Engineering plastics

The use of plastics in the manufacture of fans has increased tremendously over the last two decades, especially in small

Units of all types. There has also been an increase in their use for the blades of large axial fans up to the very largest sizes.

The plastics used may be divided into three generic types:

• Thermoplastics

• Thermosets

• Composites

As their names imply, thermoplastic polymers can be re-soft­ened by heating, in contra-distinction to thermosets where they cannot.

Many practical applications of plastics in the fan industry need to use composite grades to meet the necessary strength and durability requirements.

Thermoplastics

These are probably the most widely used group of plastic mate­rials and include the following:

• ABS (acrylonitrile butadiene styrene)

• PVC (poly vinyl chloride)

• Polyethylene

• Polyamides (nylons)

• Polypropylene

• PTFE (polytetrafluoroethylene)

Thermosets

Whilst perhaps not used so widely in their solid form, they are nevertheless recognised as important for surface coatings and finishes. Examples of thermosets are:

• Alkyds

• Epoxies

• Polyesters

• Silicones

Composites

These are expected to be the group with the most exciting fu­ture. Not only has glass fibre been used as a strengthening agent, but there is now the possibility of using carbon fibres with even greater strength properties. Grades currently popularare:

• GRP (glass reinforced plastic)

• SMC (sheet moulding compound)

But there will be many more to come in the future.

Mechanical properties of plastics

These vary enormously, not only according to type, but also from one manufacturer to another. It is best to check with the suppliers of the appropriate grades and ascertain from them how their figures were obtained and also what supporting test work they can instance. Table 7.3 is therefore given as typical only.

One unfortunate property of plastics from a fan manufacturers’ point of view is that even at temperatures just above ambient they are affected by “creep”. Thus they are subject to extension (time dependent strain) under the most moderate stresses. It is therefore important to design for a known working life.

Plastic type	Ultimate tensile strength N/mm2	Modulus of elasticity kN/mm2
Epoxies	80	8
GRP	<180	<20
Nylon	60	2
Polyethylene	20	0.6
PTFE	14	0.3
PVC	50	3.5

Table 7.3 Typical mechanical properties of plastics

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