Sifting screens

A frame over which woven wire mesh is to be stretched and secured to form a sieving screen and which can be used to screen solids from drilling mud recovered from down-hole when drilling for oil or gas. The frame is a rectilinear moulded plastics frame having edge regions by which it is secured in place in a shaker. Within the frame is a plurality of rectilinear windows formed by an orthogonal array of intersecting ribs also of moulded plastics material. Some of the ribs are internally reinforced by rigid metal members which extend orthogonally between hollow box-section members which define a sub-frame for reinforcing the four edge regions of the frame. The orthogonal members are secured at their ends to the sub-frame. The ends of the sub-frame members are joined at the four corners of the sub-frame. In this way not are only the edge regions of the frame reinforced internally but so also are some of the orthogonally intersecting ribs, so as thereby to produce a rigid frame for the screen. The internal reinforcement for those ribs which are to be reinforced may comprise hollow box-section metal members which may be of similar cross section to that of the sub-frame members, or the internal reinforcement for those ribs which are to be reinforced comprises metal I-beam cross section members, which typically have the same height as the height of the box-section members forming the sub frame.