Flat poly bags have open top that can be closed with twist tie or sealed with heat or tape. Varies from 1 to 8 Mil in thickness, choosing the right thickness depends on the types of product. 1 to 1.5 Mil flat poly bags are lightweight, economical, and can be used for temporary storage of items like food and clothing. 2 Mil flat poly bags are the most popular thickness used for light to medium weight products. 3 Mil flat poly bags are sturdy and protects medium to heavier products. 4, 6, and 8 Mil flat poly bags are extra heavy-duty for punture and tear resistance, can hold items such as industrial hardware, tools, nails, and more.