You may have heard about the traditional process of sand casting, where molten material gets poured into a mold and solidifies into a shape. However, that final product often doesn’t meet precise requirements right away – a series of machining operations are carried out to achieve the right dimensions and surface finish.
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Shell molding is a refined casting approach that tackles these challenges. Instead of relying on loose sand, it creates a thin and hard shell of sand and a thermosetting resin binder. This shell provides superior dimensional accuracy and a smooth surface finish, reducing the need for additional machining.
This article explores the process of shell molding, the materials best suited for it, and its multiple product applications across industries.
Shell molding is also known Croning process, after its inventor, Johannes Croning – a German engineer who devoted his life to developing casting techniques and machines. The discovery of shell mold casting was one of his major innovations that paved the way for more accurate building of parts.
Shell casting is an expendable mold casting technique in which a thin-walled shell of sand and thermosetting resin binder is used as a mold. This shell, unlike traditional sand molds, is strong and resin-bonded. It results in superior dimensional accuracy and a smooth surface finish.
The shell mold casting process is slightly lengthier than the sand casting process as it involves a series of preparatory steps. Here’s how the process works:
For casting you need a metal pattern that accounts for shrinkage and machining allowance. Usually, manufacturers make it via machining, and in two halves. Iron and steel are common. However, graphite is also an option for reactive metals.
This is the most critical process in the journey that starts with metal pattern heating at 230–340ºC (450–650ºF). Then, technicians lubricate it with calcium stearate so the sand shell doesn’t stick to it.
The whole pattern gets hung over a dump box containing a blend of fine silica sand and thermosetting phenolic resin. That box is flipped to cover the hot pattern with the sand-resin mixture. The heat from the pattern slightly cures a layer of the mixture, forming a firm shell around 9–20 mm thick.
After a few seconds, they invert the pattern and box again to let any loose, uncured sand fall away. This leaves just the cured shell adhered to the pattern. Finally, the shell gets placed in an oven for complete curing. This is where it gets in its tensile strength (350 to 450 psi). We require a two-part shell, so the same mold construction process gets repeated for the other half.
After the construction of both shelves, their assembly starts. Clamping or gluing are two options for joining.
For added support of the shell, technicians may place it inside a pouring jacket or surrounded by sand, gravel, or metal shot. Then, they pour the molten metal into the mold cavity. This may done by hand or by machine.
The metal cools naturally. Once it’s solidified, the workers may open or break the mold to remove the cast part. Then, the casting is cleaned or machined as per finish requirements.
You can use shell casting for most types of metals, pure and alloys – both ferrous and non-ferrous.
Cast iron was once the most cast metal due to its mechanical properties. Still, its use is prevalent in the industry. Fray cast iron, nodular iron, white cast iron, malleable iron, and alloy cast irons are popular for this casting.
Steel castings are known for their high tensile strength and toughness. The best steel options are:
Aluminum alloys are suitable for lightweight applications for instance aerospace. ASTM A356 and ASTM 360 are common aluminum alloys for mold casting.
Copper is a ductile material with excellent conductivity and corrosion resistance. So, the use case is applications requiring these properties, for instance, electrical components. However, copper alloys are expensive and offer less strength than steel.
Shell mold casting is popular for its better tolerances, surface finishes, and versatile material capabilities.
Here’s a list of advantages, the casting process has to offer:
Some downsides of shell casting include:
Shell casting is very effective in replicating metal parts with dimensional accuracy in a short time. So, the shell-molded products span across multiple industries, including automotive, aerospace, energy, medical, etc.
The presence of resins increases the surface smoothness, which eliminates the need for extensive post-processing. Subsequently, it requires less labor & time, reducing the overall production cost. For these reasons, manufacturers prefer the shell molding method for the production of small to medium-sized metal parts
The table below outlines the specific shell molding applications in various industrial sectors;
Molding is a broader technology that refers to shaping the material by melting and solidifying it in a die or mold, shell casting is one of them. Besides this, there are other closely related techniques, such as sanding casting, die casting, and investment casting.
Sand casting and shell molding are sand mold casting techniques. However, the mold creation process is different. In sand casting, the whole packed sand itself acts as a mold. Whereas, shell molding uses a thin shell of sand and resin binder as the mold.
Sand molds are loose and the likelihood of surface defects and dimensional inaccuracies is higher. In contrast, expendable molds are thin and hard, and shell-molded products have a better finish.
Investment casting utilizes a different casting approach than shell molding. It uses a wax pattern for each casting. These wax patterns are assembled into a tree structure (wax sprue), coated with a ceramic slurry, and then melted away to leave a mold cavity.
So, both the pattern and mold materials are different. Shell molds are usually metallic. Whereas, in investment casting, they have wax build. Shell casting uses sand mold whereas investment casting utilizes refractive ceramic.
Although the shell molding surface involves a better surface finish, the finish of casted products is much higher in the case of investment casting, up to 1.3 microns.
Shell and die casting are both metal casting techniques, but the major difference is the mold. Die casting uses a permanent metal mold, or die, and applies high pressure to inject molten metal into the mold cavity. Whereas, shell casting uses an expendable shell mold of sand + resin.
Unlike die casting, shell molding doesn’t require high pressure; instead, it relies on the strength of the resin-bonded shell to achieve precision. Also, die casting only works for metals with low melting points and parts with simple geometries.
Injection molding is another precise mold technique; however, it primarily focuses on replicating plastic products. In this process, molten plastic gets injected into a metal mold under high pressure, where it cools and solidifies into the final shape.
So, the major difference between shell and injection molding is their target materials and casting approach. The shell molding process involves pouring molten metal into a sand (expendable) mold. Whereas, injection molding revolves around the injection of plastics into a solid (non-expendable) mold.
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What is the shell casting process?
(a) Shell Moulding Casting ProcessThe shell moulding process is a special casting process to manufacture net shapes with high surface finish quality, complex precise shape geometry, high tolerance, excellent pourability, thin sharp corners and better dimensions accuracy casting.
This process includes making metal patterns, thermosetting sand-resin shell moulds, pouring molten metal in the shell mould cavity and solidifying the liquid molten metal into the shell mould with a shorter lead time.
This process is different from the traditional sand moulding process where sand is bonded together with moisture.
The thermosetting phenolic resin is used here instead of sand, this resin is basically plastic which melts binding the fine sand together forming a versatile shell mould and casting. Types of resins used here are polyester, phenol formaldehyde, alkynes and urea-formaldehyde.
This resin used has a faster curing rate, bonding properties, sufficient viscosity, higher shelf life, high melting point, tensile strength, compressive strength and shear strength.
What is the shell moulding process also called?
The shell moulding process is also called the shell casting process, C-process, shell casting or corning process.
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What is the step-by-step process, working and method to manufacture shell casting using the shell moulding process, explain it with diagrams?
This process uses a metal pattern that needs to be heated at higher temperatures to cure the sand-resin mixture.
I have explained the process of metal pattern-making including all processes, materials and machines used in this process that mechanical engineers should read first.
The gating system is a part of the metal pattern, this increases the cost of manufacturing the pattern as compared to another moulding process.
Commonly used metal pattern materials are cast iron, iron, grey cast iron, magnesium, aluminium, graphite, stainless steel and copper.
Once the metal pattern is made the pattern is attached with the heating elements that heats the pattern for making shell mould.
The advantage of this metal pattern is that the multiple numbers of moulds can be mass-produced using one single pattern. Shrinkage observed of the finished solidified shell mould is less with high tolerance
1st step in this process is to mix synthetic thermosetting resin and sand in the muller and fill it in a box called a dump box or blowing machine.Additives such as sulphur, iron oxide, cold dust, magnesium and magnesium oxide, calcium carbonate and plumbago are added to improve the surface finish.
Fine silica sand is the first preference when producing mould at a lower cost, but zircon sand has become very popular because is very small, round and fine, they have high thermal conductivity and faster cooling as compared to normal fine sand.
This box is partially filled with a mixture and kept ready to pour on the heated metal pattern.
This process starts shell mould-making starts with properly mixing sand and synthetic resin (up to 3%) evenly, this mixture should be dry, moistureless and free-flowing to produce high-quality porosity-free strong thin shell mould.
This process starts with heating the metal pattern from 150℃ to 380℃ as shown in the diagram (b) below.
(b) Heating Metal Pattern In Shell Casting ProcessAfter heating the metal pattern the pattern is inverted and clamped in the dump box. The base of the dump box consists of a mixture of fine silica sand and phenolic resin material.
The dump box is inverted as shown in diagram (c) below allowing the resin-sand mixture to fall on the heated metal pattern.
(c) Pouring Resin On A Hot Metal Pattern In Shell MouldingSand used in this process is zircon or silica sand which has a high melting point and having round grain structure to get a better quality high surface finish.
As soon as the synthetic resin and sand mixture comes in contact with the heat from the pattern the resin melts and fuses the sand particles together. This process takes 30 seconds to one minute to complete.
The shell takes the shape of a metal pattern, and complex small to medium-sized shape moulds can be easily mass-produced using this step.
The dump box is inverted again as shown below in the diagram to separate the excess sand-resin mixture from the metal pattern.
After this pattern is reheated from 300℃ to 350℃ in oven the gives additional strength to the shell pattern in the casting process.
Ejector pins in the metal pattern eject the shell mould and separate it from the metal pattern as shown in diagram (d) below.
(d) Shell Pattern Made From Resin-Sand MixtureShell made from this process has a gating system in it because the metal pattern used here is just not a pattern but has gating elements such as sprue, runner and gates attached part of it.
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Exactly the same shell is made again by the same process and assembled together to form one solid shell mould by clamping, clipping and using thermosetting plastic adhesive.
Mould and cores which are produced are also assembled by fasteners. Thermoset adhesive or glue can also be used to clamp the mould.
Mould is inserted into the flask and surrounded by sand, gravel and steel shot for supporting mould. These are actually called backing materials. Sprue, runner and gate are part of the mould and do not need separately produced.
Shell made from this process are thin from 0.3 mm to 0.8 mm thickness with a mixture of adhesives, silica fine sand and resin material.
This mould is better than the sand casting moulds where coarse and angular grain sand also gives uneven undesriable rough surface texture and roughness.
That is why the shell moulding process is efficient and popular for producing sound-quality products as round sub-angular fine grains are used in this process.
Now shell mould is prepared and is ready to pour molten metal into the mould to form high-quality casting with low porosity, cracks, blowhole defects, airholes, sand inclusions, hotspot defects, accuracy, cold shut and better fitment and surface finish than sand casting.
The process of making cores is similar to making any other cores in the casting process in a core box. The difference is that shell cores are heated in the core box with a fine sand and resin mixture, making it very dimensionally accurate.
Care is taken that the core sand is without any lumps and painted for a smooth flow of molten metal around it with carbonaceous material to avoid metal penetration of molten metal in the core.
Cores are baked in the oven above 100℃ to gain their strength and hardness before placing them in the mould cavity.
Cores used in this process can be solid and shell cores depending upon the final application.
These days shell core-making machines are readily available that shoot the ready pre-coated cores out of the machine which can be directly used in the process.
Cores used in this process should have low liner thermal expansion, high permeability, high thermal resistance towards temperature, geometry dimensional stability, durability, easy shakeout operation and collapsibility.
The mould used in this process is lighter than any other casting mould such as die casting, investment casting, sand casting moulds and permanent casting moulds.
Once shell mould is ready it becomes important to pour molten metal into the thermosetting sand-resin mould.
The pouring operation starts with pouring molten metal into the shell mould and allowing the metal to flow through the sprue, runner, gates, mould cavity, and all gating system sections.
This shell is very thin and fragile for molten metal and needs to be supported uniformly. This shell is placed in the sand mould in the metal flask and supported by the metal shots as shown below in diagram (e).
(e) Sand-Resin Shell In Sand MouldThis machine is called a shell moulding machine where all the above frame, sand, backing material and gravel are placed.
This is a special casting shell mould with a cavity where the pouring operation of the molten metal into the shell mould takes place where metal solidifies around fine sand giving better texture and surface finish.
Metal is melted in the induction furnace and metal is poured into the shell mould through a ladle as shown in diagram (f) below.
(f) Pouring Molten Metal In Shell MouldProper selection of the moulding sand is necessary to avoid the deformation of the shell at high temperatures.
What materials are used to manufacture shell casting in the shell moulding process?
Casting products made from this process are made from ferrous and non-ferrous metals such as carbon steel, ductile iron, magnesium, bronze, gunmetal, cast copper alloy, aluminium alloy, nickel, titanium, chrome steel, cobalt, tin, stainless steel, copper alloy, zinc, brass, carbon steel, carbon alloy, malleable iron, compacted graphite iron, corrosion resistance steel, iron, cast steel, ductile iron (SG iron), grey iron, spheroidal cast iron, ni-resist cast iron, cr-hard cast iron, ni-hard cast iron and super alloy.
Metal is allowed to solidify uniformly in the shell mould to achieve directionally solidified casting with a uniformly distributed grain structure.
Once the dwell time is finished, the box is inverted to clear the remaining sand. Now shell is ready to be taken out by ejecting the shell from the metal pattern with the help of ejecting pins after the mould is cooled up to room temperature.
Shell mould is separated from the supporting elements and broken to separate the finished product from the sand to remove highly integrated casting.
The shakeout process of shell mould is easy and takes less time to separate the solidified component from the gating system.
Shell is further inspected for any defects, cracks, pores, bends, structural deformity, surface defects and dimensional inaccuracy.
Trimming, shaving and cleaning of the casting are required to remove any excess material from the edges of the solidified casting.
All metals including cast iron, super alloy and stainless steel are used for producing casting products.
The thickness of the wall shell varies from 3mm to 10mm, lowest thickness that can be produced is 0.25 mm. For casting weight up to 30kg, length up to 450mm and a production cycle of 50 parts/ hours.
Tolerance and dimensional accuracy achieved are±0.005 mm/mm and ±0.2 mm respectively.
The tensile strength of the shell is around 2.4 mpa to 3.1 mpa.
A smooth surface finish as fine silica sand is used (up to 3 microns to 6 microns)
What are the advantages, cons and merits of the shell moulding or shell casting process?
What are the disadvantages, demerits and cons of the shell moulding or shell casting process?
What are the uses, products and applications of shell moulding or shell casting process?
What is the difference between the shell casting and the green sand casting?
The surface accuracy and finish of shell casting are higher than the green sand casting process due to the presence of fine sand as compared to the course angular sand used in the sand casting process.
Porosity is low in shell casting because binding material (resin) and sand (silica, zircon etc) are dry without any moisture content and are fused at higher temperatures.
In the sand casting process moisture is the agent that helps bind sand and clay together in the mould making casting prone to porosity, pinhole defects and other gas defects.
Shell castings can be manufactured with better draft angle than sand castings. The draft angle for green sand casting is 2° to 4° while for shell casting is up to 2°.
The flexibility of setting up a sand casting process, moulding and manufacturing is high as this process can be carried out anywhere without any special tools, equipment and setup as compared to moulding.
The flow of molten metal in the shell mould is faster, better and smoother than green sand mould due to the presence, of course, grain structure in the sand mould.
The mould handling, fettling, inspection and testing cost of shell molding is lower than the sand casting process.
There is no need for shell mold to dry as compared to the sand molding process.
What is the difference between shell casting and investment casting?
Shell moulding has a lower surface finish than investment casting or lost wax casting, but a higher surface finish than sand casting.
The mould used in investment casting is made from ceramic but because of the wax pattern accuracy of the internal mould cavity is improved.
Shell casting produces less complex casting objects compared to investment casting products.
Shell moulding process is easier than investment casting production as the steps involved are fewer as explained below and the investment required is of low tooling cost.
In the investment casting method, a die cavity is made first to produce the wax pattern. This wax pattern is assembled in the tree structure dipped into slurry and solidified. Wax is drained out and molten metal is poured into the ceramic mould.
What is the difference between the shell casting and the die-casting?
Die casting is used for manufacturing casting like engine blocks in large quantities as compared to shell moulding where mass production is difficult but castings are manufactured in batches.
Shell casting products have a higher surface finish than die casting but there are limitations over the size of the casting manufactured.
Production setup for producing die-casting components requires a higher initial investment than shell-casting products.
Flexibility in producing mould cavities is better than die casting.
This process was invented in Germany by Johannes Croning, that is the process is also called croning after the inventor's name.
Shell moulding is the process of manufacturing casting with higher dimensional accuracy and surface finish by using "shell mould" made out of sand and resin.
High-quality mould made out of fine sand and resin (binding agent) is what makes "shell mould" better than "sand casting mould."
This process is called the shell moulding process because foundry engineers manufacture shells to pour molten metal into the mould cavity made from a sand-resin mixture.
This process fills the gap between sand casting and investment casting in terms of precise dimensional accuracy, geometry tolerance, surface finish, production rate, low labour cost and short lead time.
The limitation of this process is that large components are impossible to form in this process and small to medium size casting parts with less weight are best suited.
This is because the material strength of the shell mould is not strong enough to hold the molten metal volume.
For further casting larger parts precise investment casting process is used as a replacement for the shell moulding process for casting with large sizes and higher weights.
Moulds used in this process are strong, have good collapsibility and permeability, process can be coreless, have a short lead time, are easy to make and break, are moisture-free have 50% less draft angle than sand castings.
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