Just thought Id chip in as we probably have more experience than most in the use of plastics in robots.

Polypropylene - good if you want something stiff but with the kind of impact energies in RWs, it tends to shatter, in a similar manor to polycarbonate.

HDPE - requires almost 10 x the energy density on polypro to shatter. It is softer, but will tend to plasticly deform and absorb impacts / spinning things better than anything Ive come across.

Our drum & outriggers are now made from HDPE.

As someone else mentioned, the way to glue it is with silicone sealant or mastic. In order for a laminate to work as armour from piercing, the layers need a certain degree of relative movement. The mastic provides this.

When our drum was made from Polypro, IronAwe knocked several big holes in it. The same spec of drum made from HDPE has yet to be pierced which kind of speaks for itself. Several things have gouged it, but nothing has got through!

Top this end, we are now building an axe robot, using HDPE for armour - in a way nobody has done before. Providing protection from spinners as well as axes. No laminates, just an intelegent use of the material properties!

Si