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 Mouldings

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PostSubject: Mouldings   Tue Oct 08, 2013 1:40 pm

Glassfibre moulding is not normally undertaken by many people, but it is surprisingly easy if a little time consuming and messy, and with prices of some of the more exotic composite materials coming down it is a reasonably cheap way of making high strength components for an off road vehicle, recently we made a set of lightweight carbon fibre under body protection for a 4X4.

Glassfibre or GRP (glass reinforced plastic) uses a combination of materials to make a composite, the glassfibre itself is nothing more than a polyester resin which comes as a liquid in pre-accelerated form (always use pre-accelerated) and an activator or hardener which are mixed together, once mixe they create an exothermic reaction (create heat) which cures the liquid into a solid substance. On its own it is fairly weak and brittle so another material is added to give it strength and this is called glassfibre tissue and glassfibre mat, and these are defined by their weight in ounces per square yard or grammes per square metre.
To make this composite you apply a couple of coats of polyester resin to your mould and let them dry between coats, then you add a third coat of resin and push the glassfibre tissue into it, you then add more resin to wet out the tissue fully and you see it change colour as it becomes wetted out, but you need to wet it out fully with no dry spots and force the resin fully into the glassfibre tissue. You let this harden and repeat with another layer of glassfibre tissue, then you let this dry and them repeat with a varying number of coats of glassfibre mat to build up thickness to your required thickness or strength.

Glassfibre tissue is very fine and is the first layer used as it is so fine and doesn't show through the surface in the finish, it adds little strength on its own, then you add various layers of glassfibre mat which is much stronger and gives a much stronger composite. Glassfibre mat and tissue is called rovings or CSM (chopped strand mat) as they are just dropped in a random pattern and work equally well in providing strength in all directions. There is something called glassfibre tape which is not random rovings as this is woven lengths of tape in various widths and this can be a directional tape or non directional tape which means the type of weave determines which way its strength is in the tape. Tape takes much more wetting out than CSM or tissue so allow more time to apply it and wet it out.

Once the mat or tissue is wetted out you have two options, one is to use the paint brush and force it down quickly and repeatedly which is called stippling, and just flatten any matting which stands high or proud, and rolling which is done with a metal roller, the object of stippling and rolling is to remove any trapped air bubbles which would weaken your composite.

There are other alternatives to glassfibre tissue and matting and this is either carbon fibre cloth and tape, or Kevlar cloth and tape and they are essentially the same as glassfibre tissue and mat, just a different material which takes slightly more wetting out but is much stronger and lighter, and a hybrid material which is simply a carbon fibre/Kevlar mix and comes as cloth or tape.

When working with polyester resin you will find it cures very quickly once the hardener is mixed with it the working time is short so work quickly; but you can increase the working time of your mixed polyester resin by cooling it, normally I use plastic cups and stand them in a bowl of ice cubes, sounds a little dramatic but it is very effective.

Other strengtheners are available, the most common is aluminium mesh and this comes as a very thin mesh which is very pliable, but you can also use clean timber such as softwoods, or even obscure items such as piano wire which us very light and very strong. If you have bolt holes in your moulding you can include thin steel strips for strength or even large penny washers and laminate them in as you build up your composite. If you bond porous materials such as wood in as reinforcement it is better to apply a thinned coat of polyester resin to the timber first to seal it and this is thinned with a cleaning agent called acetone, this is also used for cleaning your brushes and rolling equipment.

As well as polyester resin you can now get epoxy resins and these work in exactly the same way as polyester resins, pour out a small quantity of liquid resin and mix in your hardener and stir, this is beneficial as it gives less shrinkage than polyester resin, is around 50% stronger, has higher adhesion to most materials and has more chemical resistance, but it costs more then polyester resin. By substituting epoxy resins for polyester resins the additional cost is offset by the fact that for a given strength you use much less epoxy resin then you do polyester resin.
Epoxy resins work better with carbon fibre and Kevlar and give a lightweight component with immense strength, this is why formula 1 cars are made from it as are Aston Martin, Ferrari, Lamborghini, and many other high performance road cars.

Mould making is remarkable simple and there is a choice of materials available, you can use GRP resin and matting or you can use anything such as plaster or even liquid latex for more intricate shapes. Once a mould is made you need to seal any porous materials such as plaster and polish them with a non silicone wax, the polishing is the crucial part as the less polishing you do, the harder the part is to remove from the mould. Experience is the key to correctly polishing a mould so don't skimp on this stage, once its correctly polished you apply a few coats of PVA release agent.
Making a mould is time consuming as you need to get the finish flawless as any imperfections will show up in your finished component, so spend the time to get this right.

Plaster Moulds - you can use most types of plaster, for a fine finish use casting plaster, if the finish is not so crucial then a standard pink finish plaster as used by plasterers in your home will suffice as this is cheaper than casting plaster and it comes in decent sized bags. Take your object you wan't to take a mould from and clean it, if it is something such as steel it needs fetching back to bare metal and polishing well and you mix your plaster and apply it with a trowel and build it up in thickness, let it dry and add reinforcing to strengthen it. Remove your component and lightly sand out any inperfections in the finish, or ass a little more plaster to any holes, let it dry, then sand with fine wet and dry paper to give a perfect finish.
Seal your plaster mould in one of several ways, you can polish it with non silicone wax but this takes days as it takes time to dry and to build up the required number of coats required to seal it; or you spray it with a quick drying compound such as shellac or even car paint lacquer which is the sort used in basecoat and clear systems, this dries quickly and several coats can be applied and dry overnight. Wet sand with 1200 grit wet and dry used wet to remove any imperfections, polish, apply PVA release agent, and then mould your item.

For GRP moulds the principle is basically the same except you clean your item, polish it well, then apply PVA release agent to your item you are using to make your mould from.

Liquid latex is even simpler, clean, polish, apply PVA release agent, and for larger items you build a simple wooden frame slightly larger than your item, suspend the item in the box, mix the liquid latex and pour it into your mould around your item; allow it to harden then just prise your component used to make the mould from the latex, remember latex is very pliable and flexible and better for intricate components.

Plaster Bandages - these are the same thing they use in hospitals when you break your arm or leg, they are better for larger intricate items, follow the procedure for casting plaster, dip the plaster bandage in clean water for a couple of minutes to soak it and just wrap it around your component and force it flat into any awkward shaped areas, allow to dry and remove your component to be moulded.



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PostSubject: Re: Mouldings   Tue Oct 08, 2013 9:03 pm

More advanced mouldings are possible, but these are nothing more then finished mouldings with a finished surface and these require a slightly different approach, where a finished surface is required we apply something called a gel coat to the mould before we apply our polyester resin and glassfibre mat.

Gel coats are nothing more than a different type of resin called a gel coat and this is polyester resin which in simple terms is finer and contains a colouring agent called a tinter, this tinter is available separately so you can mix your own colour, or you can buy gel coat resin ready coloured in a variety of standard colours.

When applying a coloured gel coat you apply 2-3 coats directly to your mould and this is the finished, visible surface, let this cure slightly as it never fully cures as it requires all the air to be removed before it will cure fully (air drying gel coat is available if expensive) and once partially cured you apply your two coats of polyester resin to it, this seals it, then apply your matting in the normal manner.

Many complex shapes can be moulded with GRP and this is where glassfibre comes into its own, by making a moulding of a complex shape in one go you can save hours of fabrication time if you used steel or other metals. Complex mouldings can also be made in sections and bonded together and epoxy if fine for bonding them together, you then laminate them by using a strip of glassfibre mat and apply it over the joint of both components and wet it out, then apply a couple more pieces over the top and progressively widen these strips so they overlap and cover the previous strips.

Glassfibre does have limitations and these are that polyester resin contains something called styrene and this is poisonous if ingested, so never use it for drinking water tanks as styrene leaches out into water to contaminate it, and you always get several small pockets of uncured glassfibre no matter how careful you are, so avoid using glassfibre for water tanks.
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PostSubject: Re: Mouldings   Wed Oct 09, 2013 5:10 am

Through a sponsorship deal, a company supplied fibreglass body parts for my Range Rover Classic when competing in the 1989 Paris-Dakar Rally Raid. They are long since defunct unforntuantely, but I have a certain Chris Dawson to thank for that - he's now the man behind Silverline 4x4 incidentally! The products he supplied were ace! Although very strong, the bonnet you could lift with one finger - on a 2-door Rangie this is impossible with a steel bonnet - the front and rear wings were also lightweight and strong, and the top and bottom tailgates were equally as good. The bottom tailgate was a revelation. You could (and I would to demonstrate it's strength) jump up and down on the open tailgate, yet again it was so light.

Coupled with trimming down the steel doors by disposing of the glass and winding mechanism - the windows were replaced with custom-made sliding perspex units - all this lightening meant we stood a fighting chance of avoiding getting stuck in the sand dunes ... and it worked. Unlike most entrants, even the factory teams, I'me proud to say that we never got stuck once in the Sahara! Okay, a heavy right foot also helped as we'd skim over the worst of it rather than plough though it, but it goes to show that fibleglass panels can make a BIG difference.

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PostSubject: Re: Mouldings   Thu Oct 10, 2013 12:32 pm

Any weight reduction on a vehicle means a higher payload can be carried Tom, and of course you have mentioned the rest.

With epoxy resins and Kevlar and carbon fibre becoming cheaper people can begin to make many of their own mouldings much cheaper than they can buy them, and even make accessories for their vehicles.
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PostSubject: Re: Mouldings   Fri Oct 11, 2013 8:57 am

Never tackled a GRP moulding personally, but it can't be that hard surely ... can it? affraid 


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PostSubject: Re: Mouldings   Fri Oct 11, 2013 10:56 am

No, its like spraying, its 90% preparation.

Once you have the technique its very easy if a little messy.
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PostSubject: Re: Mouldings   Sat Oct 12, 2013 5:14 am

Sounds a lot like sex - you just know it has to be done, but the prep can be boring and you really want to get on with it!!! Twisted Evil 

As for your last sentence, although that is also oh so true, think we'll skip over that bit! affraid affraid affraid

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PostSubject: Re: Mouldings   Sat Oct 12, 2013 10:50 am

I thought you skipped straight TO that bit, not OVER it.
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PostSubject: Re: Mouldings   Sun Oct 13, 2013 12:18 am

lol! 

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