Plastics Joining A practical guide

The joining of plastics refers to the joining of semifinished parts. This includes fastening, adhesive bonding, and welding. Fastening refers to the incorporating latches, hinges and snap fits into the design of the part, or using external fasteners such as bolts and screws. Adhesive bonding means the application of an adhesive (such as epoxy) to join parts together. Welding refers to the joining of two parts via the application of heat and pressure. As the parts are already semifinished when they are prepared to be joined, many of the process specifications for joining are dependent on the manner in which the semifinished parts were made. Though joining is dependent on part shape, this process can be time consuming and incur a high labor cost.

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Handbook of Plastics Joining

A Practical Guide

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Plastics Design Library is a trademark of William Andrew, Inc

ISBN: 1-884207-17-0

Library of Congress Card Number 97-65526

Published in the United States of America, Norwich, NY by Plastics Design Library a division of William Andrew, Inc

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responsibility of the user The information provided should assist in material selection and not serve as a substitute for careful testing of prototype parts in typical operating environments before commercial production

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Introduction i

How To Use This Book ii

Plastics Joining Processes Heated Tool Welding - Chapter 1 Process 1

Processing Parameters 3

Materials 4

Weld Microstructure 4

Effects of Ageing on Weld Strength 5

Variants of Hot Tool Welding 5

Equipment 6

Advantages and Disadvantages 6

Applications 7

Hot Gas Welding - Chapter 2 Process 9

Materials 10

Joint Design 11

Equipment 11

Applications 13

Vibration Welding - Chapter 3 Process 15

Materials 17

Cross-Thickness Welding 19

Equipment 20

Orbital Vibration Welding 22

Joint Design 24

Applications 25

Spin Welding - Chapter 4 Process 29

Materials 31

Variants of Spin Welding 32

Equipment 32

Joint Design 33

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Ultrasonic Welding - Chapter 5

Process 35

Ultrasonic Weldability of Materials 40

Joint Design 45

Ultrasonic Equipment 53

Applications 56

Ultrasonic Welding Tips 57

Ultrasonic Inserting 59

Ultrasonic Spot Welding 61

Ultrasonic Staking 61

Ultrasonic Stud Welding 65

Ultrasonic Swaging 65

Ultrasonic Bonding 65

Ultrasonic Slitting 66

Ultrasonic Scan Welding 66

Ultrasonic Degating 66

Induction Welding - Chapter 6 Process 67

Electromagnetic Materials 68

Materials To Be Joined 69

Equipment 69

Work Coil Design 69

Joint Design 71

Applications 72

Radio Frequency Welding - Chapter 7 Process 75

Heat Generation 75

Equipment 76

Materials 77

Applications 78

Microwave Welding - Chapter 8 Process 79

Materials 81

Equipment 82

Resistance Welding - Chapter 9 Process 83

Materials 85

Joint Design 85

Equipment 86

Applications 86

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Equipment 89

Applications 89

Electrofusion Welding - Chapter 11 Process 91

Materials 93

Equipment 93

Applications 94

Infrared Welding - Chapter 12 Process 95

Materials 97

Microstructure 98

Variants of Infrared Welding 98

Equipment 99

Applications 100

Laser Welding - Chapter 13 Process 101

Materials 103

Joint Design 103

Equipment 104

Applications 104

Mechanical Fastening - Chapter 14 Process 105

Machine Screws, Nuts, Bolts, & Washers 105

Self-Tapping Screws 107

Molded-In Threads 113

Inserts 114

Press or Interference Fits 119

Snap-Fits 121

Rivets 133

Staking 134

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Chemical Bonding - Adhesive and Solvent Bonding - Chapter 15

Mechanism of Bonding 137

Types of Adhesives 140

Hot Melt Adhesives 140

Acrylic Adhesives 141

Epoxy Adhesives 144

Elastomer Adhesives 145

Types of Solvents 146

Surface Preparation Methods 147

Mechanical Treatments 147

Chemical Cleaning Treatments 148

Surface Modification 148

Electrical Discharge Treatments 150

Other Surface Preparation Techniques 151

Factors Affecting Adhesive and Solvent Bonding 152

Joint Design 155

Equipment and Application Methods 159

Applications 162

Adhesive Bonding Tips 163

Thermoplastics Acetal Resin Acetal Resin - Chapter 16 165

Acetal Copolymer - Chapter 17 171

Acrylic Resin Acrylic Resin - Chapter 18 175

Cellulosic Plastic Cellulose Propionate - Chapter 19 183

Fluoroplastic Fluoropolymer - Chapter 20 185

Ethylene-Tetrafluoroethylene Copolymer (ETFE) - Chapter 21 187

Fluorinated Ethylene-Propylene Copolymer (FEP) - Chapter 22 191

Perfluoroalkoxy Resin (PFA) - Chapter 23 193

Polytetrafluoroethylene (TFE) - Chapter 24 195

Polyvinylidene Fluoride (PVDF) - Chapter 25 197

Ionomer Ionomer - Chapter 26 201

Polyamide Nylon - Chapter 27 203

Amorphous Nylon - Chapter 28 209

Nylon 12 - Chapter 29 211

Polyarylamide - Chapter 33 223

Polyphthalamide (PPA) - Chapter 34 225

Polycarbonate Polycarbonate (PC) - Chapter 35 231

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Polybutylene Terephthalate (PBT) - Chapter 37 267

Polyethylene Terephthalate (PET) - Chapter 38 287

Polycyclohexylenedimethylene Ethylene Terephthalate (PETG) - Chapter 39 291

Liquid Crystal Polymer (LCP) - Chapter 40 293

Polyimide Polyimide - Chapter 41 297

Polyamideimide (PAI) - Chapter 42 301

Polyetherimide (PEI) - Chapter 43 305

Polyketone Polyaryletherketone (PAEK) - Chapter 44 313

Polyetheretherketone (PEEK) - Chapter 45 317

Polyetherketone (PEK) - Chapter 46 321

Polyolefin Polyethylene (PE) - Chapter 47 323

Low Density Polyethylene (LDPE) - Chapter 48 327

Medium Density Polyethylene (MDPE) - Chapter 49 329

High Density Polyethylene (HDPE) - Chapter 50 331

Ultrahigh Molecular Weight Polyethylene (UHMWPE) - Chapter 51 337

Polyethylene Copolymer - Chapter 52 341

Ethylene-Vinyl Acetate Copolymer (EVA) - Chapter 53 343

Polyethylene-Acrylic Acid Copolymer (EAA) - Chapter 54 345

Polymethylpentene (PMP) - Chapter 55 347

Polypropylene (PP) - Chapter 56 349

Polypropylene Copolymer (PP Copolymer) - Chapter 57 363

Polyphenylene Ether Polystyrene Modified Polyphenylene Ether (PPO and PPE) - Chapter 58 367

Polyphenylene Sulfide Polyphenylene Sulfide (PPS) - Chapter 59 383

Polysulfone Polysulfone (PSO) - Chapter 60 387

Polyethersulfone (PES) - Chapter 61 399

Styrenic Resin Acrylonitrile-Butadiene-Styrene Copolymer (ABS) - Chapter 62 407

Acrylonitrile-Styrene-Acrylate Copolymer (ASA) - Chapter 63 417

Polystyrene (PS) - Chapter 64 421

General Purpose Polystyrene (GPPS) - Chapter 65 425

Impact Resistant Polystyrene (IPS) - Chapter 66 427

Styrene-Acrylonitrile Copolymer (SAN) - Chapter 67 431

Styrene-Maleic Anhydride Copolymer (SMA) - Chapter 68 435

Styrene-Butadiene Copolymer (Styrene Butadiene) - Chapter 69 441

Polyurethane Rigid Thermoplastic Urethane (RTPU) - Chapter 70 443

Vinyl Resin Polyvinyl Chloride (PVC) - Chapter 71 445

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Plastic Alloy

Acrylic Resin/Polyvinyl Chloride Alloy (Acrylic/PVC) - Chapter 72 453

Acrylonitrile-Butadiene-Styrene Copolymer/Polyvinyl Chloride Alloy (ABS/PVC) - Chapter 73 455

Polycarbonate/Acrylonitrile-Butadiene-Styrene Copolymer Alloy (PC/ABS) - Chapter 74 457

Polycarbonate/Polyester Alloy - Chapter 75 461

Polycarbonate/Polbutylene Terephthalate Alloy (PC/PBT) - Chapter 76 463

Polycarbonate/Polyethylene Terephthalate Alloy (PC/PET) - Chapter 77 465

Polyethylene Terephthalate/Polbutylene Terephthalate Alloy (PET/PBT) - Chapter 79 469

Polystyrene Modified Polyphenylene Ether/Nylon 6 Alloy (PPE/Nylon) - Chapter 80 471

Polyvinyl Chloride Alloy (PVC Alloy) - Chapter 81 473

Thermoplastic Elastomers Generic Thermoplastic Elastomer Thermoplastic Elastomer (TPE) - Chapter 82 475

Olefinic Thermoplastic Elastomer Olefinic Thermoplastic Elastomer (TPO) - Chapter 83 477

Polyester Thermoplastic Elastomer Polyester Thermoplastic Elastomer (Polyester TPE) - Chapter 84 479

Styrenic Thermoplastic Elastomer Styrenic Thermoplastic Elastomer (Styrenic TPE) - Chapter 85 483

Urethane Thermoplastic Elastomer Urethane Thermoplastic Elastomer (TPUR) - Chapter 86 485

Thermoplastic Polyester-Polyurethane Elastomer (TPAU) - Chapter 87 487

Thermoplastic Polyether-Polyurethane Elastomer (TPEU) - Chapter 88 489

Vinyl Thermoplastic Elastomer Polyvinyl Chloride Polyol (pPVC) - Chapter 89 491

Thermosets Diallyl Phthalate Polymer Diallyl Phthalate Polymer (DAP) - Chapter 90 495

Epoxy Resin Epoxy Resin - Chapter 91 497

Phenolic Resin Phenol-Formaldehyde Copolymer - Chapter 92 499

Polyester Thermoset Polyester - Chapter 93 501

Reaction Injection Molding Systerm (RIM) Polyurethane Reaction Injection Molding System (PU RIM) - Chapter 94 503

Vinyl Ester Vinyl Ester Resin - Chapter 95 505

Polycarbonate/Glycol Modified Polycyclohexylenedimethylene Terephthalate Alloy - Chapter 78 467

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Ethylene Propylene Rubber (EPR)

Ethylene-Propylene-Diene Copolymer (EPDM) - Chapter 96 507

Fluoroelastomer Vinylidene Fluoride-Hexafluoropropylene Copolymer (FKM) - Chapter 97 509

Polyurethane Urethane (PU) - Chapter 98 511

Rubber Alloy PE Copolymer/ Fluoroelastomer Alloy - Chapter 99 513

Appendices Glossary of Terms 515

Indicies Reference Index 551

Figure Index 561

Table Index 569

Supplier Directory 573

General Index 579

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Chapter 1

Heated Tool Welding

© Plastics Design Library Heated Tool Welding

PROCESS

In hot tool or hot plate welding, a heated

platen is used to melt the joining surfaces of two

thermoplastic parts After the interfaces of the

plastic parts have melted, the heated platen is

removed, and the parts are held together under low

pressure to form a molecular, permanent, and

hermetic seal A hot plate is used for flat joining

surfaces; for curved or irregular joining surfaces,

complex tools that allow the hot surfaces to match

the contours of the joint interface are required

For accurate mating and alignment,

holding fixtures (collets, gripping fingers,

mechanical devices, vacuum cups) must support

the parts to be joined The joint surfaces should be

clean and relatively smooth to the surface of the

heated tool; weld quality is affected if the surfaces

are contaminated by mold release agent or grease

Surfaces can be treated mechanically or

chemically For a butt joint weld (Figure 1.1), the

two ends must be completely aligned before

welding begins [513, 495, 502]

In hot plate welding, the parts to be joined

are pressed against the hot platen; platens can be

coated with polytetrafluoroethylene (PTFE) to

inhibit melt sticking Welding can be performed in

either of two ways, referred to as welding by

pressure and welding by distance Both processes consist of four phases, shown in the pressure vs time diagram in Figure 1.2 [552, 521]

In welding by pressure, the parts are brought in contact with the hot tool in phase I, and

a relatively high pressure is used to ensure complete matching of the part and tool surfaces Heat is transferred from the hot tool to the parts by conduction, resulting in a temperature increase in the part over time When the melting temperature

of the plastic is reached, molten material begins to flow This melting removes surface imperfections, warps, and sinks at the joint interface and produces

a smooth edge Some of the molten material is squeezed out from the joint surface due to thermal expansion of the material In phase II, the melt pressure is reduced, allowing the molten layer to thicken; the rate at which the thickness increases is determined by heat conduction through the molten

Figure 1.1 A butt joint used for hot tool welding, shown

before and after welding

Figure 1.2 Pressure vs time curve showing the four phases of heated tool welding Parts to be welded are pressed against the hot tool in phase I, and heat is transferred to the parts by conduction Melting begins when the melt temperature of the plastic is reached

In phase II, pressure is reduced in order to increase melt thickness

In phase III, the hot tool is removed, and in phase IV, the parts are brought together under pressure to cool and solidify

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layer Thickness increases with heating time - the

time that the part is in contact with the hot tool

(usually 1 to 6 seconds)

When a sufficient film thickness has been

achieved, the part and hot tool are separated This

is phase III, the changeover phase, in which the

pressure and surface temperature drop as the tool

is removed Duration of this phase should be as

short as possible (ideally, less than 3 seconds) to

prevent premature cooling of the molten material

A thin, solid “skin” may form on the joint interface

if the changeover time is too long, affecting weld

quality In phase IV, parts are joined under

pressure, causing the molten material to flow

outward laterally while cooling and solidifying

Intermolecular diffusion during this phase creates

polymeric chain entanglements that determine

joint strength Because final molecular structure

and any residual stresses are formed during

cooling, it is important to maintain pressure

throughout the cooling phase in order to prevent

warping For semicrystalline polymers,

recrystallization occurs during this phase;

recrystallization behavior is affected by cooling

rates Joint microstructure, which affects the

chemical resistance and mechanical properties of

the joint, develops during phase IV [513, 520,

521, 495]

Welding by pressure requires equipment in

which the applied pressure can be accurately

controlled A drawback of this technique is that

the final part dimensions cannot be controlled

directly; variations in the melt thickness and

sensitivity of the melt viscosities of thermoplastics

to small temperature changes can result in

unacceptable variations in part dimensions [366]

In welding by distance, also called displacement

controlled welding, the process described above is

modified by using rigid mechanical stops to

control the welding process and the part

dimensions Parts are pressed against the hot tool

under pressure, but the displacement of the parts as

the molten material flows out during phase I is

restricted to a predetermined distance using

mechanical stops on the hot tool (melt stops) and

on the holding fixture (holding or tooling stops)

During melt flow, the part length decreases as

molten material flows out laterally; when melt

stops contact tooling stops in phase II, parts are held in place for a preset time to allow the molten film to thicken The hot tool is removed in phase III, and mechanical stops are used again in phase

IV to inhibit motion of the parts, allowing the molten film to solidify only by heat conduction and not by lateral flow Cooling time is usually 3

to 6 seconds and ends when tooling stops on supporting fixtures come into contact Total cycle time for hot tool welding is usually 20 seconds or less Steps in welding by distance are shown in Figure 1.3 [495, 366, 511]

Figure 1.3 The hot tool welding process, showing displacement stops used in welding by distance In step 1, parts are aligned in holding fixtures; tooling and melt stops are set at specified distances on the holding fixture and heating platen, respectively The platen is inserted between the parts in step 2, and parts are pressed against it in step 3 Step 3 includes phases I and II of Figure 1.2 Molten material melts and flows out of the joint interface, decreasing part length until melt stops meet tooling stops Melt thickness then increases until the heating platen is removed in step 4, the changeover phase (phase III in Figure 1.1) Parts are pressed together in step 5 (phase IV), forming a weld as the plastic cools; tooling stops inhibit molten flow The welded part is removed

in step 6

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3

PROCESSING PARAMETERS

Important processing parameters for hot

tool welding are the hot tool temperature during

phases I and II, the pressure during phase I

(matching or heating pressure), heating time,

displacement allowed during heating (heating

displacement), melt pressure during phase II,

changeover time, pressure during phase IV (weld,

joining, or consolidation pressure), duration of

phase IV (consolidation time or welding time), and

displacement allowed during phase IV (welding

displacement) In welding by distance, the

parameters should be set so that the displacement

(also called the penetration), the decrease in part

length caused by the outflow of molten material, is

large enough to control part dimensions Initially

in the welding process, there is very little molten

flow, and the molten film thickens The flow rate

increases with heating time, eventually reaching a

steady state at which the rate of outflow equals the

rate at which the material is melting; at this point

in welding by pressure, the penetration increases

linearly with time When displacement stops are

used, however, the penetration ceases when the

melt displacement stops come into contact with the

hot tool displacement stops Until the stops come

into contact, the melt will flow out laterally;

afterward, the thickness of the molten material

increases with time

Molten layer thickness is an important

determinant of weld strength If the thickness of

the molten layer is less than the melt stop

displacement, melt stops cannot contact holding

stops, part dimensions cannot be controlled, and

joint quality is poor due to limited intermolecular

diffusion In addition to contributing to weld

strength, adequate displacement in phases I and II

compensates for part surface irregularities and

ensures that contaminated surface layers flow out

before the joining phase [514]

Melt thickness increases with heating

time For optimal molten layer thickness, heating

time should be long enough to ensure that melt

thickness is as large as the melt stop displacement

High heating pressures result in larger amounts of

squeeze flow; displacement stops may not be

reached if too much material is lost by being

squeezed out of the joint, and the decreased molten

layer thickness produces a brittle weld If the molten layer thickness is greater than the melt stop displacement, molten material will be squeezed out, producing weld flash and an unfavorable molecular orientation at the interface; this reduces the quality of the joint [512, 514, 510]

The effect of parameters on weld strength has not been investigated extensively In experiments with polypropylene, tensile strength increased slightly with heating time (at 260oC, 500

o

F) up to about 30 seconds, then leveled off; optimum molten layer thickness was reached, so that further increases in heating time had no effect

on weld strength At higher heating temperatures, weld quality was sensitive to variations in heating time At 320oC, (608 oF) optimum heating time was 10 seconds; changes in either direction in heating time significantly decreased weld strength

strengths were not significantly affected by 30 second variations in heating time Strength decreased with increased heating pressures (over 0.9 MPa, 131 psi) and decreased with increasing changeover times (0.5 to 3 seconds); the effect of changeover time was greater at heating times of 30 seconds than at 40 seconds At a 60 second heating time, weld quality improved as changeover time increased to 10 seconds Lower strengths were obtained when displacement stops were increased from 0.2 mm (0.0075 in.) to 0.4 mm (0.015 in.) Weld strengths increased slightly with increasing weld times, then leveled off at about 25 seconds Highest weld strengths obtained were about 95% of the neat material Displacement (penetration) generally increases with increasing temperature and heating time and decreases with increases in changeover time [518, 510, 513]

High strength welds were obtained with acrylonitrile-butadiene-styrene (ABS) Weld strengths with flash retained were higher than those in which the weld flash was machined off; highest strengths obtained were 95% of neat ABS Weld strength increased slightly as machine

o

seconds; however, at 20 second heating times,

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(400.1 F to 425.3 F) did not affect weld strength

[514]

Optimal parameter settings are dependent

on the materials to be welded Computer-aided

parameter optimization is possible by monitoring

the viscosity of the melt zone [517]

Quality control in production can be

implemented by monitoring parameters during the

welding process; if one parameter is not within a

specified tolerance range, the welding machine

either produces a signal or stops the welding

process More sophisticated techniques include

statistical process control, in which parameters and

melt characteristics are monitored and compared

throughout the welding cycle, and continuous

process control (CPC), in which optimum

parameters are continuously calculated, with the

welding machine adjusting conditions as necessary

throughout the welding process [508]

MATERIALS

Hot tool welding is suitable for almost any

thermoplastic but is most often used for softer,

semi-crystalline thermoplastics such as

polypropylene and polyethylene and for

thermoplastic polyimides It is usually not suitable

for nylon or other materials with long molecular

chains The temperature of the molten film can be

controlled by controlling the hot tool temperature,

so that plastics that undergo degradation at

temperatures only slightly above the melting

temperature can be welded

Properties of the plastics to be welded

affect the strength of the weld Within a polymer

family such as high density polyethylene (HDPE),

attainable weld strength may depend on the grade

of the polymer and can be related to the structural

parameters of melt index and density Lower melt

index polymers produce higher melt viscosities

and can tolerate higher heating temperatures

without melt sticking to the hot tool As a result,

the size of the heat affected zone (HAZ), the part

area affected by heat, can be larger; a larger HAZ

produces a higher strength joint For a constant

melt index, increasing polymer density results in

joints with lower tensile strength Higher density

polymers have a greater proportion of crystalline

regions, which melt in a narrower temperature

range than polymers of lower crystallinity As a

result, a thinner HAZ and more brittle welds are obtained [522]

In hygroscopic materials such as polycarbonate (PC), absorbed water may boil during welding, trapping steam and lowering weld strength High weld strengths can be obtained by predrying materials; alternatively, processing parameters can be adjusted to compensate for absorbed water High strength welds can be achieved in dried PC over a wider heating temperature range (250 - 400oC, 482 - 752 oF) than

in undried PC (230 - 250oC, 446 - 482 oF) With increasing part thickness, the optimum temperature range shifts to higher temperatures [521]

Dissimilar materials having different melting temperatures can be welded in hot tool welding; instead of a single platen with two exposed surfaces, two platens are used, each heated to the melting temperature of the part to be welded Different melt and tooling displacements and different heating times for each part may be necessary, and due to different melt temperatures and viscosities, the displacement of each part will

be different Optimum processing conditions for each material must first be established, followed

by optimizing process conditions for welding the two materials together High strength welds equal

to the strength of the weaker material can be achieved [511]

WELD MICROSTRUCTURE

Weld quality is determined by the microstructure of the heat affected zone of the weld The heat affected zone consists of three zones in addition to the weld flash The stressless recrystallization zone consists of crystals with a spherulitic shape, indicating that cyrstallization occurred under no significant stress This zone results primarily from reheating and recrystallization of the skin layer and the molten layer near the joint interface The columnar zone consists of elongated crystals oriented in the flow direction; lower temperatures in this zone lead to

an increase in melt viscosity, and crystals formed during melt flow aligned with the flow direction

In the slightly deformed zone, deformed spherulites are present, resulting from recrystallization under joining pressure Higher heating temperatures result in larger heat affected

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5

zones and greater bond strength; however, too high

a temperature or pressure results in void formation

at the joint interface [513]

polypropylene welds by differential scanning

calorimetry (DSC) and Fourier transform infrared

spectroscopy (FTIR) indicated that welds with

low tensile strength correlated with the lack of a

region of deformed spherulites between the weld

and the bulk material, a wide weld region, and a

reduction in the amount of melt flow in the weld

direction Low strength welds had a lamellar

thickness distribution similar to that of the bulk

material, but a wider distribution of lamellar

thicknesses was present in high strength welds

[614]

EFFECTS OF AGEING ON WELD STRENGTH

Chemical and physical changes may occur

in polymers during hot tool welding, affecting the

durability of the weld After air oven ageing at

120oC (248oF) at times ranging from 3 to 14 days,

there was a significant reduction (>30%) in weld

cross-sectional area in ABS welds, and

degradation of the rubber segment of ABS resulted

in yellowing Elongation and tensile strength were

reduced more than in the bulk material After

tensile properties and elongation deteriorated more

in the weld than in the bulk material Whitening occurred in the weld, possibly due to free radical and hydroperoxide formation during welding which subsequently initiate degradation reactions Unsaturation in ABS decreased, along with an increase in carbonyl group concentration; both were more significant in welds than in bulk material Ageing effects should be considered when welds will be exposed to aggressive environments [516]

VARIANTS OF HOT TOOL WELDING

In direct contact hot tool welding, described above, parts are pressed against the hot tool For high temperature polymers, the hot plate temperature required for melting is too high for non-stick surfaces to be used In non-contact hot plate welding, parts are brought very close to the hot plate without actually coming into contact with

it (Figure 1.4) Heat is transferred by thermal radiation and convection The process is otherwise identical to hot tool welding: the hot plate is removed in the changeover phase, and pressure is applied to achieve intimate contact as the weld cools and solidifies

Processing parameters that influence weld strength include the size of the non-contact gap, platen temperature, heating time, change-over time, and weld pressure and duration Effects of change-over time and weld pressure and duration are similar to those in direct contact hot tool welding In non-contact hot plate butt welding of polypropylene plates, using a 1 mm (0.04 in.) non- contact gap, weld strength approached or equaled bulk strength at optimal heating times which varied with hot plate temperature Higher hot

stronger joints at shorter heating times (40 s); however, joint strength decreased at longer heating times due to excessive squeeze flow of molten material out of the joint interface and an adverse molecular orientation

Joint strengths increased with increasing duration

of weld pressure up to 60 seconds, then remained constant or decreased slightly Optimal weld

Figure 1.4 Non-contact hot plate welding Parts being welded

are placed near the hot plate, separated from it by a distance referred

to as the non-contact gap The hot plate is removed during the

change-over phase, and pressure is applied to hold the parts in

intimate contact during weld coolingand solidification

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pressure was about 0.35 MPa (50 psi); lower

pressures allowed air entrapment in the joint,

while higher pressures produced excessive squeeze

flow out of the joint and an unfavorable molecular

orientation during weld formation [613]

EQUIPMENT

A hot tool welding machine consists of the

hot tool assembly with two exposed surfaces, two

fixtures for holding parts to be welded, tooling for

bringing parts in contact with the hot tool and

bringing molten joint surfaces together to form the

weld, and displacement stops on the platen and

holding fixtures Dual platen hot tool welding

machines are used for welding dissimilar

materials Welders can accommodate a range of

varying part designs and sizes and can join parts in

either a vertical or horizontal plane In vertical

heat platens, tooling can be lifted out of the top of

the machine, both part halves can be loaded at the

same time with a single cavity tool, and nests are

in view for part loading Some welding equipment

can remove weld flash after the weld is formed

[576, 492, 493]

Equipment ranges from manually loaded

and unloaded machines to semi-automated and

fully automated in-line systems Statistical control

of weld cycles can be achieved through operator control panels that display all machine parameters and diagnostic functions, and pressure or displacement can be programmed throughout the welding cycle Parts conveyors or drawer load features are optional equipment Equipment is rugged and is designed to produce molecular, hermetic seals with consistent joint strength A typical hot tool welder is shown in Figure 1.5 Tooling provides accurate mating and alignment of parts, and displacement stops control melt and weld dimensions Tooling with displacement stops is shown in Figure 1.6 [514, 493, 492]

ADVANTAGES AND DISADVANTAGES

Hot tool welding is a simple, economical technique in which high strength joints and hermetic seals can be achieved with both large and small parts Joints with flat, curved, or complex geometries can be welded, and surface irregularities can be smoothed out during the heating phases (I and II) Dissimilar materials that are compatible but that have different melting temperatures can be welded using hot tools at different temperatures Expensive plastics can be used for only critical part components; inexpensive plastics can be welded on to comprise the remainder of the part Processing parameters can

be monitored, and the welding process can be easily automated Hot tool welding is used on compatible materials and does not introduce foreign materials to the part; as a result, plastic

Figure 1.5 A typical vertical platen hot tool welder

Figure 1.6 Tooling displacement stops in a hot tool welder, used to control melt and part dimensions

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7

parts are more easily recycled [517, 495, 513,

477, 450]

In non-contact hot tool welding,

contamination of weld surfaces is minimized,

heating is uniform, and a small weld bead is

produced, providing good, consistent weld

strengths [613]

The major disadvantage is the long cycle

time required, compared with vibration or

ultrasonic welding Welding times range from 10

to 20 seconds for small parts to up to 30 minutes

for large pipes; typical cycle times are from 12 to

22 seconds A second disadvantage is the high

temperatures required for melting Heat is not as

localized as in vibration welding, and in some

cases can cause plastic degradation or sticking to

the hot platen When hot melted surfaces are

pressed against each other in phase IV, weld flash

is produced This must be hidden or removed for

cosmetic reasons In welding by pressure, part

dimensions cannot always be controlled reliably

due to variations in the molten film thickness and

sensitivity of the melt viscosities of thermoplastics

to small temperature changes [511, 576, 552]

In the appliance industry, the welding of

glass-filled polypropylene dishwasher pump

housings, initially welded using hot tool welding,

was converted to vibration welding due to reduced

labor costs and lower power requirements This is

described in more detail under Vibration Welding,

Applications

APPLICATIONS

Hot tool welding can be used to join parts

as small as a few centimeters to parts as large as

1.5 meters (4.9ft.) in diameter It is commonly

used in load-bearing applications and for welding

large parts such as pipelines; special machines can

weld large diameter pipes on site [477, 518]

Cost reduction is possible by welding

dissimilar materials Automotive headlights, tail

lights, and blinker assemblies are made by welding

a clear polycarbonate or, more commonly,

polymethylmethacrylate (PMMA) lens to an

inexpensive plastic body made of ABS Double

cavity holding tools are used for welding rear

lights For high temperature applications, a fascia

of a relatively expensive high temperature plastic can be welded to a less expensive subcomponent

An automobile headlight joined using hot tool welding is shown in Figure 1.7 [511, 493, 508]

Stress cracking occurs in tail lights made from welding ABS to PMMA and is the most frequent cause of failure in tail lights Welding induces internal tensile stresses below the yield point in PMMA which later cause cracks to form; the time before crack formation occurs varies ABS is relatively insensitive to stress cracking due

to the soft butadiene component Exposure to surface active media such as methanol or windshield washer fluid accelerate crack formation

by reducing the cohesive surface tension of the plastic The mechanical stresses necessary for crack formation are then lowered to below the yield point, and cracks occur at low strain Figure 1.8 shows stress cracking in a tail light Higher internal stresses occur on the weld seams, which trigger cracks after exposure to a surface active medium and result in realignment of the break surface Several small cracks are present on the welded lights, due to positioning constraints on the welded-on ABS housing Susceptibility to stress cracking can be reduced by suitable processing conditions In stress cracking experiments,

Figure 1.7 An automobile headlight; parts were joined using hot tool welding

Trang 17

susceptibility to stress cracking was significantly

lower when either low (230oC) or high (420oC) hot

tool temperatures were used [616]

Other automobile parts, such as fluid

reservoirs, fuel tanks, and vent ducts, are hot tool

welded Polypropylene air ducts and mounting

brackets are hot tool welded to the main part of the

instrument panel, made of glass mat reinforced

polypropylene, on the Mercedes-Benz S-class

automobile using mechanical stops to control part

dimensions In plastic fuel tanks, function parts,

such as clips, vent lines, and filler necks, are hot

plate welded to the blow molded tank; 29 parts are

hot plate welded to the AUDI Quattro fuel tank

[508]

Plastic heat exchangers can be hot tool

welded using a hot plate with deep heatable

grooves that can be pressed against the bundles of

thin-walled thermoplastic pipes High pressure

applied to the melted region results in molecular

entanglement and high weld strength Heat

exchangers produced using this welding method

display superior thermal performance, and

production cost is competitive with traditional heat

exchangers [509]

Hot tool welding is also used in appliance

tubs, agitators, and spray arms Polyvinyl chloride

is hot tool welded in medical products, life

jackets, stationery products such as loose leaf

binders, and blister packages, and plastic window

frames are made by welding mitered, extruded

profiles of a commercial grade of

acrylonitrile-butadiene-styrene (ABS) developed especially for

window frame applications [514, 552, 495]

Figure 1.8 Stress cracking in tail lights Welding induces internal stresses which later cause cracks to form.

Trang 18

Chapter 2

Hot Gas Welding

© Plastics Design Library Hot Gas Welding

PROCESS

In hot gas or hot air welding, a heated gas is

used to heat thermoplastic parts and a filler rod to

the melting or glass transition temperature Rod

and parts then soften and fuse, forming a high

strength bond upon cooling Hot gas welding is

commonly used for fabrication and repair of

thermoplastic components and for lap welding of

thin sheets or membranes High bond strengths,

up to 90% of the bulk material, can be achieved

Hot gas welding is the earliest method of joining

thermoplastics and was first used in World War II

to repair bullet-riddled acrylic cockpit canopies

[669, 671, 670, 652]

Hot gas welding methods can be manual or

automatic; manual methods are commonly used for

short seams In manual methods, a gas flows

through a flexible tube to a hot gas gun containing

a sheathed ceramic heating element Gases used

are usually nonflammable (air, nitrogen, carbon

dioxide), although flammable gases (hydrogen,

oxygen) can also be used; air is most common

The gas is heated to the melting temperature of the

thermoplastic and is applied to the part and a

thermoplastic filler rod through a nozzle or tip

The filler rod is composed of the same material as

the part and is positioned at the joint As the

operator moves the tip along the joint, the parts

and filler rod soften and merge together, forming a

weld after solidification (Figure 2.1) Tacking,

welding just enough to hold the parts together, is

frequently performed to hold the parts in place

while a permanent weld is made A filler rod is

not used in tack welding For high speed welding,

the rod is fed through a welding tip containing a

feeding channel, making it unnecessary for the

operator to hold the rod during welding Joint

surfaces should be cleaned prior to welding, using

mild soap or chemical detergent and/or methyl

ethyl ketone (MEK) for grease removal [671,

652]

Automatic welding machines are also available and are used for overlap welding of seams or membranes No filler rod is used in lap welding, and no joint preparation is necessary A diagram of an automatic hot gas welder used for sealing sheet seams is shown in Figure 2.2 The pressure and drive rollers apply pressure to the seam and move it along as welding proceeds As

heated gas is blown between the membranes through a nozzle, escaping gas preheats the material to be sealed, and small particles (stones, sand, dust) are blown away from the surface Hot

Figure 2.1 Manual hot gas welding A heated gas flowing through the welding tip is applied to the joint interface and to the filler rod positioned at the joint As the operator moves the tip and the filler rod along the joint, the filler rod and joint surfaces soften and fuse

Trang 19

gas emerges at the tip of the nozzle, causing the

thermoplastic sheet material to melt and flow As

new material is fed through the pressure and drive

roller, the melted seam cools and solidifies

Automatic hot gas welding produces a consistent,

reproducible, high quality weld [652, 670]

Processing parameters in hot gas welding

include welding speed, welding pressure, and hot

gas temperature Gas temperatures usually range

from 200 - 600ºC, (400-1100°F) depending on the

melting temperature of the plastic material; gas

flow rates range from 15 - 600 L/min Welding

speeds vary greatly; typical speeds can range from

0.04 - 10 m/min (1.6 - 394 in./min.) or more

Because hot gas welding is frequently a manual

process, parameters are adjusted continually by the

operator, according to the appearance of the weld

Figure 2.3 depicts the appearance of good- and

poor-quality welds No charring or discoloration

should be apparent along the weld, and the filler

rod should not be stretched during welding A

good weld has a fine bead on both sides of the

weld (Figure 2.3) [671, 652, 673]

In welding high density polyethylene (HDPE)

geomembranes, temperatures of 450ºC, 500ºC, and

550ºC (840ºF, 900ºF and 1020ºF) and welding

speeds of 1.7 - 2.0 m/min (67 - 79 in./min.) were used with a joint pressure of 1250 N Highest strength seals were obtained at a temperature of 500ºC (900ºF) and a welding speed of 1.8 m/min (71 in./min) [670]

MATERIALS

Hot gas welding can be used to join most thermoplastics, including polypropylene, polyethylene, acrylonitrile-butadiene-styrene (ABS), polyvinyl chloride, polyurethane, HDPE, polyamide, polycarbonate, and polymethyl methacrylate The diameter of the filler rod selected should be similar to the thickness of the part; a 0.32 cm (0.13 in.) diameter rod should be used for a part thickness of 0.32 cm (0.13 in.) For part thicknesses greater than 0.64 cm (0.25 in.), more than one rod may be necessary to reach the required thickness [671, 507, 670]

Hot gas welding used to repair automobile bumpers composed of polycarbonate/polyester, polybutylene terephthalate (PBT), or ethylene

Figure 2.2 Diagram of an automatic hot gas welder used for sealing seams Pressure and drive rollers apply pressure and move the seam along as welding proceeds Hot gas is blown between the sheets through a nozzle to the nozzle tip, where the thermoplastic sheet melts and flows together, forming a seal

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11

Figure 2.3 Weld quality analysis by weld appearance

Figure 2.4 Joint designs commonly used in hot gas welding

Trang 21

propylene diene monomer (EPDM) produced

welds of low ductility and reduced strength from

that of the bulk material Highest tensile strength

was obtained with PBT; strength was 97%

of bulk material Tensile stengths of

polycarbonate/polyester and EPDM were 63% and

78%, respectively, of bulk material Impact

strength of all repaired bumpers was low,

especially when impact occurred at the weld face;

damage occurred in single V welds after impacts

from a distance of >0.1 m (4 in.); double V joint

designs withstood impacts dropped from 1 m (39

in.) All bumpers withstood crashes at speeds up

to 5 mph Scanning electron microscopy indicated

that low joint strength was due to a lack of

complete fusion at the joint line; contractions that

occurrred as the weld cooled could not be

counterbalanced by pressure on the weld from

surrounding material, leading to cavities and crack

formation [669]

JOINT DESIGN

Joints commonly used in hot gas welding are

shown in Figure 2.4 In repairing automotive

generally produced higher strength welds than a

single V butt weld [671, 669]

EQUIPMENT

Equipment for manual welders consists of a welding barrel heating element, gas cylinder, pressure regulator, welding tips, welding rods, and various connectors Many types of welding tips are available, depending on the application, and automatic feed tips can automatically feed filler rods to the joint for high speed welding Prices vary widely, ranging from about $150 for small welders to over $1000 (US dollars) Optional equipment, such as cleaning brushes, gas filters, and leak detectors is also available A typical manual welder is shown in Figure 2.5 [671]

Automatic welders are commonly used for welding sheet material, bitumen, and roofing membranes Air flow rates, drive speeds, and temperatures are adjustable, and temperatures and drive speeds are electronically controlled Welding seam widths vary depending on the machine and pressure roller With automatic welders, uniform pressure and precise tracking on uneven surfaces can be achieved, and membranes

Figure 2.5 A manual welder

Figure 2.6 An automatic overlap welding machine, with a welding seam width of 4 cm (1.6 in.) and welding speed up to 3 m/min (118 in./min.)

Trang 22

13

of various thicknesses can be welded Nozzles are

interchangeable on tape welding machines to

accommodate different widths of bar cover strips

on roofing membranes An automatic overlap

welding machine is shown in Figure 2.6

Hot gas welding is a fast, simple welding

process requiring inexpensive equipment It can be

used to weld components together in small,

difficult-to-weld places and can be used on-site for

fabrication of large components or for repairs

Manual welding methods, however, require a

skilled operator for optimum welds, and weld

quality is not as reproducible as with automatic

welders Temperature control can be difficult in

ambient air conditions [652, 507]

APPLICATIONS

Hot gas welding is used in a wide variety of

welding, sealing, and repair applications It is

used for fabrication and repair of chemical tanks,

pipe fittings, plastic glazing units, and large

injection molded components Other applications

include sealing sheets and membranes, such as

vinyl floor coverings, HDPE geomembranes in

landfills, and ducting and roofing membranes

[669, 507]

Trang 23

Vibration Welding

PROCESS

Vibration welding uses heat generated by

friction at the interface of two materials to produce

melting in the interfacial area The molten

materials flow together under pressure and bond,

forming a weld upon cooling Vibration welding

can be accomplished in a short time (8-15 second

cycle time) and is applicable to a variety of

thermoplastic parts with planar or slightly curved

surfaces There are two types of vibration

welding: linear, in which friction is generated by a

linear, back-and-forth motion, and orbital, in

which the upper part to be joined is vibrated using

circular motion in all directions Linear vibration

welding is most commonly used, but orbital

vibration welding makes the welding of irregularly

shaped plastic parts possible

In linear vibration welding, the surfaces to be

joined are rubbed together in an oscillating, linear

motion under pressure applied at a 90o angle to the

vibration Process parameters are the amplitude

and frequency of this motion (weld amplitude and

weld frequency), weld pressure, and weld time, all

of which affect the strength of the resulting weld

(Figure 3.1)

The welding process consists of four phases

In the first phase, heat generated through friction

raises the temperature of the interfacial area to the

glass transition temperature of amorphous

thermoplastics or the crystalline melting point of

semi-crystalline plastics In phase II, material at

the interface begins to melt and flow in a lateral

direction, and the generated heat is dissipated in

the molten polymer This viscous flow begins to

increase the weld penetration, the distance through

which the parts approach each other due to lateral

flow In phase III, melting and flow attain a steady

state, and the weld penetration increases linearly

with time At the end of phase III, the vibratory

motion is stopped, and during phase IV, the weld

penetration increases slightly as the molten film

solidifies under pressure A representative

Figure 3.2 Penetration vs time curve showing the four phases of vibration welding

I solid material friction

II non-steady state melt film formation III steady state melt film formation

IV cooling or holding phase, after vibratory motion ceases Penetration begins in phase II, reaches the threshold penetration,

ηT, at the beginning of phase III, and continues to increase until the end of phase 4, when the weld has solidified tc is the cycle time for the welding process

Figure 3.1 Linear Vibration Welding Part surfaces are rubbed together in a longitudinal direction, along the z axis, generating heat through friction Processing parameters are the weld amplitude, a, the weld frequency, n, the weld pressure, po, and weld time, t Pressure is applied along the y axis, 90o to the vibration

Trang 24

Vibration Welding © Plastics Design Library

16

penetration vs time curve showing the four phases

is given in Figure 3.2

Most industrial vibration welding

machines operate at weld frequencies of 120 to

240 Hz, although welding machines with higher

frequencies are also available The amplitude of

the vibration, produced by exciting a tuned

spring-mass system, is usually less than 5 mm (0.2 in.);

weld time ranges from 1 to 10 seconds (typically 1

to 3 seconds), with solidification times, after

vibratory motion has ceased, usually of 0.5 to 1.0

seconds Total cycle times typically range from 6

to 15 seconds, resulting in 4 to 10 cycles per

minute Lower weld amplitudes, (0.7 mm to 1.8

mm; 0.03 in to 0.07 in.) are used with higher

frequencies (240 Hz), and higher amplitudes (2

mm to 4 mm, 0.08 in to 0.16 in.) are used with

lower frequencies (100 Hz) to produce effective

welds; low amplitudes are necessary when welding

parts into recessed cavities with low clearances

[491]

Amplitude and frequency are dependent

on the geometry of the parts to be joined and are

set to attain a maximum frictional force Welding

at high frequencies requires less relative motion

between parts, while low frequencies require

greater amplitudes Generally, high frequencies

are used when clearances between parts are

restricted to less than 1.5 mm (0.06 in.) and/or

when flash (molten plastic that seeps out of the

joint area during welding) is undesirable, as in

welding brake and steering fluid reservoirs The

greater amplitudes of low frequency welding are

advantageous in welding parts with long, thin,

unsupported side walls oriented perpendicular to

the direction of vibration These parts are

susceptible to flexing, which inhibits welding;

however, the greater displacement of low

frequency welding in many cases negates the

effects of flexing, so that a weld can be obtained

[503, 504]

For high temperature thermoplastics, a

minimum amplitude of 0.5 mm (0.020 in.) is used

to increase the viscoplastic component of the

deformation Deformation behavior of the plastic

is important in converting mechanical energy into

heat during phase II Only the viscoplastic

component of the shear energy is converted into heat irreversibly; the elastic component of the deformation energy is reversible At higher amplitudes, the viscoplastic component of the deformation is proportionately higher, leading to increased heat at the joint surface [482]

Weld pressure varies widely (0.5 - 20 MPa; 72 - 2900 psi), although usually pressures at the lower end of this range are used Welding time and pressure depend on the material being welded Higher pressures decrease the welding time; however, higher strength of the welded parts is usually achieved at lower pressures due to a greater melt layer thickness Weld strength is generally not very sensitive to the frequency and amplitude of vibration, although some materials (i.e polyetherimide) require high frequencies to attain high weld strengths [491]

High mechanical strength can be obtained

at shorter weld times by decreasing the pressure during the welding cycle A high starting pressure shortens the time required to reach phase III in the welding process; the pressure can then be reduced

to obtain high mechanical strength equal to that of

a conventional, constant pressure weld The value

of the low pressure in this modified process depends on the material Figure 3.3 shows an optimized pressure profile as a function of time [478, 482]

Figure 3.3 A schematic optimized pressure profile for obtaining high strength at short welding times An initial high pressure, p1, is decreased in phase III; this lower pressure is maintained throughout the cooling period

Trang 25

The most important determinant of weld

strength is the weld penetration Static strengths

equal to that of the neat resin can be achieved

when the penetration exceeds a critical threshold

value, ηT, equal to the penetration at the beginning

of the steady state phase (III); weld strengths

decrease for penetrations below this value

Penetrations greater than the critical threshold do

not affect the weld strength of neat resins, chopped

glass-filled resins, or structural foams but can

increase the weld strength of dissimilar materials

The threshold increases with increasing thickness

of the parts to be welded; a threshold of about 0.25

mm (0.010 in.) results in high strength welds with

material thicknesses of 6.3 mm (0.25 in.) As long

as this threshold is reached, weld strengths are not

very sensitive to welding frequency, amplitude,

and pressure; however, at a constant threshold

value, weld strengths can decrease with increasing

weld pressure [366] Increasing the welding

pressure or vibration amplitude increases the

penetration rate and decreases welding time by

decreasing the time required to reach phase III

MATERIALS

Properties of the materials to be welded affect

the strength of the weld Water absorption during

storage increases the moisture content of some

thermoplastics, leading to bubble formation in the

joining area and decreased weld strength Storage

at 20oC (68oF) and 50% relative humidity leads to

a moisture content of approximately 3% in 2 mm (0.079 in) thick polyamide panels In vibration welding, polyamide panels in the joining area are heated to over 220oC (428 oF) in less than two seconds Transport of evaporated water to the surface by diffusion processes takes much longer than two seconds, so that much of the water vapor

is trapped in the polyamide melt in the form of bubbles According to the vapor pressure curve for water (Figure 3.4), water vapor formation can

be avoided (water will remain in a fluid state) at

220oC (428 oF) using higher welding pressures (about 5 MPa (725 psi) or higher) Pressure is reduced as melted materials flow into the joining area, so that bubble formation cannot be prevented completely Pre-drying reduces the amount of welding time required and leads to decreased bubble formation in the weld, but nylon and other hygroscopic resins can be welded without predrying [504, 483]

Welding behavior of materials that contain particulate or glass fillers (10 to 30%) is similar to that of neat resins, but attainment of threshold penetration generally requires slightly increased welding cycle times; times required are lower for glass than for particulate fillers Increasing filler content reduces the weld strength relative to that

of the neat resin by various amounts depending on the amount and type of filler [486] For plastics reinforced with glass fibers in the direction of the

load and for liquid crystalline thermoplastics, a pronounced weak point develops in the welds compared to the neat material due to fiber reorientation along the direction

of the weld For nonreinforced plastics, satisfactory weld strengths could be attained with optimum process parameter settings [482]

With optimal parameter settings, weld strengths of reinforced materials can be greater than those of unreinforced plastics In vibration welding experiments with nylon, maximum tensile strengths of 6 - 50% glass fiber reinforced nylon 6

Figure 3.4 Vapor pressure curve for water At a temperature of 220oC (428oF) and boiling

pressures above approximately 5 MPa (725 psi), water will remain fluid, and bubble formation

will be reduced At lower pressures, steam will form and will be trapped in the polyamide

melt, forming bubbles

Trang 26

18

were up to 17% higher than unreinforced grades;

peak strength, about 93 MPa (13,485 psi), was at

14-25% glass fiber content and was 17% higher

than that of unreinforced material Tensile

strength of nylon 66 with 33% glass fiber content

was 12% higher than unreinforced nylon 66

Optimal parameters were a weld amplitude of 1.02

to 1.80 mm (0.0402 to 0.071 in.), weld frequency

of 240 Hz, weld pressure of 0.70 MPa (102 psi),

and weld time of 4-25 seconds High weld

strengths at 14 - 25% glass fiber nylon 6 correlated

with a maximum weld zone thickness of about 300

direction of applied tensile stress, perpendicular to

the weld [615]

The thickness of parts to be welded affects

welding behavior Although the time required to

reach the melting temperature (time elapsed during

phase I) is not dependent on part thickness, lateral

flow of the molten material during phases II and III

is affected, resulting in longer times to reach a

steady-state flow (phase III); in addition, the

thickness of the weld is greater, resulting in

increased solidification times As a result, longer

welding cycle times are required for parts with

greater thickness Welding times can be decreased

by an increase in pressure A pressure increase

decreases the steady-state weld thickness and

increases weld strength by decreasing the

penetration necessary to attain the steady-state At

low pressures (0.52 MPa; 75 psi), high weld

strengths of thick parts (12.3 mm) cannot always

be achieved due to high required threshold

penetrations If penetration-based controls are

used to terminate the weld cycle, threshold

penetration must be adjusted to account for

pressure and part thickness [484]

When welding dissimilar materials, properties

of the two materials to be welded influence

welding behavior and affect weld strength

Vibration welding can generally be used to join

two materials differing in melt temperatures by up

to 38 oC (100oF) The extent of interdiffusion of

the two materials that occurs in the molten state

differs significantly for different materials,

depending on diffusion coefficients, molecular

weights, and cohesive energy densities, and affects

the morphology of the weld Shear mixing of the

two molten polymers produces mechanical

interlocking at the weld interface, the dominant mechanism for bond strength Penetration vs time curves are similar to those of welding the same material, but different melting temperatures of the two polymers can result in a steady increase in weld strength with penetration, even after the threshold penetration has been reached This results from an apparent steady state due to melting and flow of the lower melting polymer dominating the penetration in the early stages As the temperature increases, the higher melting polymer melts at a faster rate, reaching a later steady state Increasing penetration leads to greater strength; at high penetrations, weld strength equal to that of the weaker neat material can be obtained Due to this behavior, penetration

vs time curves alone cannot be used to determine optimum parameter conditions for welding dissimilar materials [481]

Mechanical properties of the weld obtained can be correlated with weld microstructure Although high pressures and amplitudes lead to shorter cycle times, impact strength and tensile strength decrease with increasing pressure Figure 3.5 shows polarization micrographs of a vibration welded polypropylene homopolymer at three pressures Low impact strength (1.1 kJ/m2; 0.52

ft•lb/in.2) at high pressure (8.0 MPa; 1160 psi) correlated with an area of weld thickness of about

opening into an area with a thickness of about 1

mm (0.04 in.) This increase in thickness results from melt flow changing from laminar to turbulent

at point K, producing a shear area This shear area

is not present in welds at lower pressures (2.0 MPa, 0.5 MPa; 290 psi, 72 psi) and higher impact strengths (1.8 kJ/m2 (0.8 ft•lb/in.2), 3.8 kJ/m2 (1.8

ft•lb/in.2), respectively) In high pressure welds (8.0 MPa; 1160 psi), the joining plane is adjacent

to an inner layer (I) whose microstructure cannot

be determined by microscopy It is separated from the bulk material (B) by a thin band of deformed spherulites (D) During vibration, shear is greatest

in this melt/solid transition area, and the ordered spherulitic structure is disturbed by flow Low pressure welds are wider and display three

Trang 27

microstructural zones : an inner zone with

microspherulites (I), a middle zone (recrystallized

zone) with spherulites of different sizes (R), and

the third layer of deformed spherulites (D) Only

thick welds with three layers have high impact and

tensile strength [480, 478]

In glass fiber-reinforced plastics, fiber

orientation during melt flow significantly affects

weld strength Low strength welds have a narrow

glass fibers align with the melt flow direction,

along the weld (Figure 3.6a) High strength glass

fiber reinforced welds have a large melt zone

thickness (200-300 µ m; 0.008-0.012 in.), which is

comparable to the fiber length This results in

increased fiber mobility, and fibers can move in

directions other than the melt flow direction With

a large melt zone thickness, fibers can align in the

tensile direction, perpendicular to the melt flow

direction, and some fibers can cross the weld plane

at the interface Figure 3.6b is an optical micrograph of a high strength weld of glass- reinforced nylon [615]

CROSS-THICKNESS WELDING

In linear vibration welding, the vibratory motion is in the longitudinal direction, along the weld seam (Figure 3.1) In cross-thickness vibration welding, used for welding closed seams

of box-like or tubular parts, the linear vibratory motion is along the thickness of the part (Figure 3.7) This motional direction exposes a portion of the entire welding seam to ambient air during each vibration, reducing the temperature Experiments using polycarbonate and PBT show that penetration vs time curves are similar to those for the normal mode of welding, except that for the same processing conditions (pressure, amplitude, frequency), the steady-state phase (III) begins at a higher penetration Also, at low pressures, phase

Figure 3.5 Polarization micrographs showing microstructures of three typical vibration welds of a polypropylene homopolymer Welding pressures are

a) 8.0 MPa (1160 psi) b) 2.0 MPa (290 psi) c) 0.5 MPa (72 psi) Weld amplitude is 0.7 mm (0.03 in.) In the upper micrographs, high pressure results

in an area of weld thickness of about 50 µm (0.002 in.) gradually increasing in thickness to 1 mm (0.04 in.); high pressure causes the melt flow to change from laminar in the boxed area to turbulent at point K At lower pressures, this area of greater weld thickness is not present In the lower micrographs, higher pressure welds (8.0 MPa, 2.0 MPa) are separated from the bulk material (B) by two zones: an inner layer (I) adjacent to the joining plane and a layer of deformed spherulites (D) Low pressure welds (0.5 MPa) are composed of three zones: an inner layer (I), a middle zone with spherulites of different sizes (R), and a third layer of deformed spherulites (D)

Trang 28

20

II exhibits some steady-state behavior The tensile

strength increases with increasing weld amplitude

and pressure up to the threshold penetration, and

polycarbonate welds could be obtained that were

equal in strength to neat polycarbonate

The intermittent cooling did affect the weld strength of semicrystalline PBT, possibly due to local solidification during each cycle, and weld strengths equal to neat PBT could not be obtained [489]

EQUIPMENT

Most equipment used in vibration welding produces linear vibratory motion Equipment is classified as low-frequency (120 to 135 Hz) or high-frequency (180 to 260 Hz) and can be variable-frequency or fixed frequency Variable- frequency equipment is electrically driven, and a dial adjustment is used to tune the frequency to match the part mass Fixed frequency equipment

is hydraulically driven, and a specific determined part mass is needed Machines operating at high frequencies need less motion and less clearance between parts Linear vibration welders can assemble parts with sizes from 6 in (152 mm) x 6 in (152 mm) to 48 in (1219 mm) long and 20 in (508 mm) wide, and multiple parts per cycle can be welded To optimize weld quality and decrease cycle times, pressure can be adjusted stepwise during a weld cycle

pre-Figure 3.7 Cross-thickness vibration welding Vibratory motion is

along the part thickness, in the x direction As in linear vibration

welding, process parameters are the weld amplitude, a, the weld

frequency, n, the weld pressure, po, and the weld time, t

Figure 3.6 Optical micrographs of welds of glass-reinforced plastics a) a low strength weld Fibers are aligned along the weld direction, perpendicular to that of the applied tensile stress b) a high strength weld of 14% glass fiber reinforced nylon 6 Some fibers are aligned along the tensile direction, perpendicular to the weld direction

Trang 29

Most vibration welding systems are

electrically driven and consist of three major

components: a vibrator assembly suspended on

springs, a power supply, and a hydraulic lifting

table The vibrator assembly is a moving element

with no bearing surfaces and consists of two

electromagnets, which provide alternating energy

to both ends of a system of springs The springs

resonate at the frequency of the electromagnetic

energy, support the vibrator assembly against

vertical welding pressures, and provide precise

alignment between parts to be welded by returning

the vibrator assembly to its home position at the

end of the welding cycle One of the parts to be

welded is attached to the vibrator assembly; the

other part is clamped onto the hydraulic table, the

stationary element For the welding cycle, the

hydraulic table, the lower tooling, rises up and applies pressure to the vibrating element, the upper tooling After solidification of the weld, the hydraulic table is lowered, and the welded part is unloaded A complete cycle normally takes 15-20 seconds For ease of handling and higher productivity, the parts should incorporate a pin or similar feature which would provide alignment when placed into the lower tooling and break away when vibration begins A schematic of a vibration welding machine is shown in Figure 3.8 [504]

Tooling used in vibration welding equipment

is simple, consisting of aluminum plates machined

to conform to the contour of the parts at the joint For complex part geometry, slides and actuators

Figure 3.8 Schematic of a vibration welding machine Energy at a particular frequency is supplied to the springs by electromagnets Parts are loaded into the lower tooling, which rises during a welding cycle and feeds the upper part to be welded into the vibrator assembly

Trang 30

22

can be incorporated using polyurethane poured

nests Multicavity tools are also possible; at least

one application in production can produce 20

components per cycle Tooling changes are rapid,

requiring 10-15 minutes [504]

The process can be easily automated due to

the modular design of the equipment The vibrator

assembly can be mounted into in line or rotary

systems with a remote power supply; currently, a

fully automatic, in line system with four vibrators

and cassette fed weld stations is welding nylon

emission control canisters in a total cycle time of 6

seconds Automated production lines can produce

1500 welded parts per 8 hour shift [504]

A broad range of vibration welding

equipment is available, with costs ranging from

$65,000 to $100,000 in US dollars, depending on

size and options Options available include

closed-meltdown distance control, statistical

process control, and pick and place, “over the

line,” and robot loading Equipment generally

includes rapid setup features, state-of-the-art

process control, safety features, and models with

increased capacity Top-of-the-line vibration

welders incorporate menu-driven industrial

computers which allow weld parameter data

collection, monitoring, and output for statistical

process control purposes and include new process

verification software for validating weld quality

on critical applications Programmable microprocessors can recall 10 sets of parameter listings, allowing for rapid tool changes Accurate table position monitoring and meltdown control during welding is accomplished using a linear displacement transducer (LDT), which eliminates the need for discrete table position limit switches and proximity sensors and improves weld quality and consistency Hydraulic, variable-speed lift and clamp systems ensure smooth, accurate positioning of the platen and make possible the use

of multiple weld and hold pressures Typical vibration welding equipment is shown in Figure 3.9 [494, 492, 493, 495]

ORBITAL VIBRATION WELDING

Orbital vibration welders are plastics assembly systems designed to weld large or irregularly-shaped thermoplastic parts, up to 10 in (254 mm) in diameter Electromagnetically-driven welding heads drive the moving platen, and the spring resonant drive has one moving element and

no wear surfaces The part clamped to the moving fixture is vibrated using circular motion (Figure 3.10) Operation at low weld amplitudes (<0.030 in., 0.76 mm for some applications) reduces part clearance requirements and enables welding of unsupported vertical walls, and low clamping pressure results in minimum part distortion Microprocessor-based equipment includes closed loop amplitude control, adjustable frequencies, ergonomically designed opto-touch sensors, and process verification software for validating weld quality Cost of a typical orbital vibration welder

is about $60,000 U.S dollars [492, 494, 461]

Advantages of vibration welding for joining large thermoplastic parts include relatively short cycle times, simple equipment, and insensitivity to surface preparation Because of the smearing action due to friction, welds can be obtained on surfaces that have been vacuum metallized, argent painted, or contaminated No additional materials are introduced, as in adhesive bonding, so the weld interface is composed of the same material as the

Figure 3.9 A typical vibration welding machine

Trang 31

parts that were welded Heating is very localized,

in contrast to hot tool welding, and material

degradation resulting from overheating at the

interface is much less likely The transmission

properties of the materials have no effect on

vibration welding, as they do in ultrasonic

welding [504, 491]

Almost any thermoplastic can be vibration

welded: crystalline, amorphous, filled, foamed,

and reinforced Parts can be welded regardless of

how they were processed (injection molded,

extruded, vacuum formed, etc.) Vibration welds

produce high strength, pressure tight, hermetic

seals; failure in burst testing of these seals

occurred outside the joint, leaving the weld intact

In transparent materials, the weld is optically clear

Vibration welding should be used only on

assemblies that do not have tight tolerances;

although the melting points of plastics are

predictable, the location at which the melt

solidifies may vary slightly [504, 552]

A fundamental problem in vibration welding

involves conversion of all the energy originating

from the vibratory movement into heat energy in

the joining zone Kinetic energy is converted into

heat energy by both internal friction and interfacial

friction The solid material friction in phase I can

cause high bending forces; proper clamping must

be used, and the thermoplastic must be rigid enough to avoid deformation This is particularly important in cross-thickness welding, in which the direction of vibration is at right angles to the component Cross-thickness welding is less favorable and less common than linear vibration welding; however, in practice it is encountered in virtually every part, since all parts to be welded possess walls both in the direction of vibration and

at right angles to it A stroboscopic photograph of

a pronounced deformation of a specimen plate is shown in Figure 3.11 In this case, no frictional movement takes place between the parts to be joined In the extreme case, in which there is excessive deflection in the parts to be joined, the plastic can start to melt directly at the clamping point, the point of maximum deflection; when this happens, a weld cannot be made [482]

Figure 3.10 Depiction of orbital welding One component is

clamped to the bottom, stationary fixture The other component,

attached to the top fixture, is vibrated using orbital motion.

Figure 3.11 Stroboscopic photograph of a specimen plate showing pronounced deformation

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24

When welding large parts that are difficult to

lock into supporting fixtures due to dimensional

variations or a lack of prominent features, relative

motion can occur between part and fixture,

resulting in part slippage and energy loss A

satisfactory weld cannot always be obtained at

high frequencies; in this case, use of lower

frequencies is necessary to attain high weld

strength [503]

A disadvantage of vibration welding is that it

is not suited to low-modulus thermoplastics, such

as some thermoplastic elastomers Linear

vibration welding is limited to nearly flat-seamed

parts or parts with small out-of-plane curvature

[491]

JOINT DESIGN

In designing parts for vibration welding,

sufficient clearance must be provided in the joint

to allow for vibratory motion between the parts,

and parts must be sufficiently rigid to support the

joint during welding Butt joints (Figure 3.12) and

tongue and groove joints (Figure 3.13) are

commonly used for vibration welding Unless

parts have thick walls, a flange is generally required in butt joints to provide rigidity and an adequate welding surface; the flange also facilitates gripping of the parts and applying uniform pressure close to the weld A U flange (Figure 3.14) may be necessary for thin or long unsupported walls; with a flange, walls as thin as 0.8 mm (0.03 in.) have been successfully welded

If weld flash is unacceptable, a flash trap can

be designed (Figure 3.15) The trap should be volumetrically sized to the amount of material displaced during welding Tongue and groove joints contain excess weld flash and are commonly used when flash is unacceptable Tongue and groove joints should have 2.54 mm (0.10 in.) wall and weld tongue thickness to ensure a weld strength equal to wall strength Weld material (0.5-0.8 mm; 0.02-0.03 in.) is added to the tongue during welding; tongue height when welded should approximately equal tongue width Flash is trapped between the tongue and groove wall Clearance for vibrational motion ( ± 0.8 mm, ±

Figure 3.12 Butt joint before and after welding

Figure 3.13 Tongue and groove joint, w/out and w/ skirted cover

Trang 33

0.03 in for high frequency; 1.5 mm, 0.06 in for

low frequency) must allow for flash volume A

gripping tab can be added to the back side of

flanges to securely hold and locate the part walls,

especially with crystalline materials with high

walls Tabs need not be continuous Weld flash

can be hidden using a skirted cover, allowing 1.27

mm (0.05 in.) clearance for vibrational motion and

tolerances [505, 504]

APPLICATIONS

Vibration welding is commonly used on large

parts, although smaller parts can be welded

economically in multiple cavity tooling Typical

part sizes range from 3 x 3 in (76.2 x 76.2 mm) to

24 x 60 in (61.0 x 152.4 cm) Vibration welding

is used when strong, leak-proof pressure or

vacuum joints are necessary

Welding is not limited to the periphery of the part Internal surfaces and dividers can be welded

to produce separate compartments, each hermetically sealed, as in batteries Although the welding joint is ideally in a single plane, parts in more than one plane can also be welded, such as polypropylene dishwater conduits [ 504]

Parts can be vibration welded to provide rigidity or eliminate sink marks Sink marks resulting from processes such as injection molding are caused by abrupt or large changes in thickness due to stiffening ribs, bosses, or other physical features These marks can be eliminated by molding these features separately, then vibration welding them in place A separate frame to provide added rigidity to large parts can be welded

to the outer surface of components such as car hoods or boots, minimizing panel thickness and weight Honeycombed structural components such

as estate car floors can be vibration welded, attaching a flat sheet to a molded honeycomb part [504]

Vibration welding is used extensively in the appliance industry for assembling washer and dishwasher pumps, particulate-filled soap

Figure 3.15 Flash trap design Left design is functional but does not hide weld flash; design on the right is cosmetic - weld flash is hidden

Figure 3.14 Butt joint with U-flange, before and after welding

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26

dispensers, and dishwasher spray arms One large

electrical appliance company converted a

production line of glass-filled polypropylene

dishwasher pump housings from hot plate welding

to vibration welding Operation was faster, easier,

more energy efficient, and easier to maintain, and

better control of strength and appearance was

achieved The vibration welding process was less

labor intensive With hot plate welding, two

welders were needed for three shifts, and a third

welder was needed half the time A separate

station was needed for leak testing, and venting

systems were necessary in order to eliminate the

gases that were generated Also, extra time was

necessary to allow for part cooling Total cycle

time was 30 seconds [491, 495, 504]

With vibration welding, only one welder was

needed A leak tester was attached to the frame of

the equipment, so that the operator could test a

part immediately after assembly, while the next

part was being welded Cooling time was not

required, since heat production was located only in

the joint area Warping in pump-housing valves

was minimized by clamping under pressure before

welding Cycle time, including testing, was 10

seconds The vibration welding process required

only 2622 kilowatt-hours of electricity per year,

compared to 38,000 kilowatt-hours for the hot

plate welding process, which uses power

constantly, even when not welding, to maintain

temperature Vibration welding requires power

only during the weld cycle Design of the part was

unchanged in shifting from hot plate to vibration

welding [504]

In the aircraft industry, the method of joining

polycarbonate air diffuser ducts was switched from

adhesive bonding to vibration welding Epoxy

bonding required sandblast preparation,

prebonding of parts, expensive fixturing devices,

and a 24 hour cure time With vibration welding,

three parts per minute are produced, and labor cost

was cut by 70% [504]

Automotive applications include the welding

of headlight, taillight, and instrument panel

assemblies using welds of polycarbonate to itself

and to acrylic, acetal gasoline reservoirs, 30%

glass-filled nylon brake fluid reservoirs,

polypropylene compartment access doors welded

in two planes, dash and trim components, air conditioning and heater ducts, vacuum reservoirs, fuel filler doors, and air flow sensors Vibration welded nylon 66 emission control canisters are less expensive to produce and weigh 0.7 kg less than metal canisters The nylon parts do not have

to be dry, and a seal is obtained even when carbon dust or oil is present on the weld surfaces prior to welding Vibration welding can be used in welding nylons in under hood applications, such as inlet manifolds and throttle housings, which must withstand high temperatures Lower costs and up

to 50% savings in weight can be achieved It has been estimated that by the year 2010, 21.4 million air intake manifolds will be produced from plastics joined by vibration welding [491,495, 504, 615]

Vibration welding was used in assembling the first all-plastic automotive bumper that could withstand a 5 miles per hour impact A two-piece thermoplastic bumper was developed in the late 1970s in order to achieve more flexibility in styling, weight reduction, reduced manufacturing costs, and better aerodynamics than conventional bumpers Thermoplastic bumpers consist of a long curved U section, commonly referred to as a front beam or facia, and a rear reinforcement, referred to

as a back beam Other techniques were evaluated for joining the two sections Hot plate welding caused a time-aggravated embrittlement problem at the joint interface, while induction bonding was not cost effective Adhesive bonding was not used due to thermal cycling and low temperature impact limitations and added weight, and holes required for mechanical fasteners would raise the stress levels Initial problems with vibration welding included the large part area (three times as large as any other previously welded part, with a four times larger joining area) and the necessity of applying clamping force in the horizontal rather than the traditional vertical direction, due to the attachment

of the back beam on the inside of the U channel legs A large part vibration welder using an electromagnetic drive, low (120 Hz) frequencies, and new fixturing for horizontal clamping forces was developed for this specific application [503, 504]

Other applications of vibration welding include joining two halves of pressure vessels,

Trang 35

reservoirs, valves, electronic modules, and sealed

containers in the medical, computer, recreation,

and toy industries It has also been used for

welding chain-saw motor housings made of 30%

glass-filled nylon, butane gas lighter tanks,

batteries, and pneumatic logic boards [491,495,

504]

Trang 36

Chapter 4

Spin Welding

PROCESS

Spin welding is a frictional process in which

thermoplastic parts with rotationally symmetrical

joining surfaces are rubbed together under

pressure using unidirectional circular motion The

heat generated melts the plastic in the joining

zone, forming a weld upon cooling It is a fast,

reliable process that requires only minimal, basic

equipment but that can be completely automated

It is generally used for small cylindrical or

spherical components

In spin welding, one of the parts is clamped,

remaining stationary throughout the cycle, while

the other is spun After the rotational speed

stabilizes, pressure is applied axially,

perpendicular to the joining surfaces (Figure 4.1)

The process consists of five phases In phase I,

external, coulombic friction occurs between the

two solid surfaces, generating heat In phase II,

friction results in abrasive forces which strip off

surface roughness and generate wear particles

When the crystalline melting point or glass

transition temperature is reached, the plastic parts

begin to melt This occurs in phase Coulombic III, an

unsteady melting state in which a dramatic rise in temperature leads to a large increase in melt thickness When melting begins, heat is generated through internal friction within the molten material As the melt thickness increases under pressure, part of the molten material is squeezed out of the bond area into a flash bead around the interface

In phase IV, a steady state friction state is reached in which generated heat is in equilibrium with heat eliminated through bond formation along the axial direction and flash generation around the interface In this phase, the weld penetration, the distance the two molten materials approach each other, increases linearly with time, and the temperature and thickness of the molten layer remain constant Weld formation in this phase results in an axial decrease in length of the parts being joined In phase V, rotation is stopped, either abruptly or by continuous ramping, and the molten material is allowed to cool under pressure Pressure is sometimes increased at this point in order to distribute the melt and squeeze out excess material In this phase, the penetration remains constant, and the temperature decreases as the weld solidifies Figure 4.2 is a representative penetration vs time curve showing the five phases

of the spin welding process

In some descriptions of the spin welding process, phases I and II are combined into a single phase including both external friction and abrasive wear In these descriptions, phases II, III, and IV are identical to phases III, IV, and V of the five phase process [497,496]

The main process parameters for spin welding are the speed of rotation, weld or axial pressure, and weld time Parameters used depend on the material and diameter of the joint In commercial machines, rotational speeds can range from 200 to

Figure 4.1 Welding Spin One part is fixed, while the other is

rotated, using unidirectional rotational motion, ω Pressure, p, is

applied axially, along the x direction r1 and r2 are the inside and

outside radii of the tubes being joined

Trang 37

14,000 revolutions per minute (RPM) Weld times

can range from tenths of a second to 20 seconds;

typical values are about 2.0 seconds, with an actual

spin time of 0.5 seconds and a cooling time, when

pressure is applied but spinning is stopped, of 0.2

to 0.5 seconds Axial pressures range from about

1.0 to 7.0 MPa (145 to 1015 psi) Pressure must

be high enough to force any contaminants or

bubbles out of the joint, and the combination of

speed and pressure must be controlled but high

enough to cause melting at the interface as

opposed to grinding A prototype evaluation is

usually done to determine optimum rotation speed,

pressure, and weld time for the particular

application [78, 496, 500, 498]

In computer-controlled machines, sensors

continuously monitor the three parameters

-rotational speed, axial pressure, and weld time - in addition to the weld penetration, weld or penetration velocity, and interfacial torque For reproducible results and consistent weld strength, process monitoring is recommended Figure 4.3 is

a sensor output trace obtained during welding of polyvinylidene fluoride (PVDF) Axial pressure and rotational speed were set at 4.3 MPa (624 psi) and 3500 RPM, respectively

In the welding cycle, rotational motion is initiated first When the rotational speed reaches its set value, axial pressure is applied, and weld penetration, velocity, and torque can be monitored Melting begins at about 1.2 seconds after the rotational motion commences The rotational speed decreases moderately at this point due to the presence of the viscous melt at the joint interface, then remains constant until termination at 4.4 seconds At the constant rotational speed, axial pressure and torque increase to 4.6 MPa (667 psi) and 55 kN•m (40,560 ft•lbf), respectively, then slightly decrease over the remaining weld time Weld penetration velocity reaches a steady state at about 2 seconds The weld penetration curve is similar to the penetration vs time curve of figure 4.2 and exhibits the five phases of the spin welding process [498]

Changes in welding parameters affect the weld penetration and weld strength Increasing axial pressure at constant rotational speed and weld time increases the penetration, the penetration velocity, and the torque and decreases

Figure 4.2 Penetration vs time curve showing the five phases

of spin welding friction in phase I strips off surface roughness in

phase II, generating wear particles Penetration begins in phase III,

reaches a steady state in phase IV, and ends in phase V, after

rotational motion has ended and the weld has solidified The

penetration velocity, shown by the dotted line, increases

dramatically in phase III, reaches a constant value during steady

state melting, then drops off abruptly in phase V as penetration

ends

Figure 4.3 Sensor output data from computer-controlled monitoring of the spin welding of PVDF When rotational speed reaches its set value, axial pressure is engaged, and weld penetration begins Interfacial torque, penetration velocity, and weld penetration are monitored throughout the spin welding cycle

Trang 38

31

the weld initiation time (also called weld induction

time), the time before any melting occurs (phases I

and II) These effects are due to increased heating

and melting rates at higher pressures Noise levels

in penetration and torque output data curves are

also reduced Mass reduction, a measure of the

amount of wear particles and weld flash fragments

dislodged during the welding process, is not

affected significantly by increased pressure,

especially at higher rotational speeds [498, 496,

499]

Effects of increasing rotational speed are

similar to those of increased pressure: increased

weld penetration and penetration velocity and

decreased weld initiation time The weld initiation

time is decreased only moderately at higher

rotational speeds (1.0 second at 2000 RPM to 0.75

seconds at 3500 RPM in polyvinylidene fluoride at

4.3 MPa (624 psi) axial pressure), in contrast to

the greater effects of axial pressure (1.25 seconds

at 2.0 MPa (290 psi) to about 0.10 seconds at 6.8

MPa (986 psi) in polyvinylidene fluoride at 3500

RPM) Mass reduction shows an almost

exponential increase with increases in rotational

speed [498]

An increase in welding time increases the

amount of time axial pressure and rotational

motion are maintained and results in an almost

linear increase in weld penetration [498]

Stopping rotation at the beginning of phase V with

an abrupt stop rather than continuous braking

results in a higher weld quality in some materials

[497]

Breaking force generally increases initially

with increasing axial pressure or rotational speed,

then reaches a maximum before decreasing The

optimum strength at the peak is dependent on the

combination of rotational speed and axial pressure

used and occurs at intermediate parameter values

At high rotational speeds, strength decreases

rapidly, possibly due to a high degree of chain

orientation aligned in the weld plane in the hoop

direction, with weak interchain bonds in the axial

direction At low axial pressure, low rotation

speeds, and low weld times, bonding does not

always occur

MATERIALS

Properties of the materials to be joined affect welding behavior Thermotropic liquid crystalline polymers exhibit low coefficients of friction in the solid state, high melting temperatures, and low melt viscosities This results in longer weld initiation times at lower pressures and a non- steady state penetration velocity; higher pressures decrease weld initiation times and result in near- steady state penetration velocities Higher rotational speeds also decrease weld initiation times, but to a lesser extent than axial pressure

Weld strength as determined by breaking force exhibits a slow, steady increase as rotational speed is increased at low (4.3 MPa; 624 psi) pressures At higher pressures, a rapid rise in strength is followed by a sharp decline, resulting in peak strength at intermediate rotational speeds The influence of rotational speed on weld strength follows the same pattern: a slow, almost linear increase in strength at low pressures and a rapid rise at higher pressures, peaking at intermediate rotational speeds Weld strength at low pressures and rotational speeds is generally lower than in other materials [499]

Addition of glass fibers to thermotropic liquid crystalline polymers increases the penetration and decreases weld initiation time; the temperature at the interface increases at a faster rate due to the lower heat capacity of glass fibers Weld strength increases significantly with the addition of glass fibers [500]

Glass fibers added to poly p-phenylene sulfide (PPPS) significantly increase the weld strength, particularly at high axial pressures, and weld strength of filled materials is not significantly affected by changes in axial pressure The level and rate of penetration decreases with increasing filler concentration due to the increase in melt viscosity with the glass fibers present; interfacial torque is not affected by fiber addition [496]

The most efficient welds can be achieved by welding the same materials Conditions must be adjusted when welding dissimilar materials A longer spin dwell and post-spin and set-pressure cooling cycle may be necessary [ 501]

Trang 39

In spin welding, heat generated at the surfaces

to be welded is localized Because thermoplastics

are poor conductors of heat, surface heat is only

slowly transferred to the interior of the part; if heat

is generated fast enough, interior alteration can be

avoided The highly localized region of thermal

influence is the heat affected zone (HAZ) Its

structure is the primary determinant of the

mechanical performance of the weld The

rotational motion of spin welding creates a

velocity differential along the radii of the weld

specimens, producing a temperature differential

across the weld interface which influences

crystallization and fiber orientation [ 498]

Two x-ray microraudiographs of the heat

affected zone of poly-p-phenylene sulfide

containing 10% glass filler at two different axial

pressures are shown in Figure 4.4 The thickness

of the HAZ decreases with increasing axial

pressure due to the molten material being squeezed

out through the edges as rapidly as it is produced There is also increased orientation of fibers parallel to the weld line at higher pressures, with weak interchain bonds in the axial direction Both these effects result in decreased weld strength Increased rotation speeds in 10% glass filled thermotropic liquid crystalline polymers results in increased HAZ thickness and a tendency for the material to flow inward in opposition to the centrifugal force generated by spinning (the Weissenberg effect) Thickness of the HAZ varies from the center to the edge of the weld; the area of greater thickness depends on the material [496, 500]

VARIANTS OF SPIN WELDING

The spin welding process can be varied to accommodate requirements of the parts to be welded Typically, spin welding uses a random spin-stop cycle, in which the orientation of the two parts is random after spinning is stopped When the two parts must be oriented upon completion of the weld cycle, a stepping motor can be used to provide a fixed spin-stop cycle Stepping motor tolerances are generally ± 2 degrees of the part diameter [ 501]

In continuous-drive welding, the speed of the rotating part being joined is controlled Other variations include the “follow” method, in which the firmly clamped part is released once the plastic has melted, and gyrating mass welding, in which a rotating mass coupled to one of the parts to be joined is accelerated to a suitable speed Axial pressure then brings the parts into contact, and the kinetic energy is converted into the heat of friction In another variation, the stationary part can be made to rotate with the moving part when spinning is disengaged, in order to counteract the inertial forces incurred in stopping the rotating part [ 497, 502]

Figure 4.4 X-ray microraudiographs of the heat affected zone

(HAZ) of a weld of poly-p-phenylene sulfide containing 10% glass

fillers at low (1.47 MPa; 213 psi) and high (6.77 MPa; 982 psi)

axial pressures Thickness of the HAZ decreases with increasing

axial pressure, and higher pressure results in increased fiber

orientation parallel to the weld line and a decrease in weld

strength

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33

quality and can be automated Small to

intermediate bench models that incorporate a

variable speed drive for applications up to 152 mm

(6 in.) in diameter are commercially available, and

units can be custom-built, with powerful torque

motors, for large-diameter parts Spin welders are

rigid machines and can be fitted with rotary tables

for integration into high-speed production lines

Cost ranges from $10,000 to $50,000 U.S dollars

A typical spin welder is shown in Figure 4.5

[495, 497, 493]

Spin welding is a simple, fast and efficient

joining method for circular or cylindrical

thermoplastic parts A strong hermetic seal is

obtained which is frequently stronger than the

material substrate itself Large diameter (10 to 24

in.; 254 to 610 mm) parts can be welded using low

to medium rotational speed and high torque, and

parts as small as 12.7 mm (0.5 in.) can be welded

using medium to high rotational speeds and low

torque conditions No foreign materials are

introduced into the weld, and no environmental

considerations are necessary, as in solvent and

adhesive bonding

The main disadvantage is that it can be used

mainly for circular cross-sectioned parts that do

not require angular alignment; however,

equipment can be modified to accommodate part

alignment

[ 477, 496, 495]

JOINT DESIGN

Joint areas of the parts to be joined must be

circular, and a shallow matching groove (tongue

and groove) is desirable in order to index the two

parts and provide a uniform bearing surface;

however, due to minimal wall thickness, a burn

mark or ring from the heat generated may cause

discoloration in the grooved part To avoid or hide

this discoloration, a skirt may be used on one of

the parts Joint design can be used to direct weld

flash A typical male/female step joint, with the

female step 30 to 50% shorter in length than the

male step, will generate flash only in the

inside

Figure 4.5 A typical spin welder

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