Profile design, specification, properties and related matters

3 Profile design, specification, properties and related matters DAVID EVANS

3.1

Introduction

Futurists have claimed that in general our technology is advancing so rapidly that we now live in a virtually new world every 11 months. While for the majority such a statement may be related only to computers, the reality is that changes are indeed everywhere, from medical and farming practices to materials selection and technology. The first design manual for structural plastics was published by the American Society of Civil Engineers in 1979,1 and in conjunction with other confirmatory evidence this had a large effect in moving ‘plastics’ from a cheap, kitchenware and children’s toy material into a serious design option for a wide variety of engineering markets and applications. However, unlike other ‘plastics’ which have also eventually found industrial, engineering and structural uses, reinforced plastic composites have increasingly confirmed that they possess a unique ability to be critically designed for optimum finished product performance, whether that be based on physical, mechanical, environmental or long in-service economic assessment. Increasingly, particularly since 1980, one of the preferred composites fabrication processes is pultrusion.

3.2

Common pultrusion materials

It is estimated that over 85% of the pultrusions currently manufactured world-wide are a composite of E-glass fibre, typically continuous unidirectional glass rovings or continuous strand (or filament) mat (CFM), but at times chopped strand mat (CSM), ‘encapsulated’ within a resin matrix – often in a fire-retardant grade – of either ortho- or, more often, iso-phthalic unsaturated polyester or vinyl ester. The balance is composed of other E or even A-glass woven fabrics and reinforcements as well as those based on carbon or aramid fibres and their respective hybrids with yet another option, three further thermosetting resins based on acrylic, epoxy or phe66

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nolic chemistry. All these many choices receive in-depth consideration in Chapters 4 and 5 and it is worth emphasising that as profile performance demands increase, there will be a continuing move away from what can currently be termed the 85% E-glass–polyester resin norm. In fact, recent marked reductions in the cost of certain grades of carbon fibre can for example be confidently expected to promote a pronounced increase in its use as a pultruded profile reinforcement. Finally, and parallel to other composites technology development and commercialisation, there is a slow but steadily increasing interest in the use of thermoplastic matrices as typified by polypropylene.

3.3

Profile: design

Pultruded profiles can be readily classified as standard or custom.

Standard profiles This term is usually reserved for those profiles (Fig. 3.1) that are classified as ‘standard’, simply because they are all manufactured by virtually every pultrusion company around the world and with a commonality that also reflects their replication of similar timber, aluminium or steel sections. As a result they clearly find extensive use, although it must be noted that the dimensional (Table 3.1), physical and mechanical properties while exhibiting close batch-to-batch consistency from any one manufacturer, may not necessarily exactly duplicate those for an otherwise totally identical profile from another manufacturer. Equally many other ‘standard’ profiles are available than the typical selection of bar, hollow, rectangular, round, channel, angle, I and H-sections illustrated. Manufacturers’ catalogues and technical literature should therefore always be consulted.

Custom profiles As the name implies, this term is usually applied to those profiles that are designed, specified and manufactured for a particular customer, who in turn may also own the tooling. Occasionally, but typically only after many years of production, such profiles may eventually find their way into a manufacturer’s standard profile catalogue. Although the ‘standards’ through their typically long case histories have very clearly proved many of the preferred attributes of design and specification, it is the custom profiles that in terms of shape complexity, dimensional tolerance and specification continue to develop both the process and technology of pultrusion. As a clear demonstration of what can now be successfully pultruded, four examples in volume production are now described

68

Pultrusion for engineers A Round 10mm ≤ A ≤ 80 mm 0.4” ≤ A ≤ 3.15” A Square 10mm ≤ A ≤ 80 mm 0.4” ≤ A ≤ 3.15”

A

A

B

Rectangle A = 2B 10mm ≤ A ≤ 120 mm 0.4” ≤ A ≤ 4.75”

A

B

Flat A = 10B 10 mm ≤ A ≤ 240 mm or more 0.4” ≤ A ≤ 10.0” or more

A Tube 10mm ≤ A ≤ 200 mm or more 0.4” ≤ A ≤ 8.0” or more

C

A Box beam, square 20mm ≤ A ≤ 200 mm or more 0.8” ≤ A ≤ 8.0” or more

A C

A

C

A

Box beam, rectangle, A = 2B 20 mm ≤ A ≤ 320 mm or more 0.8” ≤ A ≤ 12.5” or more

3.1 Selection of standard profiles (Courtesy, European Pultrusion Technology Association)

‘U’ channel, square 20mm ≤ A ≤ 200 mm 0.8” ≤ A ≤ 8.0”

B C

A

B C

‘U’ channel, A = 2B 20mm ≤ A ≤ 200 mm or more 0.8” ≤ A ≤ 8.0” or more

A H section beam 50 mm ≤ A ≤ 320 mm or more 2” ≤ A ≤ 12.5” or more

C A

A C B

I section beam A = 2B 50 mm ≤ A ≤ 480 mm or more 2” ≤ A ≤ 19.0” or more

A

L section, equal leg 20mm ≤ A ≤ 200 mm 0.8” ≤ a ≤ 8.0”

A C

A

B C

L section, unequal leg, A = 2B 20mm ≤ A ≤ 200 mm 0.8” ≤ A ≤ 8.0”

A T section 20mm ≤ A ≤ 200 mm or more 0.8” ≤ A ≤ 8.0” or more

B C B

A C

3.1 Continued

Z section, A = 2B 20mm ≤ A ≤ 200 mm 0.8” ≤ A ≤ 8.0”

Table 3.1. Indicative dimensions standard glass/polyester profiles and tolerances Section

Dimension A

Dimension B

Dimension C

Round

0.85–27.5 0.035–1.10 8.0–25.4 0.30–1.0 8.0–25.4 0.30–1.0 6.35–44.0 0.25–1.75 6.35–44.0 0.25–1.75 40.0–51.0 1.6–2.0 11.65–300.0 0.45–11.75 8.0–102.0 0.30–4.0 25.4–83.0 1.0–3.25 20.0–94.0 0.80–3.70

— — 8.0–25.4 0.30–1.0 2.75–25.4 0.10–1.0 1.60–9.50 0.06–0.35 — — 20.0–25.4 0.80–1.0 3.80–51.0 0.15–2.0 25.4–203.0 1.0–8.0 25.4–44.0 1.0–1.75 16.0–95.25 0.60–3.75

— — — — — — — — 1.25–2.54 0.05–1.0 3.0–6.0 0.12–0.24 1.75–4.0 0.65–0.16 7.0–9.5 0.275–0.375 3.18–5.0 0.125–0.20 3.45–5.2 0.13–0.20

Direction

Dimension

Tolerance

Thickness (mm)

<5 5 <0.2 0.2 <3 12 25 50 100 >100 <0.12 0.5 1.0 2.0 4.0 >4.0 <3 6 >6 <0.12 0.24 >0.24

±0.25 ±0.35 ±0.01 ±0.014 ±0.15 ±0.2 ±0.25 ±0.35 ±0.5 ±0.75 ±0.006 ±0.008 ±0.01 ±0.014 ±0.02 ±0.03 ±3 ±6 ±10 ±0.12 ±0.24 ±0.4

(mm) (inches) Square (mm) (inches) Rectangle (mm) (inches) Flat (mm) (inches) Tube (mm) (inches) Box beam/ (mm) rectangle (inches) U channels (mm) (inches) I & H sections (mm) (inches) L angles (mm) (inches) T section (mm) (inches)

Thickness (inches) Width and depth (mm)

Width and depth (inches)

Length (mm)

Length (inches)

NB Many other standard profiles are available and tolerances may differ between suppliers, who should also be contacted in respect of other dimensional tolerances, e.g. concavity, convexity, twist and straightness. Courtesy, Fiberforce Composites Ltd.

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3.2 Profile for camper construction (Courtesy, Holiday Rambler Inc)

and illustrated; many other suitable examples appear elsewhere in this book. Baggage door This component (Fig. 3.2) is only one of a large number of custom (and standard) profiles now being employed by many of the world’s established vehicle manufacturers whether for motor homes, campers, buses or trucks. This particular one-piece assembly, which might also act as an air conditioner duct, an electric light support and associated conduit, combines what was previously a multiplicity of sub-parts. Unit assemblies of this type are very common in pultruded profiles generally, their application not only reducing assembly time but also contributing to further weight savings beyond the use of separate profiles. Window lineals Profiles for window and door frame construction (Fig. 3.3), typically exhibit as here, highly complex and close-fitting cross-sections. Over recent years they have become increasingly common because their use totally eliminates the need for the thermal breaks normally associated with aluminium versions. Furthermore, and also neglecting their more optimum weatherresistant properties, the structural integrity of the composite profile offers strong competition to U-PVC extrusions. Indeed, the use of in effect standard profiles to internally strengthen the latter as a post-extrusion operation, is not unknown.

72

Pultrusion for engineers Double glazed window unit

68 mm 2.687”

68 mm 2.687” Pultruded profiles NB The respective profiles ‘click’ together on assembly.

3.3 Typical mullion/transom window frame lineal (Courtesy, Inline Fiberglass Inc)

3.4 Airfoil section (Courtesy, Cofimco USA)

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Airfoil section While impossible to classify as a high-energy wind generator, many of these small windmill units (Fig. 3.4) find excellent application as water pumps or otherwise in the generation of a domestic or farm electricity supply, typically in very isolated locations. Replacing the former three-piece spliced design which was susceptible, to rainwater, ice and snow attack, their efficiency depends on a carefully designed pultruded profile which must not degrade, twist or fail in any mode. That has all been successfully achieved with virtually no maintenance input. Modular building panel There is a slow but steadily growing use in the pultruded manufacture of modular panels (Fig. 3.5) which interlock or can be otherwise assembled into complete structures suitable for any number of applications. Typical is the housing of service company plant or, at the other end of the spectrum, electronic early-warning equipment. Little or no supporting framework is

3.5 Building constructed from pultruded profiles

74

Pultrusion for engineers

necessary and the lightweight property of each panel aids shipment to site, reduces the site and foundation work necessary, as well as enabling the building to be easily erected and if required, later dismantled. Finally, such panels can be readily foam-filled to enhance thermal insulation and the excellent environmental/weather resistance of composites has a direct influence on the low long-term maintenance input required.

The design process In addition to providing an excellent illustration of the design flexibility possible with pultrusion, none of these four profiles replicates an already existing design manufactured in an alternative material. That is important because such a procedure invariably leads to failure with pultruded composites owing to their anistropic nature. In other words, every opportunity must be taken to optimise the particular benefits and unique advantages offered not just by composites, but also by pultrusion. Indeed, in the case of the window lineal an attempt to include screw assembly pockets in the design identical to their aluminium counterparts proved much less effective and moreover significantly increased production difficulties and cost. Consequently it was necessary to develop alternative and, to ultimate advantage, more successful assembly methods. Taking a completely different example, the pultruded box beam structurally outperforms a conventional steel I-beam primarily because the composite version negates the web/ flange shear associated with the conventional configuration. In other words, very close liaison between manufacturer and customer is vital at every stage during the design and specification of a custom moulded profile, ideally starting as soon as the profile is conceived. Only in this way will the total advantages of an optimum combination of composite material properties and the pultruder’s expertise be realised in the most costeffective manner possible. Equally, that is a process that must also include, as now discussed, the resolution of such closely related matters as production rates, part-to-part reproducibility, surface finish, appearance, durability and last, but certainly not least, the agreed dimensional tolerances of the profile.

Profile thickness, production rates and part-to-part reproducibility Although the upper limit will slowly grow commensurate with further thermoset resin and cure developments, pultrusions are already produced in a variety of section thickness typically ranging from 2 to 30 mm (0.08–1.2 inches). However, it is essential to recognise that because the rate of matrix

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75

polymerisation (cure) is influenced by the profile section thickness, there is, at least theoretically, a maximum production speed possible for a particular profile at a particular matrix/reinforcement specification. Not unexpectedly then, those interrelating factors also show a very close relationship to the eventual profile price per unit length, per batch quantity. For this reason, the traditional practice with, say, aluminium and steel sections of simply increasing the thickness of any part of the cross-section as a ready means of enhancing the structural capability is not necessarily the best answer where pultrusion is concerned. If an increase in section depth is not practical, then the only alternative usually possible is a total re-design of the whole profile. Further, although thermally optimised die-heating systems2 can assist in the successful manufacture of ‘unbalanced’ profiles, ideally the design, if not also the reinforcement pack, should always be uniform about the centre-line or otherwise ‘balanced’, typically about the neutral axis. In the same fashion, the use of fillets across section changes as a way of providing additional strength, to prevent corner cracking or other in-service failure at that radius, is counter-productive with a pultruded profile. Owing to their shape and size, fillets can only be created by additional reinforcement which may be prone to cracking and thus create stress risers completely negating, therefore, the effect of the additional cross-sectional area of that fillet. At the same time, a fillet area may prove difficult to ‘fill’ with reinforcement, leading as shown by Fig. 3.6, to the possibility of resin richness and thus weakness at that part of the profile section. Moreover, resin richness can cause resin pickup on the surface of the die cavity leading to profile seizure within the die or, at worst, serious spalling damage to the cavity. Consequently, when designing pultruded profiles, every attempt must be made to maintain the same part thickness throughout the section change at the radii to avoid production or later in-service problems. However, and as also illustrated by Fig. 3.6, pultrusion in comparison with the traditional material forms offers the distinct benefit of providing a homogeneous and consistent level of ‘continuous’ reinforcement through such a ‘blended radii’ cross-section change. In effect, therefore, a stronger section change is generated than would otherwise result. Variations in the wall thickness across the profile section can also affect both the tolerance level of all the remaining dimensions and the part-topart variances. Furthermore, since it is an inherent property of the pultruded profile for dimensions to shrink by up to 2% as it cures, exits the die and cools, problems such as linear part distortion – or twist – can also occur in any direction, particularly if care has not been taken in the profile design and reinforcement pack specification. The distribution of the latter and its

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Pultrusion for engineers

Bad

Better

Better

Best

R = 1.5 mm (0.06”) R = 1.5mm (0.06”) R = 1.5 mm (0.06”)

R = 0.25 mm (0.01”)

R = 1.5 mm + thickness of profile (0.06”+ T )

Reinforcement follows through the radius

Resin-rich area – can occur as a result of inferior tool design, and also at internal radii, particularly if attempting to create a fillet (see text); placement of rovings at radii can prevent such problems.

3.6 Treatment at pultruded profile radii (Courtesy, Fibreforce Composites Ltd)

volume fraction are also important, as is the cure regime chosen for the profile. The latter is in turn related to the particular grade and type of thermoset resin employed but, as already suggested, equally to the die temperature and length (Chapter 2.2), as well as the selected pulling speed. For obvious reasons this dimensional shrinkage and possible distortion must also be taken into consideration in the design of the tooling itself, if the desired profile dimensions, angularity and configuration are to be achieved. For example, in the case of even a simple channel section where ‘closure’ across the open end can occur, it is typically necessary to machine the tooling with a 2° outward taper to compensate. One over-riding and closing comment under this sub-heading is essential. All the above is based on the still normal practice of using platen or cartridge-type heaters to raise the die temperature as necessary to initiate the cure. Even with close microprocessor control, it is difficult to achieve

Profile design, specification, properties and related matters

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that level of critical temperature control which could resolve some of these noted design and associated problems. Consequently, this is another reason for the development and recent commercialisation of thermally optimised die systems,2 whereby the profile cure cycle can be modelled and continuously monitored and controlled in such a way that it is now practical to pultrude perfect non-uniform cross-section profiles at a steady rate of reproduction.

Surface finish, part appearance and durability Unlike the majority of composite fabrication processes, under normal operating conditions the pultrusion process produces profiles with a surface that is relatively fibre-rich. For many applications and for a variety of reasons that circumstance is unacceptable. Although visually rarely not too detrimental, the eventual in-service environmental exposure can lead to a condition known as fibre-blooming, which, if severe, can ultimately result in serious deterioration of the composite structure itself. However, by completing the outer layers of the reinforcement pack with a polyester or polypropylene-based surfacing veil – or tissue – a resin-rich surface results which not only improves the depth of colour and overall appearance, but also enhances the weather/environmental performance. The action of the veil is to ‘push’ the actual reinforcement pack very slightly below the external surfaces. Further enhancement of the profile in respect of corrosion resistance is provided by the use of A and C-glass tissues together with resin matrices such as vinyl esters which are specifically formulated for that market sector. Dyed and printed veils can also be employed to further improve the surface appearance and colour depth of profiles used for decorative application or, taking another example, to provide a finish which replicates timber when the profile is to be employed in furniture manufacture. A final tissue version based on carbon will produce surface conductive profiles for static grounding applications. Like other composite components which frequently use an unreinforced resin gel-coat to provide not just a decorative, but also an environmentally resistant surface to protect the underlying reinforcement, pultruded profiles can be finished on-line with a variety of surface treatments. Typical in the manufacture of ski-poles, is on-line epoxy powder coating but urethanebased paint or other treatments applied using secondary off-line stations after the cutting-to-length operation are far from unusual. In addition to improving the visual appearance, such treatments often enhance the ultraviolet resistance under outdoor exposure conditions. In a more recent window lineal development, co-extrusion with vinyl polymers creates a sufficiently thick coating to allow separate profiles to be ultrasonically welded into larger assemblies.

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Pultrusion for engineers

Finally, and further illustrating the versatility of pultruded profiles, it is clearly practical to enhance the stiffness of any profile by the use and/or specialised placement of suitable fibres – such as carbon – within the initial reinforcement pack. Otherwise the addition of a shallow rib/s or other section changes can add rigidity to a profile.

Dimensional tolerances Through organisations such as the Pultrusion Council of the Composites Industry3 and the European Pultrusion Technology Association (EPTA),4 which support the world-wide pultrusion industry, standard dimensional tolerances have been established by the industry and accepted by such authorities as the American Society for Testing Materials (ASTM). Table 3.1 in conjunction with Fig. 3.1 has already indicated the nature of the dimensional information now widely available. Typically, however, these dimensional tolerances are fairly general and do not necessarily reflect the best that can actually be achieved by the process. Tighter tolerances can therefore apply but as this can add to the overall cost, it needs to be a matter for discussion and agreement between supplier and customer. These meetings should ensure that the end use requirements fall within the range which are reproducible by the process for a particular profile and specification.

3.4

Profile: specification and production

Like any quality product and as already intimated there is a need for early discussion and close agreement between the pultruded profile supplier and the customer. This is equally true for all reinforced plastic composites as well as pultrusions and is particularly apposite when, like pultruded profiles, the composite material itself is to be formed and the eventual product manufactured, at the same time. Further, although the guidelines that follow are more applicable to custom-moulded rather than standard profiles, certain agreed customer approvals are of course just as desirable for the latter. That requirement is best answered by relating the purchase order to a detailed and agreed profile production-supply specification whose clauses quantify those parameters that are judged important to both supplier and customer. As relevant, these clauses should in turn make close reference to the many internationally recognised standards now published covering composites technology generally and pultruded profile manufacture in particular. Commensurate with the expansion of the pultrusion market, moves to enlarge the latter category have been put in hand over recent years. One important example is the attempt by the European pultrusion industry

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through EPTA to persuade the CEN (Comité Europèen de Normalisation) to compile a standard5,6 which should provide a reliable level of performance, so that ‘industrial users and design engineers would gain confidence in using pultruded profiles’. It was judged that ‘not only were pultrusions important in their own right, but also because their off-the-shelf availability often provides a first entry point for experience of composites’. Discussion and compilation began in December 1996 and the very recently published CEN Standard has the number EN 13706. In addition and although far from a definitive list, other authoritative American (API, ASTM and MIL), French (NF), German (DIN), Japanese (JIS) and UK (BS) specification standards covering the reinforced plastics and composites sector and of particular relevance to pultrusion, are provided through an Appendix to this chapter. Many are achieving, or have already achieved either European (CEN) or international (ISO) acceptance. Although the clauses of this recommended production-supply specification may well differ from one purchase order to another, the following text outlines what should at least be considered during discussion before being rated of importance to a particular supply. Clauses typically fall into one of four classifications, each detailed below, namely product specification, physical properties, mechanical properties, and quality procedures. It is a guide offered without warranty of any kind, either expressed or implied.

Product specification It should now be clear that the composite from which the profile will be pultruded is formulated from three main constituents: matrix resin, reinforcement and additives. Each is open to total and careful selection, both necessary and vital if the required mechanical performance and environmental resistance are to be fully satisfied. Although the use of thermoplastic matrices is growing, thermosetting systems such as unsaturated polyester, vinyl ester, epoxide, phenolic and acrylic resins currently predominate. All can be purchased in a variety of distinct grades of perhaps different viscosity, ‘toughness’, environmental resistance or, with the exception of the phenolics which are firehard (i.e. they exhibit optimum fire, smoke and toxicity performance), fire retardancy. All also typically permit some selective ‘chemical’ or additive modification by the user, but again with the obvious exception of the phenolics, the use of hydrated alumina to provide the necessary degree of fire retardancy. In terms of the reinforcement, the basic choice rests between glass, carbon, aramid (or much more rarely another thermoplastic-based fibre) or their respective hybrids. That however, is far from the complete picture. Glass fibre is for example available in six formulations (A, C, E, R, S and ECR) and there is then the exact make-up of the reinforcement

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Pultrusion for engineers

pack as well as its volume fraction relative to the whole profile, to consider. Like the matrix, all receive more detailed consideration in Chapters 4 and 5. The additives within the composite may also have a profound influence on the finished properties of the profile, as already explained in the context of fire retardancy. However, depending on their weight/volume percentage addition, other typically inorganic fillers such as talc and kaolin can, given compatibility with the chosen resin matrix, enhance the compressive strength in the same way that glass, polyester or other tissues overlaying the reinforcement can improve the surface properties and environmental resistance. Finally, ultraviolet or optical stabilisers may be formulated as a part of the matrix supply or added separately just before use together with pigments and release agents. In fact all these options and the need for optimum selection and identification, are steadily leading the pultrusion industry into drawing up an internationally recognised classification system based on a six or more letter and/or digit code. The position of each would have a particular significance in comprehensively describing the composite formulation, etc, employed for the profile. One proposal is quoted in literature4 published by the EPTA. Here UP.E5.PF for example would refer to a profile comprising an unsaturated polyester matrix with approximately 50% axial E-glass reinforcement, having a corrosion-resistant polyester veil, and supplied in a fire-retardant grade.

Physical properties A listing of the physical properties which should be critically defined and agreed would include the following. It is worth emphasising that considered collectively, this stage in the whole supply process should be viewed as equally vital to ensuring a guarantee of the mechanical performance, all as a further part of the suggested profile production-supply specification: • • • • • • • • • • •

chemical resistance; coefficient of thermal expansion; cross-section dimensional tolerance; electrical conductivity; fire-retardant performance; glass to resin ratio; hardness; longitudinal dimensional, twist and warp tolerance; thermal conductivity; ultraviolet resistance; water absorption.

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Mechanical properties Like the above, this further alphabetical listing is in many respects no different from that which should be discussed and agreed in any material component-supply situation: • • • • • • • •

compressive strength; elongation; fatigue resistance; flexural modulus; flexural strength; impact resistance; tensile modulus; ultimate tensile strength.

Quality procedures Although agreement on the quality procedures relative to a manufacturing process is far from unusual in an agreed specification document between supplier and customer, the inclusion of control/approval procedures covering the raw materials employed is certainly not typical. Nevertheless, in a process where the material and the product are being manufactured at the same time, it has to be strongly recommended that the customer, irrespective of any process control measures or finished product physical or mechanical evaluation, also ensures that only the very highest raw material standards apply. It is important to have an early and agreed definition of all quality issues and to understand the required level of performance as well as the causes for finished product rejection. Once a comprehensive supply specification is in place, the ability to distinguish causes of defects and appropriate corrective responses is critical to controlling scrap levels to the benefit of both supplier and customer. Matrix Thermoset resins should typically be checked in respect of such properties as acid number, monomer content, reactivity (i.e. gel- and cure-time, plus peak exotherm temperature) and viscosity. A visual assessment of colour and the presence of contamination and gel particles can usefully be included. Reinforcement In terms of the reinforcement, the properties that should be assessed will largely be dictated by the type/s of reinforcement that it is intended to

82

Pultrusion for engineers

employ in the profile. Rovings are for example typically checked for what is known as yield, the presence of knots and fuzz and perhaps also tensile strength, whereas for mat-type materials their weight per unit area, moisture and binder content (and perhaps type) are the important parameters. Fillers The demonstration of a minimum moisture content is also vital for any fillers that are to be added to the formulation. The particle size distribution of that filler can also frequently be another critical property. Ancillary materials Last but not least in the context of raw materials, the customer should ensure that all the ancillary materials that are to be employed – catalysts, release agents, pigments, etc – satisfy, as already recommended, the most exacting standards. Tooling Where custom-moulded profiles are concerned, the tooling is, as has been seen, frequently owned by the customer who should therefore ensure that the selected tool steel and the internal finishing treatment, usually polished hard chrome, conforms to that which will provide the longest possible inservice life. Process As far as the actual pultrusion process is concerned, the monitoring, control and recording of the listed parameters below are essential in ensuring the highest product standard, at the lowest possible scrap levels. It needs to be recognised that unlike those competitive products that can be readily recycled by, for example, a re-melting or re-moulding process, the thermosetbased composite suffers the disadvantage of being irreversible. Defective material or product cannot therefore be recovered. • A high standard of raw material storage, distribution and control to the shopfloor. • Optimum supplier housekeeping to avoid product contamination. • Regular visual checks to creel, guidance and forming plates. • Regular resin impregnation monitoring including wet-bath temperature. • The careful exercise of all mandatory health and safety regulations.

Profile design, specification, properties and related matters

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• The comprehensive monitoring and recording of all production parameters, including catalyst addition, release agent percentage, die temperature profile, line speed and pulling force. • Subsequent finished product mechanical and physical evaluations as already considered. All the above are further enhanced for the customer if the supplier makes use of a comprehensive batch code system which relates raw material and process conditions to the date and time of profile manufacture. When recorded in some form of computer, card or machine-production log book, an obvious link is established to the agreed production-supply specification and moreover in a manner that enables such records to be employed by the supplier to provide the customer with a Certificate of Compliance for every batch of profiles manufactured for them. At the same time such detailed records provide important data for subsequent production runs of the same profile and the management and engineering, with critical information enabling continual assessment of the whole efficiency of the process and the performance of the shopfloor personnel.

Defect identification Comprehensive day-to-day production records also assist in both tracing the cause and implementing immediate rectification of profile defects.These can usually be classified under one of three headings: composition, process parameters or processing procedures. Composition Defects that can be traced to the reinforcement-matrix formulation of the profile can, for example, include such problems as resin reactivity, incomplete resin mixing, contamination, moisture content and low mat fibre binder, and are typically resolved through incoming or in-process raw material testing procedures. Process parameters In this case defects can result, for example, from too high (or low) a die temperature or too fast a line speed, in other words problems typically identified by process exotherm testing methods as described in Chapter 2.2. Processing procedures Suitably indicative of the several problems under this heading are the reinforcement forming arrangements prior to die entry, impregnation

84

Pultrusion for engineers

difficulties, inferior die design and profile gripping systems, all of which are open to systematic resolution by respective design improvement. In addition the obvious relationships between safety, shopfloor housekeeping, hygiene and the positive attitude of production staff to the overall company success are worth emphasis. Matters such as machine guarding, electrical grounding, elimination of debris, proper lighting, the use of safety glasses and shoes and, as a final example, correct training in the handling and use of hazardous materials such as the catalysts employed are all important factors in maintaining the scrap and waste levels as low as possible.

3.5

Profile: property prediction

What is known as the Rule of Mixtures7 can, using for example the following two equations and authoritative fibre and matrix properties (Tables 3.2 and 3.3), provide some guidance to the designer on the expected performance of any reinforced plastic composite. Longitudinal: Xc = VrXr + VfXf Transverse:

(A)

1 Vr Vf = + Xc X r X f

(B)

where Xc = desired property (tensile strength or modulus, flexural strength or modulus) Vr = volume of resin (%) Xr = property of resin (relative to Xc) Vf = volume of fibre (%) Xf = property of fibre (relative to Xc) However, the accuracy of these calculations is very closely dependent on the actual placement, type and alignment of the reinforcement within the finished laminate, and although applicable to all reinforced plastic composites, this is clearly particularly apposite in the context of pultruded profiles.

Table 3.2. Typical reinforcement fibre properties7 Property

E-glass

S-glass

Aramid

Carbon

Density (g/cm3)

2.6

2.49

1.47

1.77

Tensile strength (MPa)

3450

4585

2750

1900–3100

Tensile modulus (GPa)

72.4

86.9

62.0

Elongation to break (%)

4.8

5.4

2.3

227–379 0.5

Profile design, specification, properties and related matters Table 3.3.

85

Typical thermoset matrix resin properties7

Property

Polyester

Vinyl ester

Epoxy

Density (g/cm3)

1.13

1.12

1.28

Tensile strength (MPa)

77

81

76

Flexural strength (MPa)

123

138

115

Flexural modulus (GPa)

2.96

3.72

3.24

Elongation at break (%)

4.5

5.0

6.3

Heat distortion temperature (°C)

71

104

165

For example, if Equation A is applied to a polyester resin-based pultruded bar having a 55% by volume E-glass content, then the calculated longitudinal tensile strength ((3450 MPa ¥ 0.55) + (77 MPa ¥ 0.45)) would be 1932 MPa, whereas the published values for profiles pultruded to this specification are only in the range of 1100 MPa. In other words, the optimum or true fibre alignment required for the calculated result of these Rule of Mixture equations (A and B) to be correct and guaranteed can rarely, if ever, be achieved in production. Furthermore, the normal use of the stacked plies consisting of rovings, mats, woven, stitched or other reinforcements found in the majority of pultruded profile laminates, would distort these formulae even further. Consequently a more accurate method for estimating the properties of a pultruded composite laminate is necessary. Equation C employs a similar type of equation but instead of calculating the properties for the reinforcement and the matrix element of the composite, it estimates the performance of each individual layer or ply within the laminate constituting the profile (Table 3.4): Xc = TlXl + TmXm

(C)

where Xc = desired property (tensile strength or modulus, flexural strength or modulus) Tl and Xl = thickness and property of laminate/reinforcement type (there can be more than two reinforcement/laminate types in a pultruded profile) Tm and Xm = thickness and property of laminate/reinforcement type (there can be more than two reinforcement/laminate types in a pultruded profile) For example, a profile laminate 3 mm thick, consisting of two external plies of 300 g/m2 chopped strand mat (1.016 mm thick, or 33.9% of total laminate) and with a central longitudinal roving core (1.984 mm thick, or 66.1% of total laminate), would show a predicted ((0.661 ¥ 1100 MPa) + (0.339 ¥

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Pultrusion for engineers

Table 3.4. Typical laminate mechanical properties8,9 Roving/ polyester Glass content (%wt)

50–75

3

Woven roving/ polyester

SMCa polyester

45–60

20–35

Density (g/cm )

1.6–2.0

1.5–1.8

1.8–1.85

Tensile strength (MPa)

410–1180

230–240

50–90

Tensile modulus (GPa)

21–41

13–17

Flexural strength (MPa)

690–1240

200–270

140–210

Compressive strength (MPa)

210–480

98–140

240–310

9

a

Sheet moulding compound, a preimpregnated short (50-mm) chopped glass fibre material, formulated with polyester resin and fillers, etc, for moulding by a high-pressure, evaluated temperature (hot-press) technique between steel tools.

1.85 MPa)) longitudinal tensile strength of 727.73 MPa. As this estimated value falls within the range of standard published values for this type of laminate construction, Equation C does therefore provide a useful pultrusion design aid, and this in turn demands additional input data: • Woven roving or stitched, off-axis fibre can be applied at the reinforcement manufactured thickness • Because of their respective reinforcement constructions, neither chopped strand mat (CSM) nor continuous filament mat (CFM) exhibits a true tensile strength prior to resin impregnation and eventual cure, and it is therefore usual to consider them as high fibre-loaded SMC materials (see Table 3.4), and for CSM to apply these typical thicknesses: 150 g/m2 300 g/m2 450 g/m2 600 g/m2

= = = =

0.254 mm 0.508 mm 0.762 mm 1.016 mm

A development of this procedure is illustrated by Table 3.5, which determines the amount of reinforcement required to fill a channel section 10 cm wide ¥ 5 cm deep at a constant thickness of 5 mm, while at the same time allowing the approximate tensile modulus to be calculated. Appropriate values are entered into columns 1–4 inclusive and 6. This allows column 5 to be calculated. Using a compressibility volume fraction for each material (column 8) as determined from Table 3.6, it is then possible to calculate the respective values for columns 9 and 10. The sum of the layer cross-sectional areas is then compared with the cross-sectional area (CSA) of the profile. The reinforcement layers or weights are adjusted to give 100% die fill.

UD Carbon

Second

Third

113

600

1200

3

2

4800

Mass (g/m2)

No of ends

Tex (g/km or mg/m) 1

0.3

0.3

4

Effective width

Material/Fabric

Rovings

1082.4

180

360 1.8

2.55

2.55

6

5a 542.4

Density (g/cc)

Weight (g/m)

Fibre

453.89

100.0

141.18

212.71

7b

Volume (cc/m)

0.66

0.28

0.62

8

Local volume fraction

Considers a channel section, 10 cm wide ¥ 5 cm deep ¥ 0.5 cm thick, CSA = 10 cm2. * Global volume fraction of fibre volume is volume of fibre in the layer relative to the total volume. a Column 1 ¥ column 2 ∏ 1000, or column 3 ¥ column 4. b Column 5 ∏ column 6. c Column 7 ∏ column 8. d Column 7 ∏ CSA ∏ 100. e Column 10 ¥ column 11 ¥ column 12.

Totals

Roving

CFM x 2

First

Type

Layer

Table 3.5. Tabulation to establish profile construction and tensile modulus9

9.99

1.52

5.04

3.43

9c

Layer CSA (mm2)

0.1

0.1412

0.2127

10d

Fibre Global Vf*

2.3

70

70

11

Fibre modulus (GPa)

1

0.375

1

12

b

41.60

23

3.71

14.89

13e

Tensile modulus (GPa)

Profile design, specification, properties and related matters 87

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Pultrusion for engineers

Table 3.6. Compressibility of reinforcement – volume fraction9 Reinforcement

Fibre volume fraction (%)

E-glass roving E-glass chopped strand mat (CSM) E-glass continuous filament mat (CFM) E-glass woven roving Unidirectional carbon cloth ±45° Carbon fabric Kevlar® fabric

0.62 0.38 0.28 0.58 0.66 0.56 0.68

Table 3.7. Fibre efficiency factors (b)9 Reinforcement

b

Unidirectional Bidirectional (woven roving) Chopped strand mat

1.0 5 0.375

Column 11, the fibre modulus, can then be completed for each layer plus in column 12 an efficiency value (b) which, as shown by Table 3.7, depends on the directionality of the fibres in the particular layer. The final column 13 – the moduli of the individual layers – can then be completed, the sum of which is a reasonable estimate of the tensile modulus of that particular profile, when employing the chosen reinforcement pack. Although Equation C in particular is useful for profile feasibility studies or for other purposes such as the design of localised stiffeners or tensile flanges, the fact has to be recognised that whatever theoretical approach is employed, a pultruded profile is a composite and therefore extremely sensitive to the location of the reinforcement. Consequently, by far and away the best practice is to design around known performance values for a given configuration or composite loading. These days most pultrusion manufacturers have available a comprehensive profile/performance database such as the already referenced Design Guide7 and these can usefully provide the designer with a very respectable level of performance predication for a particular section. Similar shaped and dimensioned profiles, or even an epoxy adhesive bonded mock-up comprising, say, two or more standard and perhaps also machined profiles, can be very suitably employed to characterise and confirm the design and specification of a potential profile. The structural loading acceptance can equally be established in this way with respectable accuracy.

Profile design, specification, properties and related matters

89

Table 3.8. Recommended mechanical properties for pultruded structural shapes3 Propertya

Test method

Minimum value

Tensile strength (MPa) Longitudinal Transverse

ASTM D638

206 45

Tensile modulus (GPa) Longitudinal Transverse

ASTM D638

15.9 5.5

Flexural strength (MPa) Longitudinal Transverse

ASTM D790

206 45

Flexural modulus (GPa) Longitudinal Transverse

ASTM D790

10.3 4.8

Compressive strength (MPa) Longitudinal Transverse

ASTM D695

206 45

Apparent horizontal shear (MPa) Longitudinal

ASTM D2344

20.7

Barcol hardnessb

ASTM D2583

50

Water absorption (%wt)

ASTM D570

0.7 max

Density (g/cm3)

ASTM D792

1.6–1.9

Glass content (%wt)

ASTM D2584

50 ± 5

a

Values derived from a limited series of ‘round-robin’ testing of stock standard structural shapes, 6 mm thick. No emphasis was placed on glass loading or placement; however, the testing fell within a very close range. b The use of surface veils – tissues – can affect the Barcol value. Phenolic-based composites can also give false readings.

This practice can minimise the initial tooling investment in a much more comprehensive way than is possible with prototype tooling, where the latter is often just a shorter version of the production tool, saving basically only the cost of the tool steel. Even prototype tools have to be machined, polished and plated, operations that account for the bulk of the tooling costs. In addition and owing to their shorter length, they may not produce totally satisfactory trial profiles for evaluation, and an eventual increase in their length with tool extension pieces is also certainly not to be recommended. As already referenced, additional design and mechanical property reference data are also provided through the two authoritative associations which very actively support the pultrusion industry (Table 3.8).3,4

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Pultrusion for engineers

3.6

Process characteristics

As a continuous machine production process, pultrusion with its low direct labour requirement of often below 10% demonstrates an important advantage over many alternative composite fabrication techniques, even when they are capital-intensive. If, as is usual, the cost of manufacture inclusive of wages and benefits is related to the quantity of finished product then, as the latter increases, the allocation of those costs for a given length of pultruded profile decreases. This has the obvious potential to allow for either a price reduction to the customer or an additional profit margin to the pultruder. However, unlike other composite fabrication techniques the process allows for additional ways of enhancing productivity beyond that optimum level initially thought practical for a particular profile in respect of die design and its operating parameters such as pulling speed. In other words, because of the nature of the process, enhanced productivity can frequently be secured whenever found feasible by running multiple streams of product to the fullest extent of the available machine envelope capacity. For example, a single 100 mm (4 inch) wide profile pulled on a machine with a 600 mm (24 inch) envelope is totally inefficient. Adding a second line results in a 100% improvement in output with an important 50% reduction in the allocated labour content per unit length. Further lines have successively greater impacts, although problems of additional material handling, maintenance and inspection, etc, can eventually seriously degrade those gains. The pultrusion process also benefits from what is virtually a total conversion of the matrix and reinforcement entering the die, into a cured composite and moreover a finished product demanding even at worst very limited secondary finishing. Compared with other composite fabrication processes then, the process provides a very high percentage yield from those two raw materials. Although resin and fibre waste does occur at both startup and shut-down, and during any production run owing to processing and specification-related causes, the proportion in comparison to finished product and these other techniques is much less. Applying diligent control, thoughtful process designs and proper scheduling assist in keeping the total waste to better than 3% and often as low as 1% where long production runs are feasible. Levels approaching 10% only apply where short runs are essential, or for infrequently produced custom profiles and those which are required to meet tight tolerance requirements.

3.7

Conclusion

Pultruded profiles are increasingly becoming a more viable structural material for a wide variety of applications. Much of that recognition and accep-

Profile design, specification, properties and related matters

91

tance by the engineer and designer has accrued from the better knowledge, understanding and use of the many process and related parameters reviewed by this chapter. Further explanation and confirmation of this structural viability and wide application spectrum is the purpose of the following chapters, which collectively enable the confident prediction that the pultrusion industry will continue to grow in the foreseeable future by as much as 15% per annum. Indeed there is potential for even that figure to be exceeded as a number of developments now in hand are commercialised. The design freedom of pultrusion is a major part of the reason for its success, coupled to the unique properties of the reinforced plastic composite and the ever-expanding skills of the many companies who remain excited by the technical and market challenge offered by the process. From its humble beginnings as a means to produce kite rods and fishing rod blanks, to the current position where entire composite housing units, railcar bodies and bridges are being constructed from basically simple pultruded profiles, there is clear demonstration that for the engineer it is the composites process of the future.

3.8

References

1. Structural Plastics Design Manual, FHWA-TS-79-203, US Government Printing Office, Washington DC, 1979. 2. TOPDIETM Pultrusion Dynamics Technology Center, Oakwood Village, OH, USA. 3. Pultrusion Industry Council of the Composites Institute, Recommended Specification for Materials Used in Pultruded Shapes, Society of the Plastics Industry, USA (now merged with the Composites Fabricators Association’s Pultrusion Growth Alliance, also of the USA). 4. EPTA, Standard for Pultruded Composite Structural Profiles, European Pultrusion Technology Association, Leusden, The Netherlands, 1998. 5. Sims, Graham D, ‘Standardisation of structural pultruded composites’, Composites: Design Data and Methods, Centre for Materials Measurement & Technology, National Physical Laboratory, Teddington, UK. 6. Sims, Graham D, ‘Pulling together on European standards for pultruded profiles’, Composites: Design Data and Methods, Centre for Materials Measurement & Technology, National Physical Laboratory, Teddington, UK. 7. Creative Pultrusions, The Pultex® Pultrusion Design Manual, Creative Pultrusions Inc, Alum Bank, PA, USA. 8. Fibreglass Ltd, Design Data Fibreglass Composites, Fibreglass Limited, St Helens, UK (as this authoritative publication is now no longer published, a suitable alternative reference is provided by reference 9). 9. Quinn, J A, Composites – Design Manual, James Quinn Associates Ltd, Liverpool, UK.

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Pultrusion for engineers

Appendix: important standards General ASTM D-2563-70

Standard recommended practice for classifying visual defects in glass reinforced plastic laminate parts.

Pultrusion – directly related API Spec 11C-88 ASTM A-1405 ASTM D-3647-84 ASTM D-3917-84 ASTM D-3918-80 ASTM F-1092 BS 6128: Part 3–81 BS 6128: Part 5–81

BS 6128: Part 7–81

MPI-P-79C(2)-64

Specification for reinforced plastic sucker rods. Ladder rail specification, underwriter laboratories. Standard practice for classifying reinforced plastic pultruded shapes according to composition. Standard specification for dimensional tolerance of glass-reinforced plastic pultruded shapes. Standard definitions of terms relating to reinforced plastic pultruded products. Fiberglass handrail specification. Industrial laminated rods and tubes based on thermosetting resins specification for round pultruded rods. Industrial laminated rods and tubes based on thermosetting resins: specification for rectangular pultruded rods. Industrial laminated rods and tubes based on thermosetting resins: specification for hexagonal pultruded rods. Plastic rod and tube, thermosetting, laminated.

Pultrusion – indirectly related BS 6128: Part 1–81 DIN 40618-06.71 JIS K 6914-77 JIS K 7011-89

Industrial laminated rods and tubes based on thermosetting resins: classification and methods of test. Laminated products; laminated moulded tubes of paper-base laminate or fabric-base laminate. Laminated thermosetting tubes. Glassfibre reinforced plastics for structural use.

Raw materials ASTM D-1763-81 BS 3396: Part 3–87

Specification for epoxy resin. Woven glass fibre fabrics for plastic reinforcement – specification for finished fabrics for use with polyester resin systems.

Profile design, specification, properties and related matters

93

BS 3496-73

Specification required for E-glass fibre chopped strand mat for the reinforcement of polyester resin system.

BS 3496-89

Specification for E-glass fibre chopped strand mat for reinforcement of polyester and other liquid laminating systems.

BS 3691-69

Specification for glass fibre roving for the reinforcement of polyester and epoxide resin systems.

BS 3749-74

Specification for woven roving fabrics of E-glass fibre for the reinforcement of polyester resin systems.

ISO 3672/1-79

Plastics – Unsaturated polyester resins – Part 1: Designation.

ISO 3673/1-80

Plastics – Epoxide resins – Part 1.

ISO 3342-88

Textile glass – Mats – Determination of fracture strength by traction.

JIS K 6919-92

Liquid unsaturated polyester resin for reinforced plastics.

JIS R 3411-84

Textile glass chopped strand mats.

JIS R 3412-84

Glass rovings.

JIS R 3417-84

Woven roving glass fabrics.

MIL-G-9084C-70

Glass cloth, finished for resin laminates.

MIL-M-15617A(1)-75

Mat, fibrous glass for reinforcing plastics.

NF B 38-205-83

Textile glass – fabrics – basis for a specification.

NF B 38-301-78

Textile glass – mats for reinforcement (made from chopped or continuous strand) – basis for specification.

Terminology BS 1755: Part 1 – 1982

Glossary of terms used in the plastics industry.

BS 1755: Part 2 – 1974

Glossary of terms used in the plastics industry.

ISO 472-88

Plastics – Vocabulary.

ISO 6355-89

Textile glass – Vocabulary.

Testing – composites generally ASTM D-256

Test methods for impact resistance of plastics and electrical insulating materials.

ASTM D-570

Test method for water absorption of plastics.

ASTM D-635

Test method for rate of burning and/or extent and time of burning of self-supporting plastics in a horizontal position.

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Pultrusion for engineers

ASTM D-638-89

Standard test methods for tensile properties of plastics.

ASTM D-696

Test method for coefficient of linear thermal expansion of plastics.

ASTM D-790-86

Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials.

ASTM D-792

Test methods for specific gravity and density of plastics by displacement.

ASTM D-2343

Test methods for tensile properties of glass fiber strands, yarns and rovings used in reinforced plastics.

ASTM D-2344

Apparent interlaminar sheer strength of parallel fiber composites by short-beam method.

ASTM D-2583

Test method for indentation hardness of rigid plastics by means of Barcol impressor.

ASTM D-2584

Test method for ignition loss of cured reinforced resins.

ASTM D-2734

Test methods for void content of reinforced plastics.

ASTM D-3846

Test method for in-plane shear strength of reinforced plastics.

ASTM D-4357

Specifications for plastic laminates made from woven roving and woven yarn glass fabrics.

ASTM E84

Test method for surface burning characteristics of burning materials.

BS 2782: Pt 10-M1006-77

Methods of testing plastics: glass reinforced plastics: measurement of hardness by means of a Barcol impressor.

BS 2782: Pt 10 Method 1008A-D

See ISO 3597

BS EN ISO 527 1997

Part 4. Plastics – determination of tensile properties. Test conditions for isotropic and orthotropic fibre reinforced plastic composites. Part 5. Plastics – determination of tensile properties. Test conditions for unidirectional fibre reinforced plastic composites.

DIN EN60-11.77

Glass reinforced plastics: determination of the loss on ignition.

EN ISO 14,125 1998

Fibre reinforced plastic composites – determination of flexural properties.

EN ISO 14,129 1999

Fibre reinforced plastic composites – determination of the in-plane shear stress/shear strain, including the inplane shear modulus and strength by the ±45° tension test method.

Profile design, specification, properties and related matters

95

EN ISO 14,130 1997

Fibre reinforced plastic composites – determination of apparent interlaminar shear strength by short beam method.

ISO 15,310 1999

Fibre reinforced plastic composites – determination of in-plane shear modulus by plate twist.

ISO 2114-74

Plastics – unsaturated polyester resins – determination of acid value.

ISO 25356-74

Plastics – unsaturated polyester resins – measurement of gel time at 25 °C.

ISO 2555-89

Plastics – resins in the liquid state or as emulsions or dispersions – determination of apparent viscosity by the Brookfield test method.

ISO 3268-78

Plastics – glass reinforced materials – determination of tensile properties.

ISO 3597-77

Textile glass reinforced plastics – determination of mechanical properties on rods made of rovingreinforced resin. Part 1. General considerations and preparation of rods. Part 2. Determination of flexure strength. Part 3. Determination of compressive strength. Part 4. Determination of interlaminar shear strength.

ISO 8515

Textile glass reinforced plastics – determination of compression properties parallel to the laminate.

ISO 4585-89

Textile glass reinforced plastics – determination of apparent inter-laminate shear properties by shortbeam test.

NF T 51-514-78

Unsaturated polyester resins – conventiona determination of reactivity at 80 °C.

Testing – pultruded profiles ASTM D-3914-84

Standard test method for in-plane shear strength of pultruded glass reinforced plastic rod.

ASTM D-3916-84

Standard test method for tensile properties of pultruded glass fiber reinforced plastic rod.

ASTM D-4385-84a

Standard practice for classifying visual defects in thermosetting reinforced plastic pultruded products.

ASTM D-4475

Standard test method for apparent horizontal shear strength of pultruded reinforced plastic rods by the short-beam method.

ASTM D-4476-85

Standard test method for flexural properties of fiber reinforced pultruded rods.

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Pultrusion for engineers

ASTM D-5028

Standard test method curing properties of pultrusion resins by thermal analysis.

ASTM D-5117-90

Standard test method for dye penetration of solid fiberglass reinforced pultruded stock.

ISO/CD 1268-8

Fibre reinforced plastics – preparation of test plates, pultrusion.