Selective Modification of Film Properties through Uniaxial Stretching

The expectations placed on packaging continue to rise. It must provide enhanced product protection, greater convenience and catch the eyes of consumers while offering optimum efficiency and sustainability. Companies producing flexible packaging are responding to these challenges by developing complex, multi-layer composite films and using special raw materials. Whenever this approach reaches its limits, machine direction orientation (MDO) provides new scope for achieving unique film properties. In the MDO process, blown or cast films are continuously stretched in machine direction. The tube or flat film is directed over several heated rollers to be raised to a stretching temperature which ranges from 60 to 150 °C depending on the type of film material. The heated film is then stretched in the machine direction by means of the difference in the rotational speed of one or two pairs of temperature-controlled rollers. This stretching process reduces the film thickness and selectively changes the morphology of the film material. Stretch ratios vary with the type of material and intended application but are typically in the range of 1:3 to 1:10. To adjust the shrink properties or to ensure dimensional stability, the stretched film is relaxed on downstream annealing rollers by applying tension and heat. Finally, it is cooled to near ambient temperature on chill rollers. MDO installations can be used in-line with blown or cast film equipment or offline as autonomous units. The off-line system, consisting of unwinding, stretching and rewinding, offers maximum independence of the upstream extrusion process. By contrast, the in-line solution has a lower capital requirement and lower operating costs, and is highly efficient. Figure 1 shows a schematic layout of a blown film installation with an in-line MDO unit. The flattened tube is directed into the MDO via an optional web center guide. Unwanted air is best prevented from being drawn into the tube by making slits in one or two sides of its folded edges before it enters the MDO unit. The film exits the MDO and enters an edge-trimming station before being fed to the winding unit.