Shrink Film or Stretch Wrap - It’s easy to confuse shrink film and stretch wrap.
Shrink film is a flexible packaging film that is sealed around a product and heated in a shrink tunnel. The heat inside the tunnel causes the film to shrink tightly around the product. Shrink film can be thin for wrapping lightweight products or thick for large or heavy items.
Stretch Wrap is a strong stretchy plastic that is commonly wrapped around palletized loads or heavy bulk items like 55 gallon drums. Stretch wrap is commonly applied at the end of the packaging process before the products are shipped on to their final destination. Its primary purpose is to keep products from shifting during transport. Stretch wrap does not shrink and is not affected by heat guns or shrink tunnels.
How Is Shrink Film Made
During the manufacturing process, shrink film is heated. This orients the film molecules from an initially random pattern in to long chains. The film is then quickly cooled setting its shrink characteristics. The reheating process, commonly taking place in a shrink tunnel, causes the film to shrink back toward its original dimensions. Shrink films can be made to shrink in one direction (unidirectional or mono-directional) or in both directions (bidirectional). Today, the most common shrink films are bi-directionally oriented. This means the film shrinks evenly all around the package.
Shrink film can supplied in many forms. Flat roll stock, commonly called singlewound sheeting, can be wrapped around a product using specialized machinery. Centerfolded film is supplied on a roll where the plastic is folded in half. The product is placed in the center portion of the film, the remaining three edges are sealed using an L-Bar sealer to form a bag, and the package is heated inside a shrink tunnel causing the bag to shrink around the product for a tight clear fit.
How Is Stretch Wrap Made
There are two kinds of stretch wrap film: cast and blown. To make cast film, polymer resin pellets are melted and squeezed through a flat die horizontally. From the flat die, the melted thermoplastic goes onto a chill roll where the film becomes solid, similar to the cooling of plastic. This process makes it easier to see through the wrap, and when you unwind it from its roll, this film is generally very quiet.
Blown film is also extruded. The thermoplastic material is melted, but it is then squeezed through a specialized rotating die with a hole in the middle. The film comes out vertically as a tube. Air is pumped into the tube through the hole in the die. This causes the tube of molten film to expand and become thinner. At the top of the tube is an air ring that cools the melted material. The process is more complex than the process used to make cast stretch wrap, which is why this wrap can be more expensive. Blown stretch wrap is also not as clear as cast stretch wrap. This is because it tends to crystallize and become cloudy during the manufacturing process. Because it contains additives to make it tacky, the film sticks to itself. As it is removed from the roll, the film unsticks which causes the noise many of us are accustomed to.
Is Cast or Blown Film Better
Neither film is "better" than the other, just different. Cast film is quiet and very clear. It's easy to use and is well suited to wrapping almost anything that isn't too abrasive by hand or machine. Blown stretch film isn't as clear but it does have excellent abrasion resistance and doesn't tear easily. This makes it a particularly good choice for wrapping bricks, concrete blocks, roof tiles and other industrial products.