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Vol. 38 (Nº 48) Year 2017. Page 36

Increasing geometric homogeneity of dispersed particles of plastic materials produced by vibration assisted micro-cutting

Aumento de la homogeneidad geométrica de las partículas dispersadas de materiales plásticos producidos por el micro-corte asistido por vibración

Yuriy SERGEEV 1; Sergey SERGEEV 2; Alexander D’YAKONOV 3

Received: 30/09/2017 • Approved: 05/10/2017


Content

1. Introduction

2. Methods

3. Data, Analysis, and Results

4. Discussion

5. Conclusion

Acknowledge

References


ABSTRACT:

This article substantiates a new concept of dimensional mechanical dispersion of viscous plastic materials in condensed media by cutting. It is about increasing geometric homogeneity and forming a required shape and size of dispersed particles by forcibly introducing a specified controlled vibration action into the area of material destruction. This approach will allow us to move from the traditional (chaotic) to the new highly organized and controlled process of destructing viscous condensed media of various structures. Achieved results make it possible to produce dispersed powder and fiber particles of required quality without changing the physicochemical characteristics. They will also contribute to the processes of developing vibration-assisted cutting technologies and designing high-technology energy efficient dispersing machines.
Keywords: dimensional vibratory dispersion; particle-size distribution (PSD) control; grinding various materials; powder production; granule like chips and fibers

RESUMEN:

Este artículo sustenta un nuevo concepto de dispersión mecánica dimensional de materiales plásticos viscosos en medios condensados por corte. Se trata de aumentar la homogeneidad geométrica y formar una forma requerida y el tamaño de las partículas dispersadas mediante la introducción forzada de una determinada acción de vibración controlada en el área de destrucción del material. Este enfoque nos permitirá pasar del proceso tradicional (caótico) al nuevo proceso altamente organizado y controlado de destrucción de medios condensados viscosos de varias estructuras. Los resultados obtenidos permiten producir partıculas dispersas de polvo y fibra de la calidad requerida sin cambiar las caracterısticas fisicoquımicas. También contribuirán a los procesos de desarrollo de tecnologías de corte asistido por vibración y diseño de máquinas dispersoras de alta tecnología con eficiencia energética.
Palabras clave: dispersión vibratoria dimensional; control de distribución de tamaño de partícula (PSD); molienda de diversos materiales; producción de polvo; fragmentos y fibras similares a gránulos

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1. Introduction

A high level of requirements for modern high-tech products has necessitated the production of a wide range of composite materials with specified physical and mechanical properties. At the same time, we are able not only to control the performance properties of composites by varying the composition, ratio, orientation of filler particles different in shape and size (Chumanov et al., 2010; 2012), but also to create new ones (Chumanov et al., 2015; 2017).

The prospects for materials science development are conditioned by the demand for new materials with unique properties and efficient manufacturing technology, as the requirements for their properties are constantly changing. Powders, fibers, granules and light metal pastes are widely used in various fields of engineering: metallurgy (Gordeev, Sergeev & Chumanov, 2010), chemistry (Sergeev & Gordeev, 2009), medicine (Salernitano & Migliaresi, 2003), energy (Sandalov & Sergeev, 2012), building and construction (Lakirev et al., 2003), nuclear (Sergeev, 2011), space, military technology (Balaykin, Smelov & Chempinskiy, 2012) and other industries, as they have new (in comparison with compact materials) properties, based on specific surface properties of dispersed materials (Sergeev & Gordeev, 2010) (various metal and non-metal composite materials). They are produced by reducing metal oxides or metal salts to metal, by electrolytic deposition, by plasma spraying, by electrospark dispersion, and by mechanical methods: cutting, grinding, abrasion, and etc. The technological features of these processes determine their scope of implementation and economic feasibility. Such a variety is based on different requirements to physical, chemical, mechanical and performance properties of dispersed materials. However, all these processes are being improved to increase productivity while maintaining the physicochemical properties of materials and, mainly, producing particles of desired shape and size in stable dispersion. Therefore, all manufacturers producing components for composite materials face an acute problem of providing the required particle geometry and particle uniformity that largely determine the performance properties of a product.

Physicochemical and mechanical dispersion methods used in industry have the following drawbacks. Physicochemical dispersion of metals and plastics involves physicochemical feedstock transformations. As a result, produced material is chemically or physically different in comparison with the feedstock. Mechanical granulating methods transform the feedstock into granular-like chips, fibers and even into powder without a noticeable change in its chemical composition. There are such widely used methods as granulating through cutting (shredding machines), abrasion and grinding (ball, vortex and hammer grinders), but the last two methods are advisable to use only for treating brittle materials. The process of grinding viscous and plastic materials (zinc, aluminum, copper, great part of thermoplastics) is difficult, since they are mainly flattened, not destructed. On the contrary, the process of granulating through cutting allows treating these viscous materials. Treated metal must be granulated into chips and have, at the same time, stable size and shape, since non-uniform particle-size distribution (PSD) of granulated metal entails defects (chipping, swelling, excessive or uneven porosity) and, as a consequence, deterioration of physical-mechanical properties of the final product. At the same time, the process of granulating through cutting does not allow making a desired shape and providing a desired PSD of dispersed material while the existing granulating methods are physically highly demanding. Therefore, high cost of manufacturing powder, especially metal powder, holds the producers from wider distribution of products made of composite materials. There is a need to produce granule like chips while treating wastes, for example – metallurgical slags (Sergeev, Sergeev & Karpov, 2016) and plastics (Sergeev & Gordeev, 2010), for the second time. Meanwhile, up to 50% of secondary raw materials can be added to the feedstock while manufacturing products based on thermoplastics, and up to 20% –while sintering products made of thermoset material if the last meets the specified PSD. In treating thermoplastics, non-uniform size of the cut-off material and its significant deviations from a desired shape lead to a number of defects in products obtained of it (non-melts, burnings, cracks, etc.). In terms of injection molding machines, there are always requirements for the size and shape of used thermoplastic granules, while the allowed deviation of the final particle size from the required should not be more than 15%. In sintering products made of thermosetting plastics, it is recommended to add secondary powder materials, which degree of dispersion should be 0.02 ... 0.05 mm. The latter cannot be obtained with existing cutting mills.

Summarizing the above, we should note that the considered methods have certain requirements for the granulate particle size and particle shape. In plant operating conditions, granulate particles with specified sizes and shapes are produced irregularly while grinding raw materials and waste. High dimensional heterogeneity and existing deviations from the specified shape of the cut-off segments lead to a number of defects in products obtained of it. This problem is particularly urgent when waste is being dispersed by milling, since such method is widely used because it is simple to implement and works fine with various engineering materials combining elastic and viscous properties. These difficulties are explained by insufficient knowledge of how the chips separate while the segments are being cut. There are also no substantiated recommendations how to control the process of shaping final particles. In this regard, there is an urgent task to improve the process of forming a cut-off segment while dispersing materials.

2. Methods

Physical phenomena occurring during mechanical dispersion are peculiar, as it has not been possible (until now) to build a unified physical theory of such processes that would explain the whole set of empirically well-known facts and predict new effects and phenomena. The problems in mechanics associated with the penetration and motion of solids in the continuous (solid) media are the least studied part of mechanics. Moreover, issues related to rotating instruments, namely their rotation motion in solid media, are not considered as problems of mechanics. Thus, no one seriously studies them be means of mathematical modeling. Since the issues of modeling the movement of multiple-cutting-edge tools in a metal workpiece and the movement of a grinding tool on the surface of formation are attached to the applied branches of science, the level of knowledge of such processes is immeasurably behind the level of knowledge of mechanical motion in other media. This situation is further aggravated by the fact that modern production technologies are based more on the experience of engineering practice, rather than on scientific research. In this regard, increasing efficiency of a particular development is largely based on the knowledge of the physical nature of related processes. Numerous attempts to attract professional mathematicians to the mathematical modeling of specific engineering problems, in most cases, do not drive to success, since successful solution of the applied problem requires a deep understanding of its essence.

3. Data, Analysis, and Results

Our analysis of performed work shows that when it comes to material dispersion as the increment of new surfaces, various researchers agree only in that particle dispersion is dependent on such factors as non-homogeneity and anisotropy of media (Sumelka, Zaera & Fernández-Sáez, 2016; Ouelaa et al., 2017; Gordienko, Mustyatsa & Kovaleva, 2016). The remaining issues are highly debatable; various hypotheses and scattered empirical facts are in contradiction.

Despite a large number of research papers, they are focused mainly on studying and improving energy-consuming and costly ways to produce powders and fibers, such as reducing metal oxides or metal salts to metal, electrolytic deposition, plasma spraying and electrospark dispersion (Kalpakjian & Schmid, 2014; Şahin & Yalcinkaya, 2016; Hayes, 2014). There are attempts to study and improve mechanical methods of dispersion (Hamran & Rashid, 2017; Webster & Eren, 2014; Parveen, Rana & Fangueiro, 2013). Their results are provided mainly to increase the process productivity while maintaining the physicochemical properties of materials. As a result, used methods lag behind, and the equipment is morally obsolete.

Therefore, research on material fracture mechanics, associated with the process of developing the fundamentals of chip geometry control and its surface uniformity control while forming the rotating multiple-cutting-edge tools through forcibly changing the amplitude-phase-frequency characteristics of their transverse oscillations (Lakirev et al., 2003; Sergeev et al., 2009), will become a scientific basis for the improvement of modern vibration technologies, used in the course of solid dispersion in condensed media.

Since (Kumabe, 1979; Poduraev, 1985), cutting kinematics determines the shape and size of chips, then it is expedient to control the chip formation according to the method (Sergeev et al., 2009) – by changing the kinematic movements of the rotating multiple-cutting-edge tool through the forced controlled radial vibration. This approach is fundamentally new. According to this granulation method (Sergeev et al., 2009), movement patterns of the milling cutter teeth 1 (cutter diameter is D) are formed by combining three trajectories (Figure 1): cutting speed as a uniform rotation around the axis O0 with a frequency ωvr; uniform feed motion S; oscillatory feed motion forced in the radial direction with a frequency ωk and amplitude A (A = 2ρ) as a rotational movement of the eccentric 2 relative to Oe. As a rule, tool oscillations can be any, for example, if one uses (Sergeev et al., 2014), he/she has to consider the major property of treated material while making a mathematical model – namely, the shape stability typical for solids. In other words, the main qualitative difference between a solid, a liquid and gaseous media is that the solid media "remembers" all past impacts. Besides, the path of a penetrating element depends not only on its current location, but also on a number of its previous positions.

Figure 1
Scheme of chip formation with a vibrating cutting tool

As a result, we will have the interrupted cutting condition guaranteeing the separation of dispersed chips (Figure 2):

Figure 2– Scheme of chip formation

Figure 3 – Scheme of the changing positions of a cutting
plane under radial feed and oscillations

Figure 4 – Kinematics of grinding without radial oscillations

------

Figure 5 – Kinematics of grinding with radial oscillations

Figure 6 – Particle cross-section

Figure 7 – Facility diagram

 

4. Discussion

As the oscillation amplitude increases, the greatest value of the cut-off particle thickness increases. As the oscillation frequency decreases, the greatest value of the cut-off particle length increases.

Since mathematical models (1,15,17-20) allow us to measure the shape and size of dispersed particle hypersurface, we are able now to model and control the process of dispersion with a computer and modern software tools in real time (Sergeev et al., 2015).

The computer-based simulator of chip formation with specified shape and size through cutting is based on the mathematical logic apparatus, since our problem can be mathematically described as the process of finding constituents. Thus, problem to be solved involves the main fundamental factors affecting the cutting process. Secondly, we have had regard to the fact that shapes of particles cut off by the cutting edges (due to the influence of these factors) are complex related hypersurfaces and depend on whether there is (conjunction) a trace left by the previous cutting blade at the each moment of time or not (disjunction). In other words, mathematical model takes into account that each subsequent chip surface cut off by the i-th cutting blade has initial conditions, depending on the previous cut-off layers.

Figure 8 – Animated process of grinding a plate with a five-tooth mill

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Figure 9 – Animated process of particle formation with specified shape by the oscillatory mill movements

In the course of computer-based dispersion modeling, this program allows controlling the shape and size of dispersed particles in a wide range (Figure 10)

Figure 10 – Examples of particle shape control by changing the oscillation parameters

The control is carried out by changing the technological process parameters: forced oscillation amplitude of the mill and the ratio of rotation frequencies and its oscillations. The simulator allows not only measuring the sizes and volumes of dispersed particles, but also visualizing the entire cutting process (Figures 11 and 12).

Figure 11 – Granulation process report form. The first report sheet
(automatically generated when the simulation is complete)

Figure 12 – Granulation process report form. The second report sheet
(automatically generated when the simulation is complete)

Volume of formed particles (cubic millimeter)

7.8059

55.1151

52.7499

52.0928

55.2392

55.7833

54.4369

19.9729

54.8043

90.9609

55.1215

52.7104

52.0783

55.2545

18.8837

61.8199

53.5314

54.8591

55.0076

55.1431

52.7144

51.4513

64.9913

55.3955

61.8097

53.5274

54.8259

90.9958

31.8045

54.6571

53.9282

64.9907

55.3941

61.8292

53.5234

22.0125

55.3084

55.6384

54.6676

53.9472

64.9753

55.3926

55.1633

52.5689

52.2133

55.2808

55.6253

54.6636

53.9231

54.7765

55.0591

55.1767

52.5858

52.1609

55.2682

55.6852

15.3481

53.4453

54.7079

55.0769

55.2047

52.588

52.1085

21.0451

55.2213

61.6898

53.4452

54.7271

55.0948

55.1454

54.4016

53.8848

65.1271

55.2226

61.6728

53.4454

54.7464

55.1889

55.9097

54.4022

53.8838

65.1295

55.2093

61.6704

52.7588

52.0916

55.2332

55.9237

54.4027

53.8681

65.1464

16.8998

55.0662

52.8584

52.0765

55.2187

55.8496

54.4179

53.5572

54.8215

55.1492

55.0658

52.8539

52.0468

55.2189

55.5224

61.7484

53.5789

54.8497

55.1158

55.08

52.8201

53.8446

65.0154

55.4767

61.7648

53.6261

54.8486

55.1234

48.8037

54.55

53.8603

64.9851

55.4748

61.8105

53.6587

52.016

55.2308

55.5056

54.5526

53.8468

64.984

55.4875

55.0748

52.5481

52.0355

55.2316

55.5701

54.5845

53.8772

54.6668

55.0336

55.0882

52.5496

52.0396

55.2468

55.5756

55.4256

53.4674

54.5945

55.0634

55.1016

52.5364

52.0454

52.9726

55.4663

62.1062

53.4215

54.6103

55.0934

55.115

54.5521

53.9503

65.0553

55.4478

62.0678

53.3755

54.6127

55.3739

55.875

54.5294

53.9356

65.0282

55.4584

62.0148

52.7209

52.1591

55.4436

55.8371

54.5214

53.9355

65.0321

55.1358

55.09

52.6962

52.1585

55.4401

55.8431

54.4987

53.3933

54.9263

55.0931

55.1043

52.6861

52.1575

55.3914

55.3088

61.6021

53.3666

54.8809

55.1376

55.0602

52.7051

53.8245

64.9514

55.3421

61.6351

53.3554

54.8943

55.1237

55.7623

54.3633

53.7799

64.9534

55.331

61.6239

53.4028

52.0634

55.2093

55.7876

54.3929

53.7938

64.9845

55.3783

 

5. Conclusion

Thus, obtained results have proved the principal possibility of forming cut-off layers of required size and shape by introducing forced oscillations of a rotating multiple-cutting-edge tool into the cutting area. In the future, this opportunity made it possible to implement a fundamentally new direction in processing metals and plastics – dimensional dispersion of materials, including waste.

The process of modeling dimensional dispersion by cutting allows controlling the size and shape of dispersed particles while their PSD stability increases, as well as determining its capabilities in regards to criteria.

The major conceptual basis is the practical implementation of the very process of dimensional dispersion, since the literary sources presented this concept only as a problem statement. Currently, dimensional mechanical dispersion is a fundamentally new direction, which is only being formed under the demand of powder and fiber producers, as well as under the demand of specialists engaged in industrial and household solid waste treatment. The existing dispersers work on the principle of "coffee grinder" and do not allow controlling the process. Therefore, it is important to conceptually change the attitude towards the dispersed particle formation. In other words, one has to treat it as the process of producing parts with metrological parameters: size and shape accuracy. Therefore, cutters and grinders have to be brought to the level of vibration disperser designs, which make it possible to meet these parameters. Their designs must also have the technological parameter tuning units. Obtained mathematical models make it possible to carry out the computer-based predictive modeling of dispersed particle formation. This opens the way to the creation of automatically-operated vibration dispersers for viscous media.

Acknowledge

South Ural State University is grateful for financial support of the Ministry of Education and Science of the Russian Federation (grant No 9.7960.2017/BP).

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1. South Ural State University, Ave. Lenin, 76, Chelyabinsk, 454080, Russia.

2. South Ural State University, Ave. Lenin, 76, Chelyabinsk, 454080, Russia.

3. South Ural State University, Ave. Lenin, 76, Chelyabinsk, 454080, Russia.

Contact E-mail:  kseniabadz@gmail.com


Revista ESPACIOS. ISSN 0798 1015
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