Summary of investment casting knowledge

The principle and characteristics of investment casting

Investment casting, also known as precision casting or lost-wax casting, is a precise fusible model made of fusible materials (wax and plastic, etc.) Shell, then heat the shell to melt the lost model and then bake at high temperature to become a refractory shell, pour the liquid metal into the shell, and cool it to form a casting.

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The main advantages of investment casting compared to other casting methods are as follows:

Castings have high dimensional accuracy and low surface roughness and can cast castings with complex shapes. Generally, the accuracy can reach 5~7 grades, and the roughness can reach two Ra25-6.3μm;

It can cast thin-walled castings and castings with a very small weight. The minimum wall thickness of investment castings can reach 0.5mm, and the weight can be as small as a few grams;

It can cast fine patterns, characters, castings with fine grooves and curved pores;

The shape and cavity shape of investment castings are almost unlimited, and parts with complex shapes that are difficult to manufacture by sand casting, forging, cutting and other methods can be produced, and some assemblies and welded parts can be made after a little structural improvement. Direct casting into integral parts, thereby reducing the weight of parts and reducing production costs;

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

There is no limit to the production batch, and it can be mass-produced from a single piece to a batch.

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

Types of mold materials and performance requirements

1. Classification of mold materials

With the development of investment casting technology, there are more and more types of mold materials with different compositions. Usually, the mold material is divided into high temperature, medium, and low-temperature mold material according to the melting point of the mold material.

The melting point of the low-temperature mold material is lower than 60 ° C, and the mold material of each 50% of paraffin wax-stearic acid widely used in my country belongs to this category; the melting point of the high-temperature mold material is higher than 120 ° C, and is composed of 50% of rosin, The mold material with 20% wax and 30% polystyrene is a typical high-temperature mold material; the melting point of the medium temperature mold material is between the above two types of mold materials, and the currently used medium temperature mold materials can be divided into rosin-based and There are two kinds of wax-based mold materials.

2. Basic requirements of mold material performance

Thermophysical properties: suitable melting temperature and solidification interval, 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 change in the solid-state;

Mechanical properties: mainly strength, hardness, plasticity, flexibility, etc.;

Process performance: mainly include viscosity (or fluidity), ash content, coating, hanging properties, etc.

Molding process

According to the specified composition and ratio of the mold material, melt various raw materials into a liquid state, mix and stir evenly, filter out impurities and pour into a paste-like mold material, that is, the investment mold can be pressed. Pressed investment generally adopts the method of pressing molding. This method allows the use of liquid, semi-liquid and solid, and semi-solid molding materials. Liquid and semi-liquid molding materials are compressed under low pressure, which is called injection molding; semi-solid or solid molding materials are compressed under high pressure, which is called extrusion molding. Whether injection molding or extrusion molding, the advantages and disadvantages of filling and solidification must be considered.

1. Injection molding

The wax injection temperature of injection molding is mostly below the melting point. At this time, the mold material is a slurry or paste in which liquid and solid two phases coexist. In the slurry-like mold material, the liquid phase significantly exceeds the solid phase, so the fluidity of the liquid is still retained. In this state, the surface of the investment mold has low roughness, and surface defects caused by turbulent flow and splashing are not easy to appear. 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.

During injection molding of molding materials, the lowest molding material temperature and molding working temperature should be used as far as possible under the condition of ensuring good filling. The choice of pressure is not as high as possible. Although the pressure is high, the investment shrinkage rate is small, but the pressure and injection speed are too large, which will make the surface of the investment mold not smooth, resulting in “bubbling” (the expansion of bubbles under the investment mold skin), and at the same time, so that the mold material splashes and cold insulation defects appear. In the molding process, to prevent the molding material from adhering to the molding and improve the surface finish of the investment mold, a parting agent should be used, especially for rosin-based molding materials.

2. Extrusion

Extrusion molding extrudes the mold material in the low-temperature plastic state into the cavity and forms under high pressure to reduce and prevent the shrinkage of the investment mold. The mold material during extrusion is in a semi-solid or solid-state. The mold material is relatively hard under normal conditions but can flow under high pressure, which is characterized by high viscosity. Therefore, the 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 of the mold material to flow, so the higher the extrusion pressure is required. Using semi-solid mold material extrusion, the solidification time of the investment mold is shortened, so the productivity is increased, and it is especially suitable for the production of castings with thick and large cross-sections.

The shell making process

Shell making includes two processes painting and sanding. Investment molds must be degreased before the application of paint. The dip-coating method should be used when coating and hanging. During the painting and hanging operation, the surface of the investment mold should be uniformly coated with paint to avoid blank and local accumulation; the welding joints, rounded corners, edges, and grooves should be brushed evenly with a brush or special tools to avoid air bubbles; The floating sand on the previous layer should be cleaned before the reinforcement layer coating is applied; the coating should be stirred regularly during the coating and hanging process to master and adjust the viscosity of the coating.

Sand after coating. The most commonly used sanding methods are fluidized sanding and rain spraying. Usually, after the investment mold is taken out of the coating tank, when the remaining coating on it flows uniformly and no longer drips continuously, it means that the coating flow has terminated and the freezing has begun, and sand can be sprinkled. Spreading sand too early is easy to causes paint buildup; spreading sand too late will cause the sand to not adhere or adhere well. When sanding, the investment mold should be continuously rotated and turned upside down. The purpose of sanding is to fix the coating layer with sand; increase the thickness of the shell to obtain the necessary strength; improve the permeability and concession of the shell; prevent cracks when the shell is hardened. The particle size of the sand is selected according to the coating level and is suitable for the viscosity of the coating. The viscosity of the surface coating is small, and the sand particle size must be fine, to obtain a smooth surface cavity. Generally, the surface layer sanding particle size can be selected from the group of 30 or 21 sand; Layer bold. When making shells, they must be fully dried and hardened after each layer of coating and sanding.

Defects and prevention methods

The defects of investment castings are divided into surface and internal defects, as well as size and roughness out of tolerance.

Surface and internal defects refer to under-casting, cold insulation, shrinkage, pores, slag inclusion, thermal cracking, cold cracking, etc.; size and roughness out of tolerance mainly include elongation and deformation of castings.

The surface and internal defects are mainly related to the pouring temperature of the alloy liquid, the baking temperature of the shell and the preparation process, the design of the pouring system and the casting structure, and other factors.

The main reason why the casting size and roughness are out of tolerance is that the design of the die is related to the wear and tear, the structure of the casting, the roasting and strength of the shell, and the cleaning of the casting.

For example, when the investment casting is under-cast, the reason may be that the low pouring temperature and the mold shell temperature reduce the fluidity of the molten metal, the casting wall is too thin, the design of the gating system is unreasonable, the shell baking is not sufficient or the air permeability is poor, pouring If the speed is too slow and the pouring time is insufficient, at this time, the problem should be solved in a targeted manner and the defects should be eliminated according to the specific structure of the casting and the related processes involved.

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