Sand casting process
Sand casting is the oldest casting process that exists. It uses pattern made of either metal, wood, or wax to create a models of an items needed to be produced. Sand casting process is very tedious and gives 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 metal. Sand casting process involves creating a cavity in a mold and pouring of molten metal inside it.
In this comprehensive article, you’ll have an in-depth knowledge on sand casting process including pattern making, mold making, drag and cope clamping, pouring, cooling, removing, and trimming. All these are stages involve in sand casting.
The first stage involve in sand casting is pattern making. Pattern making seems to be tedious and intelligent work as is it the replica of an item to be produced. It is made of different materials made of 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 pattern easily wrap and deform. It can wear quicker from the sand. Whist, patterns made with metal last longer and can be reused to create same type of cavity and it helps to reduce tooling cost. But it is more expensive.
In pattern making, an allowance is added to it for thermal contraction or shrank. Patterns are typically made in four ways. That is, there are four types of pattern used in sand casting.
- 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 part needed in small quantity. However, parting line and runner system must be determined separately which can easily damage the mold.
- Split pattern – a split pattern is two separate replica of the part. The two parts are placed in the cavity, one in the cope and other in the drag (flask). Split pattern allows the flask to be easily separated and determine the parting line. This type of pattern is typically used for complex part needed in moderate quantities.
- Match plate pattern – a match plate pattern is closely similar to split pattern. It is usually made of metal and wood. It is employ 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 is used for larger quantities and often used in automated process.
- Cope and drag pattern – a cope and drag pattern is similar to a match plate pattern, but each of the halves are 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.
In this stage of sand casting, 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.
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. The four types of casting sand include:
- Green sand – 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 employs in molding in order to determine the quality of the casting which usually describes by the following:
- Strength in the sand to maintain its shape
- Permeability (ability for trap gases to escape through the sand.) it is determined by the shape and size of sand grains.
- Thermal stability in order 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 a special sand to avoid pilling of the mold to the casting. A core is a hollow part in a casting. It is prepared and placed before the cope and drag is strongly clamped together to avoid loss of any material.
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 hold in place inside the mold. However, the core may loss 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 higher melting point than the poured molten metal. Chaplets are cutoff at the end of the casting process.
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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 gate way has been cut. Gate way 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 in order to fill the entire cavity and all channels in the mold else there will be an unfilled portion in the casting.
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 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 solidified at 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 oxides layers attached to it. The sand can be removed using a shot blasting, mostly from internal surface and in reducing the surface roughness.
Remember the molten metal is poured through a channels (gate way 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’s envelope. A big casting may require a longer trimming time.
Advantages of sand casting
Sand casting has advantages over other casting process even though defects may occur. But it still offers great advantages. Sand casting gives great strength to the casting when 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 are used mostly in producing automotive products, such as engine blocks, casing or housing of an item, etc. some other advantages of sand casting include:
- Both ferrous and non ferrous metals are used for casting.
- Low cost of production.
- Low cost for post- casting tooling.
- Complex shape can be produced.
- 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
- 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.
And that is it for this article, sand casting process. just as promised all necessary information is explained from beginning on how sand casting process is performed to the end. I hope you know casting is carryout in foundry shops. read article on foundry work and processes.
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