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Kraft Paper in Packaging Printing and Book Printing

There are many types of paper applied for packaging printing and book printing. To create general ideas, from this chapter, we would like to introduce most popular ones of them. This chapter is about Kraft paper.

Kraft paper is made by the fourdrinier process for a pure chemical sulphate pulp. The term Kraft is of Scandinavian origin and means literally strong. It is only applied to paper or board made entirely from paper-pulp produced by the sulphate process. It is available either unbleached (brown), which is used for general wrapping in packaging printing, or bleached which is used for bag-making and for wrapping clothing, food, medical goods and stationary. The bleached material is also used for dyeing to various colors and may have decorative applications in ribbed and striped form. Various finishes can be imparted during the final stages of the paper-making process according to requirements. Unglazed paper is smooth on both sides and does not have a gloss finish. Machine-finished (MF) papers are treated on the machine to produce a smooth, but again not a glazed surface. Water-finished (WF) papers are produced by damping one or both sides on the machine, and this gives rise to a higher gloss and a smoother finish than MF. Machine-glazed (MG) papers have a high gloss finish on one side only, and super-calendered (SC) papers posses a high finish imparted by damping on the machine and subsequent treatment in a super-calender. This last variety is used extensively where attractive wrappers are required. All of these finishes may be supplied either plain or with a ribbed structure. In addition to wrapping application, Kraft papers are used for multi-wall sacks bags, envelopes and gunned sealing tape.

Kraft papers popularly application is due to its high strengthen, and strong water resistance. Bleached white Kraft paper is applied sometimes in book printing field, with four colors printing.

Kraft paper is produced on paper machines with moderate machine speeds. The raw material is normally softwood pulp from the kraft process. Kraft paper can be white or brown depending on the pulp if it is bleached or unbleached.

Here below are some characters of quality.

• Normal kraft paper is strong and relatively coarse. It has high tensile strength. The grammage is normally from 40-135g/m².

• Sack kraft paper, or just sack paper is a porous kraft paper with high elasticity and high tear resistance, designed for packaging products with high demands for strength and durability.

• Absorbent kraft paper is made with controlled absorbency, i.e., a high degree of porosity. It is made of clean low kappa hardwood kraft and has to have a good uniformity and formation.

• Spinning kraft paper is an especially strong about kraft paper with relatively low grammage (40g/m²). This paper requires the best possible machine direction strength and cross machine elongation. This is done by high fiber orientation on the papermachine.

• Hunting cartridge paper is a kraft paper used in shotgun shells. This paper needs a high tensile strength in the machine direction, which is the axial direction of the cartridges. In the cross direction, the cartridge is supported by the gun-pipe, but a sufficient elongation is needed. The body of the cartridge is wound of a kraft paper of 80-120 g/m², which is further covered by an outer sheet of 60-80 g/m² with colour and printing.

• Candy wrapping paper or twisting paper are thin 30-40g/m² kraft papers and is mostly flexo or offset printed. These papers requires a good strength, with highly oriented fibers. Twisting paper is mostly opaque and often supercalendered.

Kraft Paper Used for Paper Bag Printing Jobs