Resin sand, also known as self-hardening sand, is a type of sand that uses synthetic resin as a adhesive to hold sand grains together. It is widely used in foundry industry for making molds and cores, due to its special properties and advantages:

1. High dimensional accuracy: Resin sand molds have superior dimensional accuracy, which is critical for producing precise castings.

2. High strength and toughness: Resin sand molds have excellent mechanical properties, such as high strength and toughness, which can withstand the stress from pouring and solidification of molten metal.

3. Good surface finish: Resin sand molds have good surface finish, which reduces the need for post-processing and improves the final appearance of the castings.

4. Easy to use and handle: Resin sand is easy to mix, shape, and handle, which saves time and labor cost for foundries.

5. Environmentally friendly: Resin sand is a green and sustainable foundry material, which has no harmful effect on the environment and human health.

In conclusion, resin sand is a versatile and efficient foundry material, which has many advantages over traditional sand casting. Its use results in high-quality castings that meet customer requirements in terms of precision, strength, and surface finish.