Fundamental investigation understanding casting of lead sheet

Abstract

Lead sheet is widely used in the construction industry for roofing and flashing applications. The roots of this process can be tracked back to the Roman times when sandcast lead sheets were used for a wide variety of applications. Sandcast lead sheets are characterised by their superior aesthetic performance and mottled appearance. These days such sheets are used for premium roofing and flashing applications in the heritage construction industry. Lead sheet is also manufactured using a type of continuous casting process also called as the ‘Direct Method (DM)’. This thesis focuses on a fundamental investigation of both these processes used for manufacture of cast lead. Just like any casting process, sand casting of lead sheet suffers from the presence of surface defects. In this study, a surface defect type, hereby referred to as ‘grooves’, has been investigated. The focus has been laid on the identification of the main factors affecting defect formation in this process. Based on a set of screening experiments performed using Scanning Electron Microscopy (SEM) as well as the existing literature, a number of factors affecting the formation of such defects was identified and their corresponding significance was estimated. Two-dimensional Computational Fluid Dynamics (CFD) simulations have been performed to simulate the melt flow and solidification stages of the lead sandcasting process. The effects of process parameters such as pouring temperature, screed velocity and clearance between the screed and the sandbed on the final quality of the lead sheet are investigated. Sheet quality is quantified by measuring the variance and the average of the final sheet thickness over the sandbed length. The CFD model has been validated against experimental results by comparing the evolution of the lead-sandbed interface temperature against data collected by thermocouples during the evolution of the process. The direct method of casting lead is a much more energy efficient compared to the conventional rolling process which requires a casting process before rolling to achieve the required thickness. This work also looks into the energy consumption in different stages of the DM process and suggests pointers for improvement. An energy audit of the process is conducted, and the consumption is analysed at different stages and compared with rolled lead. A two-dimensional numerical model of the DM process was developed and different process parameters affecting the thickness of the final cast sheet is studied. Effects of parameters like volume flow rate, heat transfer coefficient, speed of rotation of the casting drum and its immersion are investigated. The studies were conducted in collaboration with ML Operations, a cast lead sheet manufacturer based in Derbyshire and the findings of the study were implemented successfully.