Technical Information

G-3 High-Precision Optical Glass Heating and Molding Technology

G-3 High-Precision Optical Glass Heating  and Molding Technology

The high-precision optical glass mold press machine is used for molding aspheric glass lenses in digital cameras, etc. To obtain a molded product whose lens has 0.1μm or less form accuracy, the molding knowhow of high-transferability, such as uniform heating and cooling of mold and glass, is required. Now, we introduce the molding processes of the fixed mold type molding-press machine.

[Platform]
G. Manufacturing technology based on features of molds
[Applications]
High-precision optical glass mold press machine.

[Technical points]

  • Heating with infrared lamps.
    1. Uniform heating of large-diameter mold is possible.
    2. The maximum heating temperature of 800°C can cope with almost all types of optical glasses.
  • Two-step molding technique
    Glass is pressed again near the transition point to compensate shrinkage caused during cooling.
  • Form accuracy of molded product is 0.1μm or less.

1. Introduction

The glass history is also molding history is very long. Examples are; the bottle, tableware, flower vase, electric bulb, fluorescent lamp, CRT, etc. For instance, in the technique of pressurizing molten glass which was poured in a mold is in the forming of glass bottles, however, the glass will shrink at the time of cooling and a highly form accurate cannot be expected. To obtain the high form accuracy development of sophisticated technology is essential.

2. Precision molding of glass elements

Fig. 1 Glass molding-press machine GMP-311VA

Fig. 1 Glass molding-press machine GMP-311VA

The principle of precision molding can be summarized as pressure-molding of glass in an area that allows plastic deformation at a temperature that is as low as possible to minimize shrinkage during cooling and maintain the accuracy. This technique is called the reheat press, as solid glass of predetermined size is reheated in the machine, or referred to as the isothermal press because the mold and glass are heated, pressed and cooled at an almost same temperature. To realize the precision molding of glass, harmonization of the technologies for molding-press machine, glass material and mold is very important. Even if one of these technologies is absent, the realization of precision glass molding is rather difficult. Fig. 1 is a photo showing the glass molding-press machine.

3. Mold Heating technology

3.1 Machine structure

Fig. 2 Basic structure of molding chamber

Fig. 2 Basic structure of molding chamber

Fig. 2 shows the molding chamber of a fixed mold type molding-press machine. This machine consists of the; molding chamber covered by the quartz glass tube, heating unit containing infrared lamps which are located outside the tube and the mold drive unit. In the molding chamber, the upper and lower molds are arranged and the lower mold is driven vertically by CNC control. For the motion of the lower mold, control of press force is possible due to feedback detected by the load cell equipped in the lower mold drive shaft and the press force setting can be specified more flexibly due to an electric servo press control. Air in the molding chamber is replaced with nitrogen gas to prevent oxidization of the molds. To cool the mold and molded products after pressing flow rate controlled nitrogen gas is supplied.
The fixed mold type molding-press machine allows easy setting of mold conditions, as it can control the temperature by directly measuring the mold temperature and realize consecutive processes. Thus, this machine is suited for molding large-diameter lenses with high accuracy. As it performs heating, molding and cooling in the same molding chamber, however, productivity may not be so high, compared with the moving transferred molds type machine. Additionally, a machine with a vacuum molding function is also available, which maintains vacuum in the molding chamber to prevent oxidization of the mold and gas accumulation molded products.

3.2 Heating and cooling

Fig. 3 Monitor display of actually measured values of molding conditions

Fig. 3 Monitor display of actually measured values of molding conditions

The heating system uses infrared lamps and is characterized as shown below.

  1. Uniform heating of large-diameter molds is possible.
  2. No restriction is imposed on the size of material subject to heating, which is a defect of high-frequency induction heating.
  3. Mold material (glass) is widely applied.
  4. The maximum heating temperature of 800°C can cope with almost all types of optical glasses.
  5. Setting of heating and cooling speeds is easy.
  6. With infrared lamps and nitrogen gas flow rate control, control of fine heating and cooling speeds is possible.

Fig. 3 Exemplifies the change in temperature during heating and cooling.

3.3 High-accuracy molding technique

Fig. 4 Effect of 2-step molding technique

Fig. 4 Effect of 2-step molding technique

For the high-accuracy molding, setting of optimal conditions is essential according to the profile, size, required accuracy of molded product, and the physical property, profile, etc. of the raw material. At present, the optimal conditions are specified by an estimation from the experienced past conditions for similar products achieved and through trials and errors.
The molding temperature is a key point of the molding conditions and is has been determined by fully considering the balance of glass viscosity and shrinkage due to cooling (i.e., deterioration of accuracy). As this machine allows positioning of the mold and setting of press force, however, extremely high-accuracy molding as described below is possible.
Glass will soften by heating. But if it is molded at a temperature below the transition point (Tg), the glass will crack and molding is not possible. The glass viscosity will go down at a temperature above the yield point (At) and in the vicinity of the softening point (Sp). When this happens molding is easy, but shrinkage due to cooling becomes greater and the so-called "shrinkage defect" will cause the accuracy of mold products to deteriorate. To compensate this shrinkage, this machine allows re-pressing near the transition point. That is, the glass is pressed again near the transition point to compensate the "shrinkage". As the press force is controlled in two steps, it is called the 2-step molding technique.
In Fig. 3, the green-colored line represents change in press force during the 2-step molding process. In the process of the 1stpressing, the molding temperature is 600°C and the press force is about 10kN. Likewise, in the process of the 2ndpressing, the press force is 15kN during gradual cooling.
Fig. 4 shows an example of the effect of the 2-step molding technique where biconvex lenses of 33mm in diameter were molded. From it you can find that the form accuracy has been improved dramatically.

4. Conclusion

We have introduced the heating, cooling and molding techniques. For the optimal molding conditions, we are now investigating them through trials and errors. Evolution of the diversity of applications can be expected but according to various setting methods of molding conditions.
In the past, we have been engaged mainly in molding of aspheric glass lenses. In the years to come, we will develop the molding technology for the design parts such as cover glass born from the glass characteristics, in addition to the high-accuracy parts such as lenses.