Our 4" x 30" Open Top Static Shielding Bags are made of a multi-layer laminated film which has a metal layer as its core. This 3 Mil static shield bag has a layer that functions as a barrier to electrostatic discharge or ESD which allows a charge to move around and not through the bag. The inner layer of this 4 inch static control bag provides static dissipative protection for the metal layer and also strengthens seal. The outer layer of this 3 Mil bag is a non-static generating layer that resists abrasion and puncture.