Knife for perforating plastic sheet material

An elongated perforating blade for use in apparatus for forming tear perforations in a two-ply plastic material, said blade including

• a series of spaced apart triangular shaped teeth formed along a bottom edge of the blade, said teeth having front and back surfaces,
• each tooth having a thickness, a root, a tip that lies on a common line describing the bottom edge of the blade, and a pair of linear side edges that diverge uniformly from the tip toward the root,
• said side edges of adjacent teeth being separated by lateral edge grooves that extend upwardly into the blade beyond the roots of the teeth,
• said front and back surfaces of each tooth being hollow ground so that the thickness of the tooth decreases from the root of the tooth toward the tip,
• a bevelled surface extending along the linear side edges of each tooth to form a sharp cutting edge extending from the tip of the tooth to an adjacent edge groove, said bevelled surfaces combining at the tip to form a chisel edge tip that slopes obliquely and upwardly from the back surface toward the front surface of the tooth,
• whereby each tooth pushes a top sheet of the two-ply material over a bottom sheet as the tooth penetrates into the ply, thereby preventing the plies from sticking together as the tear perforations are being formed.