Modi, Dhiren
(2008)
Modelling and active control of the Vacuum Infusion Process for composites manufacture.
PhD thesis, University of Nottingham.
Abstract
Vacuum infusion technology, even though first reported more than 50 years ago, was not popular for mainstream fibre reinforced polymer composites manufacturing until recently. Its present-day popularity is due to the increasing emphasis on the manufacturing cost as well as environmental and health concerns. As a result, novel processes such as Vacuum Infusion (VI) and Seemans' Composite Resin Injection Moulding Process (SCRIMPTM), employing the same basic technology, have been developed. As latecomers, these processes have not been investigated in detail and there exists a lack of understanding that can undermine the potential improvements in composites manufacturing offered by them. The present work is focused on (i) enhancing the fundamental understanding, and (ii) advancing the processing technology to fully exploit their potential.
Limitations of the existing analytical models for fluid flow in VI are explored. Then, improvising upon and extending these models, analytical formulations for the pressure profile and fill-times in rectilinear and radial flow VI processes are developed. An important result from this study is that with increasing reinforcement compliance, the analytical VI pressure profile diverges from the RTM pressure profile. It is found that for rectilinear as well as radial flow processes, the fill-time ratio between equivalent RTM and VI remains constant. Experimental validation for these formulations show that the pressure profile varies with flow progression in both rectilinear and radial flow VI. This leads to a dynamically changing fill-times ratio between RTM and VI. This dynamic behaviour, which is contrary to analytical predictions, is explained by hypothesising that the compliance characterisation experiments do not replicate the actual events in VI.
The issue of process control is also investigated for the VI process. A novel approach, using non-intrusive sensors and real-time flow simulations, is designed and implemented. The study gives important insights about the controllability of this process. It is found that in VI, due to low driving pressure, an optimum window of opportunity exists for process control. Reinforcements with high permeability give higher flow velocity, while low permeability reinforcements lead to lower flow velocity. Both of these cases lead to a marginal window of opportunity and poor process controllability. For reinforcements that offer good controllability, the control system is able to identify flow deviations and correct them, increasing the process efficiency.
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