Investment casting, also known as lost wax casting, is an industrial process. This process is used for metal forming and is one of the oldest metal forming methods. Previously referred to as ” lost wax casting,” it is now referred to as modern investment casting processes.
The process is about 5,000 years old and has been used in various forms ever since. In the past, beeswax was the main object for forming the pattern during the casting process. However, today we have multiple tools, alloys, waxes, and advanced refractory materials to make the desired patterns. Investment casting is famous for producing total accuracy, versatility, repeatability, and integrity components with a wide range of high-performance metals and alloys.
Although wax patterns are fragile, they must withstand extreme forces during the mold making. You can recover or reuse more than half of the wax used in investment casting. Investment casting is the modern process, but lost foam casting is its most developed form that allows you to skip some steps in the process.
It is called investment casting because the process invests (surrounds) the pattern with refractory material for mold formation, and the molten substance is cast into the mold. Casting materials include brass, stainless steel alloys, aluminum, steel, carbon, and glass. The refractory mold cavity has the exact shape required to create the desired part. With investment casting, you don’t necessarily need the secondary machining process because of the hardness of the refractory materials. The products formed in this process have exceptional surface qualities.
Investment casting is divided into different groups, of which water glass and silica sol are the two most well-known casting methods today. Although the two processes differ in many ways, the main differences are the cost of casting and the roughness of the surface. The water glass method requires high-temperature water for dewaxing. In addition, the ceramic mold used in this process is made of water glass quartz sand.
On the other hand, the silica sol method requires a flash fire for dewaxing, while the ceramic mold is made of zircon sand silica sol. The silica sol method is expensive, but the products of this method are more stable and offer a better surface than the water glass method.
Regardless of material weight, you can use this process for parts ranging from a few ounces to several hundred pounds. Comparatively, this process is more expensive than die casting or sand casting; however, the unit costs will be lower if you are casting large volumes.
Investment casting is more efficient than other methods because it can produce complicated shapes that would not be easy. Also, the finished products have exceptional surface quality, with low tolerances and minimal surface finishing or machining.
Investment Casting Process
1. Patterns Preparing
- It uses a model with the exact details as the finished part. The only difference is a margin for thermal contraction (i.e., shrinkage).
- Metal injection dyeing is a significant component of creating wax patterns.
2. Wax Pattern Assembly
- After the wax pattern is formed, it is combined with various wax components to create the metal supply system for the gate and runner.
- Once the first step is completed, if necessary, multiple models will be processed with a single tree depending on the size and configuration of the finishing component.
3. Creating Mold Shell
- A ceramic slurry is used to dip the entire wax model. The slurry is covered with sand stucco and then allowed to dry.
- The dipping and stuccoing process are repeated until the shell of the required thickness is created. The thickness depends somewhat on the size and configuration of the product.
- After the ceramic shell dries, it becomes strong enough to bear the molten metal during casting.
4. Wax Removing
- To remove the wax, the whole assembly is set in a steam autoclave so that as much wax as possible can melt.
- However, if any wax remains in the ceramic shell, it will be burned off in an oven. At this point, all remaining wax patterns and input material are entirely removed. Only the ceramic mold remains inside the cavity with the desired molded part shape.
- The strength and stability of the ceramic material also increases with this high-temperature process. Also, this process minimizes the shell and the metal reaction.
5. Melt & Cast
- The mold is first preheated to a specific temperature to create the metal casting, and then molten metal is poured into it.
- This process can produce almost any type of alloy. You can apply air melting or vacuum melting depending on the alloy chemistry. Usually, vacuum melting is used when the alloy contains reactive elements.
6. Final Operations
After the casting is cooled down to the desired temperature, they break the mold shell in a knockout operation.
After that the gates and runners are removed from the casting, and other processes like final post-processing, sandblasting, machining, and grinding, is done if required.
They also do non-destructive testing. This testing may include fluorescent penetrant, magnetic particle, radiographic, or other inspections. Once done with all the processes then final dimensional inspections, alloy test results, and NDT are done before shipment.
Investment Casting Advantages
When manufacturing complex and intricate parts, design freedom is essential because they can have multiple internal and external shapes. You can produce any part with investment casting, regardless of size, thickness, or configuration. Investment casting can adapt or transform to meet any challenge.
Of the many benefits, tighter tolerances are the most prominent. When a part is manufactured, it must meet the exact specifications of the design so that it can be easily integrated into other parts. Many investment castings meet casting tolerances (CT) of five or six.
You only need a touch of superior finishing with investment casting because the process produces fine finishes. This process is famous for producing top-notch finishes. The excellent quality of the surface finishes consistently exceeds expectations and is far better than other casting processes. None of the other methods can meet the combination of tight tolerances and exceptional finishes like investment casting.
Investment casting does not limit you to producing a certain quantity of parts. Whether it’s a small run or thousands of runs, investment casting will produce parts quickly and flawlessly. Whether it’s a 0.1 kg part or a 100 kg part, it can easily be produced and finished with investment casting.
Regardless of type and shape, investment casting can produce any metal including stainless steel. That’s why this process is considered number one. Investment casting can combine with any form of molten metal to create a reliable and valuable part, which is its exclusive property.
No other process than investment casting can produce minute, miniature components. This unique capability is the reason why investment casting has been preferred over other methods in the aerospace industry. It was also the only process used to develop the first jet aircraft. As technology advances, the accuracy of imperfection and dimensional tolerances will also increase.
Whether it’s the initial step, creating the ceramic shell, or cutting additional parts, the investment casting process produces intricate and precise details with accuracy.
It is necessary to check whether the process is environmentally friendly or not with any job. However, with investment casting, you don’t need to do that. All the products used, such as patterns, wax, ceramic paste, and stucco coatings, can be reused without producing much waste. This is another reason why this procedure is popular.
Investment Casting Disadvantages
The process is expensive, and its cost is the only disadvantage, especially in the case of short-term productions. The high cost is due to the following reasons
- Specialized equipment
- Expensive refractories
- Stages of mold making
- A lot of labor is required
- Occasional minor defects.
However, the cost is still less than other processes such as bar casting. This is why firearm manufacturing has switched to investment casting due to the lower cost of gun production.
- Items requiring cores will be challenging to cast.
- The process is expensive and sometimes limited to small parts.
- Hole size is limited: it must be larger than 1/16 in. (1.6 mm) and should not be deeper than about 1.5 times the diameter.
- The process consists of longer production runs compared to other molding processes.
- Many factors can affect the quality of the molds and castings, so maintaining the quality management system is challenging.
Investment Casting Applications
Aerospace is the premier industry that uses investment casting to produce parts with exceptional tolerances and finishes. A wide range of metals is used to create flight components, which is only possible with investment casting because it allows the necessary selection of metals. Investment casting allows for efficient production of precision parts using less energy and minimal materials and energy waste.
Aerospace components must have the property to tolerate extreme weather conditions, pressure fluctuations, and similar operational wear and tear, which requires superior durability. However, investment casting will achieve consistency, precision, and tensile strength that will exceed the requirements. This method provides aerospace components with a precision that allows the connected parts to fit together quickly and easily.
Investment casting allows firearms manufacturers to create new and unique designs. Accuracy and precision are the only requirements for firearms manufacturers. However, investment casting can achieve a clean shape from a wide selection of alloys.
Investment casting minimizes the amount of metal removed during the finishing process. CNC machining also allows us to manufacture similar parts slightly at a low cost.
Medical and Dental
Instruments and components used in the medical and dental fields must be precise and meet strict tolerances and dimensional requirements. So they use investment casting to produce surgical tools, implants, machines, stretchers, and wheelchairs.
All of these parts save lives and must be of high quality. Using the investment casting process in this industry makes it more valuable.
Locks need to be durable and strong. With investment casting, you can produce both specialized and regular locks. The lock industry uses investment casting to produce precise parts with meticulous detail.
Food and Dairy
The food industry also needs huge machines with detailed and accurate images to produce the food we eat. That’s why this industry also uses investment casting to produce components and machine parts. Parts used in the food industry are made of stainless steel or special alloys because they offer precision and durability. Meat slicers, poultry processing equipment, ice machine parts, grills, and reheating machines have components made exclusively from investment casting.
The fluid power industry requires hydraulic and pneumatic equipment to convert power into a usable form. Components used in this section include ball valve parts, steam trap parts, impellers, needle valve parts, compressor parts, and pumps. Also, the food and dairy industry uses stainless steel, aluminum, and some special metals.
Whether you want to create miniature or complex metal parts, investment casting is the most reliable and cost-effective process, with little or no downside. You can also increase the efficiency of investment casting by choosing the right partner.