Basic Hand Lay-up Techniques for Reinforced Composites

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REVISED: JUNE 2002.GC

BASIC HAND LAY-UP TECHNIQUESFOR REINFORCED COMPOSITES

Technical Data

Bulletin

Nuplex Industries Limited Fibreglass International

6 Winston Place, Henderson; PO Box 21018, Henderson, Auckland Telephone: (09) 836 9717 Facsimile: (09) 838 261655 Montreal Street, Sydenham: PO Box 7501, Christchurch Telephone: (03) 366 0409 Facsimile (03) 377 2523

WebSite:www.nuplexcomposites.co.nz- Email:composi [email protected] - Freephone 0800 80 3001

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BASIC HAND LAY-UP TECHNIQUESPage 2 of 7

BASIC HAND LAY-UP TECHNIQUES

Normally the work is carried out in a female mould a FRP (Fibre Reinforced Plastic) mould

with a polished tooling gelcoat surface on the inside. Having acquired and set up the mould ata convenient working height in the workshop, the following procedure should be adopted:

(1) Wash the mould carefully with warm water and soft soap to remove any old PVA releaseagent, dust, grease, finger marks, etc.

(2) Dry the mould thoroughly.(3) Check the mould surface for chips or blemishes. These should be repaired by filling with

polyester filler and cutting back with wet and dry paper. The odd small chip can be

temporarilyrepaired by filling with plasticine or plaster filler. If resurfacing of the mould isrequired refer to the Duratec technical data.

(4) If the mould surface is in good condition the mould release paste wax is now applied, witha circular action, using a small piece of cloth. One coat of wax is sufficient for a mouldsurface which has been previously broken in but a new mould surface will require at leastsix applications. Each application is polished up to a high shine with a large piece ofcheesecloth, after being left to harden for 40-45 minutes. Care must be taken to removeall streaks of wax from the mould surface. Polishing is correctly done when there is arubbed over mould surface. Check application with manufacturers instructions.

Note: When using PVA, you should use a mould release paste wax that contains nosilicone and therefore compatible with the PVA, eg. TR108.

(5) Apply the polyvinyl alcohol solution carefully with a piece of sponge or foam rubber.Some PVA solutions are coloured (say green or red) to assist in getting an even andcomplete coverage, but if the PVA is applied against the light and you workprogressively from one end of the mould, you should have no problems. Avoid going overthe mould surface more than once with the sponge as this may lift the layer of PVApreviously applied. PVA solution contains water and so must be allowed to dry completelybefore the gelcoat is applied. At normal room temperature (approximately 21C [70F])this takes about 3 hours (or overnight if cooler). PVA can also be sprayed from a gravityfeed gun at low pressure and applied as a mist coat.

(6) The gelcoat is now weighed out and the correct quantity of catalyst stirred in, ie. between1 - 2% by weight. In industry the gelcoat is frequently sprayed onto the mould surfaceor applied with large lambswool rollers. For amateur use, however, a soft bristle (50-120mm [2-5in]) brush is the most suitable. Care must be taken to ensure an evencoverage of gelcoat resin, free from blobs, deep ridges, shallow furrows and air bubbles.

Practice makes perfect and very quickly you will learn to work from one end of the mould,

spreading the gelcoat with light even strokes in one direction only. Vigorous paintingstrokes give too thin a gelcoat coverage and can sometimes cause the PVA to lift from themould surface. The best practice is to ensure the brush always keeps a wet edge. Thegelcoat surface is applied very thickly compared to a painted surface. To avoid serious


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gelcoat problems, it is safer to err on the slightly heavy side, rather than apply too thin acoating.

The gelcoat should have as short a pot life as possible, within reason (say about 10-15minutes), as lengthy gel time on the mould allows excessive evaporation of the styrenemonomer and this could result in permanent undercure of the gelcoat. Also a longgelation period can allow the styrene in the gelcoat, to attack the mould surface andcause a polystyrene build-up. This will spoil the highly polished appearance of both themould and the moulding.

Ideally, the lay-up of the fibreglass reinforcement should commence as soon as thegelcoat has cured sufficiently to be hard to the touch. This stage of the cure may takefrom one to four hours depending on such factors as catalyst levels, working temperature,

humidity and mould shape. One test is to touch the lowest point in the mould gently withthe fingertips. The gelcoat should feel tacky but solid, and no pigmentation should comeoff on the finger. Another useful test is to gently rub gently this part of the mould with arag dipped in acetone. Slightly pigmentation will be removed from the tacky surface butno softening of the gelcoat film should occur. The lowest point in the mould is used as anindicator since this is the last part to cure.

It is important to note that the gelcoats are normally air inhibited, that is, they remainslightly tacky during cure so that a good chemical bond can be established between thegelcoat and the first glass fibre reinforcement layer. While the gelcoat should not be

allowed to overcure, it is equally important that the lay-up of the polyester resin andglassfibre reinforcement should not be commenced until the gelcoat has reached thestage mentioned above, otherwise styrene in the polyester resin will soften and wrinklethe gelcoat, causing unsightly blemishes in the finished moulding called crocodiling oralligatoring.

Leaving the gelcoat on the mould for too long a period prior to lay-up of thereinforcement material may cause the useful tacky period of the cure to be lost. At thesame time, dust, girt and moisture (humidity) deposits, which could affect adversely thebonding of gelcoat to laminate, may settle on the surface. If a mould must be left for aperiod hours after gelcoat application it should be placed under a polythene tent or, inthe cause of a small mould, in a cupboard or if possible inverted by suspending it from awire.

If it is not possible to apply the laminating resin for some time, adhesion between gelcoatand laminate can be improved by giving the gelcoat surface a thorough acetone washprior to the glassfibre lay-up. It is essential to dry off all acetone before applying thelaminating resin.

LAYING UP A FRP MOULDING

After the gelcoat has cured, the moulding is reinforced by bonding some form of glassfibrereinforcement to the gelcoat with polyester laminating resin. In amateur mouldings the glassusually takes the form of chopped strand mat because of the cost and ease of application.However, tissue, tapes, woven rovings and cloths may also be used and each will impart


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specific strength and performance factors to the moulding. Since the lay-up technique is thesame for all types of reinforcement, we shall deal here with the more common CSM.

Note: When using woven rovings, to eliminate print through, woven rovings should be onlyapplied after 600-900grms CSM has been applied first.

The glassfibre required for the lay-up should have been previously cut to the shape of themould by means of plywood templates, paper patterns or by approximate measurements takendirectly from the mould. A steel tape measure is useful here as it can be easily flexed to followthe contours of the mould and then laid out on the cutting table and used as a straight edge.

The CSM should be cut so that when layed in the mould, it should not extend beyond the edgeof the mould by more than about 25mm (1in), otherwise the excess material may sag when

wet and pull the mould material away from the surface, leaving an air cavity between mat andgelcoat.

The lay-up takes place in the following stages:

(1) The laminating resin is weighed out, pigmented to match the gelcoat (if desired) and thencatalysed.

(2) A brush or roller is used to apply a liberal coating of resin to the gelcoat surface.(3)

A shaped layer of CSM is placed carefully in the mould and the previously applied resinworked through this. Additional resin is applied to the CSM surface at this stage and allfibres are stippled or rolled until thoroughly and evenly wetted out with resin.

The first layer is very important as it reinforces the gelcoat and prevents air pockets fromforming behind it. For this reason a layer of surface tissue or a light weight CSM such as200 or 225gm/m is often laid up first. This ensures that the CSM will not pull out of tightcorners or uneven spots in the gelcoat. In most amateur work, however, tight cornersshould be avoided and moulds with large radius curves used. In this case the standardweight CSM can be successfully used for the initial lay-up.

(4) A second layer of CSM is applied as soon as the first is thoroughly wetted out and asmuch resin as possible is stripped through the mat from the previous layer. Additionalresin is added sparingly by brush, the object being achieve a thorough wet out of the CSMwith the minimum quantity resin.

(5) Immediately after the second layer of CSM has been applied a compression roller (washerdisc or vane type) is used to compress the mat and squeeze air bubbles and excess resinfrom the laminate. This technique appreciably improves the strength of the moulding byincreasing its density and reducing its porosity on the inside surface, so it is important thatthe rollers are used firmly and evenly across the entire surface.

Note: If a high standard of finish is required with minimal print through of thereinforcement, then after the second ply of CSM or reinforcement has been applied theapplied reinforcement should be allowed to cure fully before continuing with further


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laminating.

(6) Successive layers of laminate are now applied to the mould until the lay-up is complete.Each is compression rolled as described above. The number of layers required willdepend on the type of moulding and the structural stresses it will need to withstand inuse.

(7) When the resin in the lay-up has gelled sufficiently to allow the laminate to be trimmedwith a Stanley knife or plane blade the cure has reached what is known as the greenstage. This is the ideal time to trim all edges and usually lasts for about 15-20 minutes.If trimming is delayed until the initial cure is complete a coping saw and surform will berequired. Best results are achieved by using a diamond-cutting blade. This is hard workand often takes more time than that involved in the actual lay-up. Trimming at the

green stage on the other hand, is the work of only a few minutes and if done carefullygives a clean, precise cut along the edge of the mould. At the same time excess gel fromthe flange of the mould is also removed. The only further edge treatment required then isa rub with garnet paper after the moulding has been demoulded.

NOTES ON LAY-UP TECHNIQUES

The glassfibre laminate used in any FRP moulding can be made up from different types ofglassfibre material depending on the specific function of the end product. For example wovenrovings add bulk and therefore rigidity to a moulding. Finely woven glassfibre and nylon cloths

create flexibility and resilience. Chopped strand mat gives a multi-directional rigidity, whilecontinuous roving filaments give uni-directional rigidity and strength as required in vaultingpoles. Woven glassfibre cloth and tape can be soaked in resin and would round a mandrel togive a hollow tube (filament would laminate) with great axial strength as used for fishing rods,radio aerials and yacht masts.

Any GRP laminate will generate heat during the polymerisation of the resin so it is important tolimit the thickness of the lay-up to 4 or 5 layers of 450gm/m (1oz/ft) CSM or equivalent ata time. Otherwise, there will be a risk of over heating the moulding, which could result inwarping and weakening of finished product. Even more importantly, perhaps, there is a risk ofdamage to the mould surface due to excessive heat build-up.

In addition to the reinforcing used in normal lamination, a FRP moulding can be furtherreinforced at specific points simply by increasing the thickness of the reinforcement materialduring the lay-up procedure. Overlapping two sheets of CSM on a corner, for example, offerstwice the thickness and thus greatly increased strength at that point. Additional reinforcementcan be added by using a wide range of materials, which are moulded into the job after theinitial lay-up has been cured. Some of these materials are:

Strip strengthening materials

These can be in the form of timber, carbon fibre or polyurethane foam strips, cardboard orplastic tubes, paper mache, rope etc. For suitability of use please contact your nearest Nuplexrepresentative.


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These materials, when bonded onto the inside surface of the cured laminate act ascorrugations and as such give the moulding rigidity with minimum weight gain. When weightgain is critical, tubes of polythene can be used for this purpose and pulled out of the mould

when the overlay of glassfibre has reached the toffee stage.

Area strengthening materials

These usually consist of core materials such as polyurethane foam slab stock, end grain balsaor Klegecell closed PVC Foam. Another well used product is Coremat which is treated similar toa ply of CSM as it is incorporated into the laminate. The major difference to other corematerials is that the Coremat is pre-wetted prior to application to the lamination.

FASTENINGS

These are generally metal or timber fittings that are bonded into a FRP laminate either beforeor after removal from the mould. If possible, total cure of 14 days should be allowed beforeheavy, localised fittings are bonded into a moulding, as there is a risk of local shrinkage ofbonding laminate layers. This creates a print through on the gelcoat side of the moulding,which appears as an indentation on the otherwise smooth gelcoat following the contours of thefitting. It looks unsightly and in extreme cases can cause cracking of the gelcoat andweakening of the laminate layers of the moulding.

Note: When bonding fastenings or timber fittings make sure you use a correctly formulated

putty. If you are making your own putty NEVER USE A WAXED OR LSE LAMINATING RESIN asthey contain waxes that will inhibit adequate bonding.

PULLING THE MOULDING FROM THE MOULD

In industry, compressed air or pressurised water is sometimes used between mould andmoulding to help release moulding with deep draw from the mould. Nozzles are embedded inthe bottom of the mould and covered with tape or a plug of plasticine before gelcoat lay-upcommences, and the moulding is floated or popped out of the mould on completion. Splitmoulds are also used where difficult shaped mouldings are made. The mould is in two halveswhich can be unbolted and easily levered apart, thus cracking the moulding clear of themould.

In the home workshop, however, these methods are usually neither practical nor necessary.Shallow moulds, with reasonable draw released by flexing gently. Slight twist may also beapplied to the mould to help break the bond and as a rule this is the only effort required to

crack out a shallow moulding, provided it has been correctly trimmed to the edge of the mouldand the release agents have been properly applied. Wedges of soft material can also be gentlyused.

Flexing the mould with start the moulding at one or two points and strips of soft plywood or

masonite can then be wedged between mould and moulding. The wedges are then carefullypulled along the top edge of the moulding until it is released from the mould all round. If needbe, the strips can then be pressed further down round the contours of the mould until the totalrelease is obtained. Under no circumstances should chisels, screwdrivers or plane blades be


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used to wedge or lever the moulding from the mould. This will result in scratching or evenchipping of the mould and would ruin it for future mouldings.

The moulding should be left to cure for as long as possible in its mould total cure takes about28 days, although most mouldings are rigid enough to resist warping after 24 hours. If,however, the moulding has a tendency to flop or sag slightly after removal from the mould, itshould be supported in some way to prevent permanent distortion during further curing.

FLOCOTING

After all reinforcements (if any) have been bonded into the moulding the inside surface isusually rubbed down with 80 grit garnet paper to remove any sharp fibres, the dust is removedand a flocote applied to the laminate. The flocote is polyester resin containing about 3%

paraffin wax dissolved in styrene. The functions of a flocote are:

(1) To seal the laminate surface from attack by moisture, grit, grease, chemicals, etc.(2) To remove the tacky feel from the laminating resin used in the lay-up, if an unwaxed

resin has been used for laminating.

(3) To give the moulding a clean internal finish with colour uniformity, if desired.The wax floats to the surface of the flocote and prevents air inhibition of the resin cure which

would leave the surface tacky. The result is a hard, shiny surface free from porosity and sharpfibres.

Note: By using different spray techniques and/or Webbing Solution, spatter or cob-webbingsurface effects can be achieved. For further information please contact your nearest Nuplexrepresentative.

Basic Hand Lay-up Techniques for Reinforced Composites

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