# Study of Refining Effect of Mixed Pulps Using Refiner Plates with Different Bar Patterns
## 1 Introduction  
A refining system should be selected with consideration of the properties of the end products, available pulp types, and production capacity range. Refining during stock preparation is commonly performed separately for different types of pulps because they show varying refining behaviors. However, other types of pulp are sometimes mixed and processed before refining. In either case, the number of refiners depends on the target freeness and production capacity [1]. For example, in a toilet tissue mill, only one refiner is required because the pulp fibers are lightly refined, but in the case of glassine paper, 5-6 refiners are often needed for extensive refining [2]. Currently, both independent refining systems and mixed refining systems are widely used in paper mills. New large paper machines use independent refining, and older small paper machines use mixed refining. As is well known, independent refining and mixed refining each have their own advantages. In some pulp blends, independent refining develops better strength while consuming less refining energy than mixed refining [1, 3]. However, for some pulp blends, mixed refining may give better results. In addition, mixed refining may be more suitable for some pulp blends, so the dimension and bonding ability of the fibers may determine whether independent refining or mixed refining is appropriate [2, 4]. If softwood fibers with long fiber lengths and thin fiber walls are mixed with hardwood fibers with short fiber lengths and thin fiber walls, the hardwood fibers are loosely bound to the softwood fibers [5]. As hardwood pulp fibers pass through the refiner grooves without enough modification, independent refining becomes more desirable in these pulps [1, 6]. Conversely, if short, thick softwood pulp fibers are mixed with long, thin hardwood pulp fibers, the hardwood pulp fibers are firmly bound to the softwood fibersand are protected [5]. In this case, if mixed refining is applied, good refining results can be expected due to inter-fiber contact. For this reason, a combined system of mixed refining after separate refining of different pulps provides good alternatives because the benefits from both independent and mixed refining systems can be obtained. In the refining process, the pulp fibers undergo compression and shear forces to change their properties, which will vary depending on the initial fiber properties or the specifications of the refiner [7; 8]. Therefore, refining should be applied differently depending on the properties of the fibers. In general, long fibers can be shortened through high-intensity refining (free beating) to achieve better formation, and excessive fiber shortening in short fibers can be minimized by low-intensity refining (wet beating) to preserve fiber length [9; 10; 11; 2; 2]. Currently, most paper manufacturers prefer low-intensity refining as the use of hardwood pulp and recycled pulp increases [13; 14]. The most frequently used way to evaluate refining intensity is the Specific Edge Load (SEL). Plate bar fillings are the most critical factor in determining refining intensity suitable for papermaking [15]. According to the SEL equation, a straightforward way to achieve low-intensity refining is to increase the number of bars in the refiner plate, increasing the Cutting Edge Length (CEL) [16]. However, even if the CEL is the same, the refining result may vary depending on the bar filling or bar design. In general, refiner plates have been manufactured using conventional casting technology, which requires a 3\({}^{\circ}\)-5\({}^{\circ}\) minimum draft angle for plate bars [17; 18]. This small angle helps with the removal of the pattern without damaging the sand mold. However, with the recent introduction of technologies capable of manufacturing vertical bars without the draft angle, it has become possible to use refiner plates that are advantageous in terms of bar sharpness and stock throughput. [PERSON] et al. developed a lightweight bar plate without a draft angle using different metal alloys [19].