Engineering materials are metals and plastics. Wood is used to make patterns and models. Smart materials and composites such as carbon fibre are also important engineering materials.
Plastics - engineering plastics are usually very strong or tough. and may be self lubricating.
The properties of polymers may be altered by the addition of plasticisers which improve flexibility, and fillers which increase opacity (stop light passing through), change density, change thermal properties, decrease cost if filler material is cheap e.g. talc.
Very strong, nylon can be machined and will take a fine thread. It is also slippery and can be used to make washers, spacers and bushes.
Nylon was originally developed as a textile but is available in many forms with vastly different properties. Engineering nylon grades are easy to machine with good resistance to biological attack. Unfortunately nylons can absorb moisture from the atmosphere and can degrade in strong sunlight (they are unstable in ultraviolet light) unless a stabilising chemical is added at the initial manufacture of the plastic. Nylons are easy to mould. Nylons also have a natural 'oily' surface that can act as a natural lubricant. Nylons are used for everything from clothes through to gears and bearings.
Comes in a range of thicknesses, colours and can be opaque or transparent. There are two type of acrylic extruded which is cheaper and very "plastic" and cast which machines better but is harder and less flexible.
Acrylics are available in a range of colours and can be opaque, translucent or transparent. They are available in sheet, rod, and tube for use in injection moulding, extrusion and vacuum forming. Acrylics withstand weather and are stable in sunlight. Almost any colour can be produced. Transparent acrylic can be as clear as the finest optical glass, this led them to be used in optical equipment such as cameras. It is possible to significantly strengthen the acrylic when it is being made, these high grade acrylics are used use for aircraft windows.
|PVC and uPVC||Stiff, hard,tough lightweight plastic. uPVC is stabilised for outside use and is used for plastic windows and plastic pipes. Plasticised PVC is used for flexible applications such as insulating - cables.|
|Polythene||This plastic has a range of uses from food packaging to gas pipes. The plastics can be injection moulded or extruded and is available in two forms. High-density polyethylene (HDPE) is a hard rigid plastic. A low-density grade ( LDPE )is tough and flexible.|
|Polypropylene||Polypropylene is a tough, cheap plastic, it has a slightly waxy feel. It can be bent repeatedly without breaking. Used for Medical equipment such as syringes, stacking chairs (chair shell is polypropylene), suitcases with integral hinges,|
|Polycarbonate||Used for making eye protection, machine guards and riot shields. It is not as hard as acrylic and can be cut easily but it will absorb impacts.|
|Bakelite||A thermosetting plastic. Dark brown. Used as a composite reinforced with paper or cloth. Used to make circuit boards and heat proof insulated parts in the electronics industry.|
|Epoxy resin||A two part mix which can be used as a glue (ARALDITE)or be reinforced with carbon fibre to produce a very strong and light composite materials which is used in aerospace and Formula 1|
|Melamine||A thermoset very tough and heat resistant. White but can be produced in a full range of colours.|
Now try these quizzes about plastics -
|Metals - a wide range of metals is available for use by the engineer. In school we have a more limited choice.|
|Steels||Carbon steel is sometimes referred to as 'mild steel' or 'plain
carbon steel'. The American Iron and Steel Institute defines a carbon
steel as having no more than 2 % carbon and no other appreciable alloying
element. Carbon steel makes up the largest part of steel production and
is used in a vast range of applications.
Typically carbon steels are stiff and strong. They also exhibit ferromagnetism (i.e. they are magnetic). This means they are extensively used in motors and electrical appliances. Welding carbon steels with a carbon content greater than 0.3 % requires that special precautions be taken. However, welding carbon steel presents far fewer problems than welding stainless steels. The corrosion resistance of carbon steels is poor (i.e. they rust) and so they should not be used in a corrosive environment unless some form of protective coating is used.
Disadvantages - Poor corrosion
resistance (i.e. rusts)
|Mild steel||A low carbon steel which can only be hardened by case hardening. Easy to work and generally strong. Used for the manufacture of steel products e.g. car bodies, washing machines, oil drums,filing cabinets etc.|
All cast irons consist of more than 2% carbon. This high carbon content makes them excellent materials to use for casting and at much lower temperatures than those required to cast steel. They also have better flow characteristics when molten helping them to fill the mould more easily. Grey cast iron, commonly used in engineering, is brittle and not very ductile. It can be machined reasonably easily but cannot be welded easily. Cast iron has been used for many applications are engine blocks and gears.
Advantages - Better corrosion resistance than steels
in most environments.
Disadvantages - Very brittle
|High speed steel||A high carbon steel which can be hardened and tempered to produce tools. e.g. drill bits are H.S.S.|
|High and medium carbon steel||These are also tool steels which can be hardened and tempered.|
|Silver Steel||A shiny silver coloured tool steel. We use silver steel for screw driver blades. The tip is hardened to take the wear but the shank is left unhardened to take the strain without being brittle.|
|Brass||An alloy of copper and zinc. Yellow or gold in colour. Used for decorative effect. Used to produce bushes as it is resistant to wear. Bronze (copper and tin) is also used for bushes. Brass spelter and wire is used when brazing.|
|Copper||A reddish or pink metal. Very good conductor of heat and electricity. Ductile and is used to manufacture wire for electrical use. PCBs have copper tracks on them.|
|Pewter and solder||Originally made from lead and tin. Lead free versions contain over 90% tin with various other metals such as copper, silver and antimony. These low melting point alloys are used for electronics and jewellery respectively. Solder is manufactured in a range of forms but solder wire used for electronics contains a resin flux inside the wire.|
|Zinc Alloy||A low melting pont alloy of zinc used to produce components which do not need high tensile strength such as die cast model cars.|
|Aluminium||Aluminium is a strong silver metal. It turns easily being softer than steel. Aluminium has a low melting point and it is the metal used for sand casting.|