1. Introduction
Lost foam casting is a modern casting process that has gained significant attention in the manufacturing industry. It combines the advantages of foam lost foam casting and investment casting, making it a promising technique for producing high-quality castings. In this process, the coating plays a crucial role as it affects the quality of the final product. This article focuses on the study of the lost foam casting coating and investigates the impact of mixing time and standing time on its properties.
1.1 Lost Foam Casting Process
Lost foam casting is a near-net shape casting process that uses a polystyrene foam pattern. The pattern is coated with a refractory coating and then placed in a flask. Molten metal is poured into the flask, which vaporizes the foam pattern and fills the cavity left behind. This process offers several advantages, such as the ability to produce complex shapes, reduced machining requirements, and improved casting quality.
1.2 Importance of Coating in Lost Foam Casting
The coating in lost foam casting serves multiple purposes. It provides a barrier between the molten metal and the foam pattern, preventing the metal from penetrating the foam and causing defects. It also helps in controlling the heat transfer during the casting process, ensuring proper solidification of the metal. Additionally, the coating affects the surface finish and dimensional accuracy of the casting.
2. Experimental Materials and Methods
2.1 Coating Materials
The coating used in this study consisted of a powder and a liquid component. The powder component included Al2O3,SiO2 powder as the main refractory fillers with an average particle size diameter of 300 – 800 μm. Small amounts of iron oxide powder, sodium bentonite, and sodium carboxymethyl cellulose were also added. The liquid component was a water-based solution with effective ingredients such as latex and SN thickener, having a pH value of 8 – 9 and a density of 1.2 g/cm³.
2.2 Mixing and Measuring Procedure
140 kg of the liquid component was first poured into a mixer, and then 250 kg of the powder component was gradually added while the mixer was running at a speed of 800 r/min. During the mixing process, 500 mL of the coating was taken from the mixer at regular intervals. The viscosity, rheological curve, and shear dilution ratio of the coating were measured using an LVDV – 2T viscometer with a No. 3 rotor and different rotor rotation speeds of 6, 12, 30, and 60 r/min. After 60 min of mixing, 800 mL of the coating was taken and placed, and the viscosity, rheological curve, and shear dilution ratio were measured at regular intervals during the standing time. All measurements were carried out at a natural environmental temperature of 18°C.
3. Results and Analysis
3.1 Influence of Mixing Time on Coating Properties
3.1.1 Viscosity
Under different rotor speed conditions, the viscosity of the coating decreased rapidly with the increase of mixing time in the initial stage of mixing. As the mixing time continued to increase, the viscosity value showed a slow downward trend. When the mixing time exceeded 50 min, the measured coating viscosity tended to a stable value.
| Rotor Speed (r/min) | Viscosity Change with Mixing Time |
|---|---|
| 6 | Decreases rapidly initially, then slowly, stabilizes after 50 min |
| 12 | Similar to 6 r/min |
| 30 | Similar to 6 r/min |
| 60 | Similar to 6 r/min |
3.1.2 Rheological Curve
The influence of mixing time on the rheological curve of the coating. When the mixing time reached 50 min, the coating properties tended to be stable. Continuing to increase the mixing time to 60 min, the rheological curve of the coating changed little, which was consistent with the change trend of the coating viscosity with the mixing time.
3.1.3 Shear Dilution Ratio
The shear dilution ratio of the coating under different mixing times. In the initial stage of mixing, the shear dilution ratio of the coating was less than 4.0. With the increase of mixing time, the shear dilution ratio gradually increased and reached the maximum value when the mixing time was 40 min. Continuing to increase the mixing time to more than 50 min, the shear dilution ratio slightly decreased, with a value range of 4.2 – 4.3. A higher shear dilution ratio indicates better coating application performance.
3.2 Influence of Standing Time on Coating Properties
3.2.1 Viscosity
Under different rotor speed detection conditions, when the standing time was within 7h, the apparent viscosity of the coating increased slightly overall. When the standing time exceeded 20h, the viscosity increased rapidly and reached the maximum value at about 35h. When the standing time exceeded 40h, the coating viscosity began to decrease, which may be due to the settlement of the coating and the failure of some components.
| Rotor Speed (r/min) | Viscosity Change with Standing Time |
|---|---|
| 6 | Increases slightly within 7h, rapidly after 20h, decreases after 40h |
| 12 | Similar to 6 r/min |
| 30 | Similar to 6 r/min |
| 60 | Similar to 6 r/min |
3.2.2 Rheological Curve
The influence of standing time on the rheological curve of the coating. With the increase of standing time, the shear stress that the coating can withstand generally showed an increasing trend. When the standing time exceeded 40h, the rheological curve tended to be stable.
3.2.3 Shear Dilution Ratio
With the increase of standing time, the shear dilution ratio of the coating first increased and then decreased. When the standing time exceeded 40h, the shear dilution ratio decreased significantly, but overall, the shear dilution ratio value was above 4.0.
3.3 Analysis of Coating Property Changes
The viscosity of the coating during the preparation process decreased with the increase of mixing time and finally tended to a stable value. In the initial stage of coating placement, the viscosity increased with the increase of standing time, and after a long time of placement, the viscosity began to decrease. The viscosity change of the coating was mainly due to the change of the internal structure during the mixing and standing processes.
The lost foam casting coating viscosity in this study consisted of structural viscosity and plastic viscosity. The plastic viscosity was mainly determined by the gravitational force between the molecules of the coating, the collision between the powder particles, and the internal friction, and was less affected by the shear force. The structural viscosity was affected by some special structures formed inside the coating, such as the chain structure formed by polymer and the hydrated film network structure formed by bentonite. During the mixing process, the internal network structure was destroyed under the action of shear force, resulting in a decrease in the structural viscosity. When the mixing time was long enough, the coating viscosity tended to a stable value. During the short-term placement, the coating viscosity increased with the increase of standing time because the network structure cut during the mixing process gradually recovered and some free water was re-bound and could not move freely. After a long time of placement, the coating viscosity began to decrease due to the weakening of the effective components in the coating and the settlement of the coating particles.
4. Conclusions
This study investigated the influence of mixing time and standing time on the properties of the lost foam casting coating. The following conclusions were obtained:
- To obtain a uniform coating, the mixing time should not be less than 50 min.
- To ensure the performance of the coating, the standing time after mixing should not exceed 40h.
These findings provide important technical support and theoretical references for the correct use of lost foam shell casting coatings, which can help improve the quality of castings produced by the lost foam casting process. Future research could focus on further optimizing the coating formulation and exploring other factors that may affect the coating properties.