A method of forming a filter unit comprised of a filter element and an encircling peripheral encasement frame to which the filter element is sealed. The frame is moulded in situ by solidification of a liquid, solidifiable moulding composition (e.g. a curable resin or a matter thermoplastic resin that solidifies on cooling) provided around the periphery of the filter element. The method may comprise the steps of: (i) providing a filter element having front and rear faces and a bounding peripheral edge, (ii) locating around the peripheral edge of the element a mould unit which seals against peripheral regions of the front and rear faces and which together with the peripheral edge defines a mould cavity, (iii) filling the mould cavity with a solidifiable, liquid moulding composition, (iv) effecting conversion of the liquid to a solid, and (v) removing the mould thereby producing the filter element.