of sustainable production.
The following figure shows the steam dewaxing kettle used for dewaxing:
Dewaxing with Steam Dewaxing Kettle
Ceramic Mold Shell Firing and Preheating
The resulting hollow ceramic mold shell after dewaxing is placed in a high-temperature furnace and fired at a temperature ranging from 540°C to 1100°C.
This process aims to completely burn off any remaining wax and further improve the ceramic’s strength and chemical stability through sintering.
Preheating the mold shell also effectively prevents cracking (thermal shock) caused by excessive temperature differences during pouring and ensures that the molten metal smoothly fills the entire cavity, especially complex thin-walled sections.
The following figure shows that the ceramic mold shells are fired in a high-temperature furnace:
Ceramic Mold Shell Firing
Melting and Pouring
The metal is melted in an induction furnace or vacuum furnace and precisely adjusted to the optimal pouring temperature. The molten metal is then poured into a preheated mold shell.
Common pouring methods include the most basic gravity pouring, vacuum-assisted pouring for active metals such as titanium alloys to prevent oxidation, and anti-gravity or pressure pouring, which significantly improves the filling capacity for thin walls and complex structures.
The following figure shows pouring melting metal into ceramic shells:
Melting Metal Pouring
Cooling
After pouring, the mold enters a strictly controlled cooling phase, during which the molten metal undergoes phase transformations according to a predetermined solidification sequence.
The cooling rate is precisely controlled according to the alloy characteristics: non-ferrous alloys, such as aluminum alloys, are cooled uniformly by air, while steel alloys require slow cooling to release casting stress.
The following figure shows that the poured ceramic mold shell is cooling down:
Poured Ceramic Mold Shell Cooling
Shell Removal
Once the metal has completely solidified and cooled to a safe temperature, the ceramic shell encasing the casting is broken and removed using methods such as mechanical vibration (Shell Press), high-pressure water jetting, sandblasting, or manual hammering, exposing the metal “casting tree” containing the gating system.
The following figure shows 2 methods of ceramic mold shell removal:
2 Ceramic Mold Shell Removal Methods
Cutting-Off and Separation
This process uses band saws, angle grinders, or specialized shearing equipment to precisely separate the formed casting from the gating system through mechanical cutting or liquid nitrogen cooling.
Strict control of the cutting allowance is necessary to avoid damaging the casting itself and to allow for appropriate machining allowances in subsequent grinding processes.
The figure below shows that castings are cut off via a grinder:
Cutting-Off Investment Casting
Post-Processing and Finishing
Primary Finishing
Using belt grinding, abrasive wheel grinding, or sandblasting, the gate residue on the casting is removed, and the surface is cleaned to achieve an acceptable delivery condition.
Secondary Machining
For parts with more stringent requirements, machining (such as CNC milling, drilling) may be necessary to ensure critical dimensions; heat treatment (such as solution treatment T4, aging T6) may be required to obtain the desired mechanical properties; and surface treatment (such as electroplating, spraying) may be required to enhance corrosion resistance or improve appearance.
The figure below shows the surface finished investment casting parts:
Surface Finished Investment Casting Parts
Advantages and Disadvantages of Investment Casting
Advantages
• Excellent dimensional control, maintaining casting tolerances of ±0.1mm even with complex geometries.
• Superior surface quality, with surface roughness reaching Ra 1.6-3.2μm, significantly reducing subsequent finishing work.
• Outstanding performance in forming thin-walled structures, capable of consistently producing castings with wall thicknesses from 0.8 to 1.5mm.
• Capable of one-time forming of complex internal flow channels, thin-walled structures, and deep cavity features that are difficult to achieve with traditional processes.
• Breaks through traditional limitations, achieving zero draft angle structures and seamless integrated molding, eliminating the influence of parting lines.
• Wide range of material adaptability, capable of stably machining from ordinary carbon steel to nickel-based superalloys.
• Integrated design, supporting the merging of multiple components into a single casting, simplifying product structure and assembly processes.
• Significantly higher material utilization than traditional machining, aligning with sustainable development principles.
• Relatively reasonable initial investment, with significantly lower mold costs than die casting, suitable for medium-volume production.
Limitations
• Production cycle is relatively long, typically taking 5-7 weeks from mold making to finished product delivery.
• Small-batch production is less economical per unit than sand casting, while ultra-large-batch production is less economical per unit than die casting.
• There is an upper limit to the size range; the maximum producible size is limited by equipment capacity.
• High reliance on technical personnel; professional engineers are needed for full-process management.
Commonly Used Materials in Investment Casting
Stainless Steel
• 17-4PH (630): Achieved through precipitation hardening with tensile strength up to 1100 MPa while maintaining good corrosion resistance, widely used in aerospace structural components and medical devices.
• 304(L): A typical austenitic stainless steel with excellent formability and weldability. The low-carbon version effectively avoids intergranular corrosion, suitable for chemical equipment and the food industry.
• 316(L): Contains 2-3% molybdenum, significantly improving pitting corrosion resistance, particularly suitable for corrosion-resistant components in marine environments and chemical processing equipment.
• 420: Martensitic stainless steel, heat-treated to achieve a hardness of HRC 50-55, commonly used in cutting tools, molds, and wear-resistant parts.
Tool Steels and Alloy Steels
• 4140:Medium-carbon chromium-molybdenum steel, after quenching and tempering, exhibits a good balance of strength and toughness, widely used in gears, shafts, and other mechanical structural components.
• 4340:High-strength nickel-chromium-molybdenum steel with excellent hardenability and fatigue strength, suitable for critical load-bearing components in aircraft landing gear and heavy machinery.
• A2/O2: Air-hardening tool steel. A2 steel has good wear resistance, while O2 oil-hardening steel has excellent toughness. Primarily used for cold-working dies and precision tools.
Aluminum Alloys
• A356.2:Aluminum-silicon-magnesium casting alloy. After T6 heat treatment, its tensile strength can reach 320 MPa, making it the preferred material for automotive wheel hubs and aerospace structural components.
• 355.0:Aluminum alloy containing 1.0-1.5% copper. Its strength is significantly improved after heat treatment, suitable for engine components and structural parts requiring high strength.
• 535.0:Aluminum-magnesium alloy with naturally excellent resistance to seawater corrosion, widely used in marine equipment and marine engineering equipment.
Copper Alloys
• C87500: Silicon bronze, combining good casting performance and mechanical properties, with excellent resistance to stress corrosion cracking, is commonly used in marine pumps and valves and chemical equipment.
• C90500: Tin bronze, a traditional casting material with excellent wear resistance and seawater corrosion resistance, is widely used in gears, bearings, and other mechanical parts.
• C95700: Aluminum bronze, containing iron and nickel, with significantly superior strength and wear resistance compared to ordinary bronze, designed specifically for bearings and gears under heavy-duty conditions.
• C17200: Beryllium copper alloy, with a strength reaching 1400 MPa after aging treatment, while maintaining a conductivity above 20% IACS, used in high-performance molds and electronic connectors.
• C86300: Manganese bronze alloy, with excellent wear resistance and high load-bearing capacity, particularly suitable for manufacturing bearings and gears for heavy machinery.
Titanium Alloys
• Ti-6Al-4V: Aerospace-grade titanium alloy, with a density only 60% that of steel but comparable strength, irreplaceable in aerospace and high-end medical devices.
High-Temperature Alloys
• Inconel 718: Nickel-based high-temperature alloy, maintaining high strength and oxidation resistance at 700°C, specifically used in gas turbine blades and rocket engine components.
• Inconel 625: Rich in columbite and molybdenum, it exhibits excellent corrosion resistance in both oxidizing and reducing media, making it suitable for chemical equipment and marine engineering.
• CoCrMo: A cobalt-chromium-molybdenum alloy that is not only biocompatible but also has wear resistance several times higher than stainless steel, making it an ideal material for artificial joints.
Investment Casting Applications
• Aerospace: Engine turbine blades, engine casings, aircraft frame components, combustion chamber assemblies, navigation system housings.
• Automotive Industry: Turbocharger impellers, engine valves, engine rocker arms, transmission synchronizers, steering system components.
• Medical Devices: Hip implants, surgical instruments, dental implants, medical device housings, orthopedic trauma instruments.
• Industrial Machinery: Impellers, pump housings, industrial valve bodies, reducer gears, hydraulic system components, precision parts for automated equipment.
• Energy Sector: Gas turbine blades, steam turbine guide vanes, nuclear power plant valves, oil and gas drilling components.
• Marine Engineering: Marine propellers, mooring system components, seawater pumps and valves, navigation equipment brackets, deep-sea equipment structural components.
• Jewelry and Art: Precision jewelry castings, art sculpture replicas, precious metal crafts, personalized custom jewelry.
Investment Casting Parts
Investment Casting Design Guidelines
Design Optimization Points
Uniform Wall Thickness Design
It is recommended to control wall thickness variation within ±20% to ensure a consistent cooling rate during solidification.
Uneven wall thickness can lead to hot spots, resulting in casting defects such as shrinkage cavities and porosity, severely affecting the mechanical properties and service life of parts.
Reasonable Wall Thickness Transition
When wall thickness must vary, a gradual wall thickness transition design should be adopted.
The length of the transition zone should be not less than three times the wall thickness difference, and the inclination angle of the transition surface should be controlled between 15-30 degrees.
This design ensures a smooth flow of molten metal, avoids turbulence and air entrapment, and reduces the risk of deformation due to differences in cooling rates.
The following is a schematic diagram of a reasonable wall thickness transition:
Casting Wall Thickness Transition
Rounded Corner Transitions
All internal and external corners should be designed with appropriate rounded radii.
It is generally recommended that the internal rounded corner radius be no less than 1.5mm and the external rounded corner radius no less than 2.5mm.
A reasonably rounded corner design can effectively disperse stress and avoid cracking caused by stress concentration.
At the same time, the rounded corner structure is more conducive to the flow and coverage of ceramic slurry, ensuring the quality of the mold shell.
The figure below shows the incorrect and correct corner design of castings:
Rounded Corner Design
Cost Optimization Strategies
Batch Production Planning
Appropriate production batches can significantly reduce unit costs. It is recommended to consolidate small-batch orders to fully utilize the wax tree’s assembly space.
By optimizing the wax tree layout, a single production run can accommodate 20-50 parts, effectively distributing mold and shell-making costs and achieving economies of scale.
Material Selection Optimization
While meeting usage requirements, prioritize alloy materials with mature processes and stable prices.
For example, A356 aluminum alloy has better casting performance and lower costs than A357; 304 stainless steel is more economical than 316 in non-corrosive environments.
Correct material selection can reduce material costs by 15-25% while ensuring quality.
Structural Design Optimization
Reduce subsequent machining processes by optimizing the structural design.
Unify machining datum with casting datum to avoid secondary clamping errors; cast features such as holes and slots directly as much as possible, and only perform finishing on key mating surfaces.
This design strategy can reduce machining time by 30-50%, significantly lowering overall manufacturing costs.
Tolerance Design Principles
Tolerance Grade Selection
Allocate tolerance grades rationally according to the functional requirements of the parts.
Non-mating surfaces maintain general casting tolerances (typically ±0.25mm), general mating surfaces use precision casting tolerances (±0.1mm), and only critical mating features require precision machining tolerances (±0.02mm).
This hierarchical control method ensures performance while effectively controlling production costs.
Datum System Establishment
To ensure manufacturing accuracy, a unified datum system should be established.
It is recommended to use three mutually perpendicular flat surfaces as the main positioning datums, avoiding the use of curved surfaces or centerlines.
Curved surface contact will cause positioning deviations, and centerlines require indirect measurement to determine position, both leading to error accumulation.
A clear datum system can ensure consistency in manufacturing and inspection, effectively improving product accuracy and quality stability.
Special Structural Design
Thin-Wall Structure Design
For thin-walled structures, it is recommended to set reasonable reinforcing ribs to improve rigidity.
The height of the reinforcing ribs should be controlled within 3 times the wall thickness, and the thickness should be 0.6-0.8 times the wall thickness.
While the casting process itself does not require draft angles, a properly designed stiffener with appropriate draft angles helps ensure smooth ejection of the wax pattern from the mold and improves the flowability of the ceramic slurry during shell making.
A well-designed stiffener can reduce weight while ensuring structural rigidity and strength meet usage requirements.
Hole Structure Design
The recommended minimum diameter for through holes is 2.0 mm, and the depth-to-diameter ratio for blind holes should be controlled within 2:1.
For critical holes with strict fit requirements, it is recommended to leave a machining allowance of 0.5-1.0 mm on each side, ensuring final dimensional accuracy through subsequent finishing.
Bosses or locally thickened structures should be provided at the hole edges, with a thickness of 1.2-1.5 times the adjacent wall thickness, to distribute stress and prevent cracks during casting and under load.
Comparison with Other Casting Processes
The table below compares the key characteristics of investment casting with other major casting processes:
|
Characteristic |
Investment Casting | Sand Casting | Die Casting | Centrifugal Casting | Lost Foam Casting |
| Mold/Tooling Cost | Medium to High | Very Low | Very High | Medium |
Low to Medium |
|
Production Volume |
Small to Medium Batch | Small to Medium Batch | Mass Production | Medium to Mass Production | Small to Mass Production |
| Part Size | Small | Very Wide (g to tons) | Small to Medium | Tubular, Annular Parts |
Small to Medium |
|
Dimensional Accuracy |
Very High (CT4-6) | Low (CT10-13) | High (CT6-8) | Depends on Process | High (CT7-9) |
| Surface Roughness | Ra 1.6-3.2μm | Ra 12.5-25μm | Ra 1.6-6.3μm | Ra 6.3-12.5μm |
Ra 6.3-12.5μm |
|
Material Suitability |
Most Alloys | Very Wide (Most Alloys) | Non-ferrous (Al, Zn, Mg…) | Steel, Iron, Cu-alloys | Iron, Al, Steel |
| Min. Wall Thickness | ≥ 0.8mm | ≥ 3mm | ≥ 2mm | ≥ 5mm |
≥ 2.5mm |
|
Production Cycle (with tooling) |
Long | Relatively Short | Very Short | Short | Relatively Short |
| Complex Internal Cavities | Excellent, no parting line | Capable, using cores | Limited | Not Applicable |
Excellent, no parting line |
Comparison Table of Main Casting Processes
Conclusion
Investment casting, with its unique morphology transfer principle and ability to form complex structures, continues to drive innovation in high-end manufacturing fields such as aerospace, medical implants, and the automotive industry.
This article systematically analyzes the complete knowledge system of this precision casting technology, from core principles and process flow to material selection, and from design principles to cost optimization, providing comprehensive guidance for engineers from theory to practice.
If you are looking for a suitable supplier for your casting project, we sincerely invite you to share your project requirements with our technical team.
Contact us today, and our team of casting experts will provide detailed technical solutions and free quotations within 24 hours.