Transcript Materials Selection and Material Processing in Design

Material Selection and Material
Processing in Design
Eng. R. L. Nkumbwa
Copperbelt University
2010
New Product Analysis
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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
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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
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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
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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-gym
Seat, weights, frame, wire,
handles, etc.
Product Analysis
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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
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To build a design specification, consider questions
like the following:
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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
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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:
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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
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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
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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
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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…
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Product analysis can seem to follow a fixed
pattern:
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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
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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
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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
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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
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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
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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, competing
materials can be quickly identified.
Where selection charts are really useful is in showing the tradeoff 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
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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
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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.