Pattern used in casting

Pattern used in casting

What is pattern :-
pattern is en element which is used to make cavities in the mould and molten metal is poured in to cavities to produce a casting.
Materials used for patterns:-
The common materials used in pattern making are,
1.wood
2.metal
3.plastic
4.gypsum or quick setting compounds.

1.wood:-
The most commonly used pattern material is wood, the main reason being the easy availablity and the low weight. Also it can be easily shaped and is relatively cheap. But the main disadvantage of wood is its absorption of moisture as a result of which distortions and dimensional changes occur. It is most commonly used for making pattern of sand casting. It is worked easily and readily available.
The different types of wood commonly used for making patterns are pine, teak, maple, birch and cherry.

Advantages:-
1.cost of wood is low compered to their pattern materials.
2.easily available and has low weight.
3.good surface finish can be easily obtained.

Disadvantages:-

1.dimensional inaccuracy.
2.non-uniform in structure.
3.they cannot withstand rough handling.


2.metal:-
metal being more durability and smooth surface finish. Metal patterns are extensively used for large scale casting production and for closer dimensional tolerances. Metal patterns are usually used in machine moulding.
The different types of commonly used metals are castiron,brass, aluminium..etc can be used as pattern materials aluminium and white metal are most commonly used. These are light can be easily worked and are corrosion resistant.

Advantages:-
1.good surface finish.
2.more durable and accurate in size than wood.
3.it can withstand rough handling.

Disadvantages:-
1.costly.
2.not easily repaired.
3.they get rusted.

3.plastics:-

plastic are also used as pattern materials because of their low weight easier formability,smooth surfaces and durability. They do not absorb moisture and are therefore, dimensionally stable and can be cleaned easily. The making of a plastic pattern can be done in sand clay moulds. The most commonly used plastics are cold seting epoxy, polyester resins, phenol formaldehyde… etc most widely used.

Advantages:-

1.makes production process very easy.
2.it does not absorb moisture.
3.surface of patters are smooth.

Disadvantages:-
1.comparatively higher costs of material.
2.cannot withstand high temperature.

4.gypsum:-
gypsum patterns are capable of producing castings with intricate details and to very close tolerances. The main two types of gypsum are soft plastics of paris and hard metal casting plaster. The main characteristics of gypsum are mobility, plasticity and reparability.

Construction of a pattern:-

After finishing the pattern by giving various allowances, the next step of the pattern maker is the shaping of the pieces that are to be used in the construction of the pattern. The method required to be followed depends upon the size and shape of the pattern and number of costings required.
Procedure for pattern constructions.

1.decide the location of parting surface after stusying the layout.
2.after visuolising the shape of the pattern determine the number of separate pieces to be made and the process to be employed for making them.
3.start construction of pattern from the main part of the body. Keep the direction of wood grains along the length of the pattern to get acuracy and strength.
4.different parts are provided adequate draft while cutting and shaping.
5.check all the prepared parts finally by placing them over the prepared layout.
6.outside corners should be rounded.assemble different parts in position by gglusinf or by means of dowels as the case may be:
7.fit all the fillets at desired places.
8.check the relative locations of all the parts assembled on the pattern.

Types of patterns :-

There are various types of patterns depending upon the complexity of the job, the number of castings required and the moulding procedure adopted.

(1) single piece pattern (solid) :-

A single piece pattern is made of a single piece and it is not attached to frame. It is also known as solid piece pattern.

(2) Two piece pattern ( split pattern) :-

Two piece pattern are usually made in two parts and this is the most used type of pattern for intricate castings. For making a mould using two piece pattern, kept one part of pattern in the drag and other in the cope. It is also known as split pattern.

(3) Gated pattern :- 

This is an improvement over the single pattern where the gating and runner system are intefral with the pattern a number of castings by joining gates or runners between the group of pattern.

(4) cope and drag pattern :- 

These are similar to split patterns cope and drag patterns are a combination of two or more pieces. These pattern are also known as built-up patterns. These types of patterns are used

(5) sweep pattern :- 

Sweep pattern are used to produce symmetrical moulds. It consists of a wooden board which rotate about a central axis. This types of pattern are used for very large castings.