Polymer Workpiece Research Articles

Precision electroforming of nickel nanocomposite mould for defects-free demoulding in polymer micro replication: Surface properties, performance validation and mould release mechanism

Microinjection moulding and micro/nano hot embossing are two promising manufacturing processes for the mass production of polymer workpieces with micro/nano structures. However, adhesion and friction between polymer workpieces and mould tools can cause structural distortion and surface damage. Besides, the nickel mould's lifetime is limited by its low hardness and wear resistance. To address these issues, we proposed a new methodology to codeposit micro/nanomaterials into nickel mould for microhardness enhancement and effective mould release. In this study, polytetrafluoroethylene (PTFE) microparticles, tungsten disulfide (WS2) and molybdenum disulfide (MoS2) nanosheets were incorporated into nickel mould by electroforming. Cobalt was added as a secondary phase to improve the microhardness and wear resistance of the composite mould. The result indicated that WS2 nanosheets achieved the most significant performance enhancement among three micro/nanomaterials, represented as the microhardness improved from 191 HV for pure nickel to 564 HV for nickel/WS2 nanocomposites, along with the coefficient of friction (COF) against the polymethyl methacrylate (PMMA) pin reducing from 0.72 to 0.10 in the initial stage of the sliding. With the incorporation of micro/nanomaterials, the wettability altered from hydrophilic for pure nickel to hydrophobic for the composite moulds. The study shows that the addition of cobalt improved the microhardness of the composite moulds but meanwhile induced more adhesive wear during the pin-on-disc test. Finally, a nickel/WS2 nanocomposite mould is successfully fabricated via electroforming with feature sizes from 70 to 180 μm and its lubricating properties are validated by micro hot embossing PMMA microfluidic chips with good surface quality and structural integrity.

Method for predicting tool condition using kriging analysis of normalised frequency shifts of spindle acceleration signature and validation in trimming operations of CFRPs

ABSTRACT This work presents a method that deals with the problem of determining tool wear, surface roughness, and cutter temperate in the machining of a carbon fibre-reinforced polymer (CFRP) workpiece without the need for removing the tool for inspection. The spindle acceleration signature is dominated by the cutter passing frequency and its first harmonic. Cutting parameters and tool conditions strongly correlate with shifts in the latter two frequencies. The kriging method is utilised to construct prediction models of tool conditions using cutting parameters and spectral shifts. The presented Kriging-based method is unique in its ability to generate acceptable predictions of tool conditions using a small set of experimental data to construct the prediction models. Validation experiments have shown that the proposed method predicted tool wear, surface roughness, and cutter temperature with accuracies of 91%, 86%, and 84%, respectively. Benchmarking of the proposed method against another prediction method has shown acceptable performance.

A method of designing equipment for heat processing of polymer workpieces

The paper describes a method of designing heating equipment that maintains a predetermined temperature field. The method consists in sequential solution of two problems. At the first stage, the heat generation field was calculated using the stationary heat conduction equation. At the second stage, parametric optimization of the temperature field was performed with reference to the power and configuration limits of the heaters. To test this method, the problem of maintaining a predetermined non-uniform temperature field was solved. A practical example of the application of the method for designing a uniform heating plate used in vulcanizing presses was given. To assess the efficiency of the plate, the results of modeling the heat processing of a workpiece from a rubber mixture were presented.

PVDF Sensor Foils Employed to Measure Shear Stress and Temperature of Friction Welding

Friction welding is a popular process for joining metal and polymer work pieces by rubbing them against each other. This way, friction heat is generated in a zone of the faying surfaces, thinner than 1 mm. After cooling down, the heated surfaces establish a tight and strong bond. To improve this joining process, a method is desirable allowing measuring generated temperature and shear stress in the zone between the joining work pieces. Unfortunately, this is a very difficult task because the welding zone cannot be accessed with a sensor without significantly altering the process and thereby the desired measurement results. This paper describes how shear stress and temperature change generated by rubbing polymer pieces in a friction welding machine have been measured between the faying surfaces by employing sensor foils from the piezoelectric and pyroelectric polymer polyvinylidene fluoride (PVDF). This way, heating and cooling rates, pressure rise of the pneumatic system, frequency rise of the starting machine, the duration of starting and stopping, the damping of the vibrations after the drive was stopped, and the stress generated by the pullback of the machine head have been measured. A careful characterization of the sensor was necessary to enable distinguishing the measured voltage due to straining, shearing and temperature change.

Mathematical modeling of electrothermal processes using the example of high-frequency welding of a batch of symmetric polymer workpieces

The article substantiates the necessity and practical relevance of the mathematical modeling of processes of the high-frequency electrothermy of polymers. A mathematical model of high-frequency welding of polymeric and composite products has been developed. A mathematical model of high-frequency welding of a batch of polymeric and composite products has been developed, which is a system of differential equations of unsteady-state heat conduction with internal heat sources, which allows studying the heat distribution in the entire volume of the processed product of any geometric shape. The paper presents the testing results for the developed mathematical model at high-frequency welding of a batch of polymer parts. This made it possible to identify the effect of a non-insulated electrode heating up during sequential processing of a batch of parts on the position of the coordinate of the maximum heating point and to develop a technique for its search and maintenance of the required position. This, in turn, is necessary for organizing control of the high-frequency processing operation.

Evaluation of the standard measurement uncertainty due to the ISO50 surface determination method for dimensional computed tomography

Surface determination is the process by which a CT volume is converted from an image-based representation of an object to a surface-based representation. The process of surface determination relies on well-established image processing algorithms, but these algorithms are not exact, surface determination therefore contributes a component of uncertainty to X-ray CT based dimensional measurements. The purpose of this work is to develop a method for quantifying the standard uncertainty due to surface determination such that this standard uncertainty can be combined with other standard uncertainties in order to calculate the combined standard uncertainty for X-ray CT based dimensional measurements. The proposed method relies on evaluating the mode and the 68% dispersion of the air and material grey values of a given CT data-set. These values are propagated through the ISO50 threshold calculation to give the standard uncertainty of the ISO50 threshold value from which the standard uncertainty due to the ISO50 surface determination method can be evaluated. The proposed method is verified numerically and then demonstrated experimentally for dimensional measurements of an aluminium workpiece, a polymer workpiece, and a steel additively manufactured workpiece. The results show that the measurement uncertainty due to the ISO50 surface determination method is largest for dimensions that have both internal and external surfaces such as wall thicknesses, followed by dimensions such as internal and external diameters, whilst dimensions that are not sensitive to surface determination, for example centre-to-centre distances have the smallest uncertainty due to the ISO50 surface determination method.

Efficient and Precise Micro-Injection Molding of Micro-Structured Polymer Parts Using Micro-Machined Mold Core by WEDM.

In this paper, micro-structured polymer parts were efficiently and accurately fabricated by micro-injection molding using a micro-structured mold core machined by wire electrical discharge machining (WEDM). The objective was to realize low-cost mass production and manufacturing of micro-structured polymer products. The regular micro-structured mold core was manufactured by precise WEDM. The micro-structured polymer workpieces were rapidly fabricated by micro-injection molding and the effects of the micro-injection molding process parameters on replication rate and surface roughness of micro-structured polymers were systematically investigated and analyzed. It is shown that the micro-structured polymer can be rapidly and precisely fabricated by the proposed method. The experimental results show the minimum size machining error of the micro-structured mold core and the maximum replication rate of micro-formed polymer were 0.394% and 99.12%, respectively. Meanwhile, the optimal micro-injection molding parameters, namely, jet temperature, melt temperature, injection velocity, holding pressure and holding time were 195 °C, 210 °C, 40 mm/min, 7 Mpa and 5 s, respectively. The surface roughness Ra at the groove bottom and top of the micro-structured polymer workpieces achieved minimum values of 0.805 µm and 0.972 µm, respectively.

Process Optimization for Compression Molding of Carbon Fiber-Reinforced Thermosetting Polymer.

To enhance the quality and mechanical performance of a carbon fiber–reinforced polymer (CFRP) workpiece, this paper prepares a polyacrylonitrile (PAN)-based carbon fiber–reinforced thermosetting polymer (CFRTP) laminated board through compression molding, and carries out orthogonal tests and single-factor tests to disclose the effects of different process parameters (i.e., compression temperature, compression pressure, pressure-holding time, and cooling rate) on the mechanical performance of the CFRTP workpieces. Moreover, the process parameters of compression molding were optimized based on the test results. The research results show that: The process parameters of compression molding can be ranked as compression temperature, pressure-holding time, compression pressure, cooling rate, and mold-opening temperature, in descending order of the impact on the mechanical property of the CFRTP; the optimal process parameters for compression molding include a compression temperature of 150 °C, a pressure-holding time of 20 min, a compression pressure of 50 T, a cooling rate of 3.5 °C/min, and a mold-opening temperature of 80 °C. Under this parameter combination, the tensile strength, bending strength, and the interlaminar shear strength (ILSS) of the samples were, respectively, 785.28, 680.36, and 66.15 MPa.

ALTERNATIVE STUDY OF A BEVEL PUNCH-ASSISTED ECAE SCHEME

It is a common practice in pressure forming to make an Equal Channel Angular Extrusion (ECAE) of a workpiece through a die with channel intersection angle 2θ = 90° using a standard punch of brick or cylindrical shape with 2θ0 = 90°. However Nejadseyfi et al (2015) have applied a beveled 2θ0-punch to the process of ECAE through a standard angular die of Segal geometry with 2θ = 90° and 2θ0 ≠ 2θ. The scope of the article is focused on an alternative numerical study of Nejadseyfi-ECAE-Scheme using techniques of Computational Fluid Dynamics (CFD). A finite-difference method was applied to the numerical solution of the boundary value problem for the Navier-Stokes equations in the form of a vorticity transfer equation. The complex of 2D plots for CFD-derived fields of flow lines and flow velocities and 3D plots for spatial distributions of flow velocities and tangential stresses were firstly derived for Nejadseyfi-ECAE-Scheme during viscous flow of polymer workpiece models through angular die with 2θ = 90° for the different punch inclination angles 30° ≤ 2θ0 ≤ 150°. It was found that Nejadseyfi-ECAE-Scheme provides enhancement of the rotary modes of intensive deformations during ECAE. Results provide visualization of velocity gradients and macroscopic rotation and the illustration of Nejadseyfi et al’s ideas from an alternative CFD-based viewpoint.

Study on non-isothermal hot-embossing of polymer micro-prism array with efficiency and accuracy

The isothermal hot-embossing leads to inefficiency because of heating and cooling whole polymer workpiece for precision micro-forming. Hence, a non-isothermal hot-embossing is proposed by compressing and melting a micron-scale surface layer into heated core microgrooves without changing the solid-state workpiece. The objective is to understand how the upper layer micro-prism array is formed on large-size polymer surface with regard to core microgroove parameters and hot-embossing variables. First, the micro-prism shape and sizes were in-process modeled through a hybrid of mechanical compression and hot-melt flowing inside core microgrooves; then the micro-grinding with the trued wheel micro-tip was employed to fabricate various microgrooves on macro-size die cores; finally, the micro-forming accuracy and efficiency were experimentally investigated in hot-embossing. It is shown that the integrated micro-prism array with nanometer-scale surface roughness is rapidly generated inside accurate and smooth core microgrooves with 1–3 seconds. The micro-forming accuracy is dominated by the hot-embossing variables under the limitations of core microgroove parameters. Moreover, the hot-embossed surface roughness reaches 10–30 nm through the hot-melt layer flowing even if the core surface roughness is larger. It is confirmed that the rapid micro-forming sizes and surface roughness may be predicted by the micro-nano core topographies and the hot-embossing variables.

Laser welding of vertically aligned carbon nanotube arrays on polymer workpieces

Here we demonstrate laser transmission welding of vertically aligned carbon nanotube (VACNT) arrays for joining polymer sheets. The unique characteristics of VACNTs make them suitable for use in laser welding. First, the excellent light absorption of the VACNTs induces selective heating at the contact plane with a polymer sheet, minimizing thermal damage to the polymer. Second, the porous and compliant structure of the VACNTs prevents the formation of air pockets inside the contact space. Successful welding is obtained when the laser irradiation power is at an optimal level, below which the adhesion is too weak and above which the excessive heat causes periodic damage along the scanning path. The optimized laser welding technique is expected to become a new method for implementing carbon nanotubes as mechanical linkers for various thermoplastic polymers.

Effects of Material Rheology and Die Walls Translational Motions on the Dynamics of Viscous Flow during Equal Channel Angular Extrusion through a Segal 2θ-Die: CFD 2D Solution of a Curl Transfer Equation

The present article is focused on a phenomenological description of a polymer workpiece Equal Channel Angular Extrusion (ECAE) through 2θ-dies of Segal and Iwahashi geometries with a channel intersection angle 2θ = 105° with fixed and movable external inlet and outlet die walls. The local flow dynamics, including the formation of macroscopic rotation and a dead zone appearance during the flow of plasticine, paraffin, and wax workpiece models through the subject die configuration was studied using physical simulation techniques. The present article utilizes a Computational Fluid Dynamics (CFD) numerical approach to a theoretical description of 2D viscous flow of incompressible Newtonian continuum through the stated die geometries. The boundary value problem for the Navier-Stokes equations in the curl transfer form for the local viscous flow was formulated and numerically solved with a finite-difference method. Theoretical CFD-derived plots with computational flow lines, dimensionless flow and curl functions, flow velocities, and tangential stresses for viscous material flow through the stated die geometries have been generated and described. As a first rheological approximation the derived computational results provide the theoretical description of physical simulation experiments and visualize the formation of ECAE-induced rotational modes of large deformations like macroscopic rotation and rotational inhomogeneity.

Effect of workpiece viscosity on strain unevenness during equal channel angular extrusion: CFD 2D solution of Navier–Stokes equations for the physical variables ‘flow velocities—punching pressure’

The present article addresses strain unevenness effects during equal channel angular extrusion (ECAE) of physical models of polymer workpieces with viscosity flow features through a Segal die with channel intersection angle of 2θ = 90°. Computational viscous flow lines, flow velocity fields, and material dead zone formation in the physical simulation of ECAE have been numerically derived for planar flow of viscous incompressible continua in an angular die with 2θ = 90°. This is accomplished through the introduction of Navier–Stokes equations with the following dimensionless physical variables: polymer model local flow velocities u, v and punching pressure p. Derived experimental results are grounded on the application of the following physical simulation techniques: marker method, based on harder disperse particles partially forcing into the front faces of soft workpieces; layered model production by assembling the workpiece soft model with different layers, and circular gridlines use with viscous flow of the polymer soft model. Good agreement has been found between the computational and observable physical simulation results. Based on the obtained results, recommendations are made for polymer ECAE technology enhancement and angular die design for polymer workpiece pressure forming.

Study on Vibration Transmission and Interfacial Fusion in Ultrasonic Bonding Process for Thermoplastic Micro Joint

AbstractFusion precision is an important factor to be considered for ultrasonic bonding in thermoplastic MEMS. Vibration transmission in ultrasonic bonding process through polymer workpiece was related to the interfacial fusion, which could be used for in situ monitoring. In this article, experimental platform was established to study the vibration transmission. Experiments were carried out to measure the vibration transmitted from horn onto the high‐frequency dynamic force sensor installed in the anvil. According to the peak‐to‐peak value of dynamic force, the whole bonding process could be divided into rising and falling phases. Interfacial fusion corresponding to ultrasonic transmission was also studied. From experimental results, when the peak‐to‐peak value change rate rose to 130%, some area on the surface starts to melt. When it fell down back to 60%, glass transition occurred with the melt spread gradually over the whole interface. When it fell down to 30%, defects of overflow and bubble began to generate as polymer got into viscous flow state. When the change rate reached 10%, the bubbles burst into smaller ones like floccules as the fluidity of polymer was enhanced further under continuous ultrasonic load. The experimental results gave a deep understanding in the formation of interfacial fusion and the generation of defects.

Optimization for solid polymer microstructure replication using gas-assisted hot embossing under low pressure

In order to realize the replication of high-quality polymer microstructures with vertical and smooth sidewalls, a gas-assisted hot embossing process with low pressure supplied was optimized to eliminate the swallowtail phenomenon during pattern transfer and simplify workpiece process setting. With help of passive alignment clamp, the rate of replication greater than 95.5% for vertical sidewalls was successfully obtained under the optimum process condition. Accordingly, a root mean square sidewall roughness of 4.6 nm was measured for polymer trenches in comparison with that of 5.7 nm for the silicon mold using a bevel-cut preparation technique. Furthermore, the rate of bulging of less than 15% for polymer workpiece was also obtained. The experiment has demonstrated that high-precise polymer pattern replication is related to both process settings and mold morphology sizes.

Machining polymer workpieces on a lathe after preliminary surface deformation

Preliminary surface deformation (rolling) is proposed in the turning of polymers. The dependence of the surface roughness on the rolling and cutting conditions is derived.

CFD 2D Description of Local Flow of Polymer Workpiece through a modified U-Shaped Die During Equal Channel Multiple Angular Extrusion

The present article is focused on a 2D computational fluid mechanics study of local viscous flow dynamics and the formation character of rotary modes of deformation during Equal Channel Multiple Angular Extrusion (ECMAE) of a polymer workpiece fluid model through a U-shaped die with parallel slants in channel intersection zones. The present local flow problem was experimentally analyzed using physical simulation methods and theoretically studied with numerical fluid mechanics techniques. The computational approach has been grounded on the numerical finite difference solution of the boundary value problem for the Navier-Stokes equations in the curl transfer form for the local viscous flow of incompressible Newtonian fluid through a U-shaped rectangular die with parallel slants. The derived research results allow us to draw a conclusion that the implementation of a geometric design of parallel slants within a 2-turn U-shaped die results in localization of the maximum tangential stresses within the workpiece volume to the vicinity of these parallel slants during ECMAE.

CFRP voids 3D identification and location method based on the process of backscattered signal

The porosity of carbon fiber reinforced polymer (CFRP) workpiece is tested by ultrasonic in pulse-echo mode. When the ultrasonic frequency is close to the resonant frequency of the workpiece, the resonance will occur along the thickness direction. If the CFRP workpiece contains voids, the resonant frequency will decrease. The result of ultrasonic testing experiment clearly draws the conclusion that the center frequency of the backscattered signal spectrum declines with increasing porosity. Based on the above theory and conclusion, the three-dimensional (3D) voids identification and location method is established. Firstly, the ultrasonic signals are collected and the center frequencies of the backscattered signal spectra are calculated. Then the C-scan of center frequency is generated to identify the voids. At last the B-scan of center frequency for the region containing voids is generated to determine the depth of the voids. The experimental results show that, by using this method, the voids in the CFRP workpiece can be identified and pinpointed.

Investigating the Surface Tribology of Roller-Burnished Polymer Using the Fuzzy Rule–Based Approach

Burnishing is specified as the plastic deformation cold working process applied as surface treatment and smoothing following machining to obtain a superior surface roughness finish. The present experimental study was carried out on roller-burnished polyurethane using a roller burnishing tool. An analysis was done to investigate the effect of burnishing depth, speed, feed rate, and roller width on the surface roughness of the polymer workpiece. Roughness (Ra) prediction was achieved with a fuzzy rule–based system. The results indicate 95% accuracy between the fuzzy predicted roughness values and experimental results.

Development of polymer tribocomposites on the basis of polytetrafluoroethylene with an elevated wear resistance

The results of investigations on the development of wear-resistant polymer composite materials on the basis of polytetrafluoroethylene, created with the use of the process of spontaneous impregnation of porous polymer workpieces with motor oils, are presented.