Recycling end-of-life tires (ELTs) poses an enormous challenge in waste management, as tires are not biodegradable and, if not correctly disposed of, can cause problems for human health and the environment. The transportation of ELTs from waste collection points to recycling facilities is one of the biggest problems in the recycling process, as a whole piece of ELT takes a large volume, resulting in high costs and transport delays. Therefore, a possible solution is a pre-cutting process at collection points to reduce volume and facilitate transport. Cutting processes play an essential role; hence, the devices used in this operation must be efficient to keep a minimum energy consumption. This study addresses the numerical evaluation of several blade profiles to find the most efficient regarding the force and work done for the cutting of tire tread. The numerical results regarding the performance of the profiles were validated experimentally confirming that the most efficient geometry between the evaluated correspond to a hollow profile.
Research Article
Assessment of sharpening profile effects on the efficiency of cutting tools for tire recycling: a numerical study
https://doi.org/10.21203/rs.3.rs-3324604/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 05 Sep, 2024
You are reading this latest preprint version
recycling
end-of-life tires
cutting force
cutting tools
Population and economy’s growth have increased the use of vehicles: their production augments constantly, amounting to 100 million units per year in 2018 (International Monetary Fund 2019), and they are responsible for demanding high quantities of tires whose fabrication reached 1626 million units in 2018 (Zare Mehrjerdi and Shafiee 2020), turning ELTs management into a decisive environmental issue. Despite research and regulation on their disposal, enormous volumes of this waste are stored untreated in landfills and stockpiles, especially in developing countries, becoming a threat to the environment and public health (Valentini and Pegoretti 2022). Above all, sustainable options for ELTs management are reuse, retread, recycle, and energy recovery. However, when tires cannot be reused in vehicles, they constitute waste, and the treatment options are recycling and recovery (Shulman 2011) for example to improve the rheological performance of modified asphalt (Leng et al. 2018).
In tire design and manufacturing, properties such as wear resistance, performance in harsh conditions, and high durability are relevant parameters that make it hard to manage used tires and their waste, representing an environmental hazard (Dabic-Miletic et al. 2021). The complexity varies depending on vehicle size, load unit, material, and production processes (Jovanović et al. 2014). So, it is pivotal to undertake a comprehensive review of the tire production process to achieve a sustainable circular economy, aiming to reduce the amount and variety of waste considering its lasting and nonbiodegradable materials (Ghai et al. 2022). During the tire design phase, it is suggested to bear in mind its entire life cycle so that parts can be recovered, reused, and disposed of.
Recycling used tires into raw materials (rubber, textile fibers, and steel) represents an opportunity and a challenge in the circular economy of tire management. Recycled rubber can be used in different engineering applications, such as tire-derived fuel by pyrolysis, civil engineering solutions, devulcanized rubber for new tire molded objects, and rubber-modified asphalt (Dabic-Miletic et al. 2021). In the recycling process of ELTs’ materials, tires must be collected, transported to facilities for shredding, and then separated into their components. In the first step, since the tires are very bulky, transporting them is difficult; therefore, a possible alternative solution for ELTs’ hauling is to pre-cut tires at the collection points. It reduces the size and facilitates transportation.
In addition, the transfer contributes to pollutants and greenhouse gasses, as the collection points are far from treatment facilities (Nowakowski and Król 2021). The cost of scrap tire collection is greatly influenced by the number of shipments and routes required, making it a critical factor in total costs. When collecting ELTs over short distances, it is paramount to use vehicles that can carry the minimum logistical load of tires. In some cases, large-capacity commercial vehicles may be necessary (Nowakowski and Król 2021). Therefore, preprocessing of the ELTs with cutting, packing, and baling is suitable for reducing the overall collection and transportation costs and minimizing the negative environmental impact of exhaust emissions.
Moreover, the cutting force is a factor that significantly influences the ease and accuracy of the tire pre-cutting process at collection points. Understanding cutting force is imperative as it enables the development of advanced recycling technologies and specialized equipment capable of efficiently handling ELTs. However, the literature regarding the cutting of these polymeric composite materials gathers little research. Results in neoprene and nitrile materials with normal force and sliding motion showed that the resistance to cutting of a material when a sharp object slides over it and exerts a normal force depends on the coefficient of friction (Vu Thi et al. 2005). Spagnoli et al. (2019) performed tests with different blade inclination angles and measured the cutting force. The results revealed that the increase in the angle decreases the cutting force, but this hypothesis still needs to be confirmed with greater angles. The sharpness of the tool used in the polymer cutting is a necessary variable and one of the main terms in the total applied force, and it determines the reduction of the amount of energy lost due to frictional dissipation. Regarding this, McCarthy et al. (2007) derived a quantitative index of blade sharpness through indentation tests on soft solids. They found that the condition of sharpness of the blade's cutting edge defines the indentation depth before cutting. This property leads to the Blade Sharpness Index (BSI), which is independent from the target material and the cutting rate and only related to the blade. Furthermore, using an implicit finite element model, they proved the effect of tip radius, wedge angle, and blade profile on the BSI, seeing that all these variables affect it, where the tip radius effect is more significant (McCarthy et al. 2010).
Also, the finite element method has been extended and used to analyze all kinds of phenomena, including the cutting process. In this case, it has been popular the use of explicit dynamics to simulate metal cutting. For example, Shih, (1995) simulated the material deformation during the cutting process including various factors such as elasticity, viscoplasticity, temperature, large strain, and high strain-rate effects to accurately capture the behavior of the material under these conditions. Also Özel, (2003) examined through the use of practical finite element (FE) simulations and high-speed orthogonal cutting tests the impact of edge preparation on cubic boron nitride (CBN) cutting tools. Adidionally Guo et al. (2009) in a review of the characteristics of residual stresses in machining processes performs a critical assess of the numerical modelling metal cut process using finite element method. On the other hand, the rubber cutting process has been scarcely simulated. Related to this issue, Yan and Strenkowski, (2006) developed an explicit plain strain finite element model to evaluate the orthogonal cutting of rubber using high-speed steel tools. They employed the neo-Hookean constitutive model for the hyperelastic material, predicting cutting forces, chip shape, stress and strain fields, and strain energy distribution in the chip and workpiece.
In this study, explicit 3D finite element simulations are used to evaluate several shapes of blade profiles and find the most efficient one for the required cutting force on the rubber tread material of a wasted tire. Two of these profiles (with different performances) were fabricated and tested experimentally to validate the numerical simulation's effectiveness.
2.1 Numerical experiment
The test included nine configurations based on the blade profiles obtained from Kelly, (2003). As a result, three geometrical blade profiles (straight, lenticular, and hollow) were simulated, each with three variations in one of their characteristics (Fig. 1). The straight ones have a V shape for the 20°, 30°, and 40° angles (Fig. 1a); in the lenticular, the varied property is the radius of a convex curve (Fig. 1b) whose center is 20 mm above the tip in the vertical direction where radius values of 32 mm, 50 mm, and 68 mm were used; in the hollow profile, three radii of a concave curve were defined, where the center of the arc is 20 mm below the tip, and the radius values are 70 mm, 100 mm, and 130 mm (Fig. 1c). For each blade, the tip radius was defined as 1 µm, which was based on the study performed by McCarthy et al. (2010), who evaluated the sharpness of the geometry by varying the tip radius.
The blade profiles are used to cut a 22 mm wide, 40 mm deep, and 25 mm thick piece of rubber with tire tread properties. These properties were obtained from experiments performed by Ke-Jia et al. (2009) and were used here to calibrate the 3rd-order hyperelastic Ogden model (section 2.2.2). Figure 2 shows the setup of for the cutting test. The numerical experiment was performed using the explicit dynamics solver of ANSYS mechanical software. Here to improve the simulation times, and after several tests, the cutting speed of the blade was defined at 400 mm/s downwards in a vertical direction. The simulation time was set to allow the motion of the blade 26 mm in the negative y-direction that permitted the complete cut.
2.2 Finite element model
The motion was defined via a boundary condition applied to the blade only in the negative y-direction and constraining the displacements in the x-direction and z-directions to allow the blade to cut the tire rubber. The boundary conditions for the rubber specimen correspond to restricted motion only in the y-direction on the bottom face and limited in the z-direction on the four vertical edges. These constraints are based on a simple shear test similar to that proposed by McCarthy et al. (2010). Figure 2 shows the boundary conditions applied to the test arrangement.
Since one of the objectives of this study evaluates the shape of the blade profile for cutting efficiency, the interaction between the blade and the tire rubber during that the cutting process must be included. Thus, it was necessary to perform an explicit dynamic simulation to reproduce that process. On the one hand, in order to simplify the calculations, the cutting blade was modeled as a rigid body since it is much stiffer than the tire rubber. On the other hand, Vu Thi et al. (2005) pointed out that the friction coefficient is a critical variable to consider in the analysis, so a body-friction interaction was required. In this case and according to the experimental results on styrene-butadiene rubber obtained by Setiyana et al. (2021), a 1.55 friction coefficient, a 1.6 dynamic coefficient, and a 50 decay constant were defined for all simulations. In the analysis settings of the simulation the preference was set as low velocity, where the minimum CFL (Courant-Friedrichs-Lewy) time was set to 10− 4 s, which was determined by considering the minimum mesh element size obtained after a mesh convergence study (see section 2.2.1). Additionally, the maximum element and part scaling parameters were defined at 1000 to reduce simulation times. The erosion control was set to a geometric strain limit of 1.67 obtained from the experimental tests performed by Noh et al. (2022), and to capture in detail the cutting process, 200 data points were saved.
2.2.1 Mesh convergence study
After several test simulations, it was found that the size of the mesh element under the blade tip influenced enormously the force at cut formation. Therefore, to understand this behavior, a mesh independence study was conducted. For this purpose, a 20° straight blade profile and a frictionless contact condition between the two bodies were used to gain efficiency in the simulation process. Figure 3 shows the force at cut formation as a function of mesh size in terms of the number of elements and element height under the blade tip. It can be seen that the force for the larger meshes shows a minimum difference between them, despite the large difference in the number of nodes. This two meshes correspond to element sizes of 0.03 mm and 0.01 mm, however, it was identified that including the frictional effects the resulting mesh will be inefficient for the available computational resource. Therefore, the mesh used to evaluate the blade profile shape corresponds to the one of element height 0.103 mm and 127764 elements.
Figure 4 compares the used and the finer mesh for the rubber piece. Both show a refinement zone where the cutting process occurs. This refinement is based on Schneiders’ work (Schneiders 1999) and performs a transition, dividing one element into three.
2.2.2 Hyperelastic model
Vulcanized elastomers and biological tissues are known to have a high deformation capacity without having a plastic deformation. However, this behavior is not linear, and analyzing these materials requires a hyperelastic constitutive model. There are several ways to represent hyperelasticity, and, to choose the one that best reproduces behavior of tire tread rubber, a numerical simulation was performed based on ASTM D412-16 (ASTM 2019) and on experimental data obtained by Ke-Jia et al, (2009). Namely, uniaxial, biaxial, and shear test data of the tire tread (Ke-Jia et al. 2009) were used to fit the constants of one of the available models in ANSYS software. During this process, all models were tested, finding the 3rd -order Ogden model (Ogden 1972) as the one that best fit for the experimental data. According to it, the strain energy function is based on the deviatoric principal stretches of the left Cauchy-Green strain tensor, as given in Eq. (1–3):
Table 1 shows the model fit parameters. It is noteworthy that the compressibility parameters \({d}_{p}\) have a low value since the material is assumed to be incompressible. Figure 5 demonstrates the fitted curves of the Ogden model for the experimental data obtained by Ke-Jia et al. (2009).
|
Coefficient |
i = 1 |
i = 2 |
i = 3 |
|---|---|---|---|
|
\({\mu }_{i}\) |
1.54×106 |
1.63×106 |
8.62×106 |
|
\({\alpha }_{i}\) |
4.53 |
4.51 |
3.41e×10− 5 |
|
\({d}_{i}\) |
1×10− 10 |
0 |
0 |
Figure 6 presents the numerical results using the Ogden model compared to the experimental data. It is identifiable that the chosen model adequately reproduces the behavior of the tire tread rubber.
2.3 Experimental validation tests
Cutting trials were performed under a compression test in a Shimadzu Universal Testing Machine (model AGS-50kNX) of 50 kN load capacity. In this case, the 30° straight blade (Fig. 1 (a)) and the hollow blade of radius 130 mm (Fig. 1 (b)) were fabricated to use this with the device shown in Fig. 7 (c). This device helped to reproduce the boundary conditions of the simulation in order to compare numerical and experimental results. The 30° straight blade was fabricated considering that it is a commercially available profile, and the hollow blade is a result of the analysis of the numerical data shown in section 3.2. Both blades were fabricated in a CNC machine to ensure the required profile shapes were correctly reproduced. For both cases, hardened tool steel was used as construction material.
Moreover, to ensure the repeatability of data, at least three tests were performed with each blade. The tire rubber samples were obtained by extracting a section that only contains rubber from the thicker spots in the tread of a used tire. Fig. 7 shows the fabricated blades and the cutting test assembly.
3.1 Stress and failure in the cutting process
As mentioned, to estimate the blade profile performance, the cutting process had to be modeled, then to achieve this, an erosion control was set to eliminate the mesh elements reaching a threshold equivalent to the von Mises strain at material failure. Figure 8 illustrates the stress and strain contours just when the limit value is reached, a moment after this event, and when the blade has cut the rubber piece with the 20° straight blade. It can be observed that the section of rubber is significantly deformed due to its hyperelastic behavior (Fig. 8 (a)). Then, the energy accumulated inside before the failure is released, and the two pieces resulting after the cutting process get deformed in the opposite direction (Fig. 8 (c)). McCarthy et al. (2010) obtained similar results, pointing out experimental data of the same deformed shape as the numerical simulation. In addition, the high-stress concentration under the blade tip is observed, leading to the beginning of the cutting process; then, the equivalent strain keeps close to the value defined as that where erosion occurs. Likewise, the effect of the friction force in the cut rubber is meaningful and reflected in the equivalent stress values of around 10 MPa that appear at the resulting edges after cutting, causing the split of small rubber pieces as showed in Fig. 8 (b).
Figure 9 shows the evolution of the force applied to the blade versus its displacement for the data directly obtained from the simulation and for the filtered force data. Due to the high noise level in the data, Gaussian smoothing was applied to obtain Fig. 9 (b) to compare with the results of the other blade profiles. It is observed that at around 10 mm of blade displacement, the force increases significantly, and shortly after, a reduction in its value is identified; here, the energy release indicates the start of the cutting process. A similar pattern was observed with the other profiles.
3.2 Effect of the blade profile geometry
Consequently, all proposed geometries of blade profiles in Fig. 1 were simulated following the settings described in section 2.2 and the results are showed in Fig. 10. The measured variable represents the force applied to the blade and, as a result, after placing a Gaussian filter on the data, the behavior of the forces for each blade was obtained. The identifiable maximum force is thus obtained with the lenticular profile of a 32 mm radius, corresponding to the bluntest profile (Fig. 1(b)) as expected.
Besides, the lowest peak force is obtained with the hollow profile of 130 mm radius, which, in Fig. 1 (c), coincides with the sharpest profile, as expected. Furthermore, the maximum force increases with the angle for the straight one and decreases with increasing radius for the lenticular and hollow profiles.
To reinforce the above analysis, the work corresponding to the area under the curve of Fig. 9 was calculated for each blade. Table 2 reveals the results of the work required to perform the cutting for the proposed geometries. As projected, the blunt geometry, coinciding with the 32 mm radius lenticular profile, requires the highest amount of work, similar to that obtained with the results of the maximum peak force of Fig. 9. However, unlike the case of the peak force, the 20° straight profile shows the minimum work performed, and it is close to the result of the 130 mm radius hollow profile, concluding that the best performance among the evaluated profiles corresponds to the 130 mm radius hollow.
| Blade profile | Straight 20° | Straight 30° | Straight 40° | Lenticular R 68 mm | Lenticular R 50 mm | Lenticular R 32 mm | Hollow R 130 mm | Hollow R100 mm | Hollow R 80 mm |
|---|---|---|---|---|---|---|---|---|---|
| Work done (J) | 21.78 | 30.74 | 39.28 | 26.97 | 35.92 | 39.84 | 22.12 | 28.35 | 36.67 |
3.3 Comparison between numerical and experimental results
Figure 11 shows the results of experimental tests along with the numerical results for both 30° straight blade and hollow blade of radius 130 mm. Even though the simulations no exactly reproduce the experimental tests, the values of maximum force are in the same order of magnitude for the straight blade in both cases (simulations and tests). Also, experimental tests showed that the hollow blade is more efficient than the straight blade, as was evidenced in the simulations, however, the numerical simulation overpredicts the maximum force and work done. The manufacturing process of the blade could explain the differences in these values since, at the moment of machining the blade profile, the tip radius of the blade can be less than the simulated (1 µm). This, as reported by McCarthy et al. (2007), causes an enhancement of the sharpness, therefore, the performance of the real blade would be improved.
On the other hand, an analysis of the tire rubber sample after the cutting test help to understand the qualitative behavior predicted by the numerical simulation. Figure 12 shows the cutting marks from both tested blades on the sample after the test. Here it is evident the efficiency of the hollow blade, since, it was able to perform the entire cut through the thickness of the sample, while the 30° straight blade was not able (Fig. 12 (a)). Moreover, the top view in Fig. 12 (b) shows the trace of the contact between the blade and the rubber. Here it is observed that the straight blade had a larger contact area, in which, if the contact pressure is considered, a larger force will be require to initiate the cutting process.
This is also noted in the simulation results of Fig. 13 where a comparison of the cutting process between the two blades at the same blade position is showed. It is clear that due to the blunter shape of the 30° straight blade, the rubber material is retained in the blade’s side surfaces, causing the contact area to be larger. Due to this, a larger deformation towards the blade appeared. With the hollow blade instead, the contact area and the deformation are smaller, which means less force required for to perform the cut. Evidence of this fact is observed in Fig. 11 at a displacement of 15 mm, where a difference of 800 N of force is noted, which interestingly is similar to the difference of force between the experimental data for both cases at the same displacement. The last results show that, at least qualitatively, the simulation predicts well the behavior of both profiles and can be a helpful tool to optimize the geometry of the cutting tools for tire rubber recycling.
Explicit dynamic simulations were performed to analyze the behavior of the blade profile to cut tire tread rubber. The 3rd−order Ogden model was used to simulate the hyperelastic behavior of the rubber, where it was identified that the explicit simulation provides good insight into the material behavior during the cutting process. Considering this, several blade profiles were tested, finding that the hollow ones behave at best at cutting force and work done and improve when the profile radius is broader. Moreover, based in the numerical results, two blades were fabricated to perform experimental tests, and it was found that the simulation reproduces well the qualitative behavior of the tire rubber cutting process. Although the simulation overpredicts values of force and work done, it allowed to obtain an efficient design that resulted better than the predicted by simulation.
Author contributions
Leonel Teran performed investigation, methodology, writing – original draft. Leydi Cárdenas performed conceptualization, methodology, writing – review and editing. Luis Ruiz performed visualization, writing – review and editing. Duberney Hincapié performed conceptualization, writing – review and editing.
Acknowledgments
The authors thank the Fundación Universidad de America for the support received through project number IIQ-003-2021.
Founding and/or Conflicts of interests/Competing interests: All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Data availability: Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.
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