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Saturday, 26 October 2013

Flask Technology

This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping. In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.

This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping. In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping. In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.
Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water
Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.
Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water
Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.

Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water

Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

This casting technology was originally developed for the jewellery and dental industries, for the production of comparatively small parts. As the production time is relatively short compared to conventional shell building, it is also being applied more and more for Rapid Prototyping. In the embedding machine a chalky powder is mixed with water under vacuum and poured over the wax part, housed in a small metal container, the so-called flask.

After solidification of the plaster, the flask is heated to a casting temperature of 300 – 500 °C, step by step. It is then cooled down depending on the material.

After the cast the part is de-bedded by means of a high-pressure water-jet to release the part from the plaster. The part is then sent for finishing.
Advantages:
Very good surface quality
Material is only mixed if required
No cores are necessary
Easy de-bedding by water
Disadvantages:
Only small and medium-sized parts are possible
Only low melting alloys e.g. aluminium can be cast, no steel
High material usage and lot of waste
No perfect casting quality because of the long solidification process, depending on the size of the flask
Burning takes up to several days, depending on flask size
Oven is blocked for many hours / days with (one) flask, as soon as the tempering process has been started

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Friday, 25 October 2013

Centrifugal Casting

Centrifugal casting, sometimes called rotocasting, 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. In centrifugal casting, a permanent mold made from steel, cast iron, or graphite is typically used. However, the use of expendable sand molds is also possible. The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available) and includes the following steps:

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

Centrifugal casting is used to produce axi-symmetric parts, such as cylinders or disks, which are typically hollow. 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.

Centrifugal Casting

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Die Casting

Die casting is a manufacturing process that 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, typically a non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then injected into the dies in the die casting machine. There are two main types of die casting machines - hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys with high melting temperatures, such as aluminum). The differences between these machines will be detailed in the sections 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. The steps in this process are described in greater detail in the next section.

Die casting hot chamber machine overview

Die casting cold chamber machine overview

The castings that are created in this process can vary greatly in size and weight, ranging from a couple ounces to 100 pounds. 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.

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Investment Casting

Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which 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. Lox-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 can not 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. The process steps include the following:

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.
Mold creation - This "pattern tree" is dipped into a 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.
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.
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.
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).
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

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Permanent Mold Casting

Permanent mold casting is a metal casting process that shares similarities to both sand casting and die casting. As in sand casting, molten metal is poured into a mold which is clamped shut until the material cools and solidifies into the desired part shape. However, sand casting uses an expendable mold which is destroyed after each cycle. Permanent mold casting, like die casting, uses a metal mold (die) that is typically made from steel or cast iron and can be reused for several thousand cycles. Because the molten metal is poured into the die and not forcibly injected, permanent mold casting is often referred to as gravity die casting.

Permanent mold casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness. Non-ferrous metals are typically used in this process, such as aluminum alloys, magnesium alloys, and copper alloys. However, irons and steels can also be cast using graphite molds. Common permanent mold parts include gears and gear housings, pipe fittings, and other automotive and aircraft components such as pistons, impellers, and wheels.

The permanent mold casting process consists of the following steps:

Mold preparation - 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.
Mold assembly - 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.
Pouring - 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.
Cooling - The molten metal is allowed to cool and solidify in the mold.
Mold opening - After the metal has solidified, the two mold halves are opened and the casting is removed.
Trimming - During cooling, the metal in the runner system and sprue solidify attached to the casting. This excess material is now cut away.

Permanent Mold Casting

Using these basic steps, other variations on permanent mold casting have been developed to accommodate specific applications. Examples of these variations include the following:

Slush Casting - As in permanent mold casting, the molten metal is poured into the mold and begins to solidify at the cavity surface. When the amount of solidified material is equal to the desired wall thickness, the remaining slush (material that has yet to completely solidify) is poured out of the mold. As a result, slush casting is used to produce hollow parts without the use of cores.
Low Pressure Permanent Mold Casting - Instead of being poured, the molten metal is forced into the mold by low pressure air (< 1 bar). The application of pressure allows the mold to remain filled and reduces shrinkage during cooling. Also, finer details and thinner walls can be molded.
Vacuum Permanent Mold Casting - Similar to low pressure casting, but vacuum pressure is used to fill the mold. As a result, finer details and thin walls can be molded and the mechanical properties of the castings are improved.

Advantages and disadvantages of shell molding casting

Shell molding casting is a main sand casting production process. The castings have good surface smoothness, less surface defects, and good dimensional accuracy. Our foundry has used this process for many years.

The followings are the advantages of shell molding castings:

1. Good surface quality

Because shell molding uses phenolic resin as the sand binder, so the smooth and hard surfaces of sand molds make the castings have good surface smoothness. The following photo could be taken as sample for the surface quality.

Moreover, this process have less sand residue during production, so could reduce some iron casting defects, such as sand inclusion, sand holes and air holes.

2. High rough casting dimensional accuracy

This molding material is a type of hard mold, so there will be less swell of sand molds, so the dimensional tolerance will be smaller. This advantage will be very useful for producing high accuracy rough castings, and reduce machining cost.

3. Thin wall thickness and complex castings

Less than 5mm wall thickness will be taken as very thin as for sand castings. Only shell molding process could produce these cast products.

In addition, hot shell and core molds are made by molding machines, so it could produce the castings with complex structures, especially complex inside structures.

4. Less manpower and molding skill requirements

Since the main works have been completed by the molding machines, so this process could be operated by women workers, and there is no special skill required. This is very different with green sand casting process.

The followings are the disadvantages of this process.

1. High production costs and casting prices

The phenolic resin sand is more expensive than green sand and furan resin sand, and it could not be recyclable. Therefore, shell molding castings will have higher prices.

2. High pattern costs

This process needs to use metal patterns (iron patterns), which will be more costly. So, it is not suitable for producing small quantity castings and small orders.

3. Size and weight limitation

The shells and cores of castings are produced by shell molding machines. These machines have dimensional limitation. So, most of shell molding castings will be less than 400mm length, and less than 20kg weight. Too long or too heavy can not be produced by this process.

Although shell and core molding process has these disadvantages, but its advantages are also very important. So, more and more iron foundries in China are using it to produce small and middle iron castings. As we know, in other countries, many metal foundries are using it to produce steel castings to replace lost wax casting process.

Shell Mold Casting

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.

The shell mold casting process consists of the following steps:

Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.
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.
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.
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.
Cooling - After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.
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 Mold Casting

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Thursday, 24 October 2013

CORE AND CORE BOX

A core is a preformed baked sand or green sand aggregate inserted in a mold to shape

the interior part of a casting which cannot be shaped by the pattern.

A core box is a wood or metal structure, the cavity of which has the shape of the desired core which is made therein.A core box, like a pattern ismade by the pattern maker.Cores run from extremely simple to extremely complicated.

A core could be a simple round cylinder form needed to core a hole through a hub of a wheel or it could be a very complicatedcore used to core out the water coolingchannels in a cast iron engine block along with the inside of the cylinders.

Dry sand cores are for the most part made ofsharp, clay-free, dry silica sand mixed with a binder and baked until cured;the binder cements the sand together.

When the metal is poured the core holds together long enough for the metal to solidify, then the binder is finely cooked, from the heat of the casting, until its bonding power is lost or burned out.

If the core mix is correct for the job, it can be readily removed from the castings interior bysimply pouring it out as burnt core sand.

This characteristic of a core mix is called its collapsibility.

The size and pouring temperature of acasting determines how well and how long the core will stay together.

Dry sand core with support wire.

The gases generated within the core during pouring must be vented to the outside of the mold preventing gas porosity and a defect known as a core blow.

Also, a core must have sufficient hot strength to be handled and used properly.

The hot strength refers to its strength while being heated by the casting operation.

Because of the shape and size of some coresthey must be further strengthened withrods and wires.

A long span core for a length of cast iron pipe would require rodding to prevent the core from sagging or bending upward when the mold is poured because of the liquid metal exerting a strong pressure during pouring.