Additionally, contaminants require process adjustments and can lead to increased machine run interruptions. In general, paper manufacturing and printing systems often have to operate at reduced speeds, raising costs per unit of output. Paper breaks occur more often lowering productivity. Stock preparation is an important part of the recycling process. This process involves repulping, removing contaminants and preparing recovered fibers for paper making.
Contaminants can affect finished product quality or the ability of paper machines to operate.
Heavy contaminants include metals such as nuts, bolts, wires, staples and soda cans as well as sand, rocks and glass. Light contaminants include plastic, Styrofoam, waxes, glues, adhesives, pressure-sensitive adhesives, and wood. Nearly all Northwest mills contacted for a 1992 Northwest Pulp and Paper Association survey indicated that they prefer to receive materials from total source-separated systems because they get better quality material and it requires lower capital costs for secondary fiber processing. Nearly all of them also said they prefer that the recovered paper they receive be baled. There are 43 regular and 37 specialty grades of recyclable paper, each of which has different fiber qualities and end uses.