Detlef Merklinger, of Pagendarm BTT, describes the application of the Speed-Star Vacutex dryer concept for coating temperature sensitive films
Coated and siliconized film applications – especially release liners – have steadily expanded in recent years. At the same time, technical demands on the base films have increased, especially regarding material thickness and shrinkage properties. PET, PE, HDPE, LDPE or PP films are used with either solvent based or 100 per cent silicone coatings.
Important features of the Pagendarm coating machine layout include the AGS 2500 S (System BASF) pressure chamber doctor used to apply the solvent based silicones to the web. The web tension zones necessary are achieved by the use of vacuum rollers and vacuum tables. This eliminates scratches and other damage to the web.
The Speed-Star Vacutex dryer comprises five separate sections, each having separate closed-loop air and temperature control systems for both upper and lower nozzle fields.
To describe the dryer concept, the following coating application is used: a 40 micron thick LDPE film is coated over a working width of 2,300mm with a solvent based silicone (solids content six per cent) at a production speed of 200m/min. Coating weight in the dry state is 0.6g/m2.
The system uses a combination of Vacutex nozzles for the lower nozzle field and switchable nozzles for the upper field. The switchable nozzle can be set to an impact air stream with high heat and substance transfer or a tangential air stream for gentle drying of the coating.
Air flow in the nozzle generates a partial vacuum around the integrated roller, which results in full area suction of the substrate against the guide roller. So, independently of the upper air setting, a stable web position throughout the entire dryer is guaranteed even at high speeds. This means web tension in the dryer can be kept very low (low shrinkage) without running the risk of the web becoming unstable. Values of 30N can be achieved.
And, in the event of production problems, both the vacuum effect and the air cushion below the web can be maintained. In practice this means that dryers equipped in this way have an extremely large operating window and are very reliable in use.
Upper and lower nozzle fields are heated with heat transfer oil. Thanks to the central air extraction principle in conjunction with adjustable-width nozzles, it is possible to run with large temperature differentials in order to cool the film from below and to dry or wet the coating from above.
Nozzle systems are so designed that for all operating states – maximum circulating air flow 40,000Nm3/h and minimum circulating air flow 5,000 Nm3/h – the maximum deviation in nozzle exit speed is less than 1m/s and the maximum deviation in temperature is less than 1degC.
This precision is necessary, for example, if drying is carried out with large temperature differentials between the upper and lower nozzle fields, and the longitudinal and transverse shrinkage of the product must not exceed one per cent.
Both air circulation systems in each section – upper and lower – are equipped with Vacutex switchable nozzles and the Air-Con central extraction system. This system of the Speed-Star dryer guarantees high drying performance and absolutely uniform air and temperature distribution in the entire dryer section.
Over-drying of the edges is ruled out, as every nozzle is equipped with central extraction ports distributed evenly over the entire width.
This ensures gentle and absolutely uniform transport of the drying air upwards and downwards away from the web over the working width. Consequently, no air currents transverse to the direction of travel of the web occur in the peripheral zone of the web. Fouling of the nozzles as a result of ‘fluttering’ web edges is practically impossible.
If the web breaks, the travelling web is kept on track by the partial vacuum at the nozzles, which means that any fouling of the upper nozzle field is practically eliminated.
In order to achieve web tensions of 20N in practice, vacuum tables are provided before and after the dryer. Web temperature measurement is by non contact means. As a result, LDPE films with a thickness of 40 micron can be siliconized and dried without shrinkage (0 per cent) under production conditions at speeds of 200m/min.
From an operating point of view, the exhaust air load should be as uniform as possible. This calls for adjustable-width nozzles, in order to reduce the total air quantity required for the drying process. At the same time, the use of adjustable-width nozzles offers reliable and reproducible operation of the upper and lower nozzle fields, especially at high temperature differentials.
Any mixing of the upper and lower circulating air streams is largely prevented, which means that larger temperature differentials can be set. Energy requirements for heating the circulating air are considerably reduced.
Major advantage of the system compared with conventional flotation dryers is the ability to set the air exit speed independently of web travel behaviour. This in turn results in an extremely stable web position.