What do you want to know about investment casting?

12,560 Views 2024-11-09 17:22:15

A brief introduction to Investment Casting

Investment Casting usually uses investment materials (low melting point materials such as wax, etc.) to make molten molds and form molds, and then coats the mold surface with several layers of refractory materials.

After hardening, drying and solidification, the mold is heated to melt out the mold material to form a hollow shell, and then poured with liquid metal after high temperature sintering.

After cleaning, the casting is obtained.

Because the obtained casting has high dimensional accuracy and surface finish, investment casting is also called “investment precision casting”.

At the same time, because the investment material is usually a wax-based material, it is also called “lost wax casting (Lost Wax Casting)”

Investment casting is an advanced near-net shape molding process that can produce precision and complex castings of various alloys.

The castings are close to the final shape and size of the parts and can be used directly without machining or with little machining.

Investment casting process

Investment casting mainly includes six processes:

Wax mold forming->Tree assembly->Shell making->Dewaxing and roasting->Melting and pouring->Sand cleaning and cutting to obtain the finished product.

1.1 Wax mold forming

Wax mold forming is to press the wax material into the mold cavity, cool it and take it out to form a wax mold.

Wax modeling

1.2 Tree assembly

Tree assembly is to weld multiple wax molds on the pouring system.

Tree assembly

1.3 Shell making

Shell making is to first dip the prepared silica sol coating on the surface of the wax mold, and then sprinkle refractory sand on it.

The shell is dried and hardened at a specific temperature and humidity, so that a dense refractory coating is formed on the surface of the wax mold.

Then the process is repeated 5-6 times, and finally a silica sol shell with a certain strength and refractoriness is formed.

Shell making

1.4 Dewaxing and roasting

Dewaxing is to melt the wax in the shell through high-temperature steam to discharge the wax, so that a cavity shell that can be cast is obtained.

Shell roasting is mainly to burn off the residual wax and moisture in the shell. At the same time, precision casting is poured in the red shell state.

The shell is usually roasted at about 1000 degrees for 1-2 hours.

1.5 Melting and pouring

The ingredients are prepared according to the material composition of the product, and then the molten steel is melted and the slag spectrum test is performed.

After the composition is qualified, it can be poured. It is necessary to pour the molten steel into the shell in the red shell state in strict accordance with the requirements of the process card to gradually form a blank.

Melting and pouring

1.6 Sand cleaning and cutting to obtain the finished product

After the blank obtained after pouring is fully cooled, the shell is separated from the blank by manual (hammering) or vibration shelling machine, and then the casting is cut and separated from the pouring system to obtain the finished casting.

Polishing, machining and other processes can be carried out according to needs.

Advantages and Disadvantages of Investment Casting

The advantages of investment casting include:

2.1 Extremely complex parts can be formed

As a metal forming process, investment casting can form extremely complex parts. Compared with die casting, sand casting, lost foam casting and other processes, investment casting can allow the highest part complexity.

Date stamps, text and internal spines can all be formed.

Undercuts can be formed.

Complex pipes can be formed.

Taking advantage of this feature, investment casting can be used to combine parts that were originally processed by other processes into one part, thereby simplifying the product structure, eliminating assembly processes, and achieving the purpose of cost reduction.

From another perspective, investment casting provides great design freedom, and extremely complex parts can be formed by investment casting.

Complex investment casting parts

2.2 Good appearance quality

Investment castings have good appearance quality, low surface roughness, and the surface roughness can reach Ra3.2um.

Generally speaking, the surface roughness of milled metal is Ra3.2. In other words, the appearance quality of investment castings is similar to that of milling.

The advantage is that in some cases, investment casting can eliminate secondary machining processes or surface treatment processes such as painting.

2.3 High dimensional accuracy

Compared with other casting processes, investment castings have higher dimensional accuracy.

Of course, due to the complex process of investment casting, there are many factors that affect the dimensional accuracy of castings, such as the shrinkage of mold material, deformation of investment mold, changes in the amount of wire in the shell during heating and cooling, shrinkage rate of alloy, and deformation of castings during solidification.

Therefore, although the dimensional accuracy of ordinary investment castings is relatively high, its consistency still needs to be improved (the dimensional consistency of castings using medium and high temperature wax materials should be much improved).

2.4 No restrictions on materials

There are no restrictions on metal materials that can be used for investment casting, including carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and ductile iron, etc.

The disadvantages of investment casting include:

The biggest disadvantage of investment casting is that it is relatively expensive, because the process is cumbersome and the production cycle is long.

At the same time, wax molds, coatings, refractory sands, etc. are all costs. Therefore, investment casting is more suitable for medium and large batches of products.

For small batches, you can consider using 3D printing to make wax molds.

If a simple part process is replaced by investment casting, the cost will be higher.

If investment casting is used to reduce costs, you need to consider merging multiple parts and removing machining.

The size of the casting cannot be too large. The maximum weight of the casting can be 1000Kg. It is difficult to cast a casting that exceeds the weight.

There are too many factors that affect the quality of investment casting, and it is difficult to control the process quality.

Which parts are suitable for investment casting?

Investment casting is suitable for the following application scenarios:

3.1 Part weight between 200g and 50kg

Although investment casting can also form large castings, it is more suitable for forming small and medium-sized castings.

On the one hand, when the casting is too large, the dimensional accuracy is difficult to guarantee; on the other hand, the mold cost and processing cost will be relatively high.

Sand casting is a better choice for large castings.

Investment casting is more ideal for forming small parts, the dimensional accuracy is easier to guarantee, and the cost is within an affordable range.

3.2 Medium and large batch production

Investment casting first needs to process wax molds and other tooling fixtures, which requires a certain amount of fixed asset investment, so it is more suitable for medium and large batch production.

3.3 When the dimensional accuracy requirements are high

Compared with other casting processes, the dimensional accuracy of investment casting is the highest.

Investment casting parts display

3.4 When machining needs to be reduced and eliminated

Because of the high dimensional accuracy and good surface finish of investment casting, machining can be eliminated in investment casting, and even if machining is required, the amount of machining work is less than other casting processes.

3.5 When the internal structure of the part is complex

When the internal structure of the part is complex, investment casting may be the best choice.

Investment casting can form extremely complex parts because there is no demolding problem in investment casting.

The mold used to pour the molten metal is ceramic, which can easily fall off after cooling and solidification.

The internal structure of the wax mold that is coated to form a ceramic shell can be easily obtained through creative molds, soluble wax cores or ceramic cores.

Therefore, there is no limit to the internal details of the part in investment casting.

Typical Applications of Investment Casting

Investment casting has a wide range of applications, such as aerospace, power generation, automobiles, military, commerce, food, natural gas, oil and energy industries, such as blades, impellers, guides, guide wheels of steam turbines, gas turbines, water turbine engines, etc., and small parts on automobiles, tractors, pneumatic tools, machine tools, and handicrafts.