Differences in film properties and manufacturing methods used to produce them

Let’s start with cast-film properties. In the cast-film process, a molten polymer is extruded from a slot die and pulled against the chill roll by a vacuum box, an air knife, an electrostatic pinning system or a nip roll. Each of these methods “pins” the melt to the roll surface to remove heat from the melt curtain to form the solid film. The film is then stripped from the roll over another roll which may be heated, or cooled to further set the film properties. At this point, the film morphology has been set. The morphology means the crystalline structure and any orientation caused in the formation of the film. Crystalline region size and level in the film will control the barrier properties, the stiffness (modulus) of the film, the thermoforming behavior and, to some degree, the optical properties (see internal haze posting). Cast films are generally low in orientation and give high levels of elongation, excellent tear resistance and impact resistance or toughness. This makes it a good inner layer in laminations as a sealant. Coextruded cast films can be used to enhance the seal-initiation temperature and winding characteristics.

For some polymers such as HDPE, the crystallinity will be set at film formation. For PP, the crystallinity will continue to increase over a three- to seven-day period due to secondary crystallization, and polyester can be quenched to give an essentially amorphous sheet. For the case PP, the continuation of the crystallization will cause the film to tighten as the density increases with the increasing crystallinity and perhaps form baggy lanes in the film. All of these properties are controlled by the quenching of the film during casting. Increasing or decreasing the casting temperature and the roll heat transfer will greatly modify the final film properties obtained and is a key aspect of the film property development (the structure property relationship).

In many manufacturing locations, we will find two lines which are said to be exactly the same but which inexplicitly give different results. But if both lines are exactly the same, how can this be? Well in many instances, the two lines, which are exactly the same were made at different times. Consequently, they just look the same on the outside but they are quite different on the inside, especially the cast roll. What does this matter? Well, the cast roll controls the quenching of the film, and this controls the morphology of the film, or the internal structure of the polymer molecules. This, of course, will control the physical properties as I explained above. In order to make the same film from both lines, you will need to insure that the film experiences the same thermal history. To do this you can measure the heat transfer coefficients of each roll and then design the process to give the same film temperatures. A simple way to do the same thing is to measure the actual film temperatures at various points around the cast roll on Line 1 (making “good” film). Next, measure the film on the second line and adjust roll  temperatures to give the same temperatures at the same location on the cast roll as measured on the first line at the same line speed. If you can achieve the same thermal history in the film, then you will obtain the same physical properties.