The Production Process of Hollow Ceramic Insulators

As a core component in power systems, hollow porcelain insulators serve the dual functions of mechanical support and electrical insulation. Their internal hollow structure not only reduces weight and lowers material costs but also enhances vibration resistance thanks to an innovative elastic design, making them the preferred solution for applications such as ultra-high-voltage transmission and DC power stations. This article will systematically analyze the entire process—from raw material preparation to final product testing—unveiling the intricate manufacturing secrets behind this precision component.

## I. Raw Material Preparation: The Cornerstone of Basic Performance

The performance of hollow porcelain insulators heavily depends on the purity and precise ratio of their raw materials. Production begins with the rigorous selection of materials such as kaolin, feldspar, quartz, and alumina, ensuring that their chemical compositions remain stable and impurity levels stay below 0.5%. After being crushed, these raw materials proceed to the ball-milling stage, where a rotating ball mill reduces the particles to below 200 mesh. Deionized water is added during this process to create a uniform slurry. A critical factor at this stage is carefully controlling the milling time and rotational speed, as excessive grinding could lead to particle agglomeration.

The slurry is then passed through a three-stage screening system, sequentially removing coarse particles larger than 0.1 mm, iron-containing impurities, and organic matter. The remaining material is further processed using a magnetic separator to isolate ferrous microparticles, ensuring that the raw material achieves a purity level of over 99.8%. Finally, the slurry must undergo viscosity testing, with the viscosity value carefully controlled within the range of 1500–2000 mPa·s to guarantee the stability of subsequent forming processes.

## II. Forming Process: Shaping Structural Precision

The forming process combines slurry casting with vacuum kneading. The casting molds are made from high-strength plaster and feature meticulously designed internal venting channels, ensuring that no air bubbles remain during the slurry filling stage. After the slurry is poured into the mold, it must be left undisturbed in a vacuum environment for 2 hours, allowing moisture to distribute evenly while eliminating any trapped air. Once removed from the mold, the green body still retains a moisture content of 18%–22%, requiring gradual dehydration in a hot-air circulation drying chamber: first at a low temperature of 40°C for 6 hours, then increasing the temperature to 80°C for another 12 hours, ultimately reducing the moisture content to below 1%.

The vacuum kneading machine plays a critical role at this stage, using 10 MPa of pressure to thoroughly remove air bubbles from the clay material, resulting in a solid clay segment with a density of up to 2.3 g/cm³. After kneading, the green body must undergo a trimming process, during which CNC machines are employed to precisely finish the inner walls, ensuring that the diameter tolerance of the hollow structure remains within ±0.5 mm, while wall thickness uniformity is tightly controlled within ±0.2 mm.

## III. Firing Technology: The Core of Performance Enhancement

The firing process is divided into two stages: pre-firing and high-temperature sintering. Pre-firing is carried out in an oxidation atmosphere furnace, where the temperature is ramped up at a rate of 5°C/min to 600°C, followed by a 4-hour hold to eliminate crystalline water and organic materials. During this stage, it is crucial to carefully control the heating curve to prevent cracking of the green body caused by thermal stress.

High-temperature sintering employs a combined system of electric kilns and gas-fired kilns, with sintering temperatures adjusted between 1280°C and 1350°C depending on the specific formulation. The 1100kV-class products developed by Xi'an Electric Group utilize segmented temperature-control technology: first, the temperature is ramped up at 10°C/min to 900°C to promote glass-phase formation; then, it is further increased at 5°C/min to 1320°C, facilitating the transformation into the mullite crystalline phase. Finally, the material is held at this temperature for 6 hours to achieve densification. After sintering, the resulting porcelain body must attain a density of at least 2.65 g/cm³ and exhibit a flexural strength of no less than 120 MPa.

## IV. Surface Treatment: The Key to Enhanced Functionality

The glazing process employs the dip-glazing method, with a glaze formulation comprising 80% alumina, 12% mullite powder, and 8% feldspar. After flash-firing at 1250°C, a dense glassy phase layer is formed, resulting in an water absorption rate of less than 0.1%. In the sanding step, corundum sand with a particle size of 0.5–1.0 mm is applied precisely to the areas where metal fittings are assembled. This is achieved using a high-pressure sandblasting machine, ensuring uniform adhesion of the sand particles and increasing the coefficient of friction to above 0.8, thereby guaranteeing robust adhesive strength.

In 2024, New Century Ceramics developed a patented glaze technology that, by incorporating 3% yttria and 1.5% phlogopite mica, boosts the insulator's dielectric strength to 45 kV/mm while simultaneously increasing its mechanical strength by 20%. After firing at 1300°C, the glaze achieves a surface hardness of 7H, effectively resisting both contamination and mechanical damage.

## V. Quality Inspection: Ensuring Safe Operation

Finished product inspection covers three major dimensions: mechanical, electrical, and structural. Mechanical performance testing includes a 50% tensile load test and a bending failure test, with the requirement that the failure load must be at least 2.5 times the rated value. Electrical performance testing involves an insulation resistance test conducted using a 2500V DC voltage; for new products, the insulation resistance must exceed 5000 MΩ, while for products in operation, it should remain above 300 MΩ.

Ultrasonic testing technology can detect internal defects as small as 0.1 mm, using a 40 kHz probe to receive reflected waves.