although the processing technology available for plastics

1Kunststoffe plast europe 7/2004

GOTTFRIED W. EHRENSTEIN

The difficulties and questions thatarise when attempting to decide onthe appropriate method result from

the variety of possibilities and the com-plex consequences of individual joiningtechniques on the design and entire lifecycle of a product. Computer-aided en-gineering can help with analysis and thesearch for the appropriate joining meth-od. It can be seen even from examples in-volving simple joining elements, such asscrews, plastic clips, forces applied tofibre-reinforced plastics and other fas-tening elements, that a systematicallycompiled database would represent atechnically as well as economically usefultool for modern design practice. On theother hand, the amount of work neces-sary to create one of these desirable data-bases is obvious. A critical position vis-a-vis an expert system is also based on thesignificant effort that would be required.Innovation represents a further level ofdiscussion when selecting the joiningtechnique and the joining or fastening

element. As a rule, a database presumesthat a specific type or group of joiningtechniques exists and naturally limits thepossible solutions to the existing exam-ples (Table 1). It is equally unlikely thatan expert system can replace a creative en-gineer. At the same time, a thoroughknowledge of the technical possibilities isabsolutely essential for designers whobase their work on intuition.

Screw Joints

Screw joints represent the best-knowntype of joints. Practical screw/nut combi-nations include metal/metal, metal/ plas-tic and plastic/plastic, each of which canbe found in the form of metal screws withthreaded metal inserts, direct-to-plasticjoints using self-tapping metal screws,andplastic screws. The joint strength and ef-

Joining Techniques. Although the processing technology available for plastics

permits a high degree of integration, thus reducing the need for joining processes,

it is nevertheless often necessary to combine plastic parts in assemblies with other

materials or with other plastic parts. This article provides an overview of purely

additive joining techniques for previously produced individual parts.

Direct screw joints

Fig. 1. Factors affecting properties of direct screw joints in plastic

Adhesive bonding process using radiation-cured adhesive, left: application of adhesive, centre: mounting, right: irradiation

Joint Technology

Translated from Kunststoffe 7/2004, pp. 28–34

SPEC I A L ■

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2 © Carl Hanser Verlag, München Kunststoffe plast europe 7/2004

SPEC I A L■

fort required for recycling also follow thissame sequence. With regard to piece partcosts and manufacturing, direct-to-plas-tic joints using self-tapping screws offerclear advantages over joints involvingscrews with threaded metal inserts in partbecause of the fewer number of parts.Plastic screws occupy a special positionregarding application properties, becausethey are electrically insulating, non-mag-

netic and corrosion-resistant. The lowstrength of plastic screws can be offsetsomewhat through the use of reinforcedplastics. The quality of the screw joint isdetermined also by the assembly process.For this reason, specification of the as-sembly torque is critical for the applica-tion properties of a screw joint, along withthe dimensions and overall design. Roughcalculations based on analytical equationsprovide an initial step toward under-standing the operating mechanisms of ascrew joint. Thus, when a change or im-provement in certain properties is re-quired, a starting point can be found rap-idly in this way. Specification of dimen-sions today generally occurs with the aidof a computer, backed by diagrams, tablesand numerical calculations.

For direct-to-plastic joints, a self-tap-ping metal screw with special flankgeometries is screwed into a previouslyformed cylindrical hole in the part to beassembled, thus forming the female threadin the plastic part during the assemblyprocess itself. For plastic components, thedirect screw joint using self-tapping met-al screws represents the most commonlyemployed joining technique, above all for

joints between dissimilar materials thatmust be capable of disassembly or formore stringent requirements regardingjoint strength that cannot be met withsimple snap-fit joints. Because of the rel-atively low investment for screw systems,the direct screw joint is also suitable forsmaller production quantities.

In addition to direct screw joints, screwjoints utilising threaded metal inserts with

a standard female thread are employed.Such inserts are embedded either duringproduction of the component or in a post-production secondary operation. As a re-sult of the additional assembly step, in-sert-based joints are more expensive thandirect screw joints and require additionaldisassembly steps when recycling.

In a direct comparison to other joiningtechniques capable of disassembly, directscrew joints in plastic are very economi-

cal.The primary reasons for employing themore expensive insert-based joint are theincreased reassembly capability and the re-duction in tightening force relaxation, aslong as the metal component is attachedto a metal insert. Well-designed directscrew joints with self-tapping metal screws(flank angle of 30°) easily provide,as a rule,a 10-time repeated assembly capability inaccordance with VDE 0720.

The quality of such a joint is deter-mined by a multitude of factors that canbe classified as material-based/produc-tion-related, geometrical, assembly-relat-ed and application-related (Fig. 1).

Threaded inserts that are embedded inspecially formed bosses or directly in thewall of the component are preferredwhenever the joint must be disassembledand reassembled often, e. g., for mainte-nance or repair purposes. In contrast todirect screw joints, a threaded insert witha female metric thread transmits only theresulting assembly and operating loadsfrom the screw into the surrounding plas-tic. These inserts can be embedded eitherdirectly during production of the mould-ed part, in a post-moulding secondaryoperation utilising ultrasonics or heat topress in the insert, by screwing the insertitself into the moulded part, or by ex-panding the insert with the aid of a screw(Fig. 2).

Two embedding techniques are distin-guished:■ Insert Moulding (Overmoulding)

Technique. Threaded inserts are placedin the mould prior to production of themoulded part (e. g., by injection orcompression moulding) and are sub-sequently embedded.

■ Post-Moulding Technique. Threadedinserts are incorporated into the pre-viously moulded part in a secondaryoperation.

Inserts

Fig. 2. Slight protrusion of a US threaded insert to avoid relaxation (Source: DVS-Guideline 2240-1)

Inserts

Fig. 3. Load-bearingcylindrical surface of

insert

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SPEC I A L ■

Threaded inserts are made primarily ofmetal, and occasionally more recycling-friendly reinforced plastics. The use ofthreaded inserts is more expensive thandirect screw joints, but the forces that canbe transmitted are considerably higher forthe same nominal thread diameter be-cause of the greater cylindrical surfacearea of the insert (Fig. 3).

The most important geometric factorsinfluencing the load-bearing characteris-tics of an insert are the core hole diameterand the wall thickness of the plastic partas well as the outside contour of the insert.It is noteworthy that, except for a few hot-pressed inserts,all are very sensitive to vari-ations in core hole diameter. The pull-outstrength of hot-pressed inserts is clearly su-perior to that of cold-pressed inserts. Inaddition, hot-pressed inserts generallyshow somewhat higher pull-out valuesthan spreading-type inserts.

The classical screw joint with nut andbolt is of relatively little importance forplastics, since both the bolt (if made ofplastic) and the assembled parts willcreep. This behaviour can be counteract-ed to a certain degree through the use ofribbed designs, large washers or metalbushings that are incorporated after themoulding process.

Snap-fit Joints (Snap Fits)

Snap-fit joints (also called snap fits) are avery commonly encountered,plastics-ori-ented design principle and, thanks to theirelastic deformation,provide non-positive

(fric-tional) and/or

positive locking joints. Inprinciple, snap-fit joints con-

sist of a spring-like element and adetent (undercut) that forms the

joint. They are characterised by con-venience, a small number of necessary

parts and tools,and exceptional economy.They represent a better alternative toscrew joints in a confined space and whenthe requirements regarding joint strengthand joint properties are not all too high.Here, too, the comments apply about an-alytical calculations that are intended toprovide a better understanding of the op-erating mechanisms in snap-fit joints sothat a solution can be found quickly whenchanges or improvements are necessary.With the aid of the finite-element method(FEM) elements of snap-fit joints can becalculated much more accurately today.Even a rough calculation helps to discov-er large errors attributable to, for instance,material data or computational assump-tions.

The operating environment must al-ways be taken into consideration. For ex-ample, in the hectic conditions of every-day assembly operations, the maximum

possible deflection of snap hooks is crit-ical for sizing. For this reason, it is re-commended that a limit on deflection beincorporated into the design from thevery beginning.

In this regard, it should be pointed outthat the material data from a database,e. g., Campus, represent approximate val-ues, not guaranteed values. They can thusbe used only with caution as the basis forcalculations. Like screw joints, snap-fitjoints represent a joint that can be disas-sembled and withstand multiple open-ings and closings, thus raising the ques-tion as to which is the best type of joint.This also applies regarding application-appropriate design. The above-men-tioned joint types tend to show an in-creasing economy with a decreasing num-ber of parts needed for a joint, and a de-creasing joint strength that can, however,be increased significantly through mate-rial and technological developments.

Living Hinges

A living hinge is an extremely thin, flexi-ble joint connecting two parts that mustmove with respect to one another. It ispreferably produced from a thermoplas-tic resin as a single piece in an integrateddesign.

To achieve a long service life in suchliving hinges, it is critical that the mole-cules in the thin hinge area, which is on-ly a few tenths of a millimeter thick, behighly oriented in the bending direction.This occurs during injection moulding asa result of the pronounced shear flow ofthe polymer melt in the skin layers. Thecore layer should exhibit a fine spherulith-ic structure with spherulite diameters of

Joint type Examples Forcetransmittal

Separability Disassemblytools

Disassemblycharacteristics

Snap-fit joint Snap hook Positive Separable toconditionallyseparable

None/ Screwdriver

Good/ moderate

Clip joint Frictionally aug-mented positive

Separable None Good

Screw joint Threadedinsert

Frictionally augmentedpositive

Separable Screwdriver Good

Plastic screw

Self-tappingsteel screw

Self-tappingscrew withquick fastener

Clamp joint Clamp Non-positive Separable Moderate

Riveted joint Positive Permanent Poor

Adhesive joint Material-based Permanent Div. Poor

Table 1. Important types and implementations of joints in plastic parts

V

Fig. 4. Horizontal, 4 m long heated tool buttwelding machine for sheet stock andextreme joint strengths,e. g., for PE-UHMW(Source: Wegener GmbH,

Aachen/Germany)

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SPEC I A L■ SPEC I A L■

only a few micrometers. With this mo-lecular structure, especially when usingadaptive resins, a long service life can beexpected. Living hinges are, however, sen-sitive to transverse loads. As a type ofjoint, living hinges are a typical exampleof the high functional integration possi-ble with plastic-appropriate design. Froma functional standpoint, there is, as a rule,no comparable alternative to this joint.The many variations, especially regardingproduction-appropriate design as well asweld lines and molecular orientation,should not be overlooked. It should alsobe noted that glass fibre-reinforced plas-tics are generally not capable of formingliving hinges.

Welding

As an additional processing step, thewelding process represents a repeatedthermal and rheological load on the ma-terial. Structural and geometric notchescan lead to a disruption of the usually ho-mogeneous state of the material in themoulded parts at the joints. Due to de-

sign-related changes in cross section, theflux of force in the welded joint can bedisturbed. For this reason, a welded jointcannot exhibit the same mechanicalproperties as the surrounding base mate-rial. The empirically determined weldingfactor takes this into consideration. Themany welding methods can be classifiedgenerally as either low-volume (manual)or high-volume welding techniques. Thelow-volume techniques are employed pri-marily for welding of semi-finished goodsfor process equipment, storage vessels andpiping systems as well as for landfill andwater distribution applications; these in-clude the hot gas and extrusion weldingmethods. They are flexible and requirerelatively little investment, since the ma-chines are simple in design (Fig. 4). Be-cause of this, the amount of work in-volved is greater, and there are only lim-ited possibilities for process monitoring.

The welding techniques for high-vol-ume production – these are, above all, ul-trasonic welding, vibration welding andlaser welding – are employed for massproduction in the automotive, electrical,

household appliance and packaging in-dustries.Using these techniques, injectionand blow moulded parts in particular arewelded in extremely short cycle times.Machinery and tooling require higher in-vestments,but offer seamless process con-trol when suitably equipped. The bound-aries between the two classifications areflexible; for instance, heated tool weldingand spin welding are employed success-fully for both low-volume (manual) andhigh-volume applications. In this case, themachines are matched to the particularrequirements. Table 2 briefly characteris-es and rates the most important weldingtechniques.

Adhesive Bonding

The development of various adhesives andadhesive bonding techniques means thatmany different materials, including al-most all plastics, can utilise this technol-ogy. Typical applications for adhesivebonding can be found in the constructionindustry (floor coverings, insulating ma-terials, bridges, components for prefabri-

Advantages Disadvantages Application/Part type Joint characteristics

Heated tool welding (Heat contact)

High process reliability withminimal chance for defects,extensive knowledge base available

Limited range of materials dueto sticking on heated tool,long cycle time, high energyconsumption

High-volume production: partssuch as taillights, householdappliances; manual assembly:process equipment & piping,windows

High short- and long-termstrength up to the materialstrength, unavoidable formationof flash

Heat sealing/Sealing of films andwebs

Universally applicable Problems with thick films Packaging: film, multi-layer films(paper, aluminium), substrateswith a sealing layer

Joint strength variable from“tight“ to “peel off“

Hot gas welding Simple welding equipment,highly flexible, suited fordifficult locations

Low welding speed, weldingrods available for only fewplastics

Process equipment and piping,one-off assembly from semi-finished stock, landfill linersand roofing material

Highly influenced by operator,poor process reliability/repeat-ability

Hot gas extrusionwelding

Simple welding equipment,highly flexible, large amount ofmaterial applied, may beautomated

Low welding speed, specialwelding shoe for each jointshape, welding rods availablefor only a few plastics

Process equipment and storagevessels, sealing method in civiland hydraulic engineering, wallthickness from 5 mm, repair welds

Good weld quality, less influenced by operator

Heated tool welding withnon-contact heating

No problems with melt stickingto the heated tool as with con-ventional heated tool welding

Heating time depends on pig-ment, only slight tolerance per-missible, very high temperature– high energy consumption

Welding for high-volume pro-duction of components such asfilters, floats, also for piping

As with heated tool welding

Laser welding No mechanical load, no fibrousdebris, only localised heating,complicated 3-D joints possible

For laser welding, parts to bejoined must have differentabsorption characteristics

High-volume production of sen-sitive parts: sensors, electronicsenclosures, containers

Depending on method, hermeticseals possible

Ultrasonic welding Very short cycle times, easilyautomated, energy-efficient

Special joint design necessary,part size limited for hermeticallysealed joint, hearing protectionrequired

High-volume production of smalland medium-sized parts in theauto, electrical and householdappliance industries

Low melt film thickness,resulting in only moderate jointstrength

Spin welding Short cycle time, welding eqmt.can be improvised (e.g., rotatingmachine)

Limited to parts with rotational-ly symmetrical joint

High-volume and one-off pro-duction of parts with at leastone rotationally symmetricaljoint

High joint strength possible,e.g., with larger joint surfacearea

Vibration welding Short cycle time, more than 2parts to join, suited for largeparts (bumpers), energy-efficient

Fibrous debris cannot always beavoided, hearing protectionsometimes required

High-volume production of partsin the auto, electrical and house-hold appliance industries

Good weld quality, highly repro-ducible (Fig. 5)

Table 2. Overview and rating of common plastic welding methods

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SPEC I A L ■

two techniques are very similar, there aredifferences when it comes to fields of ap-plication.

Riveting is employed largely in situa-tions where high quality standards re-garding joint integrity and mechanicalproperties must be met.

In contrast, edge forming often findsapplication for joints that are not sub-jected to high mechanical loads. It is on-ly when joining pipes for drinking wateror heating, for instance, that edge form-ing has gained a foothold in a field withhigh quality standards regarding joint in-tegrity.

Advantages of both techniques in-clude:■ Joining of similar and dissimilar ma-

terials,■ fast production process,■ simple tooling,■ low tolerance requirements,■ simple, destructive disassembly (e. g.,

for recycling).Disadvantages include:■ direct sealing function not possible,■ limited design freedom.

Press-fit Joints

Press-fit joints are joints that, depend-ing on the design, can be intended toserve as permanent joints or be capableof assembly and disassembly multipletimes. The advantages lie in the simpledesign and assembly as well as the pos-sibility to join similar and dissimilar ma-terials.

Since the press-fit joint is created as aresult of the insertion force and the asso-ciated tribological friction as well as thestiffness in conjunction with the geomet-ric conditions, the following pointsshould be observed:■ Relaxation of the insertion force, espe-

cially at elevated and fluctuating tem-peratures.

■ In materials such as PA (nylon) thejoint strength depends additionally onthe moisture uptake.

■ Internal stresses remaining from pro-duction.

Conclusion

The joint types discussed compete withone another and are also complementa-ry techniques. On the other hand, forevery specific application, only a limitednumber of techniques comes into con-sideration as a result of the particular sit-uation. The selection-determining ques-tions could be:

cated buildings), the packaging industry,shoe production, electrical equipment(housings, motors, measuring instru-ments, etc.), the automotive industry (au-to body parts, windows, brake and clutchpads, trim mouldings, interior parts, etc.),and in the aircraft industry. Adhesivebonding offers many benefits for use inactual practice. The advantages and dis-advantages with respect to welding arepresented in Table 3.

With adhesive bonding, it is possibleto join not only similar plastics, but alsodissimilar plastics and plastics to othermaterials. Successful application of ad-hesive bonding, however, requires that anumber of factors be taken intoconsideration. These include the bondgeometry, the chemical and physicalproperties of the adhesive and the mate-rials to be bonded, the chemical compo-sition and the cleanliness of the surfacesto be bonded, and the resistance to age-ing of the adhesive layer and bondedparts. Depending on the manufacturingapplication and processing, adhesivejoints, like welded joints, can also exhib-it inhomogeneities that affect the me-chanical properties and the long-termbehaviour. In adhesive-bonded joints, theshear stresses that result from differen-tial expansion of the bonded parts dur-ing temperature changes must also beconsidered.

An adhesive bond is formed with theaid of an adhesive. If the proper adhesiveis selected, it is often possible to form avery strong, reliable and hermeticallytight seamless joint between identical ordissimilar materials. The light and thinadhesive layer contributes to lightweightconstruction by exhibiting little weight it-self and permitting the use of optimal ma-terial combinations.

Reliable adhesive-bonded joints oftenrequire an overlap to achieve a large con-tact area, but offer in return the least dis-turbance of the joined surfaces.

The costs of adhesive bonding andother joining techniques are not gener-ally comparable. The material andprocess costs of adhesive bonding mustbe compared to the expenses for addi-tional components and tooling for oth-er methods.

The physical and chemical principlesthat produce a strong and durable adhe-sive bond are still not part of generalknowledge. Accordingly, experience re-mains an important foundation forproper adhesive selection and process-ing.

Riveting and Edge Forming

The joining techniques of riveting andedge forming provide permanent, posi-tive locking (interlocking) joints that areaugmented to some extent through fric-tion. With these techniques, joining ofthe parts is accomplished by shapingor deforming the joining element. Ac-cordingly, the joints formed by thesemethods are primarily localised spotjoints or segmented joints. In contrast toadhesive bonding or welding, require-ments regarding the absence of leaks inthe joints formed using these methodscannot be fulfilled directly, but usuallyonly through the use of additional sealsor gaskets.

A major advantage of these methods isthat, as a result of the mechanical bond,joining of components made of similaror dissimilar plastics as well as of com-ponents made of plastic and metal, fab-ric or paperboard is possible. Althoughthe joint-forming mechanisms of these

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Advantages Disadvantages

Adh

esiv

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– Joins dissimilar materials and thin, large-areaparts

– Not necessary to damage the parts to be joined – Possible to compensate for fit tolerances – No or only little thermal effect on the materials

of the parts to be joined– Uniform stress distribution over the joint

surface – Production of liquid- and gas-tight joints– Vibration-damping and insulating property of

the adhesive layer

– Overlapping always necessary and pretreat-ment of joint surfaces often necessary as wellas exact maintenance of process parameters

– Long cure times for thermosetting adhesives– Low strength of the adhesive bond

(may need large joint cross sections)– Time-dependent changes in the properties

of the adhesive layer (ageing, sensitivity toatmospheric and chemical factors)

– Limited thermal stability (max. ≈ 250 °C)

Wel

ding

– Production of smooth, flat joints possible– Absolutely tight joints– Joint strength of the base material– Approximately uniform stress distribution

– Only thermoplastics can be welded, bondingpossible only with somewhat similar materials

– Joined parts subject to thermal load (change inmicrostructure, warp)

– Welding flash

Table 3. Comparison of the material-bonding joining methods “adhesive bonding” and “welding”

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■ Is the joining process a step in a high-speed assembly operation?

■ Should joining take place without theuse of tools? If the answer to this ques-tion is yes, then only snap-fit joints orsnap-fit joints combined with livinghinges can be employed.

■ Must the joint be capable of assemblyand disassembly multiple times or sep-arable at the end of its service life?

■ How strong must the joint be? A highstrength requirement can exclude theuse of snap-fit joints or, in the case ofreinforced plastics, riveting and edgeforming.

■ How much design freedom is required?A requirement for a high degree of de-sign freedom could exclude welding,riveting and edge forming.

■ Is additional physical functionality re-quired, such as sealing, insulating ormagnetic shielding?

It is well-known that product costs are es-tablished to a large extent during the de-velopment and design phase. An optimaldesign is possible only if extensive infor-mation about the subsequent manufac-turing processes is available during this

phase. It should also be pointed out herethat there is often no optimum joiningtechnique for a product. Rather, an opti-misation process must take place on thebasis of actual operating conditions, i. e.,the existing know-how among plantpersonnel and the existing productionequipment must be taken into consider-ation. ■

ACKNOWLEDGEMENTS

This article represents a summary of contributions toa handbook titled “Kunststoff-Verbindungstechnik”(Plastic Joining Techniques, only available in German)appearing this autumn. A listing of individual refer-ences and literature sources is beyond the scope ofthe present article. I wish to extend special thanks toProf. Dr.-Ing. Jian Song and Dipl.-Ing. Martin Welz fortheir assistance in reviewing and rating the varioustechniques.

THE AUTHOR

PROF. DR.-ING. DR. H.C. GOTTFRIED W.EHRENSTEIN, born 1937, is Professor of PlasticsTechnology at the University of Erlangen-Nurembergand a partner in the company Neue Materialien FürthGmbH.Contact: Fax +49 (0) 91 31/8 52 97 09

Fig. 5. Etched surfaceof a vibration weldedjoint in unreinforcedpolypropylene(mikroscopy: light,differential interfer-ence contrast)

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although the processing technology available for plastics

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