Preparation of lost foam coating

2025-03-07 17:47:13

The preparation process of water-based coating is roughly to add dry materials such as refractory materials, bentonite, anhydrous sodium carbonate, etc. into the sand mixer, dry mix for about 10 minutes, add binder solution and a little water; wet mix for about 20-30 minutes, pour the wet materials into the bucket, add appropriate amount of water and stir. Finally, add various additives during the stirring process.

Production practice has proved that the speed and stirring time of the agitator will affect the coating performance and precipitation of the coating. The higher the speed of the agitator and the longer the stirring time, the more uniform the coating is mixed, the less likely it is to precipitate, and the coating effect is also very good. From a process point of view, the speed of the agitator is required to be ≥1380r/min and the stirring time is ＞1h.

For the preparation of coatings by ball mill, due to the ball milling effect, the coating can be fully mixed to reach a colloid state, and the coating effect is better. The preparation process is to mix refractory materials, activated bentonite and a little water into a paste coating (water accounts for 25% to 30% of the powder mass) and add them to the ball mill, then add appropriate amount of water to stir into coating, ball mill for 7 to 8 hours, add binder and polyvinyl acetate emulsion and then ball mill for 1 hour, pour the coating into or press it into the coating bucket with compressed air to stir it thoroughly, and add some additives depending on the air permeability of the coating.

If calcium-based bentonite is used, sodium carbonate (accounting for 5% of the bentonite) can be added to convert it into sodium-based bentonite, which is called activation treatment.

For polymer binders, they must be prepared into solutions before use. For example, sodium carboxymethyl cellulose (CMC) is prepared into a 1:40 aqueous solution; polyvinyl acetal (PVB) or phenolic resin is prepared into a certain proportion of ethanol solution.

When using a ball mill or roller to prepare the coating, a coarser particle size can be selected because the refractory particles can be crushed; while using a mixer to prepare the coating has no crushing effect, so a finer particle size refractory should be selected.

When a ball mill or roller is used to prepare water-based coatings, the coarser refractory aggregates are crushed to form a new surface and have greater activity, so that the carrier is evenly distributed on the surface of the refractory particles, so it has better stability, thixotropy and high strength.

Pulp waste liquid coating is a kind of lost foam coating water-based coating, which is usually used in the production of small and medium-sized cast iron parts and aluminum alloy castings, and the overall effect is good. Three points need to be noted during use: First, the pulp waste liquid coating needs to be sprinkled with dry quartz sand after immersion to prevent the coating from sliding down. Second, if you encounter a situation where you need to apply more than two layers of coating, you should take quick drying measures, otherwise the dried first layer of coating will easily deliquesce and fall off, affecting the operation process. Third, since there is a certain amount of humidity in the air, even if the dried coating is placed in the air for too long, it will absorb moisture. Therefore, enterprises should try to avoid placing the coating in the air for a long time.

Note: If the refractory quartz powder is replaced with zircon powder, it can be used for steel castings and thick iron castings.

Preparation of quick-drying coating

Quick-drying coatings with polyvinyl butyral (PVB) or phenolic resin as binders should be prepared in advance into a certain proportion of ethanol solution, and then refractory aggregates and various additives are added and fully stirred.

Relationship between the performance and structure of lost foam coatings

There has been a lot of experience and knowledge accumulated on the relationship between the performance and components of lost foam coatings, which has been discussed above. The understanding of the relationship between the performance and structure of coatings is very limited. Establishing the relationship between the performance and structure of coatings will help the development and use of high-performance new coatings and provide assistance for the control of lost foam casting processes.

Early studies have touched on this aspect. Sun et al. (1992) have revealed that the permeability of the coating depends on the particle size and distribution, shape, and pore characteristics of the coating. Their research has shown that a coating mainly composed of nearly round, larger, and more uniformly distributed aluminum oxide particles has higher permeability than another coating composed of irregularly shaped, smaller, and unevenly distributed silicon dioxide particles. The research of and Penumadu (2005) showed that the size and structure of the "effective pores" in the coating are closely related to the permeability of the coating. Qi et al. (2005) have tried to establish a structural model for lost foam coatings to explain their heat transfer and mass transfer characteristics. Given its complexity, there is still a lot of research work to be done on the relationship between the performance and structure of lost foam coatings.