The U-shaped bag is constructed from three body panels, one of which covers the entire length of the bag, forming the bottom and sides, and the other two pieces of fabric sewn into the larger piece to form the other sides. Similar to other construction types, U-shaped bags are square after filling, especially U-shaped bags with stretch bars. The U-shaped bag is an industry standard and is the most popular type, offering a high Safe Working Load (SWL) and solid durability.
Round bags are made of tubular fabric that forms all sides of the bag, allowing only the top and bottom of the bag to be stitched. Although the fabric is tubular, a round lain bag will still resemble a square bag when filled. There is a common misconception about the fill shape of round bags, they are not literally round, they are just constructed from a piece of fabric that makes up all four sides of the bag.
The 4-piece FIBC is constructed with four individual pieces sewn together to form a square structure. When filled, the 4-piece lain bag will maintain its square shape. In general, four-piece bags hold shape better than U-shaped bags, which hold shape better than round bags. The biggest disadvantage of 4-piece Fib Cs is that they require a lot of stitching. However, 4-piece Fib Cs are ideal for storage as they retain their shape better when filled.