Plastic captive nuts and trim panel

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 7.58Cable 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

222Repair 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 7.60Plastic capture nut

(TRW United-Carr Ltd )

Figure 7.61Plastic 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 7.62Trim panel fasteners (TRW United-Carr Ltd )

224Repair 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 7.63Quick-release fasteners, quarter turn

(TRW United-Carr Ltd )

Figure 7.64Edge 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 7.65Simplified cross-sectional representation of a typical joint (Permabond Adhesives Ltd )

Table 7.2Adhesives: 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

226Repair 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 7.3Compatibility 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 7.4Summary 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

228Repair 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.