The effect of fibrous talc on the performance of lost foam coatings

Abstract: By testing the process performance and working performance of the lost foam coating, the influence of fibrous wollastonite on the performance of the lost foam coating was studied. The results show that fibrous wollastonite can improve the process performance of the lost foam coating, such as suspension, thixotropy, and coating ability, as well as enhance the drying crack resistance, bending strength at room temperature and high temperature, and air permeability at room temperature and high temperature. Lost foam casting is an advanced manufacturing technology, and the coating technology, as one of the key technologies, is an important factor restricting its development and promotion. Refractory powder is the main component of lost foam coating, accounting for more than 50%, and it directly affects the process performance and working performance of the coating. Its physical and chemical properties largely determine the effectiveness of the coating, so researching and appropriately selecting suitable refractory powders is key to ensuring the quality of castings. Among them, the particle shape of the refractory powder is particularly important; fibrous particles belong to non-equiaxed particles and play a special role in the coating. This paper conducts a preliminary study on the influence of fibrous wollastonite on the performance of lost foam coating. 1 Experimental Materials and Methods 1.1 Experimental Materials The refractory powder for the lost foam coating used in the experiment is a combination of mullite and wollastonite, with wollastonite accounting for 0-30% (mass fraction) of the refractory powder. 1.2 Coating Performance Testing Methods 1.2.1 Coating Process Performance Testing Methods Suspension Rate: Measured using the plunger cylinder method. Coating Ability: The actual foam plastic model was dipped and then visually evaluated for coating condition. Drying Crack Resistance: Visually observed the cracking condition of the dried coating. Bending Strength at Room Temperature: After drying the coating, a self-made testing device was used to test the bending strength of the coating, with a span of 41mm. The principle is shown in Figure 3. 1. Tray balance 2. Loading object 3. Support point 4. Knife edge 5. Coating sample High-Temperature Bending Strength: After drying the coating, it was placed in a high-temperature furnace and heated at 900°C for 10 minutes. After cooling, the bending strength testing device was used for testing. It is not difficult to see that what is actually being tested here is the residual bending strength of the coating after high temperature, which only reflects the trend of the change in the high-temperature strength of the coating to a certain extent. Air Permeability at Room Temperature: The STZ type permeability measuring instrument was used to measure the air permeability of the coating at room temperature. A thin layer of petroleum jelly was applied to the end face of the standard sample tube used for measuring sand permeability, the coating was attached to the end face coated with petroleum jelly, and then the sample tube was placed on the sample seat of the permeability measuring instrument. Finally, a pressure cap with holes was pressed onto the coating (as shown in Figure 4) for testing. The height of the water column was first read, and then the permeability was obtained from the conversion table of water column pressure and permeability. High-Temperature Permeability: Given that testing high-temperature permeability is relatively difficult and that various testing methods have their pros and cons, making them less comparable, this experiment adopts the following method: first, the dried coating sample is placed in a high-temperature furnace at 900°C for 10 minutes, then cooled, and the air permeability is tested using the room temperature permeability testing method to measure the relative high-temperature permeability of the coating. This method is simple and easy to implement, has good stability and data reproducibility, and has good comparability under certain specific conditions, reflecting the trend of changes in the high-temperature permeability of the coating. 2 Experimental Results and Analysis 2.1 Process Performance of the Coating 2.1.1 Suspension Rate of the Coating. As the content of wollastonite increases, the suspension rate of the coating improves, and the suspension performance becomes better, thus having good sedimentation resistance during storage. The unique fibrous morphology of wollastonite makes it easy to form a network structure in the coating suspension, which prevents the powder particles from sinking. 2.1.2 Thixotropic Index of the Coating As the content of wollastonite increases, the thixotropic index of the coating increases, and the thixotropic performance improves. This can be understood as the micro-particles of wollastonite being non-equiaxed fibers, which tend to form interlaced network or bulk structures in the aqueous suspension system. When subjected to external forces such as shear force, this structure may be disrupted and form new structures, such as parallel arrangements. When the external force is removed, the particles rearrange and restore to a network or bulk structure, demonstrating that needle-shaped and disk-shaped particles exhibit a more pronounced thixotropic effect than spherical particles. 2.1.3 Coating Ability By observing the coating condition after dipping the actual EPS model, it can be seen that as the content of wollastonite increases, the dripping amount decreases, and the coating thickness on the EPS model increases. When the content of wollastonite is below 5%, there is severe dripping, and the coating is very thin; when the content is between 10% and 20%, the coating is smooth, shiny, and without accumulation; when the content exceeds 25%, there is basically no dripping, and the coating is very thick. The fibrous wollastonite particles have a larger specific surface area and higher surface energy than spherical particles, making it easier to form a certain network structure in the coating system, which allows more liquid to be encapsulated, reducing the free-flowing liquid and thickening the coating. 2.2 Working Performance of the Coating 2.2.1 Drying Crack Resistance The bending strength increases with the increase of wollastonite content in the coating, and the high-temperature bending strength is lower than the room temperature bending strength. At the same time, when measuring the room temperature bending strength, the deflection (or the angle of the balance pointer) observed when the coating breaks can serve as an indicator of the toughness of the coating; as the content of wollastonite increases, the toughness of the coating also improves, reaching a deflection of 5mm when the content exceeds 25%. The addition of wollastonite increases the strength of the coating because the fibrous wollastonite intersperses in the matrix of the coating, forming a network structure that acts as a reinforcing rib. The lower high-temperature bending strength compared to room temperature bending strength is related to the loss of organic binder at high temperatures while the refractory powder has not yet sintered. 2.2.3 Air Permeability at Room Temperature and High Temperature As the content of wollastonite increases, both the room temperature and high-temperature air permeability of the coating improve, with high-temperature permeability being higher than room temperature permeability. This is because when the content of wollastonite exceeds 10%, its proportion in the aggregate is relatively large, and a large amount of fibrous wollastonite easily forms a bulk network structure in the coating, creating more interconnected microscopic voids, thus improving permeability. The higher high-temperature permeability compared to room temperature permeability is related to the loss of organic binder at high temperatures. It is worth noting that when the content of wollastonite is below 10%, the dried coating has cracks, and a higher content of wollastonite means that the coating has enhanced absorption capacity for liquid EPS. 3 Conclusion (1) The method used for making the coating has good stability, thus providing assurance for subsequent measurement work. (2) Fibrous wollastonite particles belong to non-equiaxed particles, which can improve the suspension, thixotropy, and coating ability of lost foam coatings, thereby enhancing the operational performance of the coating. (3) When the content of wollastonite in the coating (mullite-wollastonite series) is below 10%, the drying crack resistance of the coating is poor; when the content exceeds 10%, with the increase in content, the bending strength and permeability of the coating improve.