1- What Is Investment Casting?
Investment casting, which is also known lost wax casting or precision casting, refers to the formation of ceramic around the wax patterns to create a multi or single part mold to receive molten metal. This process utilizes an expendable injection molded wax pattern process to achieve complex forms with exceptional surface qualities. To create a mold, a wax pattern, or cluster of patterns, is dipped into ceramic material several times to build a thick shell. De-wax process is then followed by the shell dry process. The wax-less ceramic shell is then produced. Molten metal is then poured into the ceramic shell cavities or cluster, and once solid and cooled, the ceramic shell is broken off to reveal the final cast metal object. Precision investment castings can achieve exceptional accuracy for both small and large casting parts in a wide range of materials.
2- What Are The Advantages of Investment Casting?
✔ Excellent and smooth surface finish
✔ Tight dimensional tolerances.
✔ Complex and intricate shapes with design flexibility
✔ Capability to cast thin walls therefore a lighter casting component
✔ Wide selection of cast metals and alloys (ferrous and non-ferrous)
✔ Draft is not required in the molds design.
✔ Reduce the need for secondary machining.
✔ Low material waste.
3- What Are The Steps of Investment Casting Process?
During the investment casting process, a wax pattern is coated with a ceramic material, which, when hardened, adopts the internal geometry of the desired casting. In most cases, multiple parts are cast together for high efficiency by attaching individual wax patterns to a central wax stick called a sprue. The wax is melted out of the pattern – which is why it is also known as the lost wax process – and molten metal is poured into the cavity. When the metal solidifies, the ceramic mold is shaken off, leaving the near net shape of the desired casting, followed by finishing, testing and packaging.
4- What Are The Investment Castings Used For?
Investment castings are widely used in pumps and valves, automobile, trucks, hydraulics, forklift trucks and many other industries. Because of their exceptional casting tolerance and exellent finish, the lost wax castings are used more and more. Especially, the stainless steel investment castings play a vital important role in the shipbuilding and boats because they have strong anti-rust performance.
5- What Casting Tolerance Could Your Foundry Reach by Investment Casting?
According to the different binder materials used for making the shell, the investment casting could be divided into silica sol casting and water glass casting. The silica sol investment casting process have better Dimensional Casting Tolerances (DCT) and Geometrical Casting Tolerances (GCT) than water glass process. However, even by same casting process, the Tolerance Grade will be different from each cast alloy due to their various castability. Our foundry would like to talk with you if you have special request on the required tolerances. Here in the following are the general tolerances grade we could reach both by silica sol casting and water glass casting processes separately:
✔ DCT Grade by Silica Sol Lost Wax Casting: DCTG4 ~ DCTG6
✔ DCT Grade by Water Glass Lost Wax Casting: DCTG5 ~ DCTG9
✔ GCT Grade by Silica Sol Lost Wax Casting: GCTG3 ~ GCTG5
✔ GCT Grade by Water Glass Lost Wax Casting: GCTG3 ~ GCTG5
6- What Are the Size Limits of Investment Cast Components?
Investment castings can be produced in all alloys from a fraction of an ounce, for dental braces, to more than 1,000 lbs. (453.6 kg) for complex aircraft engine parts. Smaller components can be cast at hundreds per tree, while heavier castings often are produced with an individual tree. The weight limit of an investment casting depends on the mold handling equipment at the casting plant. The majority of U.S. facilities cast parts up to 20 lbs. (9.07 kg). However, many domestic facilities are increasing their capability to pour larger parts, and components in the 20-120-lb. (9.07-54.43-kg) range are becoming common. A ratio often used in designing for investment casting is 3:1—for every 1-lb. (0.45-kg) of casting, there should be 3 lbs. (1.36 kg) to the tree, depending on the necessary yield and the size of the component. The tree always should be significantly larger than the component, and the ratio ensures that during the casting and solidification processes, the gas and shrink will end up in the tree, not the casting.
7- What Kind of Surface Finishes Are Produced with Investment Casting?
Because the ceramic shell is assembled around smooth patterns produced by injecting wax into a polished aluminum die, the final casting finish is excellent. A 125 rms micro finish is standard and even finer finishes (63 or 32 rms) are possible with post-cast secondary finishing operations. Individual metal casting facilities have their own standards for surface blemishes, and facility staffs and design engineers/customers will discuss these capabilities before the tooling order is released. Certain standards depend on a component’s end-use and final cosmetic features.
8- Are Investment Castings Expensive?
Due to the costs and labor with the molds, investment castings generally have higher costs than forged parts or sand and permanent mold casting methods. However, they make up for the higher cost through the reduction of machining achieved through as-cast near-net-shape tolerances. One example of this is innovations in automotive rocker arms, which can be cast with virtually no machining necessary. Many parts that require milling, turning, drilling and grinding to finish can be investment cast with only 0.020-0.030 finish stock. Further more, investment castings require minimal draft angles to remove the patterns from the tooling; and no draft is necessary to remove the metal castings from the investment shell. This can allow castings with 90-degree angles to be designed with no additional machining to obtain those angles.
9- What Tooling and Pattern Equipment Is Necessary for Lost Wax Casting?
To produce the wax mold patterns, a split-cavity metal die (with the shape of the final casting) will need to be made. Depending on the complexity of the casting, various combinations of metal, ceramic or soluble cores may be employed to allow for the desired configuration. Most tooling for investment casting costs between $500-$10,000. Rapid prototypes (RP), such as stereo lithography (SLA) models, also can be used. The RP models can be created in hours and take on the exact shape of a part. The RP parts then can be assembled together and coated in ceramic slurry and burned out allowing for a hollow cavity to obtain a prototype investment cast component. If the casting is larger than the build envelope, multiple RP sub-component parts can be made, assembled into one part, and cast to achieve the final prototype component. Using RP parts is not ideal for high production, but can help a design team examine a part for accuracy and form, fit and function before submitting a tool order. RP parts also allow a designer to experiment with multiple part configurations or alternative alloys without a large outlay of tooling cost.
10- Are There Porosity and/or Shrinkage Defects with Investment Castings?
This depends on how well a metal casting facility make the gas out from the molten metal and how fast the parts solidify. As mentioned earlier, a properly built tree will allow porosity to be trapped in the tree, not the casting, and a high-heat ceramic shell allows for better cooling. Also, vacuum-investment cast components rid the molten metal of gassing defects as air is eliminated. Investment castings are used for many critical applications that require x-ray and must meet definite soundness criteria. The integrity of an investment casting can be far superior to parts produced by other methods.
11- What Metals and Alloys Could Be Poured by Investment Casting at Your Foundry?
Almost most of the ferrous and nonferrous metal and alloys could be cast by investment casting process. But, at our lost wax casting foundry, we mainly cast the carbon steel, alloy steel, stainless steel, super duplex stainless steel, gray cast iron, ductile cast iron, aluminium alloys and brass. Additionally, certain applications require the use of specialized other alloys used primarily in harsh environments. These alloys, such as Titanium and Vanadium, meet the additional demands that might not be achieved with standard Aluminum alloys. For example, Titanium alloys often are used to produce turbine blades and vanes for aerospace engines. Cobalt-base and Nickel-base alloys (with a variety of secondary elements added to achieve specific strength-strength, corrosion-strength and temperature-resistant properties), are additional types of cast metals.
12- Why Is Investment Casting Also Called Precision Casting?
The investment casting is also called precision casting because it have much better surface and higher accuracy than any other casting process. Especially for the silica sol casting process, the finished castings could reach the CT3 ~ CT5 in geometrical casting tolerance and CT4 ~ CT6 in dimensional casting tolerance. For the casings produced by investment, there will be less or even no need to make the machining processes. To some extent, the investment casting could replace the rough machining process.
13- Why Is Lost Wax Casting Called Investment Casting?
The investment casting gets its name because the patterns (wax replicas) is invested with the surrounded refractory materials during casting process. The "invested" here means being surrounded. The wax replicas should be invested (surrounded) by the refractory mateials to withstand the high temperature of the flowing molten metals during casting