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