17. In the following description of how plastics are shaped, put the verbs in brackets into the Present Simple Passive.

There are many ways of shaping plastics. The most common way is by moulding. Blow-molding (a) ….. (to use) to make bottles. In this process, air (b)….. (to blow) into a blob of molten plastic inside a hollow mould and the plastic (c) ….. (to force) against the sides of the mould.

Toys and bowls (d) ….. (to make) by injection moulding. Thermoplastic chips (e) ….. first ….. (to heat) until they melt and then forced into a water-cooled mould under pressure. This method (f) ….. (to suit) to mass production. Thermoplastics can (g) ….. (to shape) by extrusion. Molten plastic (h) ….. (to force) through a shaped hole or die. Fibres for textiles and sheet plastic may (i)….. (to make) by extrusion.

Unit 12

MECHANICAL PROPERTIES

LEAD-IN

1. Look at the objects in the pictures. What materials do you think they are made of and why?

A	B	C	D

2. In pairs, discuss what is meant by mechanical properties. What mechanical properties do you know?

READING

3. Read the text and decide whether the statements that follow are true or false.

Mechanical Properties

Mechanical properties are those associated with how a material reacts when subjected to external forces.Force will cause material to become stressed. Stress relates to how much load is applied to a given area.Strain is another result of the application of force. This is defined as the amount of distortion (extension or compression) that takes place per unitof length.

Strength is the ability of a material to withstand force without breakingor permanently bending.

Elasticity is the ability of a material to bent and flex when subjected to a force and then to return to its previous form when the force is removed.Structures such as bridges and tall building must have a degree of elasticity.

Plasticity is the ability of a material to be changed permanently in shape (deformed) by the application of an external force without breaking or fractures. Most materials become more plastic when heated. The extent to which a material's shape can become changed by hammering, rolling or pressing is called malleability. Another associated term is ductility;this is the ability to undergo cold plastic deformation by bending or stretching. All ductile materials are malleable, but all malleable materials are not necessarily ductile.

Hardness is the ability to resist abrasive wear, indentation and deformation. This is the important property of cutting tools such as turning tools, drills, files and saws.

Toughness is the ability to withstand sudden stress in the form of shocksor blows without fracturing, and also to resist cracking when subjected to bending forces. Repeated bending will cause the toughness of some materials to break down such that they will eventually fracture.

Brittleness is the opposite of toughness. A brittle material will not withstand shocks or bending without fracture.

Durability is the ability to withstand wear, tear and deterioration withtime. Durability refers to both the mechanical properties and the appearance of the surface, particularly of metals, is corrosion. Plastic materials are generally less prone to corrosive effects and are thereforedurable.

Stability is the ability to resist changes in shape and size over time. Wood is particularly unstable and tends to warp and twist with changesin humidity. Metals and some plastics tend to gradually deform when subjected to stress over long periods; this process is known as creep.

1. All malleable materials are ductile, but all ductile materials are not necessarily malleable.

2. Plastics are generally less prone to corrosive effects and are therefore durable.

3. Structures such as bridges and tall building must have a degree of stability.

4. Stability is the ability to resist changes in form and size over time.

5. Chemical properties are those associated with how a material reacts when subjected to external forces.

6. Strength is the ability to resist abrasive wear, indentation and deformation.

7. Strength is the ability of a material to withstand power without breaking or permanently bending.

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