Analysis of the demolding forces during hot embossing

Abstract

Hot embossing is one of the main processing techniques for polymer microfabrication, which helps the LIGA (UV-LIGA) technology to achieve low cost mass production. When hot embossing of high aspect ratio microstructures, the deformation of microstructures usually occurs due to the demolding forces between the sidewall of mold inserts and the thermoplastic (PMMA). The study of the demolding process plays a key role in commercial manufacturing of polymer replicas. In this paper, the demolding behavior was analyzed by Finite element method using ABAQUS/ Standard. Simulation identified the friction force caused by interface adhesion and thermal stress due to shrinkage between the mold and the polymer as the main sources of the demolding forces. Simulation also showed that the friction force made a greater contribution to the deformation than thermal stress, which is explained in the accompanying theoretical analysis. To minimize the friction force the optimized experiment was performed using PTFE (Teflon) as anti-adhesive films and using Ni-PTFE compound material mold inserts. Both lowered the surface adhesion energy and friction coefficient. Typical defects like pull-up and damaged edges can be reduced.

Analysis of factors affecting the demolding of micro-structured devices in the injection molding process

“…They reported that the farther the features are from the shrinking center, the harder the demolding becomes. This is in accordance with FEM calculations for the hot embossing process conducted by Guo et al (2007), who concluded that the thermal stress is a function of the orientation of the micro-structures relative to the shrinkage direction [5]. In practice, the orientations of structures and stress barrier should be kept coincident with the shrinkage center, e.g.…”

“…This behavior was attributed to the introduction of thermal stresses into the PMMA, which caused part failure. The investigation on the design of the micro-structures confirmed favorable placement close to the gate and in flow direction in accordance to Heckele and Schomburg (2004) [4] and Guo et al (2007) [5], although the influence was minor due to the low impact of shrinkage on the demolding energy (compare Figure 3). PP displays the lowest overall demolding energies, due to its high elasticity and low tendency for surface adhesion to the stamper.…”

“…All in all, a study on all of these factors is still lacking, since each research group covered one or two influencing factors at most [2][3][4][5][6]. All factors occur to be inter-dependent, which deems an independent investigation of only one factor impossible.…”

Effect of Molding Conditions on Demolding Force During Injection Molding of Parts with Micro-features

“…또한 측벽의 표면 거칠기에 따라 이형력에 차이가 있음이 연구되었다 [2] . Yuhua Guo [3,4] 은 ABAQUS/Standard을 사용하여 Hot embossing 공정에서 금형과 polymer의 수축차이로 인한 열응력에 대하여 연 . .…”

Demoulding Force Prediction for Micro Polymer Replication: A Review of Relevant Literature

“…In addition the use of Ni-PTFE as a mould material was recommended to achieve lower surface energy and friction force. Simulated results were presented by Guo et al [24].…”