How things work
How Patterns Are Prepared For Casting, Types Of Patterns And Pattern Allowance
How things work
The following article emphases mainly on how used in metal casting are prepared, what are their types and various allowance to be provided while making a pattern. It also provides a brief explanation about the materials that are to be used for pattern making to ensure a longer life.
What is Casting?
Casting is a process which has been in use since many years; it is used to produce various shapes using molten metal. In this process metal is melted to molten form and is then poured in a mould cavity, this metal on solidification takes the shape of cavity. The obtained shape is known as casting and method is known as Founding.
Patterns and Pattern making
Pattern is a model of the object that is to be cast and is used for making mould cavity. Patterns are used to prepare a mould cavity by sand moulding. Patterns are made of different materials like wood, plastic, or metal depending on the type of casting and size of casting.
Types of patterns
Various types of patterns are used depending on the type of casting, size and method of casting. Some of the commonly used patterns are:
Single piece pattern
As the name states it is made of a single piece. It is the simplest form of parent used in casting. This type of pattern is used for the castings that are simple and symmetric in shape. Single piece patterns are usually made of wood and are used for small scale production. This type of pattern is also known as Solid piece pattern.
Split piece pattern
The patterns which are complicated in shape and whose depth is too high are split in to two or more parts to have an ease in withdrawing the pattern from the mould. For two piece patterns one piece is placed in the cope box and the other in drag box, these two boxes are aligned by using dowel pins. When shape is more complicated we can even use three piece patterns. This is also known as 'multi piece pattern'.
Loose piece pattern of casting
Loose piece patterns are used when some irregularity is present on the contour of the pattern, making a difficulty in withdrawing the pattern from the mould. So to overcome this difficulty the irregular part of the pattern is held loose against the contour of the pattern using dowel pins or wire. By using such arrangement during the withdrawal we can first withdraw the main pattern which is regular in shape and then the loose pieces can be removed without damaging the contour of the casting from the gap provided by the main pattern.
Match plate pattern of casting
In this type the actual pattern is made in two halves and is then mounted on the two faces of a universal match plate, which acts as the parting line between the two pattern pieces. Match plate is also fitted with gating system making as ease for moulder, and reducing the mounding time. When casting is very small many patterns can be mounted on the same match plate reducing the number of moulds to be prepared. Match plate is usually made of metal like aluminium. When the match plate is withdraw the cope and drag boxes can be joined to form the complete mould cavity.
Sweep pattern of casting
Sweep patterns are used for the castings which are symmetrical and regular in shape like cylinder or pipe etc. This type of pattern is used when the castings required are very large. In sweep pattern a simple sweep which is rotated about a central axis is used to produce a mould cavity. A sweep pattern is shown in the figure below.
Materials used for pattern making in casting
The most commonly used materials for pattern making are:
Wood is the widely used material for pattern making because it has following properties,
It is available very easily.
It has low weight.
It is easy to work with wood.
It is relatively cheap.
Wood also has some disadvantages like,
It shrinks when exposed to moisture.
More wear & tear due to moisture.
No proper surface finish when used in large scale production.
Metals and alloys:
Metals used for pattern making are cast iron, aluminum and copper alloy. Choose of metals depends widely upon quality and quantity of mould to be produced. For large scale production cast iron is used as it is cheap, easy to machine and wear resistant. Aluminum alloys are used for small scale production as it has lesser wear resistance.
Different pattern allowances in casting
Different types of pattern allowance are allotted before making a final pattern to overcome some problems faced by metals solidification and to obtain best casting. Following are the different types of allowance:
As we know, when the metal is heated to its liquid state and then allowed to cool after pouring it into a mould cavity, it shrinks from its actual size due to phase transformation from liquid state to solid state. To overcome this shrinkage the patterns are provided with shrinkage allowance. Table below shows the allowance required for different metals.
The draft allowance is defined as the tapering which is provided on the pattern. When the sides of the pattern are deeper, it may damage the walls of the pattern while with drawing it from the mould cavity. To overcome this problem a taper or slight inclined surface is furnished on the sides of the pattern for the easy withdrawal of pattern from the mould. Table below shows the values of taper to be provided for easy removal.
when an object is prepared using casting, the surface finish we get is very bad and rough. To overcome this issue some extra metal is provided on the surface to machine and make the surface smooth. This allowance is known as machining allowance.
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