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.