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ABSTRACT: Injection molding with microstructures was investigated both experimentally and theoretically. A series of injection molding experiments with PP and PMMA was carried out in a long and a short rectangular mold containing microchannels with the thickness of either 50 or 100 μm and an aspect ratio of 5. The filling lengths in the microchannels were affected by injection speed, mold temperature, and channel location. A high injection speed or high mold temperature resulted in a longer filling length. The filling length in the microchannels decreased as the filling time in the main flow region increased. All filling lengths can be merged into a single curve vs. Fourier number based on the microchannel thickness. Comparison was also made between the experimental measurements and numerical simulation. The mold/melt heat transfer coefficient was found to be a critical factor in determining the filling lengths. The local heat transfer coefficient provided a much better agreement than a constant heat transfer coefficient. POLYM. ENG. SCI., 45:866–875, 2005. © 2005 Society of Plastics Engineers
Polymer Engineering and Science 04/2005; 45(6):866 - 875. · 1.30 Impact Factor
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ABSTRACT: Injection molding has been used for mass production of polymer products with microstructures. Conventional Hele-Shaw 2.5D midplane simulation is unable to describe the flow pattern correctly. It tends to over-predict the effects of microstructures on global flow patterns. For the unidirectional flow, an x-z planar based on the general momentum equation is able to achieve better accuracy and to retrieve more detailed flow and heat transfer information around the microstructures. A hybrid numerical technique is developed, which can significantly reduce the nodes and computation time, and yet provide good flow simulation around the microstructures. The mold-melt heat transfer coefficient and injection speed are shown to be very important factors in determining the filling depth in microstructures. A decrease of the heat transfer coefficient and the occurrence of wall-slip are likely in microchannels. Polym. Eng. Sci. 44:1866–1876, 2004. © 2004 Society of Plastics Engineers.
Polymer Engineering and Science 10/2004; 44(10):1866 - 1876. · 1.30 Impact Factor
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ABSTRACT: In many polymer manufacturing operations, the material is processed near the glass transition temperature (Tg). Examples are thermoforming, blow molding, film blowing, hot embossing, forging, plastic welding, and de-airing in safety glass lamination. In these processes, solid-like behaviors such as strain hardening and yielding play important roles. These material properties cause the material to flow (or deform) in a way that substantially differs from a polymer melt. In order to understand the flow behavior near the Tg, polyvinyl butyral (PVB), a rubbery polymer used in safety glass lamination, was studied in this work. The flow properties of the polymer above the Tg were characterized by using both shear and elongational rheometers, and a tensile tester. The measured flow properties were described by a viscoelastic constitutive model.
Polymer Engineering and Science 04/2004; 41(2):275 - 292. · 1.30 Impact Factor
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ABSTRACT: Injection molding of thin plates of micro sized features was studied in order to manufacture micro-fluidic devices for bioMEMS applications. Various types of mold inserts—CNC-machined steel, epoxy photoresist, and photolithography and electroplating produced nickel molds—were fabricated and tested in injection molding. The feature size covers a range of 5 microns to several hundred microns. Issues such as surface roughness and sidewall draft angle of the mold insert were considered. Two optically clear thermoplastics, PMMA and optical quality polycarbonate, were processed at different mold and melt temperatures, injection speeds, shot sizes, and holding pressures. It was found that the injection speed and mold temperature in injection molding greatly affect the replication accuracy of microstructures on the metal mold inserts. The UV-LIGA produced nickel mold with positive draft angles enabled successful demolding. Numerical simulation based on the 2D software C-MOLD was performed on two types of cavity fillings: the radial flow and the undirectional flow. The simulation and experimental data were compared, showing correct qualitative predictions but discrepancies in the flow front profile and filled depth.
Polymer Engineering and Science 04/2004; 42(5):871 - 888. · 1.30 Impact Factor
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ABSTRACT: The dynamic shear viscosity and the transient extensional viscosity of polycarbonate (PC), polymethyl methacrylate (PMMA), and polyvinyl butyral (PVB) were measured at temperatures near and far above their glass transition temperatures. The temperature sensitivity of rheological properties was used to explain the displacement curves during embossing. Numerical simulation of the embossing process was also carried out to compare with the observed polymer flow patterns. It was found that the simulated flow pattern during isothermal embossing agrees fairly well with the experimental observation. The deviation between the simulated and experimental results at the late stage of embossing may be due to air entrapment between the mold feature and the polymer substrate. For non-isothermal embossing, the observed flow pattern can also be reasonably simulated, i.e. the polymer flows upward along the wall of the heated mold feature, and then compresses downward and squeezes outward. Temperature sensitivity of the dynamic shear viscosity and the transient extensional viscosity is similar for all three polymers. This correlates well with the initial displacement curves in isothermal embossing. Over a longer time, the strain hardening effect of the transient extensional viscosity seems to play a major role in the displacement curves.
Polymer Engineering and Science 04/2004; 42(3):551 - 566. · 1.30 Impact Factor
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ABSTRACT: The relationship among processing conditions, material properties, and part quality in hot embossing was investigated for three optical polymers: polycarbonate (PC), polymethyl methacrylate (PMMA), and polyvinyl butyral (PVB). A series of systematic embossing experiments was conducted using mold inserts having either single or multiple feature depths. The feature dimensions varied from 90 to 3000 μm. The processing conditions studied include embossing pressure, thermal cycles, and heating methods. The displacement profile, replication accuracy and molded-in stresses were measured experimentally. It was found that for isothermal embossing, both replication accuracy and birefringence pattern depend strongly on the processing conditions. For non-isothermal embossing, the molded parts showed excellent replication as long as the feature transfer was completed. The flow pattern under isothermal embossing resembles a biaxial extensional flow. Under non-isothermal embossing, the polymer deformation involves an upward flow along the wall of mold features, followed by downward compression and outward squeezing. Rheological characterization and hot embossing analysis are presented in Part II.
Polymer Engineering and Science 04/2004; 42(3):539 - 550. · 1.30 Impact Factor
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ABSTRACT: In this paper, we review the approaches developed in our laboratory for polymer-based micro/nanofabrication. For fabrication of microscale features, UV-LIGA (UV-lithography, electroplating, and molding) technology was applied for low-cost mass production. For fabrication of sub-micron or nanoscale features, a novel nano-manufacturing protocol is being developed. The protocol applies a novel nano-lithography imprinting process on an ultra-precision motion-control station. It is capable of economically producing well-defined pores or channels at the nanometer scale on thin polymer layers. The formed thin layers can be used as nano-filters for chemical or bio-separation. They can also be integrated into miniaturized devices for cell immunoprotection or tissue growth. For bonding of polymer-based microfluidic platforms, a novel resin-gas injection-assisted technique has been developed that achieves both bonding and surface modification. This new approach can easily seal microfluidic devices with micron and sub-micron sized channels without blocking the flow path. It can also be used to modify the channel shape, size, and surface characteristics (e.g., hydrophilicity, degree of protein adsorption). By applying the masking technique, local modification of the channel surface can be achieved through cascade resin-gas injection.
MRS Proceedings. 12/2001; 729.
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ABSTRACT: Several microfabrication methods for polymer-based CD microfluidic platforms are presented in this paper. For prototyping, both traditional CNC-machining and photolithography techniques were used. For mass production, mold inserts were made by CNC-machining of tool steel and LIGA-like processes such as UV photolithography, photolithography/electroplating, and photolithography/deep reactive ion etching (DRIE). Several molding methods were tried, including liquid resin casting, thin wall injection molding, and hot embossing. Advantages and disadvantages of each method are explained. Plastic bonding for microfluidic platforms is also briefly discussed.
Biomedical Microdevices 11/2001; 3(4):339-351. · 3.03 Impact Factor
