The car manufacturing industry, in its endeavours
to reduce weight and improve fuel economy, forecasts
a greater use of plastics in future designs
(see Section 4.12). Considering the extensive use
Figure 14.111 Before and after repair (Glas-Weld Systems (UK) Ltd)
Major accident damage 473
of plastics on current models, coupled with an even
greater anticipated usage, it is therefore important
that engineers and repairers are knowledgeable in
the various types of plastic that are being used and
are aware that some plastics can be repaired. Many
body components are manufactured in a variety of
plastics. Bumpers, grilles, light surrounds and even
complete body panels have enabled designers to
enhance vehicle aerodynamic styling and cosmetic
appeal while retaining impact resistance and eliminating
corrosion from these areas.
Plastic offers the same structural strength as steel
in a body component because of its greater elasticity.
Minor impacts that could deform steel beyond
repair can be absorbed by plastic. Where damage is
incurred it is capable of repair by welding. Cracks,
splits, warping and even the loss of material from
plastic components can be remedied with the aid of
the Leister Triac hot air welding equipment. Where
a steel component with equivalent damage would
be replaced at some cost, the repair of the plastic
Figure 14.113 Cleaning the damaged area
(Glas-Weld Systems (UK) Ltd)
Figure 14.114 Fitting injector to the windscreen
(Glas-Weld Systems (UK) Ltd)
Figure 14.112 Damaged windscreen (laminated
safety glass) (Glas-Weld Systems (UK) Ltd)
Figure 14.115 Using ultraviolet lamp to cure resin
(Glas-Weld Systems (UK) Ltd)
474 Repair of Vehicle Bodies
part can save time and expense, particularly when
winter accident periods make great demands of the
vehicle repairers parts stock. Welding plastics does
not produce fumes when the correct procedure is
followed. In a short time a plastic component can
be restored to an as new condition without the
need for fillers or special treatments. The combination
of welding and recommended paint procedures
will show no trace of a repair that should last
the life of the vehicle.
Identification of the material
There are two basic types of plastic used in cars:
thermosetting and thermoplastic (see Section 4.12.3).
Thermosetting plastics are cured by heat during
manufacture. They cannot be welded; any more
heat applied to them will break down their molecular
bonding. The way to repair them is by using
adhesive. Thermoplastic materials can melt without
breaking down, so they can be manipulated like
steel. With the right amount of heat they can be
bent, softened and welded. The skill is in the selection
of the appropriate temperature. The majority
of plastics employed in vehicle manufacturing are
thermoplastics. There are different types of thermoplastics,
each having a specified temperature
for welding operations.
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Experienced repairers of plastic may recognize
broad types of plastic by the degree of hardness or
softness of the component surface. A quick test for
identifying to which group a plastic belongs can be
made by cutting a very thin sliver off the component.
The thermoplastic curls right back on itself,
but the thermoset stays straight.
In their body repair manuals, some vehicle manufacturers
give details of plastic parts, while others
mark all plastic parts with code letters. However,
when no information or coding is available, identification
can be carried out in two ways. The first
is the method of combustion testing, where a small
strip of material is held in a butane flame and the
flame colour is noted. The other method, which
works on the majority of the most common plastic
materials, is to use an organic solvent test kit.
These kits, produced by an automotive paint manufacturer,
work by a process of solvent application
to the material surface. A normally concealed area
should be chosen and degreased, and any paint
applied to the surface should be removed from
the test area with the aid of an abrasive. Identification
is confirmed by reaction between the plastic
and test solvent in accordance with the test kit
manufacturers instructions. Gloves and face masks
should always be worn when using solvents.
The usual plastic identification codes are as
follows:
ABS acrylonitrile butadiene styrene
ABS/PC polymer alloy of ABS
PA polyamide (nylon)
PBT polybutylene terephthalate (POCAN)
PC polycarbonate
PE polyethylene
PP polypropylene
PP/EPDM polypropylene/ethylenediene rubber
PUR polyurethane (not all PUR is
weldable)
PVC polyvinyl chloride
GRP/SMC glass fibre reinforced plastics (not
weldable)
Equipment for plastic welding
The Welwyn Tool Company supply the Leister
Triac welding guns (see Figure 14.116). These
blow hot air out of their nozzles, and sophisticated
electronics make sure that the temperature of this
air is exactly right. A potentiometer is set to control
the power (Figure 14.117), a photo-electric cell
monitors the colour of the heating element and, as
a failsafe, a thermocouple double checks the output.
The guns can maintain a critical temperature
Figure 14.116 Leister Triac electric welding tool and
accessories (Welwyn Tool Co. Ltd)
Major accident damage 475
setting to _5 C, independent of any mains voltage
fluctuations. It is important to use a welding rod
of the same material as the component: hence the
importance of correct identification. The welding
rods come in various profiles, and there is a speedweld
nozzle available to suit each rod.
Repair method
Suppose a bumper has suffered a long, straight
crack which makes it particularly suitable for repair
by welding (see Figure 14.118). When carrying
out the repair process, thorough preparation of the
surfaces is important. When working on a painted
bumper it is necessary to sand off any paint in the
vicinity of the area to be welded; a mask for facial
protection is then necessary. To minimize the risk
of damage spreading, the ends of the crack are
drilled out with a 2 mm drill. The edges of the
crack are then shaped into a V with a burring bit
which has a cutting edge on both its circumference
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and its face (see Figure 14.119). This will allow
the welding rod, which in this case is supplied in
a V shape, to fit snugly into the crack. Care should
be taken not to penetrate more than two-thirds into
the depth of the material. The extent of the damage
should now be clearly visible.
Figure 14.117 Rotary control on rear of hot air tool
for temperature setting (Welwyn Tool Co. Ltd)
Figure 14.118 Split bumper
(Welwyn Tool Co. Ltd)
Figure 14.119 Rotary burring tool cutting
a V-groove (Welwyn Tool Co. Ltd)
In any welding operation it is important that the
two surfaces to be joined should remain in alignment,
so they should be tacked together using a tacking
nozzle (see Figure 14.120). The section of the
temperature chart relating to this nozzle must be consulted
to determine the correct gun setting; the temperature
can then be adjusted on the potentiometer.
After allowing the gun to heat up for at least two
minutes, the tacking operation can be carried out. The
nozzle is held at an angle of about 20 degrees and is
run along the crack without applying any pressure.
The speed-weld nozzle is fixed to the gun, and
the temperature is again adjusted prior to carrying
out the weld. The end of the welding rod is
trimmed into a point to ensure a smooth start to
the weld (see Figure 14.121). The welding rod is
476 Repair of Vehicle Bodies
inserted into the speed-weld nozzle, and as soon
as the edges show signs of melting the weld
can be carried out (see Figure 14.122). Contact
should be maintained between the nozzle and the
workpiece but, as with the tacking operation, no
pressure should be applied with the gun. The welding
rod, however, should be pushed well into the
crack. The rate at which the welding rod is fed into
the gun should match the distance travelled. It is
also imporant to operate the gun at the correct
angle. When the weld is finished, the gun is pulled
away leaving the welding rod attached. The excess
rod is then trimmed off.
The bumper is allowed to cool, and the welded
area can then be sanded down with an 80 grit
disc before being passed on to the painter (see
Figure 14.123). The bumper has a non-textured
painted finish, which will be reinstated. After
normal preparation and priming have been carried
out a filler is applied. After flatting the filler,
two further coats of primer are sprayed on and flatted.
The usual cleaning procedures should be carried
out between each process. The colour can now
be applied. Painting should always be carried out
according to the paint manufacturers instructions.
If the repair method is carried out correctly, the
component should achieve up to 90 per cent of its
original strength.
When repairing plastic materials there must
always be strict observance of health and safety
requirements, so the operator should work in a well
ventilated environment. Using the gun at an abnormally
high temperature does not assist the welding
operation, and can cause dangerous fumes.
Figure 14.120 Tack welding split in bumper
(Welwyn Tool Co. Ltd)
Figure 14.121 Welding rod trimmed to a point
(Welwyn Tool Co. Ltd)
Figure 14.122 Main plastic welding operation
(Welwyn Tool Co. Ltd)
Major accident damage 477
Welding defects
A tack weld can be broken and restarted if panel
alignment is not achieved on the first attempt.
To prove main weld strength, allow it to cool then
attempt to pull it from the groove by the attached
welding rod. If the weld stays firmly in place then
the weld has been successful.
Typical welding defects and their causes are as
follows (see Figure 14.124):
Poor weld penetration or bonding Incorrect weld
site preparation; welding speed too fast; temperature
too low; weld attempted with dissimilar materials;
poor technique.
Uneven weld bead width Welding rod stretched;
uneven pressure applied to welding rod.
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Charred weld Welding speed too slow; temperature
too high; repair area overheated.
Warping Parts fixed under tension; poor site
preparation.
Plastic repair
Duramix combines the tremendous strength and
durability of a plastic weld system with the flexibility
of chemical curing products, thus eliminating the
need for a filler or stopper to create the complete
cosmetic finish. It is compatible with all plastics,
fibre-glass and metal surfaces. Repairs can be carried
out on flexible plastic bumper bars, semi-rigid
plastic panels, ABS plastic grilles, polycarbonate
bumper bars, and thermoplastic bumper bars.
Duramix is a urethane composition. It applies
easily to vertical surfaces, resists sagging, contours
with a spreader to the original surface shape, sets
within three minutes, and is easily sanded to a highquality
repair within fifteen minutes. It produces
a tenacious bond while maintaining flexibility,
even in cold temperatures.
The repair procedure is as follows:
1 Material and equipment required: plastic cleaner,
heat gun, reinforcement patches, 3 in grinderette,
Figure 14.123 Weld dressing (Welwyn Tool Co. Ltd)
Figure 14.124 Defective plastics welds and their
causes (Welwyn Tool Co. Ltd)
(a) Weld started correctly but completed too quickly:
no wash, indicating haste or welding temperature
too low
(b) Hot air tool not allowed to attain the correct
welding temperature before starting, and weld
finished too soon, leaving a hole
(c) Weld started too late with an unprepared welding
rod end: hole and protrusion of plastic result
(d) Too much pressure applied to the welding rod,
leaving a low and deformed weld bead: filling
may be necessary
(e) Welding temperature too hot for the material,
blistering the sides of the weld; repair area may
be brittle
478 Repair of Vehicle Bodies
DA sander, plastic spreader, masking tape,
rubbing-down paper and the Duramix filling
material.
2 Identify the plastic to be repaired. Most of
the semi-rigid plastics on vertical panels on
todays vehicles are urethane, Xenoy, or other
repairable materials. A small percentage are
thermoplastic polyolefin (TPO), and these must
be repaired using a slightly different procedure.
3 Clean the plastic panel surface and back of
the repair thoroughly to remove all road dirt,
oil and grease.
4 Remove any deformations in the plastic panel.
If the panel has undergone a severe impact it
may have some residual distortion. To remove
any distortion, warm the stretched or distorted
area with a heat gun. The warm air will allow
the panel to return to its original shape.
5 Apply a structural support to the back of the
repair using a Duramix reinforcement patch.
This will provide some firmness so that the
repair area can be V-grooved and feathered back
around the repair.
6 V-groove the area using a coarse 3 in disc, then
feather back the paint around the V-groove.
7 Apply Duramix to the plastic panel. First select
a length of contouring plastic which will cover
the entire repair area. Then mask off the area
surrounding the repair to reduce the sanding
and clean-up time. Apply Duramix directly to
the V-grooved area, slightly overfilling the area;
then within 30 seconds overlay a sheet of contouring
plastic and use the flat side of a plastic
spreader to compress and smooth the Duramix
into the V-grooves. Duramix will start to set
within 50 seconds after it has been dispensed,
so the body repairer should work quickly to
apply the contouring plastic.
8 Sand the repair using a DA sander: start with a
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coarse grit disc and finish with a smooth grit
disc, feather edging the whole repair.
9 Apply a sealer, then primer surfacer, then finishing
coats to suit the plastic panel or bumper
bar being repaired.
