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Old 02-04-2007, 06:11 PM
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What kind of foam can I use to fibreglass over?

I need to make a small "Mullins" type fibreglass trailer for the back of my street rod, and would like to carve one out of some type of foam then fibreglass over it. The foam is only for the shape and would be removed after fibreglassing. I tried blue styrofoam SM, but the polyester resin chews up the foam. I could use epoxy resin, but I don't think that the extra expense is justified. Thanks for any replies. Dan

www.geocities.com/dantechfab

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Old 02-04-2007, 06:32 PM
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there are a few types but the big one thats most widely used by manufacturers and mold makers is polyurethane foam. its ridgid and dimensionally stable. easy to carve and shape. only problem for the home user is its not usually locally available and it can get pricy. i put a link below to a good company that makes the stuff. it comes in different densities measured in pounds per cubic foot. obviously the higher the weight the more dense. you will be fine with around 6-8pcf.

http://www.generalplastics.com/produ...cb65b34a3286d3
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Old 02-04-2007, 07:44 PM
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That seems like the perfect foam to use. Unfortunately I live in Canada, so getting specialty products up here is next to imposible. I will do some phoning around in the morning now that I know what type of foam I need. I was also thinking about spraying the styrofoam SM with a sprayable polyester primer then sealing it to keep the resin from attacking the foam, but it starting to sound like it's going to take 200 to 300 hours to get this trailer made. Oh well! it's a small price to pay for something that's one of a kind. Dan
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Old 02-04-2007, 08:04 PM
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poly primer will attack regular styrofoam too. there is really not much you can put on regular foam that wont attack it. i have heard of people using it then covering it with tape and glassing over that but it will only give you a general shape. you lose any kind of detail in the part you are making and that blue and pink foam doesn't shape worth a crap. the poly stuff shapes super easy with 80 grit and sureforms.
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Old 02-04-2007, 08:31 PM
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I was just on the website of a fibreglass supplier's website which is local to me, and he has 2lbs/sq ft urethane foam in 4X8 sheets, but they are only 1" thick. He sells it for around $80 per sheet. I think I would need at least 3-4 sheets to get the thickness I need.
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Old 02-04-2007, 08:39 PM
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there you go. 2lbs is pretty lightweight but will work just fine for you. thats about right price wise too.
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Old 02-04-2007, 09:03 PM
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I stumbled on a perfect site for what I want to do. This should answer any questions. http://www.rqriley.com/faq.htm
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Old 02-04-2007, 09:38 PM
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wrap your styrofoam mold in saran wrap. that will act as a barrier.
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Old 02-04-2007, 09:50 PM
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Good link, I would add that it is the styrene in resin that dissolves Styrofoam.

Urethane foam is a better choice if you want to remove the buck after laminating. It is the easiest to shape, and is lighter and cheaper. That is what we use for all of our one offs.

Polyurethane foam is denser and has a much greater strength, but is alot harder to shape. It can be used as a core material, as it has alot less tendancy to turn to dust with flexing. (surfboards use a polyurethane core)

To attach the sheets together I have used gorilla glue, or one of the other polyurethane glues that foam slightly. It expands a little to fill the gaps, but is harder than the surrounding foam and makes sanding fine details a little harder. I try to stay back from areas that I know I will be sanding/ shaping.

A hot melt glue gun works well also , but again, you need to stay away from the areas that you will shape.

That expanding foam that sprays from a can (great stuff), will only mess you up, it pushes your sheets apart as it expands, and it will dissolve under anything with styrene in it.

I usually build a wooden framework to glue my foam to. It helps keep things dimensionally stable.



Later, mikey
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Old 02-05-2007, 08:29 AM
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hemi43- If you are doing searches, spell it "fiberglass" instead of "fibreglass", you will get a bunch more search results.

Later, mikey
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Old 02-05-2007, 09:41 AM
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Thanks Mike! I guess you answered my other question about using a spray foam like Great Stuff. Up here in Canada a lot of words are a bit different than the American spelling, but all mean the same. Must be the "French" messing stuff up. Lots of words screw me up, ie. color VS colour grey VS gray etc... I don't even know which to use. Oh well!! Thanks, Dan

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Old 02-05-2007, 10:20 AM
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grouch built an egg shaped triler out of fiberglass in his journal. It jumps around as he built it . Here is a link to the begining page of the build:

http://www.hotrodders.com/forum/jour...ge=5&reverse=1
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Old 02-05-2007, 04:25 PM
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If you are using polyurethane foam, it is by far easier to shape than styrofoam, which is usually cut with a knife or a hot wire saw and template. You can do gross cuts easily with a large sharp knife, and then you can use a sanding block or a scrap piece of polyurethane to abrade against the polyurethane work piece to shape in the final detail detail. Great stuff, don't breath the dust or loose particles from the shaping process.

Some will shape both the exterior and interior of the polyurethane shell and put fiberglas on both sides, making the polyurethane foam the 'meat' in a 'sandwich'. Shape the exterior, do your exterior layup, let it cure, then shape the inside and repeat. This composite sandwich construction has several advantages - the foam in between the two fiberglas layups makes the entirely assembly rigid, and can do so with fewer layups than just by making an outer shell and then sanding out or dissolving the polyurethane. The part winds up being stronger and lighter and doesn't require as much bracing or ribbing. It is also easy to repair. This is much the same technique used to build fiberglas aircraft and some boats. My .02.
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Old 02-05-2007, 04:52 PM
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What dissolves polyurethane or urethane foam that won't dissolve the resin?

The easiest shaping foam is urethane, not polyurethane. The shaping methods you describe work well with the urethane, but will not work well with polyurethane..

Both types are used in the industry. As I said before, polyurethane is a better choice for core,(sandwich),because of its stiffness and tensile strength. This also makes it alot harder to shape. Making big cuts with a knife is very hard, most shaping is done with a sureform file, sand paper or a hot knife.

Urethane foam, when used as a core material, will turn to dust in a relatively short time, when used in an automotive envionrment. This is what is known as "core shear". The vibration kills it, leaving the 2 skins free to float around.

Styrofoam has no use in anything other than packaging, IMO.

later,
mikey
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Old 02-05-2007, 07:47 PM
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The easy-shaping material I mentioned in the above post is indeed polyurethane, and here is a link as to where we buy it :

http://www.wicksaircraft.com/catalog...382/index.html

The more difficult foam to shape as I also mentioned above is polystyrene foam, not styrofoam as I mentioned above - my error. Here is a link

http://www.wicksaircraft.com/catalog...383/index.html

There are also some higher grades of styrofoam that can be cut with a hot wire but they are slightly heavier than the polystyrene. Again, a link:

http://www.wicksaircraft.com/catalog...392/index.html

Aircraft engines produce a heavy vibration in the frame, so the choice of materials is critical especially in regard to 'core-shears' or delaminations. We have used all of the above materials in composite sandwich aircraft projects and have never experienced delaminations. Not trying to argue but that is our experience, going back to 1980.
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