The properties of both metals and plastics can be altered through the use of additives in the case of plastics, or the addition of elements or heat treatments for metals. The addition of elements is covered in the Metals hand out for Steel.

Plastics Additives

Pigments give the plastic its colour

Stabilisers prevent the environmental deterioration of plastics particularly from the sun's UV light. Common for PVC

Plasticisers added to make a plastic, such as PVC, less hard and brittle at temperatures of normal use

Fillers can be added to improve the properties. Glass often used to strengthen Nylon

Blowing agents used to expand polymers such as polystyrene

Flame retardants reduce the likelyhood of combustion, particularly in polyurethane foam based furniture

Antistatic agents Plastics tend to be poor electrical conductors and therefore build up static charge. Antistatic agents reduce this tendency

Heat Treatment of Metals

Take a thin piece of copper and bend it repeatedly and you will find that it becomes progressively harder. This is known as work hardening and its effects can be reversed by annealing ~ heating the copper to red/orange heat and cooling.
For steel it is often desirable to increase the hardness and this can be done by tempering ~ heating the steel to between 220-300°C and cooling it rapidly in either oil or water. It is sometimes desirable to harden only the surface of a steel component. This can be achieved by case hardening ~ heating the steel to red hot and imersing the component in carbon powder.

Selecting the appropriate material and manufacturing process will depend on a range of factors, some of them obvious, some of them more subtle. It is important that you foster an appreciation of these factors and use the correct terms when discussing them.

Properties

First and almost certainly foremost are the properties that are required. It is not sufficient however to talk about the properties in isolation. For example, for a given cross section, Stainless Steel has almost twice the tensile strength of Aluminium Alloy. In isolation, that figure would suggest that Aluminium is a lousy choice for aircraft manufacturers. However, when you throw density into the equation, it can be seen that for two samples of equal weight, Aluminium is the better choice. A material like Carbon Fibre Reinforced Plastic has greater Tensile Strength than even Stainless Steel and almost half the density of Aluminium ~ an even better choice surely? ~ not when manufacturing cost is taken into consideration.

Other main-stream properties that will often influence selection are elasticity, heat and electrical conductivity, ability to withstand impacts, scratch resistance, corrosion resistance. Some of the more subtle factors will include toxicity; Copper is an excellent conductor of heat, but toxic and therefore no use as a pan without a lining. The feel of a material is an ergonomic property that often needs to be considered; wood is a warm, natural material, concrete is a cold and coarse material. Finally environmental implications are becoming increasingly important and this is an issue that we will deal with at a later stage.

Manufacturing Methods

Again, it is not sufficient to simply chose a material in isolation and then decide on the way it is to be manufactured. A designer would need to consider the whole production implications in combination. A discussion might typically revolve around manufacturing an object by injection moulding from ABS or deep drawing in stainless steel. There are other factors to consider such as tooling costs and production volumes. A good example of this is any object consisting of a large flat area. Often it will require stiffening and this can be done in two ways: Increase the thickness of the material or stiffen it using bends or ribs. Increase the thickness and the weight goes up adding to transport costs. Adding bends and ribs, however, requires additional tooling.

In selecting a material for a component it is essential to chose one whose properties meet the requirements of the design. The properties of a material can be divided into its physical properties (such as density, thermal and electrical conductivity) and its mechanical properties (such as hardness, strength and toughness) which indicate how the material behaves under various loads.
An essential difference between GCSE and A-level is that with the former it is sufficient to know, for example, that an object made from plastic is likely to be lighter than an object of similar size made from metal. At A-level you should be able to back this up with data giving specific figures and units.

Physical Properties

The two key physical properties of most interest to a designer are the density and the useful working temperature range.
Density: From now on it is unacceptable for you to talk of a material as being light or heavy ~ a ton of lead is as heavy as a ton of feathers ~ you will need to describe a material as being low, medium or high density.
The density of a material is the value of its mass per cubic metre and it is these last 3 words that are crucial in explaining the difference between weight and density. A cubic metre of feathers will weigh a great deal less than a cubic metre of lead and therefore has a much lower density. The table below gives some values.
Thermal Properties: These are of major concern to a designer and cover not only the ability of a material to withstand heat but also its ability to conduct and transfer heat. In addition, materials expand and contract as a result of temperature changes ~ yet another factor the designer must consider.

Mechanical Properties

The mechanical properties of a material can be divided into static and dynamic properties.

Static Properties

Strength ~ The ability to withstand a force without breaking. Further sub-divided into Tensile Strength ~ Stretching
Compressive Strength ~ Squashing
Shear Strength ~ Equal and Opposite forces across
Elasticity ~ The ability of a material to return to its original shape after being deformed
Plasticity ~ The ability to deform to a stretched state and retain that shape after the load is removed (like plasticine, oddly enough)
Ductility ~ The ability of a material to be drawn out to a narrower cross section (closely linked to plasticity)
Malleability ~ The ability of a material to be stretched in all directions without fracture
Hardness ~ The resistance to wear or indentation of a material

Dynamic Properties ~ involve an element of time

Creep ~ A slow plastic deformation that can occur when a load is applied for prolonged periods
Fatigue ~ Is the phenomenom by which a material can fail at a lower than normal stress if the load is applied and released many times.
Toughness ~ The ability of a material to withstand sudden loading.
Brittleness ~ A term used to describe a material that lacks toughness
Not all of the above properties can be conveniently measured. On the following hand-out I will give you figures for the the Modulus of Elasticity and the Tensile Strength of several materials, but you should try to develop an appreciation of the properties of materials by experiencing them for yourselves. You should know by now that Aluminium is malleable, Lead has high plasticity, Concrete has poor tensile strength but good compressive strength, Rubber has high elasticity, Acrylic is brittle, Diamond is hard etc.


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