Abstract:
This comprehensive study delves into the formulation and role of various additives, including furan resin, curing agent, flame retardant, and silane, in the sand mixture utilized for low-pressure sand casting of magnesium alloy. By meticulously analyzing parameters such as serviceable time, mould starting time, and compressive strength, the research aims to elucidate the impact of different additive ratios on the performance of the resin sand. The findings reveal that minimizing the proportionate amounts of furan resin and curing agent, while ensuring the strength of the resin sand, can significantly reduce porosity defects, enhance high-temperature performance, improve casting quality, and lower production costs. This study provides invaluable technical support for the high-quality production of magnesium alloy castings and offers promising prospects for further optimization and application.
1. Introduction
Magnesium alloy, renowned for its low density, high strength, and excellent impact resistance, has found widespread applications in aerospace, automotive, and electronic products. However, magnesium alloy casting processes are prone to defects such as porosity and shrinkage, necessitating meticulous research into suitable casting techniques. Sand casting, the most widely used casting method in the industry, holds a pivotal position due to its high productivity, low cost, and versatility. With the continuous development of new materials and technologies, sand casting technology, particularly magnesium alloy sand casting, has seen significant advancements in enhancing product performance and quality.
Furan resin sand casting, compared to traditional clay sand casting, offers superior surface quality, dimensional accuracy, and reduced labor intensity for casting cleanup. However, improper usage of furan resin and curing agent can lead to environmental pollution issues, necessitating the exploration of reasonable ratio schemes. In China, the self-hardening furan resin sand process is the most widely applied, technologically mature, and experienced method among self-hardening sands. It significantly improves casting surface quality and dimensional accuracy while reducing labor intensity for casting cleanup. This process has achieved remarkable economic benefits in producing complex structures and high-quality castings.
This study focuses on optimizing the furan resin sand formula to reduce porosity defects, improve casting quality, and lower production costs while ensuring strength. By analyzing the effects of different additive ratios, this research provides technical support for the high-quality production of magnesium alloy castings, offering significant practical value.
2. Experimental Materials and Methods
2.1 Experimental Materials
The primary instruments used in this experiment include an SWY hydraulic universal strength testing machine, an SAC hammer-type sample preparation machine, a dedicated combined sample mold, and a balance with a sensitivity of 0.1g. The raw materials for magnesium alloy resin sand consist of silica sand as the sand grains, with a main component of silicon dioxide. Self-hardening furan resin is selected as the binder for the sand grains, while a curing agent is used to initiate the crosslinking reaction and solidification of the resin. The flame retardant, with a flame retardant element mass fraction of not less than 99%, enhances the flame retardancy of the resin sand. Silane is employed to improve the high-temperature performance and thermal stability of the resin sand.
2.2 Resin Sand Formulation
Table 1 outlines the material proportions for formulating 100kg of resin sand, with proportional adjustments made for different quantities. The amount of flame retardant may vary based on product structural requirements and can be specially stipulated if needed.
Table 1: Resin Sand Formulation Proportions
| Material | Quantity (kg) |
|---|---|
| Silica Sand | 100 |
| Flame Retardant | 1-3 |
| Furan Resin | 0.8-2 |
| Curing Agent | 0.25A – 0.50A |
| (A = Furan Resin Quantity) |
2.3 Mixing and Sample Preparation
The silica sand and boric acid are added to the sand mixer and mixed for 3-5 seconds. The curing agent is then added and mixed for an additional 5-40 seconds, followed by the furan resin for 5-30 seconds. The mixed resin sand is discharged and filled into a dedicated combined sample mold and an “8”-shaped standard tensile strength sample mold to produce tensile samples. The prepared samples are placed under specified testing conditions for 30min, 40min, 50min, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 5h, and 24h, respectively, to measure their tensile strengths and plot the relationship curve between hardening time and tensile strength.
The usable time of the furan resin sand refers to the period during which the mixed mold or core sand can be used to produce acceptable molds or cores. The mold stripping time refers to the time required for the prepared mold or core to harden to the point where it can be stripped (or removed from the core box) without damage. Additionally, within 10 minutes, a batch of cylindrical standard compressive strength specimens are quickly made from the freshly mixed mold or core sand. Then, three specimens are grouped together and placed for 1h, 2h, 5h, and 24h respectively, to measure their tensile strengths, which will yield the hourly strength and final strength of the mold or core sand.
By plotting the relationship curve between hardening time and tensile strength, as shown in Figure 1, it can be observed that the usable time of this resin sand is 40 minutes, and the mold stripping time is approximately 1 hour. This means that within 40 minutes after mixing, the resin sand can be used to manufacture acceptable cores; and after 1 hour of mixing, the cores have hardened sufficiently to be safely stripped without damage. This mold stripping time is a crucial indicator for assessing the production efficiency of the resin sand process.
By plotting the curve of compressive strength over time, it is found that the compressive strength of the resin sand gradually increases as the hardening time extends. At 40 minutes, the compressive strength reaches 0.07 MPa, meeting the requirement for usable time; and at 1 hour, the compressive strength reaches 0.14 MPa, satisfying the requirement for mold stripping time. This indicates that the ratio of resin to curing agent is reasonable, enabling the resin sand to achieve necessary strength within a short period. The final strength of the resin sand measured in the experiment is approximately 0.6 to 0.8 MPa, which can meet the strength requirements of most casting processes for cores.
Furthermore, in experiments on improving the performance of resin sand with silane, silane was added to enhance the high-temperature performance and thermal stability of the resin sand. The results showed that the addition of silane could improve the compressive strength of the resin sand, enhance casting quality, and reduce production costs. This provides technical support for the high-quality production of magnesium alloy castings.
In subsequent research, the ratio of resin to curing agent can be further optimized to ensure uniformity during the curing process of the resin sand and reduce the generation of pores. High-quality resin and curing agent should be selected to ensure stable performance and avoid the formation of bubbles during the curing process. Additional measures include strictly controlling the mixing time and temperature, ensuring sufficient mixing time to avoid uneven distribution of resin and curing agent. At the same time, the mixing temperature should be kept within an appropriate range to prevent excessive volatilization of resin and curing agent due to high temperatures. High vacuum or negative pressure molding techniques can also be adopted during the mold-making process to help vent gases and reduce the formation of pores.
Overall, by adopting the aforementioned measures, the pore defects in the resin sand can be effectively reduced, further improving the performance of the resin sand, enhancing casting quality, and increasing production efficiency.