Creating Casting Sand: A Guide To Investment Casting

Sand casting is a foundational manufacturing process that has been used for thousands of years to create metal parts. It is a cost-effective and versatile technique that can accommodate various shapes and sizes, from small intricate components to large engine blocks. The process involves creating a sand mould, filling it with molten metal, and then finishing the product to meet precise specifications. Sand casting is particularly suitable for casting ferrous and non-ferrous alloys, such as cast iron, steel, and aluminium. One of the key advantages of sand casting is its simplicity, making it ideal for producing larger, heavier parts at economical rates.

The sand casting process

The process begins with the creation of a sand mould. Sand, usually mixed with a bonding agent such as clay, is packed around a pattern or model of the final product. This pattern can be made of wood, metal, plastic, or other materials. The sand is then compacted and the pattern removed, leaving a cavity in the shape of the desired product. The two halves of the mould are then put back together and the void is filled with molten metal, usually iron, steel, bronze, brass, aluminium, or magnesium. Once the metal has cooled and solidified, the sand is removed and the product is taken out. The sand is often reclaimed and reused.

After the metal has cooled and solidified, the sand mould is broken apart to reveal the casting. This cast metal piece then undergoes a series of cleaning and finishing processes to remove any adhering sand and achieve the desired surface finish. Techniques such as grinding, sanding, or shot blasting can be used to refine the surface finish.

Sand casting is a versatile and cost-effective method for creating metal parts. It is suitable for casting a wide range of metals and alloys, including steel, iron, brass, and most non-ferrous alloys. It can accommodate various shapes and sizes, from small, intricate components to large engine blocks.

Creating a sand mixture

The physical characteristics of the sand mixture are some of the most important controls that must be maintained in the sand casting process. Grain size, moisture content, and compactibility are just a few of the variables that must be carefully monitored. The sand mixture is then packed around a pattern or model of the final product to create the mould.

The type of sand mixture used can vary depending on the specific sand casting process. For example, green sand casting uses a mixture of sand particles, clay, wood flour, and water, known as green sand because it contains moisture. Alternatively, chemically bonded no-bake sand is another option, which does not require a binder.

Sand moulding process

The sand moulding process is an ancient casting technique that has been refined over millennia. It is a cost-effective method for creating metal parts by pouring molten metal into a sand mould. The process is versatile, accommodating a range of metals and alloys, and can be used to create parts of varying sizes and complexities. Here is a detailed breakdown of the sand moulding process:

Preparation

Before the moulding process begins, the sand must be prepared. The type of sand used depends on whether it will be reused or discarded. Green sand casting, for example, uses sand that can be reused, whereas dry sand casting typically discards the sand after use. The sand is mixed with a binding agent, such as clay, and moistened with water to create a mouldable substance.

Mould Creation

The prepared sand is then packed or compacted into a moulding box or flask, which is divided into two halves: the "cope" (top half) and the "drag" (bottom half). The pattern, a replica of the final product, is placed into the sand, and the sand is rammed or compacted tightly around it. The pattern may be made of various materials, such as wood, metal, or synthetics, depending on the volume and tolerance required. For complex shapes, additional patterns called "cores" must be created to form interior cavities and features.

Assembly

Once the pattern and cores are removed, the two halves of the mould are assembled and clamped together to form a complete three-dimensional shape. A refractory coating may be applied to improve the surface finish and protect the mould during the pouring process. The mould is then ready for the molten metal.

Pouring

Molten metal is poured directly into the stationary mould through gates and risers, which are pathways that ensure the metal flows smoothly and fills every crevice. The risers also provide a continuous supply of liquid metal, allowing for controlled cooling and solidification. This method is suitable for producing almost any type of alloy, and shielding processes can be employed to protect highly reactive metals from exposure to air.

Cooling and Removal

Once the metal has cooled and solidified, the sand is broken apart, and the casting is removed. The sand used for moulding is often collected, reconditioned, and reused.

Finishing

The final step involves removing the gates, risers, and runners connected to the casting and applying any necessary finishing processes, such as sandblasting and grinding, to achieve the desired smoothness and surface finish.

Removing and finishing the product

After the poured metal has cooled and solidified, the sand mould is broken apart to reveal the casting. This cast metal piece goes through a series of cleaning and finishing processes to remove any adhering sand and achieve the required surface finish. The casting is removed from the sand mould, and the sand is shaken out. The cast product is then cleaned and finished to meet customer requirements.

Various techniques can be employed to refine the surface finish, such as grinding, sanding, or shot blasting. The surface detail achievable with sand casting depends on the type of sand used. A good facing sand can capture the exact detail of something as small as a piece of Scotch tape on the surface of a pattern. However, the surface detail can never be as fine as an investment casting because of the difference in the particle size of the media.

Sand casting leaves a rough surface on cast parts, and the parting line and roughness are often smoothed over in secondary machining processes, requiring additional time and labour. This is less of an issue with investment casting, which uses hard ceramic moulds that are destroyed to remove the part. As a result, finished parts exhibit smooth surfaces and low tolerances, and can have thin walls. After removing the gates and sprues, the cast parts often don't require any secondary processing.

Advantages and disadvantages of sand casting

Sand casting is one of the oldest forms of casting, which uses a mold made by packing sand around a pattern of the part to be cast. It is a simple, versatile, and cost-effective method for creating various part sizes and geometries.

Advantages of Sand Casting

• Suitable for mass production at economical rates
• Capable of casting large parts
• Shorter lead times in production
• Lower cost to produce
• More versatile with size and weight capabilities

Disadvantages of Sand Casting

• Requires help to hold tight tolerances
• Requires secondary machining for dimensional accuracy
• Rough surface textures require additional finishing
• Requires cores for internal voids or cavities, which can be time-consuming

Sand casting is ideal for larger, less complex parts where lower dimensional tolerance is permissible.

Frequently asked questions