Exploration of investment casting shell making process

Exploration of investment casting shell making process

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. However, faced with the dilemma of long shell production cycles and casting defects caused by surface layer shell defects. Propose process measures such as adding anti cracking agent to the surface slurry, drying the surface with high wind force, using large particle silica sol for the surface layer, canceling pre wetting before applying the second layer, and removing silica sol before applying the surface layer, in order to improve the strength of the surface layer shell, reduce defects in the surface layer shell, shorten the drying time of the coating, and improve the one-time qualification rate of the casting. The investment casting medium temperature wax all silica sol shell process is suitable for producing precision parts with low surface roughness and high dimensional accuracy, and has become a mainstream process widely used. The production cycle of this shell making process is long, especially the defects such as cracking, peeling, bulging, and layering of the surface layer shell, which cause many surface defects of castings, hindering mass production and improving surface quality. The quality of the surface layer shell is the key to determining the surface quality and one-time qualification rate of castings.

1、 Improvement of shell making process

1.1. Adding anti cracking agents to the surface slurry, the process of silica sol shell formation 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. The anti cracking agent added to the surface layer slurry is a polymer with a long bond like molecular form, which modifies and protects the surface of the silica sol, reducing the Zeta electrode potential of the silica sol colloid and weakening the mutual repulsion between the particles. During the dehydration and gelation process, due to the entanglement of polymer molecular chains, the silica sol micelles are promoted to approach each other, thereby accelerating the gelation speed.

1.2. 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. 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.

1.3. 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%.

1.4. The physical and chemical indicators of large colloidal silica sol applied to surface layer silica sol not only focus on the mass fraction of SiO2, pH value, and kinematic viscosity, but also the diameter of colloidal particles, commonly known as the particle size of silica sol, which is a parameter of great concern to us. The SiO2 content determines the strength of the shell, the pH value determines the stability of the slurry, the kinematic viscosity determines the powder liquid ratio, and the particle size determines both the stability of the slurry and the strength of the shell. For rubber particles, it is generally believed that larger particle size leads to better stability of the slurry, smaller particle size, and faster gelation. So for many years, most domestic manufacturers have tended to use small particle sizes for the surface layer and large particle sizes for the back layer, and the author holds different views on this. The average particle size of domestically produced silica gel ranges from 8-20nm to 8-15nm, while in the United States it is 12nm and 22nm. In Japan, it is 8nm for 830 and 14nm for 1430. Taking the application of Japanese silica sol in domestic precision casting manufacturers as an example, 1430 is mainly used for the surface layer and 830 is used for the back layer. According to data, most precision casting factories in Japan also mix and apply according to this particle size. For the surface layer slurry, the prerequisite is that the stability of the slurry should be considered first, because the surface layer directly determines the surface quality of the casting. In this sense, using large colloidal silica sol for the surface layer is correct. As for the speed of gelation time, the difference between large and small particles is negligible and can be ignored. There is a deviation between the theoretical and practical production control regarding the large diameter and low shell strength of silica sol colloidal particles, and the application in China and abroad is exactly the opposite. From natural drying of the surface layer to blow drying, domestic precision casting manufacturers have no longer strictly stipulated the use of large particle silica sol for the back layer and small particle silica sol for the surface layer in their process regulations. Currently, the number of manufacturers using large particle silica sol for the surface layer is increasing, and there is a trend of using small particle silica sol no longer being restricted to the surface layer.

2、 Conclusion and Discussion

2.1. Adding anti cracking agents to the surface layer slurry and drying it with air can overcome the defects of the surface layer shell, shorten the drying time, and significantly improve the first pass rate.

2.2. The pre wetting process before applying the second layer has more disadvantages than advantages. Reducing the viscosity value of the second layer slurry can provide protection and strengthening for the surface layer, and help eliminate the hidden defects of the surface layer shell.

Precision machining