These are more effective than are single-piece screens.

Two-piece screens have been designed specifically for use in filtering a granular material to remove contaminant particles that are significantly wider or longer than are the desired granules. In the original application for which the two-piece screens were conceived, the granular material is ammonium perchlorate and the contaminant particles tend to be wires and other relatively long, rigid strands. The basic design of the two-piece screens can be adapted to other granular materials and contaminants by modifying critical dimensions to accommodate different grain and contaminant-particle sizes.