MYPIBLOGs

Casting manufacturing is a process in which liquefied material, such as molten metal, is poured into the cavity of a specially designed mold and allowed to harden. 

After solidification, the workpiece is removed from the mold to undergo various finishing treatments or for use as a final product. Casting methods are typically used to create intricate solid and hollow shapes, and cast products are found in a wide range of applications, including automotive components, aerospace parts, etc.

CASTING TERMINOLOGY

Casting mold

A mold is a cavity in a material that receives liquid metal and produces a cooled object in the shape of that cavity. Molds can be simple. The forms used to create ingots of metal are like loaf pans, with the metal simply poured inside and left to cool. Most molds are for more complex shapes and are based on a pattern. The pattern imprinted into a split mold. Half of the pattern is imprinted on one side of the mold and half on the other, and then the halves are clamped together before the mold is filled. By making the mold in two parts, the pattern can be withdrawn before filling. These molds can be made with a horizontal split

Cope and drag

In horizontal molding, the top half of the mold is called the cope, and the bottom half is called the drag.

Swing and ram

In vertical molding, the leading half of the mold is called the swing, and the back half is called the ram.

Molding cores

If a mold is supposed to have internal spaces or holes, a core is often made. These cores are shaped like the internal space. The cores are usually held in place by extending past the casting and being held in place through core prints, which suspends the core like a bridge between two banks

Dimensional tolerance

One of the important factors in choosing a casting method is dimensional tolerance. Dimensional tolerance is the variation acceptable in the size of the final product. Metal shrinks when cooling, and the type of casting influences by how much.

Different Types of Casting and the Casting Process

Although casting is one of the oldest known manufacturing techniques, modern advances in casting technology have led to a broad array of specialized casting methods. Hot forming processes, such as die-casting, investment casting, plaster casting, and sand casting, each provide their own unique manufacturing benefits. Comparing both the advantages and disadvantages of the common types of casting processes can help in selecting the method best suited for a given production run.

Sand Casting

Sand casting typically relies on silica-based materials, such as synthetic or naturally-bonded sand. Casting sand generally consists of finely ground, spherical grains that can be tightly packed together into a smooth molding surface. The casting is designed to reduce the potential for tearing, cracking, or other flaws by allowing a moderate degree of flexibility and shrinkage during the cooling phase of the process. The sand can also be strengthened with the addition of clay, which helps the particles bond more closely. Automotive products such as engine blocks are manufactured through sand casting.

Sand casting involves several steps, including patternmaking, molding, melting and pouring, and cleaning. The pattern is the form around which the sand is packed, usually in two parts, the cope and the drag. After the sand is compacted enough to replicate the pattern, the cope is removed and the pattern extracted. Then, any additional inserts called core boxes are installed and the cope is replaced. After the metal has been poured and solidified, the casting is removed, trimmed of the risers and gates that were used in the pouring process, and cleaned of any adhered sand and scale.

Sand casting's main advantages as a casting process include:

1)Relatively inexpensive production costs, especially in low-volume runs.

2)The ability to fabricate large components.

3)A capacity for casting both ferrous and non-ferrous materials.

4)A low cost for post-casting tooling.

Investment Casting

Investment, or lost-wax, casting uses a disposable wax pattern for each cast part. The wax is injected directly into a mold, removed, then coated with refractory material and a binding agent, usually in several stages to build up a thick shell. Multiple patterns are assembled onto common sprues. Once the shells have hardened the patterns are inverted and heated in ovens to remove the wax. Molten metal is then poured into the remaining shells where it hardens into the shape of the wax patterns. The refractory shell is broken away to reveal the completed casting. 

Investment casting is often used to manufacture parts for the automotive, power generation, and aerospace industries, such as turbine blades. 

Some of the central advantages of investment casting include:

1)A high degree of accuracy and precise dimensional results.

2)The ability to create thin-walled parts with complex geometries.

3)The capacity for casting both ferrous and non-ferrous materials.

4)Relatively high-quality surface finish and detail in final components.

Plaster Casting

Plaster casting is similar to the sand casting process, using a mixture of gypsum, strengthening compound, and water in place of the sand.The plaster pattern is typically coated with an anti-adhesive compound to prevent it from becoming stuck against the mold, and the plaster is capable of filling in any gaps around the mold. Once the plaster material has been used to cast the part, it usually cracks or forms defects, requiring it to be replaced with fresh material. The advantages offered by plaster casting include:

1)A very smooth surface finish.

2)The ability to cast complex shapes with thin walls.

3)The capacity for forming large parts with less expense than other processes, such as investment casting.

4)A higher degree of dimensional accuracy than that of sand casting

Die Casting (Metal Casting Process)

Die casting is a method of molding materials under high pressure and usually involves non-ferrous metals and alloys, such as zinc, tin, copper, and aluminum. The reusable mold is coated with a lubricant to help regulate the die’s temperature and to assist with component ejection. Molten metal is then injected into the die under high pressure, which remains continuous until the workpiece solidifies. This pressurized insertion is rapid, preventing any segment of the material from hardening before being cast. After the process is completed, the component is taken out of the die and any scrap material is removed. A few of the major advantages provided by die casting include:

1)Close size and shape tolerances.

2)High component dimensional consistency and uniform design.

3)A reduced need for post-casting machining.

Centrifugal Casting

Centrifugal casting is used to produce long, cylindrical parts such as cast iron pipe by relying on the g-forces developed in a spinning mold. Molten metal introduced into the mold is flung against the interior surface of the mold, producing a casting that can be free of voids. Originally invented as the de Lavaud process using water-cooled molds, the method is applied to symmetrical parts such as soil pipe and large gun barrels and has the advantage of producing parts using a minimal number of risers. 

For asymmetric parts that cannot be spun around their own axes, a variant of centrifugal casting, called pressure casting, arranges several parts around a common sprue and spins the molds around this axis. A similar idea is applied to the casting of very large gear rings, etc. Depending on the material being cast, metal or sand molds may be used.

Permanent Mold Casting

Permanent mold casting shares similarities with die casting and centrifugal casting, notably the use of reusable molds. These can be made of steel, graphite, etc. and are generally used to cast materials such as lead, zinc, aluminum and magnesium alloys, certain bronzes, and cast iron. It is a low-pressure process with pouring usually done by hand using multiple molds on a turntable. As the molds rotate through the various stations they are successively coated, closed, filled, opened, and emptied. One such method is known as slush casting, where the mold is filled but emptied before the metal fully hardens. Molten metal is dumped from the casting to produce a hollow, cast shell. A similar idea is used in the molding of hollow chocolate products such as Easter bunnies. The use of metal molds induces faster heat transfer through the mold, allowing the shell to harden while the core remains liquid.