High-Efficiency and Ultra-precision Glass Molding of Aspherical Lens andMicrostructures(Reprint from Proceedigns of ISUPEN2011)

Tianfeng Zhou, Jiwang Yan, and Tsunemoto Kuriyagawa
Department of Mechanical Systems and Design, Graduate School of Engineering, Tohoku University
Aoba 6-6-01, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
Abstract: Glass molding press (GMP) process was applied to produce aspherical glass lens in mass production. In the traditional
isothermal glass molding press (IGMP), the four stages for aspherical lens fabrication, heating, pressing, annealing and cooling werecarried out one by one in the molding chamber. As a result, the molding cycle is long and the mold life is short. In this paper,
nonisothermal glass molding press (NGMP) was proposed to fabricate aspherical lenses at high productivity. The characteristics ofNGMP were studied by comparing with the traditional IGMP. Furthermore, GMP process was extended to fabricate microstructureson the glass plate, including microgrooves and micropyramids. The molding condition was optimized to improve the forming
accuracy of the molded microstructures. In this way, the high efficiency and the high precision are confirmed by the GMP
experiments for fabricating glass optical elements.
Key Words: Glass molding, Aspherical lens, Microstructure

1. Introduction of aspherical lens and microstructures
The optical lens is one of the most important parts in the electro-optic

Glass ball

Upper moldsystems, such as digital camera, camera assisted driving system, blue(a)(b)(c)(d)

ray DVD. The name of glass aspherical lens has already shown its
two characteristics. One is that the surface is aspherical and the otherLower moldis that the material is glass. The aspherical surface can suppress
chromatic spherical aberration, which can achieve a clearer image.
And also, a single aspherical lens can replace multi spherical lenses inan optical system, which can make the optical system more compact
by reducing the number of lenses [1]. Glass material has many
advantages over plastic. For example, the glass can endure theenvironmental change, because it is heat resistant and chemicalFig. 1. Process flowchart of the IGMP process: (a) heating, (b)pressing, (c) annealing, (d) cooling.

resistant. What’s more, the glass lens can increase the peripheral
brightness because of a better light permeability.

Upper mold

Glass ballMicrogrooves and micropyramids are widely used in the optical(a)system, biology equipment and so on. For example, microgrooves or
micropyramids can improve the imaging quality and brightness as the(b)Preheating bucketLower moldbacklight panels in the LCD monitor [2]. As the increasing needs ofthese kinds of optical elements, including aspherical lens and glass

Molded lensmicrostructures, we have to face the challenge: “How to fabricate(c)them efficiently?”(d)
Cooling plate

2. Innovation of glass molding press for aspherical lens
In conventional GMP method, as shown in the process flow of Fig. 1,
the glass ball is first placed onto the lower mold, and then, heatedFig. 2. Process flowchart of the NGMP process: (a) preheating, (b)pressing, (c) annealing, (d) cooling.

together with the mold. After that, the pressing is performed to form
the aspherical lens. Finally, the formed lens is cooled to room
temperature [3]. In this process, the temperature of the glass is theAffiliated unit

Robotic arm

same as the temperature of the mold, so this method is termed

Molding Chamber“isothermal glass molding press (IGMP)”. According to the thermal
cycle, a typical IGMP process can be divided into four stages: heating,Main molding unitpressing, annealing and cooling. Because all the four molding stages
are controlled successively in the time sequence in the same molding
chamber, the molding cycle is long, and it leads to a low efficiency.

Cooling unit

BucketPreheating unit

Additionally, the temperature change of the molding dies is large, andit results in a short mold life.

Fig. 3. Schematic diagram of the general structure of thehigh-efficiency and ultra-precision glass molding machine

In order to solve these problems, a new method, namelyISUPEN 20121

0. 83zA SP1
R0

proposed. In the NGMP process, as shown in Fig. 2, the temperature.1

2. 13First, the glass ball is heated to a temperature above the molding

R0temperature in a preheating bucket. Second, the high-temperature

x.1

control of the glass ball is separated from that of the molding dies.

0.1 54 4I 4.0 0“nonisothermal glass molding press (NGMP)” for aspherical lens isglass ball is pressed to form a lens by the low-temperature molding

1.1 474

ASP 2

dies. Third, the formed glass lens is annealed in the molding dies toI 4 .30

release the internal stress. Finally, the annealed formed lens is taken

I 5 .00

out from the molding dies and cooled separately down to the room
Fig. 4. Designed biconvex aspherical lens.

temperature on the cooling plate.
In the NGMP method, when one glass ball is in the pressing stage
and annealing stage, next glass ball is being heated in the preheatingstage, and last molded lens is being cooled in the cooling stage. By
controlling these stages in a parallel way, the average cycle time isremarkably reduced. Additionally, the temperature change of themolding dies in NGMP is smaller than that in IGMP, so the mold life
will be prolonged.
3. Experiments of IGMP and NGMP(a)(b)
Fig. 5. Photographs of the molds for aspherical lens: (a) upper mold,(b) lower mold.

Both the isothermal and nonisothermal experiments were carried out

the main molding unit is assembled by the upper axis, the upper mold,the lower mold, the lower axis, the load cell and the servo motor. Inused, at the same time, and a robot arm was added to transport thepreheated glass from the preheating unit to the molds and move the800

1750

700

1500

85
84
83

Temperature

600

82

1250
1000
750

500

81

Position

400

80
79

300

Pressing load

500

200

250

100

0078
77

molded lens from the molds to the cooling unit. While in the IGMP

76

experiments, only the main molding unit was used.
A biconvex lens with two aspherical surfaces, ASP1 and ASP2, was

75
050

100

150

lower die, made of tungsten carbide (WC), was generated by the200250300350

400Time (s)designed as shown in Fig. 4. Fig. 5 is the photographs of theassembled molds. The aspherical surfaces of the upper die and thePosition (mm)the NGMP experiments, a preheating unit and a cooling unit were

2000

Temperature (qC)

general structure is schematically shown in Fig. 3. From top to bottom,

Pressing load (N)by using a newly developed glass molding press machine, and theFig. 6. Time sequence of temperature, lower mold position andpressing load in a molding cycle of IGMP.

diamond grinding, and coated with diamond-like-carbon (DLC) film.
2000

800

bases (molds), and guided by two location pins to minimize the1750

700

decentration. The diameter of the mold is 40 mm.

1500

600

pressing load and lower mold position in NGMP are plotted in Fig. 7.

84
83

Temperature

82

1250
1000
750

Temperature (qC)

position are plotted in Fig. 6. The time sequences of temperature,

85

500

81400

Position

80

300

Pressing load

7978

By comparing Fig. 6 with Fig. 7, first, we can find that the molding

500

200

cycle time has been shortened from 6 minutes to 2 minutes. Therefore,

250

100

the productivity of the NGMP method is three times that of the IGMP

00Position (mm)The time sequences of temperature, pressing load and lower moldPressing load (N)Both the upper die and the lower die were assembled in a pair of die

7776
75
0method for aspherical lens fabrication. Second, the temperature

50100

150200250300350400Time (s)change of the molds in IGMP is in the range of 150ºC ~ 580ºC, while
the temperature change of the molds in NGMP is ranging from 500ºCFig. 7. Time sequence of temperature, lower mold position andpressing load in a molding cycle of NGMP.

to 580ºC, which is much smaller than that in IGMP. From thetemperature changes of the molds, we can make a simple predictionthat the mold life will be longer in NGMP.
Fig. 8 is the photograph of the molded aspherical lenses. Figs. 9(a)and (b) show the measurement results of form errors (including theupper surface and the lower surface) of a molded lens fabricated by
the IGMP and NGMP. For the lens molded by IGMP method, except
for the measurement error on the rim of the lens, the form error of theupper surface (ASP1) is within ±0.1 Pm, where a slight negative

ISUPEN 2012

Fig. 8. Photograph of the molded aspherical lenses.

2deviation at the lens center is shown (Fig. 9(a)). The total form errorof the lower surface (ASP2) is 0.26 Pm, –0.18 Pm in the center and0.5+0.08 Pm in the outer region (Fig. 9(b)). For the lens molded by

0.40.3surface is in the range of +0.15 ~ –0.5 Pm (Fig. 9(b)), which is a littlelarger than that by IGMP.
The form errors of the molded lens might be caused by theForm error (Pm)

NGMP method, the form error of the upper surface is almost the sameas that by IGMP method (Fig. 9(a)), but the form error of the lower---- IGMP---- NGMP

0.2
0.1
0.0
-0.1
-0.2

following three reasons: the manufacturing errors of the molding dies,-0.3

the shrinkage of the lens during cooling and the deformation of the-0.4

molding die during press. The form compensation of the shapes of the-0.5-2.0mold dies can improve the form accuracy of the molded aspherical

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

1.5

2.0

Distance from lens axis (mm)glass lens.

(a)
4. Innovation of glass molding press for microstructures
0.5

easy to be machined directly. Traditionally, the microgrooves and0.4

micropyramids are made by the MEMS techniques, but sharp angle

0.3

microgrooves are not easy to be fabricated. Micro cutting can produce
sharp microgrooves, but this method is time consuming and is notsuitable for mass production. Though micro injection molding andsheet nano imprinting can fabricate microgrooves and micropyramids
in mass production, it is only suitable for plastic. In order to make themicrogrooves and micropyramids on the glass plate efficiently, as
shown in Figs. 10(a) and (b), glass molding press (GMP) wasproposed to extend to fabricate the microstructures.
The mold with microstructures should be generated. WC is noForm error (Pm)As every one knows that the glass material is brittle and hard, and not---- IGMP---- NGMP

0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4-0.5
-2.0

-1.5

-1.0

doubt a good mold material in GMP, but it is too hard to be fabricatedefficiently. Nickel phosphorus (Ni-P) has a good machinability inroom temperature as well as a relative high hardness in high

-0.5

0.0

0.5

1.0

Distance from lens axis (mm)(b)
Fig. 9. Form accuracy of the molded lens by IGMP and NGMP: (a)upper surface, (b) lower surface.

temperature pressing [4]. The microgrooves are cut by a sharp
diamond cutting tool transversely fed in the horizontal direction whilethe depth of cut is kept constant. By periodically shifting the tool
perpendicularly to the cutting direction, parallel microgroove arrays
can be generated. While in the micropyramid cutting, the workpiece isGlassthen rotated for an angle of 90º after cutting an array of microgrooves,and the cross-grooving operation is performed. In this way,

Microgroove moldmicropyramid arrays can be created on the mold surface.
In the GMP for microstructures, first, microgrooves on the Ni-P
layer are fabricated by micro cutting and used as the mold in the glass
molding press. Second, a glass preform is heated and softened at high(a)temperature, and then, it is pressed between a pair of molds. In thisway, the shapes of microgrooves on the mold are replicated to theglass plate. The GMP method can produce microgrooves on the glass
plate in high productivity at a low cost. Similarly, micropyramids canbe molded by using the micropyramid mold.
In the microstructure molding, the glass material will be difficult to

Glassfill up the tiny groove channel, and results in an incomplete
replication of the mold shape. Therefore, the molding condition

Microgroove moldshould be optimized.
5. Experiments of microstructure forming
Both the microgrooves and micropyramids have the samecross-section profile, which is schematically shown in Fig. 11. The(b)Fig. 10. Schematic of glass molding for microstructures (a)microgrooves and (b) micropyramids.

depth is 5 Pm, the pitch is 10 Pm, and the cross section of aISUPEN 20123

microgroove or a micropyramid is isosceles right triangle. Fig. 12 isphotograph of the mold for microstructures manufactured by single

Molded microstructures

diamond cutting.
5 Pm

In the GMP experiments, the effects of the pressing velocity wereinvestigated. When the molding temperature was fixed at 570ºC, thestress is found to be proportional to the pressing velocity. The stress,
lower than 2 MPa, was adopted in the molding process. Therefore, themolding temperature of 570ºC and the pressing velocity of 6 mm/min
is considered to be the optimum molding condition, and used in theMold
10 Pm

Fig. 11. Schematic diagram of the cross section of microgrooves andmicropyramids.

experiments.
Microgrooves

Fig. 13 shows the photograph of one of the molded lenses. This is
the picture of the mold with microgrooves and micropyramids on it.
This is one of the molded microgrooves and micropyramids on theMicropyramids

glass plate. The 3D sectional profiles of the molded microstructures
were measured. As shown in Figs.14(a) and (b), the height of themolded microgrooves is 4.72589 Pm, and the height of the molded
micropyramids is 4.37375 Pm. Therefore, the calculated filling ratioof the microgroove forming is 94.5%, and the filling ratio of themicropyramid forming is 87.5%. The forming error of the5 mm

Fig. 12. Photograph of the mold for microstructures.

microgrooves between the molded lens and the mold can becontrolled within 0.38 Pm, and the forming error of the
micropyramids is about 0.63 Pm, which shows that the GMP method

Microgrooves

is can generate the microstructures at submicron meter level
efficiently.

Micropyramids

6. Conclusions

5 mm

Aspherical lens are fabricated by using the WC mold, andmicrostructures are fabricated by using the Ni-P mold. In comparison

Fig. 13. Photograph of the molded microstructures on glass plate.

of the aspherical lens forming with the microstructure forming, theGMP for aspherical lens molding concentrates on the smooth surface
replication with a high efficiency, and the GMP for microstructure
molding concentrates on the parallel microgroove and the periodic
micropyramid replication with a high precision. The main conclusions
drawn from this paper are as follows.
(1) NGMP was proposed to fabricate aspherical lenses, which canincrease the productivity to 3 times and prolong the life of themolding dies.
(2) Although the aspherical lenses fabricated by IGMP and NGMP
both have submicron level form accuracy, the form error of lenses(a)fabricated by NGMP method is a little larger than that by IGMP.
(3) Microgrooves and micropyramids were successfully molded on
the glass piece by using Ni-P mold. Microstructures on the glass piececan be obtained with form accuracy of several ten nanometers
References[1] Johnson R.B., Michael M. Aspheric glass lens modeling andmachining. In: Proceedings of SPIE, Current Developments in Lens
Design and Optical Engineering VI. CA, USA: pp.58740B. 2005.
[2] Yan J., Ultraprecision machining for Fresnel lenses and theirmolds, Chapter 3.1 in Design and Manufacturing of Rear-Projection
Optical Systems: Technical Information Institute, Tokyo. 73-94. 2006.
[3] Katsuki M. Transferability of glass lens molding. In: Proceedingsof SPIE, 2nd International Symposium on Advanced Optical
Manufacturing and Testing Technologies. China: pp.61490M. 2006.
[4] Masuda J., Yan J., Kuriyagawa T. Application of the NiP-plated
steel molds to glass lens molding. In: 10th International Symposium
Advances in Abrasive Technology. USA: pp.123-130. 2007.

ISUPEN 2012(b)
Fig. 14. 3D topographies of the molded microstructures on glass plate:(a) microgrooves and (b) micropyramids.

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