Milling Cycles Research Articles

An assessment of methods used for the generation and characterization of cryomilled polystyrene micro- and nanoplastic particles

There is a critical need to generate environmentally relevant microplastics (MPs) and nanoplastics (NPs) to better investigate their behavior in laboratory settings. Environmental MPs are heterogenous in size and shape, unlike monodisperse and uniform microspheres commonly used as surrogates. Cryogenic grinding, or cryomilling, was successfully utilized to transform polystyrene (PS) bulk material into heterogenous micro and nano fragments. Fourier-Transform Infrared (FTIR) spectroscopy confirmed that this approach did not alter polymer surface chemistry. The number of milling cycles (time of milling) and frequency of grinding (intensity of milling) were varied to investigate the role cryomilling parameters had on generated MP characteristics. The resulting particle size distributions of cryomilled samples were measured and compared. Coulter Counter and Nanoparticle Tracking Analysis (NTA) were used to measure the particle size distributions at the micro and nanoparticle size ranges, respectively. Microspheres were used to determine what camera settings yielded more accurate sizing and to reduce bias in the NTA analysis. Increasing milling cycles generally increased the number of smaller particles. The evolution of the measured size distributions indicated that small nanosized fragments broke off from larger MPs during cryomilling, steadily eroding larger MP fragments. The number of milling cycles was observed to more consistently impact the size distributions of fragments compared to the frequency of milling. This study offers both analysis of the cryomilling process and recommendations for generating more realistic PS MP/NPs for examining environmental fate and effects.

Do Mixtures of Beads with Different Sizes Improve Wet Stirred Media Milling of Drug Suspensions?

The impacts of bead sizes and bead mixtures on breakage kinetics, the number of milling cycles applied to prevent overheating, and power consumption during the nanomilling of drug (griseofulvin) suspensions were investigated from both an experimental and theoretical perspective. Narrowly sized zirconia beads with nominal sizes of 100, 200, and 400 µm and their half-and-half binary mixtures were used at 3000 and 4000 rpm with two bead loadings of 0.35 and 0.50. Particle size evolution was measured during the 3 h milling experiments using laser diffraction. An nth-order breakage model was fitted to the experimental median particle size evolution, and various microhydrodynamic parameters were calculated. In general, the beads and their mixtures with smaller median sizes achieved faster breakage. While the microhydrodynamic model explained the impacts of process parameters, it was limited in describing bead mixtures. For additional test runs performed, the kinetics model augmented with a decision tree model using process parameters outperformed that augmented with an elastic-net regression model using the microhydrodynamic parameters. The evaluation of the process merit scores suggests that the use of bead mixtures did not lead to notable process improvement; 100 µm beads generally outperformed bead mixtures and coarser beads in terms of fast breakage, low power consumption and heat generation, and low intermittent milling cycles.

The experiment and simulation of enhanced mechanical performance for polyethylene terephthalate/high‐density polyethylene composites via domain size control

AbstractSolid‐state shear milling (S3M) equipment is proficient for achieving room‐temperature ultrafine grinding of multicomponent polymers, demonstrating significant potential in polymer blending and waste plastic recycling. However, a systematic investigation of S3M's ability to control the domain size of multicomponent polymer is currently lacking, which was of great potential for the high‐value recycling of waste plastics that are difficult to separate and thermodynamically incompatible. As the raw material of beverage bottle, polyethylene terephthalate/high‐density polyethylene (PET/HDPE) system with tremendous production was hard to be recycled together. Therefore, in this study, we chose PET/HDPE for the study of domain size control efficiency of S3M on mechanical properties of polymer blend. We also measured the processing parameters during the fabrication of PET/HDPE blended powder to determine the optimal manufacturing parameters for the polymer composite with outstanding mechanical property. After 30 milling cycles, the powder size of PET/HDPE gradually decreased from 849.8 μm (2 cycles) to 43.2 μm, which related to PET domain size of 71.9 and 5.2 μm, respectively. Meanwhile, at 24 milling cycles, the PET/HDPE composites reached the highest tensile strength of 27.6 MPa, meeting the most proper milling condition for PET/HDPE system. Finite element computer simulation was introduced to further investigate the domain size influence on mechanical property of PET/HDPE composites, which acquired similar results with the experimental value before excessive milling cycles. Based on both the experimental results and simulation support, a theoretical basis for the domain‐size‐control analysis of S3M in multicomponent waste polymer recycling was established.

Recycling of Flexible Polyurethane Foams by Regrinding Scraps into Powder to Replace Polyol for Re-Foaming

In the context of protecting the ecological environment and carbon neutrality, high-value recycling of flexible polyurethane foam (F-PUF) scraps, generated in the production process, is of great significance to save petroleum raw materials and reduce energy consumption. In the present study, F-PUF scraps were ground into powder by strong shear regrinding using two-roll mill and then reused as a partial replacement of polyol for re-foaming. A series of characterizations were employed to investigate the effect of milling cycles, roller temperatures, and content of the powder on the properties of the powder and F-PUF containing powder. It was revealed that the mechanochemical effect induced breaking of the cross-linking structure and increased activity of the powder. The volume mean diameter (VMD) of powder prepared with 7 milling cycles, at room temperature, is about 97.73 μm. The microstructure and density of the F-PUF containing powder prepared in the above-mentioned manner to replace up to 15 wt.% polyol, is similar to the original F-PUF, with resilience 49.08% and compression set 7.8%, which indicates that the recycling method will play an important role in industrial applications.

Immobilization of Silver Nanoparticles on the Surface of Cellulose Nanofibers Using High-Pressure Wet-Type Jet Mill

Composite material immobilized silver nanoparticles (NPs) on the surface of cellulose nanofibers (CNF) was prepared using a high-pressure wet-type jet mill. A mixture both containing an aqueous silver nitrate solution and a CNF suspension was prepared as a raw starting material. The mixture was pulverized with the high-pressure wet-type jet mill at a pressure of 100 or 200 MPa. An X-ray diffraction pattern of the obtained sample revealed the presence of not only cellulose type I crystallites but also silver metal crystallites. According to observation by field-emission scanning electron microscopy, it was found that many silver NPs were immobilized on the surface of CNF. Note that almost all the silver NPs were well dispersed on the surface of CNF. It was cleared that the silver NPs had a spherical in shape with an average particle size of about 3 nm by the transmission electron microscope observation. The average size of the silver NPs slightly increased with the number of jet milling cycles, however, the change in the discharge pressure of the high-pressure wet-type jet mill did not affect the NPs size. The silver content in the composite materials increased with increasing both the number of jet milling cycles and the discharge pressure. The silver NPs were deposited by using the thermal energy released in the jet milling process, and their grain growth was then inhibited because the suspension was cooled immediately through the cooling tube. Therefore, it was assumed that silver NPs with a narrow size distribution could be immobilized on the surface of CNF.

Amorphous Form of Carvedilol Phosphate-The Case of Divergent Properties.

The amorphous form of carvedilol phosphate (CVD) was obtained as a result of grinding. The identity of the obtained amorphous form was confirmed by powder X-ray diffraction (PXRD), different scanning calorimetry (DSC), and FT-IR spectroscopy. The process was optimized in order to obtain the appropriate efficiency and time. The crystalline form of CVD was used as the reference standard. Solid dispersions of crystalline and amorphous CVD forms with hydrophilic polymers (hydroxypropyl-β-cyclodextrin, Pluronic® F-127, and Soluplus®) were obtained. Their solubility at pH 1.2 and 6.8 was carried out, as well as their permeation through a model system of biological membranes suitable for the gastrointestinal tract (PAMPA-GIT) was established. The influence of selected polymers on CVD properties was defined for the amorphous form regarding the crystalline form of CVD. As a result of grinding (four milling cycles lasting 15 min with 5 min breaks), amorphous CVD was obtained. Its presence was confirmed by the “halo effect” on the diffraction patterns, the disappearance of the peak at 160.5 °C in the thermograms, and the changes in position/disappearance of many characteristic bands on the FT-IR spectra. As a result of changes in the CVD structure, its lower solubility at pH 1.2 and pH 6.8 was noted. While the amorphous dispersions of CVD, especially with Pluronic® F-127, achieved better solubility than combinations of crystalline forms with excipients. Using the PAMPA-GIT model, amorphous CVD was assessed as high permeable (Papp > 1 × 10−6 cm/s), similarly with its amorphous dispersions with excipients (hydroxypropyl-β-cyclodextrin, Pluronic® F-127, and Soluplus®), although in their cases, the values of apparent constants permeability were decreased.

Thermal and Mechanical Stability of Immobilized Candida antarctica Lipase B: an Approximation to Mechanochemical Energetics in Enzyme Catalysis.

AbstractVery recently, several successful enzymatic processes performed with mechanical activation have been disclosed; that is, despite the mechanical stress caused by High‐Speed Ball‐Milling, immobilized enzymes can retain activity. In the present study, the effect of thermal and mechanical stress was examined as potential inducers of enzymatic denaturation, when using either free, immobilized, or ground immobilized enzyme. The recorded observations show a remarkable stability of ground immobilized enzyme. Moreover, ground biocatalyst turns out to exhibit an increase of one order of magnitude in the efficiency of the catalytic process, maintaining excellent enantiodiscrimination, without significant activity loss even after four milling cycles. These observations rule out enzyme inactivation as direct consequence of the milling process. Additionally, boosted enzyme efficiency was used to optimize a relatively inefficient chiral amine resolution reaction, achieving a 25 % faster biotransformation (in 45 min) and yielding essentially enantiopure products (ee>99%, E>500).

Cyclic heat-up and damage-relevant substrate plastification of single- and bilayer coated milling inserts evaluated numerically

Milling processes are characterized by interrupted cutting, resulting in cyclic thermo-mechanical loading conditions affecting the milling tool's service life. In the current paper, a numerical method is built to predict the transient temperature and stress fields inside coated milling inserts during a dry milling application. The investigated milling tools are hard coated WC-Co hard metal milling inserts, the cut workpiece material is 42CrMo4. The thermal shielding of the substrate by three different hard coating layers, each with a thickness of 7 μm is quantitatively evaluated numerically. The compared coatings are: (i) a TiAlN single layer, (ii) a TiCN/α-Al2O3 bilayer and (iii) a TiAlN/α-Al2O3 bilayer. The deformation behavior and thermal properties of the hard metal substrate and the hard coatings were considered as a function of temperature by experimentally parameterized material models. A remarkable new feature of the presented model is that the simulated dry milling process includes an unprecedented number of 100 load cycles. The synergetic combination of 2D and 3D finite element models gives insight into the cyclic thermo-mechanical tool load that causes stresses and inelastic strains in the substrate. The applied modeling approach considers that the heat flux between the workpiece and the milling tool is changing as the tool heats up during milling. During successive milling cycles, a decreasing heat flux into the tool is taken into account. A comparison of hard coatings with different inherent thermal properties showed a damage-relevant reduction in substrate plasticization with decreasing thermal conductivity of the coatings.

FE temperature- and residual stress prediction in milling inserts and correlation with experimentally observed damage mechanisms

In milling applications thermal and mechanical loadings are affecting the damage behavior of milling inserts. There are several open questions regarding the influence of loading and tool temperature on inelastic deformations and damage mechanisms. The aim of the current work is to simulate industrial milling processes with the finite element method and generate knowledge about the acting damage mechanisms. The validation of the results is made by two recently reported experimental milling setups. The focus of the simulations is set on investigations of the evolution of tensile residual stresses orientated parallel to the cutting edge of a milling insert. These tensile residual stresses foster the formation and growth of so-called comb cracks growing in planes perpendicular to the cutting edge which are detrimental to the performance of the tool. The insert is made of WC-Co hard metal with 8 wt.% Co-binder and an average WC grain size of 1 μm. It is coated with a 7 μm thick TiAlN layer acting as a thermal shield. The workpiece material is 42CrMo4, described by a Johnson-Cook constitutive material model. The milling process is modeled with a 2D Arbitrary Lagrangian-Eulerian (ALE) approach. Results of the 2D simulations are used to generate the temperature and contact load imposed on a 3D solid-model of the milling insert that is in turn used to predict the evolution of stress and temperature over 50 milling cycles. The simulations reproduce a shift of residual stresses toward tension in the milling insert at the same location as observed in experiments from which one failed due to comb cracks and one due to wear.

Compositional and Structural Evolution during Ball Milling of Ti-based Metallic Glass Powder

Ti 42 Zr 40 Ta 3 Si 7.5 Sn 7.5 metallic glass powder prepared for subsequent consolidation by hot pressing were produced by high energy ball milling from amorphous ribbon and both of the compositional and structural evolution on milled powders were studied using XRD and DSC analyses. By using WC vial and balls, the abrasion of milling media occurred causing WC contamination on < 105 µm milled powder after 2 cycles of milling. Based on DSC analysis, the contamination clearly reduced theΔT x of the alloy up to 48%. On the other hand, the contamination were overcome after milling using SS vial and balls, proved by no crystal peak observed on XRD pattern of all milling cycle levels. However, the thermal stability was noticed to be decreased implying the presence of nanocrystals on the amorphous powder after ball milling and the nanocrystal amount tend to be higher when more milling cycles were applied.

Data mining in resistance spot welding

Resistance spot welding is the dominant process in the present mass production of steel constructions without sealing requirements with single sheet thicknesses up to 3 mm. Two of the main applications of resistance spot welding are the automobile and the railway vehicle manufacturing industry. The majority of these connections has safety-related character and therefore they must not fall below a certain weld diameter. Since resistance spot welding has been established, this weld diameter has been usually used as the gold standard. Despite intensive efforts, there has not been found yet a reliable method to detect this connection quality non-destructively. Considerable amounts of money and steel sheets are wasted on making sure that the process does not result in faulty joints. The indication of the weld diameter by in-process monitoring in a reliable way would allow the quality documentation of joints during the welding process and additionally lead through demand-actuated milling cycles to a substantial decrease of electrode consumption. An annual, estimated reduction in the seven- to nine-figure range could be achieved. It has an important impact, because the economics of the process is essentially characterized by the electrode caps (Klages 24). We propose a simple and straightforward approach using data mining techniques to accurately predict the weld diameter from recorded data during the welding process. In this paper, we describe the methods used during data preprocessing and segmentation, feature extraction and selection, and model creation and validation. We achieve promising results during an analysis of more than 3000 classified welds using a model tree as a predictor with a success rate of 93 %. In the future, we hope to validate our model with unseen welding data and implement it in a real world application.

Nitriding of Fe–18Cr–11Mn powders using mechanical alloying method through aerating nitrogen circularly

Mechanical alloying (MA) method was applied to nitride Fe–18Cr–11Mn stainless steel powders through aerating nitrogen circularly. Both the MA process and increasing nitrogen pressure circularly caused the expansion of nitrogen solubility in Fe. The microstructure of powders was affected by the milling time. The phase transformation of α-Fe to γ-Fe occurred during the MA process. The grain size of powder decreased, while the internal lattice strain increased by increasing the milling time and cycles of aerating nitrogen. The as-milled powder could obtain a fully austenitic structure after sintering and subsequent water quenching. The sintered samples had better density and higher microhardness when the powders milled for more time. The formation mechanism of nitriding of stainless steel powders using MA method in nitrogen was presented.

Structure and performance of Poly(vinyl alcohol)/wood powder composite prepared by thermal processing and solid state shear milling technology

Abstract In this study, the Poly(vinyl alcohol)(PVA)/wood powder composite was prepared through combining the novel technologies of solid-state shear milling (S 3 M) and thermal processing of PVA, which provided a large-scale and non-solvent way to efficiently utilize the abundant biomass materials. The results showed that S 3 M can significantly promote pulverization, dispersion, mechanical activation, and interfacial compatibility of the components, endowing the materials with better processability and mechanical performances. The XRD and FTIR results showed that with the increasing of milling cycles, the hydrogen bonds of PVA molecules were partially destroyed, leading to the formation of looser crystalline structure and lower crystalline degree, while the hydrogen bonding between PVA and wood powder was strengthened. Combining with thermal processing technology of PVA, the composite treated by S 3 M had a good processability and mechanical properties. DSC curves revealed that the melting temperature of PVA/wood powder (70 wt/30 wt) composite decreased to 123 °C, providing a quite wide thermal processing window for composite. High-pressure Capillary rheology analysis showed that the shear viscosity of the composite decreased with the increasing milling cycles. Finally, the composites exhibited significant improvements in mechanical properties. The tensile strength increased from 16.6 MPa to 22.5 MPa, and the elongation at break increased from 32.0% to 120.5%, respectively.

Rapid extraction of machined surface data through inverse geometrical solution of tool path information

In the last decades, several process models have been developed for simulation of 5-axis milling cycles, where the simulation results are used for parameter selection or process improvement purposes. However, integrating the process models with milling cycles is not a trivial task especially for tool path modification purposes in 5-axis free-form milling. This is mainly due to the fact that tool path modification requires the machined surface information, i.e. surface location and surface normal vector, to be known. However, this information is not explicitly given in the tool path, i.e. cutter location source (CLS), file. In this paper, a novel and practical approach is proposed to analytically calculate the surface location and surface normal vectors directly from the already generated tool path in the form of CLS file. The proposed approach is applied on representative 5-axis milling cycles, and the results are verified through CAD model comparisons. It is shown that the proposed approach can calculate the machined surface data at a reasonable accuracy depending on the cutter location point density in the tool path file.

The Influence of High Energy Ball Milling on the Morphology of Metal-Ceramic Composite Powders

Investigations of high energy milling parameter of aluminum or siluminum based materials with ceramic particles has been conducted. Work consists of two parts. In the first part two types of composite powders with the same metal-ceramic ratio were synthesized via planetary ball milling with the same milling parameters. After SEM studies it has been shown, that despite of materials similarities significantly different milling times have been required for manufacturing fine grained powders. Planetary ball milling of the chosen aluminum matrix composite performed in two different milling cycles was the second part of the experiment. Two different times of the break between the consecutive millings were applied. It has been found that increase of the time of the break led to the finer powder after milling.

Effect of milling temperatures on surface area, surface energy and cohesion of pharmaceutical powders

Particle bulk and surface properties are influenced by the powder processing routes. This study demonstrates the effect of milling temperatures on the particle surface properties, particularly surface energy and surface area, and ultimately on powder cohesion. An active pharmaceutical ingredient (API) of industrial relevance (brivanib alaninate, BA) was used to demonstrate the effect of two different, but most commonly used milling temperatures (cryogenic vs. ambient). The surface energy of powders milled at both cryogenic and room temperatures increased with increasing milling cycles. The increase in surface energy could be related to the generation of surface amorphous regions. Cohesion for both cryogenic and room temperature milled powders was measured and found to increase with increasing milling cycles. For cryogenic milling, BA had a surface area ∼5× higher than the one obtained at room temperature. This was due to the brittle nature of this compound at cryogenic temperature. By decoupling average contributions of surface area and surface energy on cohesion by salinization post-milling, the average contribution of surface energy on cohesion for powders milled at room temperature was 83% and 55% at cryogenic temperature.

Non- and minimally invasive full-mouth rehabilitation of patients with loss of vertical dimension of occlusion using CAD/CAM: an innovative concept demonstrated with a case report.

Abrasion and erosion are two increasingly common indications for dental treatment. Thanks to modern digital technologies and new restorative materials, there are novel therapeutic approaches to restoring such losses of tooth structure in a virtually non-invasive manner. The case study in this article demonstrates one such innovative approach. The patient's severely abraded natural dentition was restored in a defect-driven, minimally invasive manner using high-performance composite materials in the posterior region, and the "sandwich technique" in the anterior region. The restorations were milled on an optimized milling machine with milling cycles adapted for the fabrication of precision-fit restorations with thin edges.

Integrated Simulation System for 5-axis Milling Cycles

Abstract Multi axis milling is a promising technology for machining of complex geometries and surfaces. Thanks to the additional rotary axis, it provides flexibility to overcome the accessibility issues. However, the process becomes more complicated in terms of geometry, mechanics and dynamics due to the additional degrees of freedom and ball nose end mills. In this study, mechanics and stability of 5-axis ball end milling operations is simulated throughout a given toolpath. The cutting tool-workpiece engagement boundary is calculated and workpiece is represented using distance field approach. Distance field approach features very low computational time, close to real-time simulation. The cutting forces are modeled using orthogonal-to-oblique transformation. Stability limits are estimated in frequency domain.

Thermo-mechanical coupled analysis of laser-assisted mechanical micromilling of difficult-to-machine metal alloys used for bio-implant

This study is focused on a numerical modeling analysis of laser-assisted mechanical micromilling (LAMM) for difficult-to-machine biomedical implant alloys, such as Ti6Al4V and stainless steels. Multiple LAMM tests are performed on these materials with 100 μm diameter endmills at various laser powers. A 3D thermal model is used to quantitatively analyze the material temperature increase due to laser heating during the LAMM process. Finite element (FE) models are developed using ABAQUS to simulate the continuous chip formation, and strain gradient constitutive material models are implemented to model the size effect. The quasi steady-state workpiece temperature after multiple milling cycles is analyzed with a heat transfer analysis based on the chip formation analysis and thermal model simulations. The modeling results in temperature, force and cutting stress are discussed and compared with the experimental results.

Thermal and mechanical modeling analysis of laser-assisted micro-milling of difficult-to-machine alloys

This study is focused on numerical modeling analysis of laser-assisted micro-milling (LAMM) of difficult-to-machine alloys, such as Ti6Al4V, Inconel 718, and stainless steel AISI 422. Multiple LAMM tests are performed on these materials in side cutting of bulk and fin workpiece configurations with 100–300 μm diameter micro endmills. A 3D transient finite volume prismatic thermal model is used to quantitatively analyse the material temperature increase in the machined chamfer due to laser-assist during the LAMM process. Novel 2D finite element (FE) models are developed in ABAQUS to simulate the continuous chip formation with varying chip thickness with the strain gradient constitutive material models developed for the size effect in micro-milling. The steady-state workpiece and tool cutting temperatures after multiple milling cycles are analysed with a heat transfer model based on the chip formation analysis and the prismatic thermal model predictions. An empirical tool wear model is implemented in the finite element analysis to predict tool wear in the LAMM side cutting process. The FE model results are discussed in chip formation, flow stresses, temperatures and velocity fields to great details, which relate to the surface integrity analysis and built-up edge (BUE) formation in micro-milling.