Pratapkumar Nagarajan

A Strategy for Rapid Thermal Cycling of Molds in Thermoplastic Processing

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 hy 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 he rapidly transferred from thin metallic stamps to polymer substrates with cycle times less than 10 s.

Rubber-Assisted Hot Embossing for Structuring Thin Polymer Film Polymeric Films

Precision structured polymer thin films with microstructures comparable to or greater than the film thickness are highly desired in many applications. Such micro-patterned thin films, however, are difficult to fabricate using the standard hot embossing technology where both halves of the mold are made of hard materials. This study investigated a rubber-assisted embossing process for structuring thin polymer films. The advantages of the rubber backup instead of a hard support include but are not limited to 1) simplifying the embossing tool, 2) protecting the embossing master, 3) facilitating embossing pressure buildup, and 4) accommodating conformal forming of microscale shell patterns. Several design and process variables including rubber hardness, embossing temperature, embossing pressure and holding time were carefully studied. Thin polystyrene films in a thickness of 25 μm were accurately patterned with microgrooves of characteristic dimensions on the order of 100 μm.Copyright

Constant Temperature Microfeature Embossing with Slowly Crystallizing Polymers

Abstract A new microfeature embossing method utilizing a slowly crystallizing mechanism was investigated to eliminate thermal cycling, as needed in standard hot embossing. Poly(ethylene terephthalate) was used as a model system for demonstration. Due to its slow crystallization, amorphous PET film can be made by melt casting onto a chilled roll. The resulting amorphous film was embossed at a constant temperature of 180°C for a period of time comparable to or longer than the polymers half-time of crystallization. During constant-temperature embossing, the film is softened first, caused by rubber softening of the amorphous phase, and is then hardened, resulting from the crystallization of the amorphous phase at the same embossing temperature. Since the embossed film is hardened under the constant mold temperature, no cooling is needed. Selected micro features, including circular microchannels and high aspect ratio microribs, were consistently patterned using a total cycle time about 40s. The embossed films were characterized using DSC and rotational rheometry to elucidate the physical mechanism for softening and hardening the polymer during constant-temperature embossing.

Two-Station Embossing Process for Rapid Fabrication of Polymer Microstructures

The hot embossing technique is becoming an increasingly important alternative to silicon-and glass-based microfabrication technologies. The advantage of hot embossing can be mainly attributed to the versatile properties and mass production capability of polymeric materials. However, because of the use of a large mass in thermal cycling, hot embossing is subject to substantially longer cycle times than those in traditional thermoplastic molding processes.1 The longer dwell time at elevated temperatures could further result in degradation of the embossing polymer, especially for thermally sensitive polymers. The problem exacerbates when thick polymer substrates are used. To address this problem, rapid thermal cycling of the tool is needed. One method for rapid thermal cycling is to employ a low-thermal-mass multilayer mold with electrical heating elements installed right beneath the mold surface.2 This method, however, is complex in nature and may be prone to problems caused by mismatching of thermal and mechanical properties between different layers.Copyright

Polymer micro hot embossing for the fabrication of three-dimensional millimeter-wave components

Millimeter-wave communication and radar systems have found numerous applications in recent years. These applications include millimeter-wave automotive collision avoidance radars, deep space communication and radar systems, w-band short range communication link, remote sensing of soil moisture and atmospheric monitoring in meteorological applications and climate sensing [1–4].

Constant-Temperature Embossing of Amorphous Poly(Ethylene Terephthalate) Films

In the standard hot embossing process for thermoplastic polymers, thermal cycling is needed in order to soften and subsequently cool and solidify the polymer. This thermal cycling, however, not only results in long cycle times but also deteriorates the quality of embossed features. A new embossing method based on slowly crystallizing polymers was investigated to eliminate thermal cycling. Poly(ethylene terephthalate) was used as a model system for demonstration. Due to its slow crystallization, amorphous PET film can be made by casting a PET melt onto a chill roll. The amorphous PET film was embossed at a constant temperature of 180°C for a period of time comparable to or longer than PET’s half-time of crystallization. During constant-temperature embossing, the film first liquefies, caused by rubber softening of the amorphous phase, and then solidifies, resulting from the crystallization of the amorphous phase. Since the embossed film is hardened under the constant mold temperature, no cooling is needed. Selected micro features, including circular microchannels and high aspect ratio rectangular microchannels, were successfully embossed using a total cycle time about 40 s.Copyright

Close-Die Embossing of Multichannel Waveguides

A combined punching and embossing process was developed for fabrication of 3-D multichannel waveguides. The close-die setup overcomes the lateral flow problem experienced by the standard hot embossing process, thereby allowing buildup of embossing pressure and replication of 3-D features. The embossing tool for this purpose includes a punching head and the to-be-replicated features in the socket behind the punching head. During the process, the embossing tool first punches out a billet from a large blank and then embosses this billet in the socket. The combined operation allows for accurate blanking and provides a close-die process for embossing pressure buildup. ABS and PMMA were chosen as two polymers for waveguide embossing. In both cases, cycle times around 7 minutes resulted. While ejection problems were experienced by PMMA, a relatively brittle polymer, 4-channel waveguides with crispy edges and accurate dimensions were successfully fabricated using ABS, the tougher polymer.Copyright