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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:

 

  1. 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.
  2. 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.
  3. 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.
  4. Cooling – The molten metal is allowed to cool and solidify in the mold.
  5. Mold opening – After the metal has solidified, the two mold halves are opened and the casting is removed.
  6. 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-small

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.

Capabilities

Typical Feasible
Shapes: Thin-walled: Complex
Solid: Cylindrical
Solid: Cubic
Solid: Complex
Flat
Thin-walled: Cylindrical
Thin-walled: Cubic
Part size: Weight: 2 oz – 660 lb
Materials: Aluminum
Copper
Magnesium
Metals
Alloy Steel
Carbon Steel
Cast Iron
Stainless Steel
Lead
Nickel
Tin
Titanium
Zinc
Surface finish – Ra: 125 – 250 μin 32 – 400 μin
Tolerance: ± 0.015 in. ± 0.01 in.
Max wall thickness: 0.08 – 2 in. 0.08 – 2 in.
Quantity: 1000 – 100000 500 – 1000000
Lead time: Months Weeks
Advantages: Can form complex shapes
Good mechanical properties
Many material options
Low porosity
Low labor cost
Scrap can be recycled
Disadvantages: High tooling cost
Long lead time possible
Applications: Gears, wheels, housings, engine components

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:

 

  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 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.
  3. 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.
  4. 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.
  5. 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).
  6. 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-small

Capabilities

Typical Feasible
Shapes: Thin-walled: Complex
Solid: Cylindrical
Solid: Cubic
Solid: Complex
Flat
Thin-walled: Cylindrical
Thin-walled: Cubic
Part size: Weight: 0.02 oz – 500 lb
Materials: Metals
Alloy Steel
Carbon Steel
Stainless Steel
Aluminum
Copper
Nickel
Cast Iron
Lead
Magnesium
Tin
Titanium
Zinc
Surface finish – Ra: 50 – 125 μin 16 – 300 μin
Tolerance: ± 0.005 in. ± 0.002 in.
Max wall thickness: 0.06 – 0.80 in. 0.025 – 5.0 in.
Quantity: 10 – 1000 1 – 1000000
Lead time: Weeks Days
Advantages: Can form complex shapes and fine details
Many material options
High strength parts
Very good surface finish and accuracy
Little need for secondary machining
Disadvantages: Time-consuming process
High labor cost
High tooling cost
Long lead time possible
Applications: Turbine blades, armament parts, pipe fittings, lock parts, handtools, jewelry

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-machine-hot
Die casting hot chamber machine overview
die-casting-machine-cold
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,

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