Aspects Regarding the Comparative Study of the Cryogenic and Conventional Milling of Vulcanized Rubber

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1 Aspects Regarding the Comparative Study of the Cryogenic and Conventional Milling of Vulcanized Rubber Bogdan MOCERNEAC 1* - Mircea LOBONTIU 2 Abstract: The manufacturing process of the molding, is widely used in the polymeric material processing industry which is an expensive and difficult process. Depending on the configuration of the parts most of the times they can be obtained more productively and inexpensively by combining molding process with the cutting process. In this situation intervenes the cutting manufacturing process, which is rarely used, because it is difficult to implement. This aspects has imposed the manufacturers of components from polymeric materials to search and find new solutions to improve the machining of these materials. The purpose of this paper is to determine some factors of the surface quality of vulcanized rubber obtained by milling, depending on the cutting conditions as to achieve an increase of productivity and application domain and to decrease manufacturing costs. Keywords: Milling, Cryogenics, Rubber, Regimes. 1. INTRODUCTION Soft polymeric material parts are common in modern products, and their manufacturing is done usually by molding, but given the variety of produced models and by the fact that they are constantly changing it is preferable to use cutting process, which is the most flexible method of processing from this point of view [Teramoto, K. (2009)]. The issue of milling vulcanized rubber and obtained surface quality throw the cutting process, was disputed by other authors too [Jin, M., and Murakawa, M. (1998); Lewis, M.A. (2002); Rivera-Moreno, et al (2013); Nayak, R., et al (2012) and Luo, J. (2005).], which through the experimental studies they concluded that the cutting of vulcanized rubber depends on many factors. The main factors mentioned are temperature of the polymeric material, the cutting regimes, the cutting tool geometry, etc. The machined surface is an important feature of a processing operation, since it is the final goal. The quality of the surface is important for the functionality of the assembly which includes rubber parts. Surface integrity is a term that involves several considerations: the surface quality, no cracks or chemical changes thermal damage and no stress in the material [Shaw, MC (2005)]. 2. INTRODUCING THE ISSUE Milling rubber is an operation which is very important for obtaining specific surfaces which are difficult to obtain (or even impossible) by other methods. Furthermore to obtain a high quality surface we must use cutting regimes, tools and machining technologies which are suitable for this type of flexible material. But if the process is optimized and a wellestablished work plan is prepared, which should not miss the increased attention on fixing the workpiece, the suitable selection of cutting tool geometry, the cutting regime and the conditions in which the cutting is carried out, the results can be significantly better, with a rate of 5% spoilage. High quality rubber parts are directly responsible for their safety and lifetime of service, because heat damaged surfaces, material fractures, irregular dimensions, etc., are often the source of occurrence of faults The issue of machining polymeric materials Turning and Milling polymeric materials must be treated with more attention than metal machining, therefore we must consider the following machining issues.( - Guide to Plastics Machining ) Turning operations require inserts with positive geometries and ground peripheries. Ground peripheries and polished top surfaces generally reduce material build-up on the insert, improving the attainable surface finish. A fine grained C-2 carbide is generally best for turning operations. At milling operations sufficient fastening allows fast table travel and high spindle speeds when end milling plastics. When face milling, use either high positive or high shear geometry cutter bodies. For the machining of the frontal surface there is used a positive rake angles (γ), from 10 to 75, and a clearance angle of 5-20 [Yan, J. (2006)]. In the machining of pieces of polymeric materials the following must be considered: - Thermal expansion is up to 10 times greater with polymeric materials than metals; - Polymeric materials lose heat more slowly than metals, so avoid localized overheating; - Softening (and melting) temperatures of polymeric materials are much lower than metals; - Polymeric materials are much more elastic than metals. 69

2 Because of these differences, it is advisable to make different experiments with fixtures, tool materials, angles, speeds and feed rates to obtain optimum results. Jin, M., and Murakawa, M. (1998) presented in their paper a series of milling experiments on a workpiece made out of vulcanized rubber (H-NBR), with a carbide end mill, using different cutting regimes. As a result of the experiment the two researchers observed that the geometry and the best quality of the workpiece were obtained by using a carbide tools (Ø 6.35 mm, z = 3) with a cutting speed of m / min and a feed of 0.42 mm / tooth. Another researcher Lewis, M.A. (2002) presented a study, in which he made several experimental milling on rubber at ambient temperature and cooled to -40 C with dry ice. The conclusion of the experiment, was that the higher quality has been obtain at cooled milled parts, because their fixation on the cutting machines was more accurate (parts are stiffer and tougher) and there were no vibrations or deviations of the shapes, due to the bending of the rubber pieces during milling. Yan, J. (2006), presents an experiment of 66 dry turning tests conducted on a conventional Republic Lagun CMZ lathe to study the mechanism of elastomer orthogonal cutting and gain a better understanding of favorable conditions that result in a good machined surface finish. Before any machining tests were conducted, a spindle speed calibration of the lathe was performed in order to ensure accurate cutting speeds. Cutting forces, including principal, thrust and transverse force components, and workpiece surface temperatures were recorded during the tests to study the effect of tool geometry and cutting speed. During the tests, chips were collected for subsequent analysis and the machined surface roughness was measured to evaluate the surface finish. The researcher concluded the following after completing the experiment: - The rake angle of the cutting tool has to be as high as possible, in order to achieve a small cutting force; Lower spindle speeds generate higher cutting force; - Increasing the feed has little influence on the cutting forces; - During cryogenic turning a greater cutting force was achieved compared to the processing at ambient temperatures, but the lateral forces exerted during turning are lower. - The high clearance angle of the cutting tool produces a better surface quality than the surfaces obtained by to use of lower angles; - Cutting speed and feed have a significant effect on the obtained surface; - At the cryogenically machining workpieces it has been observed an improvement of the obtained surface for all ranges of speeds and feeds that ware used. - It has been observed that although during of the machining high temperatures that could heat damage the machined surfaces were not reached, there were some fluctuations in temperature during the process; - The highest temperatures were obtained from the use of high speed and low feed. 3. GENERAL PROPOSALS After we studied the literature from domain, we observed that the cutting issue of polymeric materials is not well defined from some points of view: a. The cutting conditions; b. Temperatures of the workpiece product; c. Fastening; Also in a company specialized in the production of belts, which use milling of vulcanized rubber to generate the final surface of the belt, have met difficulties starting from fastening up to milling the rubber. At vulcanized rubber any error makes the workpiece to be rejected without the possibility to fix it, or not even to reuse the material. In this context an experimental study in cutting tools, materials, and manufacturing technologies identical to those identified in the factory has been developed. 4. EXPERIMENTAL CONDITIONS This chapter describes an experiment conducted in order to observe the behavior of rubber at cryogenic and conventional milling, with a feed of 0.01 mm / tooth at a range of cutting speed between 471 m / min m / min. As a result of the studies preceding to this experiment, we found that in the rubber machining industry, are gaps regarding cryogenic rubber machining. Based on this observations we developed an experiment where to reflect as accurately and objectively as possible the influence of cryogenics on the milled rubber with a specific cutting regime [Anthony, J. (2014)]. In the experiment we used two types of polymeric materials 3540 and 3003, used mainly in the production factory specialized in the manufacture of V-belts. The two types of materials are from the family of Styrenebutadiene or styrene-butadiene rubber SBR shortened (Figure 1.). Fig. 1. Rubber Polymers, [Ophardt, Charles E, (2003)]. 70

3 The material 3540 belongs to the family of synthetic rubbers derived from styrene and butadiene. [Ophardt, Charles E, (2003)]. The difference of the 3540 rubber to standard SBR is the high content of silicates (which gives the material an electrical resistance between KOhm) which completely replace the chemical compound Carbon Black (black smoke), this rubber also has in its composition a significant percentage of textile fibers [Dick, John S. (2003)]. In this experiment we used also and a second type of rubber named 3003 from the same rubber family of SBR, which is differentiates from 3540 by the fact that this rubber does not contain silicates or textile fibers, but contains a significant percentage of carbon black, which gives the material a very low electrical resistance, between KOhm. This rubber type 3003 (from SBR family) is one of the most common types of rubber used in industry, being used mainly in the construction of tires, belts, bushings, etc. These two types of rubber materials were subjected to the same machining, in the same conditions, the only difference was the vulcanization time, because of the chemical composition of the two types of rubber materials. After studying manufacturing technology and materials used in the manufacture of belts, we decided to use as samples two types of rubber that are most frequently used and has the biggest problems at milling (3003 and 3540). The rubber vulcanization process parameters are presented in Table 1. On the test pieces and specimens there have been made tests of elongation, electrical conductivity and viscosity. In the second stage we studied the possibilities of rubber machining considering the physical properties, and the fact that the fastening on the machining device is more difficult and the more fragile. Therefore, we designed a mold to vulcanize the pieces with dimensions of 80x80x22 mm. Crt. Nr 1 2 Table 1. Vulcanization parameters Characteristics Set Obtained Tolerance The temperature (upper plate) The temperature (lower plate) 170 C 168 C ± 2,5 C 170 C 169 C ± 2,5 C 3 Precision 150 Bar 149,5 Bar ± 5 Bar 4 Time duration 40 min 40 min - 5 Toughness 86 Shore A 84 Shore A Shore A 6 Weight 198,4 g 199,1 g ± 2,4 g The rubber pieces were vulcanized with an electrical laboratory press, type LaboPress P 300S, vulcanizing parameters and material characteristics can be seen in Table 1. The milling the two types of pieces was made on a 5-axis CNC mill type AWEA BM The cutting speed and feed were tested and calibrated before the experiment [Stephenson A, (2006)]. The mill used in experiment was checked and centered to ensure that the results will be accurate. The cutting tool used it was a disc mill with teeth in V shaped, teeth angle at 39 (Figure 2.). This type of mill is currently used in the machining of V-belts). Fig. 2. Mill Disc The cutting regime, materials and cutting conditions what we used in this experiment can be seen in Table 2: Table 2. Cutting regime Since that all parts used in this experiment have the same characteristics in terms of assortment, experiment design, materials, process technology and have the same values agreed conventional of the characteristics (The method of visual analyzing) we wanted to introduce in this paper the concept of quality class. Quality classes are ratings for a product category from the same range, designed or made for a specific purpose, of specific materials, through a specific process, with values between the specified conventional limits [Maxim, E, (2004)]. To differentiate the pieces, we used three quality categories for the obtained surfaces and based on approximation and comparison we gave ratings as percentages. The classification of the pieces in quality classes was achieved by following characteristics shown in Table 3. 71

4 Quality category Dimensional and shape deviations from nominal rates Thermal damage Material breakage I 1 > 0 mm 1 > 0 mm 1 > 0 mm Table 3 Characteristics of quality categories Roughness without porous surfaces (nappy) Other defects Percent range % II 0,2 > 0,1 mm 5 > 1 mm 1 > 0,5 mm slightly porous surfaces (nappy) ,9 % III > 0,2 mm > 5 mm > 1 mm porous surfaces (nappy) Other defects 0 49,9 % According to this concept we awarded percentage ratings for the obtained surface quality starting from the percentage of 0% (this value is relative because this qualification is given, only if there is a major error in machining process), 5. EXPERIMENTAL RESULTS In figure 5. you can see an example of a rubber piece that has been milled in this experiment. In this experiment, as shown in Figure 6. the percentages were in the range 50% - 92%. The worst result was obtained in conventional milling perpendicular to the fibers, with the cutting speed of 471 m / min, and the best quality was obtained at cryogenic milling perpendicular to the fibers, with cutting speeds of 2355 m / min and 2826 m / min. Fig. 3. Machining Quality Class III (Typical I), to 100% (this value is an absolute value that can t be achieved in practice, because it is a purely theoretical value). In Figure 3 you can see a typical machining with low quality results (quality class III), and in Figure 4. a typical machining with the best achieved quality (quality class I). Fig. 4. Machining Quality Class I (Typical II) Fig. 5. Example typical Quality Class II Looking at the chart in detail you can see that machining at the cutting speed of 471 m / min in all 4 milling types produced the lowest values, which run in the range 50% (for conventional milling perpendicular to the fiber direction) and 60 5% (for cryogenic milling on the direction of the fibers). The tendency shows accelerated growth up to the cutting speed of 1413 m / min, so that the differences between the values obtained using the cutting speed of 471 m / min and 1413 m / min are 25 percent higher. The only exception can be observed in conventional milling on the direction of the fibers which has an evolution of 27 percent. At the cutting speed of 1884 m / min the first differences were observed, that is the milling performed perpendicular to the fibers shows an increase in surface quality from values of 75% (conventional milling) and 80% (cryogenic milling), to 82.5% and 86.5%. Regarding the milling operations performed on the fibers 72

5 direction, you can observe that at the cryogenic milling, a decrease of the quality from 87.5% to 79%, in contrast at the conventional processing there is a minor increase from 83 5% to 87.5%. The millings performed with a cutting speed of 2355 m / min are different in terms of the temperature of the machined parts, so that the cryogenic milling shows an increase of 8 percent for the milling on the direction of the fiber and 6 percent for milling perpendicular to the fiber. In contrast to the increasing quality trend for cryogenic milling, conventional milling shows a drop of 10 percent for milling the on fiber direction, and 9 percent for conventional milling perpendicular to the fibers. Analyzing the values obtained from milling with cutting speed of 2826 m / min you can observe that the value of 83.5% is a common point where the curve of conventional milling perpendicular to the fiber direction meets, with the cryogenic milling on the direction of the fibers. The curve of conventional milling on the direction of the fibers is very close to this intersection, increasing to the value of 82%. Regarding the curve of cryogenic milling perpendicular to the direction of the fibers, the value remains at 92.5%. Even if all the qualities obtained by milling with cutting speeds of 3297 m / min and m / min suffer a slight decrease in the range from 84.5 to 89.5% for cryogenic milling Fig. 6. Quality evolution and 75-81, 5% for conventional milling it can be observed that the cryogenic milling is still higher than conventional milling. The qualities obtained from cryogenic milling with cutting speed of 3768 m / min have similar values (86% and 87.5%). Regarding conventional milling it can be observed that milling on the direction of the fibers produce a lower quality of 74.5% and the milling perpendicular to the direction of fibers showed a slight increase to 80%. In Figure 6. you can follow the evolution of obtained surface quality of the material 3540 with the feed of 0.01 mm / tooth, the variable introduced in this graph is the cutting speed used in processing. 6. ANALYSIS OF THE RESULTS Comparing the conventional to the cryogenic milling on the direction of the fibers, we can observe 8 cryogenic millings were qualitatively higher compared to the conventional machining operations, and one situation where the obtained quality of a conventional milling is superior to the cryogenic milling. The greatest difference between the obtained qualities for milling on the direction of the fiber appears at the cutting speed of 3768 m / min, where is a difference of 13 percent (in favor of the cryogenic milling). Comparing the conventional to the cryogenic milling perpendicular the fibers, 8 cryogenic millings were qualitatively higher compared to the conventional machining operations, and one situation where the obtained quality of a conventional milling is superior to the cryogenic milling. The greatest difference between the obtained qualities by milling perpendicular to the fibers, appears at the cutting speed of 2,355 m / min where is a difference of 19 percent (in favor of cryogenic processing). 7. CONCLUSIONS Given the fact that the experiment through the experiment we obtained 16 cryogenic qualitatively superior millings, and in only two cases superior quality 73

6 through conventional milling and that there had been great difference of quality rates (for cryogenic milling) even 19%, I think that for the cutting regimes used in the experiment, cryogenic rubber milling are superior to the conventional milling. The only situations where qualitatively superior conventional millings were achieved compared to cryogenic milling operations are observed at a cutting speed of 942 m / min at milling performed perpendicular to the fibers, and the second is at the cutting speed of 1884 m / min at the milling on the direction of the fibers. The rubber was in a cryogenic state for a period of time (without modifying the physical or chemical properties), hard polymeric material, which gives it a high machinability, a precise and easy fixing, and reduces vibrations in the cutting process, so that in most cases by machining the rubber in a cryogenic state superior surface quality is achieved, as compared to conventional machining operations. It requires the development of a research with multiple feed speeds and creating a catalog of standards on quality surfaces, or the identification of a directly measurable method instead of the benchmarking methods. CO. INGENIERÍA MECÁNICA TECNOLOGÍA Y DESARROLLO, 4(5), [14] Shaw, M. C. (2005). Metal Cutting Principles Oxford Series on Advanced Manufacturing. Publ. Oxford University Press, New York (USA).ASTM D , 2000, Standard Terminology Relating to Rubber, American Society for Testing of Materials. [15] Stephenson A, (2006). Methal Cutting Theory and Practice, CRC Press Taylor& Francis Group,New York. [16] Teramoto, K., Kuroishi, Y., & Yamashita, M. (2009). A framework for machining of soft objects. In The 5th International Conference on Leading Edge Manufacturing in the 21st Century (pp ). [17] Yan, J. (2006). A numerical and experimental investigation of the machinability of elastomers. REFERENCES [1] *** (2001) SR EN ISO 9000, ASRO, București. [2] *** (2001) SR EN ISO 9001, ASRO, București. [3] *** (2001) SR EN ISO 9004, ASRO, București. [4] Antony, J. (2014). Design of experiments for engineers and scientists, Elsevier. [5] Dick, John S. (2003). Basic rubber testing: selecting methods for a rubber test program. Vol. 39. ASTM International. [6] Guide to Plastics Machining, Retrived on , from [7] Jin, M., and Murakawa, M. (1998). High-Speed Milling of Rubber (1st Report) Fundamental Experiments and Considerations for Improvement of Work Accuracy, Journal of the Japan Society for Precision Engineering, Vol. 64, 6, pp [8] Lewis, M.A. (2002). End Milling of Elastomers, NCSU Master Thesis. [9] Luo, J. (2005). Machining of Elastomers (Doctoral dissertation, The University of Michigan). [10] Maxim, E (2004). Managementul calităńii, Editura UniversităŃii, Alexandru Ioan Cuza, Iaşi. [11] Nayak, R., Shetty, R., & Shetty, S. (2012). Experimental and Finite Element Analysis on Chip Formation Mechanism in Machining of Elastomers. Bonfring International Journal of Industrial Engineering and Management Science, 2(2), [12] Ophardt, Charles E. (2003). Virtual chembook. Department of Chemistry, Elmhurst, IL., Elmhurst College. [13] Rivera-Moreno, A., Hernández-Castillo, L. E., Siller, H. R., Elías-Zúñiga, A., & Rodríguez- González, C. A. (2013). Influence of Authors addresses 1 Mocerneac, Bogdan, PhD Student, Universitatea Tehnica din Cluj Napoca Centrul Universitar Nord Baia Mare, str. Victor Babeş, nr. 62A, phone: , bogdan.mocerneac@gmail.com. 2 LobonŃiu,Mircea, prof.dr.ing, Universitatea Tehnica din Cluj Napoca Centrul Universitar Nord Baia Mare, str. Victor Babeş, nr. 62, phone: , mircea.lobontiu@cunbm.utcluj.ro Contact person * Mocerneac, Bogdan, PhD Student, Universitatea Tehnica din Cluj Napoca Centrul Universitar Nord Baia Mare, str Victor Babeş, nr. 62A, phone: , bogdan.mocerneac@gmail.com. 74