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 then cool it to form a casting.
Molding material - wax mold - mold group - mold repair - painting - sanding - demoulding - roasting - pouring - cooling - falling sand - cleaning.
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 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 Molding Materials and Performance Requirements
Classification of molding materials
With the development of investment casting technology, there are more and more types of molding materials with different compositions. According to the melting point of the molding material, it is usually divided into high temperature, medium, and low-temperature materials.
The melting point of the low-temperature molding material is lower than 60°C, and the currently widely used paraffin-stearic acid molding material belongs to this category; the melting point of the high-temperature molding material is higher than 120°C, which is composed of rosin 50 %, 20% ceresin wax, and 30% polystyrene molding materials are typical high-temperature molding materials; the melting point of medium-temperature molding materials is between the above two types of molding materials, and the currently used medium-temperature molding materials Basically, it can be divided into rosin-based and wax-based molding materials.
Basic requirements for the properties of molding materials
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 a 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 proportion of the molding material, various raw materials are melted into a liquid state, mixed and stirred evenly, and the impurities are filtered out and poured into a paste molding material, that is, the investment mold can be pressed. Compression investment generally adopts the method of compression 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, called injection molding; semi-solid or solid molding materials are compressed under high pressure, called extrusion. Whether injection molding or extrusion molding, the advantages and disadvantages of filling and solidification must be considered.
Shell Making Process
Shell making includes two processes painting and sanding. Investment molds must be degreased prior to 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 evenly coated with paint to avoid blank and local accumulation; the welding joints, rounded corners, edges, and grooves should be brushed evenly with brushes 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 from the paint tank when the remaining paint on it flows evenly and no longer drips continuously, it means that the paint flow stops and the freezing begins, 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, it 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.
Conclusion
Investment casting is not a simple process. If you need it to produce castings for you, then you must choose a professional investment casting manufacturer.
Besser Casting has more than 20 years of casting history and rich experience as a manufacturer, we have successfully cooperated with hundreds of enterprises. Can provide you with professional investment casting services. Please contact us if you need.