Significance As one of the important optical components in optical systems, lenses are widely used in products for imaging, illumination, and optical communications. In general, lenses can be divided into spherical, aspheric, freeform, and microstructure surfaces according to different surface shapes. Among them, the spherical lens is the simplest optical component in terms of design and manufacturing, but the aberration problem exists when a single lens is used, and the bulky lens assembly has to be used for compensation. In contrast, aspheric and freeform lenses have more flexible surface shapes, and the specific optical performance of lenses can be improved by such methods as aberration correction, beam shaping, and field of view expansion, which are beneficial for realizing high precision and integration of optical systems. Optical glass and plastic are the main raw materials for lens manufacturing. Plastic is more economic and lighter, but glass has better mechanical properties, higher thermal stability, and higher refractive index, which meets the demanding requirements of optical performance and stability for most products. High-precision glass spherical lenses have achieved mass production by grinding and polishing, but aspheric, freeform, and microstructure lenses can only be processed one by one with the traditional subtractive manufacturing methods for their complex and irregular shapes. In some cases, even several non-traditional polishing methods such as magnetorheological finishing ( MRF) and ion beam polishing ( IBP) have to be used to produce high- precision surfaces, which is time- consuming and costly. Therefore, an effective and low-cost processing method is required for the mass production of such lenses. Molding is a classical processing method to produce parts by replicating the mold shape. However, conventional fused silica glass was not friendly to the mold life for its high transition temperature (about 1300.), which limited the development of glass molding. Until the 1980s, glass with a low transition temperature emerged, and it could be molded at 300- 700., which promoted the rapid development of glass molding. Nowadays, precision glass molding (PGM) has become a main technology to realize the mass production of glass optical components and the manufacturing of numerous aspheric lenses and partial freeform and microstructure lenses, which only takes a few minutes for one lens. PGM has significant advantages over traditional manufacturing technologies, such as low costs, low pollution, high efficiency, and net shapes, which presents superior development potential.PGM is a complex process affected by factors such as glass material properties, mold manufacturing, process parameters, and molding machines. As higher requirements for shape complexity and quality of lenses are posed, many problems have been exposed and aroused the interest of researchers. There are many advances in glass, molds, machines, and numerical simulations. Therefore, it is important to summarize the current progress and prospect the development trend and challenges of PGM. Progress This study introduces the principle of PGM for glass optical components and elaborately summarizes the recent progress in glass material development, mold manufacturing, molding process optimization combined with finite element simulation, and molding machines. Firstly, various glass materials and glass constitutive models including the Maxwell model, Kelvin model, Burgers model, and generalized Maxwell model are introduced ( Figs. 5 and 6), and the modeling methods of creep and stress relaxation are analyzed. Secondly, the properties of various mold materials are compared, among which tungsten carbide has the best overall performance (Table 1). Ultra- precision grinding, cutting, etching, and polishing are the main methods to produce highprecision molds. The newly developed off-spindle- axis spiral grinding method is employed to fabricate the aspheric microlens array (Fig. 9), and an effective laser-assisted turning method is proposed to achieve the rapid mold machining ( Fig. 12). Subsequently, the typical film materials for molds are compared (Table 2), and the high- hardness Ta- C film is deposited on the mold by the filtered cathodic vacuum arc method, which shows low friction and wear. The degradation mechanism of the noble metal Pt-Ir film and its optimized structure are presented ( Fig. 17). Thirdly, numerical simulation is applied to analyze temperature, stress, friction, and surface shape accuracy in molding. Various molding machines including the single- workstation machine, multi-workstation machine, and ultrasonic vibration- assisted glass molding machine are introduced, and it is found that the multi-workstation machine is more suitable for mass production. The profile accuracy of microgroove molding by the ultrasonic vibration-assisted machine is also improved ( Fig. 31). Finally, the pioneering studies on molding lenses with flexible and complex surfaces such as freeform and microstructure lenses, as well as lens wafer arrays are summarized. Conclusions and Prospects In summary, PGM has been widely employed to produce lenses with aspheric and various flexible surfaces. Technologies in the field of glass material, mold manufacturing, numerical simulation, and molding machines have developed comprehensively and made great progress. However, as the demand of freeform components as well as microstructure and wafer array lenses is increasing, it still faces many challenges in molding complex surfaces in mass production. In the future, these difficulties will be gradually solved, and PGM will become more advanced with highperformance glass and mold materials, effective mold manufacturing technologies, integrated numerical simulation, and external energy field- assisted molding technologies.
