Investment casting, also known as lost-wax casting, is a type of precision casting. Having evolved over a thousand years, it has become one of the indispensable core technologies in modern high-end manufacturing. Next, we will explore the process flow of modern investment casting, its application domains, and when to choose it to complete your project. We believe that through this article, you will gain a comprehensive understanding of investment casting.
Investment casting is a precision casting method that involves creating a wax model of the target part, applying multiple layers of paste to the surface of the wax mold to form a heat-resistant hard shell. Then, melting out the internal wax core, pouring in molten metal, and finally obtaining the finished product. Investment casting specializes particularly in casting components with internal cavities and complex structures, high precision, and superior surface quality. At the same time, it is suitable for mass production of small parts, which sets investment casting apart from other casting methods, making it the “master of masters” in manufacturing complex metal components.
The primary investment casting methods currently in use are the water glass method and the silica sol method. The primary material for water glass method shells is water glass, quartz sand, while the main material for silica sol method shells is silica sol, zircon sand. The former undergoes dewaxing in high-temperature water, while the latter is dewaxed in a flash furnace. Compared to the latter, the silica sol casting method incurs higher production costs but delivers superior surface quality.
Investment Casting Process
Working Principle
The principle of investment casting can be summarized as follows: First, a disposable mold (wax pattern) is created, which is a replica of the target casting. A slurry is then applied to its surface to form a sturdy refractory shell. After the internal wax pattern is melted and drained, molten metal is poured into the cavity. Once the molten metal cools, a precision casting identical to the original model is produced.
Detailed Steps
The specific steps of lost-wax casting are as follows:
Creating the Master Model
The first step in investment casting is to create a master pattern that replicates every detail of the target casting. This requires comprehensive calculations of metal solidification shrinkage and wax cooling shrinkage, followed by a thorough dimensional assessment of the target casting to finalize the master pattern’s dimensions. Master patterns are typically made from metals (such as aluminum or steel) or high-precision plastics.
Creating the Mold
Once the master pattern is complete, it is used to produce the mold for manufacturing the wax pattern. This process is commonly referred to as “mold making.” The resulting mold is characterized by high precision and durability.
Creating the Wax Pattern
This step is also known as wax injection. Medium-temperature wax is poured into the mold. After the wax model cools and is removed, it requires simple trimming to remove excess material, resulting in a wax model that is identical to the final component.
Assembling the Wax Pattern
This step, also known as tree assembly, involves mounting multiple wax patterns onto a central pouring rod to form a tree-like structure for single-pour casting. This tree structure also incorporates the pouring spout for molten metal and vent channels for gas escape, ensuring smooth casting.
Shell Formation
After tree assembly, each structure undergoes a cycle of coating, plastering, and hardening to form a rigid ceramic shell of ideal thickness.
Slurry Application: A ceramic slurry is applied to the wax surface to protect intricate details from damage.
Sand Coating: Fine sand is evenly distributed over the wet, uniform ceramic slurry layer.
Drying: After each plastering step, the wax model is placed in a controlled environment with constant temperature and humidity.
The entire shell-building process is typically repeated six to nine times to achieve the desired thickness (depending on the specific casting type). The first and second layers generally use fine sand to ensure surface quality, while subsequent layers employ coarse sand to guarantee shell strength and thickness. Each layer must be fully dried before repeating the above steps, ensuring the coating is sufficiently robust to precisely retain the wax model’s details. The resting time for each layer is usually 16 to 48 hours.
De-waxing
After shell formation, the wax pattern is placed in an autoclave to melt or evaporate the wax, leaving a hollow ceramic shell ready for pouring. In most cases, the melted wax is recycled and reused.
At the same time, the elevated temperature sinter the mold, which prevents cracking during pouring and preheats the mold, allowing molten metal to fill the mold more effectively and improve casting precision.
Pouring
Once the ceramic shell is preheated, molten metal is immediately poured through the sprue cup into the mold cavity until it is filled.
Shell Removal
After the molten metal has fully cooled, the outer ceramic shell is typically broken and removed using mechanical vibration, yielding the casting tree.
Post-Processing
Shot Blasting: The entire casting tree undergoes preliminary grinding using a hanging grinder.
Cutting: Castings are cut off from the casting tree using a cutting machine, while the remaining pouring system is recycled for reuse.
Rough Grinding: Castings undergo grinding to remove gate residues.
Barrel Finishing: Barrel finishing further refines the casting surface for greater smoothness.
Inspection: Check castings for dimensional or structural deformation.
Argon Arc Welding: Repair defective sections of castings.
Precision Grinding: Perform additional detailed grinding.
Shaping: Correct minor deformations in castings.
Second Inspection: Conduct final inspection of straightened rough castings.
Materials Suitable for Investment Casting
Investment casting offers a wide range of casting material options with few restrictions. Below are several common investment casting materials.
Stainless steel is one of the most common casting materials, with grades such as 304, 316, and 17-4H stainless steel being widely used in medical devices, the food industry, automotive, pump, valve, and aerospace applications.
Aluminum
Cast aluminum alloys are also common casting materials. Due to their low weight and high strength, they are widely used in the aerospace and automotive manufacturing sectors.
Copper alloys primarily include bronze and brass alloys. Due to their excellent electrical and thermal conductivity, they are commonly used in electronic components. It is also used in the marine industry because of its excellent corrosion resistance.
Carbon Steel
Low-carbon steel, medium-carbon steel, and high-carbon steel can all be used in investment casting to manufacture various tools, mechanical components, and structural parts.
High-temperature alloy
Representative examples include nickel-based and cobalt-based high-temperature alloys, whose exceptional performance at elevated temperatures makes them ideal for manufacturing critical components such as turbine blades and nozzles in jet engines.
Advantages of Investment Casting
High Tolerance with Complex Structure: The most prominent advantage of investment casting lies in its ability to produce components with complex internal structures with high dimensional tolerance. This precision ensures consistency across thousands of casting cycles, which is crucial for high-volume production.
Design Freedom: Due to the unique nature of the casting process, it offers exceptional design freedom. Any structural idea for a casting can be realized through investment casting.
Smooth Finish: Castings produced via investment casting typically exhibit smoother surfaces than those from other casting processes, reducing the need for post-processing.
Various Materials: There are virtually no restrictions on casting materials for lost wax casting. From stainless steel to titanium alloys, all can be converted into desired casting components through investment casting.
Form as A Single Piece: Castings produced by investment casting offer the advantage of being formed as a single piece, eliminating the complexity of component assembly. This enhances product reliability while saving assembly time and costs.
No Parting Lines: It doesn’t have the parting lines found in traditional molds, resulting in a more aesthetically pleasing surface and reducing the need for subsequent machining work.
High Material Utilization: Investment casting is a quintessential near-net-shape process with exceptionally high material utilization. The pouring system and risers created during the pattern assembly process can be recycled and reused, and the wax material itself is also recyclable. Consequently, it generates minimal waste and poses lower environmental risks.
Flexible Order Volume: Investment casting offers exceptional production flexibility, which can accommodate both small and large batch orders.
Extensive Application: Components produced via investment casting find extensive applications across diverse industries.
Material Performance Consistency: Materials retain the inherent properties of their raw forms after undergoing the investment casting process.
Limitations of Investment Casting
Compared to the significant advantages of investment casting, its limitations are minor but still noteworthy.
High Cost: The most prominent limitation of lost wax casting is its higher casting cost compared to other casting processes. It is typically used for small castings.
Strict Quality Control Requirements: The complex steps involved in lost wax casting increase the likelihood of issues, necessitating a more robust quality management system.
Size Restriction: Since investment casting produces castings that are single-piece castings, it has more restrictions on size and weight compared to other casting processes. While investment casting excels at producing small, intricate components, it is not suitable for core parts with bore diameters smaller than 1/16 inch (1.6 mm) or depths exceeding approximately 1.5 times the bore diameter.
Long Lead Time: Longer production and delivery lead times represent another significant disadvantage of investment casting, necessitating careful consideration of lead times when selecting this process.
Application of Investment Casting
Investment casting finds extensive applications across diverse fields, with high-precision and cutting-edge sectors pushing the capabilities of this process to its limits. These include:
Aerospace: Turbine blades, engine fuel nozzles, guide vanes, structural frames, engine casings, etc.
Marine: Propellers, pumps, and valves for warships and high-performance vessels.
Power Generation: Gas turbine blades, turbine disks.
Medical: Surgical implants such as artificial joints; surgical instruments.
Military: High-precision, high-strength casings, triggers, etc.
Beyond these, industrial sectors rely on investment casting for its high precision in complex components, primarily in:
Automotive: Engine camshafts, turbocharger blades, lightweight structural components, and transmission parts.
Industrial fittings: Pumps, valves, and pipe fittings.
When Shall We Choose Investment Casting
Medium-sized casting
If your castings weigh between 0.1 kg and 50 kg, investment casting is highly suitable.
Medium-ranged volum
It offers the best cost-effectiveness for small to medium production runs. If the batch size is too small, the tooling costs become prohibitively high. Moreover, if your order is extremely urgent, 3D printing offers greater economic benefits.
Complex structure
Investment casting excels at producing castings with complex geometries. When your casting features intricate shapes that other casting processes cannot achieve, investment casting is your solution.
Demanding surface finish
Compared to other casting processes, lost wax casting produces castings with superior surface finish, meeting your high standards for casting surface quality.
Stringent dimensional tolerance
Investment casting perfectly meets your high precision requirements for castings. Moreover, it ensures consistent dimensional accuracy for identical castings.
Difficult-processed Material
Investment casting offers considerable flexibility in material selection. If your casting material is difficult to process, investment casting is worth a try.
Meet the Pingheng Machinery – Professionals in Investment Casting
With the above information, we believe you are now thoroughly familiar with investment casting. If you are considering offering investment casting to produce your castings, Pingheng Machinery is your trusted choice.
If you are looking for an investment casting foundry with high expertise, Pingheng Machinery is your best choice. Founded in 1999, Pingheng has accumulated abundant experience in handling any complex issues in this field, offering a satisfactory solution for your project.
Our primary casting materials include stainless steel (304(L), 316(L), etc) and copper alloy. At Pingheng Machinery, you can enjoy a one-stop customized service. Any of your casting requirements could have a satisfactory solution with our professional technology team. Besides, we provide comprehensive services from mold making, casting, machining, to delivery, eliminating the hassle of sourcing other suppliers.
Please feel free to contact us anytime to share all your ideas. Let’s work together to advance your next casting project.
