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In this work, both experimental study and numerical simulation were carried out to investigate the replication capability and flow behavior of polymeric melt inside microchannels with high aspect ratio. For this purpose, a mold insert with microchannels having different layouts (parallel and transverse to the main flow direction) and various cross sectional shapes (triangle, rectangle, and semicircle) was designed and fabricated. The filled length and surface morphology of the microfeatures molded under different conditions were characterized using scanning electron microscope and optical microscope. It was found that the cross sectional shape had clear influence on the filled length of the microfeatures, with a dependence on the mold layout. Moreover, serious hesitation of polymeric flows in microchannels took place in the transverse layout, which led to eccentric line defects. However, such defects were not observed in the parallel layout. At last, the specific mechanisms of how the cross sectional shape affected the replication capability and how the line defects in the microfeatures' surface formed were discussed. POLYM. ENG. SCI., 51:959-968, 2011. [C]2011 Society of Plastics Engineers, INTRODUCTION Microinjection molding is one of the key technologies for fabricating micro polymeric parts for its mass-production capability. In recent years, extensive research activities in this area have been reported. [...]

This article aims to investigate the effects of five processing parameters and two geometric factors on replication quality of the microribs, as well as determination of the processing condition when different materials were used. Specifically, the considered parameters include barrel temperature, mold temperature, injection velocity, holding pressure, holding time, aspect ratio, and location of the microribs. The experiment results showed a considerable difference in replicated height of molded microribs with polymethylmethacrylate and polypropylene. Among the processing parameters, the barrel temperature and injection velocity significantly affected replication quality, whereas the holding pressure and holding time appeared to have little influence for both materials. The mold temperature was found to be a sensitive parameter only for polymethylmethacrylate. As for geometric factors, both aspect ratio and location of the microribs played important roles in achievable replication. In addition, simulation results showed good prediction on the trend of how process parameters affect replication, with a small amount of over prediction. POLYM. ENG. SCI., 50:2021-2030, 2010. [C] 2010 Society of Plastics Engineers, INTRODUCTION With rapid development of new manufacturing technologies, there is an increased trend towards product miniaturization and multifunction capability. Miniaturization greatly promotes the development of Micro-Electro-Mechanical Systems (MEMS) in different [...]

In this article, an affordable split-and-recombine polymer static micromixer and its fabrication process are described. The structure of the micromixer was designed to take advantage of the process capabilities of both ultraprecision micromachining and microinjection molding. This arrangement allows a considerably short machining time, high productivity, and flexibility as compared with many of the previous fabrication processes used in manufacturing of micromixers, for example, the cleanroom process. Moreover, in recent years, microinjection molding has become a cost effective mass production method for polymeric components with micro and nano scale features. Specifically, in this study four dominating processing parameters in microinjection molding, including melt temperature, injection velocity, packing pressure, and packing time were selected as variables to investigate their effects on replication quality. Both experimental and numerical results showed that the packing pressure and packing time played an important role on replication, followed by melt temperature while the injection velocity seemed to have no influence on replication. Finally, experiments using two colored water solutions were conducted to evaluate the device performance, and the mixing result demonstrated the successful fabrication and performance of the injection molded polymer micromixer. POLYM. ENG. SCI., 50:1594-1604, 2010. [C]2010 Society of Plastics Engineers, INTRODUCTION The application of MEMS in biology and medical technology has increased rapidly in recent years. Notable applications include microfluidic analysis, drug delivery, and lab-on-chip systems. In all these applications, [...]

The single point diamond turning process has been used extensively for direct optical surface fabrication. However, the diamond machined surfaces have characteristic periodic tool marks, which contribute to reduced optical performance such as scattering and distortion. In this paper, studies of the characteristics of diamond machined surface and scattering from the diamond machined surfaces are presented. Four different parameters, the first order optical diffraction, the zero order reflection, the surface roughness, and the residual tool mark depth, are used as indicators for the machined surface quality. Four sets of tests are presented showing the relationship between machined surface quality and machining conditions such as spindle speed, feedrate, and machining process. Finally, an empirical model is given based on the measurements. [DOI: 10.1115/1.4001037] Keywords: diamond machining, optical effect, surface finish, diffraction

Glass compression molding is an alternative manufacturing method for efficient, high-quality, lowcost optical component manufacturing. However, in compression molding, refractive index variation is inadvertently introduced to glass, which can influence optical performance of molded glass lenses, especially for lenses used in high precision applications. In order to study refractive index variation and dispersion in molded glass lenses after cooling, a group of BK7 cylindrical glass lenses were thermally treated with various heating and cooling conditions. The molded glass lenses were measured by use of an optical setup based on a Mach--Zehnder interferometer with red, green, and blue lasers separately. Using the wavefront information extracted from fringe patterns, refractive index and dispersion variation in molded glass lenses were reconstructed using a filtered backprojection algorithm. Furthermore, refractive index and dispersion variation at different cooling rates and different soaking temperatures were investigated. OCIS codes: 220.0220, 230.0230.

Compression molding is an effective high volume and net-shape fabrication method for aspherical lenses and precision glass optical components in general. Geometrical deviation (or curve change as often referred to in industry) incurred during heating, molding, and cooling processes is a critically important manufacturing quality parameter. In the compression glass molding process, there are many factors that could lead to curve change in final products, such as thermal expansion, stress and structural relaxation, and inhomogeneous temperature distribution inside the molding machine. In this research, an integrated numerical simulation scheme was developed to predict curve change in molded glass aspherical lenses. The geometrical deviation in the final lens shape was analyzed using both an experimental approach and a numerical simulation with a finite element method program. Specifically, numerical simulation was compared with experimental results to validate the proposed manufacturing approach. The measurements showed that the difference between numerical simulation and experimental results was less than 2 [micro]m. Based on the comparison, the mold curve was revised using numerical simulation in order to produce more accurate lens shapes. The glass lenses molded using. the compensated molds showed a much better agreement with the design value than the lenses molded without compensation. It has been demonstrated in this research that numerical simulation can be used to predict the final geometrical shape of compression molded precision glass components. This research provided an opportunity for optical manufacturers to achieve a lower production cost and a shorter cycle time. [DOI: 10.1115/1.3063652] Keywords: glass molding, numerical simulation, geometrical deviation, compensation, optimization

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. Residual stresses incurred during postmolding cooling are an important quality indicator for these components. In this research, residual stresses frozen inside molded glass lenses under different cooling conditions were investigated using both experimental approach and numerical simulation with a commercial finite element method program. In addition, optical birefringence method was also employed to verify the residual stress distribution in molded glass lenses. Specifically, optical retardations caused by the residual stresses in the glass lenses that were molded with different cooling rates were measured using a plane polariscope. The measured residual stresses of the molded glass lenses were compared with numerical simulation as a validation of the modeling approach. Furthermore, a methodology for optimizing annealing process was proposed using the residual stress simulation results. [DOI: 10.1115/1.2950062] Keywords: glass forming, residual stress, cooling rate, annealing, refractive index, birefringence

Compression molding of glass optical components is a high volume near net-shape precision fabrication method. In a compression molding process, a variation of the refractive index occurs along the radial direction of the glass component due to thermal treatment. The variation of refractive index is an important parameter that can affect the performance of optical lenses, especially lenses used for high precision optical systems. Refractive index variations in molded glass lenses under different cooling conditions were investigated using both an experimental approach and a numerical simulation. Specifically, refractive index variations inside molded glass lenses were evaluated by measuring optical wavefront variations with a Shack-Hartmann sensor system. The measured refractive index variations of the molded glass lenses were compared with the numerical simulation as a validation of the modeling approach. OCIS codes: 220.4610, 290.3030.

An embossing strategy involving a hot station and a cold station for sequentially heating and cooling the embossing tool was investigated to reduce cycle times in hot embossing polymer microstructures. Experimental studies showed that aluminum stamps with a thickness of 1.4 mm can be rapidly heated from room temperature to 200°C in 3 s using contact heating against a hot station at 250°C. Microchannels and microlenses were successfully embossed onto high-density polyethylene and acrylonitrile-butadiene-styrene substrates using a heating time less than 3 s and a total cycle time around 10 s. The two-station embossing process for the microlens was also numerically studied. The simulated filling behavior agreed with the experimental observation and the predicted thermal and deformation history of the polymer offered a good explanation on the experimentally observed process characteristics. POLYM. ENG. SCI., 47:530-539, 2007. © 2007 Society of Plastics Engineers, INTRODUCTION The hot embossing process involves several sequential stages. First, a preheated thermoplastic film is placed between two heated mold platens heated above the polymer softening temperature (glass transition temperature [...]

Thermal cycling of molds is frequently desired in thermoplastic processing. Thermal cycling of the entire mold with a large mass, however, requires an exceedingly long cycle time. A processing strategy for mold rapid heating and cooling, involving a thin-shell mold and two thermal stations (one hot and one cold), was investigated. Because of its low thermal mass, the shell mold can be rapidly heated and cooled through heat conduction by selectively contacting with the two stations. Numerical simulations were performed to study the effect of different design parameters, including thermal contact resistance, shell material, and shell thickness, on the thermal response at the mold surface. Experimental studies showed aluminum shell molds with a thickness of 1.4 mm can be rapidly heated from room temperature to 200[degrees]C in about 3 s using a hot station at 250[degrees]C. The method was used for thermal cycling of embossing tools. Surface micro-features can be rapidly transferred from thin metallic stamps to polymer substrates with cycle times less than 10 s. [DOI: 10.1115/1.2335855] Keywords: thermoplastic processing, rapid heating, embossing, welding, contact heating

Because of the limitation of manufacturing capability, free-form glass optics cannot be produced in a large volume using traditional processes such as grinding, lapping, and polishing. Very recently compression molding of glass optics became a viable manufacturing process for the high-volume production of precision glass optical components. An ultraprecision diamond-turning machine retrofitted with a fast tool servo was used to fabricate a free-form optical mold on a nickel-plated surface. A nonuniform rational B-spline trajectory generator was developed to calculate the computer numerical control machine tool path. A specially formulated glass with low transition temperature ([T.sub.g]) was used, since the nickel alloy mold cannot withstand the high temperatures required for regular optical glasses. We describe the details of this process, from optical surface geometry, mold making, molding experiment, to lens measurement. OCIS codes: 160.2750, 240.5770, 220.1920, 220.4610.

The precision lens molding process is numerically modeled by incorporating the characteristic Structural relaxation phenomenon of glass during the annealing stage. A review of the structural relaxation theory as explained by the Narayanaswamy model (Narayanaswamy, 1971, J. Am. Ceram. Soc., 54(10), pp. 491-498) is provided and is then implemented into the simulation model. The commercial finite element method (FEM) program MSC MARC is utilized for the analysis. Glass material is modeled as a linear Newtonian fluid during the molding stage and as a viscoelastic material exhibiting stress and structural relaxation during the annealing stage. A comparison between the final lens shape and volume results obtained by implementing structural relaxation and thermal expansion coefficient is shown. The results demonstrate the need for including structural relaxation in the simulation model for reliable predictions. The results also show that the FEM can be used to predict the process, improve performance, and also provide a deeper process understanding. Keywords: lens, glass forming, structural relaxation, thermal expansion, temperature, numerical modeling, viscosity

In recent years it has become possible to measure and in some instances to correct the high-order aberrations of human eyes. We have investigated the correction of wavefront error of human eyes by using phase plates designed to compensate for that error. The wavefront aberrations of the four eyes of two subjects were experimentally determined, and compensating phase plates were machined with an ultraprecision diamond-turning machine equipped with four independent axes. A slow-tool servo free-form trajectory was developed for the machine tool path. The machined phase-correction plates were measured and compared with the original design values to validate the process. The position of the phase-plate relative to the pupil is discussed. The practical utility of this mode of aberration correction was investigated with visual acuity testing. The results are consistent with the potential benefit of aberration correction but also underscore the critical positioning requirements of this mode of aberration correction. This process is described in detail from optical measurements, through machining process design and development, to final results. OCIS codes: 220.1920, 220.1000.

Formation of high spatial frequency laser induced periodic surface structures (HSFL) in germanium by 90 fs mid-IR pulses at a 1 kHz repetition rate with wavelengths between λ=2 and 3.6 µm was studied with varying angle of incidence and polarization. The period of these structures varied from λ/3 to λ/8. A modified surface-scattering model including Drude excitation and the optical Kerr effect explains the spatial period scaling of HSFL across the mid-IR wavelengths. Transmission electron microscopy shows the presence of a 30 nm amorphous layer above the structure of crystalline germanium. Various mechanisms including two photon absorption and defect-induced amorphization are discussed as probable causes for the formation of this layer. [ABSTRACT FROM AUTHOR]