The Role of Binders and Resins in Sand Casting
Sand casting is a process where sand forms a mold that holds molten metal as it cools and solidifies. It’s also used for the cores that create hollow regions inside the cast parts. Sand grains must stick together to create those shapes: the way they do that influences the formation of defects and the surface finish achieved. It also affects the production rate.
When a foundry is deciding how to cast a particular part, selection of the binder system – the mechanism that makes grains of sand stick together – is a big part of the process. This blog will explain what binders and resins are and why they matter.
Forming Sand Molds and Cores
Sand is a good material for making molds and cores because it’s widely available, inexpensive, and resists high temperatures. Most types of sand also have low thermal expansion, which helps in achieving consistent casting quality. However, dry sand doesn’t clump to form a shape strong enough to resist the pressure of flowing metal.
Clumping is achieved by adding a binder. Traditionally, this was a type of clay, usually Bentonite. While this resulted in sand molds with good strength, moisture in the clay could cause gas defects in the casting. (Moisture is why this type of casting is called green sand casting.)
The development of other binder systems was a big advance in the sand casting process. These are referred to as resin sand processes. Here grains of sand are coated with a resin which is activated to make them sticky. Activation can be achieved by heating or through a chemical reaction and is performed immediately before the sand goes into the flask or core box.
Type of Binder System
Heat-activated binder systems coat the sand grains in a polymer. Warming makes the polymer sticky and so binds the grains together. Heating like this is energy-intensive, which was one of the drivers behind the development of chemical binder systems.
In chemical systems the sand is coated with a resin. Phenolic urethanes are one widely used type with others including oil urethane, silicate esters and furan. (Furan is a resin made from furfuryl alcohol, formaldehyde and urea.) Some resins work better with some alloys than others.
The resin is activated by applying a catalyst immediately before the sand is deposited. In some systems a pre-catalyst is applied immediately before the catalyst itself.
Advantages of Resin Sand Systems
Molds and cores made with resin sand are stronger than those from the green sand process. Generally, the pattern releases more easily, (although this is not the case with some binder systems,) and is less prone to moving as metal is poured in. These factors allow use of smaller draft angles and casting of thinner sections.
Other advantages are superior surface finish and a lower occurrence of gas-induced defects.
Binder System Considerations
The resin sand process generally produces higher quality castings, but there are some negative factors to consider.
First, some resin systems need a longer time to reach full hardness before the pattern can be removed. This increased strip time can affect productivity.
Second, stronger molds, and especially cores, can be more difficult to shakeout. This again can slow production.
Third, there is wide variation in the price of different resins and catalysts.
Expert Process Optimization
Having decided to sand cast a part, there are still a lot of decisions to make. There’s the choice between resin and green sand casting, and if resin is the better option the next decision is the binder system. Many foundries have preferences based on what they are familiar with and their experience, but it’s still important to decide which will be optimal for the part requirements and alloy.
Impro has a lot of experience with sand casting. Our process specialists can recommend the binder system that will best meet your needs and objectives. Contact us to start that conversation.