Coextruded films dominate the packaging and other end-use markets for oriented and cast films. In many instances, it is important for both the manufacturer and the users of the films that the film properties are uniform across the width of the film.
For the film manufacturer, layer uniformity means lower product-property variations and, therefore, improved product yield across the full width of the film. For the film user, layer uniformity means improved process control on packaging machines, fewer variations from one roll to another, and improved package yields.
One of the most obvious variations in layer uniformity impacting end-use performance and yield on a packaging machine is in heat-sealing and packaging-machine performance due to variations in outer sealing-layer thickness. Manufacturers seldom measure layer thickness directly because it is a time-consuming procedure and instead will use secondary film performance to monitor film properties, which are related to layer uniformity.
Heat-sealing and cross-web uniformity
For instance, for a given heat-sealing resin, the cross-web uniformity of heat-sealing characteristics, such as seal strength and minimum seal temperature (MST), are a function of heat-seal layer thickness and, therefore, these will vary due to layer uniformity. Thicker layers will give higher seal strengths and, typically, there is a target lower seal strength that must be met and, hence, a minimum layer thickness. And so, layer thickness variations are usually measured by sealing-property uniformity across the film and hopefully also are control-charted. While thicker than required sealing layers are not necessarily detrimental to sealing performance, they impact film economics and in critical, high-speed applications will affect anti-block effectiveness and, consequently, web-handling performance.
Metallizing surface-layer uniformity is important in controlling crazing performance during extrusion lamination, and there is no easy process-control, lab test for this, so layer uniformity must be designed into the film-making process and processing equipment. Of course, there are other functional interior layers, which must have excellent layer uniformity as well to ensure a high-quality, uniform product both across the film and over time.