Material Selection and Material Processing in Design

Eng. R. L. Nkumbwa

Copperbelt University

2010

New Product Analysis

Everyday we use thousands of different products, from telephones to bikes and drinks cans to washing machines and microwaves.

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

Every product is designed in a particular way - product analysis enables us to understand the important materials, processing, economic and aesthetic decisions which are required before any product can be manufactured.

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

Getting Started

The first task in product analysis is to become familiar with 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 to consider the ergonomics, how the design has been made user-friendly and any marketing issues - these all have an impact on the later design decisions.

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?

Systems and Components

There are 2 main types of product - those that only have one component (e.g. a spatula) and those that have lots of components (e.g. a bike).

Products with lots of components we call systems. For example:

System Components

Product Components

Bike Frame, wheels, pedals, forks, etc.

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

Multi-gymSeat, weights, frame, wire,

handles, etc.

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.

Some important design questions

To build a design specification, consider questions like 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 each

part and why ?– Are there alternative materials or designs in use and can

you propose improvements?

Design Questions

These are only general questions, to act as a guide - you will need to think of the appropriate questions for the products and components 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 of

bubbles– 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 volumes of 10,000+

Choosing the Right Materials

Given the specification of the requirements on each part, we can identify the material properties which will be important - for example:

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

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.

Choosing the Right Process

It is all very well to choose the perfect material, but somehow we have to make something out of it as well! An important part of understanding a product is to consider how it was made - in other words what manufacturing processes were used and why.

There are 2 important stages to selecting a suitable process:– Technical performance: can we make this product with the

material 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 with one way of approaching it in another part of the tutorial.

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

Is there anything else to know?

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'.

Example Analysis

Is the product performance driven or cost driven? This makes a big difference when we choose materials. In a performance product, like a tennis racquet, cost is one of the

last factors that needs to be considered. In a non-performance product, like a drinks bottle, cost is of primary

importance - 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 the shape (and hence process) which is more limiting.

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

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 part of this course.

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

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

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

Material Selection

Most products need to satisfy some performance targets, 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?

Using Material Selection Charts

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

This information normally comes as tables of data and it can be a time-consuming process to sort through them.

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

The answer to both these problems is to use material selection charts.

Here is a materials selection chart for 2 common properties: Young's modulus (which describes how stiff a material is) and density.

Using Material Selection Charts

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, competing materials can be quickly identified.

Where selection charts are really useful is in showing the trade-off between 2 properties, because the charts plot combinations of properties.

For instance if we want a light and stiff material we need to choose materials near the top left corner of the chart - so composites look good.

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

Using Material Selection Charts

To find the best materials we need to use the Young's modulus - density chart from amongst the available charts. The charts can be annotated to help reveal the 'best' materials, by placing a suitable selection box to show only stiff and light materials.

What can we conclude? The values of Young's modulus for polymers are low, so

most polymers are unlikely to be useful for stiffness-limited designs. 

Cambridge Engineering Selector (CES) is the Software used for Material Selection developed by Prof. Ashby.

Other material property selection charts include:

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 not be good enough, rather than to try and identify the single best material too soon in the design process.