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Investment casting, typically involves creating investment patterns and assembling modules using low melting point materials such as wax materials. Then, several layers of refractory materials are applied to the surface of the module. After hardening and drying, the module is heated to melt out the wax material, forming a hollow shell. The shell is then sintered at high temperatures, followed by pouring molten metal into it. After cleaning, the casting is obtained. Due to the high dimensional accuracy and surface finish of the obtained castings, investment casting is also known as "precision investment casting". Additionally, since investment materials are usually wax-based, it is also called "lost wax casting".
Precision investment casting is an advanced near-net forming process that can produce precise and complex castings of various alloys. The castings are close to the final shape and size of the parts and can be used directly or with minimal machining. Below, we will introduce the process, advantages, typical applications, and suitable parts for precision investment casting.
Precision investment casting mainly includes six process steps: wax pattern forming, tree assembly, shell making, dewaxing and burning, melting and pouring, and cleaning and cutting to obtain the final product.
Wax Pattern Forming
Wax is injected into the mold cavity, cooled, and then removed to form the wax pattern.
Tree Assembly
Multiple wax patterns are welded onto the gating system.
Shell Making
The wax pattern surface is first dipped in a prepared silica sol coating and then sprinkled with refractory sand. The shell is dried and hardened under specific temperature and humidity conditions to form a compact refractory coating on the wax pattern surface. This process is repeated 5-6 times to form a silica sol shell with sufficient strength and refractoriness.
Dewaxing and Burning
Dewaxing involves melting and draining the wax out of the shell using high-temperature steam, creating a cavity shell ready for casting. Burning the shell removes residual wax and water, with precision casting typically done in the red shell state, baking the shell at about 1000 degrees Celsius for 1-2 hours.
Melting and Pouring
The material composition is prepared according to product requirements, followed by melting the steel, slag removal, and spectral testing. Once the composition is qualified, the molten steel is poured into the red hot shell strictly according to the process card to gradually form the blank.
Cleaning and Cutting to Obtain the Final Product
After the blank has cooled sufficiently, manual (hammer) or vibrating machines are used to separate the shell from the blank. The casting is then cut from the gating system to obtain the final product. Subsequent processes such as polishing and machining can be done as needed.
Can Form Extremely Complex Parts
As a metal forming process, precision investment casting can form extremely complex parts. Compared to die casting, sand casting, and lost foam casting, precision investment casting allows for the highest part complexity. Date marks, text, and internal ribs can all be formed.
This feature allows for consolidating parts originally processed by other methods into a single part using precision investment casting, simplifying product structure, eliminating assembly processes, and reducing costs.
In other words, precision investment casting provides great design freedom, allowing for the formation of extremely complex parts.
Good Appearance Quality
Investment castings have good appearance quality with low surface roughness, achieving a surface roughness of up to Ra3.2μm.
Generally, the surface roughness of milled metal is Ra3.2. In other words, the appearance quality of investment castings is comparable to milling.
This means that in some cases, secondary machining processes or surface treatments such as painting can be eliminated.
High Dimensional Accuracy
Compared to other casting processes, investment castings have higher dimensional accuracy.
No Material Restrictions
Metal materials suitable for precision investment casting are not restricted, including carbon steel, alloy steel, heat-resistant alloys, stainless steel, precision alloys, permanent magnet alloys, bearing alloys, copper alloys, aluminum alloys, titanium alloys, and ductile iron.
Precision investment casting is suitable for the following application scenarios:
Parts Weighing Between 200g~50kg
Although precision investment casting can also form large castings, it is more suitable for forming small to medium-sized castings. On one hand, when the casting is too large, its dimensional accuracy is hard to ensure; on the other hand, mold and machining costs are higher. Sand casting is a better choice for large castings. Therefore, precision investment casting is ideal for forming small parts where dimensional accuracy is easier to guarantee, and costs are within acceptable limits.
Medium to Large Batch Production
Precision investment casting requires investment in wax pattern molds and other tooling fixtures, making it more suitable for medium to large batch production.
High Dimensional Accuracy Requirements
Compared to other casting processes, precision investment casting offers the highest dimensional accuracy.
Need to Reduce or Eliminate Machining
Due to the high dimensional accuracy and good surface finish of precision investment castings, machining can be eliminated. Even if machining is required, the workload is less than with other casting processes.
Complex Internal Structures
When parts have complex internal structures, precision investment casting may be the best choice.
Precision investment casting can form extremely complex parts because there are no demolding issues. The mold used for pouring molten metal is ceramic, which easily detaches after cooling and solidifying. By using creative molds, soluble wax cores, or ceramic cores, it is easy to achieve complex internal structures in wax patterns, which are then coated to form a ceramic shell. Therefore, there are no limitations on part internal details in investment casting.
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