Fibre and shot formation processes in mineral wool manufacture by centrifugal spinning.
This work stems directly from a manufacturing company's need to improve the quality of
its product and increase competitiveness in an industry where a physical understanding of
its production process, known as centrifugal spinning, is lacking. Centrifugal spinning is
the principal method of producing mineral wool; a product that is predominantly used for
thermal and acoustic insulation. The production technique generates wool fibres from a
molten jet that is directed to impinge onto four rotating wheels. However, it sometimes,
and quite often produces mineral wool fibres of poor quality due to the presence of nonfibrous
material or shots; a problem that has so far received little attention.
This inquiry was undertaken with a VIew to providing insights into the physical
mechanisms governing the formation of fibres and shots. Theoretical analyses and an
experiment were conducted to investigate the important non-dimensional groups that
govern fibre and shot formation, but concentrating mainly on understanding the
mechanics of shot formation. It was established that fibres are formed from the break-up
of the thin layer around the rotating wheels. It was also established that shots are formed
from droplets produced as a result of jet splashing. Jet splashing is temperature
independent since it occurs in a short time-scale from jet impingement with little melt
cooling; it is largely governed by force balance and geometrical parameters. Such an
observation led to an impinging water jet experiment that was conducted at ambient
temperature and its image processed by computer software.
From this, it was determined that the significant non-dimensional group that governs jet
splashing on a rotating wheel or drum was that due to the jet impact position. The least
liquid splashing was found to occur when the jet impinged in the forward position
(relative to the direction of rotation) because the depth of fluid underneath the
impingement position was always at its minimum. The speed of drum rotation affected
jet splashing only up to a critical value, beyond which it had little influence.