The Benefits of Using shell mold casting products

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.

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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]

1. 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.
2. 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).
3. 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).
4. The hardened shell is then stripped from the pattern.
5. 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.
6. For casting, the shell mold is placed inside a flask and surrounded with shot, sand, or gravel to reinforce the shell.

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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.

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• 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.

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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.

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• 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.

References

Notes

Bibliography

What Are the Metals Used in Shell Molding?

Many ferrous and non-ferrous metals can be used in shell molding. Some of the common ones are listed and described below:

1. Carbon Steel

Carbon steel is an iron alloy that has less than 10.5% alloying elements. Carbon steel grades that work well with shell molding include: C20, C25, C30, and C45. The benefit of shell molding carbon steel is that it is very machinable and resists wear. It is also ferromagnetic and highly recyclable. However, carbon steel is more susceptible to corrosion than other forms of steel. 

2. Stainless Steel

Stainless steel contains at least 10.5% chromium. Cast stainless steel can come in several varieties: ferritic, martensitic, austenitic, precipitation-hardened, and duplex stainless steel. Stainless steel alloys are more brittle and exhibit less tensile strength than carbon steel, but they&#;re very corrosion-resistant and durable. 

3. Aluminum Alloys

Cast aluminum alloys include: ASTM A356, ASTM A413, and ASTM 360. They may be used in place of steel if weight is a bigger concern than strength. Aluminum also has great conductivity. However, it is not as strong as steel and is considered less sustainable because the refining process creates more greenhouse gasses. 

4. Alloy Steel

Common alloy steel grades that are cast using shell molding include: 20Mn, 45Mn, ZG20Cr, 40Cr, 20Mn5, 16CrMo4, 42CrMo, 40CrV, 20CrNiMo, GCr15, and 9Mn2V. Each alloy has unique characteristics. Alloy steels vary in density, corrosion resistance, strength, cost, and temperature resistance. 

5. Copper

Copper is a lightweight, ductile metal that can be cast using shell molding. All copper-based alloys are suitable for casting with the exception of brass. The use of copper is advantageous in applications that require conductivity, machinability, ductility, or corrosion resistance. However, copper alloys can be expensive and weaker than steel. 

6. Low Alloy Steel

Low-alloy steels are defined as steel with less than 8% alloying content. Low-alloy steels are very similar to carbon steels but are more hardenable. They&#;re valued because they&#;re cheap and perform well in most ordinary engineering settings. 

How Long Does It Take for Shell Molding To Finish?

Some pattern tools are complex and take a long time to produce. Once you have the pattern in hand, the slowest part of the molding process is cooling. Depending on your automation process and the size and complexity of the mold, you may produce between 5 and 50 parts in an hour.

How Accurate Is Shell Molding?

Shell molding is much more accurate than most casting processes. Though similar to sand casting, the use of resin as a binder improves the surface finish of the sand pattern. This higher-quality surface finish is then imparted onto the final part resulting in a better-quality finish and tighter dimensional tolerances. 

How Much Does Shell Molding Cost?

Shell molding typically costs around $0.25-0.30/kg. This is pricier than regular sand casting because the resin-infused sand is more expensive than ordinary casting sand. The need for more equipment to cure the resin and sand mix contributes to a higher final part price. 

Is Shell Molding Expensive Compared To Die Casting?

The relative cost depends on production volume. Shell molding is cheaper than die casting for small production runs. This is because die casting requires an expensive reusable mold, whereas shell molding&#;s sand and resin are much simpler. However, as the production volume increases, die casting becomes cheaper since the cost of tooling is shared between more parts. Shell molding, meanwhile, continues to demand new sand and resin for each batch.  

What Are the Advantages of Shell Molding?

Automating the shell molding process can be a major advantage because it minimizes labor costs. Other advantages of shell molding are:

1. High-quality surface finishes and dimensional accuracy reduce post-processing. 
2. The resin binder is burned in the casting process, making the final product easy to remove. 
3. Shell molding can be used for complex geometry. 
4. The sand and resin mixture can be reused. 
5. The tooling is cheap and lead times can be short.
6. Both ferrous and non-ferrous metals can be cast using this process.

What Are the Disadvantages of Shell Molding?

Gating systems are often needed to make the casting process go smoothly, but they&#;re expensive and generate extra waste material. Other disadvantages of shell molding include:

1. Phenolic resin is expensive (although it is used in small quantities).
2. High initial investment for equipment. 
3. Creates porosity and shrinkage of the final part.
4. Post-processing may be required despite the increase in surface quality. 
5. Labor-intensive process when not automated.

What Are Examples of Shell Molding Products?

Shell molding has a huge list of applications across different industries. Parts are usually less than 20 kg and therefore relatively small. Examples of shell-molded products are:

1. Gearboxes. 
2. Truck hooks.
3. Cylinder heads.
4. Camshafts. 
5. Body panels. 
6. Lever arms. 

What Is the Lifespan of Shell Molding Products?

The lifespan of shell-molded products depends entirely on the material and application. For example, camshafts, crankshafts, and gearboxes, which are used in cars and other piston-engined vehicles, tend to outlast their cars. These parts may spend many thousands of hours in service over decades without failure. 

What Is the Quality of Shell Molding Products?

Compared to other casting processes, shell molding creates high-quality finishes and impressive dimensional tolerance. Shell molding products&#; longevity, wear, and heat resistance, and other performance characteristics depend primarily on the metals used in the molding process. 

Are Shell Molding Products Durable?

Yes, shell molding products are durable. This is due in part to the low number of surface defects. Nearly all cracking and corrosion starts at or is accelerated by the presence of a defect. Therefore the high-quality finish of shell molding results in good durability.

What Is the Difference Between Shell Molding and Sand Casting?

Shell molds are made of resin-infused sand which results in a smoother mating face between the tool and the molten metal. Sand casting uses no such resin, so the resulting parts have lower-quality surface finishes. Additionally, in sand casting, sand fills up the whole tool container, whereas in shell molding the shell is surrounded by metal shot in a flask. 

What Is the Difference Between Shell Molding and Investment Casting?

For shell molding, sand, and resin are laid over a pair of male patterns and cured. The resulting female mold halves are then assembled into one full mold into which the molten metal can be poured. In investment casting, wax is injected into a female mold pattern tool to create a replica of the part. The solidified wax is then submerged in sand or plaster and then melted leaving a female mold for the molten metal to be poured into. Shell molding makes stronger and more intricate shapes while investment casting allows for thinner walls and results in a higher-quality finish. 

Summary

This article presented shell molding, explained it, and discussed how it works and its advantages. To learn more about shell molding, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including casting and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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