I'm building a mold for a custom rear fascia. But I'm a novice when it comes to fiberglass. Instead of trying more stuff that doesn't work (don't ask), I'd rather get some advice from people who know WTF they're doing.
The mold will consist of a wooden skeleton filled with contoured foam, which in turn will be covered in fiberglass (epoxy resin). I've built a hot-wire foam cutting device. And the wood frame is almost complete. It's time to start thinking about foam.
But I'm not sure what type of foam to use. I have a bunch of white styrofoam. The hot wire cuts it nicely. But I'm concerned about it "pebbling up" if I need to sand it. The local hardware stores sell blue foam (insulation) in big sheets. I'm not sure what type of foam it is. But it's contiguous, not little pebbles stuck together (i.e. better for sanding). Will my hot wire cut that stuff easily? Are there any other types of foam I should use instead? Please, no exotic materials. I can't afford that stuff.
Thanks for your help. Positive ratings will be awarded for advice that helps my project.
yes hotwire is ideal for the blue and pink construction foam, however polyester resin will melt it in short order.. the foam i have had the most success with has been the green floral foam bricks - inexpensive and easy to stack, can be cut with a metal ruler, or a saw, or sanded/filed.
since you have the white foam i would recommend cutting it a little smaller than you want the shape to be, then using drywall mud to build up the plug if you plan to remove the fiberglass from it - the stuff sands well..
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07:15 PM
fiero_silva Member
Posts: 1493 From: Winnipeg, Manitoba, Canada Registered: Jun 2003
I use the blue sheet foam. I also use West Systems resins and hardner ($150 a gallon around here). But the no fumes and no smell is more than worth it. I have had NO issues with the blue foam melting at all.
Used the same process for building the plug for the mold for our solar car: Plug at initial stages Plug after glass, finishing and paint: The mold:
[This message has been edited by fiero_silva (edited 12-22-2005).]
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08:34 PM
Blacktree Member
Posts: 20770 From: Central Florida Registered: Dec 2001
When I built the mould for my rear cold air intake I used a spray insulating foam for houses. You can shape it with a body file, sand it cut it until you have the shape you want. Once you have the desired shape you'll have to cover it with a seal coat so the resin doesn't eat into the foam. Then use duct tape as a release agent. For more info
I use the blue sheet foam. I also use West Systems resins and hardner ($150 a gallon around here). But the no fumes and no smell is more than worth it. I have had NO issues with the blue foam melting at all.
The blue foam is almost certainly polystyrene (trade name: Styrofoam), and the West System resin is epoxy. Epoxy will not attack polystyrene foam.
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Originally posted by ryan.hess:
blue stuff's good... don't breathe the fumes... quite poisonous
Not quite right. Polystyrene foam is not toxic and can safely be hot-wire cut ... but on general principles I would avoid breathing the fumes anyway. Conversely, urethane foams (including most expand-in-place foams) and PVC foams release poisonous cyanide gas when heated to hot-wire temperatures.
There are three general families of plastic foams widely available and commonly used for fiberglass work: polystyrene (trade name Styrofoam), urethane, and polyvinyl chloride (PVC). These plastics have very different chemical properties from each other, and are all available in many grades with different mechanical properties. Polystyrene foam can safely be hot-wire cut, but urethane and PVC emit deadly cyanide gases at hot-wire temperatures. Urethane is the most resistant to solvents, while polystyrene is the least resistant. PVC is generally the strongest but the most difficult to shape. All three foams are available in sheets and blocks, but most common expandable (expand in place) foams are urethanes.
Similarly, there are three general families of plastic resins widely available and commonly used for fiberglass work: polyester, vinylester, and epoxy. These three resin types have very different chemical and mechanical properties. Polyester resins have been used for a long time in boatbuilding, are the most widely available and the least expensive, and are pretty easy to work with, but have the poorest mechanical properties when cured. Vinylester resins combine most of the desirable properties of the polyesters with significantly improved mechanical properties, but they are more expensive, more difficult to work with, and less widely available. Epoxies are the most expensive and most difficult to work with, but generally offer the best mechanical properties when cured.
Polyesters and vinylesters are cured by addition of a very small amount of catalyst that causes the molecules of liquid resin to link up into a rigid tangle of very long molecules, and the cure rate is more or less proportional to both temperature and the amount of catalyst added. Conversely, epoxies are two-part resin systems, and epoxy cure is initiated by mixing the two components together in precise proportions; the molecules of the two parts then cross-link (link up to each other) into a tangled lattice structure. You cannot accelerate the cure of epoxies by altering the mixing ratio of the two components, but the cure rate is strongly affected by temperature.
Polyester resins will dissolve polystyrene foam, while epoxies will not. Conversely, all three resins can generally be used on urethane foam without problems.
One caution about working at low temperatures: At low temperatures, epoxy resins can partially cure to a "green" or "glassy" state and then stop curing. If this happens, the resin may never fully cure (even when subsequently heated to normal temperatures or above), may remain soft and gummy, and may never develop full strength. At sufficiently low temperatures this can happen with polyesters and vinylesters as well, no matter how much catalyst you use. Your best bet is to always observe the temperature limits specified by the resin manufacturer.
Incidentally, all of the information presented here is based on both research and many years personal experience building automobile, boat, and aircraft parts. I have extensive experience working with epoxy resins, somewhat less with polyester, and I have only recently begun working with vinylesters. Whatever resin system you use, it's important to understand the characteristics of the materials you're using and to follow the manufacturers' guidelines carefully for your first few layups. Only after you have accumulated some experience with a specific material should you begin to experiment "outside the box."
Originally posted by fiero_silva: But the no fumes and no smell is more than worth it. I have had NO issues with the blue foam melting at all.
yeah epoxy resins have so many advantages - but are generally twice as expensive (they are also a lot safer since prolonged exposure to the catalyst for polyester resins has been known to caus cancer)
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08:07 AM
Blacktree Member
Posts: 20770 From: Central Florida Registered: Dec 2001
... prolonged exposure to the catalyst for polyester resins has been known to caus cancer
I don't know about that, but the styrene monomer present in many resin formulations, especially polyester, is definitely a known carcinogen. Styrene monomer passes fairly easily through the skin and then accumulates in the liver, where it never gets broken down or excreted. Occasional or incidental exposure is probably OK, but it's still a good idea to get into the habit of wearing gloves or using a barrier skin cream (but not both) when working with any of these resins.
Another caution: Even a small drop of MEKP (methyl ethyl ketone peroxide), the catalyst used with polyester and vinylester resins, will permanently damage the corneas of your eyes ... so you should always wear some kind of eye protection. Blindness is not a good thing.
Just two more reasons why you need to read up on the manufacturers' literature for these materials before just jumping in and using them. Other than that, have fun.
[This message has been edited by Marvin McInnis (edited 12-23-2005).]
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12:16 PM
PFF
System Bot
Enterra-BC Member
Posts: 115 From: British Columbia, Canada Registered: Feb 2003
When I make plugs, I used styrofoam. When the plug is roughly the right shape, I coat the styrofoam with wallboard mud, which can be worked into final shape with sand paper or even just with water. When the shape is just right, I let the wallboard mud dry completely and fill in any cracks that may develop. When the plug is completely dry, I spray it with several coats of shellac. When the shellac dries, I wax the plug, and then it's ready to be coated with fiberglass reinforced polyester resin to make the mold. I sometimes use polyvinyl alcohol (PVA) on the outside of the plug, and I always use it as mold release in the mold.
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02:26 PM
ryan.hess Member
Posts: 20784 From: Orlando, FL Registered: Dec 2002
