What is pattern | Types and function of pattern

What is a pattern? Define pattern.

1. The pattern is a replica or model of the component to be cast.
2. The pattern is a component of casting shape without allowance.

types of pattern

Criteria for Selection of Pattern Material:

 The following are the important points to be considered while selecting pattern material.

1. The number of castings required.
2. Types of the moulding process are used whether machine or hand moulding. 
3. Size and shape of the component to be cast. 
4. Accuracy and surface finish required. 
5. Types of casting such as sand casting or investment casting. 
6. Probability in design changing. 
7. Probability of repeat order. 
8. Type of complication in manufacturing after casting. 

Requirements of Pattern Material

The following are the requirements for good pattern material : 

(I)  Required size and shape easily processed.
(ii) Light in Weight.
(iii) High strength and longer life and resistance to wear, humidity or chemical process. 
(iv) The dimension will not be changed due to heat and cold. 
(v) Easily available and cheapest in cost. 
(vi) Easily repairable.
(vii) Better surface finish can be achieved.

The following are the functions of pattern

(i) Required size and shape cavity is prepared so while pouring metal in the cavity the required dimension components are achieved.

(ii) If a hole is required in casting, the core in a mould is used. Adjustment in mould required for placing Core. Core print is used for projection in core placing.

(iii) Parting surface and line are established by the pattern in the mould.

(iv) Special locating points are established by the pattern in the mould. These points are used for the measurement of casting, machining and surface location to each other.

(v) Minimum defects in casting depend on the pattern.

(Vi) Cost is minimized by proper pattern selection.

(vii) The better surface finish on casting. 

Types of Pattens

The types of patterns used depend upon the complexity of the shape, design of casting, the number of castings required, moulding process, surface finish, and accuracy. All the above-mentioned points are to be considered and based on these different types of patterns are : 

1.  Solid or single piece pattern 
2.  Two-piece pattern or split pattern 
3.  Multipiece pattern 
4.  Match plate pattern 
5.  Skeleton pattern 
6.  Sweep pattern 
7.  Pattern with loose piece 
8.  Cope and drag pattern 
9.  Shell pattern 
10. Gated pattern 
11. Follow board pattern
12. Boxed up pattern

1. Solid or single piece pattern :

Solid or single piece pattern
Solid or single piece pattern

The solid pattern is made up of joints. The mould cavity of this pattern is either in drag or in the cope. In this type the gating system, runners/risers will be made manually. Therefore it is time consuming, single piece pattern generally used for casting of simple shape, example stuffing box of a steam engine, gland, etc.

2. Two-Piece/split pattern : 

Many types of patterns cannot be made in a single piece because of difficulties encountered in the moulding operation.
(e.g. withdrawing the pattern from the mould). 
Suppose the spherical pattern is to make, it cannot be done in a single piece pattern.

Two piece or Split pattern
Two piece or Split pattern

The split patterns are basically made in two parts as shown in fig.  That one part may produce the lower part of the component and the other will be the upper part or component.

Such objects can be cast with the split pattern only. The components like steam valve body, spindle, water valve, and small pulleys, etc. are manufactured by using this pattern. 

3. Multipiece pattern : 

Multipiece pattern
Multipiece pattern

As discussed above in two parts of the split pattern that for complicated work-piece, more than two parts are used called multipiece pattern. Under this pattern, the component is divided into three or four parts as shown in fig. This process is somewhat complicated than two parts because mould is prepared in two or more boxes. 

4. Sweep pattern :

Big shape, size, symmetrical and round cross-section components are made from sweep pattern. is not a true pattern, but a template made from Wood or metal revolving around a fixed axis in the mould shapes the sand to the desired contour. It is suitable for producing large or medium size by symmetrical castings. 

Sweep pattern
Sweep pattern

The figure shows a sweep pattern. In this process, the sweep pattern is placed inside the green sand and revolving a required mould is prepared. This is a time saving and economic process. 

5. Pattern with loose pieces :  

Certain patterns can not be withdrawn once they are embedded in the moulding sand. Such patterns are usually made with one or more loose pieces for facilitating their removal from the moulding box and are known as loose piece pattern. In this case, the main pattern is removed first and then the loose pieces.

Pattern with loose piece
Pattern with loose piece

Loose pieces remain attached with the main body of the pattern, with the help of dowel pins. When the mould is made, such loose pieces remain in the mould until the main pattern is withdrawn. Then the loose pieces are taken out separately through the cavity formed by the pattern as shown in fig.

Also Read: Single Plate Clutch

6. Cope and drag pattern :

Cope and drag is another form of split pattern. This pattern is made up of two halves, which are mounted on different plates. In this case, cope and drag parts of the mould are made separately and then assembled as shown in fig.

Cope and drag pattern
Cope and drag pattern

These are used for very large castings. In this pattern, both boxes are prepared by a separate operator. So it is economic and fast.

Each metal plate, besides the pattern, has the provision for moulding runner and gates.

7. Shell pattern :

Generally, this pattern is made of metal. It is hollow and in two parts. In this outer part is pattern and the inner part is core or core boxes. To connect both the parts the dowel and slots are provided, so it is accurately connected.

Shell pattern
Shell pattern

This pattern is useful in drainage fitting, pipes, and bends.

8. Follow Board Pattern : 

A follow board is a wooden board and is used for supporting a pattern that is very thin and fragile and which may give away and collapse under the pressure when the sand above the pattern is being rammed.

Follow board pattern
Follow board pattern

In addition to supporting a thin sectioned pattern, a follow board forms the natural
parting line of the mould.

Fig shows follow board, pattern and mouldboard. Actually follow board acts as a seat for the patterns. A mould box is filled with sand, box is accurately turned and follow board is removed.

9. Boxed up Pattern : 

The boxed up pattern is manufactured by joining small wood strips with the help of nails, glue or screw, etc. The inside portion is hollow, it is light in weight.

For large pattern, it is very economic and due to light in weight, it is convenient to handle. It is used for regular outline and rectangular heavy components.

10. Match Plate Pattern :

This pattern is used for small components that required better accuracy and in large quantity and also make the production process fast. Cope and drag parts of the pattern are mounted along with the gating system on opposite sides of a wooden or metal plate. More than one pattern is also attached to a wooden plate. 

Match plate pattern
Match plate pattern

This is used in machine moulding, it is economic and fast. Match plate can be manufactured from wood, steel, aluminium, magnesium, etc. 

11. Gated Pattern : 

In a gated pattern, gates and riser for producing casting are included in the pattern itself. The uses of gated a pattern eliminate the time required to cut the gating system by hand. These are suitable for small quantity production.

Gated pattern
Gated pattern

12. Skeleton Pattern : 

The pattern for very large castings would require a large amount of pattern material, in such cases skeleton patterns are used.

Skeleton patterns are hollow patterns and made up of a wooden frame and strips.

The framework is filled and rammed with loam sand and a strickle board is used to remove the excess sand. The Strickle board is having the same contour as desired so it can remove the extra sand by passing over the skeleton pattern.

Skeleton pattern
Skeleton pattern

Cores, if necessary can be produced separately.

A Skeleton pattern is very economical as compared to a solid pattern because it involves less material cost. 

Types of pattern allowances:

A pattern differs from casting in certain dimensions. When the pattern is prepared, certain allowances are given on the sizes of casting. These are known as pattern allowances. The following are the different pattern allowances.

  1.  Shrinkage allowance
  2.  Machining allowance 
  3.  Draft allowance
  4.  Distortion allowance
  5.  Rapping allowance 
What is pattern | Types and function of pattern What is pattern | Types and function of pattern Reviewed by Harish on April 09, 2020 Rating: 5

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