Page 54 - Plastics Engineering

General Roperties of Plastics 37

range of processing methods available for plastics. Therefore Chapter 4 will be

devoted to processing. This describes the suitability of particular plastics for

each moulding method and considers the limitations which these place on the

designer.

L4,6 Costs

It is a popular misconception that plastics are cheap materials. They are not. On

a weight basis most plastics are more expensive than steel and only slightly less

expensive than aluminium. Prices for plastics can range from about ;JE600 per

tonne for polypropylene to abut €25,000 per tonne for carbon fibre reinforced

PEEK. Table 1.5 compares the costs of a range of plastics.

However, it should always be remembered that it is bad design practice to

select materials on the basis of cost per unit weight. In the mass production

industries, in particular, the raw material cost is of relatively little importance.

It is the in-position cost which is all important. The in-position cost of a

component is the sum of several independent factors i.c raw material costs,

fabrication costs and performance costs.

It is in the second two of these cost components that, in relation to other mate-

rials, plastics can offer particular advantages. Fabrication costs include power,

labour, consumables, etc and Table 1.10 shows that, in terms of the oveniH

energy consumption, plastics come out much better than metals. Performance

costs relate to servicing, warranty claims, etc. On this basis plastics can be very

attractive to industries manufacturing consumer products because they can offer

advantages such as colour fastness, resilience, toughness, corrosion resistance

and uniform quality - all features which help to ensure a reliable product.

However, in general these fabrication and performance advantages are

common to all plastics and so a decision has to be made in regard to which

plastic would be best for a particular application. Rather than compare the basic

raw material costs it is better to use a cost index on the basis of the cost to

achieve a certain perforname. Consider again the material selection procedures

illustrated in Section 1.4.1 in relation to strength and stiffness.

Selection for Strength at Minimum Cost

If the cost of a material is C per unit weight then from equation (1.3) the cost

of the beam considered in the analysis would be

(1.12)

Substituting for d from (1.2) then the cost of the beam on a strength basis

would be

Cb 83 PC (1.13)