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Quality control of precision casting silica sol slurry

Edit:Xiangyang Liqiang Machinery Co., Ltd.UpDate:2019-08-27

The key process of silica sol process precision casting process control is shell making, and the most difficult to quantify and control in the shell making process is the performance of the slurry. Generally, precision foundries do not have complete testing methods to detect various indicators of shell raw materials. The stability of suppliers' quality control of silica sol and refractory powder directly affects the stability of precision foundry slurry performance. Most precision casting plants control the slurry only to measure the viscosity of the flow cup as a performance.

Shell performance is the main influencing factor of precision casting. As a slurry system that forms the strength and refractoriness of the shell mold, there should be more complete detection and control methods to ensure the stability of its performance.

The influence of raw materials on slurry

1.1 Silica sol

In general, ordinary silica sol is sodium ion stabilized alkaline silica sol, and the properties of silica sol itself have a greater influence on the properties of the prepared slurry. The performance index of REMET ordinary silica sol is shown in Table 1. SP. 30 particle size is strictly controlled within 7~9LLm, LP. The particle size of 30 is strictly controlled at 11~13"m. Particle size influence: The size of the particle size directly affects the stability of the silica sol itself and the slurry. si0: the same content of silica sol, the smaller the particle size, the easier it is to gel , The faster the slurry dries, the shorter the service life, and the worse the slurry stability; if the slurry does not fail, the strength of the coating will be higher, but the surface slurry consumption is small, the turnover period is long, and it is prone to failure; Once there is a slight failure, the coating strength will be greatly reduced, and there will be batches of cracks and surface layer peeling. The slight failure of the slurry is difficult to monitor, so the stability of the surface layer slurry The influence of the shell is far greater than its strength. LP-30 is also more suitable as a surface layer binder than SP-30. SP.30 is a standard back layer silica sol, and its small particle size provides higher strength to the entire shell. Drying is also faster; but due to the fast consumption of the back layer slurry, its stability is not the main factor affecting the performance of the slurry. Therefore, SP-30 is more suitable as the back layer binder. The domestic choice of 830 silica sol for the surface layer, Due to the influence of production process conditions and overall market cost, 830 silica sol has a wide particle size distribution range (8~l4m) and a large average particle size (9~12LLm), and its stability and strength are close to LP.30; 1430 has a wider particle size range (10~20um), average particle size

It is also larger (14-17um), and its coating strength and drying speed are greatly affected. More coatings and thicker shells are required to meet the requirements of overall strength. Under the current conditions of domestic silica sol, 830 silica sol is more suitable for the back layer than 1430. Compared with l430 silica sol, 830 is used as the back layer slurry, which can appropriately reduce the number of coatings. Influence of SiO content: Under the same particle size of ordinary silica sol, the higher the SiO content, the worse the stability, and the shorter the service life of the prepared slurry (without distilled water); the non-failed slurry has the same coating strength Higher. However, for a silica sol with a particle size of 8m, when its siO2 content is greater than 32% (the SiO2 content of ordinary 830 glue is greater than 33%), the gel phenomenon will begin to appear, so when the SiO content in the slurry reaches this critical value, that is Start to fail.

Impurity influence:

The impurities in silica sol mainly include excessive Na0, crystalline SiO precipitated in the colloid: and other impurities brought in during the production process and raw materials. High impurity content will affect the stability of silica sol and slurry, the wet strength, high temperature strength and refractoriness of the shell. Effect of viscosity: The viscosity of silica sol directly affects the powder-to-liquid ratio, viscosity and coating performance of the slurry.

pH value and bacterial content:

The pH value of silica sol and slurry should be greater than 9. If the pH value is less than 9, the bacterial content will increase rapidly and the stability of the slurry will decrease. In severe cases, the slurry will fail, causing a large number of defects such as cracks, cracks, and air holes. In summer, the temperature is high, the humidity is high, and the bacteria multiply rapidly. The pH value of the slurry needs to be monitored at any time. Production, storage and transportation temperature: Silica sol needs to be produced, stored and transported at a suitable temperature. Too high temperature (greater than 40°C) will accelerate the growth of bacteria in the silica sol, affecting its service life and performance: if the temperature is too low (below 0°C), the silica sol will be frozen, which will directly cause its failure.

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