Mastering the Sand Casting Process: Techniques and Best Practices

Sand casting is the oldest casting process that exists. It uses patterns made of either metal, wood, or wax to create models of items needed to be produced. The sand casting process is very tedious and gives a slow production rate due to the destruction of the sand mold when removing the sand casting (item produced). This casting process makes use of pattern, sand mold, furnace, and liquid metal. This process involves creating a cavity in a mold and pouring molten metal inside it.

Today you’ll get to know the various steps involved in a sand casting process. You’ll learn pattern making, mold making, and preparation, casting sand types, cope and drag clamping, core making, pouring, cooling, and trimming stage in sand casting. You’ll also learn the various types of sand casting tools, as well as the advantages and disadvantages of sand casting processes.

 

Pattern making

The first stage involves in sand casting processes is pattern making. Pattern making seems to be tedious and intelligent work as is it the replica of an item to be produced. It can be made of different materials such as wood, metal, synthetics, etc. depending on the volume and tolerance of the casting. Wood is the most common because it is less expensive and easy to shape. However, wood patterns easily wrap and deform. It can wear quicker from the sand. Whist, patterns made with metal last longer and can be reused to create the same type of cavity, which helps to reduce tooling costs. But it is more expensive. In pattern making, an allowance is added to it for thermal contraction or shrank.

Types of patterns

Patterns are typically made in four ways. Below are the various types of patterns used in sand casting processes.

Solid pattern:

A solid pattern is the easiest type of pattern to make. It contains a complete replica of the part in a single piece. It is used to produce parts needed in small quantities. However, the parting line and runner system must be determined separately which can easily damage the mold.

Split pattern:

A split pattern is two separate replicas of the part. The two parts are placed in the cavity, one in the cope and the other in the drag (flask). Split types of patterns allow the flask to be easily separated and determine the parting line. It is typically used for complex parts needed in moderate quantities.

Match plate pattern:

A match plate pattern is closely similar to split patterns. It is usually made of metal and wood and is employed to ensure proper alignment of the mold cavity in the flask. The runner system can be included on the match plate. These types of patterns are used for larger quantities and are often used in an automated process. Finally,

Cope and drag pattern:

Cope and drag types of patterns are similar to a match plate pattern, but each of the halves is mounted on a separate plate. It also ensures proper alignment of the mold cavities in the cope and drag. The runner is included on the plates. These types of patterns are used for larger castings and are also used for larger production quantities. It is also used when the process is automated.

Mold making

In this stage of the sand casting process, a refractory material (sand is widely applicable.) is load or packed on the flask, packed around the pattern still it fully compacted. The pattern is then removed leaving the shape in the mold cavity. The sand used to make is mold is strong enough to hold the weight of the molten metal when it is poured and should brittle enough to be broken when the casting cool and solidify. Clay and some chemical bonding agent are used to strengthen the mold in order to withstand the pouring.

If the sand is packed in the cope and drag while the pattern is in it, the cope and drag are separated so that the pattern can easily be removed. A refractory coating (heat resistance material.) is added to the surface of the cavity to produce a better surface finish and to allow the mold to withstand the poured metal. The flask is coupled back together, leaving the shape in the cavity.

Types of sand casting sand

There are different types of sand used in mold making. Typically silica sand (sio2) is mixed with type of binder in order to maintain shape in the cavity. Sand offers great benefits to the casting. There are different preparations of sand mold which lead to four types. Below are the four types of sand used in mold making.

Greensand – green sand is a mixture of sand, water, and clay or binder. The sand contains 90%, 3% of water, and 7% of clay. It is the cheapest and most used.

Skinned-dried sand – skinned-dried sand is a composition of green sand but with additional bonding materials added to it. The surface of the cavity is dried by heat in order to increase the mold strength. This helps to improve the accuracy of the casting any surface finish will be complete. These types of sand molds are more expensive and require more time when treating.

Dry sand – dry sand is also known as cold box mold. The sand is mixed with an organic binder. The mold created with dry sand is strengthened by baking in a oven. Dry sand types of molding sand gives high dimensional accuracy but it is expensive.

No baked sand – no-baked mold is sand mixed with resin and hardens at room temperature.

Casting sand is used in the mold making determine the quality of the casting which usually describes by the following:

• Strength in the sand to maintain its shape
• Permeability (ability to trap gases to escape through the sand.) is determined by the shape and size of sand grains.
• Thermal stability to withstand cracking from the molten metal.
• Compatibility of the sand. And
• Reusability of the sand for another sand molding.

Cope and drag clamping

After the preparation of the mold, which is designed to withstand molten metal when poured. The surface of the cavity is sprinkled with special sand to avoid pilling the mold to the casting. A core is a hollow part in a casting. It is prepared and placed before the cope and drag are strongly clamped together to avoid loss of any material.

Core making

Let me use this opportunity to elaborate more on core making in casting. Cores are internal holes and passages in a casting. it is typically made out of sand as it covers the portions the cores are located in the pattern. The core is produced in the mold before molten metal is poured. Core print is an inset in a pattern that allows the core to be held in place inside the mold. However, the core may lose position due to the buoyancy of the molten metal. That is why the core can be supported with chaplets which help in holding the core together with the mold. In this situation chaplets used must have a higher melting point than the poured molten metal. Chaplets are cut off at the end of the casting process.

Pouring stage

At this stage, metal is melted in a furnace at a certain temperature while the mold is already prepared. That is, it has been clamped and the riser and gateway have been cut. Gateway and riser are designed to allow the molten metal to smoothly or freely flow into the cavity. It also helps to eliminate turbulence which prevents oxides and casting defects.

The metal is been melted in a crucible (high refractory material) to some extent. It is removed from the furnace using crucible tong to the pouring shank which ensures better pouring. Though pouring can be done manually or by an automated machine. Enough metal should be melted to fill the entire cavity and all channels in the mold else there will be an unfilled portion in the casting.

Cooling stage

After the molten metal is poured into the cavity, it begins to cool and solidifies after some time. The molten metal takes the shape of the cavity and solidified obtaining such shape. The mold is a break after the cooling elapsed which can be estimated based on the thickness of the casting and the temperature of the metal. At this stage defect likely occurs. If some part of the molten metal cools too quickly, it may exhibit cracks, shrinkage or incomplete section so care should be taken when making the mold and when pouring the molten metal.

Removing stage (breaking)

The casting cools and is solidified at a predetermined time. The mold can simply be broken and the casting is obtained. This process is also called checkout. The breaking is typically done by a vibrating machine that shakes the sand and cast out of the flask. The casting will likely contain some sand and oxide layers attached to it. The sand can be removed using shot blasting, mostly from the internal surface and in reducing the surface roughness.

Trimming

Remember the molten metal is poured through a channel (gateway and riser) to the cavity which fills all portion in the mold so as the channel. The metal in the channels must be trimmed off and the casting must be trimmed to the required dimension. Casting can be trimmed manually via cutting or sawing or via trimming press. Trimming time can be determined by the size of the casting envelope. A big casting may require a longer trimming time.

Watch the video below to learn how sand casting is done:

Advantages and disadvantages of  a sand casting process

Advantages:

Sand casting has advantages over other casting processes even though defects may occur. But it still offers great advantages. Sand casting gives great strength to the casting when the clay is added to the mold, helping the sand to strongly bond more closely.

Sand casting is designed to reduce the potential for cracking, tearing, and shrinkage during the cooling stage of the casting. sand casting processes are used mostly in producing automotive products, such as engine blocks, casing or housing of an item, etc. some other advantages of the sand casting process include:

• Both ferrous and nonferrous metals are used for casting.
• Low cost of production.
• Low cost for post-casting tooling.
• Complex shapes can be produced.

Disadvantages

Despite the excellent benefits of sand casting, some drawbacks still occur. Below are the disadvantages of a sand casting process in its various applications.

• A lower degree of accuracy
• It is a tedious process
• Rough surface finish
• Mold can only be used once
• Slow production rate
• Labor intensive

Sand casting tools and their uses

Below are the various types of sand casting tools and their uses:

Hand riddle – hand riddle are generally used for separating sand and some unwanted particles in the sand.

Shovel: this sand casting tools are used for transferring the sand into the molding box or flask.

Rammer: rammer is used to pack or compact the sand uniformly around the pattern and flask.

Sprue pin – this sand casting tools are used to create vertical holes on the mold.

Strike bar – strike bar is used to remove or strike off excess sand from the top of the flask.

Mallet – mallet is used for driving draw spike into the pattern and rapping for easy separation of mold and pattern.

That is all for this article, sand casting process. Just as promised pattern making, mold making, and preparation, casting sand types, cope and drag clamping, core making, pouring, cooling, and trimming stage in sand casting are explained. The tools used in sand casting are mentioned and the advantages and disadvantages of sand casting were also discussed.

I hope you got enough from the reading, if so, kindly share with other students. Thanks for reading, see you next time.