The Effects of Blade Angle on Blade Stresses During Cutting of Different Kinds of Paper Materials Abdullah Kurt1'* - Erdogan Kose2 - Turkun Sahinbaskan3 1 Gazi University, Technical Education Faculty, Mechanical Department, Turkey 2 Gazi University, Technical Education Faculty, Printing Department, Turkey 3 Marmara University, Technical Education Faculty, Printing Department, Turkey In the printing sector knives (blades) are used during pre-printing, printing and post-printing. Therefore, paper type, blade angles, the sharpness of blades and applied pressure are all important factors in terms of cutting quality. In this study, during the cutting of different kinds ofpaper material, the effects of blade angle on blade stresses has been investigated. Using paper pulp I, paper pulp II, coated paper and Bristol paper various analyses have been made. In the analyses, according to changes to blade angles of 20, 22, 24, 26, 28 and 30°, changes have been observed in maximum principal stress, S1, minimum principal stress, S3 and von Mises stresses, SEQV. In conclusion; since Von Mises stresses and especially max. compressive stresses (min. principal stress, S3) occurred at the lowest level for paper pulp I, paper pulp II, coated paper and Bristol papers, it has been decided that for increased durability of the knife the critical blade angle in terms of abrasion/breakage of the blade should be 28 degrees rather than the more widely used angle of 24°. A blade angle below 22 degrees is not advised. © 2009 Journal of Mechanical Engineering. All rights reserved. Keywords: paper, blade, stress, ANSYS 0 INTRODUCTION Cutting jobs have an important place in the printing industry. All paper products from the smallest label to all types of posters, brochures, magazines, books, newspapers and billboards have to be prepared according to a specific size. For this reason guillotine paper cutters are required both, in pre-printing and post-printing. Guillotine paper cutters are used in the printing sector; to correctly size the paper in accordance with the printing machine that is used to carry out the printing process, to separate any extra copies that have been printed and to cut off any excess from the edges of papers that have been printed and bound. With regards to work quality and customer satisfaction, cutting is a highly delicate matter in the printing industry. All cutting jobs produced under rapidly developing technological conditions must adapt to certain standards of quality. Otherwise, printing houses face the risk of losing their customers. The most common cutting range of cutting materials used in printing is paper, paperboard and cardboard. Such cutting materials are categorized as soft, normal and hard. Copy paper, tissue paper, drying paper and silk paper are all soft materials. Valuable papers such as printing paper, cardboard, bond-bill paper are normal cutting materials. Hard cutting materials, however, are materials such as coated paper, chrome paper, paperboard, label paper and adhesion paper. Guillotine paper cutters are of steel and steel alloy. This steel alloy composition determines the life of the blade. The durability of the blade is also closely related to the cutting material used. If blades for soft and normal cutting materials are used on hard cutting materials they will quickly wear out and become blunt. The main reason for this is that hard cutting materials display great resistance during cutting. Blade angles also have an important place in cutting jobs. Blades with small angles require less cutting strength compared to blades with larger angles. Blades with small angles cannot remain strong in hard and flexible cutting materials; they will bend or eventually wear out. For example, a blade with a small angle is not forced with soft and high stowing but it is forced with hard stowing and so it is quickly worn out. A very important decision is made with the selection of the knife material and the grinding angle. Both factors are crucial for cutting quality and the number of cuts with a sharp knife. Different kinds of materials for cutting and the production requirements play an important role. *Corr. Author's Address: Gazi University, Technical Education Faculty, Mechanical Department, 06500, Ankara, Turkey, akurt@gazi.edu.tr The following knives (blades) are employed [1]: - Standard steel knives (5% share of alloy), - HSS-knives (super speed steel knives, 18% tungsten as an alloy share), - Carbide-tipped knives (75 to 95% tungsten carbide, pressed in a powder metallurgy procedure). Not only the price of the new knife should be calculated but also the average number of cuts (service life), the resulting knife changes, the price for grinding (per meter) and the average grinding of the blade in millimetres [1]. It is interesting how little work exists in literature relating to printing technology and knives used in cutting processes. Bishop and Wilson [2] discussed the method for both velocity control and deceleration by the use of a single pneumatic cylinder. In addition, a method of reducing velocity variability due to differences among the work functions of the mechanism is described, and the application of such a device to a paper-cutting mechanism is presented. The concepts and theory presented are general and therefore, apply to the entire class of spring-driven mechanisms. The design of the control device was facilitated by the use of a mathematical model. In addition to the active forces, an inertia effect of the stationary knife caused by a "scissor" action of the mechanism was taken into account (the axes of the two knife blades are slightly inclined to insure proper shearing action. As a result, the stationary knife is displaced through a small angle as the rotating knife cuts the paper). A dynamic friction force produced by the associated inertial reaction was taken into account. The finite element method, by separating complicated problems into simple lower problems, is a method of solution in which the complete solution is found by solving each problem within itself. Since the subject of this study shows similarities to mechanical engineering, especially to the process of machining, it has benefited greatly from the FEM works in this field. There has been an increase in attention given to numeric methods, especially the finite element method, due to the developments in computer technology and complex codes. While the Eulerain formulation is used in one part of the models used for the modelling of orthogonal cutting, the Lagrangian formulation, which enables chip formation from beginning to steady-state, has been used on a more widely basis. To improve the accuracy and effectiveness of the finite element method, various techniques of this method such as element separation [1] to [6], modelling of cutting tool wear [1], [2] and [4] to [6], re-meshing [5] and friction modelling [1] to [6] have all been applied. Looking at the works related to the finite element method in literature; it is observed that the process of chip formation formed during the cutting process of a large part of the work consists of two dimensional simulation [6] and [9]. For simulation, commercial software such as MARC, ABAQUS, DEFORM 2D/3D, NIKE, DYNE has been used. In this study, the effects of blade angle on blade stresses during cutting of paper pulp I, paper pulp II, coated paper and Bristol paper has been investigated with the help of the finite element method based ANSYS software. Changes to the maximum principal stress, Si, minimum principal stress, S3 and von Mises stresses (Seqv) formed on blades with different angles have been observed and the required critical blade angle in terms of abrasion/breakage of the blade as a result of cutting each paper type has been investigated. 1 MATERIAL AND METHOD In this study, in which the effect of cutting different kinds of paper on blade stresses has been investigated, technical properties of the knives used on the POLAR 115 High Speed Cutter workbench (Fig. 1) given in Table 1 have been set as the basis for the investigation. The paper cutting system and knife used is shown in Fig. 2a and b, respectively. C3 Fig. 1. POLAR 115 High Speed Cutter [2] Table 1. Technical properties of the POLAR 115 High Speed Cutter [2] Cutting width [mm] 1150 Feeding depth [mm] 1150 Feeding height max. (without false clamp) [mm] 165 Front table length [mm] 715 Table height [mm] 900 Safety clamp pressure [daN] 30 Clamp pressure, min [daN] 150 Clamp pressure, max [daN] 4500 Backgauge speed on return way (0 -...) [mm/sec] 300 Knife speed [cycles/min] 45 Knife thickness [mm] 13.8 Knife grinding reserve (HSS 18 knife), max [mm] 50 Smallest cut, manual, w/o false clamp plate [mm] 20 Smallest cut, automatically, w/o false clamp plate [mm] 25 Smallest cut, automatically, with false clamp plate [mm] 95 Noise level [dB/A] 77 Fig. 2. Paper cutting system [2] 1.1 Finite Element Analysis Blade set stresses for HSS super speed steel knives, 18% tungsten as an alloy share, with an elasticity module of 200 GPa and Poisson ratio of 0.3 are analysed using cutting strength data obtained from the workbench producing company with the help of the finite element based ANSYS 9.0 software. By taking into consideration the geometrical properties of the blade edge, the modelling process of the blade is carried out when the model which is formed in the Mechanical Desktop 6 Power Pack is transferred to ANSYS in "Ages" format. While an 8 nodal quadrilateral PLANE82 component was chosen for blade edge meshing; a SURF153 component has been used so that blade force is applied in normal and tangential load distributions over the blades main cutting surface and cutting surface accessory. The process of component separation is carried out with the "smartsize" method which automatically adjusts the most appropriate component order for solid model geometry. Accordingly, the dimensions of network components have been applied as smartsize = 4 for all blade edges. As a boundary condition, displacements in areas where the blade is mounted to the blade guider component were selected zero. In the analyses, cutting forces (F) of 16.5, 12, 17.5 and 45 kN formed during cutting of paper pulp I (80 gsm), paper pulp II (54 gsm), coated paper (170 gsm) and Bristol paper (230 gsm) all 100 cm long have been applied. These forces mean that 500 pieces (h = 48 mm) of paper has been cut for pulp I, 500 (h = 32 mm) pieces for pulp II, 250 pieces (h = 30 mm) for coated paper and 100 pieces (h = 32 mm) for Bristol. Cutting forces are applied in normal and tangential load distributions over the main cutting surface and cutting surface accessory (Fig. 3). In the analyses, while T and S forces given in Fig. 3 are accepted as 0.9- and 0.1-F, respectively, force S is later separated into components of SN = 0.1-F-sin a and SR = 0.1-F-cos a. In a proper spread out surface load manner, T, Sn and Sr forces are applied to each component on the main cutting surface and cutting surface accessory. In addition to the 24° angle used widely in the sector; different models formed with angles of 20, 22, 24, 26, 28 and 30° for each paper type have been used to investigate stresses on the blade angle (a). In the analyses friction between the blade and the paper is ignored and the fact that the blades are sharp (not blunt or used) is observed. F I Sn T| Sp. Í7j Sn S, Fig. 3. Load distributions on the blade 2 STRESS DISTRIBUTIONS ON THE BLADE According to the loading situation described above, using different models with angles of 20, 22, 24, 26, 28 and 30° for each paper type; the maximum principal stress (S1), minimum principal stress, (S3) and von Mises stresses (Seqv) formed on the blade have all been investigated. Stresses that are obtained according to changes to the blade angle for each paper type are shown in Figs. 4 to 7. When investigating maximum principle stresses formed during the cutting of 80 (gsm) Paper pulp I we find that blade angles do not have a great impact on these stresses. While a similar result is obtained with minimum principle stresses, we can also say that stresses have a tendency to increase at angles below 22 degrees. Similarly, stresses have a tendency to rise when von Mises stresses fall, especially below 22 degrees of the angle. When investigating maximum and minimum principle stresses formed during the cutting of 54 (gsm) paper pulp II we find that stresses decrease as blade angles are increased. In von Mises stresses, there is yet again a decline in stresses as the angle increases and we find that the maximum increasable angle must remain at 28 degrees. a) maximum principal stresses, S1 250 200 V150 p ËL100 50 0 195.43 195.98 195.82 195.71 195.69 195.56 -625 -620 ■¡0-615 p 2-610 -605 -600 20 22 24 26 28 30 blade angle (°) b) minimum principal stresses, S3 619.74 609 26-609.53 608 4 -609.68 -608.4-606.96 540 535 n 530 H 525 520 20 22 24 26 28 30 blade angle (°) c) von Mises stresses, SEQV 538.98 529.51 _ 527.75 527.34 — 529.74 20 22 24 26 28 blade angle (°) 30 Fig. 4. Effect of blade angle on stresses for (80 gsm) paper pulp I When investigating maximum principle stresses formed during the cutting of 170 (gsm) coated paper it is determined that while there is no difference in stresses for blade angles of 20 and 22 degrees, stresses decline as the angle increases after 24 degrees. It should be pointed out that here the effectiveness of the paper lies with it being coated. However, while no change was observed at angles of 20 and 22 degrees in minimum principle stresses, a negative relationship between the stresses was observed with an increase in the angle within the 24 to 28 degrees interval. It was also observed that increases after 28 degrees also increased stresses. A similar situation occurred for von Mises stresses. 200 ■