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
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
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.
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.
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.
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
The two key
physical properties of most interest to a designer are the density and
the useful working temperature range.
The mechanical properties of a material can be divided into static and dynamic properties.
The ability to withstand a force without breaking. Further sub-divided
into Tensile Strength ~ Stretching
Dynamic Properties ~ involve an element of time
Creep ~ A
slow plastic deformation that can occur when a load is applied for prolonged