How much do you know about investment casting?

1. The characteristics of investment casting

Investment casting is also called precision casting or lost wax casting. It uses fusible materials (wax and plastics, etc.) to make precise fusible models. The models are coated with several layers of refractory coatings, which are dried and hardened into a whole The shell is then heated to melt the mold, and then fired at a high temperature to become a refractory shell. The liquid metal is poured into the shell, and the casting is formed after cooling.

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Compared with other casting methods, the main advantages of investment casting are as follows:

The castings have high dimensional accuracy and low surface roughness, which can cast complex-shaped castings. The general accuracy can reach 5~7, and the roughness can reach two Ra25-6.3μm;

It can cast thin-walled castings and low-weight castings. 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 manufactured, and some assemblies and welding parts can be made after slight structural improvements. Directly cast into integral parts, thereby reducing the weight of parts and reducing production costs;

There are almost no restrictions on the types of casting alloys, commonly used to cast alloy steel, carbon steel 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, the production cycle is long, and it is not suitable for the production of castings with large outline dimensions.

2. Mold material types 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. Mold materials are usually divided into high temperature, medium temperature and low temperature mold materials according to their melting point.

The melting point of the low-temperature molding compound is lower than 60C, and 50% of the molding compound of paraffin wax and stearic acid currently widely used in my country belongs to this category;

The melting point of the high-temperature molding compound is higher than 120C, and the molding compound with 50% rosin, 20% ozokerite, and 30% polystyrene is a typical high-temperature molding compound.

The melting point of the medium temperature mold material is between the above two types of mold materials. The currently used medium temperature mold materials can be basically divided into two types: rosin-based and wax-based mold materials.

(2) Basic requirements for 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 properties, etc.

Three, molding process

According to the specified composition and ratio of the mold material, the various raw materials are melted into a liquid state, mixed and stirred uniformly, and impurities are filtered out and poured into a paste mold material, that is, the investment mold can be pressed. The pressing method is commonly used to suppress investment molds. This method allows the use of liquid, semi-liquid, and solid and semi-solid mold materials. Liquid and semi-liquid mold materials are pressed under low pressure, called injection molding; semi-solid or solid mold materials are pressed under high pressure, called extrusion molding. Whether it is injection molding or extrusion molding, the advantages and disadvantages of filling and solidification must be considered.

(1) Pressure injection molding The wax injection temperature of pressure injection molding is mostly below the melting point. At this time, the mold material is a slurry or paste with liquid and solid phases coexisting. In the slurry-like mold material, the amount of liquid phase significantly exceeds the amount of solid phase, so the fluidity of the liquid is still retained. Under pressure injection in this state, the surface of the investment mold has low roughness, and surface defects caused by turbulence 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 fluidity. Although there are few surface defects, it has a higher surface roughness.

When molding material is injection molded, the lowest molding material temperature and pressing working temperature should be used as far as possible under the condition of ensuring good filling. The choice of pressure is not as large as possible. Although the pressure is high, the shrinkage rate of the investment is small, but the pressure and injection speed are too large, which will make the surface of the investment not smooth and produce “bubbles” (bubbles under the surface of the investment). , Make the mold material splash and appear cold barrier defects. In the molding process, in order to avoid the adhesion of the molding material and improve the surface finish of the investment mold, a parting agent should be used, especially for the rosin-based molding material.

(2) Extrusion molding Extrusion molding squeezes the mold material in the low-temperature plastic state into the cavity and forms it under high pressure to reduce and prevent the shrinkage of the investment mold. The mold material during extrusion molding is in a semi-solid or solid state. The mold material is relatively hard under normal conditions, but can flow under high pressure, and 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 greater the viscosity of the mold material, the smaller the injection hole diameter, the larger the cavity size, the smaller the cross-sectional area, and the longer the mold material stroke, the greater the resistance of the mold material to flow, and the higher the extrusion pressure is required. The semi-solid mold material is used for extrusion molding, and the solidification time of the investment mold is shortened, so the productivity is increased, and it is especially suitable for the production of thick and large section castings.

Four, shell making process

Shell making includes two processes: coating and sanding. Before coating, the investment mold needs to be degreasing treatment. Dip coating method should be used when coating. During the coating operation, the surface of the investment mold should be evenly coated with paint to avoid blanks and local cloth accumulation; the welds, fillets, corners and grooves should be evenly painted with a brush or special tools to avoid bubbles; The floating sand on the previous layer should be cleaned before the coating of the reinforcement layer; the coating should be stirred regularly during the coating process to control and adjust the viscosity of the coating.

Sprinkle sand after coating. The most commonly used methods of sanding are fluidized sanding and rain-drenching sanding. Usually, after the investment mold is taken out from the paint tank, when the remaining paint on it flows evenly and no longer drips continuously, it means that the paint flow is terminated and freezing begins, and sand can be sprinkled. Spreading sand too early can easily cause paint accumulation; spreading sand too late will cause the sand particles to not adhere or adhere firmly. When sanding, the investment mold should be continuously rotated and upside 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 air permeability and concession of the shell; and prevent cracks when the shell is hardened. The particle size of the sand is selected according to the coating level and is compatible with 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. Generally, the surface layer sanding particle size can be selected as 30 or 21 sand; the reinforcement layer is scattered with coarser sand, preferably one by one. The layer is bold. When making the shell, after each layer of coating and sanding, it must be fully dried and hardened.

5. Defects and prevention methods

The defects of investment casting are divided into surface and internal defects, and size and roughness tolerance.

Surface and internal defects refer to under-casting, cold separation, shrinkage porosity, porosity, slag inclusion, hot cracking, cold cracking, etc. The size and roughness tolerances mainly include the elongation and deformation of the casting.

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 pouring system and the design of the casting structure.

The main reason for the excessive difference in the size and roughness of the casting is related to the design and wear of the pressure, the structure of the casting, the firing and strength of the shell, and the cleaning of the casting.

For example, when the investment castings are under-casting, the reason may be that the low pouring temperature and mold shell temperature reduce the fluidity of the molten metal, the casting wall is too thin, the pouring system design is unreasonable, the mold shell is not sufficiently baked or the air permeability is poor, and the casting The speed is too slow and the pouring is insufficient. At this time, according to the specific structure of the casting and the related processes, the problems should be solved and the defects should be eliminated.