How are plastic shopping bags manufactured?

Industrial manufacturing processes are fascinating to watch and few people know how common, everyday items are produced.

Plastic bags start out life as large, thin rolls of plastic, often produced by the bag manufacturers from plastic pellets. The plastic can be produced in different colours by adding coloured pellets to the semi-transparent raw plastic. The roll of plastic is unwound and fed into the machine that produces the bags or the plastic film is extruded at the beginning of the line.

The bags are printed using several rollers in sequence, each printing one colour at a time. A four-colour CMYK printing process (cyan, magenta, yellow, and black) is usually used.

The film is then folded over on itself and heat-sealed along the side seam. Heavier gauge bags may be made from two rolls of plastic, brought together and joined at both sides. This forms a flattened tube of plastic. Handles may be added at this stage by a stamping / heat bonding process or molded directly into the bag. The bags are then formed by heat-sealing the bottom and cutting them to shape and size. Some heavy-weight bags have plastic handles added after the bags are made. Waste plastic from cuttings is recycled into new bags.

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Footnote: if you live in Canada, I can highly recommend the show "How It's Made", broadcast on the Discovery Channel in English and on Z-télé in French (http://www.commentcestfait.com/). I don't know if it has been syndicated elsewhere. Each half-hour episode highlights four different manufacturing processes, three in depth and one very briefly. The show is made in Montréal and films industries in and around that city. They have documented everything from handmade recorders and mushroom growing to sewage processing plants and heavy industries.

Most throwaway plastic shopping bags are manufactured using oil-based resources, which are non-sustainable and pollute the environment.

Most plastic bags are shaped like two identically-sized rectangular plastic sheets fused together on three of the sides, with one side left for the opening. This type of shape allows for simple, economic manufacturing and compact storage of the bags before use. Polyethylene is an economical plastic so it is commonly used for disposable bags.

Plastic bags can be made having various colours, including translucent or even transparent in some cases. Bags made of HDPE (High Density Polyethylene) are typically not see-through or are translucent, but not fully transparent. Bags made out of LDPE (Low Density Polyethylene) can be quite clear, but are still usually not as totally transparent as other plastics. HDPE and LDPE bags are manufactured from a waste product of the petroleum industry.

http://en.wikipedia.org/wiki/Plastic_bag
http://www.kesab.asn.au/betterbag/

This is an interesting read pro and con.
http://www.epa.gov/region01/communities/shopbags.html