Pattern material – Advantage, limitation and application

1. Easily worked, formed and joined
2. Light in weight
3. Strong, hard and durable
4. Resistant to wear and abrasion
5. Resistant to corrosion, and to chemical reactions
6. Dimensionally strong and unaffected through variations in temperature and humidity
7. Available at low cost
   

Some materials used for making patterns are: wood, metals and alloys, plaster of Paris, plastics and resins, and wax.

Wood:

Teak wood, rose wood, white pine are used. Laminated wood sheets used for getting accuracy, surface end and long life.

Advantages: 

Light in weight, Economical and easily available, easy to to convert in required shape and sizes, easily smoothened through varnishes and paints, may be preserved for an extended time.

Limitations: 

Absorbs water from sand and changes its shape, nonuniform structure, high wear and tear, Not used for mass manufacturing.

Metals and its alloys:

• It is used when pattern are made in mass manufacturing with extra accuracy.
• It overcomes nearly all the shortcomings of wooden. There are some limitations additionally in the use of metals.

Limitation:

• In contrast to wooden it’s miles costlier.
• In this situation machining is needed on the way to boom the price of sample.
• It is heavier in comparison to wood sample.
• It is affected by atmospheric corrosion due to which treatment is a should which boom the price.

a. CAST IRON:

It is economical. It may be casted to any shape having true machinability, resistance to abrasion, gives higher surface finish, it is very heavy.

b. BRASS:

Used for making patterns of smaller sizes. It has greater strength, more resistance to corrosion may be machined very easily, suitable for good surface finish, may be casted into any shape. It is heavier than cast iron.

c. ALUMINIUM:

Pattern of larger sizes are made through this metal. Since the weight is much less and it is economical. It may be casted and machined without problems for higher finish. It isn’t as stronger as different metals.

Plaster:

Plaster of paris may be casted very easily to any shape. It has a totally excessive compressive strength and may be used to make patterns of smaller sizes with near dimension control. It has the property that it expands on solidification.

In case proper plaster is chosen the effect of shrinkage is automatically neutralized. Plastics and resins Plastic is used for creating a pattern because of the properties lighter in weight More strength with lesser wear, gives higher end and low shrinkage during melting also mot much costlier. Plastic which are used for pattern making are thermosetting resins and phenolic resins.

Wax:

It primarily utilized in investment castings. Good surface end and high dimensional accuracy, cost very less, only used for making small patterns. Commonly used waxes are paraffin wax, shellac wax and microcrystalline wax.

PATTERN ALLOWANCE:

Pattern allowance is a essential feature because it affects the dimensional characteristics of the casting. when the pattern is produced, certain allowances should be given on the sizes specified in the finished component drawing in order that a casting with the specific specification may be made. The choice of accurate allowances greatly allows to reduce machining costs and avoid rejections.

The allowances generally considered on patterns and center boxes are as follows:

1. Shrinkage/Contraction allowance
2. Draft or taper allowance
3. Machining or end allowance
4. Distortion or camber allowance
5. Rapping allowance.