Difficulties in the Thermal Bending Process of Curved Glass
With the rapid development of fields such as consumer electronics, automotive smart cockpits, and smart homes, curved glass has become a core component of many high-end products due to its smooth appearance, excellent optical performance, and outstanding protective capabilities. As the core forming category of curved glass, heat-bent glass has a production process whose maturity directly determines the quality and yield of products. From ordinary flat glass to heat-bent glass that meets the requirements of complex curved surfaces, the entire forming process involves technical challenges in multiple dimensions such as material properties, temperature control accuracy, and mold design. These difficulties have also become key factors restricting the large-scale and high-quality production of the industry.
1. Fundamental Process Challenges Caused by Glass Material Properties
The physical and chemical properties of glass itself are the first obstacle in the thermal bending forming process. Commonly used heat-bent glass is mostly high-aluminum-silicon glass or soda-lime glass. Although this type of glass has high strength and light transmittance, it is prone to various defects during the high-temperature thermal bending process. Firstly, there is the issue of matching the thermal expansion coefficient of glass. There are slight differences in the thermal expansion coefficients of glass original sheets from different batches. Thermal bending forming requires heating the glass to its softening point (usually in the range of 600℃-750℃). If the heating rate is uneven or the temperature fluctuates greatly, internal stress will be generated inside the glass due to different degrees of thermal expansion and contraction. After cooling, problems such as warping, cracking, or even spontaneous explosion may occur.
For curved glass, the design of its curved surface radius and curvature varies greatly. Some are single-curved surfaces, some are double-curved surfaces, and some are even 3D special-shaped curved surfaces. This places extremely high requirements on the ductility of the glass. The forming of heat-bent glass essentially involves the plastic deformation of glass in a softened state. However, glass is a brittle material. During the deformation process, if the local stress is too high or the stretching degree exceeds the material limit, defects such as surface scratches, edge chipping, and wrinkles will appear. Especially for double-curved heat-bent glass, the stress concentration at the edges and the transition areas of the curved surface is more obvious. Once the process parameters are not properly controlled, the yield rate will drop significantly. In addition, the surface cleanliness of the original glass sheet also affects the thermal bending effect. Micro-dust and oil stains on the surface of the original sheet will react with the glass at high temperatures, forming defects such as pitting and bubbles, which seriously affect the appearance and performance of curved glass.
2. Forming Defects Caused by Insufficient Precision of Temperature Control Systems
Temperature control is a core link in the heat-bent glass forming process and one of the most difficult technical challenges to overcome. The thermal bending forming of curved glass goes through multiple stages including preheating, heating, heat preservation, forming, and cooling. Each stage has strict requirements on the temperature range and heating/cooling rate. Currently, most thermal bending equipment adopts an integral temperature control system, which is difficult to achieve precise temperature control for different areas of the mold. However, different parts of curved glass (such as the arc top, arc edge, and flat transition area) require different amounts of heat during the forming process. If the temperature distribution is uneven, the softening degrees of different parts of the glass will be inconsistent, leading to problems such as curved surface radius deviation and uneven wall thickness after forming.
Taking 3D curved glass as an example, its edges need to be bent to an angle close to 90°, and this area requires a higher temperature to ensure the glass is fully softened. However, if the temperature in the middle flat area is too high, it is prone to collapse due to over-softening. If the precision of the temperature control system can only reach ±5℃, it will be unable to meet the forming requirements of complex curved surfaces, and it will be difficult to control the dimensional tolerance of the finished product within the industry standard of ±0.05mm. At the same time, the rate control during the cooling stage is also crucial. Rapid cooling will generate huge thermal stress inside the heat-bent glass, leading to micro-cracks in the glass. On the other hand, excessively slow cooling will reduce production efficiency and may cause crystallization of the glass due to its long-term exposure to high temperatures, which affects the light transmittance and strength of the glass. In addition, the stability of the temperature control system is also of great importance. If temperature drift occurs after the equipment has been in operation for a long time, the forming quality of curved glass in the same batch will be uneven, putting great pressure on subsequent quality inspection and screening.
3. Technical Bottlenecks in Mold Design and Adaptability
The mold is a key carrier for the forming of heat-bent glass. The rationality of its design and the adaptability of its material directly affect the final forming effect of curved glass, which is also a long-standing technical bottleneck in the industry. Firstly, in terms of mold material selection, the mold needs to work repeatedly in a high-temperature and high-pressure environment. It must not only have excellent high-temperature resistance and wear resistance but also ensure low adhesion with the glass. Early thermal bending molds mostly used graphite materials. Graphite molds have good thermal conductivity and high-temperature resistance but low hardness. After long-term use, they are prone to wear and deformation, leading to a decline in the dimensional accuracy of curved glass. New ceramic molds, although having high hardness and strong wear resistance, have poor thermal conductivity, which affects the uniform heating of the glass. Moreover, their high cost makes them difficult to promote on a large scale.
Secondly, in terms of mold structure design, the curved surface shapes of curved glass are diverse. The mold cavity must completely match the curved surface parameters of the product, including the radius of curvature, arc height, and opening angle. Any slight design error will cause the heat-bent glass to have an inconsistent curved surface after forming. At the same time, the design of the mold's exhaust structure is also particularly important. During the forming process of heat-bent glass, air will remain between the mold and the glass. If the exhaust is not smooth, the air at high temperatures will be compressed to form bubbles or leave indentations on the glass surface, damaging the surface flatness of curved glass. In addition, the contact method between the mold and the glass also affects the forming quality. Hard contact is likely to scratch the glass surface, while soft contact may cause adhesion due to insufficient high-temperature resistance of the material. How to balance the contact method and the forming effect is a major problem in mold design. For mass production, the service life and replacement cost of the mold also need to be considered. A set of high-precision molds is expensive, and if the service life is short, it will significantly increase the production cost of heat-bent glass.
4. Supporting Technical Shortcomings in Post-Processing Technology
After heat-bent glass is formed, it does not directly become a finished product. It still needs to go through a series of post-processing procedures such as grinding, polishing, and strengthening. The supporting technical shortcomings in post-processing technology have also become important factors restricting the quality improvement of curved glass. The surface of curved glass will inevitably have slight scratches and unevenness during the thermal bending process, which requires grinding and polishing to improve the surface finish. However, the irregular shape of the curved surface poses great challenges to grinding and polishing. Traditional flat grinding equipment cannot adapt to the complex shape of the curved surface, while specialized curved surface grinding equipment is not only expensive but also has problems such as low polishing efficiency and difficulty in controlling surface roughness. If the polishing is not in place, the light transmittance of heat-bent glass will be affected, and it will also fail to meet the appearance requirements of high-end fields such as consumer electronics.
Strengthening treatment is a key process to improve the strength of heat-bent glass. By means of chemical tempering or physical tempering, a compressive stress layer is formed on the glass surface, which can greatly improve the impact resistance and bending resistance of the glass. However, the strengthening treatment of curved glass is much more difficult than that of flat glass. During chemical tempering, the curved shape of the glass will reduce the uniformity of ion exchange. The thickness of the strengthened layer in the arc edge area is often lower than that in the flat area, making the edge of curved glass a weak point in strength. Physical tempering, on the other hand, is prone to curved surface deformation after tempering due to the uneven stress on the curved glass. In addition, the connection between the post-processing procedures of heat-bent glass is also crucial. If the glass is not cleaned properly after grinding, the remaining grinding fluid will affect the strengthening effect. If the glass has dimensional deviations after strengthening, it cannot be corrected twice and can only be scrapped, which further reduces the overall yield of curved glass.
5. Process Upgrade Challenges Under Industry Development
With the continuous upgrading of market demand for curved glass, the forming process of heat-bent glass is also facing new challenges. On one hand, the consumer electronics field has increasingly high requirements for the thinness and lightness of curved glass. The thickness has gradually decreased from the original 0.7mm to 0.3mm or even thinner. Ultra-thin glass is more prone to deformation and cracking during the thermal bending process, which places higher requirements on the stability and precision of the process. On the other hand, curved glass in the automotive field has larger sizes and more complex curved surfaces. For example, the 3D curved glass used in in-vehicle large screens not only needs to meet the forming requirements of large sizes but also needs to have special properties such as UV resistance and anti-glare. This requires integrating more functional technologies into the selection of original sheets and the forming process of heat-bent glass.
At the same time, the concept of green and environmentally friendly production has also put forward new standards for the heat-bent glass process. Some release agents and cleaning agents used in traditional processes have environmental risks, so it is necessary to develop more environmentally friendly alternative materials. However, this may affect the forming quality and production efficiency of curved glass. In addition, the trend of intelligent production requires the integration of the heat-bent glass process with technologies such as automated inspection and big data analysis to realize real-time monitoring of the production process and parameter optimization. However, the equipment and systems of most enterprises have not yet completed intelligent upgrades, making it difficult to realize the full-process quality traceability and process iteration.
Conclusion
As the core forming product of curved glass, the process difficulties of heat-bent glass run through the entire production process from raw materials to post-processing, involving multiple technical dimensions such as materials, temperature control, molds, and post-processing. With the rapid development of downstream application fields, the market demand for curved glass continues to grow, and the requirements for product quality and process level are becoming increasingly strict. Only by continuously breaking through technical bottlenecks such as temperature control precision, mold design, and post-processing support, and integrating the concepts of intelligent and green production, can we promote the continuous upgrading of the heat-bent glass forming process, meet the diversified and high-quality needs of various industries for curved glass, and help the industry achieve high-quality development.
