Casting Processes

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The Mold in Casting

Mold contains a cavity whose geometry determines part shape Actual size and shape of

cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling

Molds are made of a variety of materials, including sand, plaster, ceramic, and metal

Open Molds and Closed Molds

Two forms of mold: (a)open mold, simply a container in the shape of the desired part; (b)closed mold, in which the mold geometry is more complex and

requires a gating system (passageway) leading into the cavity.

Two Categories of Casting Processes

1. Expendable mold processes – uses an expendable mold which must be destroyed to remove casting Mold materials: sand, plaster, and similar materials, plus

binders Advantage: more complex shapes possible Disadvantage: production rates often limited by time to

make mold rather than casting itself2. Permanent mold processes – uses a permanent mold which

can be used over and over to produce many castings Made of metal (or, less commonly, a ceramic refractory

material) Advantage: higher production rates Disadvantage: geometries limited by need to open mold

Casting Processes

The different casting processes are as follows:1. Expendable mold processes

Sand Casting Shell casting Investment casting

2. Permanent mold processes Basic permanent mold casting Die casting Centrifugal casting

Overview of Sand Casting

Sand casting is a cast part produced by forming a mold from a sand mixture and then pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified

Most widely used casting process, accounting for a significant majority of total tonnage cast (?)

Nearly all alloys can be sand cast, including metals with high melting temperatures, such as steel, nickel, and titanium

Castings range in size from small to very large Production quantities from one to millions

A large sand casting weighing over 680 kg (1500 lb) for an air compressor frame

Steps in Sand Casting

1. Place a pattern in sand to create a mold. 2. Incorporate a gating system. 3. Remove the pattern. 4. Pour the molten metal into sand mold5. Allow the metal to solidify and cool. 6. Break away the sand mold and remove the

casting. 7. Clean and inspect casting8. Heat treatment of casting is sometimes

required to improve metallurgical properties

Production Steps in Sand Casting

Raw metal

Solidification and

coolingRemoval of sand mold

Pouring

Cleaning and

inspection

Finished casting

Mold making

Pattern making

Preparation of sand

Core making (if needed)

Melting

Sand

Making the Sand Mold

The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern

The mold must also contain gating and riser system If casting is to have internal surfaces, a core must be

included in mold A new sand mold must be made for each part produced

Shell Molding

Gear housing

Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder

Shell mold casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern.

The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.

Shell mold casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.

Shell Molding

Shell Molding Steps in shell‑molding: (1) a match‑plate or cope‑and‑drag

metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.

Shell Molding

Steps in shell‑molding: (2) box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; (3) box is repositioned so that loose uncured particles drop away;

Shell Molding

Steps in shell‑molding: (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern;

Shell Molding

Steps in shell‑molding: (6) two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished; (7) the finished casting with sprue removed.

1. Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Aluminum for low volume production or graphite for casting reactive materials are used.

2. Mold creation - First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.

Shell Molding

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1. Mold assembly - The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material.

2. Pouring - The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.

3. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.

4. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold.

Shell Molding

Advantages and Disadvantages

Advantages of shell molding: Smoother cavity surface permits easier flow of

molten metal and better surface finish Good dimensional accuracy - machining often

not required Mold collapsibility minimizes cracks in casting Can be mechanized for mass production

Disadvantages: More expensive metal pattern Difficult to justify for small quantities

Investment Casting (Lost Wax Process)

In Investment casting, Molten metal is poured into an expendable ceramic mold.

The mold is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mold. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mold after it has been formed.

Lost-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created.

Investment casting can make use of most metals, most commonly using aluminum alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that cannot be molded in plaster or metal.

Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries.

Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding.

Investment Casting (Lost Wax Process)

Investment Casting

Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree

Investment Casting

Steps in investment casting: (3) the pattern tree is coated with a thin layer of refractory material, (4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid

Investment Casting

Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies

Investment Casting

Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue

1. Pattern creation - The wax patterns are typically injection molded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mold cavity.

2. Mold creation - This "pattern tree" is dipped into slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mold, hence the name "lost wax" casting.

Investment Casting

4. Pouring - The mold is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mold, filling the mold cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.

5. Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the material used.

6. Casting removal - After the molten metal has cooled, the mold can be broken and the casting removed. The ceramic mold is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).

7. Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part.

Investment Casting

Investment Casting

A one‑piece compressor stator with 108 separate airfoils made by investment casting

Advantages and Disadvantages

Advantages of investment casting: Parts of great complexity and intricacy can

be cast Close dimensional control and good surface

finish Wax can usually be recovered for reuse Additional machining is not normally

required ‑ this is a net shape process Disadvantages

Many processing steps are required Relatively expensive process

Permanent Mold Casting Processes

Economic disadvantage of expendable mold casting: a new mold is required for every casting

In permanent mold casting a metal mold can be reused for several thousand cycles

Permanent mold casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness.

Common permanent mold parts include gears and gear housings, pipe fittings, and other automotive and aircraft components.

The processes include: Basic permanent mold casting Die casting Centrifugal casting

The Basic Permanent Mold Process

Uses a metal mold constructed of two sections designed for easy, precise opening and closing

•Molds used for casting lower melting point alloys are commonly made of steel or cast iron

•Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures

Steps in Permanent Mold Casting

First, the mold is pre-heated to around 300-500°F (150-260°C) to allow better metal flow and reduce defects. Then, a ceramic coating is applied to the mold cavity surfaces to facilitate part removal and increase the mold lifetime.

Step 1: Mold is preheated and coated

Permanent Mold Casting

- The mold consists of at least two parts - the two mold halves and any cores used to form complex features. Such cores are typically made from iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mold halves are clamped together.

Step 2: Mold assembly

The molten metal is poured at a slow rate from a ladle into the mold through a sprue at the top of the mold. The metal flows through a runner system and enters the mold cavity.

Permanent Mold Casting

Step 3: Pouring

The molten metal is allowed to cool and solidify in the mold.

Step 4: Cooling

Step 5: Mold openingAfter the metal has solidified, the two mold halves are opened and the casting is removed.

During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.

Step 6: Trimming

Advantages and Limitations

Advantages of permanent mold casting: Good dimensional control and surface finish More rapid solidification caused by the cold

metal mold results in a finer grain structure, so castings are stronger

Limitations: Generally limited to metals of lower melting

point Simpler part geometries compared to sand

casting because of need to open the mold High cost of mold

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Die Casting

Can produce geometrically complex metal parts through the use of reusable molds, called dies.

The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal is melted in the furnace and then injected into mold cavity under high pressure.

Pressure is maintained during solidification, then mold is opened and part is removed

Molds in this casting operation are called dies; hence the name die casting

Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes

One common application of die cast parts are housings - thin-walled enclosures, often requiring many ribs and bosses on the interior.

Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks.

Other common die cast parts include propellers, gears, bushings, pumps, and valves.

Die Casting application

Die Casting Machines

Two main types: 1. Hot‑chamber machine

(used for alloys with low melting temperatures, such as zinc)

2. Cold‑chamber machine (used for alloys with high melting temperatures, such as aluminum).

Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity

Differences between these machines on equipment and tooling. However, in both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting.

Hot-Chamber Die Casting

Metal is melted in a container, and a piston injects liquid metal under high pressure into the die

High production rates - 500 parts per hour not uncommon

Applications limited to low melting‑point metals that do not chemically attack plunger and other mechanical components

Casting metals: zinc, tin, lead, and magnesium

Hot-Chamber Die Casting

Cycle in hot‑chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber

Hot-Chamber Die Casting

Cycle in hot‑chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.

Cold‑Chamber Die Casting Machine

Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity

High production but not usually as fast as hot‑chamber machines because of pouring step

Casting metals: aluminum, brass, and magnesium alloys

Advantages of hot‑chamber process favor its use on low melting‑point alloys (zinc, tin, lead)

Cold‑Chamber Die Casting

Cycle in cold‑chamber casting: (1) with die closed, molten metal is poured into the chamber

Cold‑Chamber Die Casting

Cycle in cold‑chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.

Molds for Die Casting

Usually made of tool steel, mold steel, or maraging steel

Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron

Ejector pins required to remove part from die when it opens

Lubricants must be sprayed into cavities to prevent sticking

Advantages and Limitations

Advantages of die casting: Economical for large production quantities Good accuracy and surface finish Thin sections are possible Rapid cooling provides small grain size and

good strength to casting Disadvantages:

Generally limited to metals with low metal points

Part geometry must allow removal from die

Centrifugal casting is a metal casting process that uses centrifugal force to form cylindrical parts. This differs from most metal casting processes, which use gravity or pressure to fill the mold. A permanent mold made from steel, cast iron, or graphite is typically used.The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available).Centrifugal casting is used to produce axi-symmetric parts, such as cylinders or disks, which are typically hollow.

Centrifugal Casting

Centrifugal Casting

1. Mold preparation - The walls of a cylindrical mold are first coated with a refractory ceramic coating, which involves a few steps (application, rotation, drying, and baking). Once prepared and secured, the mold is rotated about its axis at high speeds (300-3000 RPM), typically around 1000 RPM.

2. Pouring - Molten metal is poured directly into the rotating mold, without the use of runners or a gating system. The centrifugal force drives the material towards the mold walls as the mold fills.

3. Cooling - With all of the molten metal in the mold, the mold remains spinning as the metal cools. Cooling begins quickly at the mold walls and proceeds inwards.

4. Casting removal - After the casting has cooled and solidified, the rotation is stopped and the casting can be removed.

5. Finishing - While the centrifugal force drives the dense metal to the mold walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part.

Steps in Centrifugal Casting

Due to the high centrifugal forces, these parts have a very fine grain on the outer surface and possess mechanical properties approximately 30% greater than parts formed with static casting methods.

These parts may be cast from ferrous metals such as low alloy steel, stainless steel, and iron, or from non-ferrous alloys such as aluminum, bronze, copper, magnesium, and nickel.

Centrifugal casting is performed in wide variety of industries, including aerospace, industrial, marine, and power transmission.

Typical parts include bearings, bushings, coils, cylinder liners, nozzles, pipes/tubes, pressure vessels, pulleys, rings, and wheels.