Selective laser sintering (SLS) is one of the rapid prototyping technologies that use the widest range of materials, including coated sand, metal powders, and polymer powders. Among these, coated sand is one of the most widely used molding materials in sand casting. SLS can be used to realize patternless casting of sand molds (cores). Compared with conventional casting technology, it has advantages such as fast forming speed, short manufacturing cycle, high flexibility, and no limitation by part model complexity. Most existing experiments use coated sands of different mesh sizes as raw materials, but their reinforcement effects vary. Under the condition of changing the mesh number of coated sand, coated sands with different particle sizes have different effects on the material properties of SLS-coated sand.
At the same magnification, as the mesh number of coated sand increases, the number of sand grains per unit area gradually increases, the compactness improves to some extent, and the bonding between sand grains becomes tighter. The resin neck length at the sand-grain bonding points is independent of the sand grain diameter. Different degrees of ablation were observed on the surfaces of 60–70 mesh and 90–100 mesh coated sands, but no ablation was observed on the surface of 120–140 mesh coated sand. This indicates that 120–140 mesh coated sand can achieve slight ablation on the sand grain surface by changing the curing conditions, further improving the mechanical properties of 120–140 mesh coated sand. The resin film on the coated sand surface changes with sand grain size. As the mesh number of coated sand increases, the thickness of the surface resin film gradually decreases. The resin in coated sand is proportioned by mass; under the same mass, smaller sand grains are more numerous, and the film is thinner. The bonding failure modes among sand grains for all three mesh sizes are fracture and debonding at the bonding points. The number of bonding points between 120–140 mesh sand grains and surrounding sand grains is greater than that for the other two mesh sizes. When 120–140 mesh coated sand is damaged by stress, the other bonding points with sand grains help distribute the stress, resulting in good mechanical properties.
By using the single-factor control method, the optimal sintering parameters obtained for 60–70 mesh, 90–100 mesh, and 120–140 mesh coated sands are respectively: for 60–70 mesh coated sand, 35 W, 2400 mm/s, and 80 °C; for 90–100 mesh coated sand, 35 W, 1800 mm/s, and 80 °C; and for 120–140 mesh coated sand, 40 W, 1800 mm/s, and 70 °C.