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Discussion Starter #1
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

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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/prod...19&PHPSESSID=8014237ecacef1e410cb65b34a3286d3
 

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Discussion Starter #3
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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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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Discussion Starter #5
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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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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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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Discussion Starter #11
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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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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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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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/product_cat.php/subid=1382/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/product_cat.php/subid=1383/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/product_cat.php/subid=1392/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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Cool :thumbup:
A more clear terminology can only help hemi43 in his search for materials.

I know that when we would specify urethane foam to our supplier, we got the 2# stuff that you show in your first link.(That website has some cool stuff.) I am not really sure why our supplier called it only urethane instead of polyurethane, if that is the name. It shaped as you describe.

The polystyrene you show in the second link is the same material as styrofoam, and it needs to be used with epoxy and or a barrier. I doubt that hemi43 would want to spend the money on epoxy. I guess he could tape it or put saran wrap.
I never used it.

When we would specify polyurethane we'd get a much denser, harder material that was used for core in structural parts. My experience with it was that it was miserable to shape. We used it in mostly flat or slightly compound curves. I know that I have shaped it over stations, then laid up the outside, pulled the stations out and laid up the inside with great success. Very stiff and very light.

I have seen the 2# stuff go away when used as a core in doors and hatches that had alot of flexing and vibration.

I worked full time at a fiberglass shop, (Poli-Form ind),from 1/85 until 1/97 when I opened my own fab shop. We did mostly automotive products and some industrial stuff. The aircraft stuff was left to the owner, it was his kind of fun.

I learn something new every day :thumbup:
The terminology associated with FRP use varies.

A while back I found a link with a bunch of FRP terms.
http://www.justintanks.com/def.html



Later, mikey
 

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Discussion Starter #17
Thanks for all the awesome info!!
I spoke to my local fiberglass supplier today, and he mentioned something that I have not heard of before. He recommends using a 1/4" thick honeycomb panel that he sells and glassing both sides. My concern is that this product may not bend very well into compound curves ( he claims it will ). I think I will just buy a sheet of the urethane foam as well as a sheet of the honeycomb material and experiment with them both. I don't mind spending money if I'm going to learn something new in the process. If either of these 2 products don't work well, I just may make the whole thing from aluminum. (at least I know how to use an English wheel) My concern is that a small all aluminum trailer will be too light and bounce behind the car when there is nothing in it. Dan

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hemi43
I use the pink foam (owens corning 'foamular' - comes in 4'x8' sheets up to 4 inches thick) found at Home Depot/lowes for a fraction of the price of the Urethanes. It sands as easily as the others and is easy to carve with a sawzall. You have to cover it with epoxy (very easy and cheap) before you use any polyester resin, as it is styrofoam based. Glue it together with epoxy as well.

I also use the drywall plaster that comes in the 5 gall tubs at Home Depot to finish off the plugs or to make those curves look better if I screw up with the foam and take too much off. Very easy and cheap to make large plugs. Drywall plaster sands easily, is already mixed and 5 galls. costs 10 bucks.
 

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Discussion Starter #19
Those are good tip Scrimshaw! I picked up the urethane foam today (almost $180 for 2 sheets ) but I like your idea about using drywall mud for sandthroughs. I wont be sandwiching the foam , because I'm concerned about the foam shearing away from the fiberglass .
 

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Pick up a couple books on boatbuilding from your local library, then check with your local fibreglass supplier.

There are numerous products and foams that you can use. Technically the strongest way to use foam in fiberglassing is to construct a fiberglass form, or shell and pour (in place) the foam into the form. The foam expands and fills the cavity under high pressure. Boat builders typically use this method. It is very strong and light weight.

The foam is a 2 part product, and bonds chemically to the fibreglass resin.

You can also use something as simple as the foam swim noodles (children's toys), you will find at department stores, toy stores.

I used these, to form several struts to enhance the strength and reduce the hull stress around the motor mounts when I rebuilt the rotted stringer system in my boat. (There were stress cracks on the hull)

I'd recommend to use the proper products which are designed to work with fibreglass resins. You can also use epoxy resins, which are more expensive, but don't combine the two in the same structure.
 
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