Shrink wrap
From Wikipedia, the free encyclopedia
- For the legal topic, see shrink wrap contract.
 |
Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. This article has been tagged since January 2007. |
Shrinkwrap is a material made up of polymer plastic, usually PVC with a mix of polyesters. When heat is applied to this material it decreases in size so that it forms a seal over whatever it was covering. Shrinkwrap is commonly found on CDs, DVDs, software packages, and books, but the material described is also be found as safety sealant on jars and bottles as well as to seal off exposed, soldered joints in electric wiring. It is then known as shrink tubing.
It is more common to see polyethylene used for shrink wrap. In its most basic form the film is extruded in tubular form in a vertical direction. The die used is referred to as an annular die and it creates a molten tube which may be stretched in two directions. In the vertical direction - the machine direction - it is pulled into a thinner tube by rollers. Air injected into the interior of the tube causes it to stretch in a horizontal direction - the transverse direction. During this process the molten film is cooled and remains stable until heated. It then tries to regain its shape prior to extrusion, shrinking proportionally to how much it was stretched in both directions. It is used for cases of canned goods replacing some of the corrugated paper normally used and for other forms of industrial packaging.
The film can be tailored to meet commercial needs such as clarity, printability, crispness, sealability etc. To meet these ends it can be formuslated with additives or laminated to other layers of thermoplastics.
Shrinkwrap often takes the form of a clear film applied to products to protect them from dust, pests, surface contamination and moisture. The shrinkwrap provides a tamper-evident seal that helps ensure freshness and discourage pilfering. In electrical applications, shrinkwrap is usually in the form of a tube that decreases in diameter upon heating.
Another form of shrink wrap is film that has been deliberately oriented after it is manufactured. These films, PVC or PE are mechanically stretched before cooling and will try to regain their original form when reheated. Under these circumstances the temperature at which shrinking takes place is not as near to the temperature of the molten resin. More energy can be exerted inward and these films are used when point of sale appearance is important.
