10mm HDPE will hold out most hammers and axes in the featherweight class. For heavies, 30mm ain't enough.

If you go metal armor in the featherclass. The least I can advice is 3.2mm hardox 450, and if you can afford it (weight or money), thicker. If money is a problem, you can try thicker in normal steel.
If weight is an issue, I can advice HDPE, or else , with enough funds, titanium.