Shell molding

Last updated July 02, 2024

Shell molding, also known as shell-mold casting,^[1] is an expendable mold casting process that uses resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labour requirements. It is used for small to medium parts that require high precision.^[2] Shell molding was developed as a manufacturing process during the mid-20th century in Germany. It was invented by German engineer Johannes Croning.^[3]^[4] 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, aluminium 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 aluminium for low volume production or graphite for casting reactive materials.

Mold creation - First, each pattern half is heated to 175–370 °C (347–698 °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 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.

Examples of shell molded items include gear housings, cylinder heads and connecting rods. It is also used to make high-precision molding cores.

Process

The process of creating a shell mold consists of six steps:^[2]^[5]

Fine silica sand that is covered in a thin (3–6%) thermosetting phenolic resin and liquid catalyst is dumped, blown, or shot onto a hot pattern. The pattern is usually made from cast iron and is heated to 230 to 260 °C (446 to 500 °F). The sand is allowed to sit on the pattern for a few minutes to allow the sand to partially cure.
The pattern and sand are then inverted so the excess sand drops free of the pattern, leaving just the "shell". Depending on the time and temperature of the pattern the thickness of the shell is 10 to 20 mm (0.4 to 0.8 in).
The pattern and shell together are placed in an oven to finish curing the sand. The shell now has a tensile strength of 350 to 450 psi (2.4 to 3.1 MPa).
The hardened shell is then stripped from the pattern.
Two or more shells are then combined, via clamping or gluing using a thermoset adhesive, to form a mold. This finished mold can then be used immediately or stored almost indefinitely.
For casting, the shell mold is placed inside a flask and surrounded with shot, sand, or gravel to reinforce the shell.^[6]

The machine that is used for this process is called a shell molding machine. It heats the pattern, applies the sand mixture, and bakes the shell.

Details

Setup and production of shell mold patterns takes weeks, after which an output of 5–50 pieces/hr-mold is attainable.^[7] Common materials include cast iron, aluminum and copper alloys.^[1] Aluminum and magnesium products average about 13.5 kg (30 lb) as a normal limit, but it is possible to cast items in the 45–90 kg (100–200 lb) range.^{[ citation needed ]} The small end of the limit is 30 g (1 oz). Depending on the material, the thinnest cross-section castable is 1.5 to 6 mm (0.06 to 0.24 in). The minimum draft is 0.25 to 0.5 degrees.^[1]

Typical tolerances are 0.005 mm/mm or in/in because the sand compound is designed to barely shrink and a metal pattern is used. The cast surface finish is 0.3–4.0 micrometers (50–150 μin) because a finer sand is used. The resin also assists in forming a very smooth surface. The process, in general, produces very consistent castings from one casting to the next.^[5]

The sand-resin mix can be recycled by burning off the resin at high temperatures.^[6]

Advantages and disadvantages

Advantages

Very large parts
Complex, accurate shapes with fine details and good surface finishes
Reduced machining costs
Shell molding can be completely automated for mass production.^[2]
High production rate, low labor costs if automated.
Short lead time possible.
Many material options.
Low tooling and equipment cost.
There are few problems due to gases, because of the absence of moisture in the shell, and the little gas that is still present easily escapes through the thin shell. When the metal is poured some of the resin binder burns out on the surface of the shell, which makes shaking out easy.^[1]^[5]

Disadvantages

The gating system must be part of the pattern because the entire mold is formed from the pattern, which can be expensive.
The resin for the sand is expensive, although not much is required because only a shell is being formed.^[5]
Poor material strength.
High porosity possible.
Secondary machining often required.
High labor cost if done manually.

Applications

Cylinder head, connecting rod, Engine blocks and manifolds, machine bases.

Related Research Articles

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An ingot is a piece of relatively pure material, usually metal, that is cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedure of shaping, such as cold/hot working, cutting, or milling to produce a useful final product. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods. Precious metal ingots can be used as currency, or as a currency reserve, as with gold bars.

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

<span class="mw-page-title-main">Sand casting</span> Metal casting process using sand as the mold material

Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand—known as casting sand—as the mold material. The term "sand casting" can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries. In 2003, over 60% of all metal castings were produced via sand casting.

Molding sand, also known as foundry sand, is a sand that when moistened and compressed or oiled or heated tends to pack well and hold its shape. It is used in the process of sand casting for preparing the mold cavity.

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured; this process is known as compression molding method and in case of rubber it is also known as 'Vulcanisation'. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.

Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of polymer foams to simplify the investment casting process by removing the need to melt the wax out of the mold.

Evaporative-pattern casting is a type of casting process that uses a pattern made from a material that will evaporate when the molten metal is poured into the molding cavity. The most common evaporative-pattern material used is polystyrene foam.

Spin casting, also known as centrifugal rubber mold casting (CRMC), is a method of utilizing inertia to produce castings from a rubber mold. Typically, a disc-shaped mold is spun along its central axis at a set speed. The casting material, usually molten metal or liquid thermoset plastic, is then poured in through an opening at the top-center of the mold. The filled mold then continues to spin as the metal solidifies.

A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.

<span class="mw-page-title-main">Investment casting</span> Industrial process based on lost-wax casting

Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.

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Reaction injection molding (RIM) is similar to injection molding except thermosetting polymers are used, which requires a curing reaction to occur within the mold.

Permanent mold casting is a metal casting process that employs reusable molds, usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminium, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds.

Ceramic mold casting, also known ambiguously as ceramic molding, is a group of metal casting processes that use ceramics as the mold material. It is a combination of plaster mold casting and investment casting. There are two types of ceramic mold casting: the Shaw process and the Unicast process.

<span class="mw-page-title-main">Full-mold casting</span>

Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting. It uses an expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.

Plaster mold casting is a metalworking casting process similar to sand casting except the molding material is plaster of Paris instead of sand. Like sand casting, plaster mold casting is an expendable mold process, however it can only be used with non-ferrous materials. It is used for castings as small as 30 g (1 oz) to as large as 7–10 kg (15–22 lb). Generally, the form takes less than a week to prepare. Production rates of 1–10 units/hr can be achieved with plaster molds.

Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various time setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods. Heavy equipment like machine tool beds, ships' propellers, etc. can be cast easily in the required size, rather than fabricating by joining several small pieces. Casting is a 7,000-year-old process. The oldest surviving casting is a copper frog from 3200 BC.

A core is a device used in casting and moulding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece. They are most commonly used in sand casting, but are also used in die casting and injection moulding.

A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: gas porosity, shrinkage defects, mould material defects, pouring metal defects, and metallurgical defects.

References

Notes

1 2 3 4 Degarmo 2003 , p. 310.
1 2 3 Degarmo 2003 , p. 308.
↑ Recknagel, Ulrich. "The Shell Moulding Process: A German Innovation" (PDF). Huettenes-albertus: 1/7.
↑ Recknagel, Ulrich (2008). "GIESSEREI Rundschau" (PDF). Fachzeitschrift der Österreichischen Giesserei-Vereinigungen: 17/36.
1 2 3 4 Degarmo 2003 , p. 309.
1 2 Todd, Allen & Alting 1994 , p. 267.
↑ Kalpakjian, Serope (2010). Manufacturing engineering and technology (6th ed.). New York: Prentice Hall. p. 261. ISBN 978-0-13-608168-5. OCLC 305147413.

Bibliography

Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003). Materials and Processes in Manufacturing (9th ed.). Wiley. ISBN 0-471-65653-4.
Todd, Robert H.; Dell K. Allen; Leo Alting (1994). Manufacturing Processes Reference Guide. Industrial Press Inc. ISBN 0-8311-3049-0.

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[degarmo310-1] 1 2 3 4 Degarmo 2003 , p. 310.

[degarmo308-2] 1 2 3 Degarmo 2003 , p. 308.

[3] Recknagel, Ulrich. "The Shell Moulding Process: A German Innovation" (PDF). Huettenes-albertus: 1/7.

[4] Recknagel, Ulrich (2008). "GIESSEREI Rundschau" (PDF). Fachzeitschrift der Österreichischen Giesserei-Vereinigungen: 17/36.

[degarmo309-5] 1 2 3 4 Degarmo 2003 , p. 309.

[todd-6] 1 2 Todd, Allen & Alting 1994 , p. 267.

[7] Kalpakjian, Serope (2010). Manufacturing engineering and technology (6th ed.). New York: Prentice Hall. p. 261. ISBN 978-0-13-608168-5. OCLC 305147413.

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