A plastic bag is a bag-shaped packaging material that is resistant to water and dirt and is durable. It is used for many purposes such as garbage bags, packing products, managing small accessories and protecting from dust. There are a zip type, a handheld type, and a tube type for long items. Materials for plastic bags include low density polyethylene and high density polyethylene. Low density polyethylene has low intermolecular density and is flexible and transparent and is used for simple packaging and garbage bags. High density polyethylene has a high intermolecular density and a hard texture and is opaque, and is used for cashier bags. In addition, aluminum bags made of polyethylene with aluminum foil are used for long-term storage of metals and precision parts and transportation under severe conditions because they have high light-shielding properties and prevent permeation of oxygen and moisture. There are special plastic bags to keep food fresh and antistatic plastic bags for electronic parts and PCBs.