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Handbook of

Plastics Joining A Practical Guide

Plastics Design Library is a trademark of William Andrew, Inc.  Copyright 1997. All rights reserved. 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. Information in this document is subject to change without notice and does not represent a commitment on the part of Plastics Design Library. No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, for any purpose without the written permission of Plastics Design Library. Comments, criticisms and suggestions are invited, and should be forwarded to Plastics Design Library Plastics Design Library and its logo are trademarks of William Andrew, Inc. Please Note: Although the information contained in this volume has been obtained from sources believed to be reliable, no warranty (expressed or implied) can be made as to its completeness or accuracy. Design, processing methods and equipment, environment and other variables affect actual part and mechanical performance. Inasmuch as the manufacturers, suppliers, and Plastics Design Library have no control over those variables or the use to which others may put the material and, therefore, cannot assume responsibility for loss or damages suffered through reliance on any information contained in this volume. No warranty is given or implied as to application and to whether there is an infringement of patents is the sole 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.

Plastics Design Library, 13 Eaton Avenue, Norwich, New York 13815 Tel: (607) 337-5000 Fax: (607) 337-5090

Table of Contents 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 Processing Parameters .........................................................................................................................................10 Materials ................................................................................................................................................................10 Joint Design ...........................................................................................................................................................11 Equipment..............................................................................................................................................................11 Advantages and Disadvantages ............................................................................................................................13 Applications ...........................................................................................................................................................13

Vibration Welding - Chapter 3 Process..................................................................................................................................................................15 Processing Parameters .........................................................................................................................................16 Materials ................................................................................................................................................................17 Weld Microstructure...............................................................................................................................................18 Cross-Thickness Welding ......................................................................................................................................19 Equipment..............................................................................................................................................................20 Orbital Vibration Welding .......................................................................................................................................22 Advantages and Disadvantages ............................................................................................................................22 Joint Design ...........................................................................................................................................................24 Applications ...........................................................................................................................................................25

Spin Welding - Chapter 4 Process..................................................................................................................................................................29 Processing Parameters .........................................................................................................................................29 Materials ................................................................................................................................................................31 Weld Microstructure...............................................................................................................................................32 Variants of Spin Welding .......................................................................................................................................32 Equipment..............................................................................................................................................................32 Advantages and Disadvantages ............................................................................................................................33 Joint Design ...........................................................................................................................................................33 Applications ...........................................................................................................................................................34

Ultrasonic Welding - Chapter 5 Process..................................................................................................................................................................35 Processing Parameters .........................................................................................................................................37 Ultrasonic Weldability of Materials.........................................................................................................................40 Joint Design ...........................................................................................................................................................45 Ultrasonic Equipment.............................................................................................................................................53 Advantages and Disadvantages ............................................................................................................................55 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 Advantages and Disadvantages ............................................................................................................................72 Applications ...........................................................................................................................................................72

Radio Frequency Welding - Chapter 7 Process ................................................................................................................................................................75 Heat Generation ....................................................................................................................................................75 Equipment..............................................................................................................................................................76 Materials ................................................................................................................................................................77 Advantages and Disadvantages ............................................................................................................................78 Applications ...........................................................................................................................................................78

Microwave Welding - Chapter 8 Process ................................................................................................................................................................79 Processing Parameters .........................................................................................................................................80 Materials ................................................................................................................................................................81 Equipment..............................................................................................................................................................82 Advantages and Disadvantages ............................................................................................................................82

Resistance Welding - Chapter 9 Process ................................................................................................................................................................83 Processing Parameters .........................................................................................................................................84 Materials ................................................................................................................................................................85 Joint Design ...........................................................................................................................................................85 Equipment..............................................................................................................................................................86 Advantages and Disadvantages ............................................................................................................................86 Applications ...........................................................................................................................................................86

Extrusion Welding - Chapter 10 Process ................................................................................................................................................................87 Processing Parameters .........................................................................................................................................87 Weld Microstructure...............................................................................................................................................88 Equipment..............................................................................................................................................................89 Advantages and Disadvantages ............................................................................................................................89 Applications ...........................................................................................................................................................89

Electrofusion Welding - Chapter 11 Process ................................................................................................................................................................91 Processing Parameters .........................................................................................................................................92 Materials ................................................................................................................................................................93 Equipment..............................................................................................................................................................93 Advantages and Disadvantages ............................................................................................................................94 Applications ...........................................................................................................................................................94

Infrared Welding - Chapter 12 Process ................................................................................................................................................................95 Processing Parameters .........................................................................................................................................96 Materials ................................................................................................................................................................97 Microstructure ........................................................................................................................................................98 Variants of Infrared Welding ..................................................................................................................................98 Equipment..............................................................................................................................................................99 Advantages and Disadvantages ............................................................................................................................99 Applications .........................................................................................................................................................100

Laser Welding - Chapter 13 Process ..............................................................................................................................................................101 Processing Parameters .......................................................................................................................................102 Materials ..............................................................................................................................................................103 Weld Microstructure.............................................................................................................................................103 Joint Design .........................................................................................................................................................103 Equipment............................................................................................................................................................104 Advantages and Disadvantages ..........................................................................................................................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

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 Advantages and Disadvantages ..........................................................................................................................160 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 Nylon 6 - Chapter 30..........................................................................................................................................213 Nylon 612 - Chapter 31......................................................................................................................................217 Nylon 66 - Chapter 32........................................................................................................................................219 Polyarylamide - Chapter 33 ...............................................................................................................................223 Polyphthalamide (PPA) - Chapter 34.................................................................................................................225

Polycarbonate Polycarbonate (PC) - Chapter 35 ......................................................................................................................231

Polyester Thermoplastic Polyester - Chapter 36 ...............................................................................................................265 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

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

Rubbers

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

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.

Heated Tool Welding © Plastics Design Library

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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.

© Plastics Design Library Heated Tool Welding

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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 oF) 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. At lower temperatures (200oC, 392 oF) weld 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 heating temperatures increased from 232oC (450 oF) to 246oC (475 oF ) at heating times of 10 seconds; however, at 20 second heating times, temperature increases from 204.5oC to 218.5oC

Heated Tool Welding © Plastics Design Library

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(400.1oF to 425.3 oF) 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

© Plastics Design Library Heated Tool Welding

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zones and greater bond strength; however, too high a temperature or pressure results in void formation at the joint interface. [513]

Morphological investigation of 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 immersion for 7 days in 80oC (176oF) water,

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 plate temperatures (480oC, 896oF) produced 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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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.

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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.

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.

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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 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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Figure 2.3 Weld quality analysis by weld appearance.

Figure 2.4 Joint designs commonly used in hot gas welding.

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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 bumpers, a 60º V was used; a double V butt weld 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.).

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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.

ADVANTAGES AND DISADVANTAGES 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]

Chapter 3

Vibration Welding

© Plastics Design Library 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.

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penetration vs. time curve showing the four phases is given in Figure 3.2. PROCESSING PARAMETERS

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.

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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 pre- drying. [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 (428 oF) 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.

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