treating a plastic sheet blank , preferably an acrylic resin sheet , along its edge surface by applying a groove of non - uniform depth to its edge surface along a side of the blank having a length to blank thickness ratio that exceeds a critical ratio for the given blank thickness to improve the conformance of the outline of the compression stretched sheet to one similar to the outline of the thick blank prior to its compression stretching and so stretch the blank more uniformly along its orthogonal axes during the compression stretching of said blank to form said sheet .