Hot embossing process parameters:\ud Simulation and experimental studies
Fabrication processes for the high volume production of parts with micro and\ud nano scale features are very important in the global research and industry efforts to\ud meet the increasing needs for device miniaturisation in numerous application areas.\ud Processes for the replication of surface geometries are promising technologies that are\ud capable to meet the demand of manufacturing products at a low cost and in high\ud volume. Among these technologies, hot embossing is a process which relies on raising\ud the temperature of a sheet of polymer up to its melting range and on pressing a heated\ud master plate into the polymer for triggering a local flow of the material to fill the\ud cavities to be replicated. This technique has attracted increased attention in recent\ud years in particular due to the relatively simple set-up and low cost associated with its\ud implementation in comparison to other replication techniques.\ud The present work is concerned with investigating the process factors that\ud influence hot embossing outcomes. In particularly, a detailed study of the process\ud parameters’ effect on the cavity pressure, demoulding force and uniformity of the\ud residual layer for different materials is conducted to analyse the further potential of this\ud process. A review of the current state of the art on these topics reported in Chapter 2, is\ud also used to assess the capability of this replication technology.\ud Chapter 3 presents an experimental study on the effects of process parameters\ud on pressure conditions in cavities when replicating parts in PMMA and ABS. To\ud measure the pressure state of a polymer inside mould cavities, a condition monitoring\ud system was implemented. Then, by employing a design of experiment approach, the\ud iii\ud pressure behaviour was studied as a function of different process conditions. In\ud particular, the effects of three process parameters, embossing temperature and force\ud and holding time, on the mould cavity pressure and the pressure distribution were\ud investigated. In addition, using a simple analytical model, the minimum required\ud embossing force to fill the cavities across the mould surface was calculated. The\ud theoretical value obtained was then used to inform the design of the experiments. It\ud was shown that cavity pressure and pressure distribution were dependent on both\ud materials and processing conditions. The obtained results indicate that an increase in\ud temperature and holding time reduced the pressure in the central and edge cavities of\ud the mould and the pressure distribution while the opposite effect takes place when\ud considering the embossing force. Also, it was observed that an increase of the\ud embossing force has a positive effect on cavity filling but a negative influence for\ud homogenous filling.\ud In Chapter 4, a theoretical model was proposed to predict demoulding forces\ud in hot embossing by providing a unified treatment of adhesion, friction and\ud deformation phenomena that take place during demoulding of polymer\ud microstructures. The close agreement between the predicted results and those\ud measured experimentally suggests that the model successfully captures the\ud relationship between mould design, feature sidewall, applied pressure, material\ud properties, demoulding temperature and the resulting demoulding force. The\ud theoretical results have been confirmed through comparisons with the demoulding\ud experiments. The temperature at which the demoulding force is minimised depends on\ud the geometry of the mould features along with the material properties of the mould\ud and replica. The applied pressure has an important influence on the demoulding force\ud iv\ud as the increase in pressure augments the adhesion force due to changes in material\ud dimensions and reduces the friction force due to resulting decrease in the thermal\ud stress.\ud Furthermore, the relationship between the residual layer uniformity and three\ud process parameters was investigated in Chapter 5, using simulation and experimental\ud studies when processing PMMA sheets. In particular, the characteristics of the\ud residual layer thickness of embossed parts were analysed as a function of the\ud moulding temperature, the embossing force and the holding time. Increasing the\ud moulding temperature resulted in a reduction on the average residual layer thickness\ud and on its non-uniformity. An increase in the embossing force led to a decrease in the\ud homogeneity of the residual layer. Also, an improvement of the residual layer\ud thickness uniformity was also observed when embossing with a longer holding time.\ud The results of the conducted experimental and simulation studies were analysed to\ud identify potential ways for improving the hot embossing process.\ud Finally, in Chapter 6 the results and main findings from each of the\ud investigations are summarised and further research directions are proposed.
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