‘ over-sized’ particles that can modify and shrink during firing. Resulting micro-cracks can then propagate and become areas of inherent weakness in the body during dynamic mechanical loading or even under changes in ambient temperature. The larger the quartz crystallites, the bigger these cracks and the sooner the insulator is at risk of failing.
Given the above, methods have emerged in recent years to allow producers as well as buyers to monitor microstructure of the porcelain mass to ensure freedom from crystallites formed during production. Apart from materials testing to measure physical attributes such as density and mechanical strength, structural analysis is done using x-ray diffrectometers. These allow the quartz content of different alumina porcelains to be conveniently and easily compared on charts.
The basic production technologies used for porcelain insulators have not changed appreciably for decades,
Unwanted quartz crystallite separated from vitreous phase in C-130 alumina porcelain body.
Photo Courtesy of Johannes Liebermann.
although associated equipment such as ball mills, extruders, lathes, dryers and kilns have been progressively improved or enlarged for higher productivity or reduced energy costs.
The classical wet process is still the more common manufacturing technique and sees the porcelain mass pressed and shaped while having relatively high moisture content. The main advantage is lower investment cost and that the production environment has much less of the dust caused by turning dry cylinders. The more recently developed isostatic production process, by contrast, requires expensive upstream equipment to mechanically compress the spraydried ingredients into porcelain cylinders under extremely high pressure. The main benefit is reduced processing lead times, which, in the insulator business, can prove the key decision-making factor among buyers.
Part 2 of this article will appear in Q3, 2013