8439.91 - Of machinery for making pulp of fibrous cellulosic materials:

HTS code 8439.91 encompasses parts specifically designed for machinery used in the initial stages of processing fibrous cellulosic materials into pulp. This includes components integral to the breaking down and preparation of raw materials such as wood, rags, or recycled paper into a fibrous slurry. The focus here is on the machinery that transforms these raw materials into a suitable form for papermaking, prior to the formation and finishing of the paper or paperboard itself.

This category is distinguished from its sibling, 8439.99 ("Other"), which covers parts for pulp-making machinery that are not otherwise specified and do not fall under the more granular subheadings of 8439.91. While 8439.91 details specific components involved in the stock-treating aspect of pulp production, 8439.99 acts as a residual category for other, less specific parts of pulp-making machinery.

Within 8439.91, further classification is provided by subcategories. 8439.91.10.00 specifically enumerates bed plates, roll bars, and other parts directly involved in the mechanical treatment of the pulp stock. The remaining parts designed for this stage of pulp production, which do not fit the specific description of bed plates or roll bars, are classified under 8439.91.90.00 ("Other"). This hierarchical structure allows for precise identification of components based on their function within the pulp manufacturing process.