All About Sand Casting|Echofast

What is Sand Casting?

Sand casting is also known as sand molded casting. It is a metal casting technique that involves pouring molten metal into a sand mold with a hollow cavity of the desired shape. The casting then cools and solidifies. The sand is broken away in a shakeout process.This process is hugely versatile and ideal for producing metal parts in a wide range of shapes and sizes.

Step-by-Step Process of Sand Casting

Sand casting process. Image Credit:
https://en.wikipedia.org/wiki/Sand_casting#/media/File:Haandform-e.png

1.Producing the Pattern – Desired Product

In this casting mold process, a reusable pattern with the same details as the desired finished part is utilized, which allows for thermal contraction or shrink.

2.Pattern Creation – Gates and Risers: Metal Delivery System

The pattern created in the first step also includes the metal channels, which will feed the required cast product design with proper gating and risers. This is helpful to regulate the metal flow. Gas venting is required in the process, and the inevitable thermal contraction is driven to acceptable places (other than the actual desired finished product).
Patterns are made of various materials for different requirements of the volume and tolerance. These materials include wood, metal, synthetics, expandable polystyrene (EPS), and others.

3.Creating the Mold

A refractory substance, which is stable at high temperatures, is produced around the design. During casting , the material must be strong enough to hold the weight of the weight of the liquid metal. It should also have resistance to metal reaction. However, it must be fragile enough to be separated after the casting cools.
The mold can be manufactured of different sand materials. With the addition of other elements like clay or some chemical bonding agent, the sand can be stronger to withstand the pouring process.
The molds can be produced by machining the desired shaped hollow directly into a block of sand as well. The technology is widely employed during product development because the design changes can be dealt with and applied quickly. It also can be used for portions with infrequent usage, which can avoid the storage or maintenance of a physical pattern.
The mold usually consists of two halves, namely the cope (the top half of the mold) or drag( the bottom half). The halves are separated and the pattern is removed once the sand has set (using the traditional/non-machined process). A refractory coating is applied to achieve a finer surface finish and protect
the mold from the turbulence of the poured metal . The halves are reassembled, and the cavity can be in the shape of the pattern.
The mold may also include cores,which is a method used to produce proper internal pathways in the final product.

4.Pouring the Metal into the Mold

Molten metal is injected to the static mold directly. It fills the void, defining the finished part and the risers. Liquid metal is supplied continuously from the risers to the casting. As they are the last to cool and solidify, the shrinkage and potential void are concentrated in the riser than the desired part.
Therefore, this ensures the smooth flow of liquid metal into the casting with less turbulence, which is helpful to prevent oxide formation and casting defects. Almost all the alloys can be made using this process. An argon shielding may be employed to keep air away from the molten metal when it comes to those materials particularly reactive with oxygen.

5.Shakeout Operation

The casting solidifies and cools, which includes both the desired part and the additional metal required to make it. The sand is then split up and shaken out. The sand, which is used to create the mold, is recovered, and reconditioned. Much of it can be reused.

6.Casting Final Operations

There are some final operations that can be taken for the sand castings to achieve final dimensions or tolerances.

Cut the gates, runners, and risers from the casting.
Final post-processing like sandblasting, grinding,etc.
Extra machining
Heat treatment
Non-destructive testing(Fluorescent penetrant, magnetic particle, radiographic, etc.)

Types of Casting Sand

Listed below are several types of sand methods.

1.Dry Sand

Dry sand casting is a popular molding sand for its ability to produce intricate designs with exceptionally close tolerances. In the process, sand is mixed with clay or other binding material. This creates a mold that retains its pattern-rammed shape. The strength can be increased as the mold is baked. With less steam in the fill process, the mold can withstand the pouring of molten metal. This casting method is complex and time-consuming , but it is suitable for the production of large ferrous and non-ferrous parts such as engine blocks, housings, gear boxes, and agricultural parts.

2.Resin Sand

Resin sand is also known as resin-coated sand or furan sand. Compared with green-sand molds, resin sand casting can produce accurate results due to the better strength and dimensional stability. This process can create castings with smoother and cleaner surface finishes as the mold surface defects are reduced. However, resin sand casting is a more expensive casting method and calls for specialized molding equipment.

3.Sodium Silicate or Water Glass

Sodium silicate or water glass casting involves integrating the sand by utilizing a sodium silicate binder for mold formation. It is a type of sand casting process that is better suited to more intricate and delicate patterns. This process offers some advantages, such as good dimensional stability and low cost. These molds , however, possess less resilience in use and handling in comparison to some other methods.

4.Green Sand

Green sand casting is widely used for manufacturing metal parts because of its low cost and solid overall performance. The “green “ refers to the the presence of the moisture. It is a mixture of sand, clay, water and various additives. In most cases, a green sand mixture contains 85% sand, 4% water, 11% Bentonite Clay, and some other materials or impurities. It is an economical and efficient process to produce high volumes of parts. However, molds used for green sand casting are reusable while the sand cannot be reused.The final products manufactured by green sand casting require secondary finishing due to the poorer surface finish and lesser precision.

Sand Casting Metals

Non-Ferrous Metals