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
Keep the cuts fairly rough (instead of clean cuts) as the glue bonds to the foam even better.
Step 2: Bend the Paperclips to Shape and Then Glue Them Into the Cut
Run the tip of the hot glue gun along the cut in the foam and then insert the paperclip and line the slit. Do this in parts because it is really hard to insert the paperclip into a slit that has been entirely covered in hot glue as you will definitely get hot burning glue on your hands!
To elaborate, you should cover half the slit with glue, embed the paperclip, and then wait for it to cool and harden. Then add hot glue to the rest of the slit and insert the rest of the paperclip. It is easy to embed the second half by just holding the first half through the foam. Do not worry about deforming the foam because adding the paperclip makes it really strong, and it will retain the shape of the paperclip.
Also, do not worry if the paperclip changes shape while you are doing this, because you can very easily bend it back to the correct shape through the foam. The foam will then hold to the correct shape.
Step 3: Results: Clean Up and Repeat As Necessary
Stay tuned for my Iron Man Mark 7 project, and happy building!