Film Blowing Extrusion Technology----Rewinding, Film Surface Treatment

Because of their simple, economical design, surface winders tend to be the more popular of the two. They operate by employing a motor-driven contact roll at the film roll surface. The contact roll essentially lays the film onto the rotating film roll. As the film roll diameter increases, the axis of the film roll moves in a path that separates it from the contact roll, maintaining pressure at the surface. This system is used mainly for medium and high-tension rolls.

Axial(or center)winders are often used when low-tension roll packages are required. In these systems, the central axis of the film roll is motor-driven rather than the contact roll. This type of winder tends to be more expensive than the surface winder.

A couple of other functions performed by winders include automatic roll changeover and static dissipation. Turret winders are those that have a mechanism for moving a fresh roll (core)into position on-line while moving a full roll off-line. This can be initiated manually by an operator command or automatically based on roll weight or film length. Winders are often equipped with static discharge systems to handle the large amount of static electricity that builds up during blown film winding.

Film Treatment:
Polyolefins, such as polyethylene and polypropylene, are nonpolar polymers. This means that any polar materials will not easily wet the surface of(adhere to)these polymers. One area where this has important implications is in the printing conversion process. Inks cannot absorb into most films because they are typically nonporous. The only way the ink will stay on the surface of the film is if it wets the polymer rather than puddle up. To promote the ink adhering to the surface, it becomes necessary to treat the film to make it more polar.

The most popular treatment process for blown film is the corona discharge method.Corona discharge units(Fig.3.20)are high voltage devices that ionize the air in a small region through which the film travels. The ionization creates ozone, a very strong oxidizing chemical. The result is that the film is oxidized, creating a polar surface rich with oxygen atoms.The level of oxidation needed for a given film may depend to some extent on the additive package in the raw material. Also, this type of surface treatment is not permanent. In most cases, the film must be printed within a few days of treatment because additives in the plastic may migrate(bloom)to the surface and inhibit the treatment.

Safety is a top concern where these high voltage corona discharge systems are employed. Adequate venting is necessary in the vicinity of the corona to extract the ozone created by the discharge unit. Also, a shut-off interlock should be used in the event of a film line break.