XPS Foam
Intro
Like a 3D print-smith using different filament types, a Worbla-smith using different types of thermoplastics, or a fabric-smith using a wide variety of fabrics and materials, foam-smithing is not limited only to EVA. What is one of the most common foams we use in our daily lives? We use it for disposable cups and plates, and for packaging; Styrofoam, which is a type of polystyrene foam. There are two different types of polystyrene: EPS and XPS. EPS, or Expanded Polystyrene, is the styrofoam we all know. It is an open-cell form and makes a huge mess with little foam beads when you break it apart. The other type is called XPS. This tutorial will cover how you can use XPS to create some great pieces for cosplay.
What is XPS?
XPS, or Extruded Polystyrene, is a closed-cell home insulation foam board, making it far denser than Expanded Polystyrene, which is an open-cell foam and commonly known as styrofoam. Because of this, XPS carves like wood, and sands well. You can also cut it with a blade or hot wire knife. In this tutorial, I will talk mainly about XPS. So what would we use XPS foam for?
As I said, XPS foam board is very sturdy, stiff, carves and shaves like wood, and sands very well, but the best thing about XPS foam is that it's extremely light weight. It's a perfect material for making very large pieces like my 7 foot Lance and my 7 foot Gundam Freedom wings, or even a large pauldron. To give you an idea, my Gundam wings weigh a mere 3 pounds each side, including the wood, PVC, and hardware. So where do we find XPS foam?
As XPS is home insulation foam board, you will find it at home improvement stores like Lowe’s and Home Depot. And because it’s made and sold for the purpose of adding insulation to homes while they’re being built, they typically come in slabs 2 feet wide by 8 feet long, and anywhere from half an inch to two inches thick. There are also different qualities of XPS, and differ between who makes it. In the US, they are easy to differentiate; pink vs blue. The pink I've only found at Home Depot, and (quite fittingly) the blue I've only found at Lowe’s. What is the difference? Pink XPS is not as dense, making it easier to carve with a blade and is slightly lighter weight, but as a result it is much more prone to breaking. My lance is made out of pink XPS and has broken three times. The blue, however, is much more dense, making it far more damage resistant, but it also makes it much harder to carve with a blade. Though you can cut pieces out with a blade (with 4-5 cuts), carving will require the use of a hot wire knife. What's a hot wire knife? I'll explain that in a bit. Below is a picture of the two types of XPS together after I pressed as hard as I could with my hands, showing the difference in densities. But when considering useing XPS foam, there is one critical characteristic of polystyrene foam that makes it harder to work with than EVA.
Polystyrene vs Acetone and Toluene
Many substances we use to work with EVA foam, i.e. contact cement, super glue, spray sealants and paints, all contain one or two chemicals that disintegrates polystyrene: Acetone and Toluene. Even in minute amounts in spray paints, the smallest amounts of either will eat away the foam over time, compromising the integrity of your prop. Even Krylon’s Craft Foam Primer, which is supposed to be designed for styrofoam, contains Acetone, so be careful what you use and check ingredients. Here’s a quick experiment you can try. Take styrofoam in any form (cup, plate, whatever) and brush a little contact cement on it. The cement will quickly begin to eat away at the foam. Put a styrofoam cup in a bowl of Acetone and it will melt away like the Wicked Witch of the West. Toluene's effect is even worse. If you are unsure if a product has either chemical, it will either be on the back label or it will be found, per law, in the product’s Material Safety Data Sheet (MSDS), usually found online on the maker’s website. I should also note, Polystyrene does not like high heat. So using a heat gun will melt it and high temp hot glue will melt it as well (and still not adhere). If you can’t use any of the adhesives or sealants you can with EVA, what the heck do we use? There are many alternatives, some will work effectively and permanently whereas others hardly do the job.
Adhesives
What is the one type of glue you always used at school as a kid? Elmer’s Glue or white crafting glue of some sort. Elmer’s is a type of adhesive called Polyvinyl Acetate or PVA. It contains no Acetone (which is chemically different than Acetate) and therefore will work. Unfortunately, Elmer’s could only be suitable as a sealant because it dries only by exposure to air. If you used it to glue two boards together, the glue will never cure and your pieces will come apart very easily. The most popular adhesive overall for use on XPS foam is a different yet common type of PVA glue called wood glue. It’s much stronger than white PVA and is able cure between objects, hence its intended use on furniture vs crafts in kindergarten. Wood glue is also fairly inexpensive and very easy to find. Some will do the quick and cheap way and use Super 77 spray adhesive, but cheap and quick often means it’ll come apart quick too.
Another type of adhesive comes in a caulk tube, requiring the use of a caulking gun. As XPS is typically used in construction, it’s no surprise that you’d find compatible adhesives in construction form. There are many of this type but not all are recommended for use with foam, just read the info on the back of the tube to find its intended uses. There are three that I’ve used. Loctite PL 300 is an acrylic latex adhesive specifically designed for XPS in home construction. I used this on my Altera Lance. Though very sturdy, it’s very thick, you can’t cut through it easily, and its biggest drawback is that it takes a full week to cure. Another one I’ve used is Liquid Nails Heavy Duty. This is also latex-based and acts similarly to PL 300, but it’s cure time is not one but two full weeks! Yikes! Loctite PL Premium is a polyethylene based adhesive that’s extremely strong but should be used to adhere polystyrene to other materials like wood, metal, or plastics, and not foam to foam. This adhesive was critical for the construction of my Gundam's wings. The reason I would not recommend it for foam on foam is because it is very hard when cured so you will have a hell of a time trying carve or cut through it to make your piece.
So what adhesive do I recommend for the strongest hold and ease to work with? My favorite adhesive for polystyrene is a product by a company out of California called Hot Wire Foam Factory. The product is called Foam Fusion. Like the ever-popular wood glue, it too is a PVA glue. But this adhesive has been designed specifically for Polystyrene foams and those who use it to create statues and other art. When cured, it’s still easy to cut with a blade and hot wire knife (which can be crucial), you don’t need much, and it takes only 12 hours to fully cure! It’s also very strong. They also make a much thicker adhesive called Styrogoo, which I use kind of like you would super glue, only it won’t melt your foam. So two PVA glues are currently the best: Foam Fusion (my favorite) and Wood Glue (most popular).
https://hotwirefoamfactory.com/028B_Foam_Fusion_Glue.html
https://hotwirefoamfactory.com/-028SG--StyroGoo-.html
Sealants
So now that we’ve covered adhesives, what about sealants? You can't use Plasti Dip because it contains Toluene. Fortunately, there are a lot of things you can use and I will cover some of them here. The most popular is also wood glue, either straight or diluted with water. Yes, it's good as an adhesive and a sealant. You can use paper mache and seal that with Spar Urethane as I did with both my lance and Ultima weapon (before I found other ways). I hated paper mache. It’s messy, it takes an eternity, and any details get covered up. And paper mache is a sealant for the foam but you must then seal the paper mache, hence the Spar Urethane, so it's quite redundant. A lot of people, quite surprisingly, will wrap their foam with Worbla. Sure that’s effective but it adds a lot of unnecessary weight and, more importantly, Worbla is expensive, so large props become very heavy and very expensive very quick. You can cover it with fiberglass and a polyurethane sealant, giving it a hard and sturdy protective barrier while still keeping it extremely light. Like all things, it too has drawbacks like cost (though not as expensive as Worbla), it’s ability to preserve certain details, and it’s health risks (wear proper PPE, please). You can use Gesso, an acrylic substance like a thick paint that you can brush on, let dry, sand, brush, dry, sand, and so on and so forth. This process is extremely time consuming but it is an option. There are also epoxy's that can be used. My personal favorite (though I want to try fiberglass someday) is another product by Hot Wire Foam Factory called Bounce. Bounce is a rubber-like substance that they originally developed as an additive to their All Purpose Foam Coat plaster but when I tested it by itself, I discovered it to be a great sealant by itself. You brush it on with a sponge brush and requires 2-3 coats. It is fairly self-leveling, so you still get a good even surface. However, on a porous material like XPS, you will have to lightly sand each coat once dried. I use 320 grit then 400 grit sandpaper. It's also an extremely useful sealant and clear coat for EVA foam, I discovered. On EVA foam, it dries on smooth so no sanding is required, and it's quite bomb proof. Acrylic paint also binds very well to Bounce. So there are a bunch of different ways to seal XPS foam. The method you choose depends on your budget, whether minimizing weight is a priority, and if maximizing damage resistance is a priority.
Get Rid of That Film
There is one crucial characteristic of XPS foam board you likely would not know until after the fact. XPS foam board has a thin, glossy, non-porous film on its main sides. This film acts just like the rough side on EVA floor mats in that adhesives will not stick to it for long or simply peel away that film, having not actually adhered to the foam. To prevent this and create a far stronger bond, a quick sanding with sandpaper will get rid of the film.
Cutting and Carving XPS
Like EVA foam, you can cut and carve XPS with a blade, but this is where a strong, wide, and long blade is crucial; anything less and you'll snap your blade in half. My 25mm wide telescoping snap-off blade does the job well. I'm a huge stickler on having a proper blade sharpener with EVA foam, but it's far more crucial with XPS. You think EVA dulls a blade fast? XPS will dull your blade in a single cut. So yes, XPS is fairly easy to cut, but you spend most of your time sharpening your blade. Don't even think about doing this without a sharpener. A useful feature of XPS is that it carves like wood and is easy to sand but please wear proper PPE as XPS foam dust gets in the air, your eyes, and lungs easier than EVA dust does. As I mentioned earlier, pink foam is fairly easy to carve with a blade, but blue is so dense it requires a hot wire knife, which you can also acquire from Hot Wire Foam Factory.
A hot wire knife is a wire connected to two prongs or arms and each end is connected to a power source, positive and negative. When power runs through the wire, the electricity makes it “hot,” allowing you to cut through polystyrene like butter. You can get hand-held ones and huge industrial sized tables.
EPS and Expanding EPS
The last thing I want to talk about is a type of EPS foam that is commonly used called spray expanding foam. This stuff comes in a can and is used to fill in holes in the side of your house that a woodpecker decided to make. Being that it’s EPS it’s a bit more rough a material but you can create great 3D objects out of it. You can make props or pieces of armor with it by creating a cardboard layout, 2D or 3D, and spraying the expanding foam on it roughly to shape. Then take a blade and carve away until you have your piece, seal, paint, done.