What Is the Investment Casting Process?
Investment casting, also known as lost-wax casting, is a centuries-old metal-forming process that is now an enabling technology for today’s leading industries, including aerospace, defense, and space.
This process was primarily used for art and jewelry until the development of the jet turbine engine at the end of World War II. Since that time, it has evolved into a manufacturing solution for producing precision components using the latest advances in computer technology, robotics and advanced casting techniques. The incorporation of additive manufacturing in the investment casting process has allowed manufacturers to produce high-strength, lightweight, and complex components with extremely fine detail and dimensional accuracy.
Many investment casting foundries, including Barron Industries, also use CAD software to create reliable designs, durable metal tooling that offers better repeatability during production, robots during shell-making and assembly to improve speed and repeatability, CNC machines to enhance the surface finish of castings, and other advanced technologies. Investment castings also have excellent “as-cast” surface finish, reducing or even eliminating the need for machining, and that means huge reductions in cost and lead times.
The Step-by-Step Process
Investment casting gets its name from the process in which an expendable pattern is surrounded or “invested” in a refractory ceramic coating. Here is the basic step-by-step process:
1. Pattern-Making
The “pattern” is a wax replica of the part being made. These are usually injection-molded, although more advanced foundries, like Barron Industries, use 3D printing for rapid prototypes, small-quantity production, or complex components that can’t be produced with machined tools or dies. The 3D printed patterns are made of photo-sensitive epoxy resin.
2. Pattern Tree Assembly
After the patterns are created, they are then connected to a sprue, or wax stick, to create a single mold and to provide a path for molten metal into the mold. The area between the pattern and the sprue is called the gate, which controls the direction and flow of the metal.
Attaching multiple patterns to a sprue increases the efficiency of the investment casting process. Instead of casting one part at a time, multiple patterns can go through the remaining steps together. The assembly of multiple patterns attached to a sprue is sometimes called a “tree” or “cluster.”
3. Shell-Building
The assembly is dipped or invested in a ceramic slurry and covered with a stucco sand. This process is repeated until the ceramic shell is strong enough to bear the weight of the metal that will be poured into the mold. Barron Industries uses a six-axis dipping robot during this stage to ensure consistency and uniformity, which is critical when producing intricate and complex parts.
4. Burnout
When the shell is completely dry, a flash fire oven is used to remove the wax or 3D printed pattern prior to the metal being poured into the mold. At Barron Industries, the wax is recovered, purified, and recycled.
5. Pouring
With the pattern removed, the shell is put into a furnace to completely cure the ceramic and strengthen the mold. Molten metal is poured into the top of the sprue, flowing into the individual part cavities. As the metal cools, the parts, gates and sprue become one solid casting.
6. De-gating & Finishing
Once the metal has solidified, the shell is broken apart and parts are cut away from the sprue. Individual castings are then cleaned up before being sent for finishing. Common types of post-casting finishing services include belting or hand grinding, heat treating, machining, polishing, and surface coating or painting.
Advantages of Investment Casting
- Exceptional Dimensional Accuracy: Starting with a very accurate and detailed pattern results in complex, precise, dimensionally accurate metal parts. Castings can even have thin walls and other features like undercuts, surface patterns, and letters or numbers, without sacrificing quality.
- Excellent Surface Finish: Because shell-building starts with a very fine ceramic slurry, it’s possible to achieve excellent surface finishes.
- Less Machining Needed: Part complexity and precision minimize the amount of machining work needed, which is especially advantageous when using alloys with poor machinability, like stainless steel, titanium, and superalloys.
- Reduced Part Mass: Investment casting supports efforts to minimize part mass, which is critical for aerospace and automotive manufacturers.
- Flexibility: Investment casting provides a vast array of design and metal choices for designers. Most metals can be processed via investment casting, such as aluminum, stainless steel, carbon steel, tool steel, and low-alloy steel.
- Sustainability: Investment casting is a highly sustainable manufacturing method. The process doesn’t produce any hazardous waste, metal scrap can be reused and wax patterns are reclaimed and recycled. Other green technologies include the use of induction furnaces for melting metal which does not produce any direct emissions, such as carbon dioxide or nitrogen oxides. To further promote our social responsibility, Barron Industries takes it a step further by enforcing strict air quality guidelines and uses solidification modeling to help predict a casting’s internal quality before production begins, minimizing waste.
Common Applications
Investment casting is commonly used to produce complex parts, especially those made from alloys with poor machinability, and for those going into mass-critical applications. It’s also been widely adopted in medical device and aerospace manufacturing to make intricate, near-net shape parts, such as housings and brackets, that need little final machining.
Investment casting is also used to make parts like valve bodies, connectors, and flanges that are used in chemical processing and energy generation. In these applications, investment casting allows for the use of alloys with challenging properties that offer the required corrosion resistance and strength.
Possible Process Challenges & Solutions
Lead Time
Investment casting can be a time-consuming process. Creating wax patterns and subsequent shell room operations take days or weeks, although the use of additive manufacturing for patterns can significantly shorten lead times. Barron Industries combats this challenge by offering a vertically integrated manufacturing process that includes up-front design for manufacturability, casting, machining, welding, and non-destructive testing. In addition, Barron partners with India foundries that offer high-volume capabilities while maintaining our strict quality requirements.
Optimization of Metallurgical Properties
Cooling rate determines grain size, which in turn influences mechanical properties. By utilizing the latest solidification modeling technology, Barron Industries optimizes filling and cooling before any physical parts are cast, reducing any trial and error phases.
Labor-Intensive
Investment casting can also be fairly labor-intensive, particularly during assembly and shell-building. Barron Industries overcomes this challenge by using six-axis robots that improve speed and repeatability while still ensuring quality and precision.
Barron also employs the latest in quality control technology to become better, faster, and leaner in manufacturing engineered metal components for demanding applications. We’ve expanded our engineering and inspection capabilities with the addition of a new FARO Quantum S 7-axis, high-definition laser probe scanner, and 8th-axis rotary platform.
Certified to the latest ISO standards, our innovative portable coordinate measuring machine (PCMM) also allows us to easily verify casting quality by performing 3D inspections, dimensional analysis, CAD comparison, and reverse engineering while saving time and reducing scrap and rework costs. Barron has also integrated cobots (collaborative robots) into CNC machining operations to optimize workflow and reduce downtime.
Learn More About Barron’s Process
There’s no question that investment casting is a well-proven, reliable manufacturing method that has continued to develop into a complex and technically advanced process.
Please visit our website to learn more about our unique process and capabilities or to find more investment casting-related resources.