In the manufacture of films, there are air- and water-quenched blown films, and there are chill-roll-quenched cast films and sheet-quenched for biaxially oriented films. This two-part column will focus on chill-roll quenching to form a cast film or cast sheet for subsequent stretching.
The basic process operates on typical 3-layer, cast-film lines and typical OPP-casting systems with a water bath. Other sheet-casting lines, say for OPET or OPP, can employ a cast roll without the water bath while many sheet lines use a polishing or calendering stack. Sheet for oriented tapes are often produced by casting directly into water as were some films.
Uniform heat transfer is the key
With the exception of the direct casting into water, a key feature of chill-roll casting is the level of adhesion of the melt curtain to the chill roll to give uniform heat transfer in the cooling of the melt curtain. Note that the goal is uniform heat transfer from the melt to the chill roll – and not necessarily rapid quenching. There is always an optimum cooling rate for a product to control the film or sheet morphology, which will impact the film line’s productivity and the film’s final properties.
To achieve a uniform quenching, there is generally employed a force to push the melt onto the roll surface (see Figure 5a), and this is known as “pinning” the melt to the chill roll. Pinning methods range from vacuum box, air-knife, electrostatic pinning (see Figure 5b) and nip-roll pinning, to highlight a few. Now, this is where it gets interesting, because not all molten polymers will adhere uniformly to a chill-roll surface. This can cause difficulties if the edge of the film or sheet doesn’t adhere uniformly to the chill-roll surface in cast films and pinning is employed to give as uniform a surface contact to the chill roll as possible. Ideally, the center and edges of the film or sheet are uniformly quenched.
In cast films, edge quality is less important than in sheet for orientation lines. This is simply because the edges are trimmed off and recycled before winding. However, for tenter-orientation lines, the sheet edges must survive the Machine Direction (MD) orientation without causing film breaks and then the edge must be flat enough to easily enter the tenter clips for Transvers Direction (TD) stretching. Then, after leaving the tenter, the thick edges are trimmed and recycled before winding. If the sheet edges are not quenched uniformly they may have brittle areas, which cause breaks in the MDO, and they will not be flat and straight if quenched and MD-stretched unevenly, causing the clips to miss them, or they will not enter the clip opening. This would be a source of downtime due to film breaks in the MDO or tenter.