Production composite moulding
Processes
There are many fibreglass reinforced plastic moulding
processes available to the designer. Each
process has its own characteristics, as well as its
own limitations as to part size, shape, production
rate, compatible reinforcements, and suitable resin
Reinforced composite materials 547
limitations. This means that the vehicle body structures
can be moulded in much fewer sections, minimizing
panel assembly times and costs. Lotus
mould their complete bodyshell in a few pieces,
each of which has an integral moulded structure.
This structure uses isophthalic polyester resin in
conjunction with continuous filament mat. In addition,
woven and unidirectional glass reinforcements
are used in areas where more specific loadings are
required. Kevlar aramid materials are also included
where particularly high strengths are needed.
The VARI tooling, which can be either metal
faced or constructed entirely from composite
materials, is designed so that each tool becomes its
own press. The pressure is created by vacuum
which draws the tools together and holds the male
and female throughout the moulding cycle.
Therefore no mechanical clamping mechanisms
are involved. After closing the tools the resin is
injected using a machine which dispenses precise
quantities of catalysed materials. After the curing
cycle the vacuum is reversed and the tools open,
releasing the moulded panel or body section.
16.7 Designing reinforced composite
materials for strength
The ability to design and fabricate large structures
as a whole, rather than as an assembly of components,
is one of the chief advantages that glass
resin laminates bring to the designer. This is supported
by the ease of modifying the material thickness
at specific locations and, taking advantage of
the properties of the various types of reinforcement,
by building in extra strength at any point and
in any direction (Figure 16.24). The skill of the
operator is an important factor.
Strength of glass fibre reinforced plastic laminates
in any direction is dependent on the orientation of
fibre reinforcement to that direction (Figure 16.25).
When chopped strand mat is used, its random fibre
arrangement can be expected to give roughly equal
mechanical properties in all planes; however, maximum
strength will in practice be parallel to the
plane of the laminate. Plain woven roving gives
optimum mechanical properties at right angles, while
unidirectional roving mat shows highest strength
along the roving; as the roving is continuous and
uncrimped, this last type will be stronger than other
types of reinforcement.
There is still an unfortunate tendency on the part
of designers to use a traditional design, known to
be satisfactory for wood or metal, for reinforced
plastics which have, of course, completely different
properties and processing characteristics
(Tables 16.3 and 16.4). This may give glass fibre
reinforced plastic mouldings of incorrect shape,
since although conventional materials are well
suited to straight lines and flat surfaces, the properties
of glass fibre reinforced plastic components
are improved by the introduction of curvature, and
if possible double curvature, to the design. The
specific strength (tensile strength weight ratio) of
glass-polyester laminates is high, but rigidity tends
to be on the low side. It may therefore be necessary
to design for additional rigidity rather than for
optimum tensile strength. This can be effected by
various means, of which increased overall moulding
thickness is perhaps the least desirable as it is
wasteful and may well add unnecessary weight.
The use of simple or compound curves in the
design may be the answer, or perhaps local corrugations
can be introduced as in metal designs;
these can often be incorporated into the overall
styling, especially in vehicle bodies. Localized
thickening, particularly towards the edges of a
panel, will contribute usefully to the stiffness of
the moulding. A common practice to achieve extra
rigidity is the integral moulding of ribs into the
reverse face of the laminate. These ribs, which are
often used in large boat hulls, can be solid or hollow;
for solid ribs a permanent core of glass fibre,
wood or plastic foam can be laminated in, while
hollow ribs are achieved by use of removable tube
or simple former of cardboard or similar material
(see Figure 16.14).
Large panels can be made considerably more
rigid by the employment of a sandwich form of
construction, whereby two layers of glass fibre
reinforced plastic are separated by a thick but relatively
weak lightweight material. The benefit here
derives from the fact that stiffness is a direct function
of thickness. Other advantages of this method
are increased heat and sound insulation. Stress
analysis is usually based upon the tensile strength at
which crazing of the resin matrix occurs. This corresponds
to the yield point of conventional materials.
It is usual to allow a safety factor of between
1:3 and 3:5, depending upon the conditions of service.
Frequently the stressing of a moulding is so
548Repair of Vehicle Bodies
Reinforced composite materials 549
Figure 16.22Composite moulding processes (Owens-Corning Fiberglas)
(a) Hand lay-up is a low-to-medium volume moulding method suitable for making boats, tanks, housings and
building panels and other large parts requiring high strength. The process provides only one finished surface.
(b) Spray-up is a low-to-medium volume moulding method similar to hand lay-up in its suitability for making
boats, tanks, tub/showers and other medium to large size shapes. Greater shape complexity is possible with
spray-up than with hand lay-up.
(c) Resin transfer moulding (RTM) is suitable for medium-volume production and may be regarded as an
intermediate process between spray-up and faster compression moulding methods using SMC and BMC.
RTM provides two finished surfaces. The reinforcement is placed in the bottom half of the mould. The mould
is then closed and clamped, and catalysed resin is pumped in under pressure until the mould is filled.
Moulds are usually made of reinforced plastics.
(d) Compression moulding is a high-volume, high-pressure process suitable for moulding complex,
high-strength fibreglass reinforced plastic parts using sheet moulding compound, bulk moulding
compound or preforms. Fairly large parts can be moulded in medium to high volumes with excellent
surface finish.
(e) Injection moulding is the highest volume method of any of the fibreglass reinforced plastic processes using
single or multi-cavity moulds to produce large volumes of complex parts at high production rates. Advanced
fibreglass technology makes it possible to injection mould glass reinforced thermoplastics that provide a
wider variety of mechanical, chemical, electrical and thermal properties than previously available.
(f) Thermoset moulding compounds are injection moulded with a low-temperature injection screw/plunger and
chamber, and a high temperature mould. This cures the thermoset material under heat and pressure.
Injection moulding of thermoset resins offers the capability to produce high volumes of very complex parts
with good mechanical properties and impact strengths.
(g) Pultrusion is a continuous process for the manufacture of products having a constant cross-section, such as
rod stock, structural shapes, beams, channels, pipe, tubing and fishing rods.
(h) RRIM is emerging as a leading process. RRIM provides low-cost tooling, low-pressure moulding and design
flexibility to accommodate inserts or encapsulations of structural supports.
complex that it defeats analysis. In such cases it
may be necessary to base the design upon the
known performance of a similar structure and to
produce a prototype moulding for testing under
service conditions. Where metal inserts are to be
incorporated in the moulding, allowance should be
made in the design of the joint for the greatly differing
yields of the two materials under similar
loading. A wider insert will result in lower stress
per inch at the load transfer point. The stress in the
resin laminate can be taken up to a point where
strain in both materials is similar by thickening
the laminate where it approaches and surrounds the
metal insert. Ideally the base of the insert should be
about four times as wide as it is long.
It is better to achieve load transfer by adhesion
rather than by mechanical interlocking; mechanical
methods are satisfactory, however, if only small
loads are involved. Joining of two glass fibre reinforced
plastic componenets to each other can be
effected by adhesive or mechanical means, or by a
combination of both. In adhesive joint design the
bonding area should be as large as possible. For ordinary
butt or scarf joints, extra reinforcement should
be provided by lamination of extra layers of resin
and glass over the joint on both sides; in general it is
better to use overlapping joints (Figure 16.26).
Exposure of glass fibre by roughening in each case
will enhance bond strength. Cured laminates are
commonly bonded by means of epoxy-type adhesives.
A polyester resin adhesive may be used, but
here it is necessary to ensure that the adhesive films
are thick enough to avoid problems of undercure.
This can be done by including a single sheet of
glass cloth or mat with open texture in the joint.
Most kinds of mechanical fastener (nuts and bolts,
(Facing page)
550Repair of Vehicle Bodies
Figure 16.23Cold press process: (a) 50 tonne hydraulic press (b) Kirksite metal mould (c) moulding
(d) continuous filament glass (e) polyester surface finish (Reliant Motor PLC )
self-tapping screws, rivets and others) can usually be
employed without any trouble, provided the load is
spread by means of large heads and/or large washers.
To prevent laminate crushing it is a good plan to use
spacers for bolted connections. Provided the extent
of the bonded area is taken into account, and thorough
cleaning and roughening are carried out, there
is no reason why drilled and tapped metal plates, or
special tapped inserts, should not be laminated into
the moulding or on the reverse side.
Composite theory
In its most basic form a composite material is one
which is composed of two elements working
together to produce material properties that are different
to the properties of those elements on their
own. In practice, most composites consist of a bulk
material called the matrix, and a reinforcement
material of some kind which increases the strength
and stiffness of the matrix.
Polymer matrix composites (PMC) is the type of
composites used in modern vehicle bodywork.
This type of composite is also known as Fibre reinforced
polymers (or plastics) (FRP). The matrix is
a polymer-based resin and the reinforcement material
is a fibrous material such as glass, carbon or
aramid. Frequently, a combination of reinforcement
materials is used.
The reinforcement materials have high tensile
strength, but are easily chaffed and will break if
folded. The polymer matrix holds the fibres in
place so that they are in their strongest position and
protects them from damage. The properties of the
composite are thus determined by:
• The properties of the fibre.
• The properties of the resin.
Reinforced composite materials 551
• The ratio of fibre to resin in the composite –
fibre volume fraction (FVF).
• The geometry and orientation of the fibres in
the composite.
Resin
The choice of resins depends on a number of characteristics,
namely:
• Adhesive properties – in relation to the type of
fibres being used, and if metal inserts are to be
used such as for body fitting.
• Mechanical properties – particularly tensile
strength and stiffness.
• Micro-cracking resistance – stress and age
hardening causes the material to crack, the
micro-cracks reduce the material strength and
eventually lead to failure.
• Fatigue resistance – composites tend to give
better fatigue resistance than most metals.
• Degradation from water ingress – all laminates
permit very low quantities of water to pass
through in a vapour form. If the laminate is wet
for a long period, the water solution inside the
laminate will draw in more water through the
osmosis process.
• Curing properties – the curing process alters the
properties of the material. Generally oven curing
at between 80 °C and 180 °C will increase
the tensile strength by up to 30%.
• Cost – the different materials cost different
prices.
The main types of resins are polyesters,
vinylesters, epoxies, phenolics, cyanate esters, silicones,
polyurethanes, bismaleides (BMI) and
polyamides. The first three are the ones mainly
used for vehicle body work as they are reasonably
priced (Table 16.5). Cyanates, BMI and polyamides
cost about 10 times the price of the others.
Reinforcing fibres
The mechanical properties of the composite material
are usually dominated by the contribution of
the reinforcing fibres. The four main factors which
govern this contribution are:
1 The basic mechanical properties of the fibre.
2 The surface interaction of the fibre and the
resin – called the interface.
3 The amount of fibre in the composite – FVF.
4 The orientation of the fibres.
The three main reinforcing fibres used in vehicles
are: glass, carbon and aramid. In addition, the following
are used for non-body purposes: polyester,
polyethylene, quartz, boron, ceramic and natural
fibres such as jute and sisal.
Glass is discussed separately in Section 16.4
Aramid Aramid fibre is a man-made organic
polymer, an aromatic polyamide, produced by
spinning fibre from a liquid chemical blend.
The bright golden yellow fibres have high strength
and low density giving a high specific strength.
Aramid has good impact resistance. Aramid is better
known by its Dupont trade name Kevlar.
Carbon Carbon fibre is produced by the controlled
oxidation, carbonization and graphitization
of carbon-rich organic materials – referred to as
precursors – which are in fibre form. The most
common precursor is polyacrylonitrile (PAN);
pitch and cellulose are also used (Table 16.6).