How Shrink Wrap Shrinks
Most of us don't give much thought to how shrink wrap works or how it got there in the first place. We just know it usually takes our teeth to get it off, but it actually involves a manipulation of molecules. Polymers are long molecules found in plastic, where they are usually tangled and random. When they are made into a plastic film for shrink wrapping, the molecules are stretched out almost straight. We take this film and place it around whatever we want to wrap and add heat. Once the heat is applied to the film, it shrinks because the heat causes the polymers to return to their natural tangled state. A PVC film can shrink as much as 50 percent of its original size.
The Benefits of Shrink Wrapping
Shrink wrapping makes packages more tamper-proof, and keeps out moisture and light exposure. Shrink wrapping, for example, is great for CDs because you can still see the cover. The benefits for crafters are many: You can make your own gift baskets or crafts, and dress them up with a colored shrink wrap while protecting them and keeping them clean until you give them away.
From Food to Boats
Shrink wrap is used in many applications. Food companies use it to seal a jar or bottle so that you know it has not been opened. Most of this type of wrapping is done in a machine. In the case of a bottle or can, a shrink film bag is put around the can. It is then put into a machine that seals the bag. Then it is sent down a tunnel that slowly heats the film, causing it to fill with air. There are small holes in the film, and as the film reacts with the heat, the air slowly goes out of the holes and the film shrinks to the size of the can.
In the case of a boat, the film commonly used is a co-polymer called EVA. The process to get the film over the boat is quite complicated and must be done exactly right. Once the film is applied to the boat, it is heated up and shrunk to the skeleton of the boat. They then have to go back and tape up any holes and make sure there is ventilation. The reason for the critical placement is that the boats are normally left outside. In the case of snow or ice pooling on the wrap, it can become very heavy. This can cause damage to a very expensive piece of equipment. The wrap must be placed so that rain and snow run off the boat. This type of film is much thicker than the type used to wrap a CD. It also does not become brittle in cold weather, and it is puncture- and flame-resistant.