face does not exceed 250 u00b0 c each of the outer sheet 50 and the panel 51 are secured to the frame 52 and the frame is substantially resistant to distortion of up to a temperature of about 250 u00b0 c application_number US 201113336728 A description fig1 a and 1 b illustrate a 4 panel bi folding u201c casement u201d shutter 10 in a respective fully open position fig1 a and a fully closed position fig1 b fig1 a illustrates a pair of bi fold shutters sections 11 and 12 each formed by a pair of shutter leaves 13 and 14 which are fitted to cover an opening represented by broken line 15 the opening can be closed by a window or door not illustrated the shutter leaves 13 and 14 are of equal dimension and each of the shutter leaves 13 is connected to the associated shutter leaf 14 by hinges 16 likewise each of the shutter sections 11 and 12 is connected by hinges to the frame or surrounds of the window by hinged connection of the leaves 13 with the frame or surrounds in fig1 a the shutter leaves 13 and 14 are folded open completely so that shutter leaf 14 overlies shutter leaf 13 and each shutter section 11 and 12 is fully hinged so.