Fasteners
Plastic captive nuts are available in a large range
of shapes and sizes for vehicle body applications.
They can be used in high-corrosion areas, and are
Figure 7.58 Cable clip used for latching: sectional
view shows clip in position in a panel (Forest Fasteners)
(a)
(b)
1 2 3
Figure 7.59 (a) Simple push-on clip showing clip in
position (b) tubular clip, showing fitting sequence
(Forest Fasteners)
1 Clip enters hole: cut-outs register with panel,
allowing clip to expand and hold
2 Second part of assembly is lined up and rivet and
integral stub inserted
3 Rivet pressed home, engaging turned-in end of
clip which expands to bite on rivet
222 Repair of Vehicle Bodies
designed to snap into a suitably punched hole in a
metal panel, where they will remain captive until
final assembly. The shallow head gives highly
desirable minimum clearance between panels. The
combination of design and material permits the
recommended screw to form its own thread by
displacement, resulting in high torque load and
good anti-vibration characteristics. Screws may be
removed and the nuts reused, providing the screws
are the same size and thread form. This makes
them ideal for applications such as inspection covers
and service access. They are manufactured from
glass filled nylon 66, which permits higher torque
loadings. All other parts are manufactured from
polypropylene (see Figure 7.60).
Push-in panel fasteners (W-buttons) are designed
to hold board, plastic, rubber or other soft materials
to secondary panels. They are equally suitable
for assembling components to panels. No special
tools are required, as they assemble on the principle
of a push-in rivet. Simply push the W-button through
the aligned panel holes so that it contracts on entry
and then relaxes as it snaps through the total panel
thickness, securing the panels under tension. Many
applications exist for W-buttons where a positive,
attractive, light-weight non-corrosive fastening
solution is required: to assemble painted or PVC
covered trim boards, to fasten instruction badges,
or to blank off holes. In addition, a special feature
which makes the buttons particularly suitable for
the automotive industry is the various styles of textured
and colour-moulded heads to match adjacent
panels (see Figure 7.61a).
Christmas-tree buttons are a variation of push-in
fasteners, and when they are pushed into the hole
they take up great variations in panel thickness.
These parts have a good tolerance of poor-quality
hole conditions. Although it is possible to remove
the buttons, they are not normally reusable (see
Figure 7.61b).
Trim panel fasteners are used principally in the
automotive industry to attach door and tailgate
panels to inner door panels. They are blind fixes,
ensuring a neat, attractive assembly appearance.
Figure 7.60 Plastic capture nut
(TRW United-Carr Ltd)
Figure 7.61 Plastic push-in panel fasteners:
(a) W button (b) Christmas tree button
(TRW United-Carr Ltd)
Methods of joining 223
They allow panel removal for repair, service or
inspection purposes. Easily fitted without the use
of special tools, they are positioned in a keyhole
slot in the trim panel, then firmly pushed home into
the inner door panel mounting hole. Various types
and sizes are available to suit different conditions,
and most have a flexible skirt which seals the mounting
hole against entry of dirt or water (Figure 7.62).
Retainers – where, for a technical or styling reason,
a keyhole slot is not feasible, assembly can be simply
achieved by using a separate retainer. With its
own integral keyhole slot feature, it can be attached
to the trim panel either with a suitable adhesive or
by heat staking. This will provide a secure, attractive
attachment with no evidence of fastener location.
Snapsacs provide increased retention and sealing
for trim panel fasteners, and their smooth
surface eases assembly and removal.
Quick-release fasteners (quarter turn) consist
of a stud, cam and an optional retaining washer. A
selection of lengths and head forms is available to
suit a variety of assembly conditions. The washer
allows the stud to be positively and securely
located prior to and away from the final assembly
point. Where the assembly is subject to routine
servicing, the stud, washer and cam stay securely
mounted, avoiding any risk of component loss (see
Figure 7.63).
Edge fasteners of the D type are lightweight fasteners
which are ideal for assembling leather-cloth,
soft plastics and fabrics to metal, rigid plastic or
fibreboard panels. Fitted without the use of special
tools, they are pushed or tapped on to a convenient
panel edge, using a light hammer. The barbs retain
the assembly securely in position on one side, and
the flat D side provides a neat flush appearance on
the other. The friction grip version offers good retention
without paint damage where corrosion resistance
is paramount. They are manufactured from
austempered carbon steel, and normally supplied in
phosphate and black finish (see Figure 7.64).
7.8 Adhesives
Adhesives are to be found almost everywhere in
the modern world. There are natural as well as
manmade adhesives: for example, spiders use
adhesive to spin and stick their webs and to catch
their prey, and limpets and shell fish use adhesive
to anchor themselves to rocks.
A bond between two surfaces may be regarded
as a chain of three links, the strength of the bond
Figure 7.62 Trim panel fasteners (TRW United-Carr Ltd)
224 Repair of Vehicle Bodies
being that of the weakest link. The central link is
the adhesive film between the surfaces, and the
outer links are formed where the adhesive film
meets the bonded surface. To form a strong surface
Figure 7.63 Quick-release fasteners, quarter turn
(TRW United-Carr Ltd)
Figure 7.64 Edge fasteners, D type
(TRW United-Carr Ltd)
linkage, the adhesive must thoroughly wet the surface
whilst liquid, and when dry must adhere by
penetration into the pores or fibres (Figure 7.65).
In the case of smooth, non-porous surfaces such as
glass or metal, a strong film of adhesive is best
formed between the surfaces if these are joined at
the appropriate stage in the drying of the applied
adhesive. In this process the molecules of the adhesive
and substrate are brought close together and a
variety of forces operate; the most important are
atomic forces, which can be likened to magnetic
attraction. Many variations on this simple theme
are possible and practicable, but the character of
the bond in a particular case will always be governed
by how closely the technique used approximates
to this ideal. Almost everything that is
manufactured or made will use adhesives. A good
example is the motor car. Paint has to adhere to the
metal on which it is placed. Inside the car, the
upholstery will generally be stuck together with
adhesives. All the brake shoes on the car are
bonded together. Finally, some panel assemblies
are bonded with adhesives.
Adhesive types
To facilitate the selection of adhesives, it is convenient
to classify them into the following major
groups (see Tables 7.2, 7.3 and 7.4):
Anaerobics Often known as sealants or locking
compounds. Acrylic based, they normally set in the
presence of metal and absence of air (or to be exact,
atomospheric oxygen). They are generally used to
lock, seal and retain all manner of turned, threaded
and fitted parts, and are often used to seal flanges.
Cyanoacrylates Also based on acrylic resins.
Unlike anaerobics, they require surface moisture as
the vital catalyst in hardening. Generally, they
harden in seconds. Often used in car trim applications
to bond rubber trim to metal, rubber to rubber
and also rubber to plastic.
Epoxies Based on an epoxy resin, which is mixed
with a hardener. This allows great variety in formulation.
Their strength is often employed in bonding
larger components. Single-part epoxies (ESP) are a
development in epoxide technology. Resin and catalyst
are premixed, so they give high performance
without mixing by the user. In the motor vehicle
they see a great deal of use as a supplementary
Methods of joining 225
bond to welding: the process is known as weldbonding.
Hot melts A refined form of the earliest adhesive,
hot wax. They are convenient for assembling
small, lightly loaded items for use in less severe
environments. They are too viscous for use on the
smallest parts, but are often used for the assembly
of motor vehicle trim.
Phenolics One of the earliest types of structural
adhesive. Their use often involves specialized
equipment and complex procedures. Nevertheless,
they perform well in severe environments. The
phenolics are hardly ever seen in motor vehicles,
but they do have a significant residual use in the
assembly of brake shoes.
Plastisols Based on PVC dispersions. They cure
only at elevated temperatures, and are generally
used only on large-scale work or where there is
access to a heat source, intended for another purpose.
Polyurethanes Like epoxies, they offer variety in
formulation. However, polyurethanes are difficult
Figure 7.65 Simplified cross-sectional representation of a typical joint (Permabond Adhesives Ltd)
Table 7.2 Adhesives: their general nature and uses (Permabond Adhesives Ltd)
Capital Material Process Process Solvent Heat General General
Adhesive Toxicity cost cost complexity temperature resistance resistance durability comment
Anaerobic 1 1 2–3 1 E 3 2–3 3 Assembly of
machined, coaxial
components
Cyanoacrylate 1 1 3 1 C 2 1 1–2 Almost every type
of small plastics,
metal and rubber
assembly
Epoxide 1–3 1–2 2 2 E 3 2–3 3 Usually used on
larger objects where
good performance
required
Toughened 1–2 1 2–3 1–2 E 2–3 2–3 2–3 Excellent general
acrylic performance,
particularly
structures
Toughened 1 1 2–3 1 H 3 3 3 Superb structural
epoxide adhesives
(heat cured)
Phenolic 1–2 3 2 3 E 2–3 2–3 2–3 Usually used in
large stressed and
critical structures
Polyurethane 2 2 1 2 E 2–3 2–3 1–2 Used when rapid
assembly needed in
large structures
1 low; 2 medium; 3 high
H hot; C cold; E either
226 Repair of Vehicle Bodies
to handle and usually require specialized mixing
equipment. Generally used for load-bearing applications
in dry conditions, as they are prone to
moisture attack. They can be successfully used for
bonding painted metal surfaces and polyester components
together.
Solvent-borne rubber adhesives Based on a rubber
solution, where the solvent evaporates to effect
bonding. Not suitable for loaded joints or harsh
environments.
Tapes Adequate for bonding small components,
but cannot support heavy loads. Some will withstand
quite harsh environments.
Toughened adhesives Toughened variants are
hailed as a breakthrough in adhesive technology.
They incorporate low-molecular-weight rubbers that
build in exceptional resistance to peel and impact
forces. Acrylic-based, epoxy, and single-part epoxy
adhesives can be toughened in this way. Toughening
reinforces the best features of these adhesives with
the unique shock absorption and strength of the rubber
matrix. Toughened adhesives are used in much
the same way in motor vehicles, trucks and vans but
they may be used on unpainted metal. They are
extremely durable in poor operating environments.
Selection of adhesives
Joint types involved
There are three basic joint types:
Co-axial joints, where one part fits into another,
usually require an anaerobic. Other adhesive types
may be too viscous, or lack the appropriate grades
of strength.
Plain lap joints, where the adhesive is primarily
loaded in either shear or compression. As a rule,
cyanoacrylates are better for unloaded and toughened
variants for loaded lap joints.
Butt joints, where the adhesive is in tension.
These joints are very susceptible to peel and cleavage
forces, which toughened adhesives withstand
exceptionally well.
Table 7.3 Compatibility of the principal structural adhesives with a variety of composite and associated
materials (Permabond Adhesives Ltd)
Acrylic Epoxy
Material to be bonded Pseudo one part Two part One part (heat cured) Two part PU Two part
Metal (also see paint):
Aluminium 1 1 1 1 4
Steel 1 1 1 1 4
Zinc 2 2 2 2 4
Thermoplastic:
Polyamide (nylon) 2 2 3 2 2
Polyphenylene oxide 3 3 3 2 1
Polypropylene 2 2 4 3 1
Thermoset:
Epoxy 2 2 1 1 2
Phenolic 2 2 1 1 2
Polyester: 1 1 2 1 1
hand lay
VARI 1 1 2 1 1
SMC 1 1 2 1 1
cold press 1 1 2 1 1
Polyurethane, RIM 3 3 4 2 1
Paint
Cataphoretic 1 1 1 1 1
Scale: 1 excellent, 2 good, 3 good but possible problems, 4 unsuitable.
Note: this categorization is given in relation to the types of application usually seen in association with the materials nominated.
Therefore, each line should be considered to be unique.
Methods of joining 227
Joint function
Where a coaxial joint needs dismantling for maintenance,
the weaker grades of anaerobic should be
used.
Service conditions
It is posible to indicate conditions using a threepoint
scale:
Benign Room temperature, where components
are not expected to suffer high loads or impacts.
Normal Room temperature with occasional
excursions to about 80 °C. Loads could be heavy,
and components may experience occasional shocks.
Severe High humidity and/or temperature.
Components heavily loaded, with heavy impacts
during their use or assembly.
Curing times
This has important consequences for later production
processes. Cyanoacrylates set in seconds and
reach full strength in minutes. Anaerobics, epoxies
and the toughened variants take longer to harden.
However, all three can be made to cure rapidly
using a variety of techniques.
Selection by computer
One firm, Permabond, has a computer program
called the Permabond Adhesive Locator and
Sealant Guide (PAL II). PAL links the common
engineering materials, generic adhesive types and
Permabond’s own products to assess the relationship
between materials, design, production and
use. It can assess both mechanical and structural
Table 7.4 Summary of the characteristics of the principal structural adhesives for composite bonding
(Permabond Adhesives Ltd)
Main characteristics Principal advantages Principal disadvantages
PU one part Low modulus Very simple to use. Hot melt Sensitive to moisture. Not true structural
Very low strength variants very convenient on adhesives. Slow curing. Must be
suitably sized components. applied to non-metal surface for
Fills large gaps. No mixing long-term durability
PU two part Very low to medium Fast curing possible. Very good Sensitive to moisture. Often requires
modulus application characteristics. Fills heating to achieve acceptable
Very low to medium large gaps production times. Must be applied on a
strength non-metallic surface for long-term
durability. Some versions cannot be
considered to be structural adhesives.
Must be mixed
Acrylic Medium modulus Very fast. Very easy to apply. Need good fit and narrow gaps to
pseudo Medium strength Extremely durable. Bonds metals function effectively. Best below 1 mm
one part particularly well. Copes with
contamination well. A true
structural adhesive. No mixing
Acrylic two Medium modulus Fast. Easy to apply. Benefit of Must be mixed
part (VOX) Medium strength delayed action cure (DAC).
Extremely durable. Fills large
gaps. Copes well with light
contamination. A true structural
adhesive
Epoxy one part High modulus Fast curing. Easy to apply. Needs to be heat cured
Very high strength Extremely durable, with robust
all round performance. No mixing
Epoxy two part Medium to high Easy to apply. Durable. Can be Must be mixed.
modulus speeded by warming/heating. Slow curing
Medium to high True structural adhesive
strength
228 Repair of Vehicle Bodies
joints and is specifically intended to cover such
individual issues as: the assembly of lap, sandwich,
honeycomb, and butt joints; the fitting of bushes,
bearings, splines, gears, shafts and gaskets; and the
retention and sealing of all types of threaded fittings
and pipes, together with the use of adhesive
related sealing compounds. The program selects
the adhesives that are most suitable for the job
from among different types in order to join two of
the numerous possible surfaces. It uses a variable
question-and-answer sequence to determine the
exact nature of the bond required and the conditions
under which it must operate. The program
then offers a selection of first-and second-choice
adhesives, together with notes indicating why a
particular choice has been made and which factors
have excluded which adhesives.
