People have been using sand casting for generations and, today, it’s still one of the most frequently used processes for producing cast parts. Manufacturers value it for its versatility, its cost-effectiveness and its ability to withstand high temperatures, among other beneficial characteristics. In this guide, we’ll discuss how to sand cast and when to choose it for a casting project.
What Is Sand Casting?
Manufacturers of metal parts and elements often use casting to create their products. Casting is the process of pouring molten metal or metal alloys into a pre-formed mold made out of sand, filling the inner cavity and allowing the metal to cool. Once the metal re-solidifies, the manufacturer removes the sand (mold) from around the cast part, leaving an accurate replica of the molded object.
While there are several methods for creating metal molds, sand casting, also known as sand molding, is the most widely used in a variety of industries. Sand casting is a method of forming molten metal alloys into products through the use of sand molds. It provides a highly customizable process, allowing manufacturers to create metal components of any size, from small parts that use a few ounces of alloy to large parts weighing several tons.
The process also allows the manufacturer to create an enormous variety of products out of almost any metal alloy material. Therefore, sand casting is an excellent way to create molds of complex shapes.
Types of Sand and Sand Mixtures Used
Silica-based sands are the most common type used in sand casting, although other kinds can also be used. Silica sands and other molding varieties have particularly small grains that pack closely together, providing a smooth surface free from inconsistencies and flaws.
The ideal sand for the molding process should maintain its structural integrity, even under significant mechanical stress, but still remain porous enough to allow gasses and steam to escape the mold during casting. It should also have a balanced moisture content. Low moisture content in the sand can cause it to break or crack apart, wrecking the mold. Too much moisture in the sand can trap the gasses and steam bubbles in the mold, preventing them from escaping properly.
Some of the major factors that determine how well the sand will perform as a casting mold are the size and shape of the sand grains. In addition, you should consider the sand’s refractory strength or its resistance to extreme heat, as well as how it collapses with an outside force and its reuse and conditioning capabilities.
The various types of sand mixtures used are typically categorized by the binder that holds the sand together. Some of the most common types include green sand, sodium silicate and resin sand.
The Sand Casting Process
Sand casting is a relatively straightforward process, but it still takes time, preparation and precision. The steps of the sand casting process include:
Before you begin casting, it’s important to gather all the supplies you’ll need and prepare your workspace. The most significant element is the type of sand and binder you choose. Along with deciding on the best type of sand for your uses, you have to create the shape for your mold. It’s essential to create a mold that’s easy to remove from around the metal form once you go through the casting process. While this likely won’t affect most projects, it may limit the complexity of your casings.
To prepare for the molding process, manufacturers should gather all the tools they’ll need in advance, including:
- An adequate amount of molding sand
- A mold of the object
- A kiln or oven to melt the metal alloy
- A ladle to pour the hot metal into the mold
- A block of meltable metal alloy
- A hammer, mallet or another tool to crack the sand mold
The amount of sand and metal alloy you need depends on the size of the mold you’re creating. Once you gather the necessary materials and prepare your workspace, you’re ready to begin the casting process.
2. Filling the Form
Once you have all of your objects and casting forms, you can begin creating the molds with sand. In this step, manufacturers pour sand over the object forms to create a negative impression, which, once removed, becomes the mold for your metal alloy. Check to ensure the mold is clean and that there isn’t any debris sitting on it that could affect the shape of your sand impression.
As you pour the sand, it’s crucial to continually pack it down to fill in all areas, ridges and shapes, preventing the possibility of gaps in the mold. If there are any gaps left, your molten metal will flow into them and ruin the shape of your casting. Make sure the layer of sand is also thick enough to withstand cracking or burning through with exposure to the molten metal substance.
After you’ve finished pouring and packing the sand, let it sit until it solidifies. The length of the solidification process depends on the size of your mold.
3. Removing the Mold
Once your sand mold is set, it’s time to remove it from the form. This part of the process requires precision. The packed sand is hard enough to maintain its form, but any impact or outside force is enough to crack or break it. If the mold is particularly large or unwieldy, it may benefit you to get assistance from others.
First, check the sand to see if it has hardened enough that it won’t move. If it isn’t set enough, it may fall apart or crack when you try to remove the mold. To remove the mold, tap the bottom half — known as the drag — with a hammer or mallet to loosen it from the inner form, then slowly flip it and place it in a secure area. Repeat the process for the top half of the mold, known as the cope.
4. Assembling the Mold
When you’ve removed the mold successfully, it’s ready for assembly. Before you put it together, make sure the inner forms you plan to fill with molten metal are clean and packed. If they contain any loose sand or debris, they can negatively affect your casting. Clean them lightly to ensure they provide the smoothest surface possible.
The impressions should also line up with each other to ensure the casting turns out even and free from imperfections. Align the cope and drag and bind them together tightly.
5. Pouring Molten Metal
With the sand mold ready to cast, you can begin to melt down your metal alloy. Filling the mold with molten metal can be dangerous, and it’s essential to take proper safety precautions before and during this process. The individual pouring the metal should wear protective equipment, including boots, heat resistant gloves, a heat resistant long sleeve coat, a face shield and eye protection.
Clear the area of any potential tripping or slipping hazards as well as any obstructions that might get in the way. Make sure the mold is in a stable and level position to ensure proper setting. Then, fill the ladle with the molten metal and transport it carefully to the mold. Slowly pour the molten alloy into the sand mold without spilling it onto the outer surfaces. Accuracy is essential, as multiple pours will result in metal cooling at different rates, which can cause flaws in the casting. The pouring process does not involve the application of pressure — gravity does all the work as the molten metal pours down into the mold.
6. Cracking the Mold
Once the metal is fully cooled and solidified, you can crack open the mold. The cooling time will vary based on the size of the item you’re creating and the alloy you use to make it. Take your hammer or mallet or another tool that won’t damage the object inside and crack open the sand mold. You will be left with the metal object inside. Be careful not to damage the product during the cracking process.
7. Quenching the Molded Object
Even though the metal object is solidified, it will likely still be hot, so avoid touching it without protection. To cool it off, you can use pliers to lift it and quench it in a container of cool water, or, if the object is too large to lift, you can pour water over the metal object.
After the metal is thoroughly cool and handleable, you can inspect it for any flaws or imperfections and provide adjustments if it needs them.
Sand Casting Products
Due to the versatility of the sand casting process, it’s used to create a huge range of metal products and components. These components are used across a wide variety of applications and industries. Almost any structural metal component can be produced using sand casting. A few of them include:
- Pistons and valves
- Cams, bearings and bushings
- Blowers and impellers
- Electronic equipment
- Electrical contact parts
- Engine blocks
- Medical equipment
- Pumps and pump housings
- Pipe plugs and fittings
- Lever arms and pulleys
Industries That Use Sand Casting
A few of the industries that use sand casted parts include:
- Water Filtration
- Energy generation
- Power transmission
The Benefits of Sand Casting
Sand casting is one of the world’s most widely used casting processes due to the many advantages it provides. Some of the top benefits of sand casting items include:
- Versatility: Sand casting is useful for creating a wide range of products from small, simple objects to large, complex ones. It works with almost any metal alloy, as long as they are pourable. Modifications to tools also tend to be relatively simple and cost-effective, making sand casting ideal for the prototyping and early production phases.
- Complexity: Sand casting can produce highly complex parts. You can make sand casting molds from multiple pieces and create objects with holes, undercuts, protrusions and internal channels.
- Accuracy: Sand casting processes are often extremely accurate and can reliably produce objects to precise specifications. However, sand casting is not as accurate as die casting, since the mold is made of sand instead of steel.
- Low equipment and tooling cost: Equipment costs and tooling costs are typically low. The process is relatively short, patterns can be used many times, and modifications to tools usually are relatively simple, all of which brings down costs.
- Speed: The lead time for making molds is relatively short, as is the time required to modify a tool or mold. This advantage is especially useful for short production runs.
- Recyclability: Casting sand is easy to recycle and is typically used through many production cycles. This makes sand casting more environmentally friendly and helps reduce costs.
- Ability to withstand high temperatures: Casting sand can withstand extremely high temperatures, making sand casting one of the few processes available for metals with especially high melting points such as steel, titanium and nickel.
Other Considerations of Sand Casting
Like any manufacturing process, sand casting comes with certain factors you must weigh when determining whether sand casting is the right choice for your project. These considerations include:
- Porosity: Sand casting can result in higher levels of porosity in the metal than other casting processes would. Porosity is often most prominent on the object’s surface, as gases rise up through the metal throughout the process. Before beginning the process, you need to decide on the acceptable level of porosity for your project. If needed, you can use resin impregnation, in which resin is drawn into the casting to fill the pores.
- Surface inclusions: During the casting process, the molten metal picks up some loose sand, resulting in inclusions in the surface material in the form of sand grains. Whether these inclusions are acceptable depends on the project, and you should consider this issue when specifying the machining allowance if you need the surface to be free of sand grains.
- High labor costs: There are many steps involved in making the sand mold and cleaning the cast part, which needs to be done with a saw — the process is very labor-intensive. Die casting is much quicker and usually results in even lower part prices.
- Cleaning and secondary machining needs: Following the sand casting process, you will need to clean the surface finish, and for low-tolerance projects, some secondary machining may be needed. If tolerances are tight, then sand casting always needs to be machined — this is unlike die casting, which can hold tighter tolerances. As a result, die casting part pricing is less than sand casting part pricing.
- Quantities: Sand casting is best suited for low-volume parts in the range of a few hundred to about a thousand per year. When new products are low in quantity, sand is a good casting process to start with — but when the quantity increases, the part may be a candidate for die casting.
Trust Premier Engineered Products With Your Casting Needs
Premier Engineered Products has been manufacturing components for clients for more than 70 years. If you need high-quality parts made, we’ll work with you to determine the ideal process for your needs. To get started, contact us today.