Fibreglass Construction

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AlistairM

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HI

I have been reading around this for a while now (purely out of interest, not planning any projects just yet!!) - One thing I haven't been able to work out or found anyone actually comment on as yet, is - when the "mold" is constructed, how does the very first layer of mat get applied. I assume there must be some technique or former that is used as with even the most well battened mold there will still be gaps that the mat will cover with out any support, so how do you stop it sagging at these points.

Also I have never been able to understand how you would decide on the number of layers to apply, how does one work out the thickness and then decide to stop applying further material?

Only musings so not really of any great importance but if anyone could answer these it would help to fill a "void" in my current understanding.

Thanks

Alistair
 
The mold is 'solid'. It's just a reverse of the shape you wish to construct, but is as solid and as smooth as the finished piece.

The thickness is dependent on the strength or stiffness you require. It's calculated by a variety of means from 'seat of the pants' experience through to design from first principals, depending on the application.
 
Suggest you go down the back numbers of PBO and find the series of articles on constructing the mould for the Crabber 26. A complete hull is constructed, often strip planned and given a fine external finish and then a female mould made. The hull may well then be finished as a boat. Latest techniques from big builders use solid foam male moulds cut by CNC multi axis milling machines.
 
Alistair: the thing the mould is made from is called a "plug" which, as you've read above, will have the form and shape of whatever it is you're intending eventually to make. The plug is often just a "use once and trash" affair, but it used to be fairly common to build an entire hull, usually out of wood, as a plug from which to make the mould. Some of these wooden plugs became boats themselves (although obviously built to a far higher structural standard than an ordinary plug).
 
Alistair

Moulds and plugs come in basically two types , Male /plugs or female/moulds.

If using a female mould, the shape produced inside will be the final shape of the outside of the object eg the hull in the case of a boat. The mould is coated in Gelcoat inside which produces the outside finish with any discrepancies in it hence the need for it to be near perfect. Once the Gel coat is laid / sprayed on, the glass mat follows over the top to the required thickness.

If using a male mould/Plug the finished item is the inside face.

Back in the day boat builders could use sheets of ply cut to profiles, cover frames with battens, then a layer of foam, then filler, then sand/polish to a finish and start laying up from that. This could then form the basis of the female mould.

Description is only a basic one but hope it helps in some way.
 
One-off construction in GRP is not likely to be an economic proposition. Creating the mould is a major investment; as others have noted it needs to have a very high surface finish to produce a high quality hull. Using the mould to produce a single hull would probably cost far more than simply creating a hull using the techniques used to create a mould! GRP production (like most plastics production) is only economic when the mould is used to create many copies of the article.

There are yacht designs for which the moulds are available, but I know nothing about the way in which the owners permit thier use.

There are alternative methods of composite construction, such as the "stitch and glue" method using plywood or other sheet materials, which effectively creates a wood-reinforced plastic structure.
 
Lots of replies re. moulds but nothing re. the matter of how you get the desired final thickness. I assume that it's from experience - maybe by making test samples in a mini-mould and then measuring so you know that with a certain laying-up technique you get x mm. thickness from gelcoat plus y layers of matting+resin.
 
You know how thick the mat is when you apply it so it is just a matter of adding up how many layers you have applied to get a reasonable idea of how thick the finished article will be. For most DIY jobs it's just a matter of using a bit of common sense/gut reaction to work out how many layers to use. It gets more complicated if you are talking about high performance fabrications.
 
I've just done exactly this to make a pair of cockpit instrument pods for Ariam.

I started with a wooden core of very, very approximately the correct shape (just blocks of scrap timber nailed together, not even any cutting). Then I covered this with a thick layer of clay to build up the actual shape I wanted (the core was just to reduce the amount of clay I needed to buy, and the weight to shift around). I moulded it to the right shape in situ on the boat, then brought it home for final smoothing and finishing.

I painted the clay mould with a kind of plastic floor sealing substance (a tip from E C Fibreglass) which gave a pretty good finish. I thought I could do better by then painting on a thick layer of gelcoat and sanding it - this proved to be a mistake and a lot of unnecessary effort! Next I applied a layer of mould release fluid - this dries to a plastic film, but becomes wet and slippery again if you add water, thus making sure that nothing can stick permanently (unless you leave gaps in the film).

I painted on some more gelcoat, this would be the inner surface of the mould. Then put on (I think) four or five layers of mat and resin, also bonding in some stiffeners that also worked as supports so it would sit stably on the bench while laying up inside it.

Once this was set and had been left for a while to cure, I tried to pull the plugs out, without much success. I ended up digging out the clay bit by bit. I don't think I had done a good enough job with the mould release fluid, although nothing was irredeemably bonded that shouldn't have been.

With the moulds free, I polished the insides with very fine wet-and-dry and then lots of wax polish. In theory this is all you need for a clean release, but worried by my earlier experience I added mould release as well for belt and braces. This gives a slightly less glossy surface to the finished part, but it was still quite acceptable to me. You could always polish it later.

Finally I got to lay up the parts themselves. A pot of gelcoat with plenty of catalyst (you want it to set quite quickly, not leach active molecules eager to get through the release agent and wax and bond to the mould) painted all over the inside. Then, once that was tacky, two layers of quite thick mat and one of tissue (to stop stray strands stabbing me while groping around inside plugging in instruments), interspersed with resin and thoroughly rolled out with the special rollers. I trimmed off the ragged edges once it was set but not yet totally rigid, and then left it for a couple of days with the shed heater on to cure fully.

It put up a bit of a fight, but tipping water down between part and mould did the job, converting the mould release film back to a slippery fluid so that the part was no longer stuck to anything. Both port and starboard pods popped out of their respective moulds, and are currently sitting in my shed waiting to be mounted on the boat.

If anyone's interested I could maybe upload some pictures later this evening.

Pete
 
reeac said:
Lots of replies re. moulds but nothing re. the matter of how you get the desired final thickness. I assume that it's from experience .
Click to expand...

Experience, yes, but a bit of simple arithmetic does no harm. If you know the area, working how how much gel makes a film 0.5mm thick (or whatever) is easy enough. Similarly with laminations: one layer of a particular weight of cloth will give a certain thickness, give or take. There are plenty of web sites which can suggest how much resin you need to impregnate a certain amount of cloth (although this will also vary with the type of resin used).
 
Back to East Coast FibreGlass http://www.ecfibreglasssupplies.co.uk/t-estimating_quantities.aspx

Calculating quantities

To estimate the quantities of materials for your particular project you will need to know the surface area in square metres.

For example, you want to make a part 7mm thick out of Fibreglass and have a Gelcoat finish and your mould is 3 sq mt in total.

Gelcoat is applied 600g per sq mt, so 3 x 600g = 1.8 Kg of Gelcoat is required.

Chopped strand matting: Each layer of 600g is approx 1mm thick so you would need 6 layers.
6 layers x .600g matt = 3.6 kg per sq mt x 3mt (mould area) = 10.8 Kg of 600g matt is required.

Resin is calculated between 2 – 2.5: 1 Resin to Glass ratio. So, you have 10.8 kg of fibreglass matting, 10.8 x 2 = 21.6 kg of Resin minimum, say 25kg of resin to be safe.

If the Gelcoat that is applied at just over 0.5mm and 6 layers of Glass each 1mm, your laminate should be approximately 6 -7 mm thick. This varies upon operator, some experienced laminators are very good at keeping the resin:glass ratio down to the minimum, whilst others new to laminating will use more resin, this will effect the weight of the panel and can cause varying degrees in laminate thickness.
Good consolidation of the laminate is key, use a paddle/washer or any other type of consolidating roller between each layer, draw the resin through and apply more resin only if there are any dry patches, repeat this throughout lamination for best results.
Click to expand...
 
In commercial boat construction a gel coat is first applied to the in side of the mold. This is allowed to nearly harden (gel) so that you have a barrier of resin usually coloured to stop the fibreglass strands actually getting to the surface. Often a "chopper gun is used." for lay up. This is a spray gun of resin which has a feed of fibreglass rovings which are chopped to about 5cm lengths and sprayed onto the mold. This forms a material which when hard is virtually fibreglass like chop strand mat. The operator merely sprays for longer to give a thicker end material.
It is very quick and effective in corners etc. Boats tend to need thickness of GRP for stiffness and weight is not such a major concern.

In critical structure like aircraft components or even racing boats reliable strength and stiffness at minimum weight is vital. Usually the lay up is vaccuum bagged. This often has a bleed layer of cloth. It presses down on the glass and resin to squeeze out the resin into the bleed layer which can removed after hardening. Thus you get a reliable low ratio of resin to glass but thoroughly saturated with no voids. Glass to resin ratio dictates strength to weight ratio.
However for really critical carbon fibre components on aircraft they use a "prepreg" carbon cloth. Here the carbon fibre cloth is impregnated with the precise amount of epoxy resin in the supply factory. The cloth is shipped refrigerated. When layed up it is vaccuum bagged then put in an autoclave. The pressure compresses the component and squeezes out excess resin while the heat makes the epoxy harden. Thus they get a really reliably repeatable product identical to that which was detructively tested for strength.
Note while this is carbon fibre used for max stiffness compared to glass, carbon fibre can be layed up in the same way as fibreglass for extra stiffness or similarly Kevlar for extra toughness and tensile strength. Fibreglass products can be fun to produce so try it. olewill
 
There is a company in France near Lorient ,called Bernard They build using wooden moulds with thin sheets of premade glassfibre stapled into them. The boats are laid up in the traditional manner, the wood former stripped away, outside blasted, plastered with an epoxy plaster mix, and sprayed or painted. Mostly commercial boats, but built to Veritas standards, Pilot Boats, Patrol Boats, Fishing Boats, any design you want . No tooling required at all. The finish is really very good, and because of the epoxy finish, no osmosis, and really tough..
 
Thanks for all the replies - rest assure I wont be trying anything, it is just to help me understand a bit more of what I'm reading. I hadn't thought of ec fibreglass and et-al as I figured if they were commercial organisations they wouldn't appreciate "speculative" enquirers!!!

The reading matter at the moment suggests the use of a material called "C-FLEX" but it sounds as though ply would do exactly the same and its the surface finish that is most important.

All very interesting anyway

Thanks one and all
 
AlistairM said:
HI

I have been reading around this for a while now (purely out of interest, not planning any projects just yet!!) - One thing I haven't been able to work out or found anyone actually comment on as yet, is - when the "mold" is constructed, how does the very first layer of mat get applied. I assume there must be some technique or former that is used as with even the most well battened mold there will still be gaps that the mat will cover with out any support, so how do you stop it sagging at these points.

Hi Alistair,
A typical mold is a smooth as a babies bottom with not even a pin hole, the surface is normally highly polished and a release agent added so the finished item is easy to extract.

Also I have never been able to understand how you would decide on the number of layers to apply, how does one work out the thickness and then decide to stop applying further material?

The thicker it is the stronger it is, however, it is also very heavy and not cheap. So the thickness will depend on how stiff it needs to be and if other bracing or tougher materials are included like carbon fibre.

Mass produced boats are almost all coated with a chopper gun. the fibreglass is fed in and gets chopped up into small bits and sprayed with resin as it leaves the nozzle. The operator controls the thickness by watching the mixture landing and looking for a colour marker (the glass has colour markers about every metre), so spaying a pre set area will give a known thickness. bigger area between markers = less thickness. You can pick the learners, that mark out squares and move to the next square when they see the marker.

Only musings so not really of any great importance but if anyone could answer these it would help to fill a "void" in my current understanding.

Thanks

Alistair
Click to expand...

Hope this helps.

Good luck and fair winds. :)
 
You reading a book or construction article about Roberts boats?

Bruce Roberts boats started with a plug built from formers, stringers and battens to fair the shape. The battens did not form a completely solid plug.

C-flex is/was a fibreglass mat woven or stitched to fibreglass poltruded rods. The net result was a sheet of glass material capable of spanning the gaps between the battens. The home construction process involved soaking out the c-flex, laying and stapling it in place therby forming a more solid form and then building additional glass rovings over the mould.

Google c-flex for images and some write ups. There are a few images and texts. BR used this method to reduce the investment in the plug.

The result moved the work and lots of effort into finishing the very rough outer layup of the moulding.
 
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