Hot embossing precise structure onto plastic plates by ultrasonic vibration

Hot embossing achieves excellent performance in replicating precise features onto large plastic plates. It has become a popular approach because microstructures in master molds can now be easily made using Lithographie GaVanoformung Abformung (LIGA) and micro‐electro‐mechanical‐system (MEMS) technologies. However, there are still some unsolved problems that confound the overall success of this technology. Long cycle times caused by conventional electric or hot oil heating is one of them. This research attempted to use ultrasonic vibration as a heat generator for hot embossing. The first part of this study investigated the replication capacity of ultrasonic‐heating embossing of both amorphous and semicrystalline plastic plates; the second part of this study examined the effects of various ultrasonic vibration parameters on the contour of replicated structure; and the third part of this study identified the relative significance of all these parameters on molded part quality. In addition, the temperature profiles at different depths of the embossed plates by ultrasonic vibration were measured. The experimental results in this study suggest that ultrasonic vibrated hot embossing could provide an effective way of molding precise structure onto polymeric plates. This would provide significant advantages in terms of a shorter cycle time as well as improved product quality. POLYM. ENG. SCI., 45:915–925, 2005. © 2005 Society of Plastics Engineers     

Hot embossing of discrete microparts

The hot embossing process has so far been developed mainly for replication of surface structures on thermoplastic substrates. Because of the lack of a through‐thickness action, fabrication of discrete microparts such as microgears is considered difficult. In this study, embossing molds having multiple microcavities were used in a through‐thickness embossing process with a rubber‐assisted ejection mechanism. Microparts made of HDPE and ABS with each part weighing approximately 1 and 1.4 mg, respectively, were produced. When in the mold, embossed microparts were intermittently connected to each other through thin residual films of a thickness approximately 20 μm. The residual films were detached from the microparts during a rubber‐assisted ejection stage. Because no resin delivery paths, e.g., runners and gates, are needed for microcavities on the multicavity embossing mold, this micropart fabrication process could replace micro injection molding in many applications. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Continuous infrared‐assisted double‐sided roll‐to‐roll embossing of flexible polymer substrates

This article reports a novel infrared (IR)‐assisted roll‐to‐roll embossing method, which enables the replication of microfeatures onto the surfaces of flexible polymer substrates. An IR‐assisted roll‐to‐roll embossing facility was designed and built in our laboratory especially for this study. Metallic rollers bearing micropatterns of two different feature sizes, namely 150 and 20 μm in depth, were employed. The former one was prepared by microelectric discharge machining the roller, whereas the latter was fabricated by electroplating a thin layer of nickel on the surface of the roller, followed by a diamond turning process to create the microstructures. The embossing facility was used to replicate the microstructure onto polyethylene terephthalate and polycarbonate films in the experiments. During roller embossing, the IR radiation shed on the rollers, and the energy was converted into heat to melt the polymer substrates and to replicate the microstructures. The influence of various processing parameters on the replicability of microfeatures was investigated. Under the proper processing conditions, double‐sided flexible polymer substrates with microstructures could be successfully fabricated. The proposed method shows great potential for fabrication of micro‐optical components due to its simplicity and versatility. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

A two‐station embossing process for rapid fabrication of surface microstructures on thermoplastic polymers

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.     

Flame‐assisted flash sintering: A noncontact method to flash sinter coatings on conductive substrates

We present a novel and effective method for sintering ceramic coatings onto metallic substrates. This new technique, called Flame‐assisted flash sintering (FAFS), utilizes a flame as both a heating source and a conformal, current‐carrying top electrode to facilitate flash sintering. Using this method, Yttria‐stabilized Zirconia (8 mol% Y, 8YSZ) coatings are sintered onto stainless steel substrates to controlled degrees of porosity in rapid fashion. Flame‐assisted flash sintering utilizes a dynamic soft electrode for flash sintering and has commercial potential to sinter ceramic coatings on complex‐shaped substrates for a variety of applications including tribological or thermal protection coatings.     

Through‐thickness embossing process for fabrication of three‐dimensional thermoplastic parts

The standard embossing process is limited to the fabrication of surface structures on relatively large polymer substrates. To overcome this limitation, a hybrid punching and embossing process was investigated for through‐thickness embossing of three‐dimensional parts. The embossing tool included a punching head and to‐be‐ replicated features in the socket behind the punching head. The built‐in punching head facilitated a through‐thickness action and provided a closed‐die environment for embossing pressure buildup. The method was used to emboss multichannel millimeter waveguides which requires uniform edges and accurate dimensions. With a tool temperature of 140°C, an embossing time of 3 min and a total cycle time of 7 min, discrete 4‐channel waveguides were successfully embossed from a room‐temperature ABS substrate. A computer model was established to study the flow behavior during through‐thickness embossing. It was found that nonisothermal embossing conditions help confine the polymer in the cavity and reduce the outflow into the surrounding region, thus achieving complete fill of the cavity. POLYM. ENG. SCI., 47:2075–2084, 2007. © 2007 Society of Plastics Engineers     

Hot embossing in microfabrication. Part I: Experimental

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.     

Embossing of high‐aspect‐ratio‐microstructures using sacrificial templates and fast surface heating

A new embossing method based on sacrificial templates and fast surface heating was developed, in which the de‐embossing step was avoided to prevent deformation or damage of polymer microstructures. The microstructures of interest have a high feature density, high aspect ratio, and/or undercuts. Soft lithography was used to prepare a water‐soluble mold, using polyvinylpyrrolidone (PVP) in the hot embossing process. Arrays of microchannels and microgrids with an aspect ratio greater than five were replicated on poly(DL ‐lactide‐co‐glycolide) (PLGA). In conjunction with localized surface heating of the polymer surface by a laser/IR system, this technique was able to micromold high temperature polymers such as poly(methyl methacrylate) (PMMA) with high aspect ratios. POLYM. ENG. SCI., 47:830–840, 2007. © 2007 Society of Plastics Engineers     

Rapid fabrication of various molds for replication of polymer lens arrays

This article proposes a rapid fabrication method for the production of various molds for the replication of polymer lens arrays. The method involves ultrasonic vibration embossing with a polymer substrate and small steel ball array. Only one embossing step is required in which a concave lens array pattern is directly fabricated onto the polymer substrate. The total processing cycle time is less than 20 s. The polymer substrate with a concave lens array pattern can then be used as a mold for the replication of a polymer lens array through a polydimethylsiloxane casting process. In addition, the diameter and depth of the concave lens array pattern on the surface of the polymer substrate can be changed and controlled by adjusting the processing conditions of the ultrasonic vibration embossing process. Hence, various molds with different concave lens array patterns and thus polymer lens arrays, can be effectively fabricated at very low cost and with high throughput. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers     

Modeling the Embossing Stage of the Ultrasonic-Vibration-Assisted Hot Glass Embossing Process

Ultrasonic vibration technology has recently been applied in high-temperature forming processes, such as hot upsetting and hot glass embossing. Experimental research has delineated the effects of ultrasonic vibration on reducing required forces and improving the formability of materials. The purpose of this study was to construct a finite element model of the embossing stage of the ultrasonic vibration-assisted hot glass embossing process. Traditional hot embossing experiments in which the embossing speed and temperature were varied were performed to calculate the viscoelastic dissipation caused by ultrasonic vibration, and this value was then inputted into the simulation. The consistency of the force responses in the experiments and simulation indicated that the proposed model is valid. The findings indicate that the influences of parameters such as the vibration frequency, vibration amplitude, and embossing speed on the ultrasonic vibration-assisted hot glass embossing process must be investigated further.     

Study on squeezing flow during nonisothermal embossing of polymer microstructures

A numerical simulation of the hot embossing process with nonisothermal embossing conditions was carried out to observe the flow pattern of poly (methyl methacrylate) into microcavities. The microcavity was isomorphically downsized. The ratio of the cavity width over the cavity thickness was maintained constant at 8:1 throughout the analysis, while the cavity thickness varied from 200 μm to 0.5 μm. It was found that as the microcavity was downsized, the filling mechanism varied. For larger cavity thicknesses (e.g., 100 μm), the polymer flow climbed along the wall of the heated die and was then compressed downward and squeezed outward. In contrast, for a smaller cavity thickness (e.g., 5 μm), the flow was uniform and the wall‐climbing flow was absent. This size effect was correlated with the uniformity (UNF) of the temperature distribution of the polymer substrate during the embossing process. For larger cavity thicknesses, the high temperature zone was localized in the vicinity of the die wall, and consequently localized wall‐climbing flow occurred. The size effect in nonisothermal embossing was also studied experimentally, and localized flow was observed for larger cavities but not for smaller cavities. POLYM. ENG. SCI., 45:652–660, 2005. © 2005 Society of Plastics Engineers     

Hot‐embossing experiments of polymethyl methacrylate across the glass transition temperature with variation in temperature and hold times

Hot‐embossing (HE) experiments were conducted on polymethyl methacrylate (PMMA) across its glass transition temperature from 92 to 142°C. The glass transition temperature (T_g) of the PMMA used in this study was ～ 102°C. The polymer samples were embossed to a depth of 0.8 mm (800 μm). The experiments were carried out at various temperatures for different hold times of 30, 90, and 180 sec during the embossing process. A few additional experiments were conducted at 142°C with cooling of the samples as well. The force required for embossing and the final depth of the embossed features were analyzed. Polymers, including PMMA, show significantly different material behavior around and above T_g. The same was seen in the aforementioned tests; the trends observed for the force as well as the final depth changed considerably around 122°C (T_g + 20). These findings will be used in developing material models for use in simulating the hot‐embossing process. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Technological Development of Advanced Asymmetric Magnetic Soft Mold Micro-Imprint Lithography

This study proposed an advanced micro-imprint lithography (MIL), which integrates electromagnetic field-aided hot embossing and PDMS asymmetric magnetic flexible soft mold imprinting techniques, to imprint and replicate the microlens array structure. It is similar to the continuous grayscale technique; however, it is smoother than the structural form defined by the semi-conductor grayscale mask technique, and the process is simpler with lower costs, making it a good alternative for imprinting techniques and applications. This study also employed prescale films to measure and discuss the distribution of imprinting force on asymmetric magnetic soft molds. The results indicated that the magnetic soft mold and the substrate surface can be fully contacted. Since the magnetic powder reveals a skewed distribution, the test results of prescale film indicated that color depth is related to the concavo-convex of magnetic powder. Thus, the depth and accuracy of structural molding can be controlled in advance by casting the inclining platform. Finally, the SEM observation showed that if an inclining platform is used for composite magnetic PDMS casting, the microlens array asymmetric magnetic soft mold structure, which is complementary to it, could be obtained. The asymmetric magnetic soft mold was applied together with electromagnetic field-aided hot embossing equipment to imprint and replicate different continuous and smooth microlens array structures with foreseeable depth.     

Constant‐temperature embossing of supercooled polymer films

In conventional hot embossing, a thermoplastic polymer undergoes phase transitions in liquid, semi‐solid, and solid states through cyclic heating and cooling. This paper, in contrast, describes the development of a constant‐temperature embossing process and compares its characteristics against standard hot embossing. The new process utilizes the crystallizing nature of supercooled polymer films to obtain the necessary phase transitions. By softening and crystallizing the supercooled polymer at the same temperature, the embossing and solidification stages can be carried out isothermally without a cooling step. PET, due to its relatively slow crystallizing kinetics, was chosen as a model material for this study. The embossed films with microgroove patterns of different sizes and aspect ratios were characterized for their replication fidelity and accuracy. For supercooled PET films, constant‐temperature embossing with high replication quality and acceptable demolding characteristics was achieved in a large processing temperature window between T_g and T_m of PET. A parametric process study involving changes of the embossing temperature and embossing time was conducted, and the results indicated that the optimal process parameters for constant‐temperature embossing can be derived from the crystallization kinetics of the polymer. The removal of thermal cycling is a major advantage of constant‐temperature embossing over conventional hot embossing and represents an important process characteristic desired in industrial production. POLYM. ENG. SCI., 54:1100–1112, 2014. © 2013 Society of Plastics Engineers     

Analysis of polymer viscoelastic properties based on compressive creep tests during hot embossing for two‐dimensional polyethylene terephthalate nanochannels

Hot embossing is an emerging technology, which enables cost‐effective and high‐productivity nanofabrication. It is important to optimize the processing parameters to achieve high replication accuracy nanostructure fabrication. In this article, to reveal the interrelationship between the hot embossing conditions and replication accuracy of two‐dimensional (2D) polyethylene terephthalate (PET) nanochannels, the numerical simulations were conducted by using the generalized Maxwell model. The constants of the generalized Maxwell model were calculated by a newly established fitting formula based on experimental compressive creep curves rather than tensile stress relaxation curves. Replication accuracy of 2D PET nanochannels was evaluated by a method based on weighted averages. Orthogonal method was employed during numerical simulations to optimize the hot embossing process. Under optimized hot embossing parameters, 2D PET nanochannels, ～80 nm wide, 250 nm deep and 4 mm long, were precision‐imprinted into a PET sheet. POLYM. ENG. SCI., 54:2398–2406, 2014. © 2013 Society of Plastics Engineers     

Analysis of mold insert fabrication for the processing of microfluidic chip

The microfluidic chip has been used as an example to discuss different mold insert materials by micro hot‐embossing molding. For the mold insert, this study uses the SU‐8 photoresist to coat on the silicon wafer, then uses UV light to expose the pattern on the surface of SU‐8 photoresist, and coat the seed layer on the SU‐8 structure using thermal evaporation. The micro electroforming technology has been combined to fabricate the mold inserts (Ni, Ni‐Co) followed by replicating the microstructure from the metal mold insert by micro‐hot embossing molding. Different processing parameters (Embossing temperature, embossing pressure, embossing time, and demolding temperature) for the properties of COP film of microfluidic chip have been discussed. The results show that the most important parameter is the embossing temperature for replication properties of molded microfluidic chip. The demolding temperature is the most important parameter for surface roughness of the molded microfluidic chip. The Ni‐Co mold insert is the most suitable mold material for molded microfluidic chip by microhot embossing molding. The bonding temperature is the most important factor for the bonding strength of sealed microfluidic chip by tensile bonding test. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Magnetic resonance microscopy analysis of transport in a novel Tape‐Cast porous ceramic

Freeze‐tape‐cast porous ceramics allow for tailored pore structures. The impact on transport dynamics of pore structures which vary as a function of spatial depth within a ceramic is an important consideration in designing pore structures for particular applications. In this article, the application of nuclear magnetic resonance microscopy and ¹H NMR techniques to characterize the transport in a novel tape‐cast ceramic is presented. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Rheological behavior of ethylene–vinyl acetate copolymer and fabrication of micropyramid arrays by roll‐to‐roll hot embossing on its thin films

Films of ethylene–vinyl acetate (EVA) copolymer with different vinyl acetate (VA) contents were adopted as flexible substrates for fabrication of micropyramid arrays by roll‐to‐roll (R2R) hot embossing at roller temperatures 40–80 °C. An empirical relationship between rheological behavior of EVA in rubbery state and demolding performance of the films was explored. Dependence of average forming height of micropyramid arrays on VA content and roller temperatures was investigated. Our study showed that both viscosity and relaxation behavior of EVA can be related to the ultimate forming height of micropyramids. The relationships can provide references for experimental research and industrial manufacture on R2R hot embossing. Retroreflection of micropyramid arrays was observed and transmittances of the two sides of embossed films were studied. Transmittances of the VA28 film embossed at 60 °C were 97.9% (the embossed side) and 41.8% (the smooth side), which showed potential applications in reflective film and antireflective film fields. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45228.     

Uniform shell patterning using rubber‐assisted hot embossing process. I. Experimental

In rubber‐assisted hot embossing, a softened thin thermoplastic film is pressurized between a hard mold surface and a rubber pad. The rubber pad, as a soft counter‐tool, deforms conformably to the hard mold surface, allowing feature transfer from the hard surface and formation of shell‐type structures on the polymer film. This two‐article sequence was aimed to understand the basic mechanisms affecting the pattern uniformity and replication in rubber‐assisted hot embossing. In Part I, a series of rubber‐assisted embossing experiments involving parametric studies of the effects of different processing conditions, as well as material selections, on the pattern thickness uniformity and replicated pattern height were conducted. The difference in film thickness uniformity in different experiments was explained using the mechanical and rheological behavior of the polymer film and the rubber counter‐tool under different processing conditions. Based on the experimental results, strategies for determining a feasible process window for achieving uniform shell patterning by rubber‐assisted embossing were proposed. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Finite element modeling of polymer hot embossing using a glass‐rubber finite strain constitutive model

A hyperelastic–viscoplastic constitutive model for amorphous polymers was used in finite element simulations of micro‐hot embossing across the glass transition. The model was selected for its ability to capture finite strain temperature and rate dependence over a wide range of temperatures, including across the glass transition. The simulations focused on the glass transition temperature regime, and particularly probed the effects of time and temperature during cooling and mold release. The results show that strong temperature sensitivity of the material across the glass transition leads to a wide range of required embossing force and springback. The interplay between changes in material properties upon cooling and stress relaxation can lead to significant increases in embossing force during the cooling stage, especially when high cooling rates are employed. The effects of thermal expansion also complicate the problem during rapid cooling. Nonlinear material behavior is shown to affect results in parametric hot embossing studies. Careful tailoring of embossing temperature, cooling rate, and demolding temperature is critical in acceptable feature replication. The best results are found for moderate cooling rates, which allow adequate time for stress relaxation in the material prior to mold release. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers