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186
FOOD PACKAGING TECHNOLOGY
film, a longitudinal seal is made and the end seals are neatly folded, envelope
style, prior to sealing with a hot platen which presses against the folded ends.
Shrink wrapping is similar to the overwrapping described above, except that
the packs pass through the heated tunnel once the cross seal is made – there are
no end seals. The film shrinks over the ends of the pack, the extent depending
on the width of the film used.
Another packaging format results in either flexible or semi-rigid packaging,
depending on the films used, where the film is fed horizontally and cavities are
formed by thermoforming. The plastic sheet, such as PET/PE or PA/PE, is
softened by heat and made to conform with the dimensions of a mould by pressure and/or vacuum. Where more precise dimensions for wall thickness or shape
are required, a plug matching the mould may also be used to help the plastic
conform with the mould. The plastic sheet may be cast, cast coextruded or
laminated film, depending on the heat sealing and barrier needs of the application. Products packed in this way are typically cheese or slices of bacon. This
form of packaging may be sealed with a lidding film laminate under vacuum or
in a modified atmosphere (MAP). The various methods of heat sealing are
discussed in Section 7.9.2.
7.2.4
Rigid plastic packaging
Bottles are made by extrusion blow moulding. A thick tube of plastic is extruded
into a bottle mould which closes around the tube, resulting in the characteristic
jointed seal at the base of the container (Fig. 7.8). Air pressure is then used to
1. Parison ready.
Mould ready
2. Mould moves over
parison. Mould
closes
3. Mould moves down. 4. Mould opens, bottle
Parison inflated.
is released. Cycle
Next parison
repeated
extruding
Figure 7.8 Extrusion blow moulding (courtesy of The Institute of Packaging).