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270
FOOD PACKAGING TECHNOLOGY
Outside
liner
Medium
Inside
liner
:
MD
gh
len
Figure 8.19 Components of single wall corrugated fibreboard (courtesy of The Institute of Packaging).
layers or liners separated by a corrugated inner ply known as fluting. The liner
plies are glued to the tips of the fluting. The resulting material has high bending stiffness in relation to the weight, and high compression strength when
made up in the shape of a box with glued or taped side seams and end panels.
(Side seams and closures using metal staples are not normally permitted for
food packaging applications.)
Double wall corrugated fibreboard comprising three liner and two fluting
plies is produced but this degree of strength is not normally necessary for multiples of primary food packs.
Triple wall corrugated fibreboard is a thicker and therefore stronger material and this is used with protective inner lining bags, usually made from PE
film, for the bulk packaging of free flowing food products and ingredients.
The most common lining ply material is brown kraft liner. This may be
unbleached virgin kraft liner, 100% recycled fibre, also known as test liner, or
mixtures of both types of fibre, the colour is brown. Bleached, white, liner
plies are possible with the use of bleached kraft, and mottled white/brown
liners are based on mixtures of bleached and unbleached fibres. The weights
range from 115 to 400 g/m2, though the typical values for food packaging are
125, 150 and 175 g/m2.
The fluting medium, also known as corrugating medium, may use any of several types of fibre such as mechanical, chemical or recovered recycled fibre.
Several grammages are available in the range (approximately) 100–220 g/m2.
If mechanical fibre is used it is usually of the semi-chemical type, i.e. mechanical pulp subjected to partial chemical treatment which increases the yield
compared with chemical pulp but with strength characteristics which are higher
than that of mechanical or recycled pulp of the same weight (grammage). The