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This two-article sequence on rubber-assisted embossing was aimed to understand the basic mechanisms affecting the pattern uniformity and replicability and to determine a process window for achieving uniform patterning and faithful replication. In Part I, the effects of major process and material parameters were identified and studied, and strategies for successful hot embossing with rubber as a pressure medium were proposed. In Part II, the rubber-assisted embossing process was analyzed considering the unique rheological behavior of the materials involved to develop useful predictive capabilities for this new process. Specifically, a finite-strain hyperelastic formulation was used for simulating the isothermal embossing stage, and a generalized Maxwell model was used to study the stress relaxation during the holding stage and to predict the elastic recovery after demolding. The rheological properties obtained in Part I were fitted to the constitutive models and implemented in the simulation procedures. The simulation results agreed with the major findings in the experimental work and provided a more quantitative insight into the process. POLYM. ENG. SCI., 51:601-608, 2011. [C]2011 Society of Plastics Engineers, INTRODUCTION In rubber-assisted embossing, a thin polymer film is bent and stretched to form a microshell pattern. This deformation process is similar to that experienced in conventional thermoforming where a [...]

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., 51:592-600, 2011. [C]2011 Society of Plastics Engineers, INTRODUCTION One of the most widely used techniques for conversion of polymeric materials into useful microdevices is hot embossing (1-3). It is a process variant of compression molding, particularly designed [...]

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, INTRODUCTION The standard hot embossing process is designed for imprinting surface features onto polymer substrates [1, 2]. The substrate is relatively thick as compared with the feature size. During the [...]

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

An approach of using slowly crystallizing polymers to form single-polymer composites (SPCs) was investigated. The method was demonstrated using poly(ethylene terephthalate) (PET) as a model system, with which two distinct physical forms, namely, amorphous PET films and highly crystalline PET fibers can be readily obtained. Because of the large difference between the rubbery softening temperature of the amorphous phase and the melting temperature of the crystalline phase, the new process is characterized by its wide process window and enhanced manufacturability. The heating temperature and holding time were found to play a profound role in influencing the properties of the SPC. Excellent fiber-matrix interfacial adhesion was obtained at heating temperature 180°C and holding time 10 s. The results also indicated that the heating rate plays a significant role in affecting the fusion and adhesion of the composite. With reduced heating rates, the fusion and adhesion properties are rapidly deteriorated. POLYM. ENG. SCI., 46: 1223-1230, 2006. © 2006 Society of Plastics Engineers, INTRODUCTION The original concept of a single-polymer composite (SPC), a composite with matrix and reinforcement from the same polymer, was presented by Capiati and Porter [1] three decades ago. The [...]

A polymer micro cold forging process was investigated. This method, derived from polymer solid-phase forming, resembles more the metal-forging method than the standard thermoplastic molding protocol. A micro-forging die set was designed and fabricated and a series of cold forging experiments were carried out with varied process conditions, including forging speed, forging pressure, and dwell time. High-aspect-ratio microchannels were fabricated on polytetrafluoroethylene (PTFE) substrates. The dimension of replicated channels was found to be consistent from shot to shot. Owing to instantaneous elastic recovery, the channel has a width about 30% smaller than that on the die. The final channel size is contingent upon the forging process conditions, as well as upon the post-recovery process. Upsetting experiments of PTFE cylindrical samples were carried out to study the fundamental issues on dimensional recovery so as to predict and consequently control the dimensional changes in polymer cold forging. Polym. Eng. Sci. 44:1998-2004, 2004. [c] 2004 Society of Plastics Engineers., INTRODUCTION Polymeric materials have considerable advantages in microfabrication because of their versatile properties and mass production capability. There has been much work worldwide during the past decade on adaptation of [...]

The article focuses on the experimental study involving a two-station embossing strategy for enhancing the fabrication of surface microstructures on thermoplastic polymers. The two-station embossing process uses contact heating and cooling for rapid thermal cycling of embossing molds. Based on the results of the study, good replication fidelity including local replication of sharp edges and mold surface defects can be attained with an embossing time of less than several seconds.

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 amid! 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°C in about 3 s using a hot station at 250°C. The method was used for thermal cycling of embossing tools. Surface microfeatures can be rapidly transferred from thin metallic stamps to polymer substrates with cycle times less than 10 s. [ABSTRACT FROM AUTHOR]

The article discusses the single-polymer composites based on slowly crystallizing polymers. An approach of using slowly crystallizing polymers to form single polymer composites was examined which demonstrates the method using polyethylene terephthalate as a model system. The method was used the to observed the difference in melting temperature between high density polyethylene (HDPE) matrix and its reinforcement to construct a HDPE homocomposite.

Discusses the experimental results of cold forging for replicating high-aspect-ratio microchannels on polytetrafluoroethylene (PTFE). Room-temperature mechanical properties of PTFE; Replicability of microchannels in cold forging experiments with varied forging displacement and force; Deformation behavior of PTFE during forging.