I always understood that plastic ice is where it has been through the freeze thaw cycle and stops being brittle and 'dinner plating', often turning a blue in colour. My mate was saying he thought plastic ice is where the ice is slightly wet and you can sink your axes in all the way and your crampons go in complete with the toes of your boots.
I remember encountering this on my first WI5 lead and it made life so much easier on steep ice. It was a joy to climb - the axes went it beautifully without effort, no need for exhausting kicking - it was sensational.
Plastic can be an adjective not just a noun, so like that - when the ice is pliant. Temperature makes a massive difference as does 'wetness' for the want of a better term. I climbed Introductory Gully (II) on Sunday morning and it was all on fresh water ice, yet it was warm enough to make that fresh ice soft and squidgy - exceedingly easy to climb!
made of plastic.
(of substances or materials) easily shaped or moulded.