At present, there are four widely used shell making processes in the production of precision castings in China:
1.1. Low temperature mold material water glass shell
This process has a production history of nearly 50 years in China. The advantages of the water glass brightening process are: low shell making cost, short production cycle, and convenient sand cleaning. The disadvantage is: Na The O content is high, and the high-temperature strength and resistance to deformation of the shell are not as bright as those of the silica sol mold. The surface roughness, dimensional accuracy, yield, and repair rate of the casting are all worse than the other three shell making processes.
The production equipment for water glass shell is simple, the environmental conditions are poor, the labor intensity is high, and the refractory materials for the surface and back layers are of low quality and low price. Chemical hardening such as chlorination is used, so the cost-effectiveness of castings is relatively low
Improvement direction:
a. Using silica micro powder (sand) with high silicon dioxide content instead of silica powder (sand) with low silicon dioxide content, SiO. The content requires a standard of 99.5%, and the particle size of the surface layer powder requires a bimodal gradation. The surface layer coating is prepared with a density of 128g/cm water glass fiber, and the powder to liquid ratio reaches 1.4:1
b. To strengthen the on-site quality management of the shell making process, the first priority is to separate the shell making operation site from the shell drying room in the newly designed pouring system shown in Figure 2-79. The dust removal and shell making equipment in the operation site should be increased as much as possible, and the environmental temperature and humidity in the bright drying room should be improved as much as possible.
1.2. Low temperature mold material silica sol water glass composite shell
The first layer or the first and second layers use English powder slurry, while the micro English sand back layer still uses water glass technology. The advantage is that the surface roughness of the casting is R. As the value decreases, the surface defects of castings are reduced, and the repair rate is reduced.
High alloy steel parts such as stainless steel and heat-resistant steel can be poured, and the production cycle of shell making is much shorter than that of the all silicon adhesive process, and slightly longer than that of the water glass process. The cost-effectiveness is higher than that of water glass process and lower than that of all silica sol process. The disadvantage is that the composite process can only be considered an improved version of the water glass process, and fundamentally belongs to the category of water glass shell making. The overall high-temperature strength, creep resistance, and dimensional accuracy of the shell are far inferior to those of the silica sol shell: the roasting temperature of the composite shell cannot exceed 950C, and the roasting principle from low to high temperature must be followed.
It cannot be directly entered into the roasting furnace at 1000-1200C. Due to the low roasting temperature and insufficient insulation time, it often causes defects such as porosity, insufficient pouring, and cold shut in the casting. Castings with a wall thickness of less than 3mm are difficult to apply, The production of small and extra small castings weighing less than 50g will further expose the weaknesses of the composite process. The overall quality stability is poor. Compared to the silica sol process, the cost advantage of shell making is not very significant.
Improvement direction:
a. Improving the surface quality of castings does not necessarily require the use of incorrect powder and sand. For low-temperature molds, relatively high purity silicon powder and silica sand can be used. Production practice has shown that quartz silica sol slurry can replace incorrect sand silica sol slurry.
b. Strengthen the quality management and environmental improvement of the back shell production. The back layer should use refractory materials with stable quality, excellent high-temperature performance, and relatively low cost, while also taking into account the compatibility with the expansion of the surface layer shell refractory material. For example, the back layer should use refractory clay quartz powder coating (50% each) and sprinkle granular sand. Alternatively, use fire-resistant clay particle powder coating (3:7) and sprinkle granular sand.
1.3. Low temperature mold material all silica sol shell
The low-temperature mold material all silica sol process has higher quality than the composite shell, especially with high dimensional accuracy and strong creep resistance. It can be applied to thin-walled parts, complex structural parts, and small and medium-sized parts, especially in the production of medium and large parts weighing over 5kg. Do not use high-pressure wax injection machines, as the equipment investment is low and the surface quality of wax molds is not significantly different from that of medium temperature wax molds. This is the biggest advantage of this process.
Improvement direction:
a. Emphasis should be placed on strengthening the treatment of recycled wax. In the first step, 3%~5% hydrochloric acid should be heated to boiling to ensure sufficient boiling time to completely remove saponification substances. The second step is to have sufficient settling time for the wax solution. The second step is to filter the wax solution through multiple filters after sedimentation treatment. The fourth step is to regularly measure the acidity of the mold material and supplement it with stearic acid in a timely manner.
b. By using steam dewaxing method, the steam pressure is only 0204MPa and the temperature is only 120130C, which can meet the requirements.
1.4. Medium temperature mold material all silica sol shell
It is an internationally recognized precision casting production process, which has the highest casting quality, the lowest repair rate, a surface roughness value of R083.2, and a dimensional accuracy of CT3~CT5. It is suitable for making small and extra small parts, with less production of 10~50kg large parts. Of course, the production cost is also the highest, and the equipment investment is also the highest.
Improvement direction:
a. Adopting the surface layer blowing and rapid drying process, vacuum drying, and quick drying silica sol methods to shorten the shell making time and cycle.
b. Corundum or fused silica and other surface refractory materials are used to replace quartz, reducing the cost of surface layer shell making. To solve the problem of high residual strength of shell, it is recommended that 50% mullite powder+50% quartz powder mixed slurry be used for the back layer powder, and still sprinkle mullite.
At present, some shell making processes in Chinese precision casting factories only use one type of shell, some use two types of shell making processes, and there are also four types of processes available to fully meet the different market demands for quality and price of precision castings, and improve the competitiveness and adaptability of enterprises in the market.
Casting dimensional tolerance refers to the allowable limit deviation of the size of each part of the casting. Many factors in the casting production process will affect the casting dimensional tolerance. At the same time, the casting dimensional tolerance will have a great impact on the casting production cost within a certain range. Therefore, scientific methods must be adopted. standards to coordinate the requirements of both supply and demand parties.
