The term Green Sand Casting mainly refers to a technique of casting metal using “green sand” or, more commonly understood, wet sand. Suffice it to say that despite the term having the word “green” in it, the sand isn’t green, and contrary to popular belief, the molds aren’t fashioned from green sandstone. The said it is referred to as green because of its moisture, similar to green wood. However, the molding sand dries out when hot metal is poured into it.
The significant difference between green sand casting and sand casting, apart from the moisture in the sand, is the use of clay mixed with sand in the mold. This allows the sand to stick together. Most manufacturers will mix sand and bentonite clay to ensure a strong mold that can also be forged on an assembly line.
In modern manufacturing facilities, the sand mold is made on an automated assembly line.
It is a fairly popular method used to cast everything from engine blocks to ship parts and pump housings. It is estimated that around 43% of all parts manufactured in the US are made using green sand mixture molds. However, following closely behind is no-brake, which is around 40% more popular than other methods. That said, any expert would tell you that the method used will depend on the results you want to achieve.
One of the most significant advantages of green sand mold castings is that molds can be made quickly using an automated process. This level of automation significantly reduces the cost of manufacturing parts, especially in the automotive industry, at scale possible. Speaking of parts, the process has to be both fast and economical; both boxes are ticked by green sand casting.
In addition to fine green sand castings being relatively less expensive, manufacturers can make large castings using the method. Some of these castings can be as much as 7,000 lbs., something which isn’t uncommon, but the majority of items manufactured are between 1-500 pounds.
The limitation in size stems from the compaction needed to manufacture the mold. This means that the castings can’t be extremely large, so if a very large item needs to be manufactured, another method needs to be used. No-brake casting is better for large castings. Using no-brake castings, manufacturers can make items of up to 50 tons in size which isn’t possible by loading green sand.
Green sand molds are produced by loading lots of green sand compacted into a metal or ceramic mold maker. The pattern is pressed into the sand during casting processes. The pressure created during the process of pressing the mold into the sand leads to the creation of a similar to when a cookie cutter is pressed into the dough and thus creates a shape.
The major drawback of using green sand molds for castings is that their tolerances aren’t as high as some other mold types. So, what this means is if you need a casting that has a very high tolerance or one that needs to be of a precise dimension, green sand castings will have to be machined a little more compared to other types of molds to ensure the desired results. Also, another casting process may produce a smoother finish compared to green sand casting for ferrous and nonferrous metals.
As mentioned earlier, green sand-casting molds are mainly a mixture of sand and clay. In most cases, it is:
· 85% sand
· 4% water
· 11% Bentonite Clay
· Some other materials or impurities
When molten metal is poured into the casting, some sand or silica can become airborne, which is why many manufacturing facilities have an air control system installed. These systems will pull the silica particles out of the air, ensuring that the air in the foundry is safe to breathe.
After castings are made from green sand, the sand is no longer used or useful, which means it is spent. This so-called spent green sand can’t be reused without putting it through an expensive process of removing materials. So, most manufacturers may find using new sand more economical and save a great deal of time than reclaiming spent sand.
However, reusing the sand helps to keep it out of landfills, which is where it will go after being used. That said, new or fresh sand is available at far lower prices. Also, the sand does not have any toxic substances so it can be put back into general use at the beach or in a desert.
Some of the advantages include the following:
· An easy process compared to other methods.
· A much higher rate of production, it is possible to cast up to 200 boxes a day using green sand. However, higher efficiency and speed production lines can produce 20 tons of castings daily.
· The low material costs and process ensure that green sand casting is cheaper than any other method.
· Defects are common, especially regarding sand residue, air holes, shrinkage, etc.
· The resulting product has a rough surface, requiring additional work to improve.
· Green sand casting only works for items with low size accuracy.
Green sand casting is an economical and efficient process. Using this technology, it is possible to manufacture just about any product, from counterweights to trash bins and gas burners.
The best casting processes will depend on the results you want to achieve.