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.