09.23.08

Core Materials

Posted in Fiberglass Work at 7:26 am by Administrator

Core materials are the fillings in the sandwich structures that comprise most composite structures. By sandwich structure, I’m referring to a structure that has a core material (the bologna) covered by two facings (the bread). Most people would conclude that the facings are where the strength of the structure originates, but this is only partially true. The strength also comes from the geometric arrangement of the facings — in particular, the distance between them — and that distance is established by the core materials.

Think of an I-beam. It’s composed of a web and two flanges. When the beam is put under a bending load, the web gives it a certain amount of strength. One flange is put into compression and the other flange is put into tension. The joints between the web and flanges keep the three elements aligned and from buckling. A sandwich structure works pretty much the same way. The core material takes the place of the web and the facings take the place of the flanges. The larger the web (i.e., the thicker the core material), the stronger the structure — even though the core material itself (foam) is fairly weak.

Most core materials are made from some form of foam and those are the ones we’re going to be looking at today. However, the aircraft industry uses some pretty high-tech core materials now-a-days, including aluminum or cardboard honeycomb. Check out the Aircraft Spruce website for more information on core materials.

http://www.aircraftspruce.com/menus/cm/foam.html”

Polystyrene is used to form airfoils or other shapes that need to be cut precisely with a hot wire saw or water jet. More importantly, it is the ONLY foam that you should cut with a hot wire saw. All other foams emit a poisonous gas if subjected to heat! The aircraft folks use it to build their wings, canards, elevators, rudders, etc. because it can be cut into complex airfoil shapes with templates and a hot wire saw. If you want to save some money in buying polystyrene, look for “dock billets” at your local boating supply store. It’s the same stuff — the large blue rectangular blocks of foam that are used to float boat docks. We won’t be using any polystyrene in the reconstruction of the Bradley, because we have no need to do work with a hot wire saw.

Polyurethane foam is the foam you want to use when you’re going to carve complex shapes by hand. It’s a wonderful material for carving (with a butcher knife) and sanding. The best tool to use in carving urethane foam is another piece of urethane foam. If you use one block to shape another, the two pieces will take on mirror images of each other. If you’re sanding a curved surface, your sanding block matches the curvature you’re trying to create! Folks who like to create exotic car bodies use a lot of urethane foam for this reason. However, always use a dust mask and wear a long-sleeved shirt when working with urethane foam. The small flecks of foam that you sand off are as sharp as glass. You don’t want to get them on your skin or you’ll itch as if you had been working with fiberglass insulation and you definitely do not want to breath them into your lungs.

PVC (polyvinyl chloride) foam comes in generic forms (H160, for example) and proprietary forms such as Divinycell Foam. These foams are used where large, flat sheets need to be faced with fiberglass. The airplane folks use it for parts like landing gear and fuselage bulkheads, where high strength is important. The bond strength (peel resistance) between the facings and foam is very good, and the material is easy to cut and shape. However, it’s not available in thick sheets so urethane is preferred when carving complex three-dimensional shapes. We’ll definitely be using PVC foam in repairing the Bradley. The custom overhead console that we’re going to build will be made from PVC foam.

Proprietary Foams include things like Last-A-Foam and Clark Foam (no longer available). These foams are available in sheets similar to PVC, however their chemical formulation is different. Last-A-Foam is what the aircraft folks use to form the structures around their fuel tanks because it is one of the few core materials that isn’t dissolved by aviation fuels.

One final caution about foams. The white beadboard foam that you can find in the insulaton section of your local big box hardware store is not appropriate for composite work. It’s just too weak to act as a core material.

There are a couple of ways that foam can be bonded together if the pieces that you have aren’t large enough to do the job that you need. For sheet materials that need to be bonded edge-to-edge, we’ll use 5-minute epoxy. While not nearly as strong as the epoxy that we’ll be using for the fiberglass facings, it’s still stronger than the foam itself and that’s what counts. To bond sheets together, we use the hinge method. Butt the two pieces to be joined together and run a length of 2-inch-wide masking tape along the seam. Then, bend the two pieces of foam 180-degrees back along the taped seam to open the joint. Apply 5-minute epoxy to the edges of both pieces of foam, close the hinge, wipe off any excess epoxy that squeezes out (a popsicle stick works well here) and tape the other side of the joint. Watch the epoxy in your mixing cup and when it starts to thicken (but before it gels) remove the masking tape and scrape off any excess squeeze-out. By following this procedure, you’ll get a perfect joint between the foam sheets.

If you’re bonding blocks of foam, you’re going to mix up a material that we call micro slurry. Slurry is a mixture of pure epoxy and microballoons. The amount of microballoons that you’re going to add is not critical — just add enough to get the mixture to the consistency of pancake syrup — give-or-take. Microballoons, if you’re not familiar with them, are microscopic hollow glass spheres. In bulk, they look like flour. Wear a dust mask when working with them because, if they get into your lungs, they’ll stay there for a long time. Being made of glass, they’re inert and won’t dissolve. To make slurry, first mix up some pure epoxy in a cup. Stir it for the required 2 minutes, scraping the side of the cup every 15 seconds. Only after the epoxy is mixed should you add the microballoons. Add enough to make a substance that has the viscosity of pancake syrup.

To bond foam blocks, apply the slurry to both sides of the joint with a mixing stick. Add enough slurry to fill all of the voids in the foam. Then, mound some additional slurry in the center of one of the blocks and press the two blocks together, twisting the blocks against each other to distribute the mound of slurry evenly along the joint. The idea here is to make sure that the slurry squeezes out of the joint on all sides. If you see squeeze-out along all sides of the joint, you know that you used enough slurry; if you don’t, you may have some dry areas in the joint. Separate the blocks and add more slurry, if need be. Also, make sure that the two pieces of foam are in contact with one another. You want a joint where the two pieces of foam touch — not one where they’re seperated by a zone of slurry.

To hold the blocks in position as the slurry cures, just place nails thru the foam blocks. Use just enough nails to hold the blocks in position so that you don’t have too many nail holes to fill later. Leave the heads of the nails protruding so that you can grab them with pliers to extract them after the slurry cures overnight. If you can position the blocks so that you can weight the joint, do so. Just remember that some foams crush easily. If you use a set of barbells for weights, put a board or something that won’t give between the foam and the weights.

Slurry cures to a hardness resembling plaster. The nails may be hard to extract and may pull out chunks of foam along with them. That’s OK — we’ll show you how to make repairs at the appropriate time.

Carving foam along a slurry joint is problematic. Because the slurry is hard and the foam is soft, you tend to leave slurry ridges in the carved parts. The way to get around this problem is to dig the hardened slurry out of the joint to a depth of around a quarter inch or so, do your carving, and then fill in the joint when you apply the fiberglass facings. Again, we’ll teach you the technique when we have an example that we can photograph.

Enough for now. Our next class will go into structrual and non-structural fillers and the techniques that we’ll use for bonding structures together.