Mechanism of forming defects in roll-to-roll hot embossing of micro-pyramid arrays: II. Numerical study

The hot embossing process is a cost-effective replication technology for the fabrication of polymeric micro/nano-structures. Nevertheless, there is a need to reduce the embossing temperature as industrial applications of the hot embossing process are limited by the high temperature required. For amorphous thermoplastic polymers, the hot embossing process can be conducted below the polymer’s glass transition temperature (Tg), which is promising for many industrial applications. However, it is generally difficult to achieve a high replication fidelity in such a below-Tg embossing process. The large resistance to plastic flow and the significant deformation recovery of the glassy polymer (the amorphous thermoplastic polymer in its glassy state) are two main causes. The problem of plastic flow may be solved by designing and developing a hot embossing system with large force capability, but the recovery problem may not and needs further study. In this research, the recovery of a polycarbonate (PC) glass in the below-Tg embossing process was quantified and investigated systematically. The effects of embossing depth, holding time, temperature, and loading rate on the recovery were discussed and analyzed. The research results reveal that it is the recovery (usually more than 20%) that severely limits the replication fidelity of the below-Tg embossing process. The recovery is strain-dependent due to the strain-dependent chain tension of the PC glass. As the chain tension generated in the strain-hardening region cannot be effectively reduced by increasing the embossing force, holding time, and temperature, the recovery problem poses a great challenge to be overcome. Considering the strain-rate dependence of the segmental mobility of glassy polymers, a strategy of increasing the loading rate to reduce the recovery was proposed and verified. Hence, this research advances our understanding of the below-Tg embossing process and provides new insights into how to improve the hot embossing process.

Section: Introductionmentioning

confidence: 99%

An experimental study of recovery in embossing of polycarbonate below the glass transition temperature

Wang

Deng

et al. 2020

“…Velten et al [10] presented a new R2R embossing process, which had continuous and discontinuous modes. Deng et al [11] developed a polymeric material behavior model and Wang et al [12] built a 3D FE model to reveal the underlying mechanisms for the forming defects. Peng et al [13] proposed a shape-preserving mechanism to extend the material filling time and realize a cooling effect during the demolding process.…”

Section: Introductionmentioning

confidence: 99%

Experimental studies on a novel roll-to-roll powder hot embossing for large-area fabrication of micropyramid arrays on polymers

Deng

Shu

et al. 2018

Roll-to-roll (R2R) hot embossing is regarded as a cost-effective replication technology to fabricate microstructures on polymer films. However, the characteristics of continuous and fast forming for the R2R hot embossing process limits material flow during the filling stage and results in significant springback during the demolding stage. To resolve this issue, this study proposed a novel R2R powder hot embossing process, which combines the merits of the continuous fabrication of R2R hot embossing and near-net-shape forming of powder sintering and also decreases the whole cycle of the fabrication from films to microstructures. First, the relation between the molten layer thickness and processing parameters was discussed and an analytical model was established to predict the feed of the polymeric powder during R2R powder hot embossing. Then, with the use of a micro-pyramid array mold, the impact of the process parameters including mold temperature, feeding speed and applied force on the geometrical dimension of the patterned microstructures was discussed. Last, based on the response surface analysis, a process window, in terms of the mold temperature of 132 °C –145 °C, feeding speed of 0.1–1.4 m min−1 and applied force of 15–50 kgf was determined for the continuous fabrication of completely-filled micropyramid arrays with the R2R powder hot embossing process. This research demonstrated the feasibility and superiority of the proposed R2R powder hot embossing process in continuously fabricating micropatterned structures on polymeric films.

“…As a typical process in related technologies, roll-toroll (R2R) micro/nano imprinting has been studied intensively in recent years. Though R2R imprinting technology has brought significant developments in many applications, there are still some problems that need to be solved urgently such as bubble defect [7][8][9][10][11][12], which is found in most instances.…”

Section: Introductionmentioning

confidence: 99%

Filling angle and its effect on filling process in roll-to-roll ultraviolet imprint lithography

Wang

Qiu

et al. 2018

Bubble defects are a serious problem in the roll-to-roll ultraviolet imprint lithography process. To investigate the air bubble trapping in microcavities, a numerical simulation method is proposed based on a sliding mesh and open-channel model. Such considerations in the model make it possible to include both the rotation of the roller and the flow of resin, which is different from the conventional methods. Through analysis with our model, a parameter, termed filling angle here, is found to be a significant factor in the imprinting process. With this parameter, the relationships between resin accumulation and bubble defects are studied. The differences in resin filling at different rotation velocities are also discussed. The experimental observations are in good agreement with the theoretical calculations. The results provide a deep understanding of the filling process, which can provide good theoretical guidance to eliminating bubble defects in industrial applications.