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.), coated with several layers of refractory paint on the model, and dried and hardened into a whole 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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Compared with other casting methods, the main advantages of investment casting 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 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 manufactured, and some assemblies and welded parts can be slightly structured. After improvement, it can be 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, 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, which 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.
Example 1: A venerable plate unearthed with a chime in 2400 – unparalleled and exquisite
Example 2: Yunnan wax removal method and aviation blade casting technology
The Types Of Mold Materials And Performance Requirements
1.Classification of mold materials
With the development of china 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 with 50% of each of the paraffin wax and 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 the mold material composed of 50% rosin, 20% ceresin, 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. The currently used medium temperature mold materials can basically be divided into rosin-based and 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 liquid state and no phase change in solid state;
Mechanical properties: mainly include strength, hardness, plasticity, flexibility, etc.;
Process performance: mainly include viscosity (or fluidity), ash content, coating and hanging properties, etc.
The Molding Process Of Investment Casting
According to the specified components and proportions 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, semi-solid moulding compounds. 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, and the mold material is a slurry or paste with liquid and solid two phases coexisting. 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.
When the molding material is injection molded, the lowest mold 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 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, in order 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 molding
Extrusion molding extrudes 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 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 metal castings with thick and large cross-sections.
The Shell Making Process Of Investment Casting
Shell making includes two processes of painting and sanding. Investment molds must be degreased prior to application of paint. 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 cause 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, so as 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, it must be fully dried and hardened after each layer of coating and sanding.
The Defects And Prevention Methods Of Investment Casting
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 undercasting, cold insulation, shrinkage porosity, pores, slag inclusion, thermal cracking, cold cracking, etc.;
Dimensional 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 pouring system and the design of the casting structure and other factors.
The main reason for the out-of-tolerance casting size and roughness is that the design of the die is related to 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 precision brass 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.
