Sand casting, also known as sand molded casting, is the oldest known casting process, and its history can be traced back to 1000 BC. The result of centuries of development is that sand mold casting is the most versatile, and perhaps the most widely used metal casting method.

Green sand casting is not only versatile in product size, but also can produce unusually complex or fine castings, and can be used to cast almost any metal alloy.

In sand casting foundry, molten metals such as iron, steel, bronze, brass, aluminum, magnesium and other non-ferrous alloys are usually poured into a two-piece mold. The mold is made by compacting sand (usually mixed with clay as a binder and moistened with water) around the pattern or model of the final product. The mold is separated and the pattern is removed. Put the two halves of the mold together and fill the void with molten metal. Once the metal has cooled sufficiently, open the sand casting mold, remove the sand, and remove the parts.

Several identical parts can be cast simultaneously, or a series of different parts can share a green sand casting mold. Because sand molds must be removed to remove castings, the productivity of sand castings is generally low. But new resin sand casting molds are easy to manufacture, and sand casting sand is often recycled and reused many times.

Sand casting is used to produce various metal parts with complex geometry. The size and weight of these parts can vary greatly, ranging from a few ounces to several tons. Some smaller sand cast parts include gears, pulleys, crankshafts, connecting rods, and propellers. Larger applications include housings for large equipment and heavy machine bases.

The Basic Sand Casting Process

1. Sand Casting Pattern Making

The first step in sand casting is the production of sand casting molds. The pattern is a replica of the outside of the casting, and its size is an allowance for shrinkage and finishing. If the casting is hollow, a cavity will be created in the finished product to place the core. The pattern is usually made of wood, plastic, metal, or stucco. However, if the pattern requires other specific properties, other materials can also be used. There are two main criteria for determining which material to choose for making a pattern, the number of castings made by the pressure sand casting mold and the specifications required for the finished casting.

2. Sand Casting Core Making

To produce cavities within the casting, we usually use dry sand molded cores that are inserted into the casting box after removal of the pattern. They are usually made from a mixture of sand, which is mixed with water and an organic binder called a binder and then baked to form a core. This makes the cores in sand casting strong and foldable, so they can be easily removed from the finished casting. Because the cores are made in molds, they require a pattern and mold called a core box. The core pattern is made in the same way as the casting style, but the core box is made of durable materials, such as metal or wood. Since the core is made of sand, the gravity sand casting mold cannot also be made of sand.

3. Sand Casting Mold Making

Sand casting molds are usually divided into two halves, where one side of the mold is made with one pattern and the other side is made with another pattern. Although the mold may not always be in two halves, this arrangement provides the easiest way to manufacture the mold and contact parts immediately after casting. The top part of the mold is known as the “cope” and the bottom half is the “drag”, and both are made by packing sand into a container (a “flask”) around the patterns. The operator must firmly pack the sand into each pattern to ensure there is no loose mold sand, and this can be done either by hand or by machine. After ramming, the sand casting patterns are removed and leave their exterior contours in the sand, where sand casting manufacturers can then create channels and connections (known as gates/runners) into the drag and a funnel in the cope (known as a “sprue”). These gates/runners and sprues are necessary for accurate casting, as the runners and gates allow the metal to enter every part of the mold while the sprue allows for easy pouring into the mold.

4. Clamping Sand Mold

After the precision sand casting mold is made, it must be ready for pouring molten metal. First, lubricate the surface of the cavity to facilitate the removal of the casting. The type of lubricant depends on the sand and metal materials used. Then, place the core and close the mold halves and clamp them firmly together to make a complete mold. In this step, the mold must be tightly closed to prevent any material loss. If a core for certain internal contours is required, it should be placed in the mold before the clamping step, and also check that all gates/runners are aligned.

5. Pouring Molten Metals into The Sand Mold

Now it's time to pour the molten metal into the cavity. The pouring can be performed manually or by an automatic machine. Carefully pour/cast the molten metal into the gate of the mold, where sufficient molten metal must be injected to fill the entire sand casting cavity and all channels in the mold, and then wait for complete cooling.

There are many variations of "tilt dump". This process aims to make the metal flow into the casting more smoothly, thereby eliminating turbulence. Reducing turbulence can help prevent oxides and sand casting defects. Almost any alloy can be produced using this process, some of which include iron, steel, aluminum, bronze, magnesium, zinc, and tin. For materials that are particularly reactive with oxygen, procedures such as argon shielding can be used to keep the air away from the molten metal.

6. Molten Metal Cooling

Once the molten metal injected into the presion sand casting mold enters the cavity, it will begin to cool and solidify. When the entire cavity is filled with molten metal and solidified, the final shape of the casting is formed. Different types of metals require different cooling times. The required cooling time can be estimated based on the wall thickness of the casting and the metal temperature. As the casting molten metal cools, it will return from the liquid state to the solid-state.

The sand casting mold cannot be opened until the cooling time is over. Most of the defects that may occur are the result of the curing process. If some molten metal cools too quickly, parts may shrink, crack, or have incomplete cross-sections. Precautions can be taken when designing sand casting parts and molds.

7. Remove Metal Casting

After predetermined curing time, the casting sand mold can simply be crushed and the casting removed. This step is sometimes referred to as shaking and is usually operated by a shaker that shakes the sand and pours it out of the flask. After the casting is removed, some sand and oxide layers may adhere to the surface. Sandblasting can be used to remove residual sand, especially from internal surfaces, and to reduce surface roughness.

Although foundry sand mold patterns are usually reusable, the casting molds that have been produced cannot be reused, but the casting sand can be recycled. Therefore, every time a sand casting manufacturing company wants to use sand casting to manufacture a new metal product or component, it must re-manufacture a new mold.

8. Trimming Sand Casting Parts

During the cooling of the molten metal, the casting may adhere to some of the solidified material in the mold. Therefore, the excess material must be trimmed from the casting manually or using a trimming press. The time required can be estimated based on the size of the casting shell. Generally speaking, larger castings will require longer dressing time. The scrap generated by dressing is discarded or reused in the sand casting process. However, before mixing the scrap with non-recyclable metals and reusing them, it may be necessary to readjust them to the appropriate chemical composition.

9. Polishing or Finishing Sand Casting Parts

Depending on the intended use of the final sand casting finished products, the castings can be polished or finished. Different levels of surface finish can be applied to remove the residual roughness on the casting surface. As a professional sand casting manufacturer, Yifei can provide original castings or castings that are ready for use, or they can add a second step, such as powder coating and finishing.

Sand Casting Materials

When we start a sand casting project, we need to prepare the following four important materials: base sand, a binder, additives, and a parting compound.

Base Sand

Base sand is the material to be prepared in the first step for making sand molds. To bond the base sand together, a binder is needed. The most common types of base sand are:

1. Silica Sand
Silica (SiO2) sand is the most commonly used sand. It is made by crushing sandstone or from beaches and river beds. For high melting point castings, such as steel, at least 98% pure silica sand should be used. But for lower melting metals, such as cast iron and non-ferrous metals, lower purity sand (purity between 94% and 98%) can be used.

2. Olivine Sand
Olivine is a mixture of orthosilicates of iron and magnesium from pure peridotite in mines. It does not contain silica, so it can be used with alkaline metals (such as manganese steel). And because of its low thermal expansion, high thermal conductivity, and high melting point, it is safer than silica, so it is very popular in Europe.

3. Chromite Sand
Chromite sand is a solid solution of spinel. It is characterized by low silica content, a high melting point (1,850 ° C (3,360 ° F)), and high thermal conductivity. But because the material cost is generally high, it is often used for expensive alloy steel castings and cores.

4. Zircon Sand
Zircon sand is a mixture of approximately two-thirds of zirconia (Zr2O) and one-third of silica. It has the highest melting point of all base sands at 2600 ° C (4710 ° F), extremely low thermal expansion, and high thermal conductivity. Because of these good properties, it is usually used when casting alloy steel and other expensive alloys.

5. Chamotte Sand
Chamotte sand has a melting point of 1,750 ° C (3,180 ° F) and low thermal expansion. It is an affordable material in all types of base sand, but it is still twice as expensive as silica. Due to its coarse grain structure, it will result in poor surface finish and is limited to dry sand molding. Mold cleaning can overcome surface finish problems. Chamotte sand is usually used when casting large steel workpieces.

Sand Casting Binders

After the base sand is confirmed, we have to proceed to the next step, which is to add the binder to the base sand to bind the sand particles together. The most common types of binders are:

1. Clay And Water
This is the most common method used to make sand casting binders. Because materials are more common, its economic benefits are higher. There are two commonly used clays: bentonite and kaolin.

2. Oil
Linseed oil, other vegetable oils and marine oils have been used as binders. However, due to rising oil prices, this method has been gradually eliminated. If you continue to use this method to make the sand casting binders, you need to carefully bake the oil at 100 to 200 ° C (212 to 392 ° F) to solidify. It should be noted that if it is overheated, the oil will become brittle and easily broken, thereby wasting the mold.

3. Resin
Resin binders are natural or synthetic high melting point glues. Common materials are urea-formaldehyde (UF), phenolic (PF) resin, and cold-setting resin. PF resin has higher heat resistance than UF resin, and the cost is lower. Cold-setting resins use catalysts instead of heat to solidify the binders.
Resin adhesives are very popular because different properties can be obtained by mixing with various additives. And the finished products generally have higher smoothness, other advantages include good foldability and less outgassing.

4. Sodium Silicate
Sodium silicate [Na 2 SiO 3 or (Na 2 O) (SiO 2)] is a high-strength binder for silica sand. To solidify the binders, carbon dioxide gas is used and a chemical reaction takes place. The adhesive produced by this method can be used at room temperature. However, its high-strength adhesion will cause difficulty in demolding and may cause thermal cracking in the casting.

Additives

Add additives to the molded parts to improve: casting surface smoothness, dry strength, fire resistance, and cushioning properties.

Up to 5% of reducing agents (such as pulverized coal, asphalt, creosote oil, and fuel oil) can be added to the molding material to prevent the liquid metal from adhering to the sand particles, thereby leaving them on the surface of the casting.

Up to 3% of "buffer material" can be added, such as wood flour, sawdust, husk flour, peat, and straw. It can prevent defects such as scratches and hot cracks in castings when casting high-temperature metals.

Up to 2% of cereal binders (such as dextrin, starch, sulfite lye, and molasses) can be used to increase dry strength (strength of the mold after curing) and improve surface finish.
Up to 2%, iron oxide powder can be used to prevent mold cracking and metal penetration, and fundamentally improve the refractoriness.

Casting Separation Compound

Before sand casting, the sand casting patterns need to be taken out of the mold. At this time, a casting separation compound can be applied to the pattern to facilitate us to remove the casting pattern. They can be fine powders or liquid compounds. Common powders include talc, graphite, and anhydrous silica. Common liquids include mineral oil and water-based silicon solutions. The latter is more commonly used for metal and large wooden patterns.

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