What is Investment Casting?

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What is Investment Casting?

 What is investment casting?

 Introduction of investment casting processInvestment casting, also known as precision casting or lost wax casting, is a manufacturing process of molding disposable ceramic molds with wax molds. The wax mold is made according to the precise shape of the casting. The model is coated with refractory ceramic material. Once the ceramic material hardens, turn it over and heat it until the wax melts and is discharged. The hardened ceramic shell becomes a one-time investment. The molten metal is poured into a mold and allowed to cool. Then the metal casting is taken out of the used mold.

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 The term investment casting comes from the process of "investment casting" (enclosing) a model with refractory materials. Investment casting is usually more popular than other molding methods because of its fine details and excellent as-cast surface finish. They can also be cast into thin walls and complex internal channels. Unlike sand casting, investment casting does not need draft angle.

 These process qualities can provide castings with final shape or near final shape, which saves customers a lot of costs in terms of materials, labor and processing. It can utilize the most common metals, including aluminum, bronze, magnesium, carbon steel and stainless steel. The parts made by investment casting include turbine blades, medical equipment, firearm parts, gears, jewelry, golf club heads and many other machine parts with complex geometric shapes.

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 Investment casting processInvestment casting process consists of several steps: metal mold construction, wax mold making, ceramic mold making, pouring, solidification, sand falling and cleaning.

1. Metal mold structure

 Wax mold and ceramic mold will be destroyed during investment casting, so a new wax mold is needed for each casting. Unless investment casting is used to produce a very small volume (which is a common work of art or original jewelry), molds or molds are necessary to make wax models.The size of the die must be carefully calculated; It must consider the expected shrinkage of the wax pattern, the expected shrinkage of the ceramic material covered on the wax pattern and the expected shrinkage of the metal casting itself.

2. Wax mold production

 The number of wax molds is always equal to the number of castings to be produced; Each casting needs a new wax mold.Hot wax is injected into the mold and cured. A core may be required to form any internal features. The final wax model is an exact replica of the parts to be produced. This method is similar to die casting, but wax is used instead of molten metal.

3. Mold creation

The gating system (main runner, runner and riser) is connected to the wax mold. For smaller castings, several wax molds are connected to a central wax gating system to form a tree-like assembly. Usually, a pouring cup connected to the end of the pouring rod is used to introduce molten metal into the mold.The assembled "model tree" is immersed in the fine-grained silica slurry. It is dipped repeatedly, and more and more refractory mud is coated on each dip. Once the refractory coating reaches the required thickness, it is allowed to dry and harden; The dried coating forms a ceramic shell around the mold and gating system.The thickness of the ceramic shell depends on the size and weight of the cast part and the pouring temperature of the cast metal. The average wall thickness is about 0.375 inches. (9.525mm). Turn the hardened ceramic mold upside down, put it in the oven and heat it until the wax melts and flows away. The result is a hollow ceramic shell.

4.Pour

 The mold is heated to about 1000–2000 F (550–1100 C). The process of heating further strengthens the mold, eliminates any residual wax or contaminants, and evaporates water from the mold material.The molten metal is injected into the mold while it is still hot-the liquid metal flows into the pouring cup, passes through the central pouring system, and enters each mold cavity on the tree. Preheating the mold allows the metal to easily flow through the thin and delicate parts. Since the mold and the casting will cool and shrink together, it also produces a casting with improved dimensional accuracy.

5.Cool down

 After the mold is poured, the metal cools and solidifies. The time required for the mold to cool to a solid state depends on the cast material and the thickness of the casting.

6. Sand falling

 Once the casting solidifies and the ceramic mold breaks, the casting can be taken out. Ceramic molds are usually broken by hand or water spray. After removal, separate the single casting from its gating system tree using manual impact, sawing, cutting, burning or liquid nitrogen cold crushing.

7. End

 Polishing operations, such as grinding or sandblasting, are usually used to smooth the parts at the gate and remove defects. Depending on the metal of the casting, heat treatment can be used to harden the final part.When to use investment casting?Because of its complexity and labor requirements, investment casting is a relatively expensive process-but the benefits often exceed the costs. Almost any metal can be investment cast. Parts made by investment casting are usually small, but this process can be effectively used for parts weighing 75 pounds or more.

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 Investment casting can produce complex parts with excellent as-cast surface smoothness. Investment castings don't need built-in taper to take the parts out of the mold, because the ceramic shell will fall off the parts when it cools. This production feature allows the design of castings with 90-degree angles, without built-in shrinkage allowance, and without additional machining to obtain these angles.

 Investment casting process produces parts with extremely high dimensional accuracy; Net parts are easy to obtain, and the finished shape usually does not need secondary processing. Each unique casting operation requires a new mold to produce wax patterns. Tools for investment casting can be quite expensive; Depending on the complexity, the tool cost may be between $1,000 and $10,000.

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 For large-volume orders, it is easy to make up for the cost of new molds by eliminating or reducing the time and labor saved by secondary processing. Small-scale casting is unlikely to make up the investment. Generally speaking, investment casting is a reasonable choice for 25 or more parts.

 It usually takes 7 days from a fresh wax mold to a complete casting; Most of this time is spent on manufacturing and drying ceramic shell molds. Some foundries have quick-drying ability, which can produce castings more quickly. The time and labor-intensive characteristics of investment casting not only affect the cost. The equipment and production capacity of the foundry are limited, so it is common for investment casting to have a long lead time.