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Pacotes, sacos e saquetas - vantagens e desvantagens
12. Folhas de plástico e alumínio
Linha 1:
(Wiley, 527)
Bags and their flexible cousins generally have the lowest unit cost of any packaging form. They also keep shipping costs to a minimum since they have the lowest tare weight ratio (weight of package to weight of contents). Bags can be tailored to fit snugly around the products they contain and, beyond this, will adjust to any shift in the shape of the contents. A fluffy product, which tends to settle on standing, for example, will take up less space in storage because the bag settles with the product. Bags take up a minimum of space in storage and shipment, both before and after filling. Sizes can be made to suit almost any conceivable product and, when empty, they occupy the least amount of space in disposal bins and landfills. The latter has resulted in a new burst of enthusiasm by consumers and great growth in standard and new forms of pouches and bags that will be discussed in this chapter.
FOIL, ALUMINUM
 
On the negative side is the nonsupporting character of standard bags. They may not stand as neatly on store shelves or at dispensing locations as more rigid types of packaging, and the wrinkles and folds may be unattractive for certain marketing purposes. Stacking in the warehouse or in a retail display may also present problems of slippage.
Aluminum foil is a thin-rolled sheet of pure or alloyed
aluminum, varying in thickness from about 0.00017
(4.3 mm) in. to a maximum of 0.0059 (150 m) in. (1). By
industry definition, rolled aluminum becomes foil when it
reaches a thickness less than 0.006 in. (152.4 mm) (see
Table 1). Aluminum foil is widely sold in the consumer
market in rolls of 50 cm in width and in varying lengths.
Aluminum, from which the foil is made, is a bluish
silver-white trivalent metallic element that is very malleable
and ductile. Noted for its light weight, good electrical
and thermal conductivity, high reflectivity, and resistance
to oxidation, aluminum is the third most abundant element
in the earth’s crust (1).
Aluminum always occurs in combination with other
elements in mineral forms such as bauxite, cryolite, corundum,
alunite, diaspore, turquoise, spinel, kaolin, feldspar,
and mica. Of these, bauxite is the most economical
mineral for production of aluminum. It can contain up to
60% alumina, which is hydrated aluminum oxide. It takes
about 4 kg of bauxite to produce 1 kg of aluminum (2).
Alumina is converted into aluminum at a reduction
plant or smelter. In the Hall–He´roult process, the alumina
is dissolved in a molten salt called cryolite. The action takes
place in steel boxes lined with carbon called pots. A carbon
electrode or anode is lowered into the solution, and electric
current of 50,000–150,000 A flows from the anode through
the mixture to the carbon-cathode lining of the steel pot.
The electric current reduces, or separates, the alumina
molecules into aluminum and oxygen. The oxygen combines
with the anode’s carbon to form carbon dioxide. The
aluminum, heavier than cryolite, settles to the bottom of
the pot from which it is siphoned into crucibles. The molten
aluminum is eventually processed into products.
 
(Wiley, 488)
FILM, PLASTIC
Films are continuous membranes that can separate one
area from another. These membranes can vary in thickness,
ranging from less than that for rigid containers, to
sheeting, to film, and even to thin coatings. Usually films
are considered self-supporting and less than B10 mils
(250 mm) in thickness. No definition is given for a minimum
thickness, which can vary depending on the material
of construction of the film; however, thicknesses of o1 mil
(25 mm) are common.
An important feature of most films discussed in this
article is heat sealability, which refers to the thermoplastic
property of the film, or coating on the film, which allows it
to be fashioned into pouches, bags, and overwrapped
packages by virtue of its ability to make a hermetic seal
to itself. Heat sealing is accomplished by heating up the
film areas then applying the hot areas to each other under
pressure. Sometimes these operations occur simultaneously.
During heat sealing, the polymer molecules become
entangled; the better the intermingling, the stronger
the seal. The time allowed for heat sealing in a typical
high-speed food-packaging machine is less than 1 s, during
which time heat-seal strengths of very high values can be
achieved, i.e., several pounds per inch of seal width.
Durability is sometimes borderline, in some instances, deliberately so. On the other hand, it is possible to strengthen a bag to almost any degree by incorporating additional plies, scrims (latticework layers of plastic fibers), and/or tough film laminations. This type of reinforcement adds considerably to the cost, but for very expensive or hazardous products, export shipments, and the armed services, it is sometimes necessary.
Finally, some styles are not siftproof or suitable for variable density products.
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