Summary of Investment Casting Knowledge: Principle and process analysis of investment casting, and solutions to common defects

The principle and characteristics of investment casting

Investment casting, also known as precision casting or lost wax casting, is the process of making an accurate and fusible model using fusible materials such as wax and plastic. The model is coated with several layers of refractory coatings, which are dried and hardened to form a complete shell. The shell is then heated to melt and lose the model, and then high-temperature calcined to form a refractory shell. The liquid metal is poured into the shell and cooled to form the casting. Mold material – wax pressing mold – assembly mold – repair mold – coating and hanging – sanding – demolding – roasting – pouring – cooling – sand drop – cleaning. Compared with other casting methods, the main advantages of investment casting are as follows:

1. The dimensional accuracy of castings is high and the surface roughness is low, making it possible to pour castings with complex shapes. Generally, the accuracy can reach levels 5-7, and the roughness can reach two Ra25-6.3 μ M; It can cast thin-walled castings and small weight castings. The minimum wall thickness of investment castings can reach 0.5mm, and the weight can be as small as a few grams; Can cast patterns, texts with fine patterns, castings with fine grooves and curved holes;

2. The shape and inner cavity of investment castings are almost unrestricted, allowing for the production of complex shaped parts that are difficult to manufacture using methods such as sand casting, forging, and cutting. Additionally, some combination and welded parts can be directly cast into integral parts after slight structural improvements, thereby reducing the weight of the parts and lowering production costs;

3. There are almost no restrictions on the types of casting alloys, which are commonly used to cast alloy steel parts, carbon steel parts, and heat-resistant alloy castings;

4. There is no limit to the production batch, and it can be produced in large quantities from single pieces to batches.

5. The disadvantage of this casting method is that the process is complex, the production cycle is long, and it is not suitable for producing castings with large contour dimensions.

Types and performance requirements of mold materials

With the development of investment casting technology, the types and compositions of mold materials are becoming increasingly diverse. Usually, molds are classified into high temperature, medium temperature, and low temperature molds based on their melting points. The melting point of low-temperature molds is below 60 ° C, and 50% of the widely used paraffin stearic acid molds in China belong to this category; The melting point of high-temperature molds is higher than 120 ° C, and molds composed of 50% rosin, 20% ground wax, and 30% polystyrene are typical high-temperature molds; The melting point of medium temperature mold materials is between the two types of mold materials mentioned above, and currently used medium temperature mold materials can be basically divided into rosin based and wax based mold materials.
The basic requirements for the performance of the mold material include: appropriate melting temperature and solidification range, small thermal expansion and contraction, high heat resistance (softening point), and the mold material should have no precipitates in the liquid state and no phase transition in the solid state; Mechanical properties: mainly including strength, hardness, plasticity, flexibility, etc; Process performance: mainly including viscosity (or fluidity), ash content, coating properties, etc.

Mold making process

According to the prescribed composition and ratio of the mold material, various raw materials are melted into a liquid state, mixed and stirred evenly, impurities are filtered out, and poured into a paste like mold material, which can be used to press the molten mold. The method of pressing and forming is commonly used for pressing and melting molds. This method allows the use of liquid, semi liquid, as well as solid and semi solid molds. Liquid and semi liquid molds are formed by pressing under low pressure, which is called injection molding; Semi solid or solid molds are formed by pressing under high pressure, which is called extrusion molding. Both injection molding and extrusion molding must consider the advantages and disadvantages of filling and solidification.

The wax injection temperature in pressure injection molding is often below the melting point, and at this time, the mold material is a slurry or paste of liquid and solid phases coexisting. In the slurry like mold material, the liquid content significantly exceeds the solid content, so the fluidity of the liquid is still retained. In this state of injection molding, the surface of the investment mold has a lower roughness and is less prone to surface defects caused by turbulence and splashing. The temperature of the paste mold material is lower than that of the paste mold material, and it has lost its fluidity. Although there are few surface defects, it has a higher surface roughness.

When using mold material for injection molding, the lowest mold material temperature and molding working temperature should be used as much as possible while ensuring good filling conditions. The choice of pressure is not necessarily better. Although the shrinkage rate of the investment mold is small when the pressure is high, excessive pressure and injection speed can make the surface of the investment mold not smooth, resulting in “bubbles” (expansion of bubbles under the surface of the investment mold), and at the same time, it can cause the mold material to splash and produce cold shut defects. In the process of mold making, in order to avoid adhesion and improve the surface smoothness of the mold, parting agents should be used, especially for rosin based mold materials.

Extrusion molding involves squeezing the mold material in a low-temperature plastic state into the mold cavity and forming it under high pressure to reduce and prevent mold shrinkage. The mold material during extrusion molding is in a semi-solid or solid state, which is relatively hard under normal conditions but can flow under high pressure, characterized by high viscosity. Therefore, the magnitude of pressure during extrusion depends on the viscosity of the mold material and the flow resistance in the injection hole and cavity. The higher the viscosity of the mold material, the smaller the injection hole diameter, the larger the cavity size and the smaller the cross-sectional area, and the longer the mold material stroke, the greater the resistance to the flow of the mold material, thus requiring higher extrusion pressure. By using semi-solid die material for extrusion molding, the solidification time of the investment mold is shortened, resulting in increased productivity, making it particularly suitable for producing castings with thick and large cross-sections.

Shell making process

Shell making includes two processes: coating and sanding. Before applying paint, the investment mold needs to undergo degreasing treatment. When coating, the immersion coating method should be used. During the coating operation, the surface of the mold should be evenly coated with paint to avoid gaps and local accumulation; Welding joints, rounded corners, edges, and grooves should be evenly brushed with a brush or special tools to avoid bubbles; Before applying each layer of reinforcement coating, the floating sand on the previous layer should be cleaned; During the coating process, it is necessary to stir the coating at regular intervals and master and adjust the viscosity of the coating. After coating, sprinkle sand. The most commonly used sanding methods are fluidized bed sanding and rain shower sanding. Usually, after the investment mold is taken out of the paint tank, when the remaining paint on it flows evenly and no longer drips continuously, it indicates that the paint flow is terminated and the freezing begins, and sand can be sprinkled.

Premature sanding can easily cause coating accumulation; Spraying sand too late can cause sand particles to not adhere or adhere firmly. When sprinkling sand, the mold should be continuously rotated and inverted up and down. The purpose of sanding is to fix the coating layer with sand particles; Increase the thickness of the shell to obtain the necessary strength; Improve the breathability and flexibility of the shell; Prevent cracking during shell hardening. The particle size of sanding should be selected according to the coating layer and adapted to the viscosity of the coating. The viscosity of the surface coating is low, and the sand particle size needs to be fine to obtain a smooth surface cavity. Generally, the sand particle size for the surface coating can be selected as 30 or 21 sand groups; The reinforcement layer should be sanded with coarser sand particles, and it is best to thicken them layer by layer. When making shells, after each layer of coating and sanding, sufficient drying and hardening must be carried out.

Defects and prevention methods

The defects of investment castings are divided into surface and internal defects, as well as dimensions and roughness exceeding the standard. Surface and internal defects refer to under casting, cold shut, shrinkage porosity, porosity, slag inclusion, hot cracking, cold cracking, etc; Size and roughness deviations mainly include elongation and deformation of castings. The occurrence of surface and internal defects is mainly related to factors such as the pouring temperature of the alloy liquid, the roasting temperature of the mold shell, the preparation process, and the design of the pouring system and casting structure.

The main reasons for the excessive size and roughness of castings are related to factors such as the design and wear of the die, the structure of the castings, the roasting and strength of the shell, and the cleaning of the castings. For example, when under casting occurs in investment castings, the reason may be that the low pouring temperature and shell temperature reduce the fluidity of the molten metal, the casting wall is too thin, the pouring system design is unreasonable, the shell roasting is insufficient or has poor permeability, the pouring speed is too slow, and the pouring time is insufficient. In this case, targeted solutions should be taken based on the specific structure of the casting and the relevant processes involved to eliminate defects.

 

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