If the water glass shell is cast, the dimensional accuracy of the parts is not high, and the surface roughness value is relatively large. Therefore, the requirements for the overall shape and energy of the water glass mold are relatively low, while the performance requirements for the silica sol shell are relatively low. Much stricter.
Strength Strength is the most important and basic performance of the shell. There are three
Different strength indicators, namely normal temperature strength, high temperature strength and residual strength. The shell should have sufficient room temperature strength and high temperature strength,
It is possible to successfully complete the shell making process and pour it.
The normal temperature strength of the shell usually refers to the wet strength, which is determined by the adhesion between the binder and the surface of the refractory particles and the strength of the wet state.
The cohesive force of the binder itself is superimposed by the two phases, and changes with the type of binder and refractory material and the degree of drying and hardening during the shell making process.
During dewaxing, roasting and pouring, the shell will be subjected to various stresses. If the strength is insufficient, the shell will be deformed or cracked. From the beginning of pouring to before the casting solidifies, because the shell is directly affected by high-temperature liquid metal, working conditions are extremely poor,
Therefore, the high temperature strength requirement of the shell is even more important. The high temperature strength of the shell mainly depends on the silicone gel strength of the binder at high temperature, and is related to the reaction product between the binder and the refractory material at high temperature. The high temperature strength of the water glass type filling is lower than that of the silica gel and ethyl silicate binder type shells.
Residual strength refers to the strength of the shell during shelling and cleaning after roasting and high-temperature pouring, and the residual strength affects the shelling of the shell
It has a great influence on the cleaning operation. If the residual strength is too large, it will increase the difficulty of shelling and cleaning. At the same time, the amount of cooling and solidification of castings also requires low residual strength of the shell, so that the shell has better concessions, so as not to hinder the shrinkage of the casting and cause cracks in the casting. The residual strength of the shell is generally affected by the high temperature strength. Generally, high temperature strength is high, and residual strength is also high. The shell strength index with excellent performance should take into account various factors. Therefore, the shell should have high normal temperature strength, suitable high temperature strength and low residual strength.
Gas permeability refers to the ability of gas to pass through the mold wall. Although the wall thickness of the mold shell is not large, due to its relatively
Dense, although the mold shell leaves some microcracks due to the escape of various volatiles after being squandered after roasting, its air permeability is far worse than that of sand molds. When flushing, if the air permeability of the mold shell is poor and the gas cannot be discharged quickly, the gas in the mold shell will expand rapidly under the action of the high-temperature molten metal to form a high air cushion pressure, which will hinder the smooth flow of the molten metal Filling may cause defects such as pores or insufficient pouring in the casting. Such defects are most likely to occur in thin-walled castings. In general, the air permeability mainly depends on the compactness of the shell structure, and the type and content of the binder, the properties and viscosity of the refractory material are the main factors affecting the air permeability of the shell.
The factors that are usually beneficial to improving the air permeability of the shell are often the factors that are unfavorable to the strength of the shell. penetration of different binders
There is also a large difference in gas properties. The high-temperature air permeability of the water glass shell is better, followed by the ethyl silicate shell, and the high-temperature air permeability of the silica sol shell.
Sex is poor.
Thermal Expansion The property that an object expands and shrinks with temperature changes is called thermal expansion, and the property of solids to expand when heated
It can usually be expressed by the linear expansion coefficient or the volume coefficient.
The thermal expansion of the shell refers to the expansion or contraction of the shell as the temperature rises. The increase in size when the shell is heated is the result of the shell material
The thermal expansion of the material and the transformation of the allotropic isomers, the size shrinkage is due to factors such as the dehydration of the shell during heating, the thermal decomposition of the material, the sintering of the material, the generation of the liquid phase, and the condensation of the silicone gel. The result of densifying the shell.
Thermal expansion is an important performance of the mold shell, it not only has a direct impact on the dimensional accuracy of the casting, but also affects the mold
The shell’s resistance to rapid cooling and rapid heating and high temperature deformation resistance. When the refractory material in the shell is heated up, some stars expand evenly,
Others show non-uniform expansion. There are corundum, fused quartz, and kaolinite clinker shells that show uniform expansion, and silica sand shells that expand non-uniformly. The main reason is that the polycrystalline transformation of quartz during the heating process causes its volume expansion to vary. Uniformity.
Thermal conductivity Thermal conductivity refers to the ability of the shell to conduct heat, usually expressed by the heat transfer coefficient of the shell, which is solidified
The heat transfer between the two fluids separated by the body wall is expressed by dividing the heat flux density by the temperature difference. The thermal conductivity of the shell is related to the type of shell refractory material, the porosity in the shell, and the temperature of the shell. related.
Shell-making refractories have a great influence on the thermal conductivity of the shell, and the heat transfer of corundum (Al, O,) shells and high-alumina shells
The resistance is higher than that of silica sand shell.
The thermal conductivity of the shell directly affects its outward heat dissipation. The thermal conductivity of the shell is good, and the speed of outward heat dissipation is fast, so the high-temperature liquid gold
The cooling and solidification speed of the metal is also fast, which is beneficial to the refinement of the grain and the comprehensive mechanical properties of the casting.
Thermal shock stability Thermal shock stability, also known as resistance to rapid cooling and rapid heating, refers to the resistance of the shell to rupture due to sharp bending of temperature Ability.
Generally, high thermal conductivity, small expansion coefficient, and high porosity can all improve the thermal shock stability of the material. If the elastic modulus of the material
If the amount is low and the mechanical strengt h is high, the thermal shock stability is also good.
Practice has proved that the temperature difference between the strength of the shell and the liquid metal and the thermal expansion of the shell refractory during injection are the main factors affecting the thermal shock stability. Crystal transformation is accompanied by a sudden increase in victory rate
Therefore, the stability of the type and form of silicon is poor, and the temperature and health of the shell should not be too low, and it is not suitable for cold shell injection. Kaolinite-type clinker mullite, Shangdian soil, lead soil, (ball sand and other male fire-attached clinkers have a low thermal account coefficient, so the thermal stability of the shell is high.
The shape and thickness of the shell also have an impact on the thermal shock quality. Generally, the thermal stability of the thin-walled shell is greater than that of the thick-walled shell.
Thermochemical stability refers to the chemical reaction at the interface when the shell is in contact with the commercial temperature liquid metal Ability.
The chemical stability of the shell at high temperature mainly depends on the physical and chemical properties of the cavity surface material and alloy, followed by The steam injection temperature of the gas alloy is related to the atmosphere around the cavity during the injection process. If a thermal chemical reaction occurs between the wave state alloy and the cavity surface
The reaction will produce pitting and grain shape defects on the surface of the casting, which will increase the roughness value of the casting, reduce the surface quality, and make it difficult to clean the casting.
The surface of the cavity is made of silica sand, and sand does not stick to the carbon steel when pouring, but when the high manganese steel is washed, it will make the casting
Severe chemical silt is produced on the surface of the workpiece, which is mainly si0, which is acidic and reacts with the basic oxide MnO at high temperature
Form a series of low melting point compounds, such as MnO• Si0, (melting point 1270°C), 2MnO • Si02
3MnO. SiO. (melting point 1200°C), forming a chemical sticky sand layer. In addition, the surface layer is a shell made of silica sand, and at high temperature
Chemical sticky sand is also prone to occur when washing and injecting alloy steel containing Ni, Cr, and AI, and it is easy to produce numbness when washing and injecting ZG1C+18Ni9Ti stainless steel.
Point and sticky sand defects. When the silica sand material is replaced by corundum or cobalt, a better surface quality can be obtained,
Oxidation of liquid metal at high temperature forms FeO, which is highly chemically active and has a greater wetting effect on the shell surface.
Therefore, it is also one of the important factors causing the chemical reaction at the shell interface. In practice, castings are cooled and solidified in a reducing atmosphere
When the oxidation of the molten steel is inhibited, the thermochemical reaction at the interface is effectively reduced or prevented, and the surface of the casting is improved.quality.
Therefore, investment casting should choose the appropriate shell material process according to different alloy types, regardless of the water glass shell or the silica sol shell, in short, controlling the shell making process is the most important thing.