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Putting complex bends and curves in EVA Foam armor can be tricky, since the foam is very elastic and flexible. Parts don't always like to hold their shapes, and the prevalent method (from my experience reading costume forums like therpf) for shaping foam armor parts is to use a heat gun. I didn't have a heat gun, so I decided to apply something I discovered when I was making my Iron Man Mark 3: paperclips are surprisingly good at holding form and shape.
My method basically involves cutting lines into the inside of an armor piece, lining the cut with a paperclip and then sealing it back up with hot glue. The best part about this method is that you don't have to go out and buy a heat gun if you don't have one. Additionally, it is really easy to re-shape if you so desire, and the paperclips actually do strengthen and stiffen the parts.
The materials you will need:
1) Foam armor part (pepakura is the most popular method for making foam armor)
2) Paper clips (Use large ones because they are thicker and sturdier)
3) Hot Glue gun
4) Pliers (Needlenose, and optionally diagonal cutters as well)
5) X-acto knife
In this instructable, I will show a fairly simple method to form, shape and strengthen foam armor, using an Iron Man helmet made from EVA foam floor mats.
CAUTION: Be careful when placing cut paperclip bits as these are sharp and can easily penetrate skin. I have placed this warning again at the end of the Instructable, so please follow safety precautions and exercise good judgement while building!
Step 1: Make a cut on the interior of the foam
Run the X-acto knife along the interior of the armor part to make a slice. Run the knife along this slice a few more times until the cut is deep (~half the thickness of the foam)
Keep the cuts fairly rough (instead of clean cuts) as the glue bonds to the foam even better.