Poly bags show up in a lot of familiar places. They may be in the form of a clinging, clear plastic sack at a grocery store to hold fresh vegetables; they may show up as bags to carry purchases from a department store; or they may appear as the very well known black trash bag. All these bags are created by a blown film extrusion process.
Blown film extrusion is an interesting manufacturing process. Resin beads move from a vertical container called a hopper into the extruder, which a chamber containing a threaded rod is called a screw. A screw revolves continuously, moving the material forward; heat is applied to melt the beads to form an extrudate; the extrudate moves through a screen to filter out contaminates, then out of the extruder to pass over a die, which varies in size according to what the end product will be. Compressed air blows the molten plastic and moves it upward through a wide diameter tube to form a bubble; the air flow cools the plastic and rollers form it into flat sheets. The sheets are two ply and, depending upon the end use, may, after cooling, be trimmed for use as a single layer plastic sheet good such as shrink wrap; or, after cooling it may be processed through a sealing and perforation process to create these bags.
Poly refers to polyvinyl, which is derived from petroleum. Due to concern about the longevity of poly trash bags, a very small portion of the bags now created start with a resin that comes from plants, particularly the soybean. This type of bag is designed to decompose over a short period of time into non-toxic elements; this time period may be as little as 60 days. This approach cuts down on one use of petroleum, which grows increasingly more scarce and also mitigates the amount of man made degradation to the environment. Some concern has been expressed about the non decomposition that occurs when plastic bags containing rubbish or garbage are buried in landfills, so the advent of non-petroleum based resin has been well received.
Poly bags began as a convenience but after displacing other packaging materials became very prolific. However, consumer use of and demand for poly bags increased far more rapidly than the awareness of a need to deal effectively with the different consequences of their use. However, from a standpoint of versatility, strength and cost, there are not sufficient substitute materials that can meet the standards of performance set by poly bags, and that may be required, for example, in the medical and electronics fields. Consequently, their use will be with us for the near future.
The author has spent a lot of time learning about large poly bags and other related topics. Read more about cellophane bags at Vincent O'Neil's website.
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