Using a foam plug for GRP moulding

[prv]

Boo2's post about plotter mountings reminded me of my ideas for deck instruments on Ariam. We have a scandiwegian style windscreen, with quite a lot of spare coachroof under it. My plan is to make some fibreglass "pods" fixed to the deck, outboard of the running rigging lines and slightly forward of the clutches, to take plotter and AIS displays, nav light switches, and all the other bits and bobs that you'd find on the bridge of a ship or powerboat but we sailors have a silly habit of hiding in the saloon. The space behind the pods would be specifically designed to hold spare clothes, hats, binoculars, etc.

I think the only way of getting a decent finish on the pods will be to use the traditional layup technique of a male plug, from which a female mould is taken, then lay up the two identical pods inside the mould (one after the other). I was contemplating making the plug out of cheap softwood, MDF, etc, but since most of the shaping will be effectively carving by eye, I'd have a lot of material to remove and it would be a tedious process - plus one not best suited to carrying out on board, which is where I'd want to do my plug-making to check fit.

So I thought of using foam, very easy to slice and shape with an electric carving knife and, I'm told, a drywall sanding pad. But of course most foams dissolve instantly on application of resin. Is there an easily-available one that does not? Or a suitable barrier material that can be used? I don't want to just wrap the foam in parcel tape etc because all the wrinkles, tape edges, etc will transfer to the finished work. I want to be able to smooth and polish the plug and get a smooth and polished pod at the end of it.

I was hoping to just work in (cheaper!) polyester, but perhaps I should make up a filleting mix of epoxy and cover the foam in that, then sand it? Epoxy doesn't contain styrene; does it still eat foam?

Pete

PS: No, I don't want to buy any ready-made pods, unless by some miracle they exactly match the design in my head. But that seems unlikely.

 

[Elessar]

So I thought of using foam, very easy to slice and shape with an electric carving knife and, I'm told, a drywall sanding pad. But of course most foams dissolve instantly on application of resin. Is there an easily-available one that does not?

yes foam is routinely used in boatbuilding for stringers etc for laminating over - buy it from the grp supplier. I used foam in exactly this way see my reply to Boos post.

edit - I see you're in southampton drop me a pm I'll probably be able to find an offcut for you, if you need lots marineware sells it. Lots of densities get the lightest.

 

[Boo2]

of course most foams dissolve instantly on application of resin. Is there an easily-available one that does not?

Epoxy resin is more expensive but is compatible with normal polystyrene foam. In the toy aeroplane work they call what you are thinking of doing the "lost foam" method and with a bit of care it is possible to get a fine finish without going to the bother of making a female mould, just fill and sand the result then dig out the foam (or dissolve it out with a solvent).

Thanks to you and Elessar for your thoughtful suggestions in the other thread btw,

Boo2

 

[William_H]

Foam for plug

I believe it is polyurethane foam you want. The common white polystyrene does dissolve in solvents etc.
An alternative might be to use what foam you have and cover it with plaster before liberally applying floor wax to give a release.Be careful about the design (shape) of the mold to ensure easy removal of both mold and then the end product. good luck olewill

 

[prv]

In the toy aeroplane work they call what you are thinking of doing the "lost foam" method

Ah, but I want two identical pods, so I don't want to be losing the foam

Making the mould doesn't seem like too much hardship, just paint the waxed plug with a thick layer of gelcoat then lay up a load of mat over the top. I'd be careful with the inner layer or two (probably use that thin "tissue" cloth rather than chunky mat) but then just chuck it on to help it hold shape, doesn't need much care. The shape of the pod (triangular prism) is pretty much ideal for release purposes.

Perhaps I could then offer the mould to other forumites who wanted to make the same thing

Pete

 

[prv]

yes foam is routinely used in boatbuilding for stringers etc for laminating over - buy it from the grp supplier. I used foam in exactly this way see my reply to Boos post.

Cheers. Clearly it needs to be the right foam, though - I know polystyrene dissolves, for instance. Is polyurethane the stuff I want? East Coast Fibreglass that I've usually bought resin and mat from before has it in sheets up to four inches thick, I guess I can stick a couple together.

edit - I see you're in southampton drop me a pm I'll probably be able to find an offcut for you

At a very rough guess I want to start with a block about 14" x 8" x 8" - and if I was buying it, I'd get plenty of extra in case I totally ballsed one up while carving it. Do you have that kind of amount going spare (presumably in sheets rather than blocks, of course)?

Cheers,

Pete

 

[Amulet]

I found this unpretentious little book very helpful. It gives a lot of info about different ways of making moulds (and it's fairly cheap).

I made a fridge by building a plug mould. I bought my materials from these people who were reasonably priced and helpful. I phoned up about the task and one of their blokes talked me through it very patiently and clearly.

 

[vyv_cox]

I have made quite a few bits and pieces using polyurethane foam as a plug. Kingspan building insulation is a convenient source, it comes in at least four inch thick sheets and can be glued together easily.

 

[Cookee]

Buy the right foam from a GRP supplier and some high build paint to finish which is easy to rub down to a good finish, it will then accept polish and be good for taking a mould off - We use Scott Bader who supply small quantities.

 

[fisherman]

For regular flat sided shapes we used conti board (melamine faced) as a female mould.

 

[prv]

For regular flat sided shapes we used conti board (melamine faced) as a female mould.

I thought of that stuff when I might have been laying up some flat sheet of fibreglass. But these pods are going to have quite a rounded shape, so I think carving from foam will be easier.

Cheers,

Pete

 

[knuterikt]

Ah, but I want two identical pods, so I don't want to be losing the foam

Making the mould doesn't seem like too much hardship, just paint the waxed plug with a thick layer of gelcoat then lay up a load of mat over the top. I'd be careful with the inner layer or two (probably use that thin "tissue" cloth rather than chunky mat) but then just chuck it on to help it hold shape, doesn't need much care. The shape of the pod (triangular prism) is pretty much ideal for release purposes.

Perhaps I could then offer the mould to other forumites who wanted to make the same thing

Pete

Shape the foam into a modell.
Apply epoxy & fiberglass on the outside.
Dissolve the foam and you have a form.

 

[oldsaltoz]

G'day Pete,

I have made several pods over the years and use foam, but never a mould as such.

It, not rocket science.

Foam can be cut with a simple hand saw to produce not just a curve, even compound curves.

To make a curve you make a series of cuts almost through the foam stopping about 5 to 10mm short then bend it to the required shape.

For compound curves you simply mark the areas and direction you want them to curve in and make the cuts across the sheet. Let's say you want a dome shape or shape like a scoop.
Cut the foam from edge to edge in both direction on one side of the sheet, the smaller the gap between cuts the more bend you get.

When you happy with the shape, fill in the gaps with a mix or resin and filler and then bend it as required, fix in position and let it cure.

Now your ready to glass it and add some Micro-balloons to smooth it out ready for coating with flow coat.

To make identical pods you only need a simple frame made from scrap timber to and a couple of clamps.

Google foam construction/fibreglass or something like it, it's easy fast and the finish is fantastic, and you can stand on them without a problem when fully glassed.

Good luck and fair winds.

 

[AntarcticPilot]

Ah, but I want two identical pods, so I don't want to be losing the foam

Just a thought but are you SURE you want two identical pods? I'd have thought you wanted left and right handed mirror images, which can't come from the same mould. But I can't imagine exactly what you're trying to do, so I may well be barking up entirely the wrong tree.

 

[lustyd]

If you decide to go with "handed" pods I would still use foam but in layers which can be flipped. The rounded bits can then be finished with plasticine on each side while ensuring the basic contours are identical.

 

[knuterikt]

G'day Pete,

I have made several pods over the years and use foam, but never a mould as such.

It, not rocket science.

Foam can be cut with a simple hand saw to produce not just a curve, even compound curves.

To make a curve you make a series of cuts almost through the foam stopping about 5 to 10mm short then bend it to the required shape.

For compound curves you simply mark the areas and direction you want them to curve in and make the cuts across the sheet. Let's say you want a dome shape or shape like a scoop.
Cut the foam from edge to edge in both direction on one side of the sheet, the smaller the gap between cuts the more bend you get.

When you happy with the shape, fill in the gaps with a mix or resin and filler and then bend it as required, fix in position and let it cure.

Now your ready to glass it and add some Micro-balloons to smooth it out ready for coating with flow coat.

To make identical pods you only need a simple frame made from scrap timber to and a couple of clamps.

Google foam construction/fibreglass or something like it, it's easy fast and the finish is fantastic, and you can stand on them without a problem when fully glassed.

Good luck and fair winds.

What kind of foam are you using?

 

[prv]

Just a thought but are you SURE you want two identical pods? I'd have thought you wanted left and right handed mirror images, which can't come from the same mould.

I'll find out when I can actually get on the boat again, but I *think* identical pods will be fine. There certainly is scope for them to be handed, but they don't have to be, and it'll be simpler if they're not.

Imagine the classic minicab roof sign, a big plastic toblerone across the width of the car roof. I basically want two narrower versions of that. Then make cutouts in the aft face to fit the instruments etc, and mount them on deck either side of the hatch garage. I might have to fit small waterproof hatches in the back to be able to do up the screws on the plotter etc; making them removable from the deck would be more complex.

Pete

 

[Avocet]

You can use the 2-pack expanding foam sold by the likes of Glasplies ad CFS, but the styrene does attack it (a bit). Still, it was good enough for TVR to make entire car body bucks out of! You just "cast" the foam into a rough oversized lump, then attack it with breadknife, saw, surform etc until it's about the shape you want. Pain it in a good thick coat of catalysed gelcoat and leave it to go off. You'll probably notice it sinking a bit and you will need to fill the various hollows that appear as the styrene attacks the foam. Normally, for a small, curvy shape like the pod, you could probably take your mould off straight away, but if you wanted a better finish, you could always do what TVR did and take a quick lightweight mould off your plug, then lay another one up in your mould, then prepare THAT one properly and take a second mould off that.

 

[vyv_cox]

You can use the 2-pack expanding foam sold by the likes of Glasplies ad CFS, but the styrene does attack it (a bit). Still, it was good enough for TVR to make entire car body bucks out of! You just "cast" the foam into a rough oversized lump, then attack it with breadknife, saw, surform etc until it's about the shape you want. Pain it in a good thick coat of catalysed gelcoat and leave it to go off. You'll probably notice it sinking a bit and you will need to fill the various hollows that appear as the styrene attacks the foam. Normally, for a small, curvy shape like the pod, you could probably take your mould off straight away, but if you wanted a better finish, you could always do what TVR did and take a quick lightweight mould off your plug, then lay another one up in your mould, then prepare THAT one properly and take a second mould off that.

Exactly what I did. When the first low volume competition kayaks came out in the 1970s most manufacturers blew foam into a bigger mould, then cut it down exactly as you describe.

 

[oldsaltoz]

What kind of foam are you using?

Mostly poly closed cell foams that are not effected by water ingress, available in a nimber of thichneses depending on the project or curves required.

Can be glued into blocks to make moulds.

Good luck and fair winds.