An arch shape cross section thermoplastic leaching chamber, having sidewall slot perforations which flare inwardly, is made using a mold comprised of a mold cavity part and an inwardly retractable mold core part. The core part has opposing side slides with projections that define the perforations; they are separated by a center block. The center block moves downwardly and the slides move inwardly, under action of slide rods built into the mold core, to enable a molded chamber to be removed from the mold.