a method is disclosed for making a substantially flat filter plate comprising a substantially flat thermoplastic polymer frame and substantially flat filter material seated fixedly therein . the method is characterized by its reduced exposure of the filter material to adverse mechanical stresses resultant of thermoplastic frame formation and curing . toward such end , the substantially flat thermoplastic frame is formed in at least two separate steps , in the course of which said substantially flat filter material is incorporated . the portion formed first serves as a mechanical restraint , insulating the later incorporated filter material from any propensity of the second “ embedding ” portion to shrink or otherwise warp during its formation .