Shrink Film Basics

FILM CATEGORIES

The shrink film market is comprised of two basic categories; the polyvinyl chlorides (PVC) and the polyolefins. Shrink polyethylene is not typically included in the competitive polyolefin category; “poly” lacks the clarity, gloss and the tight second skin polyolefins provide. These attributes are critical for retail appeal.

The polyolefin “family” consists of films made from polyethylene or polyethylene and polypropylene copolymer resins. All Cryovac shrink films are polyolefins.

PVC shrink films derive their properties from additives, which are primarily plasticizers. AS these films age, the plasticizers migrate and the film reverts to its brittle state, turning to a yellowish color. Also, PVC films tend to be temperaturesensitive, developing a tendency to shatter and split at cold temperatures (below freezing), and “shrink-back” and/or block when exposed to high temperatures over a period of time. The color may also change to yellow when exposed to heat (such as being stored in a non-air-conditioned warehouse over the summer).

FILM SEALING SYSTEMS

	1. As seal bar close, film in direct contact with the wire melts. Cutting action is produced as film shrinks and pulls away from each side of the wire.
	2. Melted ends are compressed and forced to fuse togther between the pressure face of the bar and the pad.
	3. Seals are produced once film pulls far enough away from the wire so they return to a plastic state.

Until recently, most sealing systems used a hot wire in order to seal and cut-off the film at the same time; now, a knife system has gained in popularity on many models, due to its durability. The wire or knife seals the film against a pad, covered with Teflon tape to protect the pad. Time, temperature and pressure are the three most critical elements affecting the seal quality.

If the sealing head is not held on the film long enough, the temperature is too low, or the pressure is insufficient or uneven, the seal quality will be poor, and/or won’t cut.

If the sealing temperature is too low, or the pressure is insufficient or uneven, the seal quality will be poor, and/or won’t cut.

If the sealing temperature is too high, the film may break just behind the seal. Following the impulse sealing cycle, a cooling cycle allows the film to “solidify” into a solid seal. This dwelltime is critical in order for a seal to hold.

PVC films require only heat in order to cut seal and cut. When run on a sealing system as previously described, build-up of carbon char (black specs and flakes) will occur, and will need to be cleaned from the sealing head on a regular basis; otherwise, the carbon will interfere with the sealing process, and will look unsightly on the sealed ends of the package.

Since only heat is needed to form a PVC seal, some machines use a hollowed out bottom platen instead of the foam pad; direct contact with the seal wire is eliminated. Other systems employ a much thinner nichrome wire (.020” diameter vs. a standard .036” to .040” for polyolefins) against a hard bottom sealing pad. This system is insufficient to create strong polyolefin seals. One other sealing system designed for PVCs is called a Universal Sealing System. Despite the name, the hot knife is typically too sharp to form a quality polyolefin seal. A rounded knife is better suited.

FILM SHRINKING SYSTEMS

Heat causes air inside the bag to expand, while the bag shrinks.
Vent holes bleed-off excess pressure to prevent the bag from rupturing.

In order for proper shrinking to take place, polyolefin films need an air evacuation hole or multiple perforations. A bag formed with PVC film does not need“artificially created” air escape holes. Because of the make-up of PVC, the seal typically is full of small pinholes; this is where the air escapes during the shrinking process. While this may seem advantageous (a step can be eliminated), the small holes actually weaken the seal area. These holes are often located near or in the package corners.

In order for proper shrinkage to occur, polyolefin film must be exposed to the correct temperature for the correct amount of time (which is controlled by conveyor speed), and also be surrounded by the correct air velocity, or wind turbulence. The air allows the film to stand away from the product, and a“bubble” is formed around it. A good, even shrink should result when this occurs.

PVC films shrink readily when exposed to heat, and therefore air velocity is not critical. In the marketplace there are low-end tunnels which have no settings to control air velocity (similar to an oven) and you may find it difficult to achieve a good shrink.

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