I. Principles and Characteristics of Investment Casting

Investment casting, also known as precision casting or lost-wax casting, involves creating an accurate, meltable model using easily meltable materials such as wax and plastics. Several layers of refractory coatings are applied to the model, which is then dried and hardened to form a complete shell. Afterward, the shell is heated to melt the model, and high-temperature firing transforms it into a refractory shell. Liquid metal is poured into the shell, and once it cools, it becomes a cast component.

The Investment Casting process steps include pattern making, wax injection, assembling, pattern repairing, coating, stuccoing, dewaxing, firing, pouring, cooling, sand removal, and finishing.

In comparison to other casting methods, investment casting offers several advantages:

1. Higher dimensional accuracy and lower surface roughness, make it suitable for casting complex shapes. Generally, tolerances can reach 5~7 grades, and surface roughness can be as low as Ra25-6.3μm.
2. Capable of casting thin-walled and small, lightweight components. The minimum wall thickness for investment castings can be as low as 0.5mm, and weights can be as small as a few grams.
3. Ability to cast intricate patterns, text, components with fine grooves, and curved fine holes.
4. Almost no restrictions on external and internal shapes, making it possible to produce complex parts that are difficult to manufacture using sand casting, forging, or machining. It can also allow for the direct casting of some assemblies or welded parts as integral components, reducing part weight and production costs.
5. Versatility in casting various alloy types, including alloy steel, carbon steel, and heat-resistant alloy castings.
6. No limitations on production batch size, ranging from single pieces to large-scale production runs.

One drawback of this casting method is its complexity, longer production cycle, and unsuitability for very large-sized castings.

Examples:

1. The unparalleled and exquisite bronze vessel, the “Zunpan,” was unearthed in 2400 BC.
2. The wax pattern technique in Yunnan and its application in aerospace blade casting.

II. Types of Mold Materials and Their Performance Requirements

1. Mold Material Classification With the development of investment casting technology, there is a growing variety of mold materials, each with its unique composition. They are typically categorized based on their melting points as high-temperature, medium-temperature, and low-temperature mold materials.

• Low-temperature mold materials have a melting point below 60°C, and widely used in China is a 50% mixture of paraffin and stearic acid.
• High-temperature mold materials have a melting point above 120°C, and a typical high-temperature mold material is composed of 50% rosin, 20% ceresin, and 30% polystyrene.
• Medium-temperature mold materials have melting points between the above two categories and are mainly divided into rosin-based and wax-based mold materials.

2. Basic Requirements for Mold Material Performance

• Thermal properties: Suitable melting and solidification temperatures, low thermal expansion and contraction, high heat resistance (softening point), and no precipitation in the liquid state or phase changes in the solid state.
• Mechanical properties: These include strength, hardness, plasticity, and flexibility.
• Process properties: Factors such as viscosity (or flowability), ash content, and coating ability.

III. Mold Making Process

The mold-making process involves melting various raw materials according to the specified composition and ratio of the mold material. These molten materials are mixed and stirred to remove impurities, forming a paste-like mold material that can be used for molding. The molding process in investment casting is typically achieved through compression molding.

1. Injection Molding: In this method, the mold material is in a liquid or semi-liquid state when injected into the mold cavity. This is known as “investment casting.” The advantage is that it results in a smoother mold surface with fewer defects caused by turbulence or splashing.
2. Extrusion Molding: In extrusion molding, the mold material is in a semi-solid or solid state and is extruded into the mold cavity under high pressure. This is referred to as “squeeze-out” molding. The choice of molding method depends on the state and characteristics of the mold material.

Regardless of the method used, it is essential to consider both the filling and solidification during the molding process. Factors such as mold material temperature and molding temperature should be carefully controlled to ensure successful mold formation.

IV. Shell Making Process

The shell-making process consists of two main steps: coating and stuccoing. Before applying the refractory coatings, the wax pattern undergoes a de-waxing process. During coating, a uniform application of the coating material is crucial, preventing blank areas and localized build-ups. Special attention should be given to welding points, corners, edges, and grooves to avoid the formation of bubbles. Between each coating and stucco layer, it is essential to clean any loose sand. Stuccoing aims to solidify the coating layers further, increase shell thickness, enhance strength, improve permeability, and prevent cracking during firing. The choice of sand grain size should correspond to the coating layer, with finer grains typically used for surface layers and progressively coarser grains for reinforcement layers.

V. Defects and Prevention Methods

Defects in investment castings can be categorized into surface and internal defects, as well as dimensional and roughness deviations.

1. Surface and Internal Defects: These include under-casting, cold shuts, shrinkage cavities, porosity, slag inclusions, hot tears, and cold cracks.
2. Dimensional and Roughness Deviations: These relate to casting elongation and deformation.

The occurrence of surface and internal defects is primarily influenced by factors such as pouring temperature, shell firing temperature, and process preparation, as well as casting system and component design.

Dimensional and roughness deviations can be attributed to factors like mold design, wear and tear on molds, casting structure, shell firing and its strength, and cleaning processes.

For instance, if under-casting occurs in investment casting, possible causes may include low pouring and shell temperatures leading to reduced fluidity of the molten metal, overly thin casting walls, poor casting system design, inadequate shell firing or poor permeability, and slow pouring. Addressing these issues requires targeted solutions based on the specific structure of the casting and the related processes to eliminate defects.