Apr. 21, 2025
Shell molding is a casting process used to produce metal parts. The mold is composed of resin-infused sand that gives the final product a high-quality surface finish and good dimensional stability. Shell molding products are common in piston engines for parts like camshafts, valves, and gears.
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This article will discuss shell molding. We’ll examine how it works, its importance, its advantages, and a few examples of shell-molded products.
Shell molding is a special form of metal casting that bears many similarities to regular sand casting. Both processes see molten metal poured into a sand mold. The pattern for shell molding, however, is formed using a mix of resin and sand, which is shaped using a mold tool. This particular mixture improves the parts’ surface finish and dimensional accuracy.
Shell molding was originally referred to as the Croning process after Johannes Croning. He patented the process in Germany in . Later, the process became known as “shell mold casting” or simply “shell molding.”
The goal of shell molding is to produce metal items with more accurate dimensions and nicer surface finishes than can be achieved with normal sand casting. It’s done at a good production rate and with relatively minimal labor. These characteristics are important for high-precision parts such as camshafts, for which sand casting is not accurate enough.
Shell molding is employed extensively in many settings within the oil and gas, aerospace, and medical industries, among others. It sees the highest use in the automotive industry where it is used for: camshafts, crankshafts, bearing housings, lever arms, valve bodies, and cylinder heads.
The shell mold itself must first be created by placing a sand-and-resin mixture around a heated “pattern” of the part and letting it cure. The mold is usually made up of two halves. With the patterns removed, the two halves are clamped together and molten metal is poured into the cavity and allowed to cool. The mold is then often broken to extract the metal part.
Shell molding is a six-step process:
The first step in shell molding is creating a pattern part. This is achieved by first machining a metal pattern tool, usually from iron or steel, in the shape of the part you want. Usually, each pattern represents one-half of the part; a second pattern is required for the other half. If you’re casting reactive metals, the pattern will need to be made of graphite rather than metal. Figure 1 is an example of a pattern tool:
Many ferrous and non-ferrous metals can be used in shell molding. Some of the common ones are listed and described below:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Automating the shell molding process can be a major advantage because it minimizes labor costs. Other advantages of shell molding are:
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:
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:
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.
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.
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.
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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.
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.
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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At its heart, a sustainability policy is an organisation’s commitment to practices and standards in order to meet its legal, and wider social, environmental obligations. This includes outlining how a business intends to promote environmentally responsible operations, mitigate its impact on, and positively contribute to the environment and society. It commits an organisation to focusing on the incredible challenges faced on a global scale and what its response will be to issues such as energy security and climate change, depletion of natural resources, water scarcity, and human rights. A realistic and action-driven sustainability policy is now a critical element to a company’s overall business strategy and can support both long-term growth and profitability.
If we take the sustainability-related issue of decarbonisation as an example, every business in the UK will have to implement sustainable measures to achieve the government’s target of reaching net zero by . Realistically, the only way to achieve this is by implementing a sustainability policy that supports short, medium, and long-term goals – this is why we believe every organisation needs one.
Many businesses are already focused on reducing their carbon footprint by implementing sustainable measures like:
Choosing renewable electricity and green gas supplies
Reducing the use of plastics
Becoming a ‘prosumer’ by generating the energy they use
Introducing LED lighting and controls
Taking part in carbon offsetting projects
In research* we conducted recently, it was encouraging to discover how many large businesses understood net zero and had a sustainability plan to achieve it. Out of 524 UK-based senior decision makers:
97% understood net zero
73% have a net zero plan
The reason for the numbers are the tangible benefits sustainability brings to a business. Here are seven of them.
1. Reduced energy usage. Relatively simple measures can increase energy efficiency and reduce energy usage, for example, through the replacement of outdated equipment.
2. Cost saving from waste reduction. A great example of this is reusing – why purchase new what you already have?
3. Increased customer base. Achieving net zero and mitigating climate change is increasingly important to customers and influences who they’re willing to purchase from.
4. Attract investors. Many investors now place sustainability at the core of their investment strategy which means more financial opportunities for businesses with a sustainability plan.
5. Position as a forward-thinking, market leader. Businesses are now ranked by their sustainability targets; we only have to consider the Dow Jones Sustainability Index to see how environmental sustainability can enhance an organisation’s brand.
6. Increased resilience. A realistic sustainability policy will help to protect a business from tighter future regulations. It also complements large business compliance requirements such as Streamlined Energy and Carbon Reporting (SECR).
7. Engagement. The development of a sustainability policy is a great opportunity to engage the internal and external stakeholders of a business.
You’ve got your new policy, now focus turns to implementing it. We all use the word ‘journey’ fairly frequently, but in terms of implementing your sustainability plan, that’s exactly how you need to think of it. It’s an accumulation of efforts that will help you get to your destination, whether that’s meeting the government’s target of net zero, or overhauling your production process to make it greener.
Start at the beginning with small improvements that can create big wins, for example, make it easier to recycle in the office, reduce single-use plastic across the business, or using lower-carbon supply via Shell Energy as a good first step.
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