webbed multiple sheets comprising two substantially parallel , substantially plane polymethylmethacrylate outer layers and at least one center layer between said outer layers , wherein said outer layers and said center layer are bonded together into one piece by webs and wherein said center layer or layers comprise a thermoplastic material having increased tenacity , increased zero viscosity or a combination thereof in molten condition as compared with the material of said outer layers , thereby reducing sagging of said center layer or layers during extrusion .