Principles and Characteristics of Investment Casting

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 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,” unearthed in 2400 BC.
2. The wax pattern technique in Yunnan and its application in aerospace blade casting.

II. Types of Mold Materials and Performance Requirements

(1) Classification of Mold Materials With the development of investment casting technology, the types of mold materials have become increasingly diverse, with varying compositions. Typically, mold materials are classified based on the melting point into high-temperature, medium-temperature, and low-temperature mold materials.

• Low-temperature mold materials have a melting point below 60°C. The widely used mold material in China, consisting of 50% paraffin and 50% stearic acid, falls into this category.
• High-temperature mold materials have a melting point above 120°C and typically comprise 50% rosin, 20% beeswax, and 30% polystyrene.
• Medium-temperature mold materials have melting points between the two aforementioned categories. Current medium-temperature mold materials can be divided into rosin-based and wax-based mold materials.

(2) Basic Requirements for Mold Material Performance Thermophysical properties: Appropriate melting and solidification temperatures, minimal thermal expansion and contraction, high heat resistance (softening point), and absence of precipitates in the liquid state and phase changes in the solid state. Mechanical properties: Mainly include strength, hardness, plasticity, and flexibility. Processability: Primarily involves viscosity (or fluidity), ash content, and coating ability.

III. Mold Making Process

According to the specified composition and ratio of mold materials, various raw materials are melted, mixed, and stirred to remove impurities, resulting in a paste-like mold material suitable for molding. Press molding is commonly used in the process, allowing for the use of liquid, semi-liquid, solid, or semi-solid mold materials. Liquid and semi-liquid mold materials are molded at low pressure, referred to as injection molding. Semi-solid or solid mold materials are molded at high pressure, known as extrusion molding. Regardless of the molding method used, considerations must be made for filling and solidification.

(1) Injection Molding During injection molding, the wax injection temperature is typically below the melting point, resulting in a slurry or paste-like state where both liquid and solid phases coexist in the mold material. In slurry-like mold materials, the liquid phase significantly exceeds the solid phase, allowing for continued fluidity. When injected in this state, the investment casting surface exhibits lower roughness and is less susceptible to surface defects caused by turbulence and splashing. Paste-like mold materials have a lower temperature, have lost fluidity, may have fewer surface defects but exhibit higher surface roughness.

During mold material injection molding, it is essential to maintain the lowest possible mold material temperature and molding working temperature while ensuring proper filling. The choice of pressure should not be excessive. Although high pressure reduces mold material shrinkage, excessive pressure and injection speed can result in an uneven mold surface, causing “bubbling” (expansion of gas bubbles beneath the mold surface) and cold separation defects. To prevent mold material adhesion to the mold, improve mold surface finish, and use a parting agent, especially for rosin-based mold materials.

(2) Extrusion Molding Extrusion molding involves the extrusion of mold materials in a semi-plastic state into the mold cavity, reducing and preventing mold material shrinkage under high pressure. During extrusion molding, mold materials are in a semi-solid or solid state and are relatively hard under normal conditions but can flow under high pressure. The key factor determining the extrusion pressure is the viscosity of the mold material and the flow resistance within the gating system and mold cavity. The greater the viscosity of the mold material, the smaller the gating orifice, the larger the mold cavity size but with a smaller cross-sectional area, and the longer the mold material travel distance, the greater the flow resistance. Thus, a higher extrusion pressure is required.

Using semi-solid mold materials for extrusion molding reduces the solidification time of the mold material, increasing productivity. This method is especially suitable for casting components with thick cross-sections.

IV. Shell Making Process Shell making comprises two processes: coating and stuccoing. Before applying the coating, the mold must undergo degreasing treatment. The coating is typically applied using the dipping method. During coating, it is essential to uniformly apply the coating to the mold surface, avoiding gaps and localized accumulations. Joints, corners, edges, and grooves should be evenly brushed with a brush or a special tool to prevent air bubbles. Before applying each reinforcing layer of coating, the sand on the previous layer should be cleaned. During the coating process, the coating should be stirred regularly, and the viscosity should be controlled and adjusted.

After coating, sand stuccoing is performed. The most common methods for sand stuccoing are fluidized bed stuccoing and rain shower stuccoing. Typically, after removing the mold from the coating tank, stuccoing is initiated once the remaining coating on the mold flows evenly and no longer drips continuously, indicating the end of coating fluidity and the start of solidification. Stuccoing should be carried out while continuously rotating and inverting the mold. The purpose of stuccoing is to fix the coating layers with sand grains, increase the shell thickness, obtain the required strength, improve shell permeability and collapsibility, and prevent cracking during shell hardening. Sand grain size for stuccoing should be selected according to the coating layer and adapted to the coating’s viscosity. Surface layers should use fine-grain sand to achieve a smooth mold cavity surface. Typically, fine-grain sand with a particle size of group 30 or 21 is used for surface layer stuccoing, while coarser sand is used for reinforcing layers. During shell making, after each coating and stuccoing layer, thorough drying and hardening are necessary.

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

Surface and internal defects include under-casting, cold shuts, shrinkage cavities, porosity, slag inclusions, hot cracks, and cold cracks. Dimensional and roughness deviations mainly refer to casting elongation and deformation.

The occurrence of surface and internal defects is primarily related to factors such as the pouring temperature of the alloy, the firing temperature of the shell, and the preparation process, as well as the design of the pouring system and the casting structure. Dimensional and roughness deviations are primarily due to mold design, wear of the molding tool, casting structure, shell firing and strength, and casting cleaning processes.

For example, if under-casting occurs in investment castings, it may be due to factors such as low pouring and shell temperatures, reduced metal fluidity, thin casting walls, inadequate pouring system design, insufficient shell firing or poor permeability, and slow pouring. In such cases, addressing the specific structure of the casting and related processes is necessary to eliminate defects.