What Is The Difference Between Resin Sand And Coated Sand Casting Processes?

In the foundry industry, sand casting is the absolute mainstay. When it comes to sand casting, “resin sand” and “coated sand” are indispensable. However, in practical applications, they each have their own characteristics and uses.

1. Resin sand: creating heavy equipment

Resin sand, as the name suggests, is a mixture of raw sand (usually quartz sand) and a synthetic resin binder. Its core technology is “self-hardening”, which means that the mixed sand will gradually harden on its own.

1. How does it work?
Workers first quickly mix raw sand with a resin binder (often with a catalyst) in a sand mixer. They then quickly fill the mixture into a mold (also called a sand box) and compact it by tamping or vibrating. Once filled, no additional heating is required. With the help of the catalyst, the resin in the sand undergoes a chemical reaction at room temperature, cross-linking and curing. After about a few hours, it becomes a solid sand block. At this point, the mold is disassembled, revealing a sand mold with an interior shape identical to the part.

2. What are its characteristics?

Flexible size, suitable for large parts: Resin sand is self-hardening and very strong, so it can be used to make very large castings. Resin sand is widely used in large machine tool beds, ship engine blocks, large valves and other “giants”.

Precision and finish: Compared with traditional water glass sand or clay sand, resin sand castings are more precise in size and have a smoother surface.

The process is relatively simple: no complex heating equipment is required, and the site adaptability is good. After molding, the sand mold has high hardness, is easy to carry, and is not easily damaged.

A bit of “smell”: The resin will emit some pungent odor during the curing process, and the ventilation requirements for the working environment are relatively high.

Cost considerations: The cost of resin binder itself is not low, but for single-piece, small-batch large castings, its overall cost is still advantageous.

In short, resin sand is good at constructing complex and large-scale blank castings from scratch, and is the cornerstone of the heavy machinery manufacturing industry.

2. Coated Sand

The “coated” part of coated sand is the essence. It’s a pre-coated sand material that’s coated with a thin layer of solid phenolic resin, creating a grain-like, “pre-coated” sand. While this sand is loose at room temperature, its formation depends entirely on heat.

1. How does it work?
First, a metal mold (usually iron or copper) with a built-in heating function is created. Dry coated sand is then injected or poured into this metal mold, which is heated to 200-300°C. The coated sand near the mold encounters the high temperature, causing the resin layer on its surface to rapidly melt and flow, then solidify again, forming a hard shell several millimeters thick. After waiting for tens to several minutes for the sand shell to fully harden, the mold is opened, and a hollow, smooth-walled “sand shell” is formed. This is what we often call a “shell mold” or “shell core.”

2. What are its characteristics?

High precision and smooth surface: This is the biggest advantage of coated sand. Because it is thermoformed in a precision metal mold, the resulting sand core dimensions are very precise, and the inner holes or cavities of the cast parts have extremely smooth surfaces, requiring almost no subsequent processing.

Ultra-high efficiency, suitable for batch production: The heating and curing process is very fast, and a sand core can usually be made in one or two minutes, which is particularly suitable for automated and large-scale production.

High strength and high gas generation: The sand shell made of coated sand is very hard. However, the disadvantage is that when pouring molten iron, the resin decomposes due to heat and produces a large amount of gas. If the exhaust design is not good, it is easy to cause porosity defects in the casting.

Expertise in creating the “core”: Coated sand isn’t primarily used to create the outer mold for the entire casting; rather, it’s specifically used to create the “sand core” placed inside the mold to form the complex inner cavity of the casting. For example, the winding water and oil channels inside the cylinder block and cylinder head of an automobile engine are all composed of individual coated sand cores.

Therefore, high temperature and metal molds are used to quickly and accurately replicate sand cores with exquisite details.

For a more intuitive understanding, let’s make a table to summarize:

Supplier

Luoyang Fonyo Heavy Industries Co., Ltd, founded in 1998, is a manufacturer in railway casting parts. Our factory covers an area of 72,600㎡, with more than 300 employees, 32 technicians, including 5 senior engineers, 11 assistant engineers, and 16 technicians. Our production capacity is 30,000 tons per year. Currently, we mainly produce casting, machining, and assembly for locomotive, railcar, high-speed trains, mining equipment, wind power, etc.
We are the railway parts supply to CRRC(including more than 20 branch companies and subsidiaries of CRRC), Gemac Engineering Machinery, Sanygroup, Citic Heavy Industries, etc. Our products have been exported to Russia, the United States, Germany, Argentina, Japan, France, South Africa, Italy and other countries all over the world.