Our 18" x 12" x 12" Heavy Duty Corrugated Boxes are manufactured from super strong 275#/ECT-44 single wall kraft corrugated carton. ECT stands for "edge crush test". These Heavy Duty Single Wall Cartons are sold in bundle quantities and ship flat to save on storage space and shipping. 18 x 12 x 12 HD (275#-C) boxes with heavy-duty construction and with 40% more strength than standard shipping containers, provides greater protection and are perfect to use when extra stacking strength is required. Our Heavy Duty Single-Wall style boxes are great for packaging and shipping those heavier items, more fragile items or for moving and storing items.