Investment casting also called precision casting or lost wax casting. This process can be returned to 5,000 years ago and has been used to make fine art and jewelry for centuries. Today, this type of casting is viewed by many industries looking for lightweight and complex parts.

Although there are different types of casting processes, most of them pour liquid materials, such as molten metal or heated plastic, into the center of the hollow mold. After the liquid material cools, the finished product is extracted from the cavity of the mold. The surface of the finished product made in this way is very smooth, with few pores or burrs. Therefore,precision investment casting has been increasingly used in industrial production in recent years.

The most important thing in investment precision casting is to construct the wax model of the finished product. This can be done in one of three ways:
1. Form a gelatin mold around the solid 3D model of the final product.
2. Create a metal mold for the final product and fill it with hot wax.
3. Engrave and process copies of the product in wax.

The Investment Casting Process Steps

1. Master Pattern

A master pattern is created from wax, clay, wood, steel or plastic by either a specialized mold maker or sculpture artists as per final product requirement. These are made considering both wax and casting material shrinkage, hence it’s called, “master pattern” for having double shrinkage allowance.

2. Master Die

A investment casting mold, known as the master die to create wax patterns is made to fit the master pattern by either casting or machining. Technology advancement in the tooling and CNC machining has enabled machining a master to die from a material such as steel and aluminum without the need for a master pattern.

The master pattern and master die must be designed taking shrinkage of pattern wax, refractory material, and the metal cast material into consideration. It’s also good practice to add machining allowances for post-machining on tighter tolerance surfaces.

3. Wax Pattern Creation

A pattern that replicates the finished part geometry is made using one of two primary methods:

Create A Wax Injection Mold
If wax molds are to be used, the first step involves designing and manufacturing metal molds from aluminum. The mold creates a wax replica of the desired part by injecting molten wax into the cavity under high pressure. According to volume requirements, the mold can be made into a simple single-cavity manual tool or a complex multi-cavity automatic tool.

3D Print Pattern
If a 3D printed pattern is to be used, send the CAD model containing the part geometry to the printer and print the part.

4. Assemble Wax Patterns to Form A Tree

The tree, formed by combining several wax patterns along with a common sprue greatly increases the efficiency of the investment casting process. These components allow multiple steps to be completed at once instead of investment casting one part at a time.

After a specified number of wax patterns are generated, the patterns are fixed on wax sticks to form trees, sometimes referred to as "clusters". The pattern is attached to the location of the gate, and the metal will enter each casting. The lost wax casting tree must be large enough to provide enough feed metal to help provide good castings during metal solidification.

The number of patterns assembled into a tree is one of the main factors that determine the unit price of the final casting: more patterns per tree can lead to a lower unit price.

5. Shell Coating

The lost wax metal casting pattern is then immersed in a finely ground refractory slurry and then drained to form a uniform surface coating called "primer". At this stage, very fine particle sizes will be deposited to form a thin and very smooth investment material layer to ensure a smooth surface and a complex and fine final product. Very fine silica and other binders (including water, ethyl silicate and acid) are commonly used as refractory materials.

After the initial layer of the shell has dried, the pattern is immersed again and coated repeatedly to increase the thickness. The second stage of the immersion process is called "stuccoing". Sand or other refractory aggregates are poured on the wet surface to increase the strength and thickness (5 to 15 mm [0.2 to 0.6 inches]).

6. Dewax

Before lost wax precision casting, the wax must be removed. Since the wax used has a much larger coefficient of thermal expansion than the surrounding cladding material, most casing failures occur at this point. When the wax is heated, it expands and generates stress. To prevent this from happening, the wax must melt quickly so that it does not have time to swell before it becomes liquid and rapidly reduces viscosity.

After the ceramic mold is fully cured, place it upside down in a furnace or autoclave to melt or evaporate the wax. In some cases, you can drill holes in the lost wax precision casting mold before heating to help reduce these stresses. Any wax flowing out of the mold can usually be recycled.

The boiler remains pressurized, and we repeat this lost wax casting process for the next batch of shells.

7. Preheat Mold

The dewaxed ceramic mold is not yet ready for an injection of molten metal alloy. Like any ceramic, it must be fired to an appropriate temperature to accept the pouring of molten metal. Put the lost wax mold in the oven, heat it to 1,900 degrees Fahrenheit, and let it cure at that temperature for a certain amount of time. This firing process also burns away the wax residues left in the mold, which can cause investment casting defects.

This warm-up investment casting process completes four tasks:
● Make sure the pattern wax is completely removed
● To further strengthen the mold wall
● Allow the molten metal to maintain its fluidity as it passes through the mold
● Improves dimensional accuracy by allowing molds and castings to shrink together when cooled

8. Casting

Use a spectrometer to confirm the chemical properties of the metal, and then start casting the mold. The casting method depends on the material used, the size of the mold and the equipment used.

Changes include: gravity casting, vacuum casting, and vacuum die casting (VPC). In gravity casting, the fired mold still at about 1,900 degrees Fahrenheit is pulled out of the oven and sent into the furnace for pouring.

The metal quickly enters the mold cavity, and as the mold is placed to cool, the metal inside the mold begins to solidify.

9. Clean Casting

Current room temperature castings are "cleaned" by removing the ceramic mold material and sawing or grinding off the gate. The once strong ceramic mold has been used up and can be shaken off by vibration and sandblasting. Cut a single casting from the metal tree, and then remove the gate (the position where the casting is fixed on the tree) through the grinding operation. The casting is then sandblasted to remove any residual ceramic and improve the surface quality for inspection.

10. Finishing Casting

After heat treatment, the investment casting parts are again sandblasted and subjected to final inspection before shipment. Many investment castings are close to mesh or mesh products, which means that their dimensional accuracy is high enough that little machining is required. If the parts require additional straightening, this process is usually performed with a hydraulic straightening press to bring the product within its tolerances.

11. Quality Inspection

Inspection is also an important step in quality control of precision castings. Our quality control department will carry out dimensional inspection according to product requirements, including 100% surface inspection, internal defect inspection and other inspection work. Only after all products have passed the inspection can we deliver to the customer.

12. Package

Usually, Yifei Machinery first pack the precision lost wax castings and plastic bags together, and then put them into cartons and standard wooden boxes. We can also provide customized packaging services according to customer requirements.

Types of Investment Casting Materials

As one of the professional investment casting companies in china, Yifei produce a variety of precision components with different investment casting materials.

Stainless Steel

Compared with many other materials, stainless steel has better durability. It can be used in aerospace and defense and other industries that require non-corrosive parts that must be able to provide the tensile strength required for demanding applications. Stainless steel is also used in the precision casting of the food industry and medical, mining and piping applications, gearbox parts in automotive applications, various gears, camp components, and golf club heads.

Low-alloy Steel

Low alloy steel has become one of the most commonly used steels in the mechanical field due to its low cost and good mechanical properties. The special heat treatment process can design parts with different characteristics in different areas of the same workpiece. For example, different heat treatments can be used to make one surface tough and impact-resistant, while the other surface becomes wear-resistant.

Aluminum Alloy

Aluminum alloy casting materials are widely used by many industries with weight requirements due to their lightweight and corrosion resistance. Aluminum can be heat treated to achieve higher strength, ductility, corrosion resistance, and workability. Aluminum alloy is the most commonly used material in custom investment casting service, especially in aerospace, avionics, electronics, military command equipment, and pump and valve applications.

Carbon Steel

Carbon steel is a common low-cost material with various grades, and its classification changes according to the carbon content. Carbon steel is economical and easy to process. It can be heat-treated to increase its strength and ductility and performance. Because carbon steel is safe and durable, it is widely used in the following industries: including mining, military, and industrial applications.

Copper Alloy

The copper-based alloy is corrosion-resistant and has a low wear rate. More common types of copper-based alloys include bronze and brass. The strongest copper alloy is beryllium copper, which has characteristics similar to high-strength alloy steel but has higher corrosion resistance for a longer period.

Like many other alloys, copper and copper-based alloys can also be formed into highly complex parts, making them ideal for precision casting. They are commonly used in applications such as propellers for ships or pumps, electrical components, and piping components.

Copper Alloy

Iron castings usually include gray iron and ductile iron and are known for their high precision and affordable prices. Ductile iron has high strength, heat resistance, and toughness, but its production process is more complicated than other steels, which also causes higher production costs than cast steel.

Nickel-based Alloys

Nickel-based alloys are known for their resistance to oxidation and corrosion in harsh environments and high-temperature applications. Nickel alloy casting materials include stainless steel, nickel-chromium alloy, nickel-copper alloy, etc. Nickel-based alloys are widely used in precision casting parts. These parts will be used in the harsh applications of oil and gas, medical, nuclear energy, and aerospace and defense industries.

Cobalt-based Alloy

Cobalt-based alloys are investment casting materials that are alloyed with nickel, chrome, and tungsten. They have excellent thermal corrosion resistance and thermal fatigue resistance, so they are very suitable for high-temperature applications. This allows cobalt-based alloy castings to be used in many industries. Including medical, aerospace, automotive, military, and other applications where oxidation problems are severe and high intensity is critical in high-temperature environments.

Advantages of Investment Casting

1. Compared with die casting, sand casting, the investment casting product has good surface smoothness. Usually about 1.3-0.4 microns RMS Ra.
2. Lost wax castings have excellent dimensional accuracy, and it is easy to achieve tighter tolerances of 0.075 mm (0.003 inches).
3. Less waste of resources, used wax can be recycled.
4. It is possible to manufacture complex shapes that are difficult to make easily by other processes.
5. The larger the finished product shape, the lower the allocated cost.
6. With shorter length or shallower depth, thin parts with a thickness as low as 0.40 mm (0.015 inches) can be precision investment cast without cold seal defects.
7. In terms of investment casting materials, it has almost no restrictions. The most common materials include aluminum alloys, cast iron, and non-ferrous alloys, especially suitable for high-temperature alloys.
8. Because there is no parting line, the investment casting product will not have any flash. But the wax mold may have parting lines with the master mold.
9. The investment casting process can be automated to produce a large number of parts.
10. It is not necessary to make a primer on the wall, but if a master mold is used to make the wax model, then the primer on the surface will make the marking process easier.
11. Investment precision casting can be used to replace die castings in the short term.

Disadvantages of Investment Casting

1. Precision casting has a high cost. In fact, it is more expensive than sand casting, permanent mold, and gypsum casting processes.
2. The high cost is also due to the need for special equipment, expensive refractory materials, and high labor costs.
3. Casting objects that require cores can be difficult.
4. The hole cannot be smaller than 1/16 inch (1.6 mm), and the depth should not be greater than 1.5 times the diameter.
5. Compared with other casting processes, precision investment castings require a longer production cycle.
6. Due to the high cost and long cycle time of this process, this process is actually not suitable for mass production.
7. The size of the castable parts is limited, and it is more suitable for the production of smaller size parts, weighing about 75 pounds, while the larger parts are usually more suitable for other casting processes.
8. Each casting requires a separate pattern.
9. The investment casting process is complex and usually requires expensive equipment and / or technicians to operate.
10. Occasionally, the finished lost wax casting supplies may have defects.

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