weldingg of plastics ASM handbook volume 06

welding of plastics

DOUGLAS AIRCRAFT COMPANY, NASA CR-2218, JAN 1973, REPRINTED AUG 1974

2 L.J HART-SMITH, DESIGN AND ANALYSIS OF ADHESIVE-BONDED JOINTS, PROC 1ST AIR FORCE CONF FIBROUS COMPOSITES IN FLIGHT VEHICLE DESIGN, AFFDL-TR-72-130, AIR

FORCE FLIGHT DYNAMICS LABORATORY, 1972, P 813-856

3 L.J HART-SMITH, ADVANCES IN THE ANALYSIS AND DESIGN OF ADHESIVE-BONDED JOINTS

IN COMPOSITE AEROSPACE STRUCTURES, 14TH NATIONAL SAMPE SYMP AND EXHIBITION,

SOCIETY FOR THE ADVANCEMENT OF MATERIAL AND PROCESS ENGINEERING, APRIL 1974,

P 722-737

4 L.J HART-SMITH, BONDED-BOLTED COMPOSITE JOINTS, J AIRCRAFT, VOL 22, 1985, P 993-1000

5 L.J HART-SMITH, ADHESIVELY BONDED JOINTS FOR FIBROUS COMPOSITE STRUCTURES,

JOINING FIBRE-REINFORCED PLASTICS, F.L MATTHEWS, ED., ELSEVIER, 1987, P 271-311

6 H.J KIM AND R.E BOHLMANN, "THERMAL SHOCK TESTING OF WET THERMOPLASTIC

LAMINATES," 37TH INT SAMPE SYMP EXHIBITION, MARCH 1992

7 D.L BUCHANAN AND S.P GARBO, "DESIGN OF HIGHLY LOADED COMPOSITE JOINTS AND

DEVELOPMENT CENTER, AUGUST 1981

8 E.T CAMPONESCHI, R.E BOHLMANN, J HALL, AND T.T CARR, "EFFECT OF ASSEMBLY ANOMALIES ON THE STRAIN RESPONSE OF COMPOSITES IN THE SPHERE JOINT REGION OF THE DARPA MAN RATED DEMONSTRATION ARTICLE," CDNSWC-SME-92/22, DEFENSE ADVANCED RESEARCH PROJECT AGENCY, ARLINGTON, VA, 4 MARCH 1992

Thermoplastic resins, on the other hand, can be softened, as a result of the weakening of secondary van der Waals orhydrogen bonding forces between adjacent polymer chains Therefore, thermoplastics can be remolded by the application

of heat, and they can be fusion welded successfully

Thermoplastics can be broadly divided into amorphous and crystalline resins, based on morphology, or structure Thesematerials are more attractive than thermosets because they:

• CAN BE JOINED USING FUSION WELDING

Table 1 lists the major thermoplastic resins Filled thermoplastics are being increasingly used in semistructuralapplications Fillers can reduce material cost, enhance mechanical properties, improve thermal properties, provide flameretardation, and so on.

TABLE 1 MAJOR CATEGORIES OF THERMOPLASTICS AND COMPOSITES

REINFORCED AND ADVANCED THERMOPLASTIC COMPOSITES

THERMOPLASTIC RESINS FILLED WITH SHORT, LONG, OR CONTINUOUS FIBERS OF GLASS, CARBON, OR ARAMID

Joining is generally the final step in any fabrication cycle Some important review articles are represented by Ref 1, 2, 3,

4, 5, and 6 The effectiveness of the joining operation can have a large influence on the application of any polymer orcomposite material A variety of polymer joining techniques are available Figure 1 provides a classification of thesedifferent methods (Ref 4)

FIG 1 CLASSIFICATION OF DIFFERENT JOINING METHODS SOURCE: REF 4

Polymers are low-energy substrates, with surface free energies of less than approximately 50 mJ/m2 (0.003 ft · lbf/ft2)(Ref 7) The creation of a successful joint depends on four factors: the chemical nature of the polymer, the surface freeenergy, the surface topography, and contamination of the polymer surface by dust, oil, and grease These factors markedlyaffect the effectiveness of the adhesive and solvent bonding methods Fusion welding, however, is much more tolerant ofaspects such as surface contamination and material variations from sample to sample

Adhesives are thermoset-type polymers that are classified as structural, nonstructural, or elastomeric Reference 7provides additional details Structural adhesives are generally used in load-bearing applications, in joints that have highstrength-to-weight ratios They are also used to improve resistance to both component fatigue and corrosion resistance.The principal disadvantages of adhesive bonding are that surface preparation is required prior to bonding, curing timescan be long, the components cannot be disassembled following the joining operation, and health and/or safety hazardsmay be involved in their use In spite of these problems, adhesive bonding is used extensively in numerous industries.Mechanical fastening and adhesive bonding can be used to join both similar and dissimilar materials For example,mechanical fastening is commonly used when joining a plastic to a metal (Ref 8), producing either permanent joints orconnections that can be opened and sealed again The advantages of this approach are that no surface treatment is requiredand disassembly of the components for inspection and repair is straightforward The main limitations of this approach areincreased weight, the presence of large stress concentrations around the fastener holes, and subsequent in-servicecorrosion problems (Ref 8) The typical applications of mechanical fastening are in the aerospace, automotive, andconstruction industry

Polymeric materials that possess similar solubility parameters can be joined using solvent or fusion welding.Interdiffusion of polymer chains plays a major role in achieving intrinsic adhesion (Ref 9) and in promoting chaindiffusion, either by applying a suitable solvent or by heating the polymer sample

As mentioned previously, only thermoplastics can be joined using the fusion-welding process The glass transition

temperature, Tg, in amorphous polymers, and the melting temperature, Tm, in crystalline polymers must be exceeded sothat the polymer chains can acquire sufficient mobility to interdiffuse A variety of methods exist for weldingthermoplastics and thermoplastic composites (Fig 2) Thermal energy can be delivered externally via conduction,convection, and/or radiation methods, or internally via molecular friction caused by mechanical motion at the jointinterface In the case of external heating, the heat source is removed prior to the application of pressure, and longer

welding times are balanced by the greater tolerance to variations in material characteristics Internal heating methodsdepend markedly on the material properties (Ref 10) Heating and pressure are applied simultaneously, and shorterwelding times are generally involved during the joining process.

FIG 2 CLASSIFICATION OF DIFFERENT WELDING METHODS FOR THERMOPLASTICS

Welding is accomplished in many stages During the initial stage, the polymer-chain molecules become mobile andsurface rearrangement occurs This is followed by wetting and the diffusion of polymer chains across the interface Thefinal stage involves cooling and solidification For linear random-coil chains, the mechanical energy required to separate

the welded substrates, G, is given by the relation (Ref 11):

References

1 G GEHARDSSON, THE WELDING OF PLASTICS, WELD REV., FEB 1983, P 17-22

2 M.N WATSON, R.M RIVETT, AND K.I JOHNSON, "PLASTICS AN INDUSTRIAL AND LITERATURE SURVEY OF JOINING TECHNIQUES," REPORT NO 7846.01/85/471.3, THE WELDING INSTITUTE, ABINGTON, UK, 1986

3 H POTENTE AND P MICHEL, THE STATE OF THE ART DEVELOPMENT TRENDS IN THE

WELDING OF PLASTICS, PROC 5TH ANNUAL NORTH AMERICAN WELDING RESEARCH CONFERENCE, EWI AND AWS, 1989

4 V.K STOKES, JOINING METHODS FOR PLASTICS AND PLASTIC COMPOSITES: AN

OVERVIEW, POLYM ENG SCI., VOL 29 (NO 19), 1989, P 1310-1324

5 R.A GRIMM, FUSION WELDING TECHNIQUES FOR PLASTICS, WELD J., MARCH 1990, P 23-28

6 G MENGES, THE JOINING OF PLASTICS AND THEIR COMPOSITES, PROC LNT CONF ADVANCES IN JOINING NEWER STRUCTURAL MATERIALS, IIW (MONTREAL), P 33-63

7 A.J KINLOCH, ADHESION AND ADHESIVES, CHAPMAN AND HALL, 1987, P 101

8 D CHANT, JOINING TECHNOLOGY FOR THERMOPLASTIC COMPOSITE STRUCTURES IN

AEROSPACE APPLICATIONS, PROC INT CONF ADVANCES IN JOINING PLASTICS AND COMPOSITES, THE WELDING INSTITUTE, CAMBRIDGE, 1991

9 S.S VOYUTSKII, AUTOHESION AND ADHESION OF HIGH POLYMERS, WILEY INTERSCIENCE, 1963

10 A BENATAR, MATERIAL CHARACTERISTICS FOR WELDING, PROC 5TH ANNUAL NORTH AMERICAN WELDING RESEARCH CONFERENCE, EWI AND AWS, 1989

11 R.P WOOL, B.-L YUAN, AND O.J MCGAREL, POLYM ENG SCI., VOL 29 (NO 19), 1989, P

The surfaces of the heated tool are usually coated with polytetrafluoroethylene (PTFE) to prevent the polymers fromsticking to the platen However, the PTFE coating generally restricts the maximum tool surface temperature to 260 °C(500 °F) Some research on heated-tool welding has involved hot-plate temperatures in excess of 350 °C (660 °F) for veryshort heating times (4 to 6 s) This thermal cycle has been made possible through the use of high-temperature aluminum-bronze heating elements (Ref 1)

The hot-tool welding process can be described by four phases (Fig 3) During phase I, the surfaces of the material arebrought in contact with the heated tool and are held under pressure This pressure is maintained until a molten filmappears In phase II, the contact pressure between the heated tool and the substrate is reduced to increase the molten-filmthickness The rate of increase of melted-film thickness depends on the thermal conductivity of the polymer Phase III isthe change-over (removal of the hot tool) time, whereas phase IV is the joining and cooling under pressure The amount

of melted-polymer displacement from the weld zone is controllable For example, computer-controlled machines willallow preselection of the pressure or displacement values that are applied during joining (Ref 15) In this connection,detailed mathematical analyses of the hot-tool welding process have already been carried out (Ref 15, 17)

FIG 3 PRESSURE-TIME AND DISPLACEMENT-TIME GRAPH SHOWING DIFFERENT PHASES OF HOT-TOOL

WELDING PROCESS PA, PE, AND PF ARE MATCHING, HEATING, AND JOINING PRESSURES, RESPECTIVELY TE ,

TU, TF, AND TK REPRESENT HEATING, CHANGE-OVER, JOINING, AND COOLING TIMES, RESPECTIVELY SF ,

REDUCTION IN LENGTH OF THE PART BEING JOINED; SA, DISPLACEMENT PATH PRODUCED DURING TIME TE SOURCE: REF 16

The key joining parameters of hot-tool welding are:

WELDED

• THE PRESSURE APPLIED AND THE DURATION OF PHASE I (HOWEVER, HIGH PRESSURE DURING PHASE IV WILL PRODUCE TOO MUCH LATERAL FLOW AND POLYMER-CHAIN ORIENTATION IN THE COMPLETED JOINT, RESULTING IN ADVERSE MECHANICAL

PROPERTY EFFECTS, AS DESCRIBED IN REF 15 AND 16)

AND WALL THICKNESS OF THE PART BEING JOINED, WHICH ARE THE PARAMETERS THAT DETERMINE THE DURATION OF PHASE II

• THE TOOL TRANSFER, OR CHANGE-OVER, TIME, WHICH MUST BE MINIMIZED SO THAT COOLING OF THE MOLTEN LAYER DOES NOT OCCUR (TO AVOID INHIBITING POLYMER- CHAIN INTERDIFFUSION)

Applications.All thermoplastic materials can be joined using hot-tool welding Components that have large, flat surfaceareas are commonly butt welded using this technique It is also possible to join dissimilar materials through the use of twoheated platens that are at different temperatures (Ref 14) The weldability factor, in this case, is the degree ofcompatibility

Hot-tool welding can be readily automated Portable equipment is primarily used for on-site weld repairs Small amounts

of joint misalignment prior to joining have negligible effects on the weld quality Hot-tool welding is used in a variety ofindustrial applications The automotive sector, for example, uses it to weld polypropylene (PP) copolymer cases forbatteries and to weld rear-light casings of acrylonitrile-butadiene-styrene (ABS) joints to polymethyl methacrylate(PMMA) or polycarbonate (PC) lenses Hot-tool welding is also employed when welding thermoplastic tanks and whenjoining large-diameter polyethylene (PE) pipeline to transport gas, water, and sewage wastes

Hot-Gas Welding

A stream of hot air or gas (nitrogen, air, carbon dioxide, hydrogen, or oxygen) is directed toward the filler and the jointarea using a torch (Ref 18) A filler rod or tape (of a similar composition to the polymer being joined) is gently pushedinto the gap between the substrates (Fig 4) A variety of nozzles are available for different applications, and eitherautomated or manual welding can be carried out During welding, the gas temperature can range from 200 to 600 °C (390

to 1110 °F), depending on the polymer being joined

FIG 4 SCHEMATIC OF HOT-GAS WELDING, SHOWING THE CORRECT POSITION OF TORCH AND FILLER ROD

FOR DIFFERENT THERMOPLASTICS SOURCE: REF 19

The key joining parameters of the hot-gas welding process are:

• GAS TEMPERATURE, WHICH DEPENDS ON THE TYPE OF POLYMER BEING JOINED, AND WHICH DETERMINES THE HEATING ELEMENT, NOZZLE DIMENSIONS, AND GAS/AIR FLOW RATES THAT ARE USED

JOINING OPERATIONS)

Applications The principal application areas involve the continuous welding of polyolefin tanks and containers, thewelding of polyvinyl chloride (PVC), ABS, PE, and PP pipe sections, the sealing of packaging materials, and the fieldrepair of PVC and other thermoplastic resins that are used in the construction and automotive industries Hot-gas weldinghas a disadvantage in that the temperature of the hot gas/air is much higher than the melting point of the polymer beingjoined Therefore, the process has poor energy efficiency, and degradation of the polymer substrate is possible unless care

is taken Polymers that oxidize at temperatures close to their melting points cannot be welded by this technique

Extrusion Welding

Extrusion welding is similar to hot-gas welding The weld area is heated using hot air, and plasticized filler material isextruded into V-shaped or lap seam joints under pressure by means of a welding shoe (Ref 20, 21) The preheating andwelding-shoe assemblies are connected to the welding head, and the welding speed is kept constant by an automatictraversing unit that has adjustable motor speeds (Fig 5)

FIG 5 SCHEMATIC OF VARIANTS OF EXTRUSION WELDING PROCESS SOURCE: REF 20

The key joining parameters of this process are the:

• WELD LENGTH AND TIME REQUIRED TO COMPLETETHE JOINT

Applications Extrusion welding is primarily used for producing long seams in thick-section polyolefin components.This joining technique is particularly useful when welding large container sections

Focused Infrared Welding

This joining technique uses a quartz lamp and focuses the infrared (IR) radiation using highly polished, parabolic,elliptical reflectors The focused IR method directs a precise, high-intensity reciprocating IR beam of a width that rangesfrom 1.5 to 3.0 mm (0.06 to 0.12 in.) onto the joint interface (Ref 22) A robotic fixture scans the IR beam back and forthover the two adherend surfaces, and when the joint line reaches the desired temperature, the heat source is removed andthe substrates are forged together in a press Sensors are mounted adjacent to the lamp fixture and are calibrated so thatthey selectively measure the adherend interface temperature only (Fig 6)

FIG 6 SCHEMATIC OF FOCUSED IR LAMP AND OPTICAL SENSOR SOURCE: REF 22

The welding of polyphenylene sulfide, using this IR technique, has been reported (Ref 23) Focused IR welding can beautomated Because this is a relatively new joining process, a detailed analysis of process operation has yet to beconducted

The key joining parameters of this welding process are the:

• RADIATION DENSITY AND TIME OF IRRADIATION

POLYMER, WHICH DETERMINE THE RATE OF THE TEMPERATURE RISE IN THE IRRADIATED MATERIAL

BEING JOINED, AND ON THE PART SIZE

PRESS (IN ORDER TO AVOID COOLING THE MELTED MATERIAL AT THE JOINT INTERFACE)

Applications This technique can be used to join both simple and complex joint configurations when a noncontactingmethod of heating is essential In addition, reinforced-fiber disruption can be minimized when advanced thermoplasticcomposites are joined.

Laser Welding

Limited information is available on laser welding Carbon dioxide (CO2) lasers induce excitation of the vibrational modesand, hence, heating of the irradiated organic material (Ref 24, 25) At low power input levels, satisfactory weldpenetration can be achieved The technique also provides high welding speeds and produces very small heat-affectedzones (HAZ) Typical joining conditions involve an energy input of 100 W using a wavelength of 10.6 m Theabsorption of laser radiation depends on the relation:

I(Z) = I0 EXP (-αZ) (EQ 2)

where z is the distance into the sample at which the laser intensity I (W/cm2) is measured, I0is the laser intensity at the

surface of the polymer sample (at z = 0), and α (1/cm) is the absorption coefficient The laser power, P (watts) required for any given material to depth a (meters) can be derived using the relation (Ref 25):

where v is the welding speed (m/s), W is the weld width (m), ρ is the density of polymer (ks/m3), Cpis the heat capacity

(J/kg · °C), Tm is the melting temperature (°C), and ∆Hmis the latent heat of melting (J/m3)

There is not much that has been published concerning the commercial application of this joining method

References cited in this section

1 G GEHARDSSON, THE WELDING OF PLASTICS, WELD REV., FEB 1983, P 17-22

14 K GABLER AND H POTENTE, WELDABILITY OF DISSIMILAR

THERMOPLASTICS EXPERIMENTS IN HEATED TOOL WELDING, J ADHES., VOL 11, 1980, P 145-163

15 H POTENTE AND P TAPPE, SCALE-UP LAWS IN HEATED TOOL BUTT WELDING OF HDPE

AND PP, POLYM ENG SCI., VOL 29 (NO 23), 1989, P 1642-1648

16 H POTENTE AND J NATROP, COMPUTER AIDED OPTIMIZATION OF THE PARAMETERS OF

HEATED TOOL BUTT WELDING, POLYM ENG SCI., VOL 29 (NO 23), 1989, P 1649-1654

17 A.J POSLINSKI AND V.K STOKES, ANALYSIS OF THE HOT-TOOL WELDING PROCESS, PROC SPE 50TH ANTEC MEETING, 1992, SOCIETY OF PLASTICS ENGINEERS, P 1228-1233

18 H GUMBLETON, HOT GAS WELDING OF THERMOPLASTICS AN INTRODUCTION, JOIN MAT., VOL 5, 1989, P 215-218

19 "RECOMMENDED PRACTICES FOR JOINING PLASTIC PIPING," DOCUMENT XVI-322-78-E, IIW

20 P MICHEL, "AN ANALYSIS OF THE EXTRUSION WELDING PROCESS," POLYM ENG SCI., VOL

29 (NO 19), 1989, P 1376-1382

21 M GEHDE AND G.W EHRENSTEIN, STRUCTURAL AND MECHANICAL PROPERTIES OF

OPTIMIZED EXTRUSION WELDS, POLYM ENG SCI., VOL 31 (NO 7), 1991, P 495-501

22 H SWARTZ AND J.L SWARTZ, FOCUSSED INFRARED A NEW JOINING TECHNOLOGY FOR

HIGH PERFORMANCE THERMOPLASTICS AND COMPOSITE PARTS, PROC 5TH ANNUAL NORTH AMERICAN WELDING RESEARCH CONFERENCE, EWI AND AWS, 1989

23 H POTENTE, P MICHEL, AND M HEIL, INFRARED RADIATION WELDING: A METHOD FOR

WELDING HIGH TEMPERATURE RESISTANT THERMOPLASTICS, PROC SPE 49TH ANTEC MEETING, SOCIETY OF PLASTICS ENGINEERS, 1991, P 2502-2504

24 W.W DULEY, IN LASER PROCESSING AND ANALYSIS OF MATERIALS, PLENUM PRESS, 1983

25 W.W DULEY AND R.E MUELLER, CO2 LASER WELDING OF POLYMERS, POLYM ENG SCI.,

Vibration Welding

Samples are clamped together and vibrated using an oscillating motion under pressure (Fig 7) Frictional heating is due toone solid being rubbed against the other, and to the shear heating of the melt Vibratory motion is applied until thepolymer melts at the bond line Then, the motion is terminated and melted polymer at the joint interface is cooled underpressure

FIG 7 SCHEMATIC OF VIBRATION WELDING PROCESS

Two types of vibration welding machines are available: linear machines, in which the center of motion lies outside of themolded part; and angular machines, in which the center of motion lies within the molded part (Ref 3) Linear vibrationwelding is more popular, because of its ability to weld long and narrow molded parts

During welding, the samples are fixed to upper and lower platens, and the lower platen is kept fixed while the upperplaten is vibrated using an electromagnetic or hydraulic drive system at frequencies ranging from 100 to 500 Hz Thevibrational amplitude ranges from 0.1 to 5.0 mm (0.004 to 0.2 in.), depending on the frequency used (Ref 26, 27) The detailed features of the vibrational welding process have been studied, along with welding parameter optimization, by

a number of investigators (Ref 28, 29, 30) It has been shown that the vibration welding process involves four distinctphases (Fig 8) Phase I comprises the initial heating of the interface to the melting point as a result of coulomb friction.Phase II involves melting and unsteady material flow in the lateral direction In phase III, melting and flow are at a steady

state, and weld penetration (the decrease in the melted zone width that is due to lateral flow) increases linearly with time.When the vibratory motion is terminated, weld penetration continues to increase until the material solidifies, whichrepresents phase IV

FIG 8 SCHEMATIC OF PENETRATION-TIME GRAPH SHOWING THE FOUR PHASES OF VIBRATION WELDING

PROCESS SOURCE: REF 26

The key joining parameters of this process are the:

• PRESSURE APPLIED DURING THE JOINING OPERATION

It has been reported that the use of a process-controlled pressure profile (high pressures during phase I and lower appliedpressures during the subsequent phases) will improve final joint mechanical properties and decrease the welding time,when compared with the time used in a constant weld pressure cycle (Ref 29)

Applications.The success of vibration welding lies in the fact that it can be used to join large parts and is limited bymachine size The joining process, which can be readily automated, can be applied to a variety of plastics, particularlysemicrystalline polymers such as PP, PE, acetal, and nylon, which are difficult to weld using ultrasonic welding Buttweld joints are the principal type of joint geometry that is used The primary application areas are in the automotive,aeronautical, and household appliance sectors

Spin Welding

Spin welding is ideally suited for joining cylindrical or circular parts One half of the component is fixed, and the otherhalf rotates at a prescribed angular velocity while pressure is applied (Fig 9) Once a desired melted polymer thickness isachieved, the rotational motion is terminated and the parts are pressed together and cooled

FIG 9 SCHEMATIC OF SPIN WELDING PROCESS

Spin-welding machines can contain a range of sensors that indicate the rotational speed, axial pressure, weld penetration,velocity, and torque during the joining operation (Ref 31) The joining process can be described as having five distinctphases (Fig 10) Solid (coulombic) friction, followed by friction and generation of wear particles, occurs during phases Iand II Phase III is the melting transition Shearing of the melt produces further heat generation, and the weld penetrationincreases linearly with time in phase IV Phase V involves cooling and consolidation of the joint under pressure