Shell Casting Process
What is the shell casting process?
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| (a) Shell Moulding Casting Process |
The 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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Shell Casting Moulding Step-By-Step-Process
What is the step-by-step process, working and method to manufacture shell casting using the shell moulding process, explain it with diagrams?
STEP 1: Metal Pattern Making
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
STEP 2: Shell Mould-Making Process
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.
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| (b) Heating Metal Pattern In Shell Casting Process |
After 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.
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| (c) Pouring Resin On A Hot Metal Pattern In Shell Moulding |
Sand 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.
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| (d) Shell Pattern Made From Resin-Sand Mixture |
Shell 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.
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.
STEP 3: Shell Core Making
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.
STEP 4: Pouring Molten Metal Into Sand-Resin Shell Mould
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).
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| (e) Sand-Resin Shell In Sand Mould |
This 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.
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| (f) Pouring Molten Metal In Shell Mould |
Proper 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.
STEP 5: Shell Casting Solidification And Shakeout Process
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)
STEP 6: Finishing Operation Of Shell Casting
Shell Moulding Vs Investment Casting
| Shell Moulding | Investment Casting |
|---|---|
| 1) A metal pattern is used here to shape the ceramic shell mould to produce high dimensional accurate casting. | 1) The wax pattern in investment casting is used to shape ceramic mould for the investment casting process. |
| 2) The quality of surface finish is better than sand moulding but less superior than investment casting. | 2) The quality of the surface finish is higher than any other casting process. |
| 3) Mass production of shell casting is not possible as investment casting as branch-style cluster castings are manufactured using one common runner and multiple mould cavities. | 3) Mass production of castings is not possible as only one casting is possible to produce at a time from shell mould. |
| 4) The intricate details and complexity of casting to be produced are inferior to investment casting. | 4) The intricate details and complexity of casting produced from investment casting are superior to investment casting. |
| 5) Large-size casting can be produced as compared to investment casting as shell mould is split and assembled together. | 5) Small size casting is produced in clusters and large size casting is not possible. |
Shell Casting Advantages And Disadvantages
Shell Casting Advantages
What are the advantages, cons and merits of the shell moulding or shell casting process?
- Best for reliable small-size casting products produced where the requirement of surface finish, texture, draft angle and accuracy is better than sand casting products.
- Out of one metal pattern, multiple sand-resin shells can be manufactured.
- The quality and complexity of the casting products are superior to the sand moulding process because of the chilling effects.
- The floor space required for this casting is less than the die-casting machine and sand moulding process.
- Moulding sand and the time required for producing casting are less in terms of quantity as compared to the sand moulding process.
- The cost of thermosetting resin and fine sand is more costly than the sands such as raw sand, bentonite sand, olivine, chromite and coal powder used in the sand moulding sand.
- Prosity, gas defects, mould drop, swell and hotspot defects produced from this process are less as the permeability of the process is high.
- The mould used in this process is lightweight and does not require heavy material handling equipment as we see in other casting processes such as sand moulding, die moulding and centrifugal casting process.
- High-accurate pre-coated shell cores can be used in this process to produce complex cavities, geometry and internal sections.
- Shell moulding produces high-quality net shape-casting having higher casting process yield with higher productivity and less material scrap.
- The shelf life of mould and cores used in this moulding process is higher than green sand mould as the sand-resin mixture is dry creating dry moulds.
- Shell and cores in this process are odourless and anti-peel best to use in the foundry.
- The process is less labour intensive and can be carried out using semi-skilled labour as compared to sand casting where the mould-making process requires a highly skilled experienced mould maker in the foundry.
- Solidified casting has lower machining, cleanup and secondary operation costs. This saves a lot of machining costs as compared to the sand casting process.
- Complex internal cavities can be produced without or with the use of shell cores.
- Shells of varying thickness can be manufactured resulting in casting with varying thicknesses that have sharp corners and turns.
- The gating system design is simple as there is no vent to escape gases out of the sand-resin mould and no need for a riser in the casting process.
- The durability of metal patterns is high compared to other pattern materials.
Shell Casting Disadvantages
What are the disadvantages, demerits and cons of the shell moulding or shell casting process?
- Limitations over the mould size to be produced, size does not match anywhere close to sand casting, die casting products and applications.
- To produce mid-size casting shell mould needs to be made separately and mould pieces need to be assembled together to form mid-size casting.
- Requires a higher initial investment than sand casting but less than investment casting.
- This process is an expandable casting process where the shell is broken to separate it from the mould.
- For every casting to be produced shell mould needs to be manufactured separately, multiple products cannot be cast from the same mould.
- Versatile expensive complex moulding tools are required to produce pattern and shell mould.
- Manufacturing metal patterns is not cost-effective for production orders of small-quantity casting as the pattern has gating elements also included in it.
- Shell moulds produced are for small-size casting, the production of dense casting with heavy weight cannot be produced as the shell mould will break and cannot sustain the high-weight castings that traditional dry sand moulds can easily handle.
- This process is not environmentally friendly as compared to sand mould because the sand-resin mixture has a high quantity of synthetic thermosetting resin material as compared to sand mould which is made from 100% natural sand taken from the river bed with few naturally occurring additives such as sawdust and charcoal.
- To seriously reuse natural fine sand, the resin needs to be burned at a high temperature evaporating the resin and recyling the ramining fine silica sand.
- The availability of resin material is less than other binding agents used in the sand moulding process such as clay.
- The process can be costly if the shell mould-making method is done manually without a machine as experience labour costs will increase.
- Less draft angle, better projection, better microstructure of casting metal and good details are possible.
Shell Casting Application
What are the uses, products and applications of shell moulding or shell casting process?
- Small spur gears for machines, gearboxes and rocker lever arms.
- Cable clamps, parts of the compressor, generators and injection system.
- Hydraulic water pumps body and cylinder head.
- Bevel gears, spur gear and gear blanks.
- Electric motors casing of electrical car.
- Sculpture, artefact, steel eyes and antique models manufacturing.
- HVAC, calliper and engine cooling systems.
- Piston, pinions, piston ring, flanges yoke and connecting rods.
- Ball bearing caps, mounting brackets and heads.
- Air-cooled cylinders, housing, base blocks and join unions.
- Chain wheel, drum sprocket, trap body and wheel cylinder
- Bushings of impellers, truck hooks and aluminium collectors.
- Elevator gears, turbine housing, crankcase and crankshafts.
- Camshafts, liner and body panel auto components.
- End yoke, damper shaft and combination square.
- Machine base, pipe bands, snap gauge body and transmission components.
- Hydraulic butterfly valve, disc and pneumatic valve bodies.
- Brake parts such as drums and turbocharger components, oil pumps and cartridge parts.
- Defence, automotive and agricultural equipment components.
- Part of oil refinery machines, aviation, marine, material handling, agricultural, mining machines and railways.
Shell Mould Casting FAQ
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.
Sell Mold Casting Process Conclusion And Summary
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.