In order to mould or laminate a polyester resin,
the resin must be cured. This is the name given to
the overall process of gelation and hardening,
which is achieved either by the use of a catalyst
and heating, or at normal room temperature
by using a catalyst and an accelerator. Catalysts
for polyester resins are usually organic peroxides.
Pure catalysts are chemically unstable and
liable to decompose with explosive violence.
They are supplied, therefore, as a paste or liquid
dispersion in a plasticizer, or as a powder in an
inert filler. Many chemical compounds act as
accelerators, making it possible for the resin-containing
catalyst to be cured without the use of
heat. Some accelerators have only limited or specific
uses, such as quaternary ammonium compounds,
vanadium, tin or zirconium salts. By far
the most important of all accelerators are those
based on a cobalt soap or those based on a tertiary
amine. It is essential to choose the correct
type of catalyst and accelerator, as well as to
use the correct amount, if the optimum properties
of the final cured resin or laminate are to be
obtained.
Pre-accelerated resins
Many resins are supplied with an in-built accelerator
system controlled to give the most suitable
gelling and hardening characteristics for
the fabricator. Pre-accelerated resins need only
Figure 16.12 Production of unsaturated polyester resin (Scott Bader Co. Ltd)
Reinforced composite materials 541
the addition of a catalyst to start the curing reaction
at room temperature. Resins of this type are
ideal for production runs under controlled workshop
conditions.
The cure of a polyester resin will begin as soon
as a suitable catalyst is added. The speed of the
reactions will depend on the resin and the activity
of the catalyst. Without the addition of an accelerator,
heat or ultraviolet radiation, the resin will take
a considerable time to cure. In order to speed up
this reaction at room temperature it is usual to add
an accelerator. The quantity of accelerator added
will control the time of gelation and the rate of
hardening.
There are three distinct phases in the curing
reaction:
Gel time This is the period from the addition
of the accelerator to the setting of the resin to a
soft gel.
Hardening time This is the time from the setting
of the resin to the point when the resin is hard
enough to allow the moulding or laminate to be
withdrawn from the mould.
Maturing time This may be hours, several days
or even weeks depending on the resin and curing
system, and is the time taken for the moulding or
laminate to acquire its full hardness and chemical
resistance. The maturing process can be accelerated
by post-curing.
Fillers and pigments
Fillers are used in polyester resins to impart particular
properties. They will give opacity to castings
and laminates, produce dense gel coats, and
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impart specific mechanical, electrical and fire
resisting properties. A particular property may
often be improved by the selection of a suitable
filler. Powdered mineral fillers usually increase
compressive strength; fibrous fillers improve tensile
and impact strength. Moulding properties can
also be modified by the use of fillers; for example,
shrinkage of the moulding during cure can be considerably
reduced. There is no doubt, also, that the
wet lay-up process on vertical surfaces would be
virtually impossible if thixotropic fillers were not
available.
Polyester resins can be coloured to any shade by
the addition of selected pigments and pigment
pastes, the main requirement being to ensure thorough
dispersion of colouring matter throughout the
resin to avoid patchy mouldings.
Both pigments and fillers can increase the
cure time of the resin by dilution effect, and the
adjusted catalyst and promoter are added to
compensate.
Releasing agents
Releasing agents used in the normal moulding
processes may be either water-soluble film-forming
compounds, or some type of wax compound.
The choice of releasing agent depends on the size
and complexity of the moulding and on the surface
finish of the mould. Small mouldings of simple
shape, taken from a suitable GRP mould,
should require only a film of polyvinyl alcohol
(PVAL) to be applied as a solution by cloth,
sponge or spray. Some mouldings are likely to
stick if only PVAL is used. PVAL is available as a
soultion in water or solvent, or as a concentrate
which has to be diluted, and it may be in either
coloured or colourless form.
Suitable wax emulsions are also available as a
releasing agent. They are supplied as surface finishing
pastes, liquid wax or wax polishes. The
recommended method of application can vary
depending upon the material to be finished. Hand
apply with a pad of damp, good quality mutton
cloth or equivalent, in straight even strokes.
Buff lightly to a shine with a clean, dry, good
quality mutton cloth. Machine at 1800 rpm using
a G-mop foam finishing head. Soak this head in
clean water before use and keep damp during
compounding. Apply the wax to the surface.
After compounding, remove residue and buff
lightly to a shine with a clean, dry, good quality
mutton cloth.
Wax polishes should be applied in small quantities
since they contain a high percentage of wax
solids. Application with a pad of clean, soft cloth
should be limited to an area of approximately 1
square metre. Polishing should be carried out
immediately, before the wax is allowed to dry. This
can be done either by hand or by machine with the
aid of a wool mop polishing bonnet.
Frekote is a semi-permanent, multi-release,
gloss finish, non-wax polymeric mould release
system specially designed for high-gloss polyester
mouldings. It will give a semi-permanent release
542 Repair of Vehicle Bodies
interface when correctly applied to moulds from
ambient up to 135 C. This multi-release interface
chemically bonds to the moulds surface and
forms on it a microthin layer of a chemically
resistant coating. It does not build up on the mould
and will give a high-gloss finish to all polyester
resins, cultured marble and onyx. It can be used
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on moulds made from polyester, epoxy, metal or
composite moulds. Care should be taken to avoid
contact with the skin, and the wearing of suitable
clothing, especially gloves, is highly recommended.
These products must be used in a well
ventilated area.
Adhesives used with GRP
Since polyester resin is highly adhesive, it is the
logical choice for bonding most materials to GRP
surfaces.
Suitable alternatives include the Sika Technique,
which is a heavy-duty, polyurethane-based joining
compound. It cures to a flexible rubber which
bonds firmly to wood, metal, glass and GRP. It is
ideal for such jobs as bonding glass to GRP or
bonding GRP and metal, as is often required on
vehicles with GRP bodywork. It is not affected by
vibration and is totally waterproof. The Araldite
range includes a number of industrial adhesives
which are highly recommended for use with GRP.
Most high-strength impact adhesives (superglues)
can be used on GRP laminates.
Most other adhesives will be incapable of bonding
strongly to GRP and should not be used when
maximum adhesion is essential.
Core materials
Core materials, usually polyurethane, are used in
sandwich construction, that is basically a laminate
consisting of a foam sheet between two or
more glass fibre layers (Figure 16.13). This gives
the laminate considerable added rigidity without
greatly increasing weight. Foam materials are
available which can be bent and folded to follow
curved surfaces such as vehicle bodies. Foam
sheet can be glued or stapled together, then laminated
over to produce a strong box structure,
without requiring a mould. Typical formers
and core materials are paper rope, polyurethane
rigid foam sheet, scoreboard contoured foam
sheet, Termanto PVC rigid foam sheet, Term PVC
contoured foam sheet, and Termino PVC contoured
foam sheet.
Formers
A former is anything which provides shape or form
to a GRP laminate. They are often used as a basis
for stiffening ribs or box sections. A popular material
for formers is a paper rope, made of paper
wound on flexible wire cord. This is laid on the
GRP surface and is laminated over to produce reinforcing
ribs, which give added stiffness with little
extra weight. The former itself provides none of
the extra stiffness; this results entirely from the box
section of the laminate rib. Wood, metal, or plastic
tubing and folded cardboard can all be used successfully
as formers. Another popular material is
polyurethane foam sheet, which can be cut and
shaped to any required form (Figure 16.14).
16.6 Moulding techniques for reinforced
composite laminates
Contact moulding
This is the oldest, simplest and most popular fabrication
technique for the automotive, reinforced plastic
body industry. It is normally used for relatively
short runs, but it has also been adapted successfully
for series production. It is the only production
method which takes full advantage of the two
Figure 16.13 Typical sandwich construction (Scott
Bader Co. Ltd)
Reinforced composite materials 543
most important characteristics of polyester resin,
namely that it can be set without heat and without
pressure. A considerable industry has been built
around contact moulding, which has facilitated the
cost effective production of large one-piece mouldings,
particularly for low production runs. Contact
moulding advantages are that a minimum of equipment
is required, tooling is inexpensive, there are
practically no size restrictions, and design changes
are easily made. Disadvantages are that the labour
context is high and the quality of the moulding
depends on the skill of the operators. The lay-up and
curing times are comparatively slow, and only one
good surface finish is achieved.
The contact moulding process is carried out in
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the following manner. A master pattern or model is
made, representing in all its dimensions the finished
product. This could be, for example, a full-size
wood model of the type of body shell or cab shell
required, or it might equally well be a steel or aluminium
panel-beaten structure of composite type,
or even a plaster model reinforced with wire mesh.
From this is made a master mould, which must be
female or concave for the most part, and this would
in all probability be made in reinforced plastics
similar to those used for the final product. It is
important to differentiate, as a matter terminology,
between mould and moulding, one being the
production tool and the other the product itself.
An important aspect of the process is that the surface
of the mould will inevitably be faithfully reproduced
in the moulding, and accordingly if the mould
is bumpy or rough so will be the final article. It does
not usually matter if the unseen or partly hidden side
of the moulding is rough (indeed, it usually is), but
for the displayed surface to be unsightly is not normally
tolerable. This is why a female mould is usually
used. If the original pattern was very smooth, so
will be the inside of the mould and, therefore, the
outside of the moulding. This is why so much trouble
is taken over the pattern. If in wood it is tooled
and sanded to perfection, and if in metal it is panel
beaten with the greatest possible skill, then ground
and polished if necessary. It is of the greatest importance
that separation of the moulding from the
mould be easy. A special compound or a polish such
as carnauba wax and/or silicone lubricant can be
used; plastics film is occasionally used as a separating
membrane, as it will not adhere strongly to
either the mould or the moulding. For contact
moulding, the equipment is relatively simple and
inexpensive. Contact moulding can be further subdivided
into hand lay-up and spray lay-up.
The application of the release agent to the mould
is followed by brush or spray application of a gel
coat of resin. There has been a constant improvement
and development in polyester resins, and
among other things this has led to the introduction
of successful semi-flexible gel coats, which are of
particular interest to the motor industry. The gel coat
is a continuous skin on the working face of a moulding.
It is almost pure resin and its object is to give a
good finish as well as to protect the working surface
from corrosion and other damage. It also hides the
fibre pattern of the reinforcement. The gel coat can
be colour impregnated or otherwise specially formulated,
e.g. for extra abrasion or impact resistance. It
should be as even in thickness as possible, as thicker
areas are very susceptible to accidental damage,
while thin patches can lower the resistance of the
structure to moisture and to chemical attack.
Even in hand lay-up the spray method may be
used for this stage and for the application of the separating
agent, so that there is a small element of mechanization.
A fine surfacing tissue may be applied to
Figure 16.14 Typical formers: (a) metal tube
(b) paper rope (c) cardboard tube (d) foam strip
(e) wood (f) folded cardboard (Scott Bader Co. Ltd)
544 Repair of Vehicle Bodies
the gel coat while wet, or it may simply be allowed
to gel as it is. Further resin is sprayed on or brushed
on, and mat or woven cloth, which has been carefully
cut to patterns, is laid in position. Consolidation and
air removal are then effected by manual means. It is
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customary to use rollers made up of split washers for
this operation, which is an extremely important one
if consistency and strength of the moulding are to be
obtained. More mat or cloth is added in order to
build up the requisite thickness of reinforced plastics,
and the moulding is then allowed to set or cure.
Curing normally takes place at room temperature,
but sometimes under a certain degree of heat if the
process is to be accelerated. It should be remembered
that curing is itself a heat-producing process. Contact
moulding can also be carried out by simple mechanical
means, but the general principle is always that of
bringing the materials into contact with the mould,
without the use of any dies.
The following is a summary of the contact
moulding process:
1 The master mould must be spotlessly clean.
2 A release agent is applied to the entire surface
of the mould face (Figure 16.15).
and the drawing-through process is repeated
until the required thickness is achieved. It is
critical that all air bubbles be removed by the
brushing and rolling (Figure 16.18).
Figure 16.15 Application of release agent to mould
surface (Scott Bader Co. Ltd)
Figure 16.18 Rolling impregnated mat (Scott Bader
Co. Ltd)
Figure 16.17 Impregnation of glass mat with resin
(Scott Bader Co. Ltd)
Figure 16.16 Application of gel coat covering (Scott
Bader Co. Ltd)
3 Gel coat covering is applied by brush or spray
(Figure 16.16).
4 Catalyst is added to the resin and the catalysed
resin is smoothed over the gel-coated mould.
5 Glass fibre mat, precut to exact size, is laid on
the mould and a further small quantity of resin
is poured on to the mat. With brushes and hand
roller, the resin is drawn through the mat
(Figure 16.17). A second layer of mat is applied
Reinforced composite materials 545
6 The mould is allowed to cure naturally or heat
is used to speed up the curing process.
7 After curing, the moulds are broken and the
completed sections are removed (Figure 16.19).
8 They are then trimmed and ready for use.
single-nozzle gun, but liquid catalyst is metered
into the resin in the gun itself. A glass rovings
chopper delivers the reinforcement to the mould
surface as in the former system.
Although much of the manual labour of the hand
lay-up is eliminated by using the spray process,
thorough rolling is still necessary, not only to consolidate
the deposited glass resin mixture, but also
Figure 16.19 Final mould and moulding (Scott
Bader Co. Ltd)
Figure 16.20 The spray-up technique (BP
Chemicals (UK) Ltd)
Spray-up technique
A development from the basic manual contact
process which is employed with increasing frequency
in the automotive body industry is known
as spray-up (Figure 16.20). In this method, rovings
are automatically fed through a chopping unit and
the resultant chopped strands are blown or carried
by the sprayed resin stream on to the mould. The
glass and resin mix applied in this way is consolidated,
and the air pockets or bubbles are removed
by manual rolling, as in simple hand lay-up.
There are several commercial spraying systems
available, where the glass fibre and resin are
deposited simultaneously on the mould face. They
consist of two principal types. In the twin-pot system
a twin-nozzle spray gun is used, and in order
to prevent gelation in the gun the resin is divided
into two parts, one of which is catalysed and the
other accelerated. The two streams of resin spray
converge near the surface of the mould simultaneously
with a stream of glass fibre ejected by a glass
rovings chopper. In the other type, the catalyst
injector system, accelerated resin is sprayed from a
(b)
546 Repair of Vehicle Bodies
to ensure that the accelerated and catalysed portions
of the resin are adequately mixed. Considerable
skill is needed to control the thickness of the laminate
when using the resin glass spray gun. Spraying
reduces labour costs, especially when the volume of
production is large enough to keep the equipment in
constant use.
Hot press moulding
This process involves the use of chopped strand
glass mat, pre-impregnated with polyester resin,
which is then in general principle formed in presses
in a similar manner to that used for forming steel
sheet (Figure 16.21). In this case, however, the dies,
which are preheated, have to remain closed for the
curing cycle of the pre-impregnated mat, which may
involve a period from 15 to 30 seconds. Hot press
moulding using matched dies have good finish on
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both surfaces. Further, this method enables high
glass content and uniform dimensional properties
and appearances to be achieved at lower cost than
by other methods for runs above 1000 units. Such a
process reduces the labour content of producing
panels, but much increases the initial tooling charge.
systems. The most common moulding processes are
as follows (Figure 16.22): hand lay-up, spray-up,
resin transfer moulding, compression moulding,
injection moulding thermoplastics, injection moulding
thermosets, pultrusion, and reinforced reaction
injection moulding.
The three techniques used in the production of
body panels are as follows:
Cold press mouldings This is used in the manufacture
of the boot lid. The boot lid is formed by
cold pressing a mineral reinforced resin-coated
glass fibre mat in a gel-coated mould, forming a
component which is very stiff in relation to its
weight (see Figure 16.23).
Reinforced reaction injection moulding The RRIM
technique is used for all the vertical body panels
such as the front and rear wings, front grille and
bumper assembly, and rear panel and bumper
assembly. RRIM polyurethane has the properties of
good recovery from deformation, outstanding resistance
to wear, impact and abrasion, and a fast cycle
time in manufacture. The use of this material for all
exposed corners of the car helps to reduce minor
body damage repair (see Figure 16.22h).
Pressure assisted resin injection moulding The
bonnet is a pressure assisted moulding of sandwich
construction with polyester resin exterior on either
side of a rigid urethane core. The underside of the
moulding is an intumescent fire barrier which is a
major safety factor for an engine compartment
cover (see Figure 16.22e).
Lotus Cars Ltd
Lotus have been producing reinforced composite
motor cars since 1957. In 1973 the company introduced
the vacuum assisted resin injection (VARI)
system for vehicle body manufacture. The first
VARI moulded production car was the Lotus Elite
introduced in 1974; since then developments have
continued in the processes, tooling and techniques
of producing composite vehicle bodies.
The Lotus VARI process provides a method of
moulding fibre reinforced composite panels from
matched tooling. The process can be used to manufacture
large body panels with integrated foam
structures and captive metal fixings, using relatively
low-cost tooling. As there is no dependency upon
platen size and press tonnage an obvious limitation
of other processes there are no panel size
Figure 16.21 Hot press moulding
