Exploration of Precision Casting Shell Process in Investment Casting

Exploration of Precision Casting Shell Process in Investment Casting

The medium temperature mold material silica sol shell making process is currently the mainstream process in China and abroad, used for the production of precision castings. Its process is suitable for producing precision castings with low surface roughness and high dimensional accuracy. However, faced with the dilemma of long production cycles for shell making and casting defects caused by surface layer shell defects. Today, precision casting manufacturer Tianyao Metal will discuss the improvement methods of investment casting precision shell technology for everyone. So as to improve the strength of the surface layer shell, reduce the defects of the surface layer shell, shorten the drying time of the coating, and improve the one-time qualification rate of the casting.

1、 Add an anti cracking agent that accounts for 8% of the mass fraction of silica sol to the surface layer of zirconium powder slurry, and stir evenly. Alternatively, a powder to liquid ratio of 3.3-3.4:1 can be used to prepare the surface layer slurry in the order of adding silica sol, stirring the powder for 8 hours, stirring the anti cracking agent for 5 hours, stirring the wetting agent for 2 hours, and stirring the defoamer for 1 hour. After the module is coated with slurry, sprinkle 100 mesh zircon sand or 100 mesh white corundum sand. In a dry surface environment with a temperature of 23 ℃ and a relative humidity of 60%, blow air at a speed of 4-6 (m/s), and the drying time of the surface layer is ≤ 2 hours. The surface layer dried by strong wind not only greatly shortens the drying time, but also has no cracks, peeling, bulging, or layering in the coating. The wet strength of the surface layer is significantly improved, and the inner cavity and outer shape of the module are dried at the same time, resulting in better casting quality. The shelling process of silica sol shell is actually a process of establishing strength within the shell. When the coating is fully dried and the silica sol gel is completed, the wet strength of the coating is established.

2、 After the pre wetting treatment surface layer shell is fully dried, the module is immersed in silica sol for a while, and then the second layer shell is coated. This is a traditional silica sol pre wetting treatment process. The purpose of pre wetting is to increase the mass of silica sol on the surface layer, thereby increasing the high-temperature strength of the surface layer and preventing many casting surface defects caused by surface layer defects. However, the negative impact of pre wetting cannot be ignored. Firstly, the drying of the surface layer is achieved through a long period of natural drying under strict temperature and humidity environmental conditions. After immersing the dried surface layer shell in diluted silica sol (with a mass fraction of SiO2 of 25%), the entire surface layer is dampened, restoring it to the initial state of the coating before the surface layer is dried. In on-site control, the drying time of the second layer is often much longer than that of the surface layer. In fact, for the drying of the second layer shell, it is necessary not only to fully dry the second layer itself, but also to re dry the dampened surface layer. Therefore, extending the drying time of the second layer is inevitable. Second, during the drying and hardening process, the silica sol coating has the characteristics of reversible physical changes, such as water loss, gelling and hardening, and re dissolution and softening by wet gel. There are variables that cannot be observed and hidden troubles of shell defects occur during the re drying process of the pre wetted surface layer shell. The author believes that pre wetting treatment before applying the second layer has more drawbacks than benefits and should be improved. Improvement ideas and methods: The second layer of shell is generally referred to as the transition layer. This layer not only serves as a reinforcement and protection for the surface layer, but also serves as a connecting layer between the surface layer and the back layer. It is a crucial layer of special significance in preventing the dissolution of silica sol in the surface layer and preventing defects in the surface layer shell. The second layer slurry can be either mullite powder or corundum powder. The viscosity value of the slurry is controlled between 17-18 seconds (measured by a 4 # Jensen cup), or even 16-17 seconds. Compared with the slurry of the same type of refractory powder used for pre wetting, the viscosity value of the slurry is much lower, which means that the slurry becomes thinner. Because the thinner second layer slurry is more likely to penetrate into the sand layer with larger particle size in the surface layer shell, it not only increases the strength of the surface layer sand, but also provides strong support for the slurry layer in the surface layer. The second layer of sand is sprinkled with 60-80 mesh mullite sand or 60-80 mesh corundum sand. In a dry surface environment, the air is blown at a speed of 4-6 (m/s). If the product structure is not too complex and there are no deep holes, the drying time for the second layer should be ≤ 8-10 hours. However, applying pre wetting treatment to the back layer is quite necessary and feasible.

3、 Some precision casting manufacturers dip their molds in silica sol before the surface layer of the mold is coated, because it is easy to operate. The author holds a negative attitude towards this. There are three basic requirements for the mold material: thermal physical properties, mechanical properties, and process performance. In terms of process performance, good coating properties are closely related to shell making and should be pursued. Whether it is wax based or resin based molds, their common characteristic is hydrophobicity. The hydrophobicity of resin based molds is particularly prominent, and the coating performance of molds is considered based on the angle of contact between the investment mold and the adhesive. The surface tension of water glass binder is 60 (10-3N/cm), and the surface tension of water glass+non-ionic wetting agent is 37 (10-3N/cm). The surface tension of silica sol binder is 71.6 (10-3N/cm), and the surface tension of silica solution adhesive+non-ionic wetting agent is 36 (10-3N/cm). It should be emphasized that both water glass and silica sol are water-soluble binders, containing a considerable amount of water by mass, and the hydrophobicity of the melt mold is prominent. Obviously, wetting agents play an important role in reducing the surface tension of binders. The wetting mechanism is that after adding non-ionic surfactants to the surface layer silica sol slurry, one end of the oleophilic group is attracted by the melt and arranged in a directional manner, while the hydrophilic group is attracted by water molecules and stays at the slurry interface, forming a monolayer composed of surfactant molecules, thereby reducing the interfacial tension between the slurry and the melt, and combining the slurry and the melt to achieve good coating performance. Based on the above analysis, it can be concluded that the use of silica sol in the module does not improve the wetting effect between the silica sol slurry and the investment mold. On the contrary, it will reduce the local viscosity value of the slurry. After the silica sol on the investment mold comes into contact with the surface layer slurry, it still needs to rely on the wetting agent in the surface layer slurry to have an affinity with the investment mold. It feels easy to operate, but only the local slurry instantly becomes thinner and has better fluidity. The improvement of coating performance is not achieved by silica sol, but by lubricating the wetting agent. Production practice has proven that for surface slurry with a powder to liquid ratio of 3.3-3.4:1, the appropriate amount of wetting agent is 0.04-0.042%. The calculation method is as follows: wetting agent%=wetting agent mass fraction/(silica sol mass fraction+powder mass fraction) * 100%.

Precision machining