Also sold as buoyancy billet, blue foam is a closed-cell extruded polystyrene foam normally used for constructing floating structures. As a prototyping material, blue foam is an easily shapable rigid foam useful for exploring complex shapes and forms. Used in both looks-like and works-like models, blue foam is a key sketch modeling material.
Compared to blue foam, pink foam is a bit lighter and easier to get a hold of quickly. Normally used in housing insulation, pink foam is carried by most hardware stores. Pink foam is easier to cut and shape than blue foam, but it is also less rigid and easier to dent. Combining both types of foam for the modeling task at hand is usually the best approach.
Using foams is often compared to sketching in 3D because you can quickly shape foams into complex shapes. The hot wire cutter is our main tool for shaping foam because it leaves a nice finish that glues well. Compared to cutting with tools like saws, using a hot wire is cleaner because there are no chips or shavings to clean up. After using the hot wire, you can sand and join pieces of foam together to create prototypes.
2D Patterns: Hot wire cutters can slice through foam but will not cut paper. You can use this to your advantage by using double sided tape to attach cardstock patterns to the top and bottom of your foam piece to help you cut out precise shapes. Patterns on both the top and bottom are important so that you get straight up and down cuts (with just one pattern the wire can flex, resulting in an angled cut). Make sure to use edge markings and a straight edge to keep your patterns lined up! 2D cutouts can be stacked and sanded to create complex 3D shapes.
3D Patterns: Patterns can be applied to multiple sides of your foam piece as well. If you are starting with a rectangular foam block, you can consider putting patterns on all 6 sides of the foam (the example here shows patterns on 4 sides). Cut one side of the foam block, and then, while still holding all of the foam pieces together, rotate the block and cut using the next set of patterns. Think of all the shapes you can make!
Sandpaper: Foams are easily sandable using sandpaper. You can grab a piece of sandpaper and sand by hand, but keep in mind you will likely introduce a natural curvature from hands. If you want to sand straight sections, consider using a sanding block. Here we have a hook-and-loop style sanding block, which can be used with a variety of sandpaper grits.
Sanding Blocks: It's also easy to make custom sanding blocks using wood, or even more foam! Sandpaper can be attached to a custom sanding block using double sided tape (I recommend double sided brown coated paper tape, as it is effective and has the longest name). In this way, you can make sanding blocks with custom curvatures to sand to exactly the right shape.
Surform Plane Rasps: To remove a large amount of material quickly, you may want to use a rasp. Rasps are like sturdy cheese graters for use on wood and foams. This process is fast, but gets very messy as charged bits of foams will get all over the place and start sticking to surfaces and clothes. You'll want a vacuum nearby to help clean up afterwards.
Chemical means can be used fix pieces of foam together using adhesives from tapes and glues. When working with foam you need to be careful, as some types of adhesives will melt the foam, degrading the finish. Some adhesives that work well with foams are listed here, but if you want to use something else, you may want to do some tests on scrap material first to prevent harming a part you have already invested time into.
Double-sided Brown Coated Paper Tape: this tape has a few great things going for it. It's sticky, thin, and pretty quick to use. Slice a piece off, attach it to your model, and peel the green checkered protective film off to expose the second sticky side. This is a good general purpose tape, but it can be a bit pricey so you're not going to want to waste it.
Scotch Permanent: Permanent double sided office tape is another useful tape to have around for foams. It isn't quite as sticky as paper tape, but it also comes in a handy dispenser and does not have a protective film, so it is even faster to use. This tape is good choice for sticking on patterns, and despite it's name doesn't actually form a permanent bond to foam.
Spray Glue: Kind of like spray paint, spray glue is an adhesive spray that is useful for covering large areas. You can use it to attach large or complex patterns or to stick pieces of foam together. Spray glue is messy business, so use it in the fume hood and take care to mask off pieces of your model that you don't want to be sticky. Wait around 30 seconds for the glue to become tacky after spraying.
Rubber Cement: Gooey and sticky goop applied with a brush, rubber cement contains polymer chains (plastic bits) suspended in a solvent. The goop gets sticky as the solvent dries away. To use rubber cement to stick pieces of foam together, you should use a technique called dry mounting. Paint both surfaces to be attached with rubber cement and leave them to dry for a few minutes. Next, press the surfaces together to form a bond.
Sometimes glues and tapes aren't enough to get the job done, especially when the attachment between foams (or between foam and another material) will experience high shear stresses (sliding stresses relative to each other). Mechanical attachment methods can help here. Stakes can be used to puncture foam to secure it, and then tape can be used to keep the foam in place. Try not to undo and redo mechanical attachments, as the foam will crush and the attachment will start to come loose.
You can also use tools to cut foam on tools like a bandsaw, similar to working with wood. These tools are faster, but leave a mess that is tougher to clean up and have a rougher surface finish.
To make quick, rough breaks in pieces of foam, you can score foams with a utility knife and then snap them to a more usable length.
One potential hangup when using a straight edge to guide straight cuts is to think that the straight edge needs to be square to the table. Because the wire cuts in any direction, the only thing that matters is the distance the straight edge is from wire.