What are the causes of internal bubble defects in glass balls and how can the process be improved to reduce the bubble rate?
Release Time : 2026-02-10
The formation of internal bubbles in glass balls is the result of multiple factors, including raw materials, processes, equipment, and operations. Its root cause can be traced back to the physicochemical reactions and physical changes throughout the entire glass melting process. Starting from the raw material preparation stage, uneven sand particle size, insufficient clarifying agent, or excessively low feeding temperature in the batch can lead to insufficient gas escape during the initial melting stage, with residual microbubbles becoming the source of subsequent defects. For example, if carbonates in the raw materials are not completely decomposed, the resulting carbon dioxide gas may form initial bubble nuclei in the melt; if the moisture content of the batch is not properly controlled, water vapor will convert into hydroxyl groups (OH⁻) at high temperatures, entering the silicon-oxygen tetrahedral structure. If these are not completely expelled during the clarifying stage, they may re-precipitate as bubbles later due to temperature changes.
Fluctuations in the melting process are the core cause of bubble formation. During the glass melting stage, if the furnace temperature regime is unstable, such as insufficient clarifying zone temperature or an unreasonable cooling curve after the hot spot, it will lead to an imbalance in gas solubility. As temperature rises, gas solubility decreases, causing gases dissolved in the molten glass to precipitate and form tiny bubbles. If the cooling rate is too rapid, the melt viscosity increases sharply, preventing the bubbles from rising and causing them to remain in the molten glass. Furthermore, improper furnace pressure control exacerbates the bubble problem—a sudden pressure drop disrupts the gas-liquid balance, causing gas molecules to aggregate and nucleate, forming new bubbles. For example, in cathode ray tube glass production, when temperature fluctuations exceed a certain range, bubble density increases significantly, directly demonstrating the crucial role of process stability in bubble control.
The quality and corrosion of refractory materials are hidden factors in bubble formation. If the refractory lining of the furnace has pores or a rough surface, it may adsorb air or release its own gases (such as sulfur dioxide and carbon dioxide). At high temperatures, these gases enter the molten glass and form bubbles. More seriously, chemical reactions between the refractory material and the molten glass, such as the dissolution of silica and alumina into the melt, can alter the local composition, leading to sudden changes in gas solubility and inducing bubbles. For example, in some systems, the bubbles generated by refractory material erosion are relatively large in diameter and often accompanied by streak defects, forming the typical characteristic of "bubbles wrapped in hemp fibers."
Operational procedures and equipment condition directly affect the bubble rate. If air is introduced into the gaps between powder particles during feeding, or if gas is entangled during stirring, external air bubbles will directly form. When removing material, the elongated material fibers flowing into the molten glass will also generate bubbles due to the low temperature and absorption of surrounding air. Furthermore, equipment aging or design flaws, such as worn agitator blades or an unreasonable flow channel structure, can lead to unstable molten glass flow, making it difficult to expel trapped bubbles. For example, frequent removal from the same location can cause material fatigue, forming a swirling, ring-like motion that buries surface gas in the molten glass, creating a defect where streaks and bubbles coexist.
Improving the process to reduce the bubble rate requires coordinated optimization across multiple stages. Regarding raw materials, the particle size distribution of sand should be strictly controlled, the dosage of clarifying agents (such as cerium oxide and sodium nitrate) should be precise, and the mixing uniformity of the batch should be optimized to reduce gas residue. In the melting process, a stable temperature regime needs to be established. During the refining stage, the temperature should be appropriately increased to promote the escape of large bubbles. Simultaneously, a gradient cooling zone should be set after the hot spot to control the cooling rate and prevent the precipitation of small bubbles. By adjusting the furnace atmosphere composition (e.g., maintaining stable water vapor partial pressure), the gas partial pressure should be matched with the melt solubility to suppress physical precipitation. For refractory material selection, materials with stable quality and strong corrosion resistance should be prioritized to reduce reaction with the molten glass. Furnace-building materials that are less prone to bubble formation should be used near the forming section to reduce the risk of corrosion.
Operating procedures and equipment maintenance are equally crucial. Strengthen employee training, standardize operations such as feeding, stirring, and unloading, and avoid artificially introducing gas. Regularly check the equipment status, replace worn parts promptly, and optimize the flow channel design to reduce glass flow fluctuations. In addition, auxiliary processes such as "bubbling" technology can be introduced to accelerate bubble rise through forced stirring; or electric melting facilities can be used to improve melting efficiency and reduce gas residence time. For existing bubbles, annealing can be used to eliminate thermal stress and prevent bubble enlargement or rupture due to stress concentration.
Controlling internal bubble defects in glass balls requires a comprehensive approach across the entire chain, from raw materials and melting to operation and equipment. By precisely controlling process parameters, optimizing raw material and equipment selection, and strengthening operational standardization, the bubble rate can be significantly reduced, improving the light transmittance, mechanical strength, and yield of glass balls, thus meeting the high quality requirements of high-end applications.