Materials; Laser cutting

[bigsi]

HDPE is not suitable for a HW spinner, its to malleable and flexible. It can be used in FW's but you are looking at at least 20mm thick.

Excuse me?
Our drum was made from 10mm thick HDPE. Although it took some cosmetic damage, it was only actually pierced once and then only a 5mm hole.

As someone else pointed out, if it is mounted in such a way that it is allowed to flex, it will distribute the force over a larger area meaning the energy is absorbed by much more material. With rigid armour, the energy is concentrated, giving a higher probability of puncture / fracture.

Polycarbonate and Polypropylene are less suitable as they will fracture at lower energy conentrations than polyethylene. To determine how much energy (pressure) a polymer will withstand, it's inversely proportional to it's freezing point (the point where it becomes solid (rather than plastic) and brittle. A low freezing point material will absorb more energy before it fractures.

It's worth looking at Armox instead of Hardox for similar reasons. Hardox, despite the name is intended as a wear resistant steel (things like teeth on digger buckets). It is tough, but will fracture at relatively modest impact energy concentrations. It's not considered ideal for Armour.
Armox is intended for balistic protection. It's available with the same hardness as Hardox but will absorb higher impact energy concentrations without fracturing / penetration.

For structural elements, my preference is Domex / Docal. It has 3x the tensile modulus of mild steel, is easily welded & folded - It is less likely to suffer stress fractures than Hardox as it's elasticity & ductility allow the force to be distributed over a wider area (lower stress concentrations). It's cheaper too! Compared to mild steel, you can achieve a 66% weight saving. It is generally about 3x the price of mild steel - but for the same price you achieve the same strength with 1/3 the mass.
Quick guide here:
http://www.ceweld.com/sites/default/...e%20steels.pdf