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Glass Optical Element made by High precision Mold Press Forming
Toshiba Machine Co.,Ltd.
1. Aspherical Glass Moulding Machine Development
Some lens production way are changing from normal polishing method to press forming.
We call this technology "Glass Mold Press" (High precision glass press forming).
In 1987, Toshiba Machine developed "Glass Mold Press Machine" by high frequency induction heating. But because of this heating system, mold size is limited less than 64mm in diameter, and it is difficult to heat mold uniformly.
We solved these problems by research work of new heating system, and completed new machine GMP-211 in 1993.
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Fig.1¡@GMP-211
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Fig.2¡@Comparison of lens making, normal polishing method and by GMP-211
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2. Introduction to GMP-211
2.1 Aspherical Glass Moulding
GMP-211 is reheat press machine, glass material is heated up to glass transition temperature and press formed.
GMP-211 means G (Glass) M (Mold) P (Press), Maximum Press force is 2,000kgf, and Maximum Mold size is 110mm in diameter.
2.2 Construction of GMP-211 and standard operation
Fig.3 shows construction of GMP-211.
There is the chamber, center of the machine, covered with quartz glass tube. Both sides (top and bottom) of the tube are sealed for protection against the mold oxidation by air, and air in the tube is purged by N2 gas. In the chamber upper and lower mold are held on the upper and lower shafts. Upper shaft is fixed, and lower shaft is actuated by AC-servo motor and controlled. Infrared ray lamps arranged outside of the chamber make heating.
Carry the glass material on the mold, heating and pressing automatically, cool the molds, open chamber and finished work is took out. |
Fig.3 Construction of GMP-211 |
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3. Feature of GMP-211
3.1 Heating
Uniform heating of the mold is the most important for high precision pressing. GMP-211 realized it by Infrared ray lamp.
(1) Possible to heat large diameter mold uniformly; Fig.4 shows mold temperature which size is 72mm in diameter, setting temperature is 580¢J, there are no temperature differences between inside and outside of the mold.
(2) Possible to adopt the mold from 30 to 110mm in diameter, and has versatility of to mold optical parts, such as dimension, form, and quantity of cavities.
(3) Control upper and lower mold temperature independently.
(4) Possible to use ceramic mold.
(5) Maximum heating temperature is 800 degree, so various glass materials is available.
(6) Possible to control cooling slope.
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Fig.4 Mold temperature |
3.2 Press control
For high precision press forming, it is necessary to control press shaft position and press force precisely with high repeatability. This technology is realized by development of Toshiba Machine's precise actuator and control engineering.
(1) Possible to control press shaft position precisely.
(2) Press force is stable.
(3) Possible to set various mold pressing process.
Setting of multiple pressing by combination control of press force and shaft position enable to produce high precision optical parts.
3.3 The characteristic of GMP
Application of above engineering, Strong point of GMP-211 is "Production of High Precision Mold Press parts is achieved with high repeatability and stable condition".
4. Mold press example
4.1 High precision mold press process "2 step pressing"
Important points of high precision mold press are temperature and pressing process. Fig.5 shows relationship between glass temperature and thermal expansion.
Glass is cracked bellow glass transition temperature (Tg). Therefore press process must be performed over (Tg). But Near glass soften temperature (Sp), glass deform easily. And transition point of thermal expansion is also on (Tg). Therefore if press operation is completed near (Sp), high precision pressed parts are not obtained by thermal shrinkage.
Above reason, we proposed, the first, pressing mold near (Sp) forming near final shape as possible, then cool near (Tg) and perform final pressing. By this "2 step pressing" high precision mold parts are obtained as Fig.6 right photograph parts.
Fig.5 Glass temperature and thermal expansion |
Fig.6 2 step pressing |
4.2 Multiple cavity process
Fig.7 is example of 16 cavities of 13mm in diameter lens.
(1) Arrangement of cavity
(2) Evaluation by ZYGO
4.3 Large diameter and thickness lens
(1) Maximum diameter : 86mm (mold of plastic spectacles lens, for example CR39)¡@Fig.8
(2) Maximum thickness : 20mm¡@Fig.9
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4.4 f-feta lens
Fig.10 shows this example; in this case 4 surfaces are formed.
4.5 Element of optical communication system
Fig.11 shows V groove alley which is used for optical communication. Interval of V groove is 250£gm¡Ó0.1£gm.
Fig.10 f£c lens |
Fig.11 V groove alley |
5. Mold
Mold is key technology of glass mold press.
(1) Mold materiel, general speaking, hard metal and ceramic are used and practiced coating on the mold surface.
6. Glass material
Forms of glass material, following pre-formed material are used.
(1) Precision sphere
(2) Gob material
(3) Polished sphere lens
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HKPOMA Secretariat
LG1, HKPC Bldg, 78 Tat Chee Avenue, Kowloon, HK
Tel: (852) 2788 6335 / 2788 5544
Fax: (852) 2788 5002
hkpoma@hkpc.org
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Honorary Chairman
Glass Molding
