There are two basic categories in the shrink film market; the Polyvinyl chlorides (PVC) and the Polyolefins (POF). Shrink polyethylene is not typically included in the competitive polyolefin category; since polyethylene lacks the clarity, gloss and the tight second skin. These attributes are critical for retail appeal.
The polyolefin category consists of films made from polypropylene and polyethylene copolymer resins. Since it is compose by different or the same polymers films that extrude together, it contains a unique characteristic from the component polymers.
The additive, mainly plasticizers, has given the PVC shrink films properties. If the film grows old, the plasticizers will expatriate and the film returns its brittle state and yellowish color. Furthermore, PVC is a thermo sensitive and have a tendency to fragment and break at cold temperature. When the film expose to high temperature, PVC may “shrink-back” and change to yellow color. Therefore PVC films need to store at a warehouse under 25℃ to maintain the film quality.
A heat knife sealing system is a common model nowadays. It has a higher durability than the traditional sealing wire system. A knife or wire seals the films against a pad which is covered by Teflon tape for protection. Three most critical elements affecting the seal quality, they are Time, Pressure and Temperature.
If the sealing time is too short, the temperature is not high enough and uneven or insufficient of seal pressure, the film will not cut or poor seal line will occur. Reversely, if the sealing time is too long and temperature is too high, the film may break just behind the seal.
A carbon char will build-up on the PVC film sealing system and need to be cleaned from the sealing head. The carbon char will retard the sealing process and make an unsightly appearance on the sealed ends of the package.
For a proper shrinking:
POF film needs an air evacuation hole for hot air to escape the bag. Expose to correct temperature for a correct period of time is essential. Multiple perforations are important for the hot air inside the bag to escape for proper shrinking. Furthermore, the POF surrounded by the correct air velocity is important factor too. Air flow allows the film to stand away from the product and a “balloon” is formed. This will finally achieve a good and even shrink appeal when the “balloon” formed.
PVC film does not need an artificially created air evacuation holes for hot air escape. It is because the seal of the PVC film is full of small pinholes. These small pinholes are acted as the air escape hole in the shrinking process. It seems an advantageous since a pin-perforation step is eliminated; however, the small holes in the seal area actually weaken the seal and eventually broken the package.
Beside the small holes in the seal area, PVC films shrink rapidly when exposed to heat. Therefore air velocity is not important to shrink PVC films but POF does. Some low-end tunnels which have no settings to control air velocity are difficult to achieve a good shrink of POF.
When the product wrapped with shrink films and enters the tunnel, heat energy causing the air inside the bag to expand. At the time the bags start to shrink, air vents holes allow the air to bleed-off from the bag to release the pressure, prevent the bag from rupturing.