A
paper sack is a type of paper bag that can be constructed of one or several layers of high quality
kraft paper, usually produced from virgin fibre. Paper sacks can also be referred to as industrial paper bags, industrial paper sacks, shipping sacks or multi-wall paper bags or sacks. They are often used for packaging and transporting dry powdery and granulated materials such as fertilizer, animal feed, sand, dry chemicals, flour and cement. Many have several layers of
sack papers, printed external layer and inner plies. Some paper sacks have a
plastic film, foil, or polyethylene
coated paper layer in between as a water-repellant, insect resistant, or rodent barrier. Multi-wall paper sacks are designed to provide strong product protection, with high
elasticity and high tear resistance, for products with high demands for safety and durability. Information such as instructions, logos or trademarks can be printed on the resistant outer surface.
Plastic films or different dispersions are sometimes used as inner layers or coatings to provide a barrier against moisture, water vapour, grease, oxygen, odours and bacteria. Paper sacks are produced on paper sack converting machines consisting of tuber and a bottomer. There are two basic designs of bags: open-mouth bags and valve bags. An open-mouth bag is a tube of paper plies with the bottom end sealed. The bag is filled through the open mouth and then closed by stitching, adhesive, or tape. Valve sacks have both ends closed and are filled through a valve. A typical example of a valve bag is the
cement sack.
Properties Paper sacks are usually made of
Kraft paper, which has the advantage of being soft and strong at the same time. The stretch or
elongation increases the energy required to break the material. They can carry and protect products up to 50 kg, and adapt easily to the nature of their contents and to handling constraints. Depending on the product, the weight ratio of a paper sack to its contents can be up to 1:250. The strength is due to the arrangement of the fresh fibres used in kraft paper production. The bonding together of the fibres during production improves not only the strength, porosity and elasticity, but also the tear-resistance. One of the natural and unique characteristics of sack kraft paper is its porosity. Acting as a filter material, high-porosity paper enables the air used in the filling process for dry powdery goods to escape very quickly, without the need for air extraction systems. This makes it possible to achieve filling speeds of up to 3.5 sec for a 25 kg sack. Paper sacks also provide a medium for promotional messages and sophisticated printing designs. Due to their natural, non-slippery texture and their construction, paper sacks can safely be handled, stacked, palletized and stored. User-friendly opening systems, such as a tear-open flap, allow quick and clean access to the contents without the use of tools such as knives. ==Construction==