Thermoplastics and thermosetting

Plastics

The simplest way of classifying plastics is by their

reaction to heat. This gives a ready subdivision into

two basic groups: thermoplastics and thermosetting

plastics. Thermoplastic materials soften to become

plastic when heated, no chemical change taking

place during this process. When cooled they again

become hard and will assume any shape into which

they were moulded when soft. Thermosetting

materials, as the name implies, will soften only

once. During heating a chemical change takes

place and the material cures; thereafter the only

effect of heating is to char or burn the material.

As far as performance is concerned, these plastics

can be divided into three groups:

General-purpose thermoplastics

Polyethylene

Polypropylene

Polystyrene

SAN (styrene/acrylonitrile copolymer)

Impact polystyrene

ABS (acrylonitrile butadiene styrene)

Polyvinyl chloride

Poly(vinylidene chloride)

Poly(methyl methacrylate)

Poly(ethylene terephthalate)

Engineering thermoplastics

Polyesters (thermoplastic)

Polyamides

Polyacetals

Polyphenylene sulphide

Polycarbonates

Polysulphone

Metals and non-metals used in vehicle bodies 149

Table 4.13Physical properties of polymers

Coefficient

Melting Specific-heat Thermal of linear

Density (softening) capacity conductivity expansion

Material P or S (kg/m3) range (°C) (J/kg/K _ 103) (W/m/K) (K _ 10_6)

LD polyethylene P 0.01–0.93 80 2.3 0.13 120–140

HD polyethylene P 0.04–0.97 90–100 2.1–2.3 0.42–0.45 120

Polypropylene P 0.90 100–120 1.9 0.09 120

GFR polypropylene P 1.00–1.16 110–120 3.5 – 55–85

Polyvinylchloride P 1.16–1.35 56–85 0.8–2.5 0.16–0.27 50–60

Polystyrene P 1.04–1.11 82–102 1.3–1.45 0.09–0.21 60–80

Polystyrene P 0.99–1.10 85 1.4–1.5 0.04–0.30 60–130

copolymer (ABS)

Nylon 66 P 1.14 250–265 1.67 0.24 80

Nylon 11 P 1.04 185 2.42 0.23 150

PTFE (Teflon) P 2.14–2.20 260–270 1.05 0.25 100

Acrylic (Perspex) P 1.10–1.20 70–90 1.45 0.17–0.25 50–90

Polyacetals P 1.40–1.42 175 1.45 0.81 80

Polycarbonates P 1.20 215–225 1.25 0.19 65

Phenol formaldehyde S 1.25–1.30 – 1.5–1.75 0.12–0.25 25–60

Urea formaldehyde S 1.40–1.50 – 1.65 0.25–0.38 35–45

Melamine formaldehyde S 1.50 – 1.65 0.25–0.40 35–45

Epoxies S 1.20 – 1.65 0.17–0.21 50–90

Polyurethanes R S 3.2–6.0 150–185 1.25 0.02–0.025 20–70

Polyurethanes F S 4–8 150–185 1.25 0.035 50–70

Polyesters S 1.10–1.40 – 1.26 0.17–0.19 100–150

Silicones S 1.15–1.8 200–250 – 0.17 24–30

GFR glass fibre reinforced; P thermoplastic; S thermosetting; R rigid; F flexible; LD low density; HD high density

Table 4.14Typical mechanical properties of representative plastics

Modulus of elasticity Tensile strength

Material E (MN/m2) (MN/m2) Compressive strength Elongation (%)

LD polyethylene 120–240 7–13 9–10 300–700

HD polyethylene 550–1050 20–30 20–25 300–800

Polypropylene 900–140 32–35 35 20–300

GFR polypropylene 1500_ 34–54 40–60 5–20

Flexible PVC 3500–4800 10–25 7–12 200–450

Rigid PVC 2000–2800 40 90 60

Polystyrene 2400–4200 35–62 90–110 1–3

ABS copolymer 1380–3400 17–58 17–85 10–140

Perspex 2700–3500 55–75 80–130 2–3

PTFE 350–620 15–35 10–15 200–400

Nylon 11 1250–1300 52–54 55–56 180–400

GFR nylon 6 7800–800 170–172 200–210 3–4

150Repair of Vehicle Bodies

Modified polyphenylene ether

Polyimides

Cellulosics

RIM/polyurethane

Polyurethane foam

Thermosetting plastics

Phenolic

Epoxy resins

Unsaturated polyesters

Alkyd resins

Diallyl phthalate

Amino resins

Amorphous and crystalline

Plastics

An alternative classification of plastics is by their

shape. They may be crystalline (with shape) or

amorphous (shapeless).

Amorphous plastics

Amorphous plastics basically are of three major

types:

ABS: acrylonitrile butadiene styrene

ABS/PC blend

PC: polycarbonate.

Amorphous engineering plastics have the following

properties:

High stiffness

Good impact strength

Temperature resistance

Excellent dimensional stability

Good surface finish

Electrical properties

Flame retardance (when required)

Excellent transparency (polycarbonate only).

In the automotive industry use is made of the good

mechanical properties (even at low temperatures),

the thermal resistance and the surface finish. The

applications are:

1 Body embellishment

2 Interior cladding

3 Lighting where, apart from existing applications

of back lamp clusters, polycarbonate is

expected to replace glass for headlamp lenses.

Semi-crystalline plastics

Semi-crystalline plastics are in two basic types:

Polyamide 6 and 66 types

Polybutylene terephthalate (PBT).

Semi-crystalline plastics have the following properties:

High rigidity

Hardness

High heat resistance

Impact resistance

Abrasion, chemical and stress crack resistance.

The semi-crystalline products find major application

in the automotive sector, where full use is

made of the mechanical and thermal properties,

together with abrasion and chemical resistance.

Examples include:

1 Underbonnet components

2 Mechanical applications

3 Bumpers, using elastomeric PBT for paint

on-line

4 Body embellishment (wheel trims, handles,

mirrors)

5 Lighting, headlamp reflectors.

Blended plastics

Blended plastics have been developed to overcome

inherent specific disadvantages of individual plastics.

For large-area body panels, the automotive

industry demands the following properties:

Temperature resistance

Low-temperature impact resistance

Toughness (no splintering)

Petrol resistance

Stiffness.

Neither polycarbonate nor polyester could fulfil

totally these requirements. This led to the combination

of PC and PBT to form Macroblend PC/PBT,

which is used for injection moulded bumpers.

Plastics applications

Plastic products can be decorated by vacuum metallizing

and electroplating. They have replaced

metals in a lot of automotive applications, such as

mirror housings, control knobs and winder handles

as well as decorative metallic trim. It is a field

which uses their advantages to the full without

relying on properties they lack.

Metals and non-metals used in vehicle bodies 151

Thin parts must be tough and resistant to the occasional

impact. They must be impervious to attack by

weather, road salts, extremes of temperatures and all

the other hazards that reduce older forms of body

embellishments to pitted, rusted, dull, crumbling

metal. They do not need high tensile strength or flexural

strength as they do not have to carry heavy

stresses. They must be cheap and capable of being

formed into highly individual and complex shapes.

All these requirements are satisfied by thermoplastics

and thermosetting resins. They can be pressed,

stamped, blow moulded, vacuum formed and injection

moulded into any decorative shape required.

Apart from their decorative properties, the mechanical

properties of acrylic resins are among the

highest of the thermoplastics. Typical values are a

tensile strength of 35–75 MN/m2 and a modulus of

elasticity of 1550–3250 MN/m2. These properties

apply to relatively short-term loadings, and when

long-term service is envisaged tensile stresses in

acrylics must be limited to 10 MN/m2 to avoid surface

cracking or crazing. Chemical properties are

also good, the acrylics being inert to most common

chemicals. A particular advantage to the automotive

industry is their complete stability against petroleum

products and salts.

Acetal resins are mostly used for mechanical parts

such as cams, sprockets and small leaf springs, but

also find application for housings, cover plates, knobs

and levers. They have the highest fatigue endurance

limits of any of the commercial thermoplastics, and

these properties, coupled with those of reduced friction

and noise, admirably qualify the acetal resins for

small gearing applications within the vehicle.

Plastics can be self-coloured so that painting costs

are eliminated and accidental scratching remains

inconspicuous, and they can be given a simulated

metal finish. For large-scale assemblies, such as

automobile bodies, painting is necessary to obtain

uniformity of colour, especially when different types

of plastics are used for different components.

Plastics can also be chrome plated, either over a special

undercoating which helps to protect and fix the

finish, or by metal spraying or by vacuum deposition

in which the plastic part is made to attract

metal particles in a high-vacuum chamber. The use

of a plastic instead of a metal base for chrome plating

eliminates the possibility of the base corroding

and damaging the finish before the chromium plating

itself would have deteriorated. The chrome coating

can be made much thinner and yet have a longer

effective life, with a consequent saving in cost.

Until fairly recently polymer materials were

joined only by means of adhesives. Now the thermoplastic

types can be welded by using various forms

of equipment, in particular by hot gas welding, hot

plate machines which include pipe welding plant,

ultrasonic and vibration methods, spin or friction

welding machines, and induction, resistance and

microwave processes. Lasers have been used experimentally

for cutting and welding. The joining of

metals to both thermoplastic and thermosetting

materials is possible by some welding operations

and by using adhesives.

Future of plastics in the

Automotive industry

The automotive industry has grown to appreciate

the potential of plastics as replacements for metal

components within their products. The realization

that plastics are, in their own right, engineering

materials of high merit has led to rapid advancement

of material and application technology, with

the end result that plastics have gained a firm and

increasing footing in the motor vehicle. Many factors

have aided the adoption of plastics by the

automotive industry, which uses them in the following

areas: body, chassis, engine, electrical system,

interior and vehicle accessories. Lower costs

of plastics parts must, of course, be the major contributing

factor in the replacement of existing parts,

and this is closely followed by the ease with which

modern plastics can be formed by comparatively

inexpensive tooling. The inert properties of synthetic

materials also contribute greatly; properties

like corrosion resistance, low friction coefficients

and light weight are of prime importance.

The use of plastics in the automotive industry continues

to accelerate at a phenomenal rate as research

into plastic technology results in new developments

and applications. The future growth of plastics in the

automotive industry will be controlled by two factors:

the growth of the industry itself, and the greater

penetration of plastic per car. A key constituent in

world growth, therefore, is the developing nations

who are involved in the assembly and production

of motor vehicles. They will consequently favour

the use of plastics as a first choice, rather than as a

replacement for metal.

152Repair of Vehicle Bodies

Figure 4.3Applications of plastics in automobiles (Motor Insurance Repair Research Centre)

1 Front bumper (Pocan S7913)

2 Front spoiler (Santoprene grade 123–50 and 121 with

aluminium insert)

3 Fog lamp blanking plate (Xenoy EPX500)

4 Lower front grille (Xenoy CL100)

5 Front number plate plinth (Xenoy CL100)

6 Front bumper insert (PVC and EB-type Nylar)

7 Front grille (moulding, ABS; Benzel, MS Chrome)

8 Bonnet/boot lid/tailgate badges (ABS, aluminium and

PU skin)

9 Underbonnet felt (moulded felt)

10 Door mirror casing RH and LH (polyamide, 15% glass

reinforced)

11 Door mirror mounting RH and LH (polyamide, 15% glass

reinforced)

12 Front/rear wheel trims RH and LH (cap, Noryl 731;

moulding, Bayer Duretan BM30X, ICI Maranyl TB570)

13 Front/rear mudflaps RH and LH (front, rubber to BLS.22

RD.27 Ref. 421; rear EPDM mix 4080)

14 Scuttle grille/mouldings (ABS)

15 Front/rear screen upper and side mouldings (PVC with

stainless steel co-extrusion)

16 Front/rear wing splashguards RH and LH (PP)

17 Front wing waist moulding RH and LH (Noryl)

18 Front door waist moulding RH and LH (Noryl)

19 Rear door waist moulding RH and LH (Noryl)

20 Rear wing waist moulding RH and LH (Noryl)

21 Front/rear door outer handles RH and LH (body, Xenoy;

flap, Glass-filled nylon)

22 Rear quarterlight moulding RH and LH (4-door,

PVC/Stainless steel extrusion; 5-door and coupé, PU

with stainless steel moulding)

23 Boot lid moulding (ABS)

24 Rear spoiler (PU core and polyester skin)

25 Rear number plate plinth (ABS)

26 Rear bumper insert (PVC and EB-type Nylar)

27 Rear bumper (Pocan S7913)

28 Front/rear door upper mouldings RH and LH (PVC with

stainless steel moulding)

29 Front/rear door outer weatherseals RH and LH (PVC

with stainless steel co-extrusion)

30 Fog lamp bezel (PP)

Metals and non-metals used in vehicle bodies 153

Over the past years, the natural applications for

plastics in automobiles (interior fittings, cushioning

and upholstery, trim, tail lights, electrical components)

have become saturated. The growth for the

future can be expected to come from the use of

plastic for bodywork and some mechanical components.

Already there is a widespread use of plastics

for front and rear bumpers. We can expect to see

bonnets, boot lids and front wings in plastics.

All the major volume producers of cars are

engaged in long-term development work towards

the all-plastic car. Whether or not such targets can

be realized remains to be seen. Factors such as

energy costs and availability of resources may play

a greater part in the total picture than simple

objects like vehicle weight reduction.

Abbreviations for automotive plastics

4.13 Plastics repair

A new car is made up, by weight, of about 65 per

cent steel, 5 per cent non-ferrous metal, 15 per cent

plastics material and 15 per cent other non-metallic

materials. Plastics materials are very light, so in

terms of bulk the percentage is much larger, but

nobody appears to have worked out the figures for

this yet! What we do know is that the plastics parts

are often damaged in even a minor accident and

the replacement of these parts costs insurance

companies and private owners dearly. That’s how

we make a profit, you might say; but you could

make more profit by repairing the damaged plastics

part, this would reduce the cost to the customer

and reduce the waste of precious natural resources.

Abbreviations for automotive plastics

Abbreviation Full name

ABS Acronitrile butadiene styrene*

PP Polypropylene*

PE Polyethylene*

PC Polycarbonate*

PA Polyamide*

PBT Polybutylene tetraphtalate*

PU Polyurethene (thermoset)

UP Unsaturated polyester (thermoset)

CS Chopped strands

SMC Sheet moulding compound

MF Milled fibres

WR Woven roving

* Can be repaired by welding

SPI materials coding system

Material type Code

Polyethylene terephthalate 1PETE

High density polyethylene 2HDPE

Vinyl 3V

Low density polyethlene 4LDPE

Polypropylene 5PP

Polystyrene 6PS

Other 6OTHER

Types of plastics

The word plastics is being used here because it is

technically correct to describe the range of manmade

materials. Plastic, without the ‘s’, is used to

describe the material state where it can be deformed

and it will remain in that state after the force has

been removed. In conversation it is normal to say

plastic for both cases as it is unlikely that there will

be any confusion.

The two main classifications of plastics are:

thermoplastics and thermosetting plastics (which

are referred to as thermosets). One of the key

areas of knowledge needed to repair plastics

components is an understanding of these two

classifications and being able to identify them in

a vehicle component.

A thermoplastic is one which melts when it is

heated up. If you get a carrier bag from the supermarket

and warm it slightly it will become soft

and pliable. It is a thermoplastic. At this point you

must remember that plastics are made from petroleum-

based chemicals and are therefore easy to

set on fire and burn at very high temperatures, so

avoid matches and other naked flames when handling

them. Conversely, if you put the same carrier

bag in a freezer it would go stiff and make a

crackling noise when you handle it. Vehicle

thermoplastic components are made to operate

normally over a wide temperature range, so obviously

they need to get very hot before they will

melt and very cold before they become brittle.

Now if you heat up a thermoplastic to a high

enough temperature it is going to melt, this means

that you can repair a thermoplastic component by

welding. Before you dash out to try to weld that

bumper assembly which is sat on the bench, there

are a few more things which you need to know.

First, that you need a special plastics welder and

154Repair of Vehicle Bodies

second, if it is not a thermoplastic bumper you

may well damage it beyond repair and set the

workshop on fire too. You cannot simply identify

thermoplastics just by warming them up; we’ll

look at ways of identifying plastics later in the

article.

A thermoset is one which uses or generates heat

during its setting stage. The first thermoset was

Bakelite, the heavy dark brown plastics material

which was used for distributor caps and ignition

coil ends. It does not go soft when you try to heat it

up; if you subject it to a flame it will burn and char.

It is a brittle material and easily chips. Thermosets

cannot be welded; most can be bonded using a suitable

bonding agent or glue.

Reinforcement

On their own plastics materials have only a limited

amount of strength – interior trim, dashboard panels

and lamp lenses are examples of non-reinforced

plastics. Apply a strong force from a mechanics

hand and these components will break. So, components

which are going to take structural loads

within the vehicle or be capable of withstanding

impact, such as a bumper assembly, need some

form of reinforcement. The most common reinforcement

material is glass.

You are probably familiar with glass reinforced

polyester (GRP) where a piece of glass matting is

layed up with a mixture of polyester resin and catalyst

(hardener) to effect a body repair, or for the

manufacture of kit cars and small boats.

Incidentally, GRP is also used to cover all glass

reinforced plastics. If you look at a strand of the

glass from this matting through a microscope, or

with a very strong magnifying glass, you will see

that the glass is in fact made from very small diameter

round tubes. A round tube gives very high

strength, but the length to diameter ratio of these

tubes is such that they can bend without collapsing,

so that they can be layed up on curved surfaces,

then when the resin sets they are firmly held in

place like roof beams for maximum strength.

The glass reinforcement used for vehicle components

is made to suit the application and can vary

between the woven cloth like material which is

sandwiched in layers in a bumper assembly and the

finely powdered glass particles which are used to

strengthen a lamp cluster.

Increasingly materials other than glass are

being used to reinforce plastics, although often

this is in addition to glass. Carbon fibre is used

either in the form of a continuous thread which is

wound around the component or as a woven matting

similar to glass reinforcement. Where two

different materials are used they are referred to as

composites.

So, when it comes to identifying a plastics

material check to see if it is reinforced, and if it is

which type of reinforcement is used. If a glass

matting is used you can usually see the woven layers

of glass on the underside of the component.

Carbon fibre can be recognized by its graphite

grey colour.

Identification markings

Many manufacturers now mark their products

with a code which will enable the identification

of the type of plastics used. The reason for this

coded marking is mainly for the identification of

genuine parts and recycling purposes. Currently

there is no standard system in Europe, nor indeed

the UK, for identifying plastics. The British

Plastics Federation are trying to encourage all

European plastics manufacturers to use the

American Society of the Plastics Industry (SPI)

material code system. This uses a number and a

series of abbreviation letters. The number and

abbreviation letters identify a classification of

plastics. The code letters used by manufacturers

outside the SPI system are often registered trade

marks, this creates legal problems as well as identification

problems. Again, the SPI classification

code is intended mainly for recycling purposes,

but it is very useful for general identification

information.

Manufacturing processes

There are many different methods of manufacturing

plastics components. After a little experience

you will be able to work out how components are

best manufactured, this is usually a good guide as

to the type of material used. Injection moulding

(Inj) is used for items such as grille panels, air

vents, dashboard panels, wheel covers and lamp

units. The material is likely to be a thermoplastic.

These items usually have a smooth surface finish

and carry markings which show that they have

Metals and non-metals used in vehicle bodies 155

been in a mould, typically the lettering is raised.

They also tend to be fairly flexible. Injection

moulding can also be carried out with thermosets.

In this case the material is much stiffer but still

shows the mould lines and has a smooth surface

finish with raised lettering. The thermoset injection

moulding may use a resin (RIM) or a bulk

material (BMC). In some cases the BMC may be a

recycled material filler, but this will have low

strength. Bumper assemblies, wheel arch extensions

and rear lamp holders are typical applications

of injection moulded thermosets. Thermosets

in the form of GRP using woven glass fibre or

woven carbon fibre may be hand layed-up (HLU)

or compression moulded (com). To effect a good

bond with a carbon fibre material an autoclave is

needed to control the finishing process. The texture

and colour will allow you to identify GRP

and carbon fibre materials.

Safety

Having described the different plastics materials

and the manufacturing processes you should be

able to start identifying them. As with all things,

you will need to practise until you become

skilled, and some mistakes are inevitable. When

you try repairing a few items you will get a feel

for the job, just like tightening nuts and bolts.

You will soon become aware of which parts on

which vehicles can be repaired. But before you

start to work on plastics materials, you need to

look at safety. As well as the normal workshop

safety procedures, there are a number of specific

hazards relating to plastics materials which

you must take extreme care with, let’s have a

look at them before discussing some of the repair

procedures.

Plastics materials are made from petroleumbased

products, this means that they are highly

flammable so you must avoid high levels of heat

and naked flames. The most common reinforcing

material is glass, but other equally problematic

materials may be used. If you start to grind plastics

components you will get powdered glass as well as

the plastics dust. The powdered glass can cut the

blood vessels inside the lungs and stomach. The

plastics powder can cause respiratory diseases and

the dust from carbon fibre can cause lung and other

internal diseases. So ensure that you are using the

correct masks or other breathing apparatus to suit

the situation. Heating plastics materials, or using

solvents, or bonding agents, can give rise to volatile

organic compounds (VOCs); breathing protection

is obviously needed in this case. To prevent a

buildup of fumes and dust in the workshop the use

of an extractor system is advised. Solvents and

bonding agents should not come into contact with

your skin, gloves and safety goggles are a first line

of defence, and you are reminded to consult the

COSHH sheet supplied by the manufacturer with

all of these products.

Repair procedures

As a general rule thermoplastics can be welded and

thermosets bonded. We’ll have a look at a few procedures

in detail.

Starting with something simple. Often in an

accident repair a plastics headlamp binnacle is

scrapped because one of the lugs is broken off. If

you apply a small amount of acetone to both of the

broken surfaces you will often find that the lug can

be bonded back into place. The acetone (also used

as nail varnish remover and not popular amongst

mechanics) actually melts the plastics material,

pressing the two parts together causes them to

bond and dry.

Dashboards and some flexible bumper assemblies

which are made from thermoplastics can be welded.

There are two ways of welding thermoplastics. One

is to use a hot air welding gun and the other is to use

a soldering iron. The hot air welding gun blows out

a stream of air which will melt the plastic, the temperature

is over 100 °C. The paint should be cleaned

off about 20 mm (3/4 in) on both sides of the joints.

The welded joint is made using the blow gun and a

plastics filler rod in the same way as you would

oxy-acetylene weld steel. If it is a long joint you

should tack weld first. The two parts can be held

together with strips of masking tape on the reverse

whilst you carry out the welding. If a component

has cracked, like a bumper assembly, and internal

stress might cause the crack to continue during or

after the repair, it is a good idea to drill a small hole

at each end of the crack. Usually 4 mm (3/16 in)

holes at each end of the crack will be sufficient to

remove the internal stress. These should then be

filled after the welding has been completed. If you

are using a filler rod it is a good idea to ‘vee’ the

156Repair of Vehicle Bodies

edges of the joint to accommodate the filler, this can

be done using a file.

Thin thermoplastic items which will melt without

a great deal of heat being applied can be

welded using a soldering iron (without the solder).

The procedure is as follows. Remove any paint

within about 15 mm of the joint, a P40 disc is usually

ideal; drill stress relieving holes at each end if

it is a crack; hold the gap closed with tape on the

underside; run the soldering iron over the joint so

that the material melts and fuses together. When

the repair has cooled, remove any excess or

unevenness with the P60 and then finish to feather

into the existing paint using P600.

If the component, say a bumper, has been

holed, it is possible to weld in a piece from a

scrap bumper of the same shape. Cut the damaged

section out of the bumper, a round or oval shape

will prevent further cracking. Then cut a piece out

of the scrap bumper which will just fit into the

hole. Remove the paint from all the edges and

weld in as in the previous examples (blow gun or

soldering iron). Finish with P600 production

paper. If the joint or the final contour is not satisfactory

this can be corrected using body filler in

the normal way.

To repair a thermoset component you need to

bond or glue on a patch. If the damage is a crack,

such as in a bumper assembly, the procedure is to

clean up the damaged area on both sides of the

crack, drill stress relieving holes at each end, then

whilst the crack is held closed bond a patch to the

underside of the component. Complete the repair

using body filler and P600 paper in the normal way.

The patch is preferably the same material as the

damaged item; the bonding could be by a number

of materials, including superglue. If the bumper has

been holed, then cut a patch to fill the hole as you

would for a thermoplastic bumper and additionally

cut another patch which is larger than the hole. The

larger patch is then bonded to the underside so that

it attaches to both the original item and the piece

which is filling the hole. The job is again completed

using body filler and P600 to feather in the paintwork.

A small amount of body filler on the underside

to blend the patch into the surrounding

material will make the job look neat.

Be aware, that not all plastics can be repaired.

Those which have a waxy finish will not even let

superglue stick to their surface.

Painting

Plastics materials require a suitable keying primer

and/or undercoat. You should use the one which is

recommended by the vehicle manufacturer. A coat

of underseal on the rear of any panel which is open

to the elements will give added protection.

4.14 Safety glass

More and more glass is being used on modern cars.

Pillars are becoming slimmer and glass areas are

increasing as manufacturers approach the ideal of

almost complete all-round vision and the virtual

elimination of blind spots. Windscreens have

become deeper and wider. They may be gently

curved, semiwrapped round, or fully wrapped. With

few exceptions they are of one-piece construction,

sometimes swept back as much as 65° from the

vertical. Styling trends, together with a growing

knowledge of stress design in metal structures, have

resulted in a significant increase in the glazed areas

of modern car body designs. As a result of this move

towards a more open style, the massive increase in

the cost of energy has brought growing pressure on

vehicle designers to achieve more economic operations,

principally in respect to lower fuel consumption

through better power/weight ratios. An outcome

of these two lines of development has been a situation

in which although the area of glass has

increased, the total weight of glass has remained

constant or even decreased.

Broadly speaking, motor vehicle regulations

specify that windscreens must be of safety glass.

To quote one section: ‘On passenger vehicles and

dual-purpose vehicles first registered on or after

1 January 1959, the glass of all outside windows,

including the windscreen, must be of safety glass’.

The British Standards Institute defines safety glass

indirectly as follows: ‘All glass, including windscreen

glass, shall be such that, in the event of shattering,

the danger of personal injury is reduced to a

minimum. The glass shall be sufficiently resistant

to conditions to be expected normal traffic, and to

atmospheric and heat conditions, chemical action

and abrasion. Windscreens shall, in addition, be

sufficiently transparent, and not cause any confusion

between the signalling colours normally used.

In the event of the windscreen shattering, the driver

shall still be able to see the road clearly so that he

can brake and stop his vehicle safely.’

Metals and non-metals used in vehicle bodies 157

Two types of windscreen fulfil these requirements –

those made from heat-treated (or toughened) glass,

and those of laminated glass. In addition there are

plastic coated laminated or annealed safety glasses.

Most windscreens and some rear windows fitted in

motor vehicles are of ordinary laminated glass. For the

main part, toughened glass is confined to door glass,

quarter lights and rear windows where the use of more

expensive laminated products has yet to be justified.

However, laminated glass is being increasingly used

on locations other than windscreens for reasons of

vehicle security and also for passenger safety (containment

in an accident), especially in estates with seating

in the rear. Note that the applicable EEC Directive

(see later) has effectively banned the fitment of toughened

windscreens from the end of 1992.

Ordinary laminated safety glass is the older of the

two types and is the result of a basic process discovered

in 1909. Some years earlier a French chemist,

Edouard Benedictus, had accidentally knocked down

a flask which held a solution of celluloid. Although

the flask cracked it did not fall into pieces, and he

found that it was held together by a film of celluloid

adhering to its inner surface. This accident led to the

invention of laminated safety glass, made from two

pieces of glass with a celluloid interlayer. An adhesive,

usually gelatine, was used to hold them together

and the edges had to be sealed to prevent delaminating.

However, despite the edge sealing, the celluloid

(cellulose nitrate plastic) discoloured and blistered;

hence celluloid was replaced by cellulose acetate

plastic, but this, although a more stable product than

celluloid, still needed edge sealing.

Nowadays a polyvinyl butyral (PVB) self-bonding

plastic interlayer is used; no adhesive is necessary

and the edges do not need sealing, making it quite

practical to cut to size after laminating. When producing

glasses to a particular size, however, the glass

and vinyl interlayer are usually cut to size first. In the

process the vinyl plastic interlayer is placed between

two clean, dry pieces of glass and the assembly is

heated and passed between rubber-covered rollers to

obtain preliminary adhesion. The sandwich of glass

and interlayer is then heated under pressure for a

specified period in an autoclave. This gives the

necessary adhesion and clarity to the interlayer,

which is not transparent until bonded to the glass. If

a piece of laminated glass is broken, the interlayer

will hold the splinters of glass in place and prevent

them flying.

Plastic coated laminated safety glass is an ordinary

laminated glass which has soft elastic polyurethane

films bonded on to the inner surface to

provide improved passenger protection if fragmentation

occurs. There is some interest in the

use of bilayer construction which uses 3 mm or

4 mm annealed glass bonded with a load bearing

surface layer of self-healing polyurethane.

Uniformly toughened glass is produced by a

completely different process, involving heating of

the glass followed by rapid cooling. Although

patents were taken out in 1874 covering a method

of increasing the strength of flat glass sheet by

heating and cooling it in oil, toughened glass was

not in common use until the 1930s. Modern toughened

glass is produced by heating the glass in a

furnace to just below its softening point. At this

temperature it is withdrawn from the furnace and

chilled by blasts of cold air. The rapid cooling

hardens and shrinks the outside of the glass; the

inside cools more slowly. This produces compressional

strain on the surfaces with a compensating

state of tension inside, and has the effect of making

the glass far stronger mechanically than ordinary

glass. If, however, the glass does fracture in use, it

disintegrates into a large number of small and

harmless pieces with blunted edges. The size of

these particles can be predetermined by an exact

temperature control and time cycle in the toughening

process, and manufacturers now produce a uniformly

toughened safety glass which will, when

broken, produce not less than 40 or more than 350

particles within a 50 mm square of glass. This conforms

to the British Standard specification.

The main standards for the UK are now:

BS 857

ECE R43 (UN regulation)

EEC Directive (AUE/178) (A common market

regulation)

BS 857 glazing is still valid but is seldom used

because ECE R43 is accepted throughout Europe,

Japan and Australia.

There are other types of safety glass – mostly

crossbreeds of the pure toughened glass screen –

which are designed to combine vision with safety.

These are modified zone-toughened glasses, having

three zones with varying fragmentation characteristics.

The inner zone is a rectangular area directly

in front of the driver, not more than 200 mm high

158Repair of Vehicle Bodies

and 500 mm long. This is surrounded by two

other zones, the outer one of which is 70 mm wide

all round the edge of the windscreen. This type of

windscreen has been fitted to various vehicles since

1962. As a result of ECE Regulation 43 this type of

windscreen has been superseded by the fully zebrazoned

windscreen. Many countries, including the

USA but with the exception of the UK, legislate

against toughened windscreens.

Although sheet and plate glass are manufactured

satisfactorily for use in doors, rear lights and windscreens,

float glass has now largely superseded their

use for reasons of economy and improved flatness.

Most laminated windscreens used in the motor

vehicle trade are 4.4 mm, 5 mm, 5.8 mm or 6.8 mm

in overall thickness, with a 0.76 mm PVB interlayer.

However, 4.4 mm is the thinnest laminated

glass available, and as this has to be made from two

pieces of glass it needs very careful handling during

manufacture and is therefore expensive. Windscreens

made from float glass should be a maximum of

6.8 mm thick, whether toughened or laminated.

However, some large coaches and lorry windscreens

are 7.8 mm thick (4 mm glass _ 0.76 mm PVB

interlayer _ 3 mm glass). This gives immense

strength and robustness against stone impact. Other

body-glasses, because they can be made from sheet

glass and also can be toughened safety glass, are

usually between 3 mm and 4 mm thick.

From our brief look at the history of glass manufacture

it is obvious that the curving of glass presents

no problems; in fact the problem has been to produce

flat, optically perfect glass. However, to curve

safety glass and still retain its optical and safety

qualities requires careful control. Glass has no definite

melting point, but when it is heated to approximately

600 °C it will soften and can be curved.

Curved glasses should be specified as 6.8 mm

thick, as it is more difficult to control the curving of

4.4 mm glass. Even 6.8 mm thick glasses will have

slight variations of curvature. To accommodate this

tolerance, all curved glasses should be glazed in a

rubber glazing channel, of which there are many

different sections available. Glazed edges of glasses

should be finished with a small chamfer known as

an arrissed edge, while edges of glasses that are visible

or which run in a felt channel should be finished

with a polished, rounded edge. Should a glass

be required for glazing in a frame, a notch will usually

be required to clear the plate used to join the

two halves of the frame together. The line of this

notch must not have sharp corners because of the

possibilities of cracking. Although laminated safety

glass can be cut or ground to size after laminating,

toughened safety glass must be cut to size and edge

finished before the heat treating process.

Nearly all fixed glazing is now glazed using adhesive

systems. Shapes are becoming more complex,

needing very good angles of entry control to meet

bonding requirements. The trend is towards aerodynamical

designs involving flush glazing and the

removal of sudden changes in vehicle shape; therefore

corners must be rounded rather than angular as

in older vehicle designs. Glass is often supplied with

moulded-on finisher (encapsulation). Consequently

bending processes are becoming very sophisticated.

Adhesive glazing (polyurethane is the adhesive normally

used) has added considerably to the complexity

of vehicle glazing in a scientific sense. It has

many advantages, however, if carried out correctly: it

will reduce water leaks, it suits modern car construction,

it results in a load bearing glazing member, and

it lends itself to robotic assembly in mass production.

As a consequence of adhesive glazing, all the associated

glazing is now printed with a ceramic fired-in

black band to protect the polyurethane adhesive from

ultraviolet degradation, and also for cosmetic reasons

so that the adhesive cannot be seen.

By a Ministry of Transport regulation, safety

glass was made compulsory in 1937 for windscreens

and other front windows. As already indicated,

with effect from 1 January 1959 the Road

Traffic and Vehicle Order 359 has demanded that

for passenger vehicles and dual-purpose vehicles,

all glass shall be safety glass. For goods vehicles,

windscreens and all windows in front of, or at the

side of, the driver’s seat shall be safety glass.

Questions

1 What would the following alloy steels be used for:

(a) high-tensile steel (b) manganese steel

(c) chrome-vanadium steel?

2 List the properties of commercially pure aluminium.

3 Explain why, in the construction of a motor vehicle,

commercially pure aluminium has a very limited

application.

4 Identify the grades of hardness in aluminium

sheet and state how the hardness is achieved.

Metals and non-metals used in vehicle bodies 159

5 Explain how you would identify the following:

(a) low-carbon steel (b) aluminium alloy

(c) stainless steel.

6 Describe the difference between laminated safety

glass and toughened safety glass.

7 Give three requirements of a body sealing

compound, and describe one type of sealer used

in vehicle repair.

8 Suggest reasons why stainless steel is

sometimes used for trim and mouldings.

9 Explain the difference between hide and PVC

materials.

10 Explain what is meant by micro-alloyed steel or

HSS.

11 Give reasons why the car manufacturers are

using zinc-coated steels.

12 Name the three main groups of stainless steel.

13 Explain the following terms in relation to plastic:

(a) monomer (b) polymer (c) copolymer.

14 Explain the difference between thermoplastics and

thermosetting plastics.

15 Which safety glass, used for vehicle

windscreens, shatters into small segments on

impact?

16 Describe the basic properties required of a body

joint sealing compound.

17 Identify the group of plastics that can be softened

or remoulded by the application of heat.

18 Steel panels can be strengthened without adding

weight. Name and explain the process.

19 Describe three different ways in which the surface

of steel can be protected.

20 State the reasons why certain metals need to be

protected from the effects of the atmosphere.

21 Describe how some metals can resist attack by

the atmosphere.

22 What is the alloying effect when zinc and copper

are added to aluminium?

23 Explain the different properties of heat-treatable

and non-heat-treatable aluminium alloys.

24 State the reasons, other than weldability, why

low-carbon steel is chosen in preference to

aluminium as a vehicle body shell material.

25 Define the term ‘HSLA steel’.

26 Define what is meant by the term ‘non-ferrous

metal’.

27 Explain the difference in properties between lowcarbon

steel and alloy steel.

28 Describe the two processes which can be used

to join plastic.

29 Explain where plastic can be used on a vehicle.

30 State the applications where natural rubber has

been replaced by synthetic materials in the

automobile industry.

31 Describe two ways of attaching a windscreen to a

vehicle body.

32 Describe how to replace a glass in an opening

quarter-light frame.

33 Why should you never hit a hammer with another

hammer?

34 Which material should not be used for axle stand

pins?

35 Why is brass often used for drifting bearings?

Metal forming

processes and

machines

5.1 Properties of metals

Metals and alloys possess certain properties which

make them especially suitable for the processes

involved in vehicle body work, particularly the

forming and shaping of vehicle body parts either

by press or by hand, and some of the jointing

processes. These properties are described in the

following sections, and some typical values of

characteristics are shown in Tables 5.1 and 5.2.

Malleability

A malleable metal may be stretched in all directions

without fracture occurring, and this property

is essential in the processes of rolling, spinning,

wheeling, raising, flanging, stretching and shrinking.

In the operation of beating or hammering a

metal on a steel block (such as planishing) an

action takes place at each blow wherein the metal

is squeezed under the blow of the hammer and is

forced outwards around the centrepoint of the

blow. The thinner the metal can be rolled or hammered

into sheet without fracture, the more malleable

is the metal.

After cold working, metals tend to lose their

malleable properties and are said to be in a work

hardened condition. This condition may be desirable

for certain purposes, but if further work is to

Table 5.1Physical properties of metals and alloys

Coefficient

Melting Specific heat Thermal Electrical of linear

temperature Density capacity/ conductivity conductivity expansion/

Metal range (°C) (kg/m3) (J/kg/K _ 103) (W/m/K) (% IACS) (K _ 10_6)

Aluminium 660 2.69 0.22 218 63 23

Al-3.5 magnesium 550–620 2.66 0.22 125 25 23

Duralumin type 530–610 2.80 0.21 115–140 20–36 23

Copper 1085 8.92 0.39 393 101 17

70/30 brass 920/950 8.53 0.09 120 17 19

95/5 tin bronze 980/990 8.74 0.09 80 12 17

Lead 327 11.34 0.13 35 8 29

Magnesium 650 1.73 1.04 146 35 30

Nickel 1455 8.90 0.51 83 21 13

Monel 1330/1360 8.80 0.43 26 3 10

Tin 232 7.30 0.22 64 13 20

Titanium 1665 4.50 0.58 17 3 8.5

Zinc 419 7.13 0.39 113 26 37

Iron 1535 7.86 0.46 71 7 12

Mild steel 1400 7.86 0.12 45 31 11

be carried out the malleability may be restored by

annealing. Annealing, or softening, of the metal is

usually carried out before or during curvature work

such as raising and hollowing, provided the metal

is not coated with a low-melting-point material.

However, the quality of the modern sheet metal is

such that many forming operations, such as deep

drawing and pressing, may be carried out without

the need for an application of heat.

The following are examples in which the properties

of malleability are most evident:

Riveting Here the metal will be seen to have

spread to a marked degree. If splitting occurs the

metal is insufficiently malleable or has been overworked

(work hardened).

Shaping The blank for a dome consists of a flat

disc which has to be formed by stretching and

shrinking into a double-curvature shape. The more

malleable and ductile the material of the blank is,

the more readily it can be formed; the less malleable

and ductile, the more quickly does the metal

work hardened thus need more frequent annealing.

The degree of malleability possessed by a metal is

measured by the thinness of leaves that can be produced

by hammering or rolling. Gold is extremely

malleable and may be beaten into very thin leaf. Of

the metals used for general work, aluminium and

copper are outstanding for their properties. The

property of malleability is used to advantage in the

manufacture of mild steel sheets, which are rolled

to a given size and gauge for the motor industry.

It is also evident in the ability of mild steel and

aluminium panels to be formed by mechanical

presses into complicated contours for body shells.

Malleability and ductility are the two essential

properties needed in order to mass produce vehicle

body shells by pressing.

The order of malleability of various metals by

hammering is as follows: gold, silver, aluminium,

copper, tin, lead, zinc, steel.

Ductility

Ductility depends on tenacity or strength in tension

and the ease with which a metal is deformed, and

is the property which enables a metal to be drawn

out along its length, that is drawn into a wire. In

wire drawing, metal rods are drawn through a hole

in a steel die; the process is carried out with the

metal cold, and the metal requires annealing when

it becomes work hardened.

Ductile properties are also necessary in metals

and alloys used in the following processes:

Pressed components Special sheets which have

extra deep drawing qualities are manufactured especially

for press work such as that used in modern

motor vehicle body production. These sheets undergo

several deformations during the time they are being

formed into components, yet because of their outstanding

ductile properties they seldom fracture.

Welding electrodes and rods Ductility is an essential

property in the production of electrodes, rods

and wires. The wire drawing machines operate at

Metal forming processes and machines 161

Table 5.2Typical mechanical properties of metals and alloys

Modulus of elasticity Tensile and compressive

Material E (kN/mm2) strength (N/mm2) Elongation (%) Hardness (HV)

Pure aluminium 68–70 62–102 45–7 15.30

Aluminium alloys 68–72 90–500 20–5 20–80

Magnesium 44 170–310 5–8 30–60

Cast irons (grey) 75–145 150–410 0.5–1.0 160–300

SG cast iron 170–172 370–730 17–2 150–450

Copper 122–132 155–345 60–5 40–100

Copper alloys 125–135 200–950 70–5 70–250

Mild steel 190 420–510 22–24 130

Structural steels 190 480–700 20–24 130

Stainless steel 190 420–950 40–20 300–170

Titanium 100–108 300–750_ 5–35 55–90

Zinc 90 200–500 25–30 45–50

162Repair of Vehicle Bodies

exceptionally high speeds and the finished product

conforms to close tolerances of measurement; frequent

failure of the material during the various

stages of drawing would be very costly.

The order of ductility of various metals is as follows:

gold, aluminium, steel, copper, zinc, lead.

Tenacity

A very important property of metals is related to its

strength in resistance to deformation; that property

is tenacity, which may be defined as the property

by which a metal resists the action of a pulling

force. The ultimate tensile strength of a metal is a

measure of the force which ultimately fractures or

breaks the metal under a tensile pull. The ultimate

tensile strength (UTS) of a material is normally

expressed in tons per in2 or MN/m2, and may be

calculated as follows:

In this case the load is the maximum required to

fracture a specimen of the material under test, and

the calculation is based on fracture taking place

across the original cross-sectional area. In ductile

materials a special allowance must be made for

wasting or reduction of original cross-sectional

area.

High-carbon steels possess a high degree of

tenacity, evidence of which can be seen in the steel

cables used to lift heavy loads. The mild steels

used in general engineering possess a small

amount of tenacity, yet a bar of metal of one inch

square (6.5 cm2) cross-section, made from lowcarbon

steel, is capable of supporting a load in

excess of 20 tonnes.

Methods of increasing tensile strength

It is possible to increase the tensile strength of both

sheet steel and pure aluminium sheets by cold

rolling, but this has the added effect of reducing

their workable qualities. In the manufacture of vessels

to contain liquids or gases under pressure it is

not always possible to use metals with a high tensile

strength; for instance, copper is chosen to

make domestic hot water storage cylinders because

this metal has a high resistance to corrosion. In this

case, the moderate strength of copper is increased

UTS _

tensile force in N

cross-sectional area in mm

by work hardening such as planishing, wheeling

and cold rolling. Work hardening has the added

effect of decreasing the malleability.

The order of tenacity of various metals in tons

per in2 (MN/m2) is: steel 32 (494); copper 18 (278);

aluminium 8 (124); zinc 3 (46); lead 1.5 (23).

Hardness

When referring to hardness, it should be carefully

stated which kind of hardness is meant. For example,

it may be correctly said that hardness is that

property in a metal which imparts the ability to:

1 Indent, cut or scratch a metal of inferior hardness

2 Resist abrasive wear

3 Resist penetration.

A comparison of hardness can be made with the

aid of material testing machines such as those

used to carry out the Brinell or Vickers Diamond

tests. Hardness may be increased by the following

methods:

Planishing In addition to increasing the tensile

strength of a metal, planishing also imparts hardness.

Heat treatment Medium- and high-carbon steels,

such as those used in many body working tools,

can be hardened by heating to a fixed temperature

and then quenching.

The order of hardness of various metals is as follows:

high-carbon steel, white cast iron, cast iron,

mild steel, copper, aluminium, zinc, tin, lead.

Toughness

This property imparts to a metal the ability to resist

fracture when subjected to impact, twisting or

bending. A metal need not necessarily be hard to

be tough; many hard metals are extremely brittle, a

property which may be regarded as being opposite

to toughness.

Toughness is an essential property in rivets.

During the forming process the head of the rivet is

subject to severe impact, and when in service rivets

are frequently required to resist shear, twist and

shock loads. Toughness is also a requisite for steel

motor car bodies, which must be capable of withstanding

heavy impacts and must often suffer severe

denting or buckling without fracture occurring.

Further, when repairs are to be made to damaged

Metal forming processes and machines 163

areas it is often necessary to apply force in the direction

opposite to that of the original damaging force;

and the metal must possess a high degree of toughness

to undergo such treatment.

Compressibility

Compressibility may be defined as the property by

which a metal resists the action of a compressing

force. The ultimate compressive strength of a metal

is a measure of the force which ultimately causes

the metal to fail or yield under compression.

Compressibility is related to malleability in so far

as the latter refers to the degree to which a metal

yields by spreading under the action of a compressing

or pushing force, while the former represents

the degree to which a metal opposes that

action.

Elasticity

All metals possess some degree of elasticity; that

is, a metal regains its original shape after a certain

amount of distortion by an external force. The elastic

limit of a metal is a measure of the maximum

amount by which it may be distorted and yet return

to its original form on removal of the force.

Common metals vary considerably in elasticity.

Lead is very soft yet possesses only a small

amount of elasticity. Steel, on the other hand, may

reveal a considerable degree of elasticity as, for

example, in metal springs. The elasticity of mild

steel is very useful in both the manufacture of

highly curved articles by press work and in the

repair of motor car bodies.

Fatigue

Most metals in service suffer from fatigue.

Whether the metal ultimately fails by fracture or