Sunday, 7 August 2011

FACTORS INFLUENCING MATERIALS SELECTION

A number of questions need to be answered before a decision can be made as to the specification required of a material and hence a decision as to the optimum material for a particular task. The questions can be grouped under four general headings:
1.    What properties are required?
2.    What are the processing requirements and their implications for the choice of material?
3.    What is the availability of materials?
4.    What is the cost?
The following indicate the type of questions that are likely to-be considered in trying to arrive at answers to the above general questions.

Properties:
1.  What mechanical properties are required?
This means consideration of such properties as strength, stiffness, hardness, ductility, toughness, fatigue resistance, wear properties, etc. Coupled with this question is another one: Will the properties be required at low temperatures, about room temperature or high temperatures?
2.  What chemical properties are required?
This means considering the environment to which the material will be exposed and the possibility of corrosion.
3.  What thermal properties are required?
This means consideration of such properties as specific heat capacity, linear coefficient of expansion and thermal conductivity.
4.  What electrical properties are required?
For example, does the material need to be a good conductor of electricity or perhaps an insulator?
5.  What magnetic properties are required?
Does the material need to have soft or hard magnetic properties or perhaps be essentially non-magnetic?
6.  What dimensional conditions are required?
For example, does the material need to be capable of a good surface finish, have dimensional stability, be flat, have a particular size, etc.

Processing Parameters:
1.    Are there any special processing requirements which will limit the choice of material?
For example, does the material have to be cast or perhaps extruded?
2.    Are there any material treatment requirements?
For example, does the material have to be annealed or perhaps solution hardened?
3.    Are there any special tooling requirements?
For example, does the hardness required of a material mean special cutting tools are required?

Availability:
1.  Is the material readily available?
Is it, for example, already in store, or perhaps quickly obtainable from normal suppliers?
2.  Are there any ordering problems for that material?
Is the material only available from special suppliers? Is there a minimum order quantity?
3.  What form is the material usually supplied in?
For example, is the material usually supplied in bars or perhaps sheet? This can affect the processes that can be used.

Cost:
1.    What is the cast of the raw material? Could a cheaper material be used?
2.    What quantity is required?
What quantity of product is to be produced per week, per month, per year? What stocking policy should be adopted for the material?
3.    What are the cost implications of the process requirements?
Does the process require high initial expenditure? Are the running costs high or low? Will expensive skilled labour be required?
4.    What are the cost penalties for over specification?
If the material is, for example stronger than is required, will this significantly increase the cost? If the product is manufactured to higher quality than is required, what will be the cost implications?

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