Can 4mm polycarbonate be cold bent into shape of windscreen ? took cover off today and after a short period since it was fitted small internal hairline cracks have appeared on the curves luckily I have got enough material to make 2 more but do not want the same happening again

It can be cold formed but for 4mm sheets the minimum bend radii is quite large between 400 and 700mm depending on grade. See page 15:-
http://www.theplasticshop.co.uk/plastic ... _guide.pdf

If you can make even the most rudimentary former you could have a go at hot forming it using a heat gun. Any sort of permanently formed curve wil help to reduce the stresses in the screen.

Cheers Floater Will give it a go with a bit of heat,... it will give me chance to get my posh heat gun out of it's box !