Stretch wrap or stretch film is a highly stretchable plastic film that is wrapped around items. The elastic recovery keeps the items tightly bound. In contrast, shrink wrap is applied loosely to an item and shrinks tightly with heat.
It is frequently used to unitize pallet loads but also may be used for bundling smaller items. Types of the stretch film include bundling stretch films, hand stretch films, extended core stretch films, machine stretch films, and static dissipative films.
The most common stretch wrap material is linear low-density polyethylene or LLDPE, which is produced by the copolymerization of ethylene with alpha-olefins, the most common of which are butene, hexene, and octene. The use of higher alpha-olefins (hexene or octene) gives rise to enhanced stretch film characteristics, particularly in respect of elongation at break and puncture resistance. Other types of polyethylene and PVC can also be used. Many films have about 500% stretch at the break but are only stretched to about 100 – 300% in use. Once stretched, the elastic recovery is used to keep the load tight.
There are two methods of producing stretch wrap.
1) Blown: the resin is melted and extruded through an annular die, it is air-cooled. This is a slower process but provides for higher quality in all the areas listed below. The cost of production is also higher due to the quantity that can be produced per hour.
2) Cast: the film is extruded through a slot die, then passed over cooling rollers. This makes the cooling process quick. The quality is not as good as blown but more can be produced in an hour with lower costs.
Other properties such as break strength, cling, clarity, tear resistance, static discharge, etc. are also important.