Armour protects the internal components of the robot from damage and flying debris that has been dislodged by an attack. Antweights started off using cardboard and plastic sheeting as armour, however today there is a great variety of (much stronger) materials that provide suitable protection.
Some popular materials include:
HDPE - the most recently introduce material, provides a good balance between strength, weight and cost.
Titanium - has great strength while remaining light, however is often very expensive and difficult to find in thicknesses suitable for Antweight use.
Polycarbonate - light, cheap and easily sourced. Provides decent protection, however it struggles to stand up to the most powerful spinning weapons.
Aluminium - heavier than Titanium and not as strong, however it is cheaper and easier to work with.