I am looking to melt some leftover acrylic into a mould, at home. Are there fumes that I should be worried about?
There are lots of links that describe toxic fumes from burning or laser-cutting acrylic.  Apparently, the risk also applies to the molten plastic, as you would encounter with 3D printing or molding.  According to Everything You Need To Know About Acrylic (PMMA):


  ...fumes from 3D printing with acrylic or from the molten material used during injection molding should not be inhaled and ... should be conducted in a well ventilated facility to avoid the potentially hazardous effects of gaseous polymer.