Improvement and discussion of current shell-making technology in investment casting by Novartis

2020-04-17 16:45:20 1116

The silica sol shell making process of medium temperature wax pattern is the mainstream process at home and abroad, which is used to produce precision castings. However, facing the problems of long production cycle of shell making and casting defects caused by surface layer shell defects. It is proposed that the surface slurry should participate in the process behavior of anti cracking agent, strong wind force and monotony of the surface layer, large particle size silica sol should be selected for the surface layer, pre wetting before removing the second layer, and silica sol should be dipped before removing the surface layer, and then the strength of the surface layer shell should be improved, the defects of the surface layer shell should be reduced, the monotony time of the coating should be shortened, and the one-time qualification rate of castings should be increased.






The investment casting process of medium temperature wax silica sol crusting is suitable for producing precision parts with small surface roughness and high scale accuracy, and has become a mainstream process and is widely used.






This shell making process has a long production cycle, especially the defects of surface layer shell cracking, falling, bulging and layering, which constitute the occurrence of many surface defects of castings, perplexing mass production and the journey of surface quality. The quality of surface layer crust is the key to select the surface quality and one-time qualification rate of castings.






1. Improvement of shell making process






1.1 Anti crack agent used in surface slurry






In the surface layer of zircon powder, join in the anti cracking agent accounting for 8% of the mass fraction of silica sol, and mix evenly. The powder liquid ratio of 3.3~3.4:1 can also be used to prepare the surface layer slurry according to the sequence of adding silica sol → mixing powder for 8h → mixing anti crack agent for 5h → mixing wetting agent for 2h and defoamer for 1h.






After the module is stained with slurry, sprinkle 100 mesh zircon sand or 100 mesh white corundum sand, and under the monotonous environment condition of the surface layer with temperature of 23 ℃ and relative humidity of 60%, blow the air, wind speed of 4-6 (m/s), and monotonous time of the surface layer ≤ 2h.






The monotonous surface layer after blowing strong wind not only greatly shortens the monotonous time, but also has no cracks, falls, bulges or delaminations. The wet strength of the surface layer is significant, and the inner cavity of the module is monotonous with the outer shape. The casting quality is shown in Figure 1


Improvement and discussion of current shell-making technology in investment casting by Novartis

The crusting process of silica sol mold shell is actually the process of establishing the strength in the mold shell. When the coating is sufficiently monotonous and the silica sol gelling is completed, the wet strength of the coating will be established.






The anti crack agent put into the surface layer slurry is a kind of polymer with long bond molecular shape. The surface of silica sol is modified and maintained, which reduces the Zeta electrode potential of silica sol gel and weakens the mutual repulsion between colloidal particles. In the process of dehydration and gelling, due to the entanglement of polymer molecular chains, the silica sol micelles are closer to each other, and then the gelling speed is accelerated. Production has proved that under the action of polymer, silica sol can start the gelling process sensitively with less water evaporation, and finish the rapid monotony of the surface layer.






At the same time, because the polymer is alkaline and water-soluble, it is also a binder, which can play a role in physical reinforcement of the surface layer in the silica sol gelling process. Because of the excellent film-forming property of the organic polymer, the binder can form a uniform and continuous adhesive film between the refractory materials of the shell, and the bonding effect of the adhesive is complete, so that the shell can finally establish the same room temperature strength as the back layer.






Moreover, the good flexibility of polymer molecular bonds can improve the coating of the binder in the powder and sand, effectively prevent and reduce the tendency of surface layer cracking, bulging and delamination, and play a role in maintaining the surface layer. It can be proved convincingly from the electronic microscope photos of organic polymer shell cross section comparison. Now, the process of increasing crack inhibitor is being used by investment casting manufacturers.






1.2 Prewetting






After the surface layer shell is fully monotonous, the module is immersed in the silica sol, and then the second layer shell is coated, which is the traditional silica sol pre wetting treatment process.






The purpose of pre wetting is to increase the quality of silica sol on the surface layer, and then increase the high temperature strength of the surface layer to prevent many casting surface defects due to surface defects. However, the negative effects of pre wetting cannot be ignored.






First, the monotony of the surface layer is achieved through natural monotony for a long time under severe temperature and humidity conditions. After the monotonous surface layer shell is immersed in diluted silica sol (the mass fraction of SiO2 is 25%), all the surface layers will be wetted, and the initial condition of the coating will be recovered when the surface layer is not monotonous. In the field control, the monotonic time of the second layer is often much longer than that of the surface layer. In practice, for the monotony of the second layer shell, not only the second layer itself is fully monotonous, but also the wetted surface layer is monotonous from the beginning. Therefore, The monotonous time of extending the second floor is certain.






Second, in the process of monotonic hardening, the silica sol coating has the characteristics of reversible physical changes such as water loss, gelling hardening, and re dissolution softening by wet gel. The surface layer shell after pre wetting has the variable of unobservable and risk of shell defects in the process of monotonic hardening from scratch.






The author thinks that pre wetting treatment before coating the second layer has more disadvantages than advantages and should be improved. Ideas and methods for improvement: The second layer of molded shell is generally called the transition layer. This layer not only plays a role in strengthening and maintaining the surface layer, but also is the connecting layer between the surface layer and the back layer. It is a very important and special key layer for preventing the surface layer silica sol from back dissolving and preventing the surface layer molded shell from occurring defects.






The second layer of slurry can be mullite powder or corundum powder. The viscosity value of the slurry is controlled within 17~18s (measured by 4 # Jens cup), or even 16~17s. Compared with the slurry of the same type of refractory powder for the second layer of pre wetting, the viscosity value is much lower, that is, the slurry becomes thinner. Because the thinner two-layer slurry is easier to penetrate into the sand layer with larger particle size of the surface layer shell, it not only increases the strength of the surface layer sand, but also plays a powerful role in supporting the slurry layer of the surface layer. 60~80 mesh mullite sand or 60~80 mesh corundum sand shall be used for sand spreading in the second layer. Under the monotonous environment of the surface layer, the wind speed shall be 4-6 (m/s). If the product structure is not too messy and there is no deep hole, the monotonous time of the second layer shall be ≤ 8-10h.






However, it is appropriate, necessary and feasible to apply pre wetting treatment to the back layer.






1.3 Molded silica sol






Some precision casting manufacturers dip the silicon sol before the surface slurry is applied to the module. The reason is that it is easy to operate. The author holds a negative attitude towards this.






There are three basic requirements for the pattern material, namely, thermophysical function, mechanical function and technological function. As far as technological function is concerned, better coating performance is related to the proximity of shell making, which is to be sought.






Whether wax based or resin based, their common feature is hydrophobicity. The hydrophobicity of resin based is particularly excellent. The coating performance of the mold is considered by the angle between the investment mold and the binder. The surface tension of sodium silicate binder is 60 (10-3N/cm), and that of sodium silicate+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 glue+non-ionic wetting agent is 36 (10-3N/cm). It is emphasized that both water glass and silica sol are water-soluble adhesives, which contain appropriate mass fraction of water, and the hydrophobicity of the investment mold is prominent. Remarkably, the wetting agent plays an important role in reducing the surface tension of the binder. The wetting mechanism is: after the surface layer silica gel slurry is involved in the non-ionic active agent, one end of the lipophilic group is attracted by the mold and placed in a direction, and the hydrophilic group is attracted by the water molecules and left at the slurry interface, forming a monolayer composed of surfactant molecules, and then reducing the interfacial tension between the slurry and the mold, so that the slurry and the mold are combined, Good coating performance is achieved.






According to the above analysis, the wetting effect of the silica sol on the module does not improve the wetting effect between the silica sol slurry and the investment mold. On the contrary, the viscosity of the slurry will decrease. After the silica sol on the investment mold contacts the surface slurry, it is still necessary to rely on the wetting agent in the surface slurry and the investment mold to play an affinity role. I feel that the operation is good, but that part of the slurry becomes thinner instantaneously, and the fluidity is better. The improvement of the coating property is not silica sol, Instead, it depends on the smoothing wet agent. The production practice has proved that 0.04-0.042% of wetting agent is appropriate for 3.3 ~ 3.4:1 powder to liquid ratio of surface layer slurry. The accounting method is: wetting agent%=mass fraction of wetting agent/(mass fraction of silica sol+mass fraction of powder) * ).






1.4 Large colloidal silica sol applied to the surface layer






In addition to focusing on SiO2 mass fraction, PH value and kinematic viscosity, the physical and chemical objectives of silica sol also focus on the diameter of colloidal particles, which is often referred to as the particle size of silica sol.






The SiO2 content selects the shell strength, the PH value selects the stability of the slurry, the kinematic viscosity selects the ratio of powder to liquid, and the particle size selects both the stability of the slurry and the shell strength. As for the colloidal particles, it is generally believed that the particle size is large, the stability of the slurry is good, the particle size is small, and the gelling is fast. Therefore, for many years, most domestic manufacturers prefer to use small particle size for the surface layer and large particle size for the back layer. The author has different ideas about this.






The average particle size of silica gel produced in China is 8~20nm, but also 8~15nm, 12nm and 22nm in the United States, 8nm and 14nm in Japan 830. Taking the application of Japanese silica sol in domestic precision casting factories as an example, 1430 is used for the top layer and 830 is used for the back layer. According to the material introduction, most precision casting factories in Japan also prepare and use slurry according to this particle size.






As for the surface layer slurry, the prerequisite is that the stability of the slurry should be considered first, because the surface quality of the casting is directly selected for the surface layer. In this sense, it is correct to select large colloidal silica sol for the surface layer. As for the gelation time, the interval between large colloidal particles and small colloidal particles is small and can be ignored.






There are errors in the theory and practice of production control on silica sol colloidal particles with large diameter and low shell strength. The application at home and abroad is just the opposite.






From the natural monotony of the surface layer to the monotony of blowing, the domestic precision casting manufacturers have no rigid regulations on the process of using large colloidal silica sol for the back layer and small colloidal silica sol for the surface layer. Now, more and more manufacturers choose large colloidal silica sol for the surface layer, showing the practice and trend that it is no longer necessary to use small colloidal silica sol for the surface layer.






2. Conclusion and discussion






2.1 The anti crack agent is used in the surface layer slurry, and the blowing is monotonous, which can defeat the defects of the surface layer shell, shorten the monotonous time, and greatly improve the one-time pass rate.






2.2 The pre wetting process before coating the second layer has more disadvantages than advantages. Reducing the viscosity value of the second layer slurry can maintain and strengthen the surface layer and help eliminate the defect risk of the surface layer shell.






2.3 The application of large colloidal silica sol in surface slurry will not affect the surface quality of castings. Dip the module with silica sol before making the shell, which is not beneficial to the quality of the surface layer shell.


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