Lecture5 Materials Selection and Material Processing in Design

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Material Selection and MaterialProcessing in Design

Eng. R. L. NkumbwaCopperbelt University

2010


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New Product Analysis

Everyday we use thousands of different products, fromtelephones to bikes and drinks cans to washing machinesand microwaves.

But have you ever thought about how they work or the waythey are made?

Every product is designed in a particular way -productanalysisenables us to understand theimportant materials, processing, economicand aesthetic

decisionswhich are required before any product can bemanufactured.

An understanding of these decisions can help us in designingand making for ourselves.


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Getting Started

The first task in product analysis is to become familiarwith the product! What does it do? How does it do it?

What does it look like? All these questions, and more, need to be asked before a

product can be analysed.

As well as considering the obvious mechanical (and

possibly electrical) requirements, it is also important toconsider the ergonomics, how the design has beenmade user-friendlyand any marketingissues - these allhave an impact on the later design decisions.


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Let's take the example of a bike

What is the function of a bicycle?

How does the function depend on the type of bike

(e.g. racing, or about-town, or child's bike)?

How is it made to be easily maintained?

What should it cost?

What should it look like (colours etc.)? How has it been made comfortable to ride?

How do the mechanical bits work and interact?


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Systems and Components

There are 2 main types of product - those

that only have one component (e.g. aspatula) and those that have lots of

components (e.g. a bike).

Products with lots of components we

call systems. For example:


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System Components

Product Components

Bike Frame, wheels, pedals, forks, etc.

Drill Case, chuck, drill bit, motor, etc.

Multi-gymSeat, weights, frame, wire,

handles, etc.


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Product Analysis

In product analysis, we start by considering

the whole system. But, to understand why various materials and

processes are used, we usually need to 'pull

it apart' and think about each component as

well. We can now analyse the function in more

detail and draft a design specification.


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Some important design questions

To build a design specification, consider questionslike the following:

What are the requirements on each part (electrical,mechanical, aesthetic, ergonomic, etc)? What is the function of each component, and how do they

work? What is each part made of and why? How many of each part are going to be made?

What manufacturing methods were used to make eachpart and why ?

Are there alternative materials or designs in use and canyou propose improvements?


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Design Questions

These are only general questions, to act as a guide - you willneed to think of the appropriate questions for the products andcomponents you have to analyse. For a drinks container, a

design specification would look something like: provide a leak free environment for storing liquid

comply with food standards and protect the liquid from health hazards

for fizzy drinks, withstand internal pressurisation and prevent escape ofbubbles

provide an aesthetically pleasing view or image of the product

if possible create a brand identity

be easy to open

be easy to store and transport

be cheap to produce for volumesof 10,000+


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Choosing the Right Materials

Given the specification of the requirements

on each part, we can identify the materialproperties which will be important - for

example:


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Choosing the Right Materials

Requirement Material Property

must conduct electricity electrical conductivity

must support loads without breaking strength

cannot be too expensive cost per kg


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Material Selection

One way of selecting the best materials would be to look up values

for the important properties in tables. But this is time-consuming, and

a designer may miss materials which they simply forgot to consider. A better way is to plot 2 material properties on a graph, so that no

materials are overlooked - this kind of graph is called a materials

selection chart (these are covered in another part of the tutorial).

Once the materials have been chosen, the next step is normally to

think about the processing options.


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Choosing the Right Process

It is all very well to choose the perfect material, but somehow wehave to make something out of it as well! An important part ofunderstanding a product is to consider how it was made - in otherwords what manufacturingprocesseswere used and why.

There are 2 important stages to selecting a suitable process:

Technical performance:can we make this product with thematerial and can we make it well?

Economics:if we can make it, can we make it cheaply enough?

Process selection can be quite an involved problem - we deal withone way of approaching it in another part of the tutorial.

So, now we know why the product is designed a particular way, whyparticular materials are used and why the particular manufacturingprocesses have been chosen.

Is there anything else to know?


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Wrap Up

Product analysis can seem to follow a fixed

pattern: Think about the design from an ergonomic and

functional viewpoint.

Decide on the materials to fulfil the performance

requirements. Choose a suitable process that is also economic.

Whilst this approach will often work, design is

really holistic- everything matters at once - so be

careful to always think of the 'bigger picture'.


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Example Analysis

Is the productperformancedriven or costdriven?

This makes a big difference when we choose materials.

In a performance product, like a tennis racquet, cost is one of thelast factors that needs to be considered.

In a non-performance product, like a drinks bottle, cost is of primaryimportance - most materials will provide sufficient performance (e.g.although polymers aren't strong, they are strong enough).

Although we usually choose the material first, sometimes it is theshape (and hence process) which is more limiting.

With window frames, for example, we need long thin shapedsections - only extrusion will do and so only soft metals or polymerscan be used (or wood as it grows like that!).


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Choosing between Different Materials

There are three main things to think about when choosing materials(in order of importance):

Will they meet the performance requirements?

Will they be easy to process? Do they have the right 'aesthetic' properties?

We deal with the processing aspects of materials in a different partof this course.

For now it is sufficient to note that experienced designers aim tomake the decisions for materials and processes separately togetherto get the best out of selection.

The choice of materials for only aesthetic reasons is not thatcommon, but it can be important: e.g. for artists.

However, the kind of information needed is difficult to obtain and wewon't deal with this issue further here.


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Material Selection

Most products need to satisfy someperformancetargets, which we

determine by considering the design specification e.g. they must be

cheap, or stiff, or strong, or light, or perhaps all of these things... Each of these performance requirements will influence which

materials we should choose - if our product needs to be light we

wouldn't choose lead and if it was to be stiff we wouldn't choose

rubber!

So what approach do we use to select materials?


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Using Material Selection Charts

So what we need is data for lots of materialproperties and for lots of materials.

This information normally comes as tables ofdata and it can be a time-consuming processto sort through them.

And what if we have 2 requirements - e.g.our material must be light and stiff - how canwe trade-off these 2 needs?

The answer to both these problems is touse material selection charts. Here is a materials selection chart for 2

common properties: Young's modulus(whichdescribes how stiff a material is) and density.


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On these charts, materials of each class (e.g. metals, polymers) form'clusters' or 'bubbles' that are marked by the shaded regions.

We can see immediately that:

metals are the heaviest materials, foams are the lightest materials, ceramics are the stiffest materials.

But we could have found that out from tables given a bit of time,although by covering many materials at a glance, competingmaterials can be quickly identified.

Where selection charts are really useful is in showing the trade-offbetween 2 properties, because the charts plot combinationsofproperties.

For instance if we want a light and stiff material we need to choosematerials near the top left corner of the chart - so composites lookgood.

Note that the chart has logarithmic scales- each division is a multipleof 10; material properties often cover such huge ranges that logscales are essential.


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To find the best materials we need to use theYoung's modulus - density chart from amongst the

available charts. The charts can be annotated tohelp reveal the 'best' materials, by placing asuitable selection boxto show only stiff and lightmaterials.

What can we conclude?

The values of Young's modulus for polymers are low,so most polymers are unlikely to be useful forstiffness-limited designs.

Cambridge Engineering Selector (CES) is theSoftware used for Material Selection developed byProf. Ashby.

Other material property selection charts include:


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Wrap Up

By considering 2 (or more) charts, the properties needed to

satisfy the main design requirements can be quickly assessed.

The charts can be used to identify the best classes of materials,and then to look in more detail within these classes.

There are many other factors still to be considered, particularly

manufacturing methods. The selection made from the charts

should be left quite broad to keep enough options open.

A good way to approach the problem is to use the charts to

eliminate materials which will definitely notbe good enough,

rather than to try and identify the single best material too soon in

the design process.

Lecture5 Materials Selection and Material Processing in Design

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