Fine Mesh Straining within the mixingline: Gear pump technology opens new possibilities
The process requirements for the tyre industry are continually growing , while cost pressure remains high. Clean compounds, not impurified by dirt, or not dispersed ingredients, are the prerequisite for meeting these requirements. On the other hand, efforts are underway to utilise more cost-effective raw materials. Impurities lead to an increase in the reject and waste rates due to process interferences and to final products that do not feature the required quality properties. This problem can be counteracted by straining of the compounds. But the processing of sensitive compounds poses new challenges for the rubber and tyre industry to deal with. Due to the relatively large mixing capacity in the lines, straining systems with high throughputs are required. The roll-ex® gear pump extruder technology from UTH facilitates meeting the above mentioned requirements because it allows the gentle treatment of materials under high pressure, constant temperature and the use of fine mesh filter screens. This process is also referred to as ‘fine mesh straining’ because it differs in essence from conventional straining capabilities and it offers many advantages for efficient straining in the mixing line. The Gear Pump Technology is a particularly material friendly solution for straining in the mixing line since the compound is filtered in a warm state and a cooling process and plasticising procedure is eliminated. In the last two decades UTH GmbH from Fulda (Germany) did major pioneer work in the field of development and production of gear pumps and gear extruders for the rubber processing industry.
Fine-Mesh Straining with Two Roll Feeder
With the roll-ex two roll feeder (TRF) model the gear extruder is integrated directly into the mixing line. Due to a very low temperature increase, this solution is especially appropriate for straining of final compound. The feeding takes place with warm material as strips come directly from the two roll mill (Figure 2) or as sheets from the twin-screw-sheeter (Figure 3). The TRF feeds the compound into the gear pump according to the speed of the mill. After straining the compound is formed as slab or strips and is then movig directly into the batch-off.
Fine mesh straining with double screw extruder (DSE)
In the tyre industry the DSE (double screw extruder) is already well known from twin-screw-sheeters. Combined with the roll-ex® gear pump technology it is also a proven technique for fine mesh straining in the mixing line. Therefore the fine mesh straining unit is placed after the mill (figure 5) or directly after the internal mixer (figure 4). Latter is especially suitable for the straining of masterbatch. The DSE supplies the material continuously to the gear pump. After the straining process, the material is formed out as slabs or strips. The crossover from batch process into continuous process is taking place within the machine. For straining of final compound, the gear pump with DSE-feeding unit can also be placed after the two roll mill (Figure 5). In this process step the DSE also serves as a buffer in the line.
Applying a fine mesh strainer in the mixing line can reduce mixing time under certain conditions. With the additional shear, provided by the strainer system, the mixing batch time can be reduced and if there are large undispersed particles they will be retained on the fine mesh screen. This ensures totally continuous processes. Thanks to the patented roll-ex® gear pump technology, the temperature increase of the compound remains low, even under high pressure up to 500 bar. The straining process is completely integrated in the mixingline and with throughputs up to 10000 kg/h (Figure 6) even large lines can be served. Due to different feeding devices, the technology is suitable for the straining of different mixing stages. In the last two decades it has been proven, that the total product quality can be improved and the scrape rate minimized. The Straining units are placed before the batch cooling device. This means that semi-finished products such as stripes and sheets can be extruded. This opens up possibilities for many different shapes that can lead to cost reductions in subsequent processes. Furthermore in many extrusion applications including tyre components large undispersed particles can block the die at thin and sensitive sections. This is a major problem as the line must be stopped, the die cleaned and then restarted. When fine mesh filtered compound is used such costly process interruption can be minimized. This makes a significant impact on efficiency.