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Shell Pattern In Casting

Shell Pattern In Casting

In the sand casting process, there is a requirement for producing small and medium size casting with higher surface finish, dimensional accuracy, complex shape and tolerance.

At this point using a regular pattern does not serve the purpose of the customer. To overcome this customer's requirement shell patterns in the casting process are used.

As the name implies this process consists of manufacturing a shell made from a mixture of thermosetting resin and fine sand producing highly accurate and complex shape casting.

I have written a detailed article below about the pattern and shell mould used to produce the casting. 

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What is a shell pattern and what it is used for?

Shell pattern can be defined as a pattern used to produce large-size casting like drainage pipes and fitting by the mounting metal pattern on a plate on both sides along parting lines dowelled together accurately.

The pattern is used to make bent hollow pipes with a desired thickness.

The outside of the pattern is used to make a mould cavity while the inside of the cavity is used as a core called a block pattern.

The shell pattern makes shell mould whereas molten metal is poured into the shell to make the final casting.

Shell moulds are a replica of the metal pattern, for low volume production aluminium as a patterned material is used for producing casting.

The pattern used in the shell moulding process is made from metal.

Steps used to make shell mould using shell pattern.

• Step 1- Shell mould is made by making a split metal pattern made of cast iron and steel. Graphite is also a patterned material used to reduce the reactiveness of molten metal with it.
• Step 2- The metal pattern is heated at approximately 232 ℃.
• Step 3- Thermosetting resin and sand are poured on the heated metal pattern by shell making machine as shown below in diagram (a). This step is called the "investment cycle".

• 

  (a) Resin On Metal Pattern

  
  
• Step 4- Thermosetting resin, sand and pattern are heated in the oven.
• Step 5- The mould is allowed to cool and the mould shell as shown below in diagram (b) is taken out of the metal pattern.

  

  (b) Shell Pattern

  
  
• Step 6- Cores are attached to this shell mould further in the flask when both sides of the shell are assembled together.
• Step 7- Molten metal is poured through sprue to make hollow pipes and fittings using the shell mould in the sand casting process as shown below in diagram (c).

  

  (c) Shell Mould

Shell Pattern In Casting Advantages

• Hollow pipes of desirable thickness can be manufactured.
• The metal patterns can be reused multiple times to make resin-sand shell mould.
• The metal pattern gives higher dimensional accuracy than any other pattern material.
• Smooth large, curved bend pipes can be quickly produced because of the resin mould.
• Multiple shell moulds can be mass-produced on an automated moulding machine.
• Complex geometry shapes can be produced as compared to loose piece patterns and match plate patterns.
• The gating system and feeders are part of the shell pattern-making process.
• The surface finish obtained from this pattern is high compared to all the patterns because molten metal is not directly poured by pattern impression but in the mould made from resin.
• Better dimensional accuracy can be achieved than match plate pattern and cope & drag pattern.
• Complex geometry parts can be produced if metal patterns are manufactured in CNC taking care of dimensional accuracy and tolerance.
• The shell pattern is moisture-free as it is cured and free from moisture-related defects, as seen in the wooden pattern.

Shell Pattern In Casting Disadvantages

• The process needs to be automated for higher volume production.
• Multiple steps are required to produce a pattern-making process length and time-consuming.
• The thermosetting resin used for making patterns is expensive.
• Asymmetrical parts are difficult to produce.
• Equipment cost is high compared to all patterns used in the casting process.
• Used to produce small to medium-sized casting. Large-size casting cannot be delivered as we see in the skeleton pattern.
• Prone to porosity, shrinkage defect and distortion defect.
• If the process is automated on the mould machine there are limitations over the size of the final pattern.
• The pattern material should have high resistance to thermal gradient.

Shell Pattern In Casting Applications

The shell pattern is used to create shell mould. The following products are made from those moulds. Examples of products produced by shell patterns are as follows:

• Drainage hollow pipes.
• Bend and curved pipes.
• Pipe fitting.
• Connecting rod.
• Gear housing.
• Cylinder heads.
• Lever arms.

Shell Pattern MCQ

1) Which of the following patterns produces sand-resin mould?

a) Metal pattern.
b) Wax pattern.
c) Sand pattern.
d) Plastic pattern.

Answer - a) Sand resin mould can be produced only using a metal pattern, this is because the pattern needs to be heated to a certain temperature so that sand-resin slurry mixtures cures and binds together.

 

2) Arrange the following steps in sequence for making shell mould using the metal pattern in the casting process.

a) Withdraw mould from the machine.
b) Pour slurry on the metal heated pattern.
c) Prepare a mixture of sand and resin to make a consistent slurry uniform paste.
d) Allow resin and sand to fuse to produce a strong thin shell mould.

Answer - 

c) → b) → d)→ a)

 

3) In which of the following patterns thin mould is movable and placed in a sand mould box while the heated metal pattern remains static.

a) Solid pattern.
b) Boxed-up pattern.
c) Follow board pattern.
d) Shell pattern in sand casting.

Answer - d) This is a very interesting process in which metal is poured into an assembled split mould made by a metal pattern.

 

4) Which of the following terms is not associated with the shell pattern. 

a) Split metal pattern.
b) Heated metal pattern.
c) Match plate.
d) Dumb box.

Answer - c) There is no relation between the match plate and shell pattern. A match plate is associated with a match plate pattern.

 

5) The following types of final casting qualities are produced using a shell pattern in the casting process.

a) Higher surface finishing than the traditional sand casting process.
b) Better accuracy and dimensional tolerance reduction after casting machining.
c) Complex casting production qualities for intricate small and medium-size casting.
d) All of the above options are right for shell casting.

Answer - d) All the above points exactly describe the qualities of the final casting produced by shell moulding using a shell pattern.

Shell Pattern In Casting Summary

The shell pattern is one of the patterns among 17 types of pattern in sand casting.

The shell moulding process has practically one pattern and one thin resin-sand mould, heated metal pattern and shell mould. This shell mould (Which stays in the sand mould and is not withdrawn) is placed in the sand mould box to make a final casting. In this process, sand is only used to back the present shell mould (pattern).

The shell pattern is a replica of the final casting to be produced which is not dependent on the craftmanship for every pattern.

A mixture of thermosetting resin is poured on the heated metal pattern to make a shell mould in the manufacturing process which gives better dimensional accuracy than the conventional sand casting process.