Improving quality in thin plastic film production

SML Extrusion Technology operates in the field of high technology extrusion and the production of breathable thin plastic film. This breathable film, only around 15 to 200 µm thick, is used in a variety of applications ranging from nappies to under-roof laminates.

The SML production line is equipped with a series of rollers that can accommodate material up to 3.2 m wide. The first six rollers at the beginning of the production line use circulating water heated to 140°C to preheat the plastic. From that point, the heated plastic is stretched and drawn until it reaches the correct thickness.

There are a number of factors that can affect the quality and consistency of a breathable thin film. These include the running accuracy of the bearings, the quality of the intermediate adapter sleeve and the stiffness of the bearing arrangement.

Though these factors are important, the one factor that is most difficult to control, and that can have the greatest impact on product quality, is maintaining a consistent tension on the plastic film throughout the manufacturing process. To maintain this tension, the friction torque of the bearings must be constant and without variation. This presents a unique challenge, particularly in the preheating section because axial expansion of the heated rollers can significantly increase the friction torque of a traditional bearing arrangement.

With a traditional bearing arrangement, the non-locating bearing must be able to move axially in the housing to accommodate thermal expansion of the shaft. When these sliding movements occur, the friction between the outer ring of the bearing and the housing induces an internal thrust load on the bearing arrangement, which increases friction torque to the point that it can impact product quality.

To virtually eliminate internal thrust loads, SKF engineers replaced the non-locating bearing with their proprietary CARB toroidal roller bearing type C2218 K/C3 + H 318 E, and the locating bearing with a CARB type 22218 EK/C3 + H 318. Both bearings used the same type of housing (SNL 518-615) and both were grease lubricated. Bearing speed is 350 m/min.

The CARB bearing proved to be the solution because it accommodates axial expansion internally to ensure precise film thickness. In addition, the CARB and spherical roller bearings meet the precision tolerances for P5 running accuracy, which results in small heating-roll run out, a particularly important feature for this application. Another feature is that the SKF adapter sleeves have the same high quality as the bearings, with tight tolerances for wall thickness, roundness and straightness. Also, with a self-aligning bearing system there is no reduction of bearing life if misalignment occurs, so reliability is increased and there is less down time.

A CARB bearing can accommodate thermal elongation of the shaft internally much like a cylindrical roller bearing, where the inner ring moves independently of the outer ring. However, a CARB bearing can accommodate more expansion than a cylindrical roller bearing because the roller set also moves. This allows a CARB bearing to accommodate axial expansion up to 20% of its width. And, unlike a cylindrical roller bearing, a CARB bearing can accommodate misalignment like a spherical roller bearing.