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Electrical porcelain troubleshooting

ELECTRICAL PORCELAIN TROUBLESHOOTING

  • Shaping defects
  • Drying defects
  • Firing defects
  • Poor vitrification

During the production of large electrical porcelain specimens the shaping, drying, and green handling stages are extremely sensitive. Serious defects can occur at these stages if the body is not sufficiently strong, plastic, or has poor drying properties. For example, cracks may appear that require repair/reprocessing work, or the piece may not be recoverable at all.

Shaping defects

The key to achieving good shaping properties is to use clays with a high level of workability or plasticity.

For this reason, Imerys has developed a test to assess the workability of clays and/or bodies. The test characterizes the amount of work energy that can be applied to a clay (or full body formulation) without causing the clay to crack. This indicates either how hard a clay (or body) can be worked, or how likely the clay (or body) is to crack under given conditions. High workability can also control the formation of cracks during drying – see Drying defects below.

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

  • UK: Hymod Blue
  • US: M & D
  • Thailand: Thaiplas BC2
  • India: PowerClay ND10

Drying defects

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

The balance of workability – see Shaping defects 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 offers a range of products with mineralogies that provide a good balance of workability and drying performance allowing faster production and higher yields.

  • UK: Hymod Blue
  • US: M & D
  • Thailand: Thaiplas BC2
  • India: PowerClay ND10

Firing defects

Electrical porcelain specimens are often very large. This makes their firing difficult and sensitive to several potential defects.

Blue/Black coring

Large electrical porcelain specimens must be fired very carefully in order to allow all gases generated during firing to be released so as 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 fewer blue/black core defects.

Imerys offers a range of low-carbon products allowing faster production and higher yields:

  • UK: Hymod Blue
  • US: M & D
  • Thailand: Thaiplas BC2
  • India: PowerClay ND10

 

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 use clays with low silica content in order to maximize fired strength – and minimize 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 in the body because this can lead to microscopic defects in the structure of the fired specimen, leading to weak areas which may be difficult to detect in laboratory testing. These defects can also lead to poor dielectric characteristics – see Poor Vitrification section.

Imerys offers a range of products designed to have as low silica as possible:

  • UK: Hymod Blue
  • US: M & D
  • Thailand: Thaiplas BC2
  • India: PowerClay ND10

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 use clays with good alkali levels (feldspathic materials) otherwise they may be refractory and prevent good vitrification. Clays with good levels of alkali can help reduce firing times/temperatures as well as problems associated with these microscopic defects.

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

  • For optimizing the dielectric properties of the final product – with potassic materials typically being superior to sodic materials.
  •  For minimizing pyroplastic deformation – again, with potassic materials 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 of electrical porcelain over time.

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

  • UK: Hymod Blue
  • US: M & D
  • Thailand: Thaiplas BC2
  • India: PowerClay ND10

 

Still having issues with your electrical porcelain production? Have a question you need to ask us?

Please contact us. We will be only too happy to help.

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