How to properly preheat a foam ceramic filter during use?

As the core auxiliary material for metal liquid purification in the foundry industry, foam ceramic filters play a significant role in enhancing the purity of castings and reducing defects, thanks to their high porosity, high temperature resistance, and corrosion resistance. However, in practical production applications, many enterprises have encountered issues such as cracking and fragmentation of foam ceramic filters due to neglecting preheating operation specifications or improper preheating methods. These problems not only affect the filtering effect but also increase production costs and slow down production progress. Industry experts point out that correct preheating operation is the prerequisite for the stable performance of foam ceramic filters. It is necessary to combine the characteristics of casting processes and product material properties, and follow scientific procedures to avoid usage risks and help improve the quality and efficiency of casting production.
According to research data from the China Foundry Association, 35% of foam ceramic filters are damaged due to non-standard preheating operations. Among these, over 60% of the cracks are caused by no preheating or insufficient preheating. This results in an approximately 8% increase in casting defect rates, causing significant economic losses to the industry. Especially in high-temperature casting processes, if the foam ceramic filter is not preheated properly and suddenly comes into contact with high-temperature molten metal, it is prone to thermal stress due to uneven thermal expansion and contraction, leading to cracking. Additionally, if the moisture inside the filter is not fully discharged, it may cause gas porosity in the molten metal, affecting the quality of the casting.

"The core purpose of preheating is to remove the moisture adsorbed inside the foam ceramic filter, alleviate thermal shock, gradually bring the filter temperature closer to the temperature of the molten metal, and avoid stress damage caused by sudden temperature changes." According to industry technical experts, foam ceramic filters adsorb a certain amount of moisture during preparation and storage. If directly put into high-temperature casting conditions, the moisture evaporates rapidly, generating steam, which can damage the internal pore structure of the filter. At the same time, when the filter at room temperature comes into contact with high-temperature molten metal (typically between 700°C and 1700°C), the instantaneous temperature difference can lead to inconsistent thermal expansion and contraction inside and outside the filter, resulting in significant thermal stress and ultimately causing cracking and fragmentation. In addition, standardized preheating can optimize the filtering performance of the filter, reduce the flow resistance of the molten metal, and ensure more uniform impurity filtration.

In combination with industry practice and the requirements of the GB/T25139-2010 standard "Foam Ceramic Filter for Foundry", the correct preheating operation of foam ceramic filters should follow the entire process of "preliminary preparation - gradient temperature rise - heat preservation and constant temperature - standardized completion". Each step has clear operational points, and adjustments need to be made flexibly based on different casting processes and filter materials.

Preheating preparation is fundamental and directly affects the preheating effect and the service life of the filter. Firstly, it is necessary to conduct a visual inspection of the foam ceramic filter, eliminating products with pits, scratches, or damages on the surface to prevent further damage to such structurally defective filters during the preheating process. Secondly, the dust and impurities on the filter surface should be cleaned to prevent impurities from melting and adhering to the filter surface at high temperatures during preheating, which could affect the filtering effect. At the same time, according to the installation position of the filter, the sprues and runners should be cleaned in advance to ensure smooth installation after preheating and avoid stress caused by improper installation. Furthermore, the selection of preheating equipment should also be compatible with the filter specifications. Common preheating methods include electric heating and gas heating. Special preheating furnaces can be used for small filters, while large filters can be preheated simultaneously with the mold to ensure uniform preheating temperature.

Gradient heating is a crucial step in the preheating operation, and the heating rate must be strictly controlled to avoid thermal stress caused by rapid heating. Industry practice has shown that the heating rate of foam ceramic filters should be controlled between 50°C/h and 100°C/h, and the heating rate should be slowed down during the low-temperature stage (below 500°C). If the heating rate is too fast, it may lead to rapid evaporation of moisture inside the filter, generating steam pressure, which in turn can cause cracking and pulverization. The requirements for gradient heating vary slightly for foam ceramic filters made of different materials: filters made of silicon carbide have strong high-temperature resistance, and the heating rate can be controlled between 80°C/h and 100°C/h; filters based on alumina and mullite have relatively moderate thermal shock stability, and the heating rate should be controlled between 50°C/h and 80°C/h; high-end zirconia-based filters used in high-temperature casting processes must strictly follow the slow heating standard of 50°C/h to ensure structural stability.

Heat preservation and constant temperature are the guarantees of preheating effect, and the heat preservation time and temperature need to be reasonably set according to the casting process temperature and filter thickness. Generally, the preheating temperature should be controlled at 60% to 80% of the metal liquid temperature, and the heat preservation time needs to be adjusted based on the filter thickness. For filters with a thickness of about 50mm, the heat preservation time is generally 15 to 30 minutes to ensure uniform temperature inside the filter and complete moisture drainage; for filters with a thickness exceeding 80mm, the heat preservation time needs to be extended to 40 to 60 minutes to avoid uneven internal temperature. For example, in aluminum alloy casting, the metal liquid temperature is about 720℃±10℃, and the preheating temperature of the foam ceramic filter can be controlled at 430℃ to 580℃, with a heat preservation time of 20 to 30 minutes; in steel casting, the metal liquid temperature can reach over 1500℃, and the preheating temperature of the filter needs to be increased to 900℃ to 1200℃, with a heat preservation time of 30 to 60 minutes, to ensure that the temperature difference when in contact with the metal liquid is controlled within a reasonable range.

It is worth noting that some special specifications of foam ceramic filters, such as small filter discs made of zirconia and alumina materials, do not require preheating. Forcing preheating may affect their structural performance. These products must be operated strictly in accordance with the instructions provided by the supplier to avoid damage caused by blind preheating. In addition, the temperature needs to be monitored in real-time during the preheating process. A thermometer can be used to track the surface and internal temperatures of the filter to ensure uniform temperature and avoid local overheating or insufficient preheating.

The operational guidelines after preheating are equally crucial, as they directly impact the performance of the filter. After preheating is complete, the filter must be installed within 10 minutes to prevent rapid cooling and subsequent thermal stress. During installation, the filter should be handled gently to ensure proper fit with the sprue and runner, avoiding assembly stress caused by excessive pressure. Once installed, the pouring process should commence promptly to minimize the filter's exposure to air and prevent sudden temperature drops. Additionally, during the pouring process, the flow rate of the molten metal should be controlled to prevent direct impact on the filter from high-temperature metal, thereby further protecting the structural integrity of the filter.

Industry experts remind enterprises that when carrying out preheating operations for foam ceramic filters, they need to combine their actual production conditions and develop targeted preheating plans. On the one hand, it is necessary to strengthen operator training to enable them to proficiently master the preheating parameters and operational procedures for filters of different materials and specifications, thus avoiding human operational errors. On the other hand, a preheating operation ledger can be established to record information such as filter model, preheating temperature, holding time, and usage, summarizing experience and continuously optimizing the preheating plan. In addition, enterprises should choose suppliers with technical strength to obtain professional preheating guidance, and select qualified foam ceramic filters to reduce the risk of damage during the preheating process from the source.

As the foundry industry transitions towards green and precision manufacturing, the application of foam ceramic filters has become increasingly widespread, and the importance of standardized preheating operations has also become increasingly prominent. The relevant person in charge of Foshan Feite New Material Co., Ltd. stated that scientific preheating operations can not only reduce the breakage rate of filters but also enhance filtration efficiency, reduce casting defects, and help enterprises reduce production costs. In the future, with the application of new preparation technologies, the performance of foam ceramic filters will be further improved, and the industry will gradually refine preheating operation standards, providing more targeted technical guidance for foundry enterprises and contributing to the high-quality development of the foundry industry.