Pattern Materials

 PATTERN MATERIALS

The following factors are to be considered for the selecting pattern materials.

● Design of casting

● Number of castings to be produced

● Degree of accuracy and surface finish required

● Shape, complexity and size of the castings,

● Casting or moulding method adopted.

Patterns are made by different materials which have their own advantages, limitations and their field of applications.

The commonly used pattern making materials are given below

1. Wood - Teak wood, mahogany, white pine, etc.

2. Metals - Cast iron, brass, aluminium, white metal etc.

3. Plaster

4. Plastics

5. Wax.

1. Wood

Wood is the material commonly used for pattern making. Generally, teak wood, mahogany, white pine, rosewood are used for making patterns. The laminated wooden sheets are also used for getting accuracy, surface finish and long life. These woods should not contain more than 10% moisture to avoid wrapping and distortion during subsequent drying. Metal spray coating up to 0.25 mm thick may be given on wooden pattern. Zinc and aluminium are used for coating the metals on the wooden surface to avoid moisture absorption and good surface finish.

Advantages:

1. It is light in weight, cheap and easily available.

2. It is easy to work, easy to cut and easy to fabricate.

3. It can easily be repaired.

4. It can easily be smoothened by varnishes and paints.

5. Good surface finish can be obtained.

Limitations:

1. It absorbs water from sand and changes its shape.

2. It has non-uniform structure.

3. It has high wear and tear by sand. Hence, it cannot be used for mass production.

4. It cannot be used in machine moulding.

5. It is not possible to produce moulding of high accuracy.

2. Metals

The metal pattern is used when a larger number of castings are to be made. Metal patterns can either be cast from a masterpiece or may be machined by the usual method of machining. These patterns are generally used in machine moulding.

Advantages:

1. It has long life and accurate in size.

2. It has smooth surface.

3. Mass production is possible.

4. It does not absorb moisture and deform in size.

5. It can be used for rough handling.

6. It is resistance to wear, tear, abrasion and corrosion.

Disadvantages:

1. It is costlier and heavier than wood.

2. It cannot be easily repaired.

3. Ferrous patterns can get rusted.

4. It is difficult to make the required shape.

The materials commonly used for pattern making are as follows:

1. Cast iron:

Cast iron having fine grain can be used as a pattern material. It has high resistance to sand abrasion and has smooth surface but it is heavier and difficult to work. Its cost is less and more durable than other metals. It is brittle and can be easily broken. It will get rusted by moisture unless it is protected.

2. Brass:

It may be easily worked and built up by soldering or brazing. It is used only for small size patterns because of high cost. It is very strong and not affected by moisture. It provides a very good surface finish and withstands wear and tear. The product can be made with very high accuracy.

3. Aluminium:

Aluminium is the best material because it is light in weight, strong and easily machined. It can be made with high accuracy and good surface finish. It is not affected by moisture and gets rusted and also melts at low temperature. It is very soft and easily damaged by rough surfaces. It is subjected to shrinkage and also wears by abrasion.

3. Plaster

Plaster of paris or gypsum cement is used for preparing hollow boards for moulding the work. Plaster can be easily made into difficult shapes i.e. easily worked. It has high compressive strength. It is used only for small patterns. It is affected by moisture.

4. Plastics

A plastic pattern has many advantages over other materials. The plastic pattern is cast from a wooden pattern called master pattern. It is light in weight but strong. It is not affected by moisture and more resistant to wear. It has very smooth glassy surface. It does not shrink much, it has high dimensional accuracy and it is more economical than other metals.

The following plastics are widely used for pattern making with the composition based on epoxy, phenol formaldehyde and polyester resins.

● Poly acrylates

● Poly ethylene

● Poly Vinyl Chloride (PVC) etc.

5. Wax

Wax pattern is primarily used in investment castings. The commonly used waxes are paraffin wax, shellac wax and microcrystalline wax. It has good surface finish and high dimensional accuracy. It will not absorb moisture and easy to work. The cost is low but it can be used for making small patterns only.