Page 367 - Introduction to Marine Engineering

338 Engineering materials

Polyvinylchloride (PVC)
A vinyl plastic which is chemically inert and used in rigid form for
pipework, ducts, etc. In a plasticised form it is used for sheeting, cable
covering and various mouldings.

Resin
Resins are hard, brittle substances which are insoluble in water. Strictly
speaking they are added to polymers prior to curing. The term 'resin' is
often incorrectly used to mean any synthetic plastic. Epoxy resins are
liquids which can be poured and cured at room temperatures. The
cured material is unaffected by oils and sea water. It is tough, solid and
durable and is used as a chocking material for engines, winches, etc.

Rubber
A tree sap which solidifies to form a rough, elastic material which is
unaffected by water but is attacked by oils and steam. It is used as a
jointing material for fresh and sea water pipes and also for water
lubricated bearings. When combined with sulphur (vulcanized) it forms
a hard material called 'ebonite' which is used for bucket rings (piston
rings) in feed pumps. Synthetic rubbers such as neoprene and nitrile
rubber are used where resistance to oil, mild chemicals or higher
temperatures is required.

Joining metals

Many larger items of engineering equipment are the result of combining
or joining together smaller, easily produced items. Various joining
methods exist, ranging from mechanical devices, such as rivets or nuts
and bolts, to fusion welding of the two parts.
It is not proposed to discuss riveting, which no longer has any large
scale marine engineering applications, nor will nuts and bolts be
mentioned, since these are well known in their various forms.
Brazing and soldering are a means of joining metal items using an
alloy (solder) of lower melting point than the metals to be joined. The
liquefied solder is applied to the heated joint and forms a very thin layer
of metal which is alloyed to both surfaces. On cooling the two metals are
joined by the alloy layer between them.
Welding is the fusion of the two metals to be joined to produce a joint
which is as strong as the metal itself. It is usual to join similar metals by