1. The casting size is out of tolerance
1) Influence of mold material and molding process on casting size
The deviation of the investment mold size is mainly caused by the stability of the molding process department, such as the size of the molding force, the wax pressing temperature (the higher the wax pressing temperature, the greater the shrinkage of the investment line), the injection pressure (the injection pressure is large, the melting The smaller the mold line shrinkage rate), the holding time (the longer the holding time, the smaller the air shrinkage), the molding temperature (the higher the molding temperature, the greater the line shrinkage), the opening time, the cooling method, the room temperature and other factors Variation in investment mold size due to fluctuations.
2) Influence of shell-making material and shell-making process on casting size
Shell thermal expansion affects casting size. The thermal expansion of the shell is related to the shell material and process
3) Influence of pouring conditions on casting size
The mold shell temperature, the molten metal pouring temperature, and the position of the casting in the mold shell will all affect the casting size during pouring. To prevent the casting size from being out of tolerance, attention should be paid to many factors affecting the dimensional accuracy of the casting, and the material quality and processing technology should be strictly controlled to stabilize the casting size.
2. The surface of the casting is rough
1) Factors affecting the surface roughness of the investment mold
The surface roughness of the investment mold is related to the surface roughness of all the moldings, the pressing method (paste molding compound pressing or liquid molding compound pressing), and the selection of pressing process parameters.
- Influence of the surface roughness of pressing
- The influence of the pressing method
- Influence of pressing process parameters
2) Factors affecting the surface roughness of the shell
- The coating does not wet well with the investment mold
- The surface coating powder is less dense than the surface of the embedded shell
Under the condition that the surface roughness of the investment mold is qualified, the surface roughness of the mold shell will become an important part of the roughness of the casting.
To embed the surface roughness of the shell, first of all, it should be ensured that the surface coating can well wet the investment mold and copy the investment mold; secondly, the surface layer should be dense, and the coating powder-liquid ratio should be high enough (using bimodal graded powder).
Mixing powder is a powder with reasonable particle size distribution configured according to certain requirements. The particle size of this kind of powder is coarse or fine, distributed and dispersed, and the average particle size is moderate so that the coating can still have suitable viscosity and good fluidity under the condition of a high powder-to-liquid ratio.
3) Factors affecting the accurate replica of molten metal
- Factors of shell temperature on molten metal replication
- Factors of pouring temperature on molten metal replica
The ability of the molten metal to copy the details of the working surface of the shell, that is, the filling ability; is referred to as the “replicating” ability here. To make the molten metal can be accurately reproduced, it is necessary to have a high enough mold shell temperature and molten metal temperature and to ensure that the molten metal has enough pressure head.
Increasing the temperature of the mold shell has a good effect on improving the flowability of the molten metal and the ability to reproduce, so the temperature of the mold shell is a factor to be paid attention to. The silicon melt mold shell is used for investment casting, and its firing temperature is 1150-1175 °C. After the shell is released, it is poured quickly, so that the contour of the casting is clear and the surface roughness is embedded.
4) Other factors that affect surface roughness
During the process of pouring and molten metal solidification, due to the high temperature, the surface of the casting will be oxidized, and the oxide layer will be uneven. In addition, the metal oxide on the surface of the casting may interact with the oxides in the mold shell, which will cause the surface of the casting to fall off unevenly, and significantly. increase the roughness of the casting surface.
Cooling castings in a protective atmosphere is an important part of obtaining a high-quality surface. (For example, after the casting is poured, it is protected by an inert gas, or cooled under the protection of a reducing atmosphere, to prevent oxidation and prevent surface decarburization, until the surface of the casting does not reach the oxidation temperature. Shelling of castings with good surface protection Afterwards, its surface is silver-gray, silver-white, or colored oxide color.)
In addition, cleaning has a great influence on the surface roughness of investment castings. The as-cast surface roughness of investment castings is originally low, and the surface should be cleaned by sandblasting or water sand cleaning. The sandblasting method is more than 2 grades better than the shot blasting to clean the surface roughness of the casting. Cleaning with shot blasting often damages the as-cast surface. Conditional use of alkali boiling and alkali blasting cleaning methods to improve the surface quality of castings and reduce the surface roughness of castings.
In short, many factors affect the surface roughness of investment castings. Only by implementing a set of strict process measures from raw materials, pressing to cleaning can reduce the surface roughness of castings.
3. Surface defects of castings
1) Sticky sand
(1) Features: A layer of metal and mold compound or mold shell material adheres to the surface of the casting. It is further divided into mechanical and chemical sticky sand
(2) Formation reason: 2Fe+O2=2FeO
(liquid, solid) (gas) (liquid, solid)
2FeO+SiO2=2FeOSiO2 (ferrous silicate)
2Mn+O2=2MnO
(3) Preventive measures:
a. To correctly select the shell refractory
b. The surface shell refractory material should be pure
c. When the alloy is smelted and poured, oxidation should be avoided as much as possible and fully deoxidized to remove oxides in the molten metal
d. Appropriately reduce the pouring temperature of molten metal under possible conditions
e. Improve the heat dissipation conditions of the shell to prevent local overheating
2) Slag inclusions, rat tails, and depressions
(1) Features: There is a tongue-shaped metal scar on the surface of the casting, and there is a flaky shell between the scar and the piece, which is called sand; The surface is locally irregularly concave, which is called a concave defect.
(2) Reasons for formation: The reasons for the formation of the three types of defects are the same. The internal cause is the delamination of the shell, and the external cause is the defect caused by the large temperature difference between the inside and outside of the shell during casting and the different thermal expansion inside and outside.
(3) Preventive measures:
a. All measures to prevent the delamination of the shell and strengthen the bonding force between layers are beneficial to eliminating the sand inclusion, rat tail, and depression defects in investment castings
b. Increase the temperature of the dewaxing medium and shorten the dewaxing time
c. When the shell is too wet, it is not suitable for high-temperature roasting
d. Try to avoid the large plane structure of the casting, and avoid laying the large plane flat or pouring upwards. If necessary, process ribs and process holes should be provided to prevent defects.
3) Zebra
(1) Features: There are raised and irregular patterns on the surface of the casting.
(2) Causes of formation: Cracks are formed when the shell surface layer is dry. When pouring, the molten metal enters the crack, causing irregular raised spots on the surface of the casting. The reasons for the formation of drying cracks in the surface layer may be; that the relative temperature is too low, the drying time is too long, the wind blows against the shell locally, and the ambient temperature changes.
(3) Preventive measures: strictly control the paint
4) Pitting
(1) Features: There are many dense circular shallow depressions on the surface of the casting called pitting defects.
(2) Formation reason: it is formed by the chemical reaction between the oxide in the molten metal and the oxide in the shell material.
(3) Preventive measures:
a. Reasonable selection of shell refractory materials; (for example, stainless steel castings use corundum powder sand as the surface layer)
b. Prevent and reduce metal oxidation
c. Appropriately increase the shell roasting temperature and pouring speed to appropriately reduce the pouring temperature of the molten metal.
5) Metal thorns (burrs)
(1) Features: There are scattered or dense tiny spurs on the surface of the casting.
(2) Causes of formation: The surface of the shell is not dense, and there are many hole defects. When pouring, the molten metal enters the holes of the shell to cause metal thorns.
(3) Preventive measures:
a. The topcoat should have a high enough powder-to-liquid ratio
b. Improve the wettability of coatings and easy investment molds
c. The surface coating should be thick enough, and the sanding particle size should be compatible with the viscosity of the coating and the thickness of the coating.
d. The paint should be fully stirred
6) Metal beads (iron beans)
(1) Features: There are prominent spherical metal particles on the surface of the casting, which often appear in the grooves or corners of the casting.
(2) Causes of formation: The surface paint contains air bubbles or the paint is not well coated on the wax mold, leaving air bubbles.
(3) preventive measures;
a. To improve the wettability of the surface coating, a defoamer should be added at the same time after adding a surfactant (it should be noted that a surfactant with less foaming should be used)
b. When the paint is prepared and stirred, the gas should be prevented from being involved, and the prepared paint should have a long enough time for the gas to escape.
c. The fusible mold should be fully degreased to improve its coatability
d. When the surface coating is applied, the air bubbles remaining in the corners, grooves, etc. of the mold can be blown out with compressed air.
e. Use vacuum undercoating when possible
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