Sand casting is one of the oldest fabrication methods around, and it’s not hard to see why the technique has stood the test of time. Using sand has some definite benefits compared to other casting methods, such as plaster or die casting.
One of the biggest perceived benefits of this method is that casting sand is reusable. This makes it both a cheaper and more efficient choice for certain production runs and parts.
However, sand casts don’t stay intact when the molded object is removed. Instead, the cast must be broken to release the new piece. This makes sand casting far better suited for projects with lower production runs. Take a look at some of the benefits and uses of sand casting to see why it is the most widely used casting method around today.
What Is Sand Casting?
Sand casting involves creating a mold out of sand to cast molten metal into solid parts. First, the sand is packed into a pre-made mold to allow for the shrinkage that typically occurs during cooling. This flexibility helps prevent the risk of cracking and other flaws that may occur during the casting process.
Sand casting is a favorite among fabricators due to its relatively low cost. The method also works with both ferrous and non-ferrous metals, and can create large pieces. This makes sand casting a widely popular choice for creating automotive parts.
Steps in the Sand Casting Process:
There are six main steps in the sand casting process. While the details of each step or repetition requirements will vary based on each piece and run specifically, the main outline of the process is as follows.
- Make the mold. Casting molds consist of two pieces that a fabricator will press together before introducing the molten metal. Molds can be made of a variety of materials—metal, wood, and plastics included—which are then packed with sand to create the usable version of the mold.
- Clamp the pieces together. Clamps hold the two pieces firmly together to ensure the mold contains the molten metal completely. Leaks caused by a weak seal can create defects in the finished product, which may require additional machining. This adds time and money costs to the process.
- Pour the molten metal. The metal is then introduced into the mold, and the cooling process begins.
- Allow the metal to cool. As the molten material cools, it will start to solidify into the shape of the final product as detailed by the sand lining the mold. Proper cooling techniques, including temperature stability, will help prevent porosity and other defects from forming in the completed part.
- Remove the product. Fabricators then break apart the mold, essentially turning it back into loose sand, to free the newly formed part.
- Trim the product. It may be necessary to trim excess pieces of solidified metal from the resulting product. Additionally, some pieces may require machining to create a higher level of detail or certain desired surface finish.
Alloys: Most casting methods have limitations regarding the use of ferrous metals. However, sand casting does not. It is an effective method for nearly any alloy, ferrous and non-ferrous alike, making it incredibly versatile and able to handle a variety of projects.
Cost: This form of casting is far cheaper than many alternative methods for smaller production runs. The casting process itself is taxing on molds, which are typically made from metal, plastic, or wood—sand can help buffer this and preserve molds for a much longer time. The low cost of small batch production also makes sand casting the perfect choice for single runs or unique castings. There is no need to commission an extensive run to make the cost of the process worthwhile.
Versatility: Everything from the size of a screw to a 100-ton machine part can be casted using sand—the options are virtually endless. Shapes can vary greatly as well, though more intricate designs will likely require additional machining post-casting.
Speed: The sand casting process itself typically produces a result faster than other methods such as investment casting—intricate dies into which molten metal is pumped, pressurized, and left to cool to create detailed parts. The additional machining often required after sand casting will add some time to this process, but it is still largely considered to be a quicker method.
High Temperatures: Sand casting is most useful for metals with extremely high melting points, such as steel, nickel, and titanium. The temperature of these molten metals is high enough to melt most other materials used to make molds. However, sand is able to withstand the heat without damage, making it the perfect choice for the toughest metals.
Post-Casting Machining: Sand castings can tend to produce less detailed parts with rougher surface finishes compared to other casting methods. While there are certain parts these qualities may be acceptable for, sand-cast pieces often require additional machining to create a suitable final result. This can tack on additional time and monetary costs, somewhat negating the casting method’s popularity as a fast and cheap alternative.
Defects: Sand molds can sometimes shift or crack during casting, causing surface defects or even cracks threatening the piece’s structural stability. Improper cooling technique will also heighten the risk of additional defects, such as hot cracking. Porosity in resulting products is yet another risk, especially when using sand with low permeability.
Storage & Maintenance: As mentioned before, sand casting is an efficient option for small production runs. However, this also means that molds are not in use as consistently and that a fabricator will likely have many molds to cycle through. Sand casting patterns also tend to be bulky compared to those used in other casting forms. This makes proper storage and maintenance a bit more extensive of a process.
Types of Sand Casting:
Casting sand comes in a wide variety of options. An extensive list of characteristics including grain sizes and shapes, strengths, permeability, moisture content, and more, all affect categorization. The combinations are seemingly endless. Each variety is better suited to different casting methods and often requires unique conditioning treatments for reuse. While the specific sands you can choose from are numerous, the are basic categories of sand that can be used in casting—each brings with it unique strengths.
The “green” in green sand casting isn’t really about the color of the sand. Rather, green sand is simply wet sand. Sometime referred to as clay, green sand casting is cost-effective and reusable. After use, a sand cast is considered “spent” and has to be rejuvenated before the next use. In order to rejuvenate the material for another casting, fabricators need to separate out the materials left behind from the casting process and add moisture back into the mix.
One major concern with green sand is that it provides for a softer mold. Because of this tactility, molds can move or collapse in areas during the casting process, leaving behind a less than ideal result. However, this risk isn’t great enough to outweigh the benefits of this tried and true process that has stood the test of time for centuries.
For projects that require more detail or a smoother finish, green sand may not be a fabricator’s first choice. However, fabricators do use green sand to cast nearly half of all parts made in the U.S.
Sodium Silicate (Water Glass)
Sodium silicate, or water glass casting, combines sand casting and investment casting principles into one method. Forgers add the liquid material to the sand to form the solid core necessary for the specific casting process. Designs incorporating cavities used this method most often.
While forgers don’t solely use sodium silicate as a binder to creates cores for water glass casting, it does possess a distinct advantage over alternatives. Sodium silicate is a dehydrator. The material turns from liquid to solid form rapidly and at room temperature—the only additional step required is exposure to carbon dioxide. This removes the need to bake a mold or core prior to casting.
It is essential to follow a precise formula to ensure the solidified part can be easily freed from the mold. If a forger uses an improper combination of sodium silicate and sand, the part may stick and become damaged upon release.
Resin sand casting provides the smoothest finish of all the sand casting methods. As opposed to the soft green sand molds, the heat applied to this typically quartz sand and resin mixture creates a very solid, hard mold with few surface imperfections. Forgers must mix, form, and burn each resin sand mold to ensure the highest quality result.
Consequently, it is a more expensive and time-consuming process than green sand casting. Not only does resin sand itself cost more than green sand, but the sand also requires rejuvenation after every use to ensure the best result possible. These added costs make resin sand casting the most expensive out of the three methods mentioned, though the quality of the finish on the resulting products often makes additional machining unnecessary. Fabricators working with resin sand are less likely to encounter casting defects, residual sand on a product, or errors in details than with other forms of sand casting.
Is Casting Sand Reusable?
The short answer is yes, casting sand can be reused. However, rejuvenation is often required to ensure the best result possible on castings incorporating previously used sand. As mentioned before, these three types of casting sand are only broad categories. Within each category exists a wide variety of characteristic combinations that alter each sand’s properties, so the rejuvenation process will look a little different for each type of casting sand. While various factors, such a grain size, moisture content, and more, will change the process before reuse, there are general guidelines for each category of casting sand.
In order to reuse green sand, you must first separate it from the impurities left behind by the metal from previous castings. It is important to filter out any material left behind by the last casting process prior to reuse to ensure the best possible product. Failure to do so may result in a variety of casting defects. A system of magnets and screens separate residual metal and other impurities from the sand. Once the sand is pure again, you’ll have to re-hydrate it to keep the clay-like consistency necessary for casting.
Resin sand is similar to green sand in that it can be reused after rejuvenation. The main difference between the two process is that resin sand does not require the re-hydration green sand does. Resin sand only requires filtering before it is ready for reuse.
Sodium silicate differs from these two a bit as it is an incredibly strong binder. Consequently, it’s necessary to grind up sodium silicate cores in order to reuse the sand. This process essentially reverses the sodium silicate’s binding ingredient, allowing the process to start over again with, relatively, fresh sand. However, the sand itself is so cheap that many fabricators opt to simply replace it and start over. In theory, sodium silicate bound sand is reusable, though doing so isn’t a common practice.
The process to rejuvenate and replace casting sand can often be more expensive than the cost of the sand itself. For this reason, most fabricators will opt to recycle the sand elsewhere and simply purchase fresh, new sand for following castings. Around 70% of used casting sand is recycled rather than thrown away. Recycled sand may end up as part of a road or other construction project or be used in more technical agricultural or geological applications.
Sand casting is a cheap, fast, and efficient fabrication method for less detailed projects with short production runs. While the ability to reuse sand does provide a benefit, most fabricators will opt to recycle instead. Casting sand is cheap enough that repurchasing can often be as cost effective as rejuvenating used sand. However, larger scale fabricators possessing the tools and machines necessary for the rejuvenation process can often purify used sand both in-house and cost effectively.
Weigh the costs of purchasing fresh sand over rejuvenating a used batch to decide which is best for you.