During the production of large electrical porcelain specimens the shaping, drying, and green handling stages are extremely sensitive. Severe faults and defects can occur in these stages if the body is not sufficiently strong, plastic, or has a poor drying characteristic.

For example, cracks may appear that either require repair/reprocessing work, or may not be recoverable at all.

Shaping Faults

Key to have good shaping characteristics is to use clays with a high level of workability – what some call plasticity. For this reason, Imerys has developed a test that assesses the workability of clays and/or bodies. The test characterises the amount of work energy that can be applied to a clay (or full body formulation) without cracks occuring. This can indicate either how hard a clay (or body) can be worked, or how likely the clay (or body) is to crack under given conditions.

A high workability can also help resist the formation of cracks during drying – see section Drying Faults

For more details about this test, please see the article on workability in our Technical Documentation section.

Imerys’ portfolio contains a range of products, with good to high levels of workability, that can improve the shaping characteristics of your body – allowing faster production or higher yields.

Drying Faults

Using clays with a good mineralogy, in terms of drying shrinkage, can help reduce the build up of drying stresses during the drying process – allowing faster drying and/or reduced drying cracks.

The balance of workability – see the Shaping Faults section – to drying performance is key in the use of clays for electrical porcelain production. A clay with high workability may still lead to drying cracks if the workability comes from minerals with poor drying characteristics.

Imerys’ portfolio offers a range of products with mineralogies that provide a good balance of workability and drying performance – allowing faster production or higher yields.

Firing Faults

Often electrical porcelain specimens are very large. This makes their firing difficult and sensitive to several potential faults.

Blue/Black Coring

Large electrical porcelain specimens must be fired very carefully in order to allow all gases generated during firing to be released, in order to prevent large firing defects such as blue/black coring. For this reason, it is important to use clays with low amounts of gas forming material – for example, low carbon. This allows specimens to be fired faster and/or with less blue/black core defects.

Imerys’ portfolio offers a range of products with low carbon – allowing faster production or higher yields.

Low Fired Strength

Electrical porcelain insulators require high fired strength in order to resist the stresses to which they will be subjected during their lifetime. It is important to utilise clays with low silica in order to maximise fired strength – and minimise the amount of alumina that may have to be added to the body, to compensate.

In addition, it is important that there is no coarse silica present because this can lead to microscopic defects in the structure of the fired specimen – leading to localised low strength, which may be difficult to detect in laboratory testing. These defects can also lead to poor dielectric characteristics – see Poor Vitrification section.

Imerys’ portfolio offers a range of products designed to have as low silica as possible.

Poor Vitrification

It is important for electrical porcelain to have a very homogeneous fired structure. This is because microscopic variations (defects) in the structure of the fired porcelain can lead to low fired strength (see section Low Fired Strength), or poor dielectric characteristics (e.g. lower electrical breakdown).

For this reason, it is important to utilise clays with a good level of alkali elements (feldspathic material), otherwise they may be refractory and prevent good vitirification. Clays with good levels of alkali elements can help reduce firing times/temperatures, and reduce the problems associated with these microscopic defects.

Furthermore, the nature of the feldspathic material is important for two reasons:

1. For optimising the dielectric characteristics of the final product – with potassic based materials typically being superior than sodic.
2. For minimising the pyroplastic deformation – again, with potassic being preferable to sodic.

These phenomena make it important to select clays that contain mainly potassic rather than sodic material.

Finally, poor vitrification can also lead to weathering and crazing problems in long-term use of electrical porcelain.

Imerys’ portfolio offers a range of products designed to have a good level of mainly potassic bearing feldspathic material.

Contact us about Common faults in electrical porcelain

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