Model Boat Mayhem - Forum

Dry Dock / Shipyard: Builds & Questions => Submarines => Topic started by: Silent Running on November 20, 2015, 01:35:35 AM

Title: HD polyurethane foam sheets?
Post by: Silent Running on November 20, 2015, 01:35:35 AM
I've been watching a programme on the telly about two blokes building an 11 foot HMS Fearless and they've used this stuff [HD polyurethane foam sheets] on the body to shape the ship, can it be used on a U boat or isn't it suitable? Thanks. ps. if not is there an alternative that is?
Title: Re: HD polyurethane foam sheets?
Post by: D108 on November 20, 2015, 09:26:00 AM
I acquired a sheet of yellow foil sided insulation foam, it's very easy to carve you get a very nice finish. It is very buoyant ,as you'd expect, not sure how would get around that. Once you have skinned your hull with grp you could remove the foam.
My foam was yellow and is softer than the blue stuff, it's the stuff you can buy in Wickes etc.
Title: Re: HD polyurethane foam sheets?
Post by: Crossie on November 20, 2015, 12:34:33 PM
 Hi Silent runner,
                                                       although quite soft and easy to carve, I personally  would advise against using polyurethane foams for a hull that you intend to sheath in grp unless you either want to leave most of the foam inside, or can easily remove it mechanically. This is because unlike polystyrene foam, ordinary solvents such as thinners and acetone will not dissolve it, there are some quite dangerous and toxic things that will though I don't recommend them. It's a good foam to use as a plug to make a mould on though, because unlike polystyrene foam you will be able to use cheaper polyester resins without dissolving your hard work!

Title: Re: HD polyurethane foam sheets?
Post by: Davy1 on November 21, 2015, 09:57:53 AM
Hi All,

Interesting thread this, not least because I'm looking for easy ways to make a plug for grp moulding for my next submarine project.

How good a surface finish can you get with these types of foam? ( Actual user experience, please. )

I have used polystyrene foam, in the past, (With care as Trevor points out.) also clay (but it shrinks) The other material on my "possibles" list is MDF.

The perfect way to produce a plug nowadays  would be to send off a computer file to have the plug milled from foam. Sadly my pockets are'nt deep enough!

PS going back to the original question. Best to avoid buoyancy in sub hulls. It all comes back as extra weight carrying it to the pond!
Title: Re: HD polyurethane foam sheets?
Post by: Crossie on November 21, 2015, 11:37:36 AM
Hi David,
                  to answer your 'how easy' question from practical experience, several times I have used polyurethane foam as a plug and found benefits and drawbacks in that it does facilitate the forming of a plug for a female mould using cheaper polyester resins, though it needs filling to get a smooth surface. It is a very tender substance, so after the initial carving of the foam block it needs quite careful sanding to avoid minor dings from corners of sanding blocks or fingernails. The same is true of course for all foam plugs, indeed I am currently working on a model plug  built up from depron sheet, an altogether firmer material, which was cheap, easy, fast and at face value provided a smooth surface which is fine for really angular subjects but much more difficult where curves and fillets had to be blended in, where you have to be really careful not to 'dig in' or form pressure dents when sanding. I prefer 3M or other good quality wet and dry papers in various grades, carpet spray glued onto sponge rubber blocks for sanding. A build log might follow eventually- it's a sort of submarine  :-))
            A big advantage withusing urethane foam is that you can use fast drying hi build primers directly on the foam to get a good surface unlike styrene foams where I use alkyd resin enamel type which will not dissolve the foam but it needs to be left several days before it is really cured enough for dry sanding.  I often use a thin plaster coat over  foam of whatever type to provide a firmer skin which after sanding is primed - even polyfiller is okay for this as it is cheap and easy to use.
            I hope this might help to answer your question,
Title: Re: HD polyurethane foam sheets?
Post by: Arrow5 on November 21, 2015, 11:58:19 AM
Brown wrapping paper saturated with waterproof wood glue gives an egg-shell type barrier on all foams that is proof against most solvents used in fibre-glassing.  A couple of coats brings up a sand-able surface when dry.   Messy but cleans-up with water and low odour.
Title: Re: HD polyurethane foam sheets?
Post by: Davy1 on November 23, 2015, 11:38:13 AM
Thanks All!

So it looks like polyurethane foam with appropriate surface finishes is a good way to go.

For my contribution, another method of final surfacing, I have used, is the use of aluminum foil "Duct" tape. It is good for reproducing steel plate and rivet effects and can be painted.

This thread from Nigel E shows it's use together with tin plate (which is easier to work with than you might think. You can cut it with scissors and it solders easily.)

Thanks again for the detailed suggestions.

Title: Re: HD polyurethane foam sheets?
Post by: Umi_Ryuzuki on November 23, 2015, 07:10:36 PM
I have used polyurethane foam for model bases for architectural models.
Most of them were 3-4 pound density and were a bit soft, and could be crushed, but
not so soft that they couldn't take a bit or working. Talking to another modeler, they have
switched to nothing less than 10 pound density. It is a lot finer, and takes less filling should
you want to create a fine finish. Locally, there is a company called General Plastics that would
cast the blocks in quantity, and cut a 4 x 8 foot sheet for our use. They call it "Last a Foam".
Most plastic distributors should have some sort of sample of Polyurthane foam or Tooling foam.

Surf board foam may be another term to research.

For smaller master parts, I use a dense material called RENSHAPE. it can be machined and
sanded to a fine finish, then primed and sanded and molded for master parts.

One Health and safety note, one friend said that if you look at the dust from the polyurethane
foams under a microscope, it looks like small spikey balls, like a world war II underwater mine.
Best to keep a dust mask handy when working the material.
Title: Re: HD polyurethane foam sheets?
Post by: Subculture on November 24, 2015, 11:48:34 AM
As Umi says, PU sheet comes in different densities, and that affects the working qualities.

The sort of stuff you find in builders merchants, which is probably what most people have experience of using, is very soft- it's designed for insulation not pattern making.

Even still you can get by with it if you skin it with resin and cloth after shaping, then finish with polyester fillers. I used this process for constructing the Yellow submarine plug. The keel and formers were cut from wood, and the bits in between blocked out in PU foam.

Denser PU boards are available which work like soft woods, but without the problems of grain, splitting etc. The snag with these is easy availability and cost, in a nutshell you can only get them from specialist suppliers, and they're far more expensive than foam sheet from a building suppliers.

Pink and blue polystyrene foam represents a good cheap pattern making material. It's dense enough to hold a pattern well, whilst still remaining easy to carve and shape. It does tend to be a bit more difficult to find than some of the lower density foams though.

You can give the foam a good coating of PVA adhesive, or oil paint or epoxy based paint to act as an effective barrier if coating it with polyester resins.

Having said all that I still find wood an economic and very practical way of making a plug especially if the shape is relatively straightforward.