CHAPTER
Brazing,
Soldering,
Adhesive-Bonding,
and
Mechanical-Fastening
Processes
'
This last chapter of PartVI describes various joining, bonding, and fastening processes that involve mechanisms unlike thosein the preceding twochapters.° Brazing and soldering are different from welding in that no diffusion takes
place at the interface, thus bondstrength depends on adhesive forces.
° Brazing andsoldering aredifferentiated bythe temperatureatwhich filler
met-als melt: Brazing takes place above 450°C and produces stronger joints than
soldering, while soldering involves lower temperatures and is widelyapplied in
theelectronics industry.
'
Adhesive bonding is versatile, and a Wide variety of adhesives is available for numerous applications.'
Mechanical joining processes are then described, such as using bolts, nuts,rivets, snap fasteners, orshrink fits inassembly.
° The chapter ends with a discussion of economic considerations in joining
operations.
32.l
Introduction
Inmost ofthe joiningprocesses describedin Chapters30 and31, themating surfaces ofthe components areheated toelevated temperatures byvarious external or internal
means, tocause fusion and bonding at thejoint. Butwhat ifwewanttojoin apair of
materials that cannot withstand high temperatures, such as electronic components? What if the parts to be joined are fragile,intricate, or made of two or morematerials with verydifferent characteristics, properties, sizes, thicknesses, andcross sections?
This chapter first describes two joining
processes-brazing
andsoldering-that require lower temperaturesthan those used forfusion welding. Fillermetals are
placed in or supplied tothe joint and aremelted by an external source ofheat; upon
solidification, a strong joint is obtained. Brazing and soldering are distinguished arbitrarily by temperature. Temperatures for soldering are lower than those for brazing, and the strength of asoldered joint is muchlower.
The chapter also describes the principles and types of adhesive bonding. The
ancient method of joining parts with animal-derived glues (typically employed in
bookbinding, labeling, and packaging) has beendevelopedinto an importantjoining technology for metallic and nonmetallic materials. The process has wide application
32.I Introduction 92|
32.2 Brazing 922
32.3 Soldering 926
32.4 AdhesiveBonding 93|
32.5 Mechanical Fastening 939
32.6 joiningPlastics,Ceramics,
and Glasses 942
32.1 Economicsofjoining
Operations 945
EXAMPLE:
32.l SolderingofComponents ontoaPrinted Circuit
Board 929 CASESTUDY:
32.l LightCuring Acrylic
Adhesivesfor Medical
Products 937
2 Chapter32 Brazing,Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes
innumerous consumer and industrial products, as well as in the aircraft and
aero-spaceindustries. Bondingmaterials such as thermoplastics, thermosets, ceramics, and
glasses, either to each other orto othermaterials, present various challenges.
Although all of the joints described thus far are of a permanent nature, in
many applications joined components have tobe takenapart for replacement,
main-tenance, repair, or adjustment. But how, for example, do we take apart a product
without destroying the joint? If joints are required thatare notpermanent, but still
must be as strong aswelded joints, the obvioussolution is to usemechanical
fasten-ing, such asfastening with bolts, screws, nuts,or a varietyof other fasteners.
32.2
Brazing
Brazing is a joining process in which a #Her metal is placed between the faying sur-faces to bejoined (or attheir periphery) andthe temperatureis raised sufficiently to melt the fillermetal, but notthe components (the base
metal)-as
would bethe casein fusion welding. Thus, brazing is a liquid-solid-state bondingprocess.Upon
cool-ing and solidification of the filler metal, a strong joint is obtained (Fig. 32.1). Filler metals used for brazing typicallymelt above45 0°C,which is belowthe meltingpoint (solidus temperature) of the metals to be joined (see, for example, Fig. 4.5). Brazing is derived from the word brass, an archaic word meaning “to harden,” and the process was first used as far back as 3000 to 2000B.C.
In a typical brazing operation, a filler (braze) metal wire is placed along the
periphery ofthecomponents tobe joined, as shownin Fig.32.2a. Heatis then applied
(H)
(D) (C)
(Ol) (G)
FIGURE 32.I Examples of brazed and soldered parts. (a) Resistance-brazed light-bulb
filament; (b) brazed radiator heat exchanger; (c) soldered circuit board; (d) brazed ring
Section 322 Brazing 2 |='|| i Fillefmeral .... viiirgrmea (thickness exaggerated) if';'l" ":` I
Tiff
'le' <a> <b>FIGURE32.2 An exampleoffurnace brazing(a) before and (b)after brazing. The filler metal
isa shapedwire and moves into the interfaces by capillaryactionwiththe applicationofheat.
,
to
_
aFIGURE 32.3 joint designs commonly used in brazing operations. The clearance between
the two parts being brazed is an important factor in joint strength. lf the clearance is too
small, themolten braze metal willnotpenetratetheinterface fully. lf itis toolarge, there will
beinsufficientcapillary action forthemolten metal tofill the interface.
by various external means, melting the braze metal and, by capillary action, filling the closely fitting space (joint clearance) at the interfaces (Fig. 32.2b).
In braze welding, filler metal (typically brass) is deposited at the joint by a technique similar to oxyfuel-gas welding (see Fig. 3O.1d); the major difference is thatthe base metaldoes notmelt. The main application of braze welding is in repair
work, typicallyon partsmade of caststeels and irons. Because of the widergaps
be-tween the componentsbeing welded (as in oxyfuel-gas welding), more braze metal
is used than in conventionalbrazing.
In general, dissimilar metals can be assembled with good joint strength.
Examples ofjoints madeare shown in Fig. 32.3. Intricate, lightweight shapes can be
joined rapidly and with little distortion.
Filler Metals. Several filler metals (braze metals) are available with a range of
brazing temperatures (Table 32.1). Note that, unlike those for otherwelding opera-tions, filler metals for brazing generally have a composition that is significantly
dif-ferent from those of the metals to be joined. They are available in a variety of
shapes, such aswire, rod, ring, shim stock, and filings. The selection of the type of fillermetal andits composition are important inorder to avoidenibrittlement ofthe
joint by (a) grain-boundary penetrationof liquid metal (Section 1.5.2); (b) the
for-mation ofbrittle interinetalliccompoundsatthe joint(Section 4.2.2); and(c) galvanic corrosionin the joint (Section 3.8).
Because ofdiffusion betweenthe fillermetal andthe base metal,the mechanical and metallurgical properties of a joint can change as a result of subsequent
process-ing or during the service life ofa brazed part. For example, when titanium is brazed with pure tinas the filler metal, itis possible for thetin to diffuse completely intothe
Chapter32 Brazing,Soldering,Adhesive-Bonding, and Mechanical-Fastening Processes
3
“hs C 'D “’ ‘% rn S6 Qze
Q @~9r S"""Qrh Jointclearance->
TABLE 32.lTypical FiilerMetals forErasing Various Metals
andhiioys
Brazing temperature
Base metal Filler metal (°C)
Aluminum and itsalloys Aluminum-silicon 570-620 Magnesium alloys Magnesium-aluminum 580-625 Copperand its alloys Copper-phosphorus 700-925 Ferrous and nonferrous Silver and copperalloys, 620-1150
(exceptaluminum copper-phosphorus
and magnesium)
Iron~, nickel-, and Gold 900-1100
cobalt-basedalloys
Stainless steels, nickel- and Nickel-silver 925-1200
cobalt-basedalloys
titanium base metal when itis subjected to subsequent aging or to heat treatment. Consequently, the jointno longer exists.
Fluxes. The use of a flux is essential in brazing; a flux prevents
oxidation and removes oxidefilms. Brazing fluxes generally are made of borax, boric acid, borates, fluorides, and chlorides. Wetting agents may be added to improveboth the wetting characteristics of the moltenfiller
metal and the capillary action.
Itisessentialthatthe surfaces tobebrazedbecleanandfreefrom rust,
oil, and other contaminantsinorder (a) for properwetting and spreading
of the moltenfiller metal in the joint and (b) to develop maximum bond strength. Sand blasting also may be used to improve the surface finish of the faying surfaces for brazing. Because they arecorrosive, fluxes mustbe
removed after brazing, usually bywashing withhotwater.
FIGURE 32 4 The effect of joint
clear-ance on the tensile and shear strength of
brazed joints. Note that, unlike tensile
strength shear strength continually
de-creases asthe clearance increases.
Brazed
joint
Strength. The strength of the brazed joint depends on(a) joint clearance, (b) joint area, and (c) the nature of the bond at the
interfaces between the components and the filler metal.
joint
clearances typically range from 0.025 to0.2 mm. As shown in Fig. 32.4, thesmall-er the gap, the higher is the shearstrength ofthe joint. Theshear strength ofbrazed joints can reach800 MPa byusing brazing alloyscontaining silver (silver
solder). Note thatthere is an optimum gap for achievingmaximum tensile strength
ofthe joint.
Because clearances are very small, roughness of the mating surfaces becomes important. The surfaces to be brazed must becleaned chemically or mechanically to
ensure full capillary action; thus, the use of afluxis also important.
32.2.l
Brazing
Methods
The heating methods used in brazing identify the various processes.
Torch Brazing. The heat source intorch brazing (TB)is oxyfuelgaswith a
carburiz-ingflame (seeFig. 30.1c).Brazingisperformed byfirstheatingthe joint with thetorch
and then depositing the brazing rod or wire inthe joint. Suitable partthicknesses are typically inthe range from0.25 to 6 mm. Torch brazingis difficult tocontrol and
re-quires skilled labor; however, it can be automated as a production process by using multiple torches. Torch brazing can also be usedfor repairwork.
Section32.2 Brazing
Furnace Brazing. Thepartsin furnace brazing (PB) are
first cleaned and preloaded with brazing metal in appro- Guide _ _
5%
priate configurations; then the assembly is placed in a _
Q
5'
:iifurnace, where it is heated uniformly. Furnaces may be
either batch type, for complex shapes, or continuous
f;,j
jtype, for high production
runs-especially
for small Eirfééobe\
_,;, 5*partswith simple jointdesigns. Vacuum furnaces or neu-
\
tral atmospheres are used for metals that react with theenvironment. Skilledlabor is notrequired, and complex
shapes can be brazed because the Whole assembly is
heated uniformly inthe furnace.
Induction Brazing. The source of heat in induction
brazing (IB) is induction heating by high-frequency AC current. Parts are
pre-loaded with filler metal and are placed near the induction coils for rapid heating
(see Fig. 4.26). Unlessa protective (neutral) atmosphere is utilized, fluxes generally
are used. Part thicknesses usually are less than 3 mm. Induction brazing is
particu-larlysuitable for brazing parts continuously (Fig. 32.5).
Resistance Brazing. In resistance brazing (RB), the source of heat is the electrical
resistance of the components to be brazed. Electrodes are utilized in this method, as they are in resistance Welding. Parts typically with thicknesses of 0.1 to 12 mm either are preloaded with filler metal or supplied externally with the metal during brazing. As ininduction brazing, the processis rapid, heating zones can beconfined to very small areas, and the process can be automated to produce reliable and
uni-form quality.
DipBrazing. Dip brazing (DB) is carried outby dippingthe assembliestobe brazed into either amoltenfiller-metalbath or amolten salt bath (Section4.12) ata temper-ature just above the melting point of the filler metal. Thus, all component surfaces
are coated with the fillermetal. Consequently, dip brazing inmetal baths is typically used for small parts (such as sheet, wire, and fittings), usually less than 5 mm in thickness or diameter. Molten salt baths, which also act as fluxes, are used for com-plex assemblies of various thicknesses. Depending on the size of the parts and the
bath size, as manyas 1000 joints can bemade at onetime by dip brazing.
Infrared Brazing. Theheat sourceininfraredbrazing (IRB) isa high-intensityquartz
lamp. Theprocess is particularly suitable for brazing verythin components, usually
less than 1 mm thick, including honeycomb structures(Section 16.12). Theradiant
en-ergyis focused on thejoint, and brazing can be carried out in avacuum. Microwave heatingalso can beused.
Diffusion Brazing. Diffusion brazing (DFB) is carried out in afurnace where, with
proper control of temperature and time, the filler metal diffuses into the faying
surfaces ofthe components to bejoined. The brazing time required may range from
30 minutes to 24hours. This process is used for stronglap or buttjoints and for dif-ficultjoining operations. Because the rate of diffusion at the interface does not
de-pend on thethickness ofthe components, partthicknesses may range from foil to as
much as5 0 mm.
High-energy Beams. For specialized and high-precision applications and with
high-temperature metals and alloys, electron-beam or laser-beam heating may be
used (see also Sections 27.6 and 27.7).
Braze Welding. The joint in braze welding is prepared as it is in fusion welding,
described in Chapter 30. While an oxyacetylene torch with an oxidizing flame is
FIGURE 32.5 Schematic illustration of a continuous
induction-brazingsetup for increasedproductivity.
925 Induction coil Insulating board Ejector
926 Chapter32 Brazing, Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes
Good Poor Comments
ff ti' Too littlejoint
'====;==f¢' Improved design when fatigue loading is afactor tobe considered Insufficient bonding area
FIGURE 32.6 Examples of good and poor design for
brazing. Source:American WeldingSociety.
used, filler metal is deposited at the joint (hence the term welding) rather than drawn in by capillary action. As a re-sult, considerably more fillermetal is used than in brazing.
However, temperatures in braze welding generally are
lower than in fusion welding, and thus part distortion is
minimal. The use of a flux is essential in this process. The
principal use of braze welding is for maintenance and
repair work, such as work on ferrous castings and steel
components, although the process can be automated for
mass production.
32.2.2
Design
forBrazing
As in all joining processes,jointdesignis important in braz-ing. Some design guidelines are given in Fig. 32.6. Strong joints requirealarger contact area for brazingthan for weld-ing. A variety of special fixtures and work-holding devices may be required tohold the parts together during brazing;
some willallowfor thermal expansion andcontraction
dur-ingthe brazingoperation.
32.3
Soldering
Insoldering, the filler metal (calledsolder) melts at arelatively low temperature.As in brazing, the solder fills the joint by capillary action between closely fitting or closelyplaced components. Twoimportant characteristics of solders are low surface tension and high wetting capability. Heat sources for soldering are usually soldering irons, torches,or ovens. The word“solder” is derived fromthe Latin solidare, meaning
“tomake solid.” Soldering withcopper-gold and tin-lead alloys wasfirst practiced as far backas 4000 to 3000 B.C.
32.3.l
Types
of Solders
and Fluxes
Solders meltatatemperaturethatistheeutecticpointofthesolderalloy(see,for exam-ple, Fig.4.7). Solderstraditionally havebeentin-leadalloysinvariousproportions.For
example, a solder of 61.9% Sn-38.1% Pb composition melts at 188°C, whereas tin
melts at 232°C and lead at327°C. For special applications and higher joint strength
(especially at elevated temperatures), othersoldercompositions aretin-zinc, lead-sil-ver,cadmium-silver, andzinc-aluminumalloys (Table322).
Because of the toxicityof leadand itsadverse effects on theenvironment, lead-free solders are beingdeveloped continuously and arecoming into wider use.Among the various candidate materials are silver, indium, and bismuth eutectic alloys in
TABLE 32.2
TypesofSolders and Their Applications
Tin-lead General purpose
Tin-zinc Aluminum
Lead-silver Strength at higher than room temperature Cadmium-silver Strength at high temperatures
Zinc-aluminum Aluminum, corrosion resistance Tin-silver Electronics
Section 323 Soldering 2
combination with tin. Three typical compositions are 96.5% Sn-3.5% Ag, 42%
Sn-58% Bi, and 48% Sn-52% In. However, none of these combinations are suit-ablefor every soldering application.
Fluxesare usedinsolderingandfor the samepurposesasthey are inweldingand brazing,as described inSection 32.2. Fluxes for soldering are generallyoftwo types:
I. Inorganic acids or salts, such as zinc-ammonium-chloride solutions, which
clean the surface rapidly. To avoid corrosion, the flux residues should be
re-moved after soldering bywashing the joint thoroughly with water.
2. Noncorrosive resin-based fluxes, used typicallyin electrical applications.
32.3.2
Solderability
Solderability may be defined in a manner similar to weldability (Section 30.9.2).
Special fluxes have beendevelopedto improve the solderability ofmetals and alloys.
As a general guide,
° Copper, silver, and goldare easyto solder
° Iron and nickel are more difficult to solder
° Aluminum and stainless steels are difficult to solder because of their thin, strong oxide films
° Steels, cast irons, titanium, and magnesium, as well as ceramics and graphite, can besoldered by first plating themwith suitable metallic elements to induce interfacial bonding. This method is similar to that used for joining carbides
and ceramics (see Section 32.6.3). A common example of the method is
tinplate, which is steel sheetcoatedwith tin, thusmaking it very easy tosolder.
Tinplate is a common materialused in making cans for food.
32.3.3
Soldering
Techniques
The following soldering techniques are somewhat similar to brazing methods:
a. Torch soldering (TS).
b. Furnace soldering (FS).
c. Iron soldering (INS) (with the use ofa soldering iron). d. Inductionsoldering (IS).
e. Resistance soldering (RS).
f. Dip soldering (DS).
g. Infrared soldering (IRS).
Other soldering techniques, for special applications, are:
h. Ultrasonic solderin in which a transducer sub'ects the molten solder to
g 1
ultrasonic cavitation. This action removesthe oxide filmsfromthe surfacestobe
joined and thuseliminates theneedfora
flux-hence
thetermfluxlesssoldering).i. Reflow (paste) soldering (RS).
j. Wavesoldering (WS).
The lasttwo techniques are widelyused for bonding and packagingin
surface-mounttechnology,as described in Section28.11. Becausethey are significantly
dif-ferent from other soldering methods, they are describednext in greater detail. Reflow Soldering. Solder pastes are solder-metal particles held together by flux,
binding, and wetting agents. The pastes are semisolidin consistency, have high vis-cosity, and thus are capable of maintaining their shape for relatively long periods.
28 Chapter32 Brazing,Soldering,Adhesive-Bonding, and Mechanical-Fastening Processes Squeegee Tensioned screen _ _
_
_ _ _ §<=L@<a1m2§_@iaL'ia§e_
_e___S
é
..y ,E 7
_
__.
if if iiii'”"C5}§i;f¢fgr¢‘g"`iPaste deposited EmU|S|0n
oncontact area
(8)
Copper |and Gull wilpg lead Cgpperland W6-fied SOldel'
'Ag
Plating or Coat*
it \ coatingJT
Oilorair_
¢¢
Circuit board"AV"; ArA '5 il” Turbulent zone
IC Ieads (oil prevents dross)
a e Solder ,_“__ ‘,',' Turbulent zone (dross formed inair) (D) (C)
FIGURE 32.1 (a) Screening solder paste onto a printed circuit board in reflow soldering.
(b) Schematic illustration of the Wave-soldering process. (c) SEM image of wave-soldered
joint on surface-mountdevice. Source: (a)After VSolberg.
The pasteis placed directly onto the joint, or on flat objects for finer detail, anditcan be applied via a screeningor stenciling process, as shown inFig. 32.7a. Stenciling is
commonly used during the attachment of electrical components to printed circuit
boards. An additional benefit of reflow soldering is that the surface tension of the
paste helps keep surface-mount packagesaligned on theirpads; this feature improves
the reliability ofthe solder joints. (See also Section
28.ll.)
Once the paste has been placed and the joint assembled, it is heated in a fur-nace and soldering takes place. In reflovv soldering, the product is heated in a con-trolled manner, so that the following sequence of events occurs:
I. Solvents present inthe paste are evaporated.
2. The flux in the paste is activated, and fluxing action occurs.
3. The components are preheatedcarefully.
4. The solder particles are melted, andthey Wet the joint.
5. The assembly is cooled ata lovv rate to prevent thermal shock and fracture of the solder joint.
Section32.3 Soldering 92
Although this process appears to be straightforward, there are several process vari-ables for each stage, and good control over temperatures and exposures must be
maintained ateach stage inorder to ensureproper joint strength.
WaveSoldering. Wai/esolderingis acommon technique for attachingcircuit
com-ponents to their boards (see Section28.11). To understandthe principle ofwave
sol-dering, it is important to note that molten solder does not wet all surfaces. The
solderwill notstickto mostpolymer surfaces, and itis easyto remove while molten.
Also, as can be observed with a simple handheld soldering iron, the solder wets
metal surfaces and forms a good bond only when the metalis preheated toa certain temperature. Thus, wave soldering requires separate fluxing and preheating opera-tions before itcan be completed.
A typical wave-solderingoperation is illustratedin Fig. 32.7b. A standing
lam-inar wave of molten solder is generated by a pump. Preheated and prefluxed circuit boards arethen conveyed overthe wave. The solderwets the exposed metal surfaces,
but it does not remain attached to the polymer package for the integrated circuits,
and itdoes not adhere tothe polymer-coated circuit boards.An airknife (basicallya high-velocityjet of hot air) blows excess solder away fromthe joint to prevent bridg-ing between adjacentleads.
When surface-mount packages are to be wave soldered, they must be bonded
adhesively to the circuit board before soldering can commence. Bonding usually is
accomplished by (1) screening or stenciling epoxy onto the boards, (2) placing the
components intheir proper locations, (3) curing the epoxy, (4) inverting the board,
and (5) performing wave soldering. A scanning-electron-microscope (SEM)
photo-graph ofa typical surface-mount joint is shown inFig. 32.7c.
EXAMPLE32.1 Soldering of Components onto a Printed CircuitBoard
The computer and consumer electronics industries
place extremely high demands on electronic compo-nents. Integrated circuits and other electronic devices are expected to function reliably for extended
peri-ods, during which they may be subjected to
signifi-cant temperature variations and to vibration. In
recognition ofthis requirement,it is essential thatthe
solder joints used to attach such devices to circuit
boards be sufficiently strong and reliable and also
that the solder joints be applied extremely rapidly with automatedequipment.
A continuing trend in the computer and the
consumer electronics industries is toward the
reduc-tion of chip sizes and increasing compactness of
circuit boards. Furtherspace savings are achieved by mounting integrated circuits into surface-mount
packages, which allow tighter packing on a circuit
board. More importantly, the technique allows
com-ponents to be mounted on both sides of the board.
A challengingproblem ariseswhenaprinted cir-cuit board has bothsurface-mount andin-line circuits
on the same board and itis desiredto solderall ofthe
jointsvia areliable automatedprocess. Itis important
to recognize that,forefficiency of assembly, all of the in-line circuits should be restricted toinsertion from oneside ofthe board.Indeed, there is noperformance requirementthatwould dictate otherwise, andthis
re-strictiongreatly simplifiesmanufacturing.
The basic steps insolderingthe connections on such a board areas follows (see Figs. 32.7b and c):
I. Apply solder paste to one side.
2. Place the surface-mount packages onto the board, and insert in-line packages through the primaryside of theboard.
3. Reflow thesolder.
4. Apply adhesive to the secondary side of the board.
5. Using the adhesive, attach the surface-mount
devices onto the secondary side. 6. Cure the adhesive.
7. Perform awave-soldering operation on the
sec-ondarysideto produce an electrical attachment
ofthe
surfacemounts and the in-line circuits to930 Chapter32 Brazing, Soldering,Adhesive-Bonding, and Mechanical-Fastening Processes
Applying solder paste is done with chemically
etched stencils or screens so that the paste is placed
only onto the designated areas of a circuit board.
(Stencils are used more widely for fine-pitched de-vices and produce a more uniform paste thickness.) Surface-mount circuit componentsare then placedon the board, and the board is heated in a furnace to
around 200°C to reflow the solder and form strong
connections between the surface mount and the
cir-cuit board.
At thispoint,the componentswith leads are
in-serted into the primary side of the board, their leads
are crimped, and the board is flipped over. A dot of
epoxyatthecenter ofa surface mount component lo-cation is printed onto the board. The surface-mount
packages are then placed onto the adhesive by
high-speed automated, computer-controlled systems, The
adhesiveis thencured, the boardisflipped, andwave solderingis done.
The wave-soldering operation simultaneously
joins the surface-mount components to the
second-ary side and solders the leads of the in-line
compo-nents from the board’s primary side. The board is
then cleaned and inspected prior to the performance
ofelectronic quality checks.
32.3.4
Soldering
Applications
and Design
Guidelines
Solderingis used extensivelyin theelectronics industry. Note,however, thatbecause solderingtemperatures are relatively low,a soldered joint has very limited utility at
elevatedtemperatures. Moreover,since solders generally donothavemuchstrength, the processcannotbeused forload-bearing (structural) members.]ointstrength can
be improved significantly by mechanical interlocking of the joint, as illustrated in
Fig. 32.8.
Soldering can be used to join various metals of different thicknesses. Copper and precious metals such as silverand gold are easyto solder. Aluminum and stain
less steels are difficult to solder because of their strong, thin oxide film. However,
these and othermetals can be soldered with the aid of specialfluxes that modifysur faces. Although manual operations require skill and are time consuming, soldering
speeds can behigh withthe use of automatedequipment.
/
/
//
ML’
1
(a) FlangedT (b) Flush lap (0) Flanged corner (d) Line contact
Q
-fe~~~i». BO"'l"1l'l »»..,,,, _M
(e) Flat lockseam (f) Flanged bottom (g) Combination joint
Crimp
-\e=~<
PC board Wife
-/
(h)Through (i) Crimped (j)Twisted
hole connection combination joint wirejoint
Section32.4 Adhesive Bonding 93| Designguidelines for solderingare similartothose for brazing.Somefrequently
used joint designs are shown in Fig.32.8. Notethe importanceof large Contact sur-faces (because of the low strengthof solders) for developing sufficient joint strength in soldered products. Sincethe faying surfaces generally would be small, solders are
rarely used to make buttjoints.
32.4
Adhesive
Bonding
Numerous parts and components can be joined and assembled by adhesives rather
than by one or more ofthe joining methods described thus far. A common example
of adhesive bonding is plywood, where several layers of wood are bonded with wood glue. Modern plywood was developed in 1905, but the practiceof adhesive bondingwood layers dates back to 3500B.C.
Adhesivebonding has gained increased acceptance inmanufacturing ever since its first use on a large scale: the assembly of load-bearing components in aircraft during WorldWar II (1939-1945). Adhesivesare available invarious forms: liquid, paste, solution, emulsion, powder, tape, and film. When applied, adhesives typically
are about 0.1 mm thick.
To meetthe requirements of a particular application,an adhesive may require
one or more of the following properties (Table 32.3):
'
Strength: shear andpeel° Toughness
° Resistance tovarious fluids and chemicals
° Resistance to environmental degradation,including heat and moisture
° Capability to wet the surfaces to be bonded.
TABLE 32.3
Typical Properties andCharacteristicsofChemically Reactive Structural Adhesives
Epoxy Polyurethane Modified acrylic Cyanoacrylate Anaerobic
Impact resistance Poor Excellent Good Poor Fair
Tension-shear 15-22 12-20 20-30 18.9 17.5
strength, MPa
Peel streI1gth”‘,N/m <523 14,000 5250 <525 1750
Substrates bonded Most Most smooth, Most smooth, Mostnon- Metals,glass,
nonporous nonporous porous metals thermosets
or plastics
Servicetemperature
range, °C -55 to 120 -40 to90 -70 to120 f55 t0 80 -55to 150
Heatcureormixing Yes Yes No No No
required
Solvent resistance Excellent Good Good Good Excellent
Moistureresistance Good- Fair Good Poor Good
Excellent
Gaplimitation, mm None None 0.5 0.25 0.60
Odor Mild Mild Strong Moderate Mild
Toxicity Moderate Moderate Moderate Low Low
Flammability Low Low High Low Low
932 Chapter 32 Brazing, Soldering,Adhesive-Bonding, and Mechanical-Fastening Processes
32.4.l
Types
of
Adhesives
and
Adhesive
Systems
Several types of adhesives are available, and more continue to be developed that
provide adequatejoint
strength-including
fatigue strength (Table 32.4). The three basic types of adhesivesare the following:a. Natural
adhesives-such
asstarch, dextrin (agummy substance obtainedfrom starch), soya flour, and animal products.b. Inorganic adhesives--such assodium silicate and magnesium oxychloride.
c. Synthetic organic adhesives-whichmay bethermoplastics (usedfor
nonstruc-turaland some structural bonding) or thermosettingpolymers (usedprimarily for structural bonding).
TABLE 32.4
GeneralCharacteristicsofAdhesives
Type Comments Applications
Acrylic Thermoplastic;quick setting;tough bondat room temper- Fiberglass and steelsandwich bonds,
ature;two components; good solventchemical and impact tennisracquets, metal parts,and plastics
resistance;short worklife; odorous; ventilation required
Anaerobic Thermoset;easyto use; slowcuring; bonds at roomtem- Close-fitting machine parts,such as
perature; curing occursin absence ofair; will notcure shafts and pulleys, nuts and bolts, and
whereair contacts adherents;one component; not good bushings and pins onpermeablesurfaces
Epoxy Thermoset;oneor two components; tough bond; Metal, ceramic, and rigid plastic parts
strongest ofengineeringadhesives; high tensile and low
peelstrengths; resists moisture and high temperature;
difficult to use
Cyanoacrylate Thermoplastic;quick setting;tough bond at room temper- “CrazyglueTM”
ature; easy to use;colorless
Hotmelt Thermoplastic; quick setting; rigidor flexiblebonds;easy Bonds mostmaterials; packaging,
toapply; brittle at lowtemperatures; based on ethylene book binding, andmetalcan joints
vinylacetate, polyolefins, polyamides, and polyesters
Pressure sensitive Thermoplastic variable strength bonds; primer anchors Tapes, labels, and stickers adhesive to roll tapebacking material-a release agent on
the back of webpermits unwinding;made ofpolyacrylate esters and various natural and synthetic rubbers
Phenolic Thermoset; oven cured; strong bond; high tensile and Acousticalpadding, brakelining and
low impact strength; brittle;easy to use;cures by solvent clutchpads, abrasive grainbonding,and
evaporation honeycomb structures
Silicone Thermoset; slowcuring; flexible; bondsat room Gaskets and sealants
temperature; high impactand peel strength;rubber-like
Formaldehyde Thermoset; strongwith wood bonds; ureais inexpensive, Woodjoints,plywood,and bonding
Urea is availableaspowder orliquid, and requiresa catalyst;
Melamine melamineis more expensive, cureswith heat, and the
Phenol bond iswaterproof; resorcinol forms a waterproof bond
Resorcinol at room temperature. Types can becombined
Urethane Thermoset; bonds at room temperature or ovencure; Fiberglass bodyparts, rubber, and
good gap-filling qualities fabric
Water-based Inexpensive, nontoxic, nonflammable Wood, paper, fabric, leather, and
dry-Animal sealenvelopes
Vegetable Rubbers
Section32.4 Adhesive Bonding
Because of their strength, synthetic organic adhesives are the most important
adhesives in manufacturing processes, particularly for load-bearing applications.
Theyare classified as follows:
° Chemically reactive: polyurethanes, silicones, epoxies, cyanoacrylates,
modi-fied acrylics, phenolics, and polyimides. Also included are anaerobics, which
cure in the absence of oxygen,such as Loctite®for threadedfasteners (see also Case Study 32.1).
° Pressure sensitive: natural rubber, styrene-butadiene rubber, butyl rubber,
ni-trile rubber, and polyacrylates.
° Hot melt: thermoplastics (such as ethylene-vinyl acetate copolymers,
poly-olefins,polyamides, and polyester) and thermoplastic elastomers.
° Reactive hot melt: a thermoset portion (based on urethane’s chemistry) with
improved properties.
° Evaporative or diffusion: vinyls, acrylics, phenolics, polyurethanes, synthetic rubbers, and natural rubbers.
° Film and tape: nylon epoxies, elastomer epoxies, nitrile phenolics, vinyl pheno-lics, and polyimides.
° Delayed tack: styrene-butadiene copolymers, polyvinyl acetates, polystyrenes, and polyamides.
° Electrically and thermally conductive: epoxies, polyurethanes, silicones, and polyimides. Electricalconductivityis obtained bythe additionoffillers,such as
silver (used most commonly),copper, aluminum, and gold. Fillers that improve
the electrical conductivity of adhesives generally also improve their thermal conductivity.
Adhesive Systems. These may be classifiedon the basis oftheirspecificchemistries:
° Epoxy-basedsystems: These systems have high strength and high-temperature
properties to as high as 200°C. Typical applications includeautomotive brake
linings and bondingagents for sand molds for casting.
° Acrylics: These adhesives are suitable for applications with substrates that are
not clean.
° Anaerobic systems: The curing of these adhesives is done under oxygen
depri-vation, and the bond is usually hard and brittle. Curing times can be reduced by external heat or by ultraviolet(UV) radiation.
° Cyanoacrylate: The bond lines are thin and the bond sets within5 to40s.
° Urethanes: These adhesives have high toughness and flexibility at room
tem-perature, and they are used widely assealants.
° Silicones: Highly resistant to moisture and solvents, these adhesives have high impact andpeel strength; however, curing timesare typically inthe range from
1 to 5 days.
Many of these adhesives can be combined to optimize their properties, such as
the combinationsofepoxy-silicon, nitrile-phenolic, and epoxy-phenolic. The least ex-pensive adhesives are epoxies and phenolics, which are followed in affordability by polyurethanes,acrylics, silicones, and cyanoacrylates. Adhesivesforhigh-temperature
applications in a range up to about 26O°C (such as polyimides and
polyben-zimidazoles) are generally the most expensive. Most adhesives have an optimum
temperature (ranging from about room temperatureto about 200°C) formaximum shear strength.
32.4.2
Electrically
Conducting Adhesives
Althoughthe majority of adhesive bonding applications require mechanical strength, a
Chapter32 Brazing,Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes
adhesives to replace lead-based solder alloys, particularly in the electronics industry. These adhesives require curing or setting temperatures that are lower than those
re-quiredfor soldering.
In electrically conducting adhesives, the polymer is the matrix and contains
conducting metals (fillers) in forms such asflakes and particles (seealso Section 7.3
on electrically conducting polymers). There is a minimum proportion of fillers
necessary to make the adhesive electrically conducting; typically, itis inthe range
of40 to 70% byvolume.
The size, shape, and distribution of the metallic particles, the method of heat and pressure application, and the individual conducting particle contact geometry canbe controlled to impartisotropic or anisotropic electrical conductivity tothe
ad-hesive. Metalsused are typically silver, nickel, copper, and gold, as well as carbon. More recent developments include polymeric particles (such as polystyrene) coated with thin metallic films of silver or gold. Matrix materials are generally epoxies,
al-though thermoplastics also are used andare available as film or aspaste. Applications
of electrically conducting adhesivesinclude calculators, remote controls, and control
panels. In addition, there are high-densityuses in electronic assemblies,liquid-crystal
displays,pocketTVs, and electronicgames.
32.4.3
Surface
Preparation,
Process
Capabilities,
and
Applications
Surfacepreparation is very important in adhesive bonding. ]oint strength depends
greatly on the absence of dirt, dust, oil, and various other contaminants. This
de-pendence can be observed when one is attempting to apply an adhesive tape over a
dusty or oily surface. Contaminants also affect the wetting ability of the adhesive
and prevent even spreadingofthe adhesive overthe interface. Thick, weak, or loose
oxidefilms onworkpiece surfaces are detrimental toadhesive bonding. On the other
hand, a porous or a thin and strong oxide film may be desirable-particularly one
with some surface roughness to improve adhesion or to introduce mechanical
lock-ing. However, the roughness must not be too high, because air may be trapped,in
which case thejoint strength is reduced. Various compounds and primersare
avail-ablethat modify surfaces to improve adhesive-bond strength. Liquid adhesivesmay
be applied by brushes, sprayers, or rollers.
Process Capabilities. Adhesives can be used for bondinga wide variety ofsimilar
and dissimilar metallic and nonmetallic materials and components with different
shapes, sizes, and thicknesses. Adhesive bonding can also be combined with
me-chanical joining methods (Section 32.5) tofurther improve the strengthof the bond.
joint designand bonding methods require care and skill. Special equipment is usually required, such as fixtures, presses, tooling, and autoclaves and ovens for curing.
Adhesive joints are designed to withstand shear, compressive, and tensile
forces, butthey should not be subjected to peeling (Fig. 32.9). Note, for example, how easily you can peel adhesive tape from asurface, yet be unable toslide it along
Peeling force
1;
5
r
... _”({'~~ ,,,, ,, \L.._;,__ li .._..
rrr.. <a> <b>FIGURE 32.9 Characteristic behavior of (a) brittle and (b) tough adhesives in a peeling test.
Section 32.4 Adhesive Bonding
the surface. During peeling, the behaviorof an adhesive may be either brittleor
duc-tile and
tough-requiring
high forces topeel the adhesive.Applications. Major industries that use adhesive bonding extensively are the
aero-space, automotive, appliances, and building products industries. Applicationsinclude
automotive brake-lining assemblies, laminated windshield glass, appliances, helicop-terblades, honeycomb structures, and aircraft bodies andcontrol surfaces.
Animportantconsideration inthe use ofadhesivesinproduction is curingtime,
which can range from afew seconds (at hightemperatures) toseveral hours (atroom
temperature),particularly for thermosettingadhesives. Thus, production rates can be
low compared with thoseof other joining processes. Furthermore, adhesive bonds
forstructural applications rarely are suitable for service above 25O°C.
Nondestructive inspection of the quality and strength of adhesively bonded
components can be difficult. Some of the techniques described in Section 36.10
(such as acoustic impact (tapping), holography, infrared detection, and ultrasonic
testing) are effective nondestructive-testing methods for adhesives.
The major advantages of adhesive bonding are as follows:
° The interfacial bond has sufficient strength for structural applications, but is also used for nonstructural purposes, such as sealing, insulation, the preven-tion of electrochemicalcorrosion between dissimilar metals, and the reduction
of vibrationand of noise(by means ofinternal dampingatthe joints).
° Adhesive bondingdistributes the load at an interface and thereby eliminates
localized stresses that usuallyresult from joiningthe components with
mechan-ical fasteners, such as bolts and screws. Moreover, structural integrity of the
sections is maintained (because no holes are required).
° The externalappearance of the bonded components is unaffacted.
° Very thin and fragile components can be bonded withoutsignificant increase
in their weight.
° Porous materials and materials of very different properties and sizes can be
joined.
° Because adhesive bonding usually is carried out at a temperature between
room temperature and about 200°C, there is no significant distortion of the
components or change in their original properties. Avoiding distortion is im-portant, particularly formaterials that are heat sensitive.
The majorlimitationsof adhesive bondingare the following:
° There is alimited range of servicetemperatures. ° Bonding time can belong.
° There is aneed for great care in surfacepreparation.
° Bonded joints are difficult to test nondestructively, particularly for large structures.
° The limited reliabilityofadhesivelybonded structures during theirservicelifeand under hostile environmental conditions(suchas degradationbytemperature,
ox-idation,stresscorrosion, radiation, or dissolution) may beasignificant concern. The cost of adhesive bonding depends on the particular operation. In many
cases, however, the overall economics of the process make adhesive bonding an
attractive joining process. Sometimes it is the only one that is feasible or practical. The cost of equipmentvaries greatly, depending on the size and type of operation.
32.4.4
Design
for
Adhesive
Bonding
° Designs for adhesive bonding should ensure that joints are subjected only to compressive, tensile, and shearforces and not to peelingor cleavage.
Chapter32 Brazi ng, Soldering, Adhesive-Bonding,and Mechanical-Fastening Processes
° Several joint designs for adhesive bondingare shown in Figs. 32.10 through
32.12. They vary considerably in strength; hence, selection of the appropriate design is importantand should includeconsiderations such as the type of
load-ing andthe environment.
Poor Good
Adhesive
\/
(H) (blFIGURE 32.10 Various joint designs in adhesive bo
largecontactareas between the members tobejoined.
Simple Simple
Beveled Beveled
Badiused Ftadiused
(H) (D)
Single taper Single
Double taper Double
Increased thickness Beveled
(C) (Ci)
FIGURE 32.lI Desirable configurations for adhesively
bonded joints: (a) single lap, (b) double lap, (c) scarf, and
(d) strap.
Verygood
*`
(C)
nding.Note that good designs require
hesive
itque,
ysyyyi
%
Rivet (H) "`“*‘eS'“e A ‘fféea Spot weld bead (blFIGURE 32.|2 Two
exam-ples of combination joints,
for purposes of improved
strength, airtightness, and
Section32.4 Adhesive Bonding 937 ° Butt joints require large bonding surfaces. Simple lap joints tend to distort
under tension because ofthe force couple atthe joint. (SeeFig. 31.9a.)
° The coefficients ofthermal expansion of the components tobe bonded should
preferably be close to each other in order to avoid internal stresses during
ad-hesivebonding. Also,situations in whichthermal cycling cancause differential movement across the joint should beavoided.
CASE
STUDY 32.I
Light
Curing
Acrylic
Adhesives
for Medical
Products
Cobe Cardiovascular, Inc., is a leading manufacturer of blood collection and processing systems, asWell as
extracorporeal systems for cardiovascular surgery.
The company (like many otherdevicemanufacturers)
traditionallyused solvents for bonding device
compo-nents and subassemblies. However, several federal
agencies began to encourage industries to avoid the
use ofsolvents, and Cobeparticularlywantedto
elim-inate its use of methylene chloride for environmental
and occupational safety reasons. Towards this goal,
the company began to redesign most of its assemblies
to accept light-curing (ultravioletor visible) adhesives. Most of the company’s devices were made of
trans-parent plastics. Consequently, its engineers required
clear adhesive bonds for aesthetic purposes and with
no tendency forstress crackingor crazing.
As an example of a typical product, Cobe’s
blood salvage or collection reservoir is an oval poly-carbonate device approximately 300 mm tall,
200 mm in major diameter, and 100 mm deep (Fig.
32.13). The reservoir is a one-time use, disposable
device; its purpose is to collect and hold the blood
during open-heart and chest surgery or for
arthro-scopic and emergency roomprocedures. Up to 3000 cc of blood may be stored in the reservoir While the
blood awaits passage into a 250-cccentrifuge, which
cleans the blood and returns itto the patientafter the
surgicalprocedure is completed. The collection
reser-FIGURE 32.l3 The Cobe Laboratories blood reservoir. The lidis bonded
to the bowl with an airtight adhesive joint and tongue-in-groove joint. Source: Courtesyof Cobe Laboratories.
938 Chapter32 Brazing,Soldering,Adhesive-Bonding, and Mechanical-Fastening Processes
voir consists of a clear,polycarbonate lid joined to a
polycarbonate bucket. The joint is a tongue-and-grooveconfiguration; the goal was to create astrong, elastic joint that could withstand repeated stresses
with no chanceof leakage.
Light-curedacrylicadhesivesofferarangeof
per-formancepropertiesthatmake themwellsuitedforthis
application.Firstand foremost, they achievehigh bond strength to the thermoplastics typically used to form medical-device housings. For example, Loctite® 3211
(seeanaerobic adhesives,Section32.4.1)achievesshear
strengthsof1 1MPaonpolycarbonate.Asimportantas
the initial shearstrengthmay be, itis evenmore
impor-tant thattheadhesive beabletomaintainthehighbond strength after sterilization. Fortunately, disposable medicaldevicesaretypicallysubjectedtoveryfew
ster-ilizationcyclesduringmanufacturing.Also, these adhe-sives canendure a limited number of cycles ofgamma
irradiation, electron beam irradiation, autoclaving, ethylene oxide,orchemicalimmersion.
Another consideration that makes light-cured
adhesiveswellsuited for this applicationis their
avail-ability in formulations that allow them to withstand
large strains prior to yielding; for example, Loctite® 3211 yields at elongations in excess of 2()0%. This flexibility is critical, because the bonded joints are typically subjected to large amounts of bending and
flexingwhenthe devices arepressurized during qualifi-cationtestinganduse. Ifan adhesiveis toorigid,itwill
fail this type of testing, even if it offers higher shear
strength than a comparable and more flexible
adhe-sive.Finally,light-curedacrylics arewidely availablein formulations that meet international qualitystandard certification (ISO,seeSection36.6),which meansthat,
when processed properly,they willnot cause biocom-patibility problemsinthe final assembly.
While these performance features are attractive,
the adhesive alsomustmeet certain processing
charac-teristics during manufacturing. Light-curedacrylic
ad-hesives havefound wideuse inmedical-deviceassembly/
joiningoperations, becausetheir processing character-istics are compatible with the high-speed automated
manufacturing processes employed. These adhesives
are available in a wide variety of viscosities and are
dispensedeasily through either pressure-time or
posi-tive-displacement dispensing systems. Gnce dispensed
on the part, they can remain in contact with even highly stressed plastic parts for several minutes or
longer withoutcausing stress cracking or degradation
of the plastic. Forexample, Liquid Loctite® 3211 can
remain in contact with polycarbonate that has been
bent to induce stressesupto 17MPa formorethan 15_
minutes withoutstress cracking. Finally, the adhesive
can be converted completely from a liquid to a solid state in seconds when exposed to light of the proper
intensity and wavelength.
Since Loctite® 3211 absorbs light in the visible
as well as the ultraviolet range, itcan beused success-fully on plastics that contain UV blockers, such as
many grades of polycarbonate. The ability to have a
longopen timewhen parts can bepositioned, yet cure the adhesive on demand, is a unique benefit to
light-curing adhesives, dramatically reducing scrap costs.
The equipment used to irradiate the part with
high-intensitylighttypically requires aspace of 1 X 2 m2on
a production line, which generally is much less than
that required for the ovens used byheat~cured
adhe-sives orthe racking shelvesrequired for slower curing
adhesives. Since floorspace carries a cost premiumin clean~roomenvironments, this is a significant benefit. It also is important that the joint be designed
properly to maximize performance. Ifthe enclosureis
bonded with ajoint consisting oftwoflat facesin
inti-matecontact,peelstresses (see Fig. 32.9)will be acting
on the bondwhen thevessel is pressurized.Peelstresses are the most difficult type for an adhesive joint to withstand, due to the fact that the entire load is
con-centrated on the leadingedge ofthejoint. The
tongue-and-groove design that the company used addressed
this concern, with the groove acting as a reservoir for
holding the adhesive during the dispensing operation. When the parts are mated and the adhesive is cured,
this design allows muchofthe load on thejoint(when the device is pressurized) to be translated into shear
and tensile forces, which the adhesive is much better
suited to withstand. Thegap between the tongue and
the groove can vary widely, because most light-cured
adhesives quickly can becured to depths inexcess of
5 mm. This feature allows the manufacturerto havea
robustjoiningprocess (meaningthat wide
dimension-altolerances can beaccommodated).
Withthenewdesign and with the use of this ad-hesive, the environmental concerns and the issues
as-sociated with solvent bonding were eliminated, with
the accompanying benefit of a safer, faster, and more
consistent bond. The light-curing adhesive provided
theaesthetic-bond linethe company
wanted-one
thatwas clear and barely perceptible. It also provided the
structural strength needed and thus maintained a com-petitive edgefor the company inthe marketplace.
Section 32.5
32.5
Mechanical
Fastening
Two or more components may have to be joined or fastened insuch away that they
canbetaken apartsometime duringthe product’s servicelife or lifecycle. Numerous products, such as mechanical pencils, watches, computers,appliances, engines, and
bicycles, have components that are fastened mechanically. Mechanical fastening may bepreferred overother methods forthe following reasons:
° Ease ofmanufacturing.
° Ease ofassembly and transportation.
° Ease ofdisassembly, maintenance, partsreplacement, or repair.
in creating designs that require movable joints such as hinges, sliding ° Ease
mechanisms, and adjustable components and fixtures.
° Lower overall costof manufacturing the product.
The mostcommonmethod ofmechanical fastening is bythe use ofbolts, nuts,
screws, pins, and a variety of other fasteners. Also knownas mechanical assembly,
mechanical fastening generally requires that the components have holes through
which the fasteners are inserted. These joints may be subjected to both shear and
tensile stresses and should bedesigned toresist such forces.
Hole Preparation. An important aspect of mechanical fastening is hole prepara-tion. As described in Chapters 16, 23, and 27, a hole in a solid body can be
pro-duced by several processes, such as punching, drilling, chemical and electrical
means, and high-energy beams. Recall from Parts II and III that holes also may be
produced integrally inthe product during casting, forging, extrusion, and powder
metallurgy. For improved accuracy and surface finish, many of these hole-making
operations may be followed by finishing operations, such as shaving, deburring,
reaming, and honing, as describedin various sections ofPart IV.
Because of the fundamental differences in their characteristics, each of the hole-making processes produces holes with different surface finishes, surface
properties, and dimensional accuracy and characteristics. The mostsignificant
in-fluence of a hole in a solid body is its tendency to reduce the component’s fatigue life bystress concentration. For holes, fatigue life can be improved best by
induc-ing compressive residual stresses on the cylindrical surfaceof the hole. These
stress-es usually are developed by pushing a round rod (drift pin) through the hole and
expanding it by a very small amount. This operation plastically deforms the
sur-face layers of the hole in a manner similar to that seen in shot peeningor inroller
burnishing (Section
342).
Threaded Fasteners. Bolts, screws, and nuts are among the most commonlyused threadedfasteners. Numerousstandards and specifications (includingthread
dimen-sions, dimensional tolerances, pitch, strength, and the quality of the materialsused
to make these fasteners) are described in the references at the end of this chapter. Bolts and screws may be secured with nuts, or they may be self-tapping-whereby the screw eithercuts or forms thethread into thepartto befastened. The self-tapping method isparticularlyeffective and economical inplasticproducts inwhich fastening
does not require a tappedhole or a nut.
If the joint is to be subjected to vibration (such as in aircraft, engines, and
machinery), several specially designed nuts and lock washers are available. They
increase the frictional resistance in the torsional direction and so inhibit any vibra-tional loosening ofthe fasteners.
0 Chapter32 Brazing,Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes
i
(3) (bl (Cl (d)
FIGURE32.|4 Examples of rivets: (a) solid, (b) tubular, (c) split or bifurcated, and
(d) compression.
L1
Poor
alll
s,e,Good
<a> im <C> <d>
FIGURE 32.l5 Design guidelines forriveting. (a) Exposed shank is too long; the resultis
buckling instead of upsetting. (b) Rivetsshould beplaced sufficiently far from edges to avoid
stress concentrations. (c) joined sections should allowample clearance for the rivetingtools.
(d) Sectioncurvature should notinterfere withtherivetingprocess. Source: After].G.Bralla.
Rivets. The most common method of permanent or semipermanent mechanical joining is byriveting (Fig. 32.14). Hundreds of thousands of rivets may be used in
the construction and assembly of one large commercial aircraft. Riveting may be
done either at room temperatureor at elevatedtemperatures. Rivets may be solid or
tubular. Installing a solid rivet takes two steps: placing the rivet inthe hole (usually punched or drilled) and deforming the end of its shank by upsetting itUveading; see
Fig. 14.11). A hollow rivet is installed byflaring its smaller end (see Section 16.6). Explosives can be placedwithin the rivet cavity and detonatedto expand the end of
the rivet. The riveting operation also may be performed byhand or by mechanized
means, including the use ofprogrammable robots. Some design guidelinesfor
rivet-ing are illustrated in Fig.32.15.
32.5.1
Other
Fastening
Methods
Numerous othertechniques are used injoining and assemblyapplications. The most
common types are described here.
Metal Stitching and Stapling. The process of metal stitching and stapling
(Fig. 32.16) is muchlike that oftheordinary staplingofpapers. The operation is fast,
Section 32.5 Mechanical Fastening 94|
includingwood. Acommon exampleis the stapling of _; i
cardboard containers. In clinc/Qing, the fastener
mate-rial must be sufficientlythin and ductile to withstand
the large localized deformation during sharp bending. (3)
Searning. Seanfiing is based on the simple principle
of folding two thin pieces of material together, much
like the joining oftwopieces ofpaper byfoldingthem
at the corners (Fig. 32.17). Common examples of
seaming are seen at the tops of beverage cans (see last (C)
illustration in Fig. 16.3O), in containers for food and householdproducts,and inheating and air-conditioning ducts. In seaming, the materials should be capable of
undergoing bending and folding at very small radii;
otherwise, they will crack. The performance and reliability of seams may be
im-proved as well as making them impermeable bythe addition of adhesives or
poly-meric coatings and seals or bysoldering.
Nonmetal
Metal channel
Crimping. The crimping process is a methodof joining Without using fasteners. It
can be done with beads or dimples (Fig. 32.18), which can be produced by
shrink-ing or swaging operations. Crimping can be done on both tubular and flat parts,
providedthatthe materials are sufficiently thin and ductile towithstand thelarge
lo-calized deformations. Metal caps are fastened to glass bottles by crimping just as someconnectors are crimped to electrical wiring.
Spring and Snap-in Fasteners. Several types of spring and snap-in fasteners are
shown in Fig. 32.19. Such fasteners are used widely in automotive bodies and
household appliances. They are economical, andthey permiteasy andrapid
compo-nent assembly.
Shrink and Press Fits. Components also may be assembled by shrink fitting and
press fitting. Shrink fittingis based on the thermal contractions oftwo components.
Typical applications are assembling die components and mounting gears and cams
onto shafts. Inpress fitting, one component is forced over another; when the
com-ponents are designed properly,this process results in high joint strength.
Shape-memory Alloys. The characteristics of these materials were described in
Section 6.13. Recall their use as fasteners because of their unique capability to
re-cover their shape upon heating. Several advanced applications include their use as
coupling in the assembly of titanium-alloy tubing for aircraft.
@i:~w
. ..(H) (D)
FIGURE32.I8 Twoexamplesofmechanical joiningbycrimping.
Standard loop
FIGURE 32.|6 Typicalexample
Flat clinch (bi ld) s ofmetal stitching. 1 2. 3. 4.
FIGURE32.I1 Stagesinform
Chapter32 Brazing,Soldering,Adhesive-Bonding, and Mechanical-Fastening Processes
Spring clip
.:...:
Rod-end attachment pUSh`°n
to sheet-metal part fastener
(H) (D) (C)
‘lik /'
f-+
...W
s ‘ssr‘r rrrrr '35 ifrrss~ff
.,i
ssrr.
.Deflected Rigid
Sheet metal cover Sheet-metal cover Integrated snap fasteners
(dl (9) (f) (Q)
FIGURE32.l9 Examples of spring and snap-in fasteners used to facilitate assembly.
32.5.2
Design
for
Mechanical
Fastening
The design of mechanical joints requires a consideration of the type of loading to
which the structure will be subjected and of the size and spacing of holes.
Compatibility of the fastener material with that of the components to be joined is important. Incompatibility may lead to galvanic corrosion, also known as crevice
corrosion (Section 3.8). Forexample,in asystem inwhich asteel boltor rivetis used
tofastencoppersheets, the bolt is anodic andthe copperplate is cathodic; this com-binationcauses rapid corrosion and loss ofjoint strength. Aluminum or zinc
fasten-ersoncopper productswill react in asimilar manner.
Other general design guidelines for mechanical joining include the following
(see also Section 37.10):
° It is generally less costly to use fewer, but larger, fasteners than to use a large
number of small ones.
° Part assembly should be accomplished with a minimum number of fasteners.
° The fitbetweenpartstobejoined shouldbe as loose aspossibleto reducecosts
and to facilitatethe assembly process.
° Fasteners of standardsize shouldbe used whenever possible.
'
Holes should not be too close to edges or corners, to avoid the possibility oftearing the materialwhen itis subjected toexternal forces.
32.6
joining
Plastics, Ceramics,
and Glasses
Plastics can be joined by many of the methods already described for joining metals
and nonmetallic materials, especially adhesive bonding and mechanical fastening.
32.6.l joining
Thermoplastics
Thermoplastics canbe joinedbythermal means,adhesive bonding, solvent bonding,
Section 32.6 joining Plastics,
Thermal Methods. Thermoplastics soften and melt as the temperature is
in-creased. Consequently, they can be joined when heat is generated at the interface
(from either an external or internal source), allowing fusion to take place. The heat softens the thermoplastic at the interface to aviscous or molten state and ensures a good bond with the application of pressure.
Because of the low thermal conductivity of thermoplastics, the heat source
may burn or char the surfaces ofthe components if appliedattoo high arate. Such
burning or charting can cause difficulties in obtaining sufficiently deep fusion for
proper joint strength. Oxidation also can be a problem in joining some polymers
(such as polyethylene), because it causes degradation. Typically, an inert shielding
gas (such as nitrogen) is used to prevent oxidation.
External heat sources may be chosen from amongthe following (the choice
de-pends on the compatibility ofthe polymers to be joined):
° Hotair; inertgases, or a filler materialof the same type is also used.
° In aprocess knownasloot-tool welding or loot-platewelding, heated tools and
dies arepressed againstthe surfacesto bejoined and heat them by the
interdif-fusion of molecular chains. This process commonly is used in butt-welded
pipesand (end-to-end) tubing.
° Infrared radiation (from high-intensity quartz heat lamps) is focused into a
narrowbeam onto the surface to bejoined.
° Radio waves are particularly useful for thin films; frequenciesare in the range from 100 to 500 Hz.
° Dielectric heating at frequencies of up to 100 MHZ are effecive for the through heating ofpolymerssuch as nylon, polyvinyl chloride,polyurethane, and rubber.
° Electrical-resistance elements (such as wires or braids, or carbon-based tapes, sheets, andropes) are placed at the interfaceto create heatbythepassing of elec-trical
current-a
process known as resistive-implant welding. Alternatively, in induction welding, these elements at the interface may be subjected toradio-frequency exposure. In both cases, because they are left in the weld zone, the
el-ementsat the interface must be compatible with the use ofthe joined product.
° Lasers emitting defocused beams at low power prevent degradation of the
polymer.
Internal heat sources aredeveloped by the following means:
° Ultrasonic welding is the most commonly used process for thermoplastics,
particularly amorphous polymers such as acrylonitrile-butadiene-styrene
(ABS) and high-impact polystyrene; frequencies are in the range from 20 to
40 kHz. The ultrasonic welding process illustrated in Fig. 31.2 is still
appli-cable, but note that the tool can apply vertical motion, causing a
released-compression loading. Due tothe high hysteresis ofpolymers in a loadingcycle, the heat for welding is developed in the polymer andnot at the interface.
° Friction welding (also called spin welding for polymers) and linear friction welding (also called vibration welding) are particularlyuseful for joining poly-mers with a high degree of crystallinity, such as acetal, polyethylene, nylons,
and polypropylene.
° Orbitalwelding is similar to friction welding, except thatthe rotarymotion of one componentis inan orbital path.
Thefusion method is particularlyeffective withplastics that cannotbebonded
easily by means of adhesives. Plastics (such as PVC, polyethylene, polypropylene,
acrylics, and ABS) can be joined in this manner. For example, specially designed
portable fusion-sealing systems are used to allow in-fieldjoining ofplastic pipe (usu-ally made ofpolyethylene and used for natural-gas delivery).
Chapter32 Brazing,Soldering, Adhesive-Bonding, and Mechanical-Fastening Processes
Coextruded multiple food wrappings consistofdifferenttypes of films,whichare
bondedbyheat duringextrusion(Section 19.2.1).Each film has adifferent
function-forexample, one film may keep out moisture,another may keep out oxygen, and a
third film may facilitate heat sealing during the packaging process.Some wrappings haveasmanyas seven
layers-all
bondedtogetherduringproduction ofthe film. Adhesive Bonding. This method is best illustrated in the joining of sections of PVC pipe (used extensively in plumbing systems) and ABS pipe (used in drain, waste, and vent systems). A primer that improves adhesionis used to apply the ad-hesivetothe connecting sleeveandpipe surfaces (astepmuch like thatusing primers inpainting), and then the pieces arepushed together.Adhesive bonding of polyethylene, polypropylene, and
polytetrafluoroethyl-ene (Teflon) can be difficult, because adhesives do not bond readily to them. The surfaces of parts made of these materials usually have to be treated chemically to improve bonding. Theuse ofadhesive primers or double-sidedadhesive tapes also is effective.
Mechanical Fastening. This methodis particularly effective for most thermoplas-tics (because of their inherent toughness and resilience) and for joining plastics to metals. Plastic or metal screws may be used. The use of self-tapping metal screws is
a common practice. Integrated snapfasteners have gained wide acceptance for
sim-plifying assembly operations; fastener geometries are shown in Figs. 32.19f and g.
Because the fastener can be molded directly atthe same time as the plastic, it adds
verylittle to the costofthe assembly.This technique is very cost effective, because it reduces assembly time and minimizes the number of partsrequired.
SolventBonding. This method consists of the following sequence of steps:
I. Rougheningthe surfaces with an abrasive;
2. Wiping and cleaningthe surfaces with a solventappropriate forthe particular
polymer;
3. Pressing the surfaces together and holding them together until sufficient joint strengthis developed.
ElectromagneticBonding. Thermoplastics also may be joined by magnetic means
by embedding tiny particles on the order of 1 um in the polymer. A high-frequency
field thencauses induction heatingofthe polymer and meltsitat the interfaces tobe
joined.
32.6.2 joining
Thermosets
Thermosetting plastics (such asepoxy and phenolics) can bejoined by the following techniques:
0 Threaded or other molded-in inserts.
° Mechanical fasteners, particularly those using self-tapping screws and
inte-grated snap fasteners.
° Solventhonding.
0 Co-curing, inwhich the two components tobe joined are placed togetherand
cured simultaneously.
° Adhesive honding.
32.6.3 joining
Ceramics and Glasses
Awide variety and numerous types of ceramics and glasses are now available with
