metal , fiber or plastic panels are fabricated by the attachment of planar imperforate and / or partially perforated outer sheets to a specially formed center member comprising a web or sheet having a deformed surface defining an array of adjacent triangular projections and depressions . the center member is produced by the longitudinal cutting and folding of the web as it passes between two forming cylinders which mesh with one another , the cylinder peripheries carrying complementary arrays of spaced triangular - shaped tooth elements having their base lines aligned in the opposing cylinders , to shear and thereby relieve the passing web laterally so as to produce intermittent parallel cuts and angular folds in the web while gathering the web longitudinally thereby to form triangular , flat - topped cells in the center member . instead of using two outer sheets , the formed center member can be used per se , e . g ., as a packing medium , or it can be combined with only one outer sheet to form a single - face product .