What’s Shell Molding: Process, Materials, and Applications

Sand casting is an ancient casting process. Molten metal is poured into a mold, where it cools and takes shape. However, the finished product does not always meet precise requirements immediately. Therefore, it is often necessary to sand it several times to achieve the right size and surface finish. Shell casting is a more advanced casting method that solves these problems.

Shell molding does not involve the use of loose sand. Instead, it uses a thin, hard shell made from sand and a thermosetting resin binder. This shell is more dimensionally accurate and has a smoother surface. Additional processing is therefore not required. This article describes the shell molding process and how it works. You will also learn about the materials suitable for this process and its many applications.

What is Shell Molding?

Shell molding, also known as the Croning process, is named after the German engineer Johannes Croning, who invented the process. Croning devoted his life to improving casting processes and machines. One of his most important achievements was the invention of shell molding, which made it possible to manufacture more precise parts.

In shell molding, thin-walled shells made of sand and thermosetting resin glue are used as molds, which can be discarded after use. Unlike most sand molds, these shells are sturdy and held together with resin. In addition, they improve the precision and surface smoothness of the parts.

How Long Does it Take to Complete a Shell Casting?

It can take a long time to produce some complex moulds. Once the mould is in place, the most time-consuming part is cooling. How many parts you can produce in an hour depends on your automation methods and the size and complexity of the mould. You could produce 5-50 parts in an hour.

How Much Does Shell Molding Cost?

In most cases, shell molding costs between 0.25 and 0.30 USD/kg. Since resin-injected sand is more expensive than normal casting sand, shell molding is more expensive than normal sand molding.

Is Shell Molding More Expensive than Die Casting?

The relative cost depends on the production volume. For small production runs, shell molding is less expensive than die casting. This is because die casting requires expensive molds that can only be used to produce this type of part. In contrast, the mold for shell molding only requires sand and resin.

Die-casting, on the other hand, becomes cheaper as the number of parts produced increases. This is because the high cost of the die is spread over a greater number of parts. On the other hand, Shell mold casting requires new sand and resin every time.

To learn more about die casting costs as well as price data, check out the metal die casting costs article.

How Are Shell Molds Made?

The shell mold casting process is more time-consuming than the sand mold casting process. This is because more preparation is required before the former can begin. The process for this casting process is as follows:

Step 1: Machining the Metal Pattern

If something is to be cast, a metal model is needed that takes shrinkage and machining allowances into account. It is usually machined and comes in two halves. Molds are usually made from iron and steel. Graphite is also an option for reactive metals.

Step 2: Making the Mold

The first step is to heat the metal mold to a temperature of 230–340°C (450–650°F). This is the most important step in the process. After that, calcium stearate is used to prevent the sand shell from sticking to the mold.

The entire model is suspended in a tipping box filled with fine silica sand and thermosetting phenolic resin. The tipping box is then turned upside down so that the sand and resin mixture covers the hot model. The heat from the metal model causes the mixture to harden. This forms a hard shell about 9–20 mm thick.

After a while, they turn the model and box over so that the loose sand that has not yet hardened falls out. Now only the hardened shell remains attached to the model. Finally, the shell is placed in the oven for drying and curing. Since we need a shell that consists of two parts, the other half of the mold is made using the same procedure.

Learn more about die casting mold design and get mold design advice.

Step 3: Assembling the Molds

Once you have made both molds using the same procedure, you can assemble them. You can attach them by clamping or gluing.

Step 4: Pouring Process

The technician may place the shell in a pouring sleeve or cover it with sand, gravel or metal shot. This adds support to the shell to make it stronger. The molten metal is then poured into the mold. You can do this by hand or with a machine.

Step 5: Cooling and Removing the Casting

After being left for a while, the metal cools down on its own. Once it has hardened, the worker can break it or open the mold to remove the casting. The casting is then cleaned or finished depending on its appearance.

Materials that Can Be Used for Shell Casting

Shell casting can be used with most metal materials, including pure metals and a wide range of alloys.

Cast Iron

In the past, cast iron was the most common metal used because it is very strong. It is still used by many in the industry today. Many people use abrasion-resistant cast iron, ductile iron, white iron, malleable iron and alloyed cast iron for casting.

Steel

Steel castings are known for their strength and high mechanical strength. The following types of steel are common options:

• Carbon Steel: Carbon steel has a low content of alloying elements, is workable, and is not prone to wear. However, it rusts more easily than other types of steel.
• Alloy Steel: These steels have additional alloying elements added to make them stronger and harder. 45Mn and 40Cr are two steels that are well suited for casting.
• Stainless Steel: Thanks to its high chromium content (10.5%), stainless steel does not rust and has a long service life. On the other hand, it is brittle breaks easily, and is not as strong as carbon steel.

Aluminum Alloys

Aluminum alloys are used for lightweight applications such as aerospace. Aluminum metals such as ASTM A356 and ASTM 360 are commonly used for die casting.

Copper Alloys

Copper is a ductile metal with good electrical conductivity and does not rust. These properties are needed in situations such as electronic components. However, copper alloys are expensive and not as strong as steel.

Advantages of Shell Molding

People like shell molding because it can handle a variety of materials. In addition, it can improve the accuracy and surface finish of the machined parts.

Here are some advantages of the shell mold casting process:

• More Precise Dimensions: Shell mold casting products have very precise dimensions. This is because the solidified shell is heated and pressurized to hold its shape. Parts manufactured using this process can have tolerances of just ±0.25 mm between them.
• Better Surface Finish: Unlike sand casting, the liquid metal comes into contact with a dry, closed shell with a smooth surface and no pores. The surface finish of the cast part is therefore better and has fewer defects, ideally up to 2.5 microns.
• Short Delivery Times: Shell molds can be used again and again with the same results. This not only reduces waiting times but also makes it cheaper to produce more of the same product.
• Reduced Gas Defects: More binder is used than with other methods. Despite this, thin-walled hollow cores provide a natural channel for air to flow through. Coarser sand grains in the shell mold make it easier for the air to pass through, resulting in better casting.
• Automation: The shell mould casting process is easy to automate and is suitable for producing large quantities of products at the same time. There are machines that do all the important work. For example, heating the metal moulds, making the shell moulds and putting them together.
• Supports a Wide Range of Materials: Shell molding can be used to produce ferrous and non-ferrous metals. This means it can be used for many different materials.

Disadvantages of Shell Molding

Everything has its advantages and disadvantages. Here are the disadvantages of shell molding:

• Limited Weight Handling: Due to the very thin wall thickness of shell molds, they can only accommodate medium-sized and heavy parts. The process can handle parts weighing between 30 grams and 10 kilograms (25 pounds).
• Expensive Resin: The thermosetting phenolic resin used for shell mold casting is more expensive than the binder used for conventional sand casting.
• High Equipment Costs: The process requires different heating and curing units, which makes it more expensive. If an automated version is used, the capital costs are even higher.

Applications of Shell Molding

Shell molding technology makes it easy to reproduce precise metal parts in a short time. This is why the casting process is widely used in many fields such as medicine, aerospace, and the automotive industry.

The addition of resin makes the surface of the part smoother, so there is no need for extensive post-processing. Because of this, it requires less time and labor, thereby reducing overall production costs. This is why shell molding is the best way to manufacture small and medium-sized metal parts.

Specific uses of shell molding in different industries:

• Automotive: Engine blocks, cylinder heads, camshafts, crankshafts, transmission cases
• Aerospace: Turbine blades, gearboxes
• Industrial Machinery: Gears, valves, pump housings, lever arms
• Electrical Equipment: Electrical system housings and components
• Oil and Gas: Valve bodies, custom precision parts

Shell Molding and Other Casting Processes

Molding is a broader term, and shell molding is one of the molding methods. This method involves shaping molten and solidified material in a mold. There are other very similar methods, such as sand casting, die casting and investment casting.

The Difference Between Shell Molding and Sand Casting

There are two methods of producing objects using sand molds: sand casting and shell mold casting. The processes used to create the moulds differ between the two methods. In sand casting, the filled sand itself forms the mould. In shell mould casting, the mould is a thin shell made of sand and resin.

As the sand mould is loose, it is more prone to surface defects and dimensional errors. The shell mould, on the other hand, is thin and rigid, and the products made using it look more aesthetically pleasing. It is also more precise.

The Difference Between Shell Molding and Investment Casting

In casting, investment casting is not the same as shell molding. A wax model is used every time investment casting is used. The wax model is assembled into a tree-like structure (wax sprues), covered with ceramic slurry, and then melted to form the mold cavity.

This means that the design and the mold are made from different materials. Most shell molds are made of metal. In investment casting, they use wax molds. Investment casting uses refractory ceramics, while shell molding uses sand molds.

One of the advantages of shell molding is the good surface finish. However, the finish of investment casting products is even better, up to 1.3 microns.

The Difference Between Shell Molding and Die Casting

Shell molding and die casting are both metal casting processes. The main difference between them is the production and use of the mold. Die casting uses a permanent metal mold to press molten metal into the mold cavity under high pressure. Shell molding, on the other hand, uses a shell mold made of sand and resin, which can only be used once.

Shell molding does not require the high pressures of die casting. Instead, it uses the strength of the resin to bond the shell to achieve precise results. In addition, die casting is only suitable for simple parts made of easily fusible metals.

The Difference Between Shell Molding and Injection Molding

Another precise molding technique is injection molding. However, it is mainly used to produce duplicates of plastic items. In this method, molten plastic is pressed under high pressure into a metal mold. The plastic is then allowed to cool and harden into its final shape.

The main difference between shell molding and injection molding is therefore the type of material used and the method of manufacture. With shell molding, molten metal is poured into expendable sand molds. Injection molding, on the other hand, involves pressing plastic into permanent solid molds.

Continue reading:What’s the difference between die casting and sand casting

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