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2022.3.8
Investment casting, also known as precision casting or wax loss casting, is an accurate fusible model made of fusible materials (wax and plastic, etc.), coated with several layers of fire-resistant coating on the model, dried and hardened into an integral mold shell, then heated the mold shell to melt the loss model, and then baked at high temperature to become a fire-resistant mold shell. The liquid metal is poured into the mold shell, and the casting is formed after cooling. Here’s everything about the investment casting.
Compared with other casting methods, the main advantages of investment casting are as follows:
– The casting has high dimensional accuracy and low surface roughness. It can pour castings with complex shapes. The general accuracy can reach grade 5 ~ 7 and the roughness can reach grade 25-6.3 μ m.
– Thin wall castings and castings with small weight can be cast. 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 grooves and curved pores.
– The shape and inner cavity shape of investment casting are almost unlimited. It can produce parts with complex shapes that are difficult to be manufactured by sand casting, forging, cutting and other methods, and can make some assemblies and weldments directly cast into integral parts after slight structural improvement, so as to reduce the weight of parts and reduce production costs.
– There are almost no restrictions on the types of cast alloys, which are commonly used to cast alloy steel parts, carbon steel parts and heat-resistant alloy castings.
– There is no limit on the production batch, and mass production can be carried out from single piece to batch.
The disadvantage of this casting method is that the process is complex and the production cycle is long. It is not suitable for producing castings with large outline dimensions.
1. Classification of mold materials
With the development of investment casting technology, there are more and more kinds of mold materials with different composition. Usually according to the melting point of the mold material, it is divided into high temperature, medium temperature and low temperature mold material.
The melting point of low-temperature mold material is lower than 60 ° C, and the mold materials with 50% paraffin stearic acid, which are widely used in China, belong to this category; The melting point of high-temperature mold material is higher than 120 ° C. the mold material composed of 50% rosin, 20% wax and 30% polystyrene is a typical high-temperature mold material; The melting point of medium temperature molds is between the above two types of molds. The currently used medium temperature molds can be basically divided into rosin based and wax based molds.
2. Basic requirements for mold material performance
– Thermophysical properties: appropriate melting temperature and solidification range, small thermal expansion and contraction, high heat resistance (softening point) and no precipitate in liquid state and no phase transformation in solid state.
– Mechanical properties: mainly including strength, hardness, plasticity, flexibility, etc.
– Process performance: mainly including viscosity (or fluidity), ash content, coating, etc.
The technological process of investment casting includes mold material, wax mold pressing, mold assembly, mold repair, coating, sanding, demoulding, roasting, pouring, cooling, sanding and cleaning. Due to the use of special high-precision refractory shell, this process has some obvious advantages over other ordinary casting processes.
The castings obtained by investment casting have high dimensional accuracy and low surface roughness, so they can be poured into castings with various complex shapes. The accuracy of castings can generally reach grade 5 to 7 and the roughness can reach 6.3 microns. In addition to casting high-precision castings, investment casting can also be used to cast thin-walled castings and small castings with small volume and weight. The minimum wall thickness of investment casting can reach 0.5mm and the weight can be as small as a few grams. In addition, investment casting can also cast various special-shaped parts, such as fine patterns, characters, castings with fine grooves and curved pores, etc.
Using investment casting process, the alloy type, shape and inner cavity shape of the target casting are almost not limited. Whether alloy steel parts, carbon steel parts or heat-resistant alloy castings, even those complex shapes that are difficult to achieve by sand casting, forging and cutting can be obtained by investment casting. Moreover, some assemblies and weldments can be directly cast into integral parts after slight structural improvement, so as to reduce the weight of parts and reduce the production cost. Investment casting can be used to produce large quantities of castings or single products. It is a very flexible production process.
1. Surface and internal defects
The surface and internal defects of castings mainly include under casting, cold shut, shrinkage porosity, porosity, slag inclusion, hot crack and cold crack, which are mainly caused by the problems in the pouring temperature of alloy liquid, the baking temperature of mold shell, preparation process, pouring system and the design of casting structure.
2. Out of tolerance of size and roughness
The out of tolerance of dimension and roughness of castings mainly includes two forms: elongation and deformation. This is mainly caused by problems in the design and use of die, wear, casting structure, baking and strength of mold shell, and cleaning of castings.
For example, when the casting temperature is too low and the casting shell temperature is too low, it will lead to a decrease in the fluidity of the casting shell. In addition, under casting may also be caused when the casting wall is too thin, the design of gating system is unreasonable, the baking of mold shell is insufficient or the air permeability is poor, the pouring speed is too slow and the pouring is insufficient. This requires us to find and solve problems and eliminate casting defects according to the specific structure of castings and related processes.
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