The main raw materials of paper bag machine are pulp and paperboard, which are produced by pulp mills and paper mills. Therefore, pulp mills and paper mills can be considered as upstream customers of paper bag machines. Manufacturers who provide machinery and equipment can also be considered as upstream customers of paper bag machines. Paper bag machine manufacturers need to buy various parts and raw materials to make paper bag machines, such as steel, electronic components, rubber products and so on. Therefore, suppliers who provide these raw materials and parts can also be regarded as upstream customers of paper bag machines.