Study on the Moulding Mechanism of Isothermal Hot Embossing in Solid-like State
Author:Sun Jing Yao
Supervisor:wu da ming
Micro-nano manufacturing is a method of product designing,processing and manufacturing on micro/nano scale.It can meet the requirements in biomedicine,micro-electronics,micro-optics,intelligent system,and other frontier domains.Manufacturing plays an important role in the field of national public sector and national defense security.The developmental level of manufacturing,especially the high-end precision manufacturing,has already become a direct representation of a country’s comprehensive strength.The fabrication of polymer micro/nano-structure device is a front area in the field of micro-nano manufacturing.This dissertation is mainly focus on the innovative methods for polymer micro-nano fabrication.The article will analyze the advantages and disadvantages of traditional hot embossing process,and proposed improvement method.The new problems and challenges encountered,new model established,and new inner mechanism will also be studied systematically.The hot embossing method was first proposed by Chou S.Y.from Princeton University in 1995.They created vias and trenches with a minimum size of 25 nm onto thermoplastic polymer substrates.Althouth this method has rhe advantages of simple operation and high replication accuracy,it has not been applied in large scale.The reason is the traditional hot embossing method hasn’t get out from the thought of "molding at high temperature,and demolding at low temperature".The periodic heating and cooling of mold during the whole fabrication processes leads to low processing efficiency(molding cycle longer than 15 minutes),which makes hot embossing has no benefit advantages comparing with injection molding,extrusion,etc..Researchers around the world had made may efforts to solve this problem,e.g.ultrasound assisted heating,rapid heating/cooling with graphene coating,etc..However,there still exists many problems in these new methods,such as processing difficulties and device dependence.This paper opens another way on the basis of previous studies and a new method called Isothermal Hot Embossing in Solid-like State(IHESS)was proposed to cut down the cycle time.Quite different from traditional hot embossing,the IHESS method get out from the thought of "molding at high temperature,and demolding at low temperature" after considering the rheological behavior,constitutive relation,and stress relaxation characteristics of polymer comprehensively.The IHESS method can significantly reduce the hot embossing cycle from 15 min down to 20 s."Isothermal" represents the mold temperature keep constant during the whole hot embossing cycle,and embossing temperature is the demolding temperature."Solid-like state",quite different from high elastic state,is a state that deformation can happen and completely maintain only when the material is under appropriate processing conditions(above glass transition temperature,below crystalline melting point or viscous flow temperature).The inner mechanism of the IHESS method is researched systematically in this paper and the results are listed below:1.Build the theoretical model of IHESS process for amorphous polymers and crystalline polymers,and obtain their constitutive relations.Build the mathematical model of filling and molding process in IHESS and get specific hoe embossing technologies,which are suitable for the manufacturing of micro/nano structures on the surfaces of different types of polymers.2.Study the response characteristics of internal stress on embossing pressure,the variation of stress relaxation with molding temperature and holding time,and the influence of stress relaxation during holding and demolding process on the reproduction rate of micro/nano structure.The principle and method of residual stress controlling and structural stability improving during IHESS,and obtain the geometric stability model of micro/nano structure based on the stress relaxation and creeping during IHESS.3.In order to improve the cognition of IHESS process together with method innovation,macro simulation and molecular dynamics simulation are performed at the same time based on actual processing parameters.The evolution mechanisms and rules of polymer aggregation structure during embossing process under solid-like state,and the embossing process itself are discussed in-depth.4.A new method of differential hot embossing is furtherly proposed after IHESS method.The processing temperature range and application area of hot embossing is widened using the temperature difference between the upper and lower molds.Products with double-side micro/nano structures also can be obtained in one embossing cycle with the help of differential hot embossing.5.Typical micro/nanostructured functional devices,including polymer-metal composites micro structure heat exchanger,microfluidic chip,and flexible sensor,are prepared using the IHESS method.A new method of Isothermal Hot Embossing in Solid-like State is proposed in this paper.Varying mold temperature during embossing process is replaced by constant mold temperature and significantly reduced the embossing cycle of hot embossing process.This new method has the potential to become one of the mainstream processing methods for the fabrication of polymer micro/nanostructured functional devices.